Processing of Industrial Wastewater

#### **Chapter 1**

## Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing Wastewater

*Nadim Reza Khandaker, Mohammad Moshiur Rahman and De Salima Diba*

#### **Abstract**

This chapter will introduce the concept of a novel application of acetogenic pretreatment of textile processing wastewater. Acetogenic pretreatment is traditionally limited to high solids, easy to degrade wastewater to enhance degradation for methane generation. The application of the acetogenic process to a complex wastewater from textile processing facilities is novel and has the potential to remove color, chemical oxygen demand, biological oxygen demand in an energy efficient manner compared to the existing extended aeration processes applied in the industry. The application of the acetogenic process can be achieved to existing treatment facilities with minimum retrofit. The acetogenic operation will ensure the treatment process becoming greener with a small carbon footprint to achieve the goal of efficient wastewater treatment.

**Keywords:** Acetogenic, Pretreatment, Textile Processing Wastewater

#### **1. Introduction**

Anaerobic treatment of industrial wastewater was from its inception limited to wastewater that has a high concentration of biodegradable solids. The anaerobic biodegradation process is a multistep process where the first step in hydrolysis where extracellular enzymes secreted by microorganisms under anaerobic conditions solubilize the biodegradable solids, the subsequent steps being the conversion of the soluble organics in multiple steps to methane and carbon dioxide gas more commonly known as biogas [1]. The sequential transformation of solids to biogas is simply summarized in **Figure 1** below. It is important to note that the transformation process is sequential and complex, and more often than not, the rate limiting step determines the kinetics of the reaction and in most cases, this being the hydrolysis step where complex organics are broken down to soluble products such as organic acids alcohols, that are then converted to the common intermediator of acetic acid, which is then further transformed by methanogenic bacteria to methane and carbon dioxide [1–3].

#### **Figure 1.**

*Simplified schematic diagram of sequential transformation of organic compounds under the anaerobic condition to methane and carbon dioxide.*

#### **Figure 2.**

*Schematic of split transformation of acetogenic methanogenic reactor application.*

In the era of sustainable development, the recent trends have been to optimize the process to enhance the production of biogas from biodegradable solids or high solids wastewater as a source of sustainable renewable energy, meaning biogas. Researchers have demonstrated that splitting the anaerobic process and its application as a twostep process and operating reactors as two-stage reactors (**Figure 2**). In the two-stage operation, the first stage reactor is followed by the methanogenic reactor to produce methane. The advantage of this split mode of operation is that more solids solubilized in the first step will increase the production of biogas in the second step. This split mode of application has been applied successfully to high solids wastewater, where the first reactor, referred to as acetogenic reactor, operates at a low hydraulic detention time in hours, generally between two to four days, followed by the methanogenic reactor with a hydraulic detention time of twenty days [4–6].

Acetogenic pretreatment has been limited in its application to high solids waste or wastewater in two-stage anaerobic reactors to enhance the hydrolysis of solids [7–9]. With a greener operation in mind, researchers have further progressed the acetogenic operation to optimized for hydrogen generation not just as a byproduct of gas of hydrolysis/acetogenesis but to produce hydrogen gas from organic waste solids. Hydrogen being a green fuel that can directly be used to generate electricity by using fuel cells [reference]. The thrust of the research has been to negate any methanogens in the acetogenic reactor, thereby increasing hydrogen yield. This chapter introduces the further progression of application of acetogenic operation of anaerobic reactors dedicated to the treatment of textile processing wastewater. At the laboratory level, progressive researchers have been applying the concept of the acetogenic process to pretreat textile processing wastewater in the hypothesis that anaerobic acetogenic operation of a reactor dedicated to textile wastewater will produce in the reduction of color, chemical oxygen demand, and total dissolved solids in an energy efficient manner [10].

*Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing… DOI: http://dx.doi.org/10.5772/intechopen.99107*

#### **2. Justification for application to textile wastewater**

Textile wastewater is deleterious, containing complex organics, chroma, and also high in dissolved solids. If allowed to be realized to water bodies can be destructive to the aquatic environments. In recent decades the textile industries have been moving to developing economies to take advantage of the cheaper cost of production and deficiencies in regulatory requirements. Case in point Bangladesh, which is a developing industry and the second largest producer of readymade garments in the world. A forty-billion-dollar industry the largest employer of women and a progressive force that had bought the country from a least developed country to a middle income country in a few decades [11]. The flip side to all this is the negative impact on the environment of Bangladesh. Unabated discharge of untreated wastewater from the textile industries has severely affected the water bodies in the areas where the industries are located. The situation is so acute that in sections, the once ecologically sound rivers are highly polluted, and all aquatic life is dead. The picture below shows the unabated release of textile dye in a river in Bangladesh (**Figure 3**) [12, 13].

The reason more often than not for noncompliance by the industries is the cost of treatment [13, 14]. The convention wastewater treatment that is currently7 applied as the industry norm is chemically mediated settling to remove solids, the addition of decoloring agents to remove chroma, and extended biological activated sludge treatment (extended aeration with hydraulic detention times of greater than 13 hrs). The schematic flow diagram of the extended aeration chemically aided process currently used in Bangladesh and other countries is to treat textile processing wastewater shown in **Figure 4** below [11].

The extended aeration process is dependent on chemicals for the settling of solids and also chroma removal; the secondary biological treatment is energy intensive, requiring 7.0 kWh of energy per Kg of BOD5 stabilized due to the aeration required by the aerobic microorganisms in the extended aeration process for operation of the blowers required for aeration. If we can negate the requirement of chemicals for chroma removal and solids removal and also reduce the BOD5 loading to the secondary extended aeration system, this would call for a cheaper and energy efficient process and not to mention the reduction of greenhouse gas emission due to reduced energy requirements of the operation. Acetogenic pretreatment would provide an option of pretreatment that would remove color and solubilize solids and also reduce BOD5 in the wastewater and thereby reduce the BOD5 loading to the secondary aerobic treatment and reducing aeration requirement and thus savings in energy. The schematic of the proposed process retrofit using acetogenic pretreatment is shown in **Figure 5** below.

In the subsequent sections, the efficacy of the acetogenic pretreatment when applied to textile wastewater will be elucidated, along with the potential

**Figure 3.** *The picture shows the unabated release of textile dye in a river in Bangladesh.*

#### **Figure 4.**

*The schematic flow diagram of the extended aeration chemically aided process currently used to treat textile processing wastewater.*

**Figure 5.**

*The schematic of the proposed process retrofit using acetogenic pretreatment.*

reduction in energy consumption in treatment will be highlighted. The discussion will be based on actual wastewaters from two textile processing industries.

#### **3. Understanding acetogenesis as applied to textile processing wastewater pretreatment**

The process of maintaining an acetogenic reactor is to curtail the growth of methanogenic microorganisms in a reactor and thereby stopping the conversion of fatty acid generated in the reactor to methane and carbon dioxide. Researchers reported controlling methanogenic microorganisms in an acetogenic reactor by shortening the hydraulic retention time greatly, usually at 2 to 4 days or even lower, in essence maintaining the reactor in a washout mode, thus limiting the growth of methanogens [15, 16]. Another method of controlling methanogenic microorganisms in an acetogenic reactor is oxygen shocked [17, 18]. Of the methods tried by prior researchers, the one that would be easier to apply in existing plants with minimum retrofitting. This method would be converting the existing basins such as equalization basins or parts of the extended aeration to the acetogenic reactor with retention time between 2-4 days and periodic shock aeration using existing in plant aeration capacities and equipment by nominal retrofitting [10]. To investigate this concept in the bench scale, acetogenic reactors were operated as proof of concept using actual textile processing wastewater. Two candidate wastewaters were used, one from a denim processing wastewater and another from com composite fabric processing wastewater. The findings of the bench scale study are summarized in the sub-headings below [10, 19].

*Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing… DOI: http://dx.doi.org/10.5772/intechopen.99107*

#### **3.1 Acetogenic reactor operation**

The acetogenic reactors operated with the textile wastewaters were operated in a semi continuous batch mode with dally waste feeding; at a hydraulic retention time of 4.0 days, the reactor food to microorganism ratio (F/M) was constantly changing through the substrate loading was kept constant, thereby operating in a washout mode with a constant decrease of MLSS over the period of operation. This washout mode of operation ensures acetogenic conditions in the reactor operated under non forced aeration conditions. However, the periodic burst of shock aeration (dissolved oxygen raised to 2.0 mg/L once a day) to kill any growth of methanogenic microorganisms. The reactors used in this experimental program were flat bottomed class vessels in volume between 500 ml to 2000 ml. The test reactors were continually steered by means of a magnetic stirrer. The test reactors were plumed for sample withdrawal and feeding, along with plumbing and air diffuser systems for aeration. The aeration was provided using simple fish tank aerators through a fine air diffuser. The reactor vessel is insulated with temperature control. There is provision for temperature, dissolved oxygen, and pH monitoring in the reactor. The reactors were operated under the following conditions:


The raw textile wastewaters used in both the case studies reported in the following sections were obtained from textile processing facilities from the equalization basin. Time proportioned composite sampling procedure was used for the collection of the sample over a twenty-four-hour period of operation of the wastewater treatment plant [10, 19].

#### **3.2 Acetogenic reactor seed source**

The culture for the laboratory acetogenic cultures for both the case studies were from the sludge thickening tank that unaerated with a solids content of around 2%. The thickening tank contained waste activated sludge from the secondary clarifier of the extended aeration wastewater treating the complex wastewater in case study two discussed below.

#### **3.3 Process operation parameters**

The reactor per process operation parameters that were monitored were dissolved oxygen level during purging, reactor mixed liquor suspended solids, reactor pH, reactor temperature, reactor effluent color, total dissolved solids (for test case run 2), and reactor effluent chemical oxygen demand. Day two and day twenty reactor effluent sample for the second test case was sent for Furrier Transformation Inferred Spectroscopy.

#### **3.4 Analysis procedures**

The biological Oxygen Demand was measured using the serial dilution method HACH Method 8043, Chemical Oxygen Demand was measured by HACH method 8000 Digestion Method using preset vials 0-1500 mg/L rang, and the color was measured by HACH Method Platinum-Cobalt adapted from. Standard Method 8025 for the Examination of Water and Wastewater [20].

Total Dissolved Solids was measured using EC/TDS/NaCl probe and meter by HANNA Instruments HI 2300 system, and pH was measured HANNA HI 2211 pH/ ORP probe and meter.

Total Suspended solids were determined by Standard Method 2540D, where a well-mixed volume of a sample was filtered through a pre-weighed glass fiber filter (pore size 0.45 micro meter). The filter was dried at 104o C and then weighed. The mass increase divided by the water volume filtered is equal to the Total Suspended solids (TSS) in mg/L [21].

The Fourier Transform Infrared (FTIR) spectrum of the reactor effluent was recorded using Bruker Vortex 70 FTIR. The spectra were taken in the range 400 to 4,000 cm−1.

#### **3.5 Case study acetogenic application to denim processing wastewater**

The denim processing wastewater was characterized to have high total Chemical Oxygen Demand and high pH. The subject wastewater had a Chemical Oxygen Demand (COD) of 371 ± 37. mg/L, the color of 660 ± 66 ptco pH = 8.6 ± 0.6, and a five-day biological oxygen demand divided by the Chemical Oxygen Demand (BOD5/COD) ratio of 0.62, indicating wastewater with a substantial organic fraction that should be biologically degradable. The wastewater was directly fed into the acetogenic reactor (Liquid volume 500 ml) without any adjustment, and the reactor operated as mentioned earlier in a waste feed more of semi-batch operation for a period of nine days. The results experimental program showed that after a period of acclimation, the acetogenic culture was able to completely remove the color and also produced substernal removal of chemical oxygen demand shown by respective parameters effluent concentrations decreasing with reactor operation (Refer to **Figures 5**–**8**). This clearly proved the efficacy of the process with ninety percent removal of color and greater than eighty percent removal of chemical oxygen demand for application for pretreatment of textile processing wastewater as an alternated to the chemical intensive decoloring and solids removal processes currently being employed. Reactor operating parameters also showed that beyond food to microorganism (F/M) operating ratio of 0.1, the system performance starts to decrease; this implies that for long term sustainable operation of acetogenic reactors, periodic reseeding with acclimated culture would be necessary [10]. Also, first order

**Figure 6.** *Picture of raw and treated wastewater showing clearly the efficacy of the process.*

*Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing… DOI: http://dx.doi.org/10.5772/intechopen.99107*

**Figure 7.**

*Treated effluent color profile denim processing wastewater from the acetogenic pretreatment process for the denim processing wastewater.*

#### **Figure 8.**

*Treated effluent Chemical oxygen demand profile denim processing wastewater from the acetogenic pretreatment process for the denim processing wastewater.*

#### **Figure 9.**

*Picture of raw and treated wastewater showing clearly the efficacy of the process for the complex wastewater.*

rate kinetics defined both the color and chemical oxygen demand reduction and increased with days of operation and can be attributed to culture acclimation [10].

#### **3.6 Case study acetogenic application to complex textile processing wastewater**

The complex wastewater was from a composite factory where different fabrics are woven, dyed, textured, and finished stitched readymade garment products are produced. The facility that produces wastewater is varied and complex and was thought would be more of a challenging substrate to test the efficacy of the acetogenic process. Characteristics of the composite textile wastewater were color of 3540 ± 353 ptco, the chemical oxygen demand of 5186 ± 138 mg/L, BOD5/COD ratio of 0.4, and pH of 9.6 ± 0.3 [19]. The proof of the efficacy of the acetogenic process in the treatment of textile processing wastewater is further illustrated in **Figure 9**, where the colloidal suspension is completely removed, indicating the extent of visual color removal. Furrier The acetogenic process was able to achieve for the complex textile processing wastewater with the color, and chemical oxygen removal was greater than 90 percent, along with a

**Figure 10.**

*Color removal efficiency for complex textile processing wastewater from the acetogenic pretreatment process.*

**Figure 11.**

*Chemical oxygen demand removal efficiency for complex textile processing wastewater from the acetogenic pretreatment process.*

#### **Figure 12.**

*Total dissolved solids removal efficiency for complex textile processing wastewater from the acetogenic pretreatment process.*

reduction in total dissolved solids. The removal of total dissolved solids by the acetogenic process is an additional benefit as most textile processing wastewaters treated effluents have a hard time meeting the regulatory standers for total dissolved solids without employing expensive membrane systems [19]. **Figures 10**–**12** clearly illustrate the efficacy of the process with its high levels of color, Chemical Oxygen Demand, removal along with the removal of Total Dissolved Solids. Transformation Inferred spectroscopy further illustrates the efficacy of treatment where an effluent sample from day one (**Figure 13**) of the acetogenic reactor is compared to effluent from the acetogenic reactor on day 20 (**Figure 14**). The acetogenic reactor was operated for 20 days and the reactor operating liquid volume was 1000 m operated in a semi batch mode with daily waste feeding. The comparison clearly shows that with the reactor operating at a steady state prolonged operation, the complex organic peaks seen in the

*Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing… DOI: http://dx.doi.org/10.5772/intechopen.99107*

#### **Figure 13.**

*Inferred spectroscopy effluent acetogenic reactor day 2.*

**Figure 14.**

*Inferred spectroscopy effluent acetogenic reactor day 20.*

effluent water were completely degraded by the acclimated acetogenic culture. They are again illustrating that the acclimated acetogenic culture can break down complex organics that are found in textile processing wastewater [19].

#### **4. Energy savings potential and sustainable operation**

Application of acetogenic pretreatment by reducing the biochemical oxygen demand/degradable chemical oxygen demand loading to the aerobic treatment system, which in return reduce the aeration volume, thus and reduce the electric energy requirement for running the aeration blowers. Aerator's energy consumption can range from 4.0 – 6.0 kWh/day-(kg of BOD5/day) based on the type of blowers and aerators used [22]. Based on the database of the existing treatment plant wastewater characterization and laboratory study outlined for the case study one for the denim processing wastewater, the estimated energy requirement at a daily average flow of 722 m3 /day and the BOD5 value of 228 mg/L the calculated BOD5 loading to the existing aerobic basin at present is 164 kg of BOD5/day. The plant uses fine bubble air diffusers with an energy rating of 4.0 kWh/day-(kg of BOD5/ day energy requirement of 656 kWh/day. Based on the 85% BOD5/COD removal efficacy of the acetogenic process, this would lead to loading of only 41 kg of BOD5/ day and a blower energy requirement of 164 kWh/day, a net savings in energy of 495 kWh/day, a substantial saving of energy for any developing economy, case in point the energy requirement of an emerging economy like Bangladesh has a per capita annual energy requirement of 320 kWh [23].

#### **5. Potential for industrial application**

The acetogenic operation works when applied to pretreatment of textile processing wastewater for removal of color, reduction of COD, BOD5, and TDS. The process only requires periodic purging with air in contrast to the aerobic extended aeration process requiring constant aeration with substantial energy to operate the blowers. The proposed process can be applied to existing extended aeration

wastewater treatment systems already existing in textile wastewater treatment facilities with nominal retrofitting. The existing aeration basin aerators could be modulated for just shock aeration, cutting aeration time from 24 hours a day to few minutes producing huge savings in electrical by limiting blower operation. The existing infrastructures also have built in secondary clarifiers and sludge storage and recycling systems; thus, added capital investment would be limited. It is anticipated that acetogenic pretreatment could be introduced with just process operational changes. Besides savings in energy, there would be a huge windfall in chemicals cost saving, for there would be no need for pH adjustment, activated carbon for color removal. All in all, acetogenic operation, with its reduced energy requirements and negating the needs of operating chemicals, makes it a greener viable option for textile wastewater treatment.

### **6. Conclusions**

In an overall prospective the following conclusions can be drawn with regards to the application of acetogenic process to textile processing wastewater:


### **Author details**

Nadim Reza Khandaker\*, Mohammad Moshiur Rahman and De Salima Diba North South University, Dhaka, Bangladesh

\*Address all correspondence to: nadim.khandaker@northsouth.edu

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Acetogenic Pretreatment as an Energy Efficient Method for Treatment of Textile Processing… DOI: http://dx.doi.org/10.5772/intechopen.99107*

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[6] Pavan, P., Battistoni, P., Cecchi, F., & Mata-Alvarez, J. (2000). Two-phase anaerobic digestion of source sorted OFMSW (organic fraction of municipal solid waste): performance and kinetic study. *Water Sci Technol, 41*(3), 111-118.

[7] Bo, Z., & Pin-jing, H. (2013). Performance assessment of two-stage anaerobic digestion of kitchen wastes. *Performance assessment of two-stage anaerobic digestion of kitchen wastes*, 1277-1285.

[8] F. Baldiai, Pecorini, I., & Iannelli, R. (2019). Comparison of single-stage and two-stage anaerobic co-digestion of food waste and activated sludge for hydrogen and methane production. *Renewable Energy*, 1755-1765.

[9] Zuoa, Z., Wu, S., Qi, X., & Dong, R. (2015). Performance enhancement of leaf vegetable waste in two-stage anaerobic systems under high organic loading rate: Role of recirculation and hydraulic retention time. *Applied Energy*, 279-286.

[10] Sarker, M. S., Rahman, M. M., & Khandaker, N. R. (2018, May). Defining the kinetics of the novel application of anaerobic acetogenics for treating textile dyeing wastewater. *Earth Syst. Environ., 4*.

[11] Khandaker, N., & Talha, M. A. (2016). The new nexus and the need for understanding textile wastewater treatment. *BUFT Journal*, 19-26.

[12] Hossain, L., Sarker, S. K., & Khan, M. S. (2018). Evaluation of present and future wastewater impacts of textile dyeing industries in Bangladesh. *Environmental Development*, 23-33.

[13] Sakamoto, M., Ahmed, T., Begum, S., & Huq, H. (2019). Water Pollution and the Textile Industry in Bangladesh: Flawed Corporate Practices or Restrictive Opportunities? *Sustainability*.

[14] Khandaker, N. R., Afreen, I., Samina, D. D., Huq, F. B., & Akter, T. (2020a). Treatment of textile wastewater using calcium hypochlorite oxidation followed by waste iron rust aided rapid filtration for color and COD removal for application in resources challenged Bangladesh,. *Groundwater for Sustainable Development*.

[15] Kotsopoulos, T. A., Fotidis, I. A., Tsolakis, N., & Martzopoulos, G. G. (2009). Biohydrogen production from pig slurry in a CSTR reactor system with mixed cultures under hyperthermophilic temperature. *Biomass and Bioenergy*, 1168-1174.

[16] Ramos, L. R., de Menezes, C. A., Soares, L. A., Sakamoto, I. K., Varesche, M. B., & Silva, E. L. (2020). Controlling methane and hydrogen production from cheese whey in an EGSB reactor by changing the HRT. *Bioprocess and Biosystems Engineering*, 673-684.

[17] Chae, K.-J., Choi, M.-J., Kim, K.-Y., Ajayi, F. F., Park, W., Kim, C.-W., & Kim, I. (2010). Methanogenesis control by employing various environmental stress conditions in two-chambered microbial fuel cells. *Bioresource Technology*, 5350-5357.

[18] Rafieenia, R., Pivato, A., & Lavagnolo, M. C. (2018). Effect of inoculum pre-treatment on mesophilic hydrogen and methane production from food waste using two-stage anaerobic digestion. *International Journal of Hydrogen Energy*, 12013-12022.

[19] Khandaker, N. R., Rio, F. F., Sarkar, L., & Sharmin, A. (2020b). Acetogenic Aerobic Sequential Batch Reactors in Series Operation for Textile Wastewater Treatment. *Journal of Environmental Treatment Techniques*, 766-769.

[20] HACH. (2013). *Calorimetr Procedure Manual.* Denver: HACH Chemical Company.

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[23] Khandaker, N., Sarker, M., & Rahman, S. D. (2017). Acetogenic Pretreatment of Textile Wastewater for Energy Conservation. *International Journal of Science and Research*, 423-426.

#### **Chapter 2**

## Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic Digestion of Waste Streams from Grain Processing

*Jagannadh Satyavolu and Robert Lupitskyy*

### **Abstract**

Globally, Anaerobic Digestion (AD) industry is booming and biogas, the most sustainable biofuel, produced via AD is in an exponential market growth curve. According to a November 2020 report from US Energy Information Administration (EIA), "25 large dairies and livestock operations in the United States produced a total of about 224 million kWh (or 0.2 billion kWh) of electricity from biogas". However, the growth of AD and the cost-effective use of the generated biogas are hindered by the inconsistencies (composition, suspended solids, flow rate, etc.) of the incoming waste stream and the associated biogas quality (due to the presence of hydrogen sulfide gas). A pretreatment step prior to an AD unit can promote consistency in the incoming stream, minimize the suspended solids; and thereby insures the efficiency of AD. In this study, we evaluated the method of pretreatment of waste streams from three grain processing industries, where 1) we adjusted the pH of a stream corresponding to its isoelectric point (zero zeta-potential), 2) removed solids (and their corresponding COD) that precipitated, and 3) produced a consistent composition stream to feed the AD process. For grain processing industry, the precipitated solids can be returned to their process – thus integrating the pretreatment with the rest of the process. The pH pre-treatment should not add any additional cost to the plant since the pH of the waste streams from grain processing plant needs to be raised per plant permits prior to disposal. Our lab and pilot AD studies showed a positive effect of such pretreatment on these waste streams in terms of increased biogas production (11–60%) and COD removal (12–60%), and in some instances reduction in H2S content in biogas (8%). This study clearly demonstrated that such a pretreatment method is economical and is effective to improve AD performance on waste waters from grain processing industries.

**Keywords:** anaerobic digestion, biogas, wastewater treatment, pH adjustment, grain processing

#### **1. Introduction**

Handling and treatment of industrial waste water has become one of the biggest problems of the last century due to constantly increasing industrial activity [1]. The amount of the industrial waste water is rapidly exceeding the biological treatment capabilities of the natural ecosystems. Hence, the treatment of industrial effluents became an important topic.

Anaerobic digestion (AD) is potentially an efficient and economically beneficial method of neutralization of industrial waste [2, 3]. Although anaerobic treatment was known for a long time, the process has not been successfully implemented owing to disadvantages, such as low sludge activity, low reactor capacity, unsuitability of the process and inhibitory effects [4]. The introduction of modern reactor designs where hydraulic retention time is uncoupled from the solids retention time led to a world-wide acceptance of the anaerobic technology as a cost-effective alternative to conventional waste water treatment methods. A number of reactor configurations have been developed leading to high biomass concentrations, such as upflow anaerobic sludge blanket (UASB) reactor, anaerobic contact filter, down flow stationary fixed film and anaerobic fluidized bed reactor (AFBR) systems [5]. In AFBR reactors, the sludge granules are fluidized by high up-flow fluid velocities generated by a combination of the influent and recirculated effluents. The fluidized bed process claims various potential advantages over other high rate anaerobic reactors [6]. These are: high sludge activity, high treatment efficiency, no clogging of reactors, no problems of sludge retention, least chance for organic shock loads and gas hold up as well as small area requirements. Currently, this anaerobic technology removes 70–90% of organic pollutants (expressed as chemical oxygen demand, COD).

In order to ensure high efficiency and high throughput of wastewater treatment using AFBR reactors, certain parameters, such as suspended solids, fat-oil-andgrease, complex organics (fiber, proteins), toxic compounds, should be minimized [7]. Pretreatment of industrial wastewater using physical and chemical methods can significantly improve efficiency of wastewater treatment using anaerobic technology [8]. One immediate impact of these pretreatments on the operation of an anaerobic digester is that its hydraulic retention time (HRT) can be lowered. HRT directly impacts the tank volume of the AD (capital cost) as well as the throughput from the digester. Hence the pretreatment methods can not only lower the capital cost of the anaerobic digestion, but also impact its operating cost.

Various physico-chemical pretreatment methods have been used to improve the anaerobic digestibility of the industrial waste streams. Filtration is used to decrease COD content, remove suspended solids, and toxic compounds [9, 10]. Enzymatic pretreatment is often used to improve digestibility of waste streams with high lipids content, such as dairy wastewater [11, 12]. Oxidative treatment with ozone is used to remove toxic organic compounds from the waste stream and improve anaerobic digestion [13]. Electrochemical treatment is often used for the destruction of recalcitrant organics and increase BOD5/COD ratio [14–16]. pH adjustment has also been successfully implemented for various purposes as a pretreatment method. pH adjustment using Ca(OH)2 was used to force ammonia stripping [17]. pH adjustment was also done to improve sludge dewatering after AD [18]. Alqaralleh [19] demonstrated the use of alkaline pretreatment to enhance the solubility of organics in the waste prior to AD. pH adjustment as a pretreatment method was also employed to precipitate proteins from wastewater [20, 21]. In another work, Cui and Jahng [22] removed proteins from disintegrated waste sludge prior to anaerobic digestion using pH adjustment to the corresponding isoelectric point (IEP) of the proteins.

Control of pH is a key operating parameter during anaerobic digestion process. However, industrial effluents very often have a pH that is not suitable for discharge or further processing. Hence pH adjustment of the waste stream to the discharge permit levels is done as an operating procedure prior to discharging the stream to

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*

further treatment. If, on the other hand, pH adjustment to bring the pH close to IEP can also serve as a pretreatment method, then we can reduce solids and other organics loading in the stream. This reduction will benefit a waste treatment process such as AD. Further, as discussed above, this pretreatment will not add any extra cost to the plant.

Solubility of many compounds depends on the IEP of the solution. Depending on the type of material being precipitated by adjusting to IEP, several advantages can be gained, such as decrease in COD, toxic compounds, complex organics, sulfates etc. This can lead to improved digestibility of the wastewater, as well as increased quality of the biogas [22, 23]. Delgenès et al. studied changes in anaerobic digestibility of industrial microbial biomass after thermochemical pretreatment. It was determined that the observed poor biodegradability and biotoxicity of the solubilized microbial biomass is due to high molecular compounds (>100 Da). Removal of these compounds using absorbent resins and precipitation by pH adjustment improved the biogas production. An increase in biogas production and biogas quality was observed as a result of the deproteination using pH adjustment to IEP [22]. In our study, we used pH adjustment to bring zeta-potential of waste streams from grain processing industries, such as distillery, soy protein processing, and oat fibers processing to near IEP as a pretreatment method. The objective is to reduce organic and solutes loading in the stream and thereby improve COD reduction, biogas yield and quality during anaerobic digestion of the waste streams.

#### **2. Materials and methods**

#### **2.1 Materials**

Calcium chloride, magnesium chloride, ammonium chloride, potassium phosphate monobasic, sodium sulfate were used as minerals and nutrients for anaerobic digestion tests and were purchased from Sigma-Aldrich. Sodium bicarbonate (Sigma-Aldrich) was used to adjust alkalinity. A proprietary inorganic salt mix (Respirometer Systems & Applications LLC, Fayetteville, AZ, USA) was used as a source of trace elements. Sodium hydroxide and hydrochloric acid (Sigma-Aldrich) were used for pH adjustment. Ethanol was purchased from Sigma-Aldrich and was used as a model source of COD. Granular anaerobic sludge was kindly provided by Anheuser-Busch (St. Louis, MO). The concentration of the bacteria in the sludge was measured as Volatile Suspended Solids (VSS) content and was determined to be 52.0 g/L.

#### **2.2 Anaerobic digestion tests**

Experimental set-up for laboratory-scale batch anaerobic digestion tests was acquired from Respirometer Systems & Applications LLC, Fayetteville, AZ, USA, and is shown in **Figure 1A**. It consists of a water bath placed on a 8-position magnetic stir plate, external pump and temperature controller, and a pulse flow respirometer PF-800. 500 ml glass bottles were used as reactors. Up to 8 bottles can be accommodated in the water bath. Trace elements, minerals, nutrients, and NaHCO3 were added to each bottle as described elsewhere [24]. Substrates were added to the bottles in the predetermined amount so that the COD load was the same in each bottle. Bottles were inoculated with granular anaerobic sludge in the quantity so that the ratio between the substrate (expressed as mg/L COD) and the anaerobic bacteria (expressed as mg/L VSS) was 1:2. Bottles with ethanol substrate were used as a control. Ethanol is quickly and easily digested by methanogenic archaea and is

**Figure 1.**

*Experimental set-up used for anaerobic digestion tests: (A) laboratory-scale batch unit; (B) continuous pilot-scale unit (main reactor only), and (C) block-scheme of continuous pilot-scale unit.*

therefore used as a benchmark for substrate digestibility [24]. The pH after adding the biomass, substrates, and nutrients was 7. The bottles were degassed with nitrogen for 1 min to ensure anaerobic conditions. The anaerobic digestion tests were conducted under mesophilic conditions (35 <sup>o</sup> C). The volume of the biogas produced was measured and recorded by the pulse flow respirometer. The test was conducted for two feeding cycles. Each feeding cycle constitutes a reaction time frame during which all nutrients are consumed and gas production stops. After the first feeding cycle ends, the nutrients are replenished and the second feeding cycle starts. For each following feeding cycle, the biomass in the bottle was not removed or added. All the lab tests were performed in duplicate. These lab tests are done prior to pilot tests in order to evaluate the activity of the biomass for each of the streams, digestibility, and biogas quality. The lab tests helped us to better plan and design pilot tests.

Pilot-scale anaerobic digestion tests were performed on 60 L 2-stage Anaerobic Fluidized Bed Pilot Reactor (Voith Meri Environmental Solutions Inc., Appleton WI) shown in **Figure 1(B** and **C)**. In this reactor design, acidogenic and methanogenic stages are spatially separated: acidogenesis occurs mainly in the

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*

preacidification tank and the methane formation happens in the main reactor. It is designed to optimize the methane formation. First, the waste water is pumped from a 10 gallon storage tank into the preacidification tank, where it is kept until the acidification degree (ratio between volatile fatty acids content and COD content) reaches approximately 30% (**Figure 1C**). Then, the acidified wastewater is fed into the main reactor from the bottom, where granular anaerobic sludge resides. The stabilized wastewater is recirculated back at the 200 l/h rate. The recirculation is required to fluidize the granular sludge bed. The excess of the stabilized wastewater (effluent) is removed via the overflow channel and discarded. The gas is collected from the top of the reactor and, after passing through the moisture trap and gas meter, is discharged into the exhaust pipe. The reactor was inoculated with 40 L of anaerobic granular sludge. Each test was conducted for a 2-week period. Samples were taken on a daily basis and analyzed. The reactor was maintained at COD load of 3.0 ± 0.2 g-COD/L/day (feed rate 0.75 l/h; HRT 80 hours). The temperature in the preacidification tank and the main reactor was maintained at 36 ± 3 <sup>o</sup> C. The pH in the preacidification tank was automatically maintained at 5.5 by dosing NaOH. The pH in the main reactor was self-maintained at 6.8.

AD at lab and pilot scale was evaluated on at least two types of streams for each waste water type - a control (no pH adjustment) sample and a pretreated sample. Repeats and additional tests are conducted as needed. The data presented is a compilation of the multiple runs for each stream.

#### **2.3 Analytical methods**

Chemical analysis of the waste water was performed spectrophotometrically using commercial test kits and DR 3900 Spectrophotometer (Hach Company, Germany). Gas analysis was performed on SRI 8610C Gas Chromatograph (SRI Instruments Inc., Las Vegas NV) using HayeSep D column (Restek Corporation) and thermal conductivity detector (TCD) for methane and carbon dioxide detection; MXT-1 column (Restek Corporation) and flame photometric detector (FPD) was used for hydrogen sulfide detection. Z-potential measurements were performed on 90 Plus Particle Size Analyzer (Brookhaven Instruments Corporation, Holtsville NY).

#### **3. Results and discussion**

#### **3.1 Wastewater characterization**

Three types of wastewater streams from local grain processing industries have been used in our experiments: distillery, soy protein processing, and oat fiber processing. These streams have been analyzed for their chemical composition, physicochemical properties, and solids content (**Table 1**). Samples from each operation were received 3–4 times a week for a three-week period in order to assess variability in the wastewater content. Therefore, some of the data in the table are presented as a range, representing the amplitude of variation of a particular parameter.

The solids in the distillery waste stream were separated by centrifuging at 1000 rpm for 15 min. The resulting liquor had a suitable mineral composition: sufficient nitrogen and phosphorus content and low sulfates. Soy protein processing wastewater had suitable COD content, low suspended solids, sufficient nutrients, but had very high sulfates content, which was in the range of toxicity for methanogenic archaea [25, 26]. Oat fiber processing wastewater had a high COD content, suitable mineral composition, but had a very high initial pH.


#### **Table 1.**

*Summary of the wastewater characterization.*

#### **3.2 Wastewater pretreatment**

All three waste streams have initial pH that is not suitable for anaerobic digestion, which should be in the 6.5–7.5 range. Distillery and soy protein processing waste streams come at pH 3.9–4.6, which is too low, whereas oat fiber processing waste stream has pH of 11.3, which is too high. Adjusting pH prior to anaerobic treatment not only ensures the proper conditions for methanogenic archaea, but also makes the stream more consistent, eliminating any possible upsets in the AD reactor. Yet another advantage of pH adjustment is the possibility to precipitate colloidal solids by bringing the system close to its isoelectric point. We studied the pH-induced precipitation in these streams by changing pH in increments from 0.5 to 1.0 and measuring the zeta-potential as a function of pH to determine the IEP of the stream (**Figure 2**). Sodium hydroxide and hydrochloric acid were used for pH adjustment throughout the study. For distillery and soy protein processing streams, the pH-induced precipitation was studied in the range from original pH (~4) until 9. For both streams a precipitation was visually observed upon reaching pH of ~6.0 and ~ 5.4 for distillery and soy protein processing streams respectively. The extent of precipitation as a function of pH was studied by measuring COD at different pH points (**Figure 3**) after the sample has been centrifuged at 4000 rpm for 15 min. The highest decrease in COD content was observed at pH ~7 for distillery sample (6.5% COD decrease) and at pH ~6 for soy protein processing sample (10.3% COD decrease). Both points of highest COD decrease are either close or within the range of optimal pH for anaerobic digestion. It is noteworthy that these pH points are in the vicinity of the corresponding isoelectric points measured for these waste streams (**Figure 2A** and **B**). This suggests that the precipitated material is most likely a fraction of water soluble proteins. Oat fiber processing waste stream also showed pH-induced precipitation. In this case pH was reduced gradually from original pH of 11.3 to 2. After pH was decreased below 5, a significant precipitation was visually observed. The graph in **Figure 1C** shows pH-dependent COD decrease

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*

**Figure 2.** *Z-potential of the waste stream from (A) distillery, (B) soy protein processing, and (C) oat fiber processing as a function of pH.*

*Total COD as a function of pH of the wastewater from (A) distillery, (B) soy protein processing, and (C) oat fiber processing.*

for this waste stream. A slight decrease in COD is observed as pH decreases from 11.3 to 6, followed by a rapid decrease in the pH range from 5 to 3. Overall, adjusting pH from 11.3 to 3 resulted in the removal of nearly 50% COD. Constant increase in precipitation throughout the entire pH range studied, combined with no isoelectric point in this range (**Figure 2C**) suggests that the precipitated material is most likely an alkali-soluble polycarbohydrates.

We also studied changes in the mineral composition of the waste streams upon pH adjustment (**Table 2**). Removal of dissolved solids upon pH adjustment in the soy protein processing wastewater resulted in the decrease of sulfates content by 16% and phosphates by 11%. Reduction of sulfates concentration is beneficial because

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*


**Table 2.**

*Changes in the chemical composition of the wastewater upon pH adjustment.*

high concentration of sulfate ions cause sulfide toxicity during anaerobic digestion process [25] Ammonia content did not decrease significantly. The above minerals in the other two waste streams did not change noticeably upon pretreatment.

#### **3.3 Batch anaerobic digestion tests**

We performed a laboratory-scale batch anaerobic digestion study in order to evaluate the effect of pretreatment on the anaerobic digestion of the wastewater in terms of biogas production, its quality, and possible inhibitory effects on the biomass activity. Pretreatment of the waste streams was performed by adjusting pH to the value that resulted in maximum decrease of COD content (**Figure 3**). Thus, the pH of the distillery and soy protein processing streams was adjusted to 7 and 6 respectively. The pH of the oat fiber processing waste stream was first adjusted to 3 to induce precipitation and, after removal of the precipitate, the pH was increased to 6 to bring it within the range suitable for methanogenic archaea. In all AD tests, separation of the precipitated solids was performed by carefully decanting the liquid after the precipitate was allowed to settle.

#### *3.3.1 Distillery wastewater*

Results of batch digestion test for the distillery wastewater before and after pretreatment are summarized in **Figure 4**. The experiment was conducted for two feeding cycles. Cumulative biogas production over each feeding cycle is presented in **Figure 4A** and corresponding specific methane production is shown in **Figure 4B**. For both feeding cycles, a clear increase in gas production is observed from the pretreated sample. The total biogas production from the pretreated sample after 40 hours of digestion was 18% and 11.5% higher for 1st and 2nd feeding cycle respectively, compared to the non-pretreated sample (**Table 3**). As a result of pretreatment, COD reduction during the second feeding cycle increased from 80.2% to 89.4% (compared to control). Analysis of biogas samples (**Table 3**) indicated a slight decrease (8%) in H2S concentration after the pretreatment, which may be due to the removal of the fraction of soluble proteins upon pH adjustmet. Protein-rich streams are known to have increased levels of H2S in biogas [27]. In addition, corn gluten is particularly rich in sulfur-containing aminoacids, compared to other seeds [28]. The biogas composition, presented in **Table 3** and subsequent tables, does not add up to 100%, because biogas contains other minor components (typically hydrogen, nitrogen, oxygen, and moisture). Since, the emphasis of the study was on COD conversion, biogas production, and methane content as a function of pretreatment, elucidation of the complete biogas composition was beyond the scope of this manuscript.

**Figure 4.** *Total gas production (A) and specific methane production (B) for the distillery wastewater.*

#### *3.3.2 Soy protein processing wastewater*

Chemical analysis of the soy protein processing wastewater showed that it contains high concentration of sulfates. High sulfate concentration has adverse effect on anaerobic digestion for two reasons: it decreases the content of methane in the biogas, because reduction of sulfur competes with methanogenesis; second, inhibition of methanogenic archaea with hydrogen sulfide can occur [26]. Typically, a safe level of sulfates is considered to be when the ratio of COD to sulfates is at least 10. In our case this ratio is 3–3.5. Thus, the inhibition of anaerobic activity may be expected. Adjustment of pH from original 4 to 6 resulted in the decrease in sulfates concentration by 16.2%. For control, we performed additional removal of sulfates

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*


#### **Table 3.**

*Biogas yield, % COD reduction, and biogas composition after 40 hours of digestion of the distillery wastewater.*

by adding BaCl2. BaCl2 selectively precipitates sulfates by forming insoluble salt BaSO4. As a result of this treatment, 86.4% of sulfates have been removed (sulfates content decreased from 4970 to 600 mg/L).

We performed anaerobic digestion tests of this waste stream using three samples: 1) non-pretreated at initial pH, 2) treated by adjusting pH to 6, and 3) treated with BaCl2 (after pH was adjusted to 6), which is referred to as "w/o sulfates". Results of the test are summarized in **Figure 5** and **Tables 4** and **5**. During the first feeding cycle the biogas production from the non-pretreated (pH 4) and pretreated (pH 6) samples is nearly the same. During the second feeding cycle, a significant decrease in the gas production is observed for the non-pretreated sample. The amount of biogas produced after 24 hours from the non-pretreated sample decreased by 40% during the second cycle. The biogas production from the pretreated sample decreased only by 7%. Such a decrease in biogas production can be attributed to the expected inhibition of methanogenic archaea by high sulfates concentration. This assumption is supported by the fact that the sample treated with BaCl2 had higher biogas production than the pretreated sample, and no decrease in the biogas production was observed during the second feeding cycle.

Nearly 60% decrease in methane content during the second feeding cycle was observed in the non-pretreated sample (pH 4). The pretreated sample (pH 6) had lesser (25%) decrease in the methane content during the second cycle. On the other hand, the methane content in the biogas from the sample treated with BaCl2 did not change. These results again suggest the inhibitory effect of the high sulfates concentration.

#### *3.3.3 Oat fiber processing wastewater*

Results from the anaerobic digestion test of the oat fiber processing waste stream are summarized in **Figure 6** and **Table 6**. The results show that the pretreatment significantly improves the digestibility of this stream. The amount of biogas produced after 40 hours during the first feeding cycle is 87% higher for the pretreated sample. An increase in digestibility for both samples is observed during the second feeding cycle (53% and 31% for the non-pretreated and pretreated sample, respectively). Upon the pretreatment, COD reduction during the second feeding cycle increased from 48.3% to 77.6%.

The gas quality, however, decreased upon the pretreatment (**Table 6**). The methane content decreased by 20%, carbon dioxide increased by 50%. The reason for this decrease in quality can be high concentration of NaCl, which accumulated as a result of pH adjustment with HCl and NaOH [25, 29].

All three waste streams, especially soy processing wastewater, contain fairly high amount of hydrogen sulfide, which, although unavoidable, is highly undesirable as it decreases the quality of biogas, causes corrosion of the piping, turbines, and other equipment [30]. It also forms a greenhouse gas SO2 during combustion of H2S-containing biogas. There is a number of methods to decrease or remove the H2S

**Figure 5.** *Total gas production (A) and specific methane production (B) for the soy protein processing wastewater.*


#### **Table 4.**

*Biogas yield and % COD reduction after 24 hours of digestion for the soy protein processing wastewater.*

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*


**Table 5.**

*Biogas composition after 24 hours of digestion for the soy protein processing wastewater.*


**Table 6.**

*Biogas yield, % COD reduction, and biogas composition after 40 hours of digestion for the oat fiber processing wastewater.*

content in biogas. They are broadly divided into two categories: 1) post-treatment of biogas and 2) prevention of H2S formation during the AD process. The first category includes absorption, adsorption, and membrane filtration, and biological filtration techniques [31]. The second category includes in-situ chemical removal and in-situ bioconversion using microaeration [32–34]. Each individual method has its advantages and disadvantages. Therefore, best strategy is integration of several technologies to achieve a balance between efficiency, feasibility, and cost.

#### **3.4 Pilot-scale anaerobic digestion tests**

In order to verify that results of batch studies are transferrable on a larger scale, we performed AD tests on a continuous upflow fluidized bed pilot reactor using only one of the tested streams. We selected for this purpose the oat fiber processing wastewater, as it seemed to benefit the most from the pretreatment. Non-pretreated and pretreated wastewater was fed continuously for a 2-week period. The anaerobic sludge in the reactor was preliminary activated by feeding with a standard nutrient solution [24] using ethanol as a source of COD at ~2 g-COD/Lday volumetric loading rate (VLR) for one week. Prior to feeding the wastewater, the biomass in the reactor was starved for 2 days. The COD content of the wastewater was adjusted to 10.0 g/L by dilution with tap water. The wastewater was supplemented with nitrogen in the form of ammonium chloride (10 g per 50 L every second day). COD of influent and effluent, as well as biogas production were measured daily. The results of this test (**Table 7**) indicate that the pretreatment of the wastewater by pHinduced precipitation resulted in the increase of biogas production by 23.1% and increase of the COD removal efficiency by 25.2% compared to the original wastewater. We attribute this improvement to the decrease in the amount of the poorly digestible compounds, such as alkali-soluble polycarbohydrates and lignins, which were precipitated and removed. Methane content, however, was slightly lower in the case of pretreated wastewater, which is consistent with the results of the batch tests. The reason for this is most likely the same as in batch studies – high level of NaCl. Although batch studies did not reveal any adverse effects of this waste stream on the anaerobic biomass, the operation of the pilot reactor was not stable in both non-pretreated and pretreated streams. While the volumetric loading rate (VLR) was kept constant at fairly low level, the volatile fatty acids (VFA) concentration in


**Table 7.**

*Summary of anaerobic digestion of the oat fiber processing wastewater using a continuous pilot-scale reactor.*

*Evaluation of Physical and Chemical Pretreatment Methods to Improve Efficiency of Anaerobic… DOI: http://dx.doi.org/10.5772/intechopen.98321*

both cases was constantly increasing throughout the entire feeding period, suggesting a possible toxic effect. Elucidation of the long-term effects of the above waste stream on anaerobic biomass was, however, beyond the scope of this study.

#### **4. Conclusions**

In this study, pH-induced precipitation has been evaluated as a method of pretreatment of industrial effluents in order to improve anaerobic treatment efficiency. The pH adjustment was done to bring the pH of the solution close to its isoelectric point. Such pretreatment resulted mainly in the removal of suspended and dissolved solids. The effect of the pretreatment was studied on the laboratory and pilot scale using wastewater from local grain processing industries: distillery, soy protein processing, and oat fiber processing plants. The anaerobic digestibility of all three waste streams benefited from the pretreatment. Lab-scale batch AD tests showed the increase in COD reduction from 80.2% to 89.4% for the distillery waste stream, from 39.3% to 60.7% for the wastewater from the soy protein processing, and from 48.3% to 77.6% for the oat fiber processing wastewater. Benefit of the pretreatment was further verified on the pilot scale using an upflow fluidized bed reactor with the oat fiber processing wastewater as a feed. After two weeks of continuous feeding, an increase in the daily biogas production by 23% and COD removal efficiency by 25% has been observed as a result of the pretreatment.

Our lab-scale and pilot-scale AD studies showed a positive effect of the pH-induced precipitation on these waste streams in terms of increased biogas production (11–60%) and COD removal (12–60%), and in some instances reduction in the H2S content in biogas (8%). This study clearly demonstrated that pH-induced precipitation is an effective pretreatment method to improve AD performance on wastewaters from grain processing industries.

#### **Author details**

Jagannadh Satyavolu1 \* and Robert Lupitskyy2

1 Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY, USA

2 Ingredion Incorporated, MD, USA

\*Address all correspondence to: jagannadh.satyavolu@louisville.edu

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### **Chapter 3**

## Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp and Paper Industry to Produce Biogas - A Case Study

*Alfredo de Jesús Martínez-Roldán, María Dolores Josefina Rodríguez-Rosales, Sergio Valle Cervantes and Thania Azucena Mendez-Perez*

#### **Abstract**

Many organic residues are being wasted since they are not given a comprehensive management; anaerobic digestion is an alternative to reduce the impact of these residues, and to produce biogas. The chapter includes the state of art about biogas and energy production, and later, the analysis of a study case focusing on the use of pulp and paper wastes to produce biogas. The study was carried out through anaerobic digestion at a bench scale using three temperature phases to treat primary and secondary sludge, establishing operational parameters such as temperature, retention time, and organic loadings. Monitoring of volume, methane concentration in the biogas, volatile solids reduction, volatile fatty acids during the process, the performance of the process in function of methane produced per volatile solids removed is calculated. This case study shows that it is feasible to use the sludge from the company's wastewater treatment plant (WWTP) for the generation of biogas, thus reducing waste management problems.

**Keywords:** anaerobic digestion, biogas, bioenergy, biosolids, paper industry

#### **1. Introduction**

The indiscriminate use of raw materials and fossil fuels has led to an infinity of environmental problems, such as water reservoir pollution, acidification of the oceans, loss of ecosystem diversity, and a concentration increase in certain gaseous pollutants in the atmosphere [1]. To reduce dependence on oil and decrease the CO2 concentration to revert the climate change, it is necessary to use renewable energy sources [1, 2]. Among the possible renewable energy sources biogas, stands out especially, when biogas is obtaining from waste produced from different productive activities [3]. The biogas generation through anaerobic digestion generates several environmental benefits, such as reducing greenhouse

gas emissions, depletion in the residuals environmental impact, the clean energy generation, and the possibility of using the generated biosolid as a soil improver or fertilizer, among others [4].

#### **1.1 Anaerobic digestion of waste**

Under anaerobic conditions decomposition of matter produces a gaseous mixture known as biogas. Methane is the main fuel gas in the biogas mixture, and to be used as a fuel, its content must be above 45% of the total composition of biogas [5]. Biogas general characteristics are listed in **Table 1**.

Biomasses such as food industry waste, animal excreta, straws, residual planta and municipal waste under anaerobic digestion are able to produce biogas [4, 8, 9]. Through several biochemical steps the macromolecules of organic matter are transforming into CH4, CO2 and H2S under anaerobic digestion [8–11]. However, the organic matter characteristics must allow being used as an energy source for a set of microorganisms that will make the digestion process possible. Therefore, not only a supply of the main nutrients (carbon and nitrogen) is necessary, but also a balance of micro and macro nutrients [12]. Carbon and nitrogen are the principal sources of food for methanogenic microorganisms, and the proportion between these nutrients must be adequate for the correct operation of the process. It is known that the approximate proportion of carbon (C) and nitrogen (N) consumption by bacteria is 30:1 (C/N), this being the optimum point. On the other hand, if there is a ratio of 35:1 the process is inhibited due to a lack of nitrogen, and if it is 8:1 the inhibition occurs due to the formation of ammonia [4, 13].

Anaerobic biodegradation of complex organic materials is a multi-stage process where solid materials are first hydrolyzed, polysaccharides to sugars and alcohols, proteins to polypeptides and amino acids, lipids to long-chain fatty acids (LCFA), and glycerol. From these, the fermentative bacteria produce short-chain fatty acids (SCFA), hydrogen (H2), and carbon dioxide (CO2), and ammonia producing by the fermentation of amino acids. Subsequently, acetogenic bacteria from non-acetic FA and neutral materials such as ethanol produce H2, acetate, and CO2, which are used by methanogenic bacteria to produce CH4, CO2, and H2O [10, 12, 14]. The process can be divided into four steps according to the proposed models and the complex inter-microbial relationships that carry it out.


#### **Table 1.**

*Common characteristics of biogas [6, 7].*

#### *Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

In the specific case of using waste sludge from Wastewater Treatment Plants (WWTP), the nutrients are in the necessary proportions and concentrations. However, the sludge generated in the plants of the forestry or paper industry contains high concentrations of cellulose, and this unbalances the C/N ratio [15]. Besides, lignin can cause toxicity problems and decrease the efficiency of the anaerobic digestion process [10, 15, 16].

In the hydrolysis process, macromolecules such as proteins, lipids, carbohydrates, and nucleic acids are transformed into oligomers (fatty acids, carbohydrates, amino acids, nitrogenous bases, and aromatic compounds) [17, 18]. The bacteria involved in the process are a very complex mix of many genera, most of which are obligate anaerobes; however, some facultative anaerobic bacteria such as streptococci and other enteric microorganisms may be present. This type of microorganisms ferments a great variety of complex organic molecules such as polysaccharides, lipids, and proteins, turning them into a wide range of end products such as acetic acid, a mixture of H2 and CO2, mono carbon compounds, organic acids with more than two carbon atoms, and compounds such as propanol, and butanol [10, 19]. The optimum pH for hydrolysis varies according to the substrate. For easily degradable carbohydrates, hydrolysis proceeds in an accelerated manner at pH between 5.5 and 6.5 [17].

In acidogenesis, the pH value decreases, going from 7.0 to values around 5.0; in this stage, the bacteria ferment the soluble products of hydrolysis, mainly hydrogen and volatile fatty acids, and long-chain fatty acids also produce acetate or propionate by ß-oxidation. Thus, together, hydrolytic and acidogenic bacteria convert complex substrates to precursors of methanogenesis: H2, CO2, and acetate, in addition to AGV and other reduced compounds, ethanol, lactate [9, 10].

In the acetogenesis stage, organisms that favor an acidic environment participate; during this stage, volatile fatty acids and nitrogenous compounds are slowly transformed. During this stage, the pH value increases from values around 5.0 to values around 6.8. The metabolic products of acetogenic bacteria are converted into substrates for methanogens by the activity of the acetogenic bacteria constituting the third level or trophic group in the population sequence that occurs in anaerobic digestion. The metabolic result of this group is the formation of acetate, H2, and CO2. These bacteria are known as hydrogen obligate acetogenic bacteria. This trophic group must have a symbiotic relationship with hydrogenophilic archaea, since they consume the hydrogen produced by the former, thus avoiding its inhibition by-product accumulation [17, 19].

In the last digestion stage, known as methanogenesis, the volatile fatty acid content drops to less than 500 ppm. The pH value increases from 6.8 to 7.4, producing large volumes of gases with 65 to 70% CH4, around 30% CO2, and other inert gases such as N2. Methanogenic archaea are responsible for producing methane from various substrates, with acetate being responsible for approximately 73% of the methane produced. Methanogenic archaea are strict anaerobes, very sensitive to oxygen as they require negative oxidoreductive potentials lower than −50 mV to grow [20]. The main products of this type of treatment are biogas and biosolids, which are used as a source of energy and as a fertilizer respectively. An additional benefit of this type of processing is that a load of pathogenic organisms in the sludge is very low, as is the mass of the sludge. The main uses of sludge from bioreactors are soil conditioning, use as fertilizer, and use for the generation of vegetation cover in sanitary landfills or for the recovery of degraded soils or sites, and also in their bioremediation [17].

#### **1.2 Factors involved in anaerobic digestion**

Biomass has a varied composition that includes different organic and inorganic compounds. To optimize the anaerobic digestion process and biogas production,

parameters such as chemical composition, operational parameters such as temperature, pH, loading rate, alkalinity, biodegradability, bioaccessibility, bioavailability, and the initial characterization of substrates [11, 21].

#### *1.2.1 Temperature*

Temperature is one of the principal survival factors of microorganisms during the anaerobic digestion process [10]. The management of the temperature range is useful to differentiate the type digestion processes. Three operating ranges can be used in an anaerobic digester: psychrophilic (~ 25° C), mesophilic (~ 35° C), and thermophilic (~ 55° C). Microorganisms grow best in temperature ranges between 35 and 55° C. An increase in temperature has a positive effect on the metabolic rate and accelerates the degradation of biomass; however, the use of a thermophilic range is difficult to control and generates energy consumption to maintain the constant temperature of the reactor. In general, the mesophilic process often involves a diversity of microorganisms and is more stable than the thermophilic process. Temperature is one of the principal parameters for microorganisms to grow, degrade organic matter, and consequently, biogas to be produced [11, 21].

#### *1.2.2 pH*

The pH value is one of the main operational factors that can affect the anaerobic digestion process. That is because most of the microorganisms prefer a neutral pH range. In the biogas production process, some organisms require a different growth pH. However, the most favorable pH range to obtain maximum biogas production is 6.8 to 7.2. In the anaerobic digestion process, methanogenic microorganisms are too sensitive to pH variations and prefer a pH of around 7.0 [11, 22].

Acidogenic microorganisms are less sensitive to pH and are tolerable in the 4.0–8.5 range. However, the optimal pH for hydrolysis and acidogenesis is between 5.5 and 6.5 [11, 22]. The pH value is an important factor because it influences the ratio of ionized and non-ionized forms. This is because excessive hydrogen, sulfur, fatty acids, and ammonia are toxic in their non-ionized forms. Generally, the pH value indicates a healthy environment for the digester microorganisms [11, 22, 23].

#### *1.2.3 Alkalinity*

Alkalinity is the ability of a system to maintain a certain pH. It is a measure of the buffer capacity of the system. The higher the alkalinity, the better the pH despite an increase in H+ generation. In systems where anaerobic digestion is performed, the buffer system is due to the presence of carbonates, in particular the presence of the bicarbonate ion HCO3 − . Since acidogenic bacteria have a higher activity than methanogenic bacteria, they are capable of causing acidification in the reactor, in case of organic matter overloads. This acidification can be avoided by maintaining an optimal buffer capacity in the digester. Alkalinity is useful for buffering purposes, at typical operating pH values [21, 22].

#### *1.2.4 Volatile fatty acids*

The concentration of volatile fatty acids (VFA) product of the fermentation has great importance in the anaerobic digestion process. This because the VFA can acidify the reactor, causing the failure of the process. Under normal operating conditions, the concentration of VFA in the effluent must be very low or negligible, less than 100 mg L−1. On the contrary, if there is a high concentration, it can cause

*Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

inhibition of methane-forming archaea. The VFA/alkalinity ratio is also an indicator of stability. A ratio greater than 0.4 indicates an immediate failure [21, 24].

#### *1.2.5 Chemical composition of substrates*

Substrates chemical composition characterization is useful to identify the appropriate substrates to carry out the anaerobic digestions. Substrates contain the full range of simple and complex chemical compounds, and the proportion of them will depend on their sources (agricultural agriculture and animal manure, municipal, food, and industrial waste). Specific organic compounds may predominate. Although, most of the time the exact composition of the substrates is difficult to determine [22, 24].

#### **1.3 Temperature regimes in anaerobic digestion**

As commented in a previous section, the temperature regime is important when looking for the conditions that allow increasing the degradation of organic matter and the production of biomass. For this reason, each of the possible regimes will be briefly analyzed.

#### *1.3.1 Mesophilic anaerobic digestion*

It is the type of conventional anaerobic digestion carried out in a temperature regime ranging from 33 to 35° C, which can have a system that allows mixing of the sludge. In this configuration, the retention times are usually long, VS reduction reaches around 40 to 48%. It presents a problem of foam generation, and destruction of the pathogens is not carried out. The quality of the biogas in this type of digestion is good, however, the volumes generated are not so considerable, which in terms of profitability makes it inefficient [25, 26]. It has been founding that for retention times between 5 and 55 days, the methane concentration can be between 62 and 66%, and the reduction in volatile solids can reach 32 to 40% for retention times between 15 and 30 days [27].

#### *1.3.2 Thermophilic anaerobic digestion*

The waste sludge treatment process in thermophilic terms is one of the most studied at present. This type of process, carried at a temperature of 50 to 55° C, allows an improvement in the deployment of retention times, and destruction of pathogens. Popat et al. (2010) report that the reduction of most pathogens can occur between 13 and 15 days at constant temperatures between 51 and 55°C. However, the energy cost resulting from the treatment puts it into consideration [28]. The VS reduction percentages are around 50 to 60%, which makes it a point of study for its improvement in energy terms [26, 29]. Besides, Wahidunnabi & Eskicioglu (2014) and Yu et al. (2014) reported that VS removal efficiencies for thermophilic systems range from 40 to 50%. Regarding the production of biogas, with values around 0.30 m3 CH4 (kg of VS fed)−1 [26, 30].

#### *1.3.3 Three-phase temperature anaerobic digestion*

This digestion is a combination of acid/gas phases and temperature phases, from which a good removal of volatile solids is obtaining, it does not produce fetid odors, and the retention times are shorter [29, 30]. Riau, de la Rubia, & Pérez (2010) carried out a configuration for this type of digestion, where the phases are delimiting

by time and temperature. The mesophilic from 1 to 3 days, the thermophilic from 5 to 15 days, and a mesophilic with a retention time from 5 to 15 days; The results of his research were 55% SV reductions, coliform and pathogen reduction, as well as a volumetric gas production of ~5.5 LCH4 (kgVS fed)−1 [31]. Similarly, the experiments carried out by Kim, Novak, & Higgins (2011), affirm the effectiveness of the combination of three temperature phases. In their results, they obtained a VS reduction of about 57% [32].

#### **1.4 Sewage sludge and its use to produce biogas**

Most conventional wastewater treatment systems generate large amounts of waste products, which are called sludge. The composition and quantity of the sludge depend on the raw wastewater characteristics and the wastewater treatment process. The main constituents of wastewater disposed of in treatment plants include garbage, sand, foam, and sludge. The sludge extracted and produced in wastewater treatment operations and processes is generally a liquid or a liquid-semi-solid with a high solids content between 0.25–12% [33, 34]. The different treatments to process sludge vary according to the source and type of wastewater from which they are deriving, the process used to treat the wastewater, and the final disposal of the sludge. Sludge is by far the constituent with the highest volume removed in wastewater treatment, so its treatment and disposal are probably the most complex problem [34].

The biological wastewater treatment process produces different types of sludge within each of the individual processes, such as (1) primary sludge produced during the primary wastewater treatment processes; this occurs after sieving and de-sanding. The composition of the sludge depends on the characteristics of the wastewater. It mainly contains large undissolved solids that generally carry on a large amount of organic material, vegetable matter, paper, and other materials. (2) Activated sludge coming from the removal of dissolved organic matter during aerobic or anaerobic treatment of wastewater. This sludge is generally in the form of flocs that contain living and dead biomass. (3) Tertiary sludge, which is produced through subsequent treatment processes, with the addition of flocculating agents [35]. The processes for treating sludge vary according to the type of wastewater from which they are deriving, the process used to treat them, and the last disposal method to which the sludge will be destining. The sludge treatment main objectives are to reduce mass and volume, to handling it easily and to increase its biological stability in order to produce a sufficiently harmless material for its disposal [35, 36].

#### **1.5 Biogas in México**

Energy is a vital supply for the development of any society, but when talking about energy, it encompasses aspects such as use and abuse, source of supply, pollution generated in its generation, danger to society in cases of accidents, etc. Global energy consumption has doubled in the last 25 years. Estimation for the next 25 years shows that there will be an increase of 70%. In developing countries, the above will be reflected mainly due to globalization, population growth, and economic growth. Besides, the consumption of fossil fuels is no longer sustainable due to its early depletion, the increase in its price, and the damage it has caused to the environment [37]. México has an enormous potential in renewable resources, and thanks to the reforms implemented in the energy sector, barriers that impede the development of new projects and clean technologies have been eliminated, achieving increases in a clean generation far above fossil energy. According to the clean energy progress report, from 2016 to 2017, fossil generation grew by 2.07% and clean by 6.98% [38].

#### *Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

In México, the production of electrical energy is based mainly on the consumption of different fossil fuels, reaching more than 90% of the total, highlighting the use of oil, natural gas, and coal; on the other hand, the fraction of energy obtained by renewable means is 7.5%, and biogas only contributes 0.02% [39, 40]. However, it is important to note that the percentage covered by renewable energies increases every year, although without yet becoming one of the most important sources [39]. However, the National Energy Strategy aims for approximately 35% of the country's consumed energy to be renewable origin by 2024 and marks that 50% of the consumed energy in 2050 be clean [41]. Experts estimate that the generation of biogas from waste has great potential in México, specifically for the use of livestock waste. It is estimated that from anaerobic digestion of them, little more than 100 million cubic meters of biogas could be generated per year, which would allow covering little more than 8% of the national energy demand [42, 43]. On the other hand, in the case of the wastewater treatment plants, the potential is slightly lower, reaching projections of around 75 million cubic meters per year for 2024; however, studies on wastewater treatment plants of the industrial sector are still needed since their effluents and operating conditions are specific, making it difficult to generalize about possible production values and biogas yields [42].

#### **1.6 Waste from the paper industry to produce biogas**

The pulp and paper industry produces large amounts of highly polluting waste; this cause the wastewater and consequently the treatment plant sludge to have particular characteristics [44]. It is estimated that up to 1 m3 of residual sludge can be generated per ton of paper produced, which will contain between 45 and 55% organic matter in addition to the presence of other pollutants and COD between 4,000 and 15,000 ppm depending on whether it is primary or secondary sludge [44, 45]. Due to its high content of organic matter, it can be used for biogas generation from anaerobic digestion, being able to achieve high values of biogas production as well as high conversion efficiencies [45].

The primary sludge is producing when clarifying the wastewater from the process. This sludge has a high content of lignocellulosic material; the fiber content is variable depending on the type of process, and the dewatering of this type of sludge is relatively simple [44, 46]. The solids content can be up to 48%, while the volatile solids and total organic carbon can reach values of 33 and 19%; the presence of heavy metals such as chromium, zinc, nickel, among others stands out [46]. On the other hand, secondary sludge is the sludge generated when carrying out the biological treatment (aerobic, anaerobic, activated sludge) of the wastewater generated in the process. The secondary sludge is recovered in the clarification phase from the treated water and is normally mixed with the primary sludge to incinerate it or to deposit it in a landfill [44, 46]. Traditionally, the sludge generated in the pulp and paper industry is mixed (primary and secondary), later they are dried, and finally, they are used as fuel when incinerated, another alternative is to place them in landfills. However, the large amount of organic matter causes its weathering to generate a great amount of greenhouse gases, so this strategy is not currently allowed in many countries. [46–48]. Since the majority fraction of the industry and paper sludge is organic matter, its use in an anaerobic digestion process has been proposed to recover energy from them. [46]. However, the process is not very efficient because a large part of the organic matter is composed of cellulose and lignin, for which various authors propose the use of pretreatment strategies that allow the breaking of the fibers and increase the efficiency of the process of anaerobic digestion [46, 49, 50]. However, the sludge characteristics depend on the operating conditions, the raw material, among others therefore the anaerobic digestion process must be adjusted and specifically designed.

#### **2. Case of study: the use of wastes of the pulp and paper industry to produce biogas a case of study**

This study was carried out to treat residual sludge from a paper-producing industry. A company and leader in the manufacture of paper and cardboard packaging, which treats a flow of 80 to 100 L s−1 of wastewater, which results in annual production of primary and secondary sludge of 5,400 to 6,000, and of 4,300 to 5,000 tons yr.−1, of primary and secondary sludge, respectively.

This papermaking company uses recycled paper as raw material to manufacture paper with three quality grades: linerboard paper for corrugated packaging, medium paper for corrugated packaging, and white top paper for corrugated packaging. That generates a variation in the wastewater characteristics resulting from the process, making it difficult for the company to treat activated sludge. This wastewater treatment consists of screening and desander pretreatment, primary clarification of primary treatment, and biological treatment. The solids from the primary settler and the flotation process are mixed and concentrated through a sludge press. The effluent from the primary clarification is neutralized and transported to the activated sludge treatment. The mixed liquor flows from the reactor to the secondary settler, where the produced sludge and the clarified effluent are separated. Primary and secondary sludge do not receive any treatment and are disposing on the land of the company. For all the anterior, this study of anaerobic digestions is the first step taken to research giving added value to the generated sludge and avoiding contamination in soils and phreatic levels.

The use of these residual sludge for the generation of biogas was studied through anaerobic digestion, using three bioreactors, one operating at mesophilic temperature (M), another at thermophilic temperature (T), and another at three temperature phases (mesophilic, thermophilic, and mesophilic) (M-T-M).

#### **2.1 Methodologies**

Primary and secondary sludge were sampling in the industry. The primary sludge was taking before the sludge press, and the secondary sludge from the sludge return line to the oxidation lagoon. Samples were transporting to the laboratory for their characterization. A mixture of primary and secondary sludge was preparing in a 50:50 ratio, thickening and concentrating the sludge to prepare an organic loading of 1.4 kg m−3 d−1.

Total solids (TS) and volatile solids (VS), pH, alkalinity, total nitrogen, volatile acids, chemical oxygen demand (COD) and total and fecal coliforms were measured according to the Standard Methods [39].

Elemental composition (C, H, N, S) and protein were conducted according to the procedure ISO-16948: 2015 [40].

Gas production was measured by displacement of an acidified brine solution (NaCl and H2SO4) in graduated cylinders. +.

Volatile fatty acids (VFA) was reassured by titration according to [41]. Biogas composition by a LandTec® gas analyzer.

#### **2.2 Biodigester operation**

Three stainless steel bioreactors (14-L each) were used to carry out the experimental anaerobic digestion process. The bioreactors had inlet and outlet valves for feeding and collecting biogas. Also, bioreactors had mechanical stainless-steel propeller-type stirrers, driven by an Arrow brand motor, model 350. The shakers were programmed to shake the content for three minutes every twenty minutes to keep

*Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

the sample homogeneous by shaking the reactors 20 times per day for 3 minutes the intervals between each shaking were 20 minutes. The digesters were providing with submersible electrical resistance and temperature control. The bioreactors were operating with an organic load mixture of 1.4 kg m-3 d-1 of primary and secondary sludge, in a ratio of 50:50 and a retention time of 30 days. One reactor was operating at a mesophilic temperature (M) of 35°C, another at a termophilic temperature (T) of 55°C, and the other at three temperature phases (M-T-M), mesophilic 35°C, thermophilic 55°C, and mesophilic 35°C. The reactors were operating in semibatch mode, feeding, and removing substrate every third, day and performing the analysis of Total and volatile solids, pH, alkalinity and acidity, volatile acids, total Kjeldhal nitrogen, total coliforms, fecal coliforms and measuring the volume and biogas composition.

#### **3. Results and discussion**

#### **3.1 Initial characterization of residual sludge**

**Table 2** shows the results obtained from the physicochemical and biological characterization for the primary sludge, secondary sludge, and the 50:50 mixture. The percentage of total solids is within the range of 5 to 9% according to [35]. The analysis were carried out in triplicate for each of the parameters analyzed. The cellulose present in the primary and secondary sludge is the result of the fact that its recovery is not total during the flotation process, and there is a great loss of these residues and has the potential to be reused for obtaining energy due to their high


#### **Table 2.**

*Physicochemical and biological characterization of the primary, secondary, and mixed sludge.*

calorific content. The cellulose concentration in the secondary sludge is due to the low biodegradability of its biological wastewater treatment [42]. The results of the alkalinity in the sludge are determined to give sludge buffer capacity, because that the anaerobic digestion process needs to withstand the changes in pH as the process progresses [43]. The content of carbon, hydrogen, nitrogen, total nitrogen, and proteins are necessary substrates for the reproduction of microorganisms and the generation of biogas [22]. The concentration of coliforms present in the sludge exceeds the Official Mexican Standard NOM-004-SEMARNAT-2002, so they require treatment for their disposal [44].

The results obtained for the 50% sludge mixture (primary sludge and secondary sludge) indicate that the combination of both substrates maintains conditions of total solids, volatile pH to carry out anaerobic digestions, having 42.5 g L−1 which, corresponds to 46% of SV of organic matter to be degraded, contained in the mixture of substrates.

#### **3.2 Volatile solids**

**Figure 1** shows the VS results for the 50:50 ratio of substrates with organic load (OL) of 1.4 Kg m−3 d−1 with a retention time of 30 days. It can be seen that, during the digestions in the three treatments, there was the removal of solids, the final removals of SV (%) for each treatment in its temperature phase were M = 52.49, T = 57.76, and M-T-M = 58.61.

#### **3.3 pH, alkalinity y total volatile acids**

**Figure 2A** and **B** show the behavior of pH and alkalinity parameters, respectively, during anaerobic digestion. **Figure 2A** shows that the T and M-T-M bioreactors managed to increase their pH to 7.3. The opposite case occurred with the mesophilic bioreactor where the increase of pH was only 6.7. **Figure 3B** shows that in bioreactor M there was a variation in alkalinity due to the low pH obtained values. For the T and M-T-M bioreactors, there was a decrease in alkalinity, reaching concentrations of 900 mg L−1 after 20 days.

**Figure 3** presents the VFA concentration, which decreased during the digestion process. During the digestions, there was no accumulation of VFA therefore, the process was not destabilized. It shows that the concentrations of VFA were decreasing throughout the process in the three bioreactors. Starting with 7100, 7800, and

**Figure 1.** *Volatile solids concentration during anaerobic digestion processes.*

**Figure 2.**

*Behavior of pH (A) and alkalinity (B) during anaerobic digestion processes.*

840 mg L−1 for the M, T, and M-T-M bioreactors, respectively. At the end of the treatments, the concentrations of 540, 640, 610 mg L−1 for the M, T, and M-T-M bioreactors, respectively. The buffer capacity in the digesters, neutralized the possible accumulation of volatile acids and maintained the pH values to stabilize the anaerobic digestion.

#### **3.4 Organic and ammonia nitrogen**

**Figure 4** shows the results of ammonia nitrogen during the experimentation, and it is observing how the ammonia nitrogen increased through the process for the three different temperatures. The increase in the concentration of ammonia nitrogen was not inhibitory for the development of the digestion process because all the bioreactors at the different temperatures presented biogas production.

**Figure 5A** and **B** shows the behavior of the biogas volume and methane fraction. It is showing that the T and M-T-M reactors generated a greater volume of biogas

**Figure 3.** *Concentration of total volatile fatty acids during anaerobic digestion processes.*

**Figure 4.** *Concentration of N-NH4 during anaerobic digestion processes.*

than the M bioreactor, which presented too low biogas volumes. The methane fraction was higher in the M-T-M bioreactor where a value above 60% was obtained.

**Figure 6** presents the methane yields in the anaerobic digestion processes. The M-T-M bioreactor resulted in a higher methane yield until day 24, after this time there was a decrease in methane yield. Methane yield was very low for the M and T bioreactors because of the conditions, but for the M bioreactor the yield was the lowest.

According to the literature review, there are research studies on different biomasses that can be processed in anaerobic digestions, such as agro-industrial, livestock, forestry residues, sludge from sewage treatment plants, industrial residues, where the biogas and methane yields are reporting when digesting these substrates. However, for particular wastes from industry using recycled paper raw materials, there are no studies to date. There are studies of the pulp and paper industry where other types of pollutants are generated from the chemical process, a case that does not apply to this industry. There is research on anaerobic digestions, always seeking to obtain high methane yields for reuse as biofuel or energy production. There is

*Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

**Figure 5.** *Behavior of the biogas volume (A) and methane fraction (B) during the anaerobic digestions.*

**Figure 6.** *Methane yields in the anaerobic digestion processes.*

also too much difference in the investigations carried out on AD in the production of methane or biogas because it is not only the substrates or cosubstrates but also the influence on the operating conditions of the bioreactors such as temperature, time retention, organic loads, pH, and C / N ratio, among others. The scope in AD is to optimize the process with the best conditions that result in high methane yields. For this reason, this study serves as the basis for future research to work on the combination of lignocellulosic waste from a primary treatment in combination with secondary sludge from an activated sludge process, both resulting from the treatment of wastewater from the industry under study. During the anaerobic digestions, the mixture into the bioreactors was agitated because one of the AD main problems is the mixing of the substrates into the bioreactor. Bad mixing results in low methane production due to the non-homogenization of the substrates [45].

The reduction of volatile solids was observed in the three treatments reaching yields greater than 50%, being greater in the digestion of the three stages M-T-M. The biodegradation of the organic compounds depends on the substrates to digesting.

During the anaerobic digestions, the pH remained between 5.8 and 6.5 in the first 18 days later after day 20 the thermophilic and three-stage MTM digestions the pH increased above 7, which did not happen with the mesophilic digestion that reached a pH of 6.6. The variation of the buffer capacity was influenced so that pH in the mesophilic anaerobic digestion did not increase its pH beyond 6.6. At pH lower than 6.6, the growth rate of methanogens was reduced and the activity of archaea-methanogenic bacteria is reduced both at low and high pH's [46]. Even so, in anaerobic digestion, M was produced in biogas, with 45% methane. For the other T and M-T-M digestions above 50 and 60% methane were obtained, respectively.

The samples in the three anaerobic digestions showed ammonia concentrations lower than 5000 mg L−1, which represented avoiding the inhibition of the VFA during the digestions [47]. It is observing that during the digestions the VFA decreased during the process however, in the M digestion the production of VFA was lower.

The temperature influences VFA production. It has been reporting that at thermophilic temperatures, VFA yields are higher due to faster acclimatization and more active acidogenesis, than at mesophilic temperatures [48].

However, there are other authors [49] that at thermophilic temperatures of 45 to 70°C it does not affect the production of VFA, finding controversies and inconsistencies in research due to the difference between microbial species, raw materials or substrates to be digested. Likewise, the use of different methodologies in AD affects the methane yield in equivalent substrates, making their comparison difficult [50–52]. The thermophilic process presents a better performance at the beginning than the mesophilic digestion due to the accelerated process of hydrolysis [53]. Higher methane yields are produced in the T and M-T-M, the latter having an advantage over the thermophilic since more than 60% of methane was obtained, which is considered biogas rich in methane [54]. Fecal coliform analyzes were performed during anaerobic digestions to determine their stabilization. Thermophilic digestion is a proven technology to produce class "A" biosolids, NOM-004-SEMARNAT-002. Where it turned out that the T and M-T-M digestions manage to obtain a biosolid with fecal coliforms lower than the norm.

#### **4. Conclusions**

The sludge generated from the paper process contains a high content of cellulose, which can be used by some microorganisms present in the secondary

#### *Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

sludge. These microorganisms could be used as a potential raw material for the production of methane [15, 42].

Anaerobic digestion of the primary and secondary sludge showed promising results for methane production. The research carried out with a mixture of primary and secondary sludge is to increase the yield of biogas and methane, since each one of the substrates provides different physicochemical and biological characteristics. The primary sludge calorific value is high compared to the secondary sludge, and mixing both sludge benefited the anaerobic digestion process.

A higher methane yield was obtained in the digestion of three M-T-M phases with a value of 24.75 L of methane (gr of VS)−1, also, a higher volume and percentage of methane, with values of 7000 mL and 67%, respectively.

The three-phase M-T-M process started with a pH value of 6.2 and was increased through digestion, reaching a pH of 7.6. The alkalinity was kept between 800 and 900 mg L−1, making the digestion process tolerate changes during the anaerobic digestion phases. That allowed no accumulation of organic acids, which diminish the production of methane gas [55].

The reduction of volatile solids occurred in the three digestions, with the thermophilic phase presenting a larger removal with 52%, followed by the three phases with 47%, and finally the mesophilic with 30%.

It was found that thermophilic and three-phase digestion have advantages over mesophilic digestion related to the destruction of bacteria and pathogens [56] in this study. The thermophilic and three-phase digestion stabilized the sludge by destroying bacteria since in the thermophilic process and the three-phase M-T-M process, fecal coliforms were eliminated on days 15 and 12, respectively, classifying these sludge as Class A according to the official Mexican standard NOM 004-SEMARNAT.

#### **Acknowledgements**

The authors would like to thank the TecNM/IT Durango for their help and financial support.

#### **Conflict of interest**

The authors declare no conflict of interest.

*Anaerobic Digestion in Built Environments*

#### **Author details**

Alfredo de Jesús Martínez-Roldán1,2, María Dolores Josefina Rodríguez-Rosales2 \*, Sergio Valle Cervantes2 and Thania Azucena Mendez-Perez<sup>2</sup>

1 CONACyT-TecNM/TecNM/IT de Durango, Master on Environmental Systems, Durango, Dgo, México

2 TecNM/IT de Durango, Master in Environmental Systems, Durango, Dgo, México

\*Address all correspondence to: mdjoserr@itdurango.edu.mx

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Employment of Organic Residues for Methane Production: The Use of Wastes of the Pulp… DOI: http://dx.doi.org/10.5772/intechopen.97111*

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### Section 2
