**2. The biological wastewater treatment process**

Municipal solid wastes are attracting more obstructive legislation with respect to landfill disposal of the biodegradable fraction [4, 8]. The treatment process for these organic fractions is biological wastewater treatment. These technologies maximises the recycling and recovery processes of waste components. The biological treatment is regarded as an important and vital aspect of wastewater treatment and is a technique employed for municipal or industrial use for soluble organic components [9]. Among all, the most widely employed method for sludge treatment is anaerobic digestion [9, 10].

#### **2.1 Anaerobic digestion (AD) process**

In this process, a large fraction of the organic matter (cells) is broken down into carbon dioxide (CO2) and methane (CH4), and this is accomplished in the absence of oxygen. About half of the amount is then converted into gases, while the remainder is dried and becomes a residual soil-like material. Kougias and Angelidaki [11] reported that the end products of organic assimilation in anaerobic treatment of waste are CH4 and CO2 as depicted in **Figure 1**. The AD technology has encountered significant recognition in the last few decades with the applications of separately configured high rate treatment processes for industrial wastewater streams. In the wastewater treatment settings, the AD has been employed in several instances throughout the world for bioremediation and biogas production [8, 12, 13]. Biogas, a well-known and common renewable source of energy, is produced via the AD process, consisting largely of CH4 and CO2. As an alternative source of energy, the AD process produces biogas that can be chiefly used as fuel in combined heat and power gas engines [11, 12, 14]. There has also been a rapid adoption of anaerobic codigestion, where two or more different feed stocks are digested together in anaerobic biodigesters with the core aim of improving the biogas yield [8, 11–13, 15–17]. Other advantages ensured in the anaerobic systems include lower energy requirements, a safer and more convenient way of converting "waste" to useful products associated with urbanisation, being a predictive tool for the fulfilment of the UN Sustainability Goal to meet Global standards, having excellent nutrient recovery and high organic removal efficiencies. Drawbacks include longer hydraulic retention times (2–4 months) and high alkalinity requirements [11, 18, 19]. The aerobic system presents merits such as high organic removal efficiencies, excellent effluent quality

*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

**Figure 1.**  *Modified stages of the anaerobic digestion process, adapted from [11, 12].* 

 and shorter start-up times (2–4 weeks). Demerits include longer hydraulic retention times, pretreatment requirements for delignification of lignocellulosic biomass, odour built-up in bioreactors, costs associated with CO2 upgrading, no nutrient recovery and high energy requirements [8, 12, 17]. Research conducted by Kainthola et al. [19] details the major differences between the anaerobic and aerobic systems of wastewater treatments, as depicted in **Figure 2**, demonstrating the mechanism with species required and products formed. The anaerobic/aerobic systems have also been employed largely at both municipal and industrial levels as a method for wastewater treatment for many years. It presents advantages such as a lower consumption of energy, low chemical consumption, low sludge production, its enormous potential for the recovery of resources, simplicity of the operation and the requirement of less equipment. Some advantages of the biological treatment method over other treatment techniques such as thermal and chemical oxidations are capital investments required and costs in operation of the processes [8, 11, 12, 14].

#### *2.1.1 Operating parameters of AD process*

Some operating parameters which are usually monitored and optimised to maximise the performance and operation of AD include organic loading rate, pH, hydraulic

#### **Figure 2.**

*Schematic diagrams of (a) aerobic treatment principle and (b) anaerobic treatment principle, adapted from [11, 12].* 

retention time, temperature, carbon to nitrogen ratio and many more [15, 20]. As a result, any sharp variation in these parameters could adversely affect the substrate concentration in the biodigesters. Some of the operating parameters are discussed in Sections 2.2.1–2.2.4.

#### *2.1.1.1 Organic loading rate (OLR)*

This is generally expressed in terms of the amount of chemical oxygen demand (COD) or volatile solids (VS) of digester volume in a day and denoted as KgCOD/m3 d or KgVS/dm3 d. Most favourable COD removal of the canning industry effluent was found to be between 89 and 93% at OLRs of 9.8 and 10.95 kgCOD/m3 d at an HRT of 10 h at a pH of 5.5 [15, 18, 19]. This prediction becomes viable during the selection of the reactor-type and other process parameters such as pH control. OLR has been found to increase with decreasing biodegradation of the volatile solid and the subsequent bioenergy produced. The performances of bioreactors decrease when the OLRs increase with energy production [10, 15, 17].

 Furthermore, the pH range suitable for AD is reported to be within the range of 6.8–7.2 [13]. This is achieved by charging the AD at an optimum OLR to obtain a higher yield of biogas. There is usually a variation in pH during AD especially during acidogenesis where volatile fatty acids such as propionate, butyrate and acetates are produced [19]. The presence of phosphates (PO4 <sup>−</sup>) in most wastewater treatment facilities renders the pH adjustment with calcium hydroxide possible to a pH of about 7.2, even at high concentrations. The growth of microorganisms in AD is largely dependent on the pH of the substrates undergoing the overall biodegradation [8]. In the treatment of wastewaters, the observed pH range of 6.0–7.1 was been reported in a study where a mixed batch reactor produced larger quantities of biogas at an average value of 0.405 m3 /d [11].

#### *2.1.1.2 Temperature*

Temperature conditions during the AD process for bioenergy production includes psychrophilic (<30°C), mesophilic (30–40°C) and thermophilic (50–60°C) [8, 12]. The anaerobes are found to be more active under both mesophilic and thermophilic temperatures as compared to psychrophilic temperatures. Comparatively, thermophilic temperatures are considered suitable for the enhancement of biomethanation by accelerating the hydrolysis of the polymeric feedstock and other metabolic pathways [12, 13]. However, several studies have shown that thermophilic digesters suffer from poor process stability due to volatile fatty acid accumulation during the acidogenesis process, most especially propionate [13, 17].

#### *2.1.1.3 Hydraulic retention time (HRT)*

This is the measure of the time required to achieve the complete biodegradation of an organic matter associated with process parameters such as the temperature of the medium and the waste composition [12]. The HRTs observed in AD under mesophilic and thermophilic temperatures are 15–30 days and 12–14 days respectively [13, 17]. Temperature and HRT effects on the methanogenesis process have been observed in a study by Shah et al. [13]. In the same study, the working temperature was adjusted from 30 to 55°C following an HRT of 8–12 days.

*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

#### *2.1.2 Applications of the anaerobic digestion process*

The full-scale application of the AD technique in the treatment of industrial wastewater depends on the hydrodynamic configuration of the AD reactor. There have been different types of AD reactors applicable in the wastewater treatment settings, which includes continuous stirred tank reactor (CSTR), the anaerobic sequencing batch reactor (ASBR), upflow hybrid anaerobic sludge-filter bed (UASFB), upflow anaerobic sludge blanket reactor (UASBR), expanded granular sludge bed (EGSB), anaerobic baffled reactor (ABR), anaerobic fixed-bed reactors (AFBR) and integrated bio-membrane reactors [8, 13, 15–17]. For instance, **Figure 3** depicts schematic cross section view of the upflow hybrid anaerobic sludge-filter bed and the upflow anaerobic sludge blanket reactor. The integrated anaerobic-aerobic bioreactors have been most preferred in the past few decades due to its ability to meet stringent constraints in terms of mitigating odorant compound release and minimising sludge production.

#### *2.1.2.1 Limitations of adapting AD process in large scale*

 Research has shown that among the various reactors used in the performances for the treatment of wastewaters, the UASBR configuration is the most widely used with a high-rate anaerobic reactor for the treatment of high-strength wastewater [8, 12]. Several modifications have been carried out in the design of bioreactors to enhance both the consistency and the efficiency of the reactors. The AD process does encounter failures causing serious environmental hazards [8]. In addition, some of the aforementioned operating parameters as previously discussed (Section 2.2) can affect the performances of the microbes responsible for the biodegradation of organic matter in the wastewater settings. Further drawbacks observed in AD large scale operation include microbial shift, process instability, low yield of biogas production and poor water quality [8, 10, 12, 14, 19]. For instance, monodigestion of energy crops still struggle to meet the reduction targets concerned with the drawbacks in AD compared to anaerobic co-digestion (AcoD) such as a mixture of slurry and energy crops [5, 20]. In response, pretreatment techniques for cellulose enhancement and the use of energy crops as feed stocks have been found to increase the efficacy of biogas production via AD [12]. Some of the improvement techniques which have gained attention in terms of research for the betterment of AD process design and the optimisation includes

#### **Figure 3.**

*(a) Upflow-hybrid anaerobic sludge-filter bed and (b) upflow-anaerobic sludge blanket reactor [8].* 

evaluating the AD process kinetics and dynamics, nitrification-denitrification, recycling of the centrate back to the AD reactor, wastewater characterisation, optimisation of operational and environmental parameters, and microbial community shift.
