**5.7 Hybrid bioreactor**

Hybrid bioreactor represents the modern production of reactor with possibility to incorporate the benefits of both suspended solid and biofilm reactors. These types of reactors provide the benefits of the UASB concept related to the ones of the anaerobic filters, and nowadays can be considered more appropriate for the treatment of a sequence of soluble or partially soluble wastewater than other reactor systems. Hybrid reactor (combination of the basic types) and anaerobic baffled reactor (ABR) fall under this category [143–145].

Anaerobic digestion may consist of a single-stage operation, or a two-stage process. Single-stage operation is less efficient, but most commonly used because of its simplicity. Traditional single-stage digester is generally larger, and hence takes more energy to mix and heat compared to a two-stage digester; while, a two-stage digestion is more efficient overall compared to a single-stage process [50, 146–150]. Many different configurations and operational systems have been developed for anaerobic digesters for use in different applications. The goals normally are to shorten the start-up period, reduce operational instabilities, decrease washout of active biomass, and attempt to better accommodate the inevitable variations in feed composition. Operation, maintenance, and installation cost are other factors that substantially impact the economics of biogas generation.

Single-stage digesters are most typically utilized on account of their simplicity, but overall two-stage digesters are more effective. There is no specific digester kind can be recommended as being internationally appropriate. The selection in a given scenario has to consider a lot of factors involving the following: the prospects for disposal of the digestate and the effluent; nature and strength of the waste stream; the availability and skills level of the local workforce; local climatic conditions, infrastructural support and cost of energy; and the expense of construction and operation. Generation of biogas by AD is a helpful method to recover energy from organic waste, whereas considerably reducing the environmental effect of the

waste [50]. In addition, the CSTR design is normally performed in single-stage systems, there the reactor operates, favoring both methanogenic and acidogenic microorganisms. These types of systems have lower capital and operating costs and are simple to operate, making them attractive for a broad range of applications through the last decades [151, 152]. Furthermore, the conversion of organic material to biogas is implemented during a series of biochemical reactions, which do not inevitably have the identical optimal environmental conditions. Singlestage digesters have simple design with less technical failure. In the other hand, it has higher retention time, and form foam and scum leading to potential failure [50, 117, 153]. In order to get higher reaction rates and hence a higher biogas yield, two- and multistage systems have developed to give optimal conditions for the various groups of microorganisms included in the degradation process [50, 153]. Four processes (hydrolysis, acidification, acetogenesis, and methanogenesis) in AD are separated in two-stage reactors. Thus, the first stage can be operated at lower pH, which is more favored for the growth of acidogenic and hydrolytic microorganisms; whereas, the second phase is operated to prefer the growth of methane forming microorganisms [154]. In the second step, the rate of limiting factor is normally the rate of microbial growth [155] since longer generation times for methane-producing archaea, and thus longer biomass retention times are required in this second stage, which in turn improves the biogas yield [118]. These kinds of digesters usually have a more steady performance than single-stage digesters, since they do not bear from the process disturbances caused by ammonia accumulation and the changes in the pH [155, 156]. Best phase extraction option can be given in multistage reactors, which can provide optimization and process control for each conversion point, leading to raised methane generation [50, 157]. Two-stage reactors increase in biomass digestion due to recirculation, it has constant feeding rate to methanogenic stage, and it is more robust and less susceptible to failure. In contrast, it has complex design and expensive to build and maintain, and solid particles need to be removed from the feedstock in the second stage [117].

## **6. Biogas technologies**

There are undesired compounds and other gases contained in biogas are unwanted and are considered as biogas pollutants [11]. The concentrations of these impurities are dependent on the composition of the substrate from which the gas was produced [158]. The removal of these harmful components and other non-combustible gases makes biogas a more viable and economical alternative renewable energy source [96, 159]. The energy content of methane described by the Lower Calorific Value (LCV) is 50.4 MJ/kg CH4 or 36 MJ/m3 CH4 (at STP conditions). Therefore, the higher the CO2 or N2 content is, the lower the LCV in biogas [11, 160]. Developing the quantity and quality of biogas often needs pretreatment to maximize methane yields and/or post-treatment to take out H2S, which includes higher costs and considerable energy consumption. Therefore, scientific research has performed to develop a low-cost desulfurization process and improve AD conversion. Appealingly, there are a lot of techniques that have been approved to enhance the anaerobic digestion process, like pretreatment procedures using acidic/alkaline, ultrasonic, thermal methods [161–163]. Lately, there are various treatments targeting at get rid of the trace contaminants and undesired components from the biogas expanding its range of applications [11]. Biomethane involves two major treatment processes; cleaning and CH4 enrichment (biogas upgrading). The cleaning of the biogas contains elimination of impurities and acidic gases; whereas, the enrichment process is for extraction of CO2 from biogas [11, 96]. There are three major reasons for gas cleaning;

### *Recent Advances of Biogas Production and Future Perspective DOI: http://dx.doi.org/10.5772/intechopen.93231*

fulfill the requirements of gas appliances (gas engines, boilers, fuel cells, vehicles, etc.), increase the heating value of the gas, and standardization of the gas [58]. Biogas cleaning treatment process includes removal of undesired materials (such as, NH3, siloxanes, H2S, volatile organic compounds (VOCs), and CO) to increase the quality of biogas. However, it is practically only H2S which is mainly targeted and many current biogas plants have H2S elimination units normally rely on biological H2S oxidation by aerobic sulfate oxidizing bacteria [11]. Biogas must be desulfurizated and also dried before usage to stop destroys the use of gas units. The concentration of H2S between 100 and 3000 ppm in biogas generated by cofermentation of manure with harvesting debris or energy crops, in order to prevent an expensive deterioration of lubrication oil and excessive corrosion [21, 22]. CHPs are used for the utilization of biogas need generally levels of H2S below 250 ppm. The existence of H2S not only affects the quality and quantity of the biogas generated which can restrict its application, but also produces dangerous environmental emissions and corrodes the motors of biogas purification machinery [20, 23]. Nowadays, biological desulfurization process mainly used to remove of H2S [21, 22]. Recent study conducted by Register Mrosso [164] reported that red rock (RR) is an available material for biogas purification which used to remove hydrogen sulfide from biogas [164]. The quality of raw biogas can be further improved via various upgrading techniques to remove the non-combustible components and as a result increasing the methane content to approximate natural gas quality (75–98% methane) [96]. Biogas has been upgraded to natural gas composition via methanation using renewable hydrogen [165]. The higher the methane content, the richer the biogas is in energy [12]. Biogas upgrading aims to increase the low calorific value of the biogas, and convert it to higher fuel standard [35]. In case the upgraded biogas is purified to specifications similar to natural gas, the final gas product is called biomethane [11, 166]. Biomethane is a gaseous fuel with physicochemical properties similar to those of natural gas, which makes it possible to inject it into the gas grid [96]. Currently, the specifications of the natural gas composition are depending on national regulations and in some countries >95% methane content is required [11].

Technological development plays an important role in biogas upgradation and purification processes in large-scale commercialization of biogas. There are various cleaning and upgrading techniques to improve the quality of raw biogas which can be categorized into physiochemical and biological technologies. Some of these techniques are conventional methods, including physical absorption, chemical absorption, membrane infiltration and biological methods, and others are considered as new technologies including cryogenic upgradation, membrane enrichment, multistage-, and high-pressurized AD [62, 96, 167, 168].

Physiochemical technologies for cleaning of biogas and its subsequent CH4 enrichment can be grouped as follows: absorption process (physical and chemical absorption), Hybrid solution (mixed physical and chemical solvent), and physical separation (adsorption on solid surface; membrane; cryogenic) [96]. Novel technologies, such as cryogenic separation, in-situ upgrading, hydrate separation, and biological methods, represent the recent developments in biogas upgrading technologies. Biogas can be used as fuel for domestic stoves, boilers, internal engines, gas turbines, cars, and fuel cells, or injected into natural gas grids to replace gaseous fuel [35]. These techniques have been reported to yield biomethane typically containing 95–99% CH4 and 1–3% CO2. At this quality, the spectrum of applications for biogas widens, it can be used to serve the same applications as natural gas [96]. Gas upgrading and utilization as renewable vehicle fuel or injection into the natural gas grid is of increasing interest because the gas can be used in a more efficient way [21]. Types of upgrading plants are available in Sweden, and shows that around 70% of the biogas purification plants apply water-washing technologies [169].

### **6.1 Physiochemical technologies include**
