10.5. Methods for enhancing the efficiency of anaerobic digestion
10.5.1. Pretreatments
The hydrolysis of substrate into soluble organics is the rate-limiting step for AD (Zhang et al., 2014). As a result, AD often suffers from long solid residence time and low conversion efficiency (Quiroga et al., 2014). Therefore, the effects of different pretreatment methods such as mechanical, thermal, chemical and enzymatic pretreatments have been investigated for improving the hydrolysis of biomass.
10.5.1.1. Mechanical pretreatments
Mechanical pretreatment is usually required for high solid content feedstocks before any other kind of pretreatment, and actually refers to milling, through which particle size reduction of feedstocks is accomplished. Mechanical pretreatment disintegrates the solids and/or decreases the particle size of the biomass and so increases the surface area, resulting in increased substrate availability for the microorganisms. Consequently, microbial growth and AD process are improved (Ariunbaatar et al., 2014).
Grinding and/or milling are mainly used for municipal solid waste pretreatment in large-scale applications, which increase biogas yield by 18–36% (Subramani & Ponkumar, 2012; Zhu et al., 2009). Using lignocellulosic biomass with a particle size in the range of 0.003–30 mm, methane yield improved up to 30% (Zheng et al., 2014). It should be noted that the particle size reduction process should be optimized as the excessive grinding will be energy-intensive. And it may also have negative effects on the AD process due to the fast VFA accumulation (De la Rubia et al., 2011; Izumi et al., 2010).
On the other hand, ultrasonication seems to be a promising pretreatment method to improve the hydrolysis and AD of sludge. It disintegrates sludge by disrupting the microbial cell walls in sludge, resulting in the release of organic substances of sludge into the liquid phase. The full-scale ultrasonication provides up to 50% increase in the biogas generation (Pilli et al., 2011). The effect of ultrasonication depends on the sludge characteristics and ultrasonication parameters, therefore, not all studies confirm the enhancement of hydrolysis or improved biogas production by ultrasonication (Sandino et al., 2005). There are also some reports explaining that ultrasonication and microwave improve AD of food waste in lab scale (Marin et al., 2010; Quiroga et al., 2014). However, the effectiveness and feasibility of ultrasonication for the pretreatment of high-solid FW in large-scale applications should be evaluated. Therefore, the effectiveness, feasibility, and cost analysis of these pretreatment methods should be assessed for each biomass prior to large-scale applications.
10.5.1.2. Thermal pretreatments
Thermal pretreatment is another pretreatment method, which is applied for the enhancement of AD processes from various kinds of biomass in large scale. It disintegrates the cell membranes and so enhances the solubilization of COD, and may subsequently improve the AD process and shorten the retention time in the digester. Moreover, it helps to remove the pathogens and reduces the viscosity of the biomass. However, the effects of thermal pretreatment depend on the substrate type, pretreatment temperature, and duration. Thermal pretreatment of sludge, even at a lower temperature (70°C), has a significant effect on pathogen removal (Skiadas et al., 2005). However, pretreatment at 70°C for 60 min provided a negligible improvement in biogas production from sludge (Appels et al., 2010), while it did not provide an improvement in biogas production from household waste and algal biomass (Chamchoi et al., 2011; Gonzalez-Fernandez et al., 2012). Pretreatments at higher temperatures enhance the COD solubilization and biogas production, even though the increase in solubility may not be proportional to the increase in biogas production (Wang et al., 2006). Aggressive thermal pretreatments can increase the solubilization of COD but inhibit biogas production (Tampio et al., 2014). Inhibitory Maillard reactions may take place above 100°C between the carbohydrates and amino acids, which causes problems in protein hydrolysis (Vavouraki et al., 2012). Rafique et al. (2010) reported that biogas production decreased when pig manure was pretreated at temperatures higher than 110°C, possibly due to the occurrence of Maillard reactions. Tampio et al. (2014) reported that autoclaved (160°C, 6.2 bar) FW had lower ammonium and hydrogen sulfide concentrations than untreated ones, possibly due to reduced protein hydrolysis as a result of formation of Maillard compounds. As a result, untreated FW provided 5–10% higher methane yields than pretreated FW. On the other hand, steam explosion is one of the most promising pretreatments for lignocellulosic substrates. It is operated at high temperatures (150–250°C) for a few seconds to minutes, which is followed by a sudden pressure drop (Weiland, 2010). It helps to reduce the crystallinity, release soluble compounds and improve the biogas yield (Shafiei et al., 2013).
10.5.1.3. Chemical pretreatments
Chemical pretreatment with strong acids, alkalis, or oxidants are applied to enhance the hydrolysis rate and subsequent biogas production. The effect of chemical pretreatment depends on the type of method applied and the characteristics of the substrates. Concentrated acid pretreatment is not a suitable option for easily degradable substrates which have high amount of carbohydrates, due to the accumulation of VFA and the degradation of soluble sugars into inhibitory compounds such as furfural and hydroxymethylfurfural (Vavouraki et al., 2012). Therefore, dilute acid pretreatment is generally used and coupled with thermal pretreatment for enhancing the hydrolysis and biogas production from such biomasses, ie, food waste (Ariunbaatar et al., 2014). On the other hand, these methods facilitate hemicellulose hydrolysis and modify the complex structure of lignin, improving the accessibility of the cellulose to microbial attacks in lignocellulosic biomasses. Therefore, the hydrolysis of and biogas production from lignocellulosic biomasses significantly is improved by chemical/thermochemical pretreatments (Zheng et al., 2014). However, it should be noted that the cations released from the salts and hydroxides, such as potassium, calcium, and sodium, can be inhibitory to AD at certain concentrations. Moreover, the pH of the pretreated biomass should be neutralized before AD. Therefore, the cost of the chemicals and the addition of neutralization agents should be taken into account in large-scale applications.
Ozonation is another chemical pretreatment option for enhancing the biogas production. It has some advantageous over alkaline or acid pretreatment because it does not release any inhibitory compounds like furfural and hydroxymethylfurfural or discharge any cations. It is also useful for the disinfection of the pathogens. Therefore, it is particularly used for sludge and wastewater treatment to improve their hydrolysis and AD process. However, a large amount of ozone is generally required for ozonation, which makes this method quite expensive.
Although thermal/thermochemical pretreatments might promote better results in solubility and methane yield, the energy required for the pretreatment process should be taken into account before including them in the AD system. Ariunbaatar et al. (2014) demonstrated that the enhanced methane production could cover the energy required for thermal pretreatment. However, Liu et al. (2008) reported that there was no energy surplus when the energy consumed during the pretreatment process was taken into account.
10.5.1.4. Biological pretreatments
Biological pretreatments using enzymes may improve the solubility of the biomass without producing any inhibitory compounds. Commercial enzymes including amylases, proteases, and lipases have been reported to improve the hydrolysis of FW, while the pretreatments using cellulolytic and hemicellulolytic enzymes intensified the biogas production from lignocellulosic agricultural residues (Moon and Song, 2011; Parawira, 2012). The pretreatment of complex biomasses with multiple commercial enzymes appeared to be more efficient than that with a single commercial enzyme (Kim et al., 2006; Moon and Song, 2011). However, it should be realized that commercial enzymes are costly and generally available in single-type form. In order to make the enzymatic hydrolysis more cost-effective, enzymes can be produced on-site from a cheap feedstock. In the study of Uçkun Kiran et al. (2015), a fungal mash rich in glucoamylase and protease was produced from cake waste and was applied directly for enzymatic hydrolysis of mixed FW. The enzymatic pretreatment using this fungal mash was shown to be more efficient than commercial enzymes. The biomethane yield and production rate from FW pretreated with the fungal mash were found to be, respectively, 2.3 and 3.5 times higher than without pretreatment, indicating that direct use of the fungal mash without any purification is a promising option for FW treatment.
Biological pretreatments by the addition of microorganisms have also been found to improve biogas production from cattle manure and agricultural residues (Angelidaki and Ahring, 2000; Chen et al., 2010; Zhong et al., 2011). The bacteria and fungi basically degrade lignin and hemicellulose and increase the accessibility of cellulose in an environmentally friendly way. However, the main drawback of microbial pretreatment is the long incubation time, which hinders its applicability in large scale. Furthermore, it should also be noted that most of these lignocellulolytic microorganisms can utilize cellulose beside hemicellulose, which negatively affects the final biogas yield. Another biological pretreatment method is ensiling, which is particularly applied for energy crops. It is applied using starter cultures or enzymes to convert soluble sugars to organic acids such as lactic and acetic acids in order to inhibit the growth of undesirable microorganisms during the storage (Weiland, 2010).
As explained above, pretreatments might improve the solubility and AD of FW when they are applied under optimized conditions. However, most of the pretreatment studies have been conducted in lab- or pilot-scale so far. In order to have a more realistic idea about their effectiveness and feasibility, these processes should be evaluated in an integrated system considering the capital costs and applicability.
10.5.2. Anaerobic codigestion
AD has great potential for energy recovery from and stabilization of the waste biomass (Zhang et al., 2014). However, AD of single substrates in long-term operations presents some negative aspects linked to the substrate characteristics, ie, the imbalance of the nutrients in the system. For example, sewage sludge has low organic solid content, which leads to low methane production. Animal manures have high nitrogen concentrations that may inhibit the methanogens. FW has high concentrations of sodium and has low buffering capacity. Agro-industrial wastes are seasonal biomass and are lacking in nitrogen. Slaughterhouse wastes contain high concentrations of long-chain fatty acids and nitrogen, which inhibit the AD. Anaerobic codigestion of feedstocks is a promising option to overcome the drawbacks of monodigestion and to improve a plant's economic feasibility. Therefore, currently there is a dramatic increase in codigestion research, particularly using sludge/manure with agro-industrial residues, organic fraction of municipal solid waste (OFMSW), algae and fats, oils, and greases (FOG). The effects of anaerobic codigestion of different biomass are summarized in Table 10.1. Traditionally, anaerobic codigestion between sewage sludge and OFMSW has been applied in many codigestion plants. However, nowadays most of the research interest is on anaerobic codigestion of sewage sludge with OFMSW and FOG, because both of these substrates can be obtained at the same wastewater treatment plant (Mata-Alvarez et al., 2014).
Sewage sludge has high water content, providing a low methane yield probably due to its low VS/TS ratio and high ammonia content (Dai et al., 2013). However, codigestion of sludge with easily degradable high carbon content biomass improves the methane yield. OFMSW and sewage sludge mixture show a complementary and synergistic effect in AD (Kim et al., 2011). OFMSW balances the lack of carbon source in sludge, while sludge provides ammonia and dilutes the harmful and excessive sodium, which inhibits the growth of anaerobic microorganisms in FW. FOG originated from wastewater treatment plants and industrial processes is also successfully used together with the sludge due to its high methane potential. On the other hand, the presence of sludge will moderate the high LCFA concentration, which may cause failure of AD (Zonta et al., 2013).
The process parameters such as composition of the substrates and their mixing ratio are very important for the performance of the codigestion process. The ratio of substrates is generally optimized based on C/N ratio, still, as mentioned in Section 10.2, other parameters such as macro- and micronutrients, pH and free ammonia and inhibitory compounds should be considered (Dai et al., 2013; Koch et al., 2015; Zhang et al., 2011). Hence, before the codigestion applications, it is better to conduct lab-scale experiments to detect the presence of inhibitory compounds/dosages and optimize the mixing ratio and substrate loading rates to prevent any inhibition and improve the effectiveness of AD.
Table 10.1
Co-digestion of various feedstocks for improving performance of AD
| Feedstock | Action of codigestion | Influencing factor | References |
| DS and FW | Enhance system stability | Less inhibition from sodium | Dai et al. (2013) |
| DS and FW | Improve methane yield | Less inhibition from lipids and potassium | Carucci et al. (2005) |
| SS and OFMSW | Afford high organic loading rate | High buffering capacity from ammonia | Kim et al. (2011) |
| SS and FW | Improve methane yield and production rate | High BMP potential | Koch et al. (2015) |
| SS and OFMSW | Allow higher organic loadings | Trace elements supplement | Parry and Evans (2012) |
| SS and FOG | Improve methane yield and production rate | High BMP potential, less inhibition from LCFA | Noutsopoulos et al. (2013) |
| WW and FW | Improve biogas productivity and process stability | Trace elements supplement | Zhang et al. (2011) |
| WW and FW | Improve methane yield and TOC utilization | High buffering capacity | Wang et al. (2013) |
| CM and FW | Improve methane yield and system stability | High buffering capacity and trace elements supplement | Zhang et al. (2013) |
| CM and FW | Improve biogas production | High buffering capacity from ammonia | Marañón et al. (2012) |
| Table Continued | |||
| Feedstock | Action of codigestion | Influencing factor | References |
| CM and FW | Improve methane yield | Nutrient balance | El-Mashad and Zhang (2010) |
| CM and OFMSW | Increase energy returns and reduce GHG emission | Nutrient balance | Banks et al. (2011b) |
| OFMSW and livestock waste | Improve methane yield and VS reduction | High buffering capacity | Kim and Oh (2011) |
| CM, FW, and FOG | Improve methane yield | High buffering capacity | Neves et al. (2009b) |
| CM, FW, and FOG | Improve methane yield | Lipid supplement | Neves et al. (2009a) |
| CM, OFMSW, and card packaging | Allow higher organic loadings and gave a more stable process | Trace elements supplement | Zhang et al. (2012) |
| FW and yard waste | Improve methane yield | Less VFA accumulation | Brown and Li (2013) |
| FW and distiller's grains | Increase biogas production | High buffering capacity from ammonia | Wang et al. (2012) |
| FW and green waste | Improve VS reduction | C/N ratio | Kumar et al. (2010) |
Revised from Zhang, C., Su, H., Baeyens, J., Tan, T., 2014. Reviewing the anaerobic digestion of food waste for biogas production. Renewable & Sustainable Energy Reviews 38, 383–392.
Acclimation is another possible approach to overcome the inhibition and prevent the lag phase during codigestion. Carucci et al. (2005) reported that AD of FW was inhibited due to the high potassium (55 g/kg dry FW) and lipid content (13%) of the FW when unacclimated inoculum was used. After a long acclimation period, the AD of a mixture of FW and sludge (60% and 40%, respectively) provided significantly higher methane yield (53%).
Thermophilic AD usually provides a faster metabolic rate and higher system performance than mesophilic AD (Zhang et al., 2014). However, Kim et al. (2011) reported that a temperature-phased anaerobic sequencing batch reactor provides faster metabolism at high organic loading rates compared to two-stage mesophilic systems. This might be due to the enhanced stability of thermophilic methanogens and alleviated alkalinity by improved protein degradation. In long-term operations, they obtained 44.2–76.5% VS removal from the codigestion of sewage sludge and FW while producing 0.2 m3 CH4/kg VSadded at organic loading rate of 6.1 g VS/L/d with short HRT (7 days) through the synergy of sequencing-batch operation, codigestion, and temperature-phasing. In another study, the codigestion of cattle manure with cheese whey in a two-stage system provided a 40% higher methane yield than a one-stage system (Bertin et al., 2013).
The biomethane yield can be increased significantly by codigestion of organic wastes (Parry and Evans, 2012). However, the transportation of waste influences this effect negatively. In order to achieve a sustainable waste treatment strategy, the transportation of the wastes should be minimized.
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