Anaerobic digestion (AD), a highly recognised technology, involves the degradation and stabilization of organic materials under anaerobic conditions (in the absence of oxygen) by microbial organisms and leads to the formation of biogas (a mixture of CH4 and CO2, a renewable energy source) and microbial biomass (Chen et al., 2008;
Mao et al., 2015). Anaerobic digestion offers numerous significant advantages, such as low sludge production, low energy requirement, and possible high-quality energy and fertiliser and finally converts a waste management issue into a profit centre.
Despite these benefits, however, poor operational stability still prevents anaerobic digestion from being widely commercialized and research study on the improve of the process is continuing.
The anaerobic digestion process comprises of four stages, namely hydrolysis, acidogenesis, acetogenesis and methanogenesis in which the organic substrates are converted to methane and CO2. Figure 3-3 shows the main phases of AD.
Figure 3- 3: Main stages of anaerobic digestion (Li et al., 2011)
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3.2.1 Classification of anaerobic digestion
Anaerobic digestion can be classified according to the total solids concentration, digester feeding process, steps involved or numbers of substrates. Table 3-4 summarises all the process. All the classified AD systems work on the same mechanism as four basic stages (Kothari et al., 2014), which is shown in Figure 3-3.
In the present study, AD focused on TS concentration inside the digester, feeding frequencies and number of substrates for single stage AD process under mesophilic conditions.
Table 3- 4: Classification of anaerobic digestion system
AD system characteristics
1. Total solids basis(Karthikeyan and Visvanathan, 2013; Kothari et al., 2014) (a) Wet AD Total solids inside the reactors <10%
(b) High solid AD Total solids inside the reactors <10-20%
(c) Dry AD Total solids inside the reactors <20-40%
2. Feeding frequency basis (Kothari et al., 2014)
(a) batch The reactors feed with substrates and inoculum before sealing the reactors for biogas production. At the end of the process, the reactors were emptied for new feed.
(b) continuous The digester is continuously feed and wasted 3. Process steps basis (Mata-Alvarez et al., 2000)
(a) Single stage All digestion steps occur in one digester
(b) Multi stage AD process consists of several reactors for hydrolysis and methanogenesis
4. Temperature basis (Saratale et al., 2018)
(a) Psycophilic Digestion temperature <20°C
(b) Mesophilic Digestion temperature 20-45°C, usually 37°C (c) Thermophilic Digestion temperature >45°C, usually 55°C 5. No of feedstock basis
(a) Single-substrates digestion
Only single substrates digested with inoculum
(b) Co-digestion Two or more substrates are mixed to improve the digestion process by maintaining mixture ratio, C/N ratio, composition
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3.2.2 Wet and dry anaerobic digestion
TS concentration is one of the most important parameters during AD which determines the efficiency assessment (Zhang et al., 2018). It is widely defined and practiced including wet, semi-dry, and dry anaerobic digestion, when TS of substrate are < 10, 10–15, or > 15%, respectively (Li et al., 2011; Liotta et al., 2015; Zhang et al., 2018). Wet anaerobic digestion (W-AD) is applied mainly to wastewater treatment plants, livestock and poultry breeding wastewater, food waste and energy crop due to high methane yield per unit substrate, low level of sludge generation and convenient operation and maintenance (Zhang et al., 2018).However, for feedstock with low moisture content, such as straw, dry manure, and agricultural wastes, dry anaerobic digestion (D-AD) is a better choice because of low consumption of water, small reactor requirement and high volumetric methane production (Shahbaz et al., 2018). However, the microbial source that accelerates the start-up of D-AD reactors has aroused the concern of researchers (Li et al., 2014b). Different parameter and conditions for dry and wet anaerobic digestion is shown in Table 3-5.
There are several AD companies, successfully treated municipal wastes. Most AD facilities incorporate the four stages described in figure 3.3 with some differences in the pre-treatment processes and to a lesser extent in the post-treatment of the products. Structurally, the AD chambers are mostly similar, though operating parameters also vary between treatment processes. Some of the technologies are BTA, Valorga, BRV, DRANCO, Kompogas, WASSA, Maltin, HIMET, ArrowBio.The WAASA process, developed by Citec in 1984, has operations in Finland, Sweden, Japan, Spain, France and the Netherlands (Citec, 2004). In this process, digestion occurs in a vertical digester at either mesophilic or thermophilic temperatures with 10-15% TS content. The digester is a single vessel which is subdivided to two separate chambers for two stages. Mixing is done through biogas injection at the base and sometimes through top mixing for household waste. The digestate is dewatered and aerobically composted. The WASSA process reduces 60% waste volume and 50-60% weight (Citec 2004).
Organic Waste Systems of Belgium developed an AD demonstration plant (DRANCO) in 1984 in Gent, Belgium.The DRANCO process is employed as part of the SORDISEP process (SORting, DIgestion and SEParation) of municipal and
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industrial waste for a maximum recovery of recyclables and energy. The feedstock is mixed with digested material, to form a mix of 15-40% TS content. DRANCO is a single stage, vertical gravity driven plug flow system, where the waste fed at the top of the chamber and digested were collected at the bottom with no mixing. The system is run at low pressures and thermophilic temperatures for 15-30 day. Biogas production ranges from 100 to 200 m3 /ton of waste. The solid digestate is dewatered to about 50% and then aerobically processed for two weeks to stabilize the material (Organic Waste Systems 2004).
Table 3- 5: Parameters for dry and wet AD(Karthikeyan and Visvanathan, 2012b; Kwietniewska and Tys, 2014)
Parameter Wet Anaerobic Digestion Dry Anaerobic Digestion
Total Solids (TS) <15% 20-40%
Feedstock Type e.g. Wastewater sludge e.g. OFMSW
Water Requirement Med-High Low
Operational Mode Single, two, multi- Single, two, multi-
Volatile Solids (VS) Loss High Minimal
Organic Loading Rate 2-5 kg VS / m3 day 5-12 kg VS / m3 day Max Biogas Yield 0.417 (WASSA process) 0.622 (DRANCO process) Volume/Heating
Requirement Large volume, high heating Smaller volume, less heating
Dispersion of Inhibitors Shock loads, more dispersion Less mixing, less dispersion
Digestate Dewatering High requirements Low requirements Digestate Characteristics Less stable with high VS More stable than wet
process Wastewater/Compost More wastewater, less
compost
Less wastewater, more compost
Maintenance
Abrasion from sand/grit, clogging and deposition, mixers lead to short circuiting
Not susceptible to abrasion, less moving parts limits short circuiting
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