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Wet anaerobic digestion
An anaerobic digestion is referred to as wet anaerobic digestion when the suspended solid content in the digester is less than 12 percent of dry matter.
As the dry matter content of the substrates in the input to the digester can be adjusted upfront with process water usually recirculated from the posterior solid-liquid separation, wet anaerobic digestion plants present a high flexibility for different types of organic residues with a wide range of dry matter content. Thus, wet anaerobic digestion plants cannot only treat those organic residues which due to the their low structure and dry matter content cause difficulties to composting or dry fermentation facilities, but also can process organic residues with a high dry matter content like source segregated organics or even the organic fraction of municipal solid waste.
Due to this, wet anaerobic digestion has a wide field of applications beyond the well-known agricultural biogas plants for manure:
biomethanization plants for source-segregated organics
Different types of wet pre-treatment systems are on the market, e.g. hammer mills, presses, pulper systems, grit removal systems to provide a suspension for wet anaerobic digestion.
Particular attention should be paid to the selection of the pre-treatment in relation to its flexibility, efficiency and selectivity. Furthermore, special attention should be given to the removal of the grit fraction to avoid difficulties with sedimentation within the tanks.
Usually, within the wet pre-treatment systems the suspended solids content is adjusted with recirculated process water down to values below 12% before feeding to the anaerobic digestion step.
The anaerobic digestion step can be executed as single-stage or multi-stage digestion (for more information please refer to the page "Anaerobic Digestion Systems”), in mesophilic or thermophilic range. The retention time usually lays within a range between 14 – 30 days in function of technology and substrate.
In the single stage fermentation, the four anaerobic digestion steps take place in one reactor, i.e. they are not separated in time or in space. These types of plants have the advantages of being simple and easy to operate, and they require low investment costs. On the other hand, the biogas output is lower in comparison to multi stage fermentation. The retention time of this type of fermentation oscillates generally between 14 and 28 days depending on the feed and operating temperature (Verma, 2002).
The process scheme of a single stage plant is shown in Figure 2. The initial step is feeding the system with wet biowaste. A screw mill opens the bags and reduces the size of bigger waste components. Then the waste is sent to the pulper and mixed with process water, where the light fraction (plastics) and the heavy fraction (metals, stones and batteries) are removed. After this the hydrodynamic grit removal systemseparates the solids (glass fragments, grit, egg shells, gravel) from the liquids, producing a clean, homogenous pulp ready for digestion. The pulp is heated and enters the reactor where hydrolysis, acidogenesis, acetogenesis and methanogenesis take place. The digestor contents are continuously mixed using compressed biogas (Figure 3). The biogas is burned in a CHP to produce thermal and electrical energy, while the rest of the substrate is mechanically dewatered and sent to post-composting (CCI BioEnergy, 2009).
Figure 2: Process scheme of the BTA® Single-stage wet fermentation. Source: BTA International GmbH
Figure 3: Scheme of the BTA® Single-stage digestion reactor. Source: BTA International GmbH
In multi stage fermentation, two or more reactors are utilized to make the anaerobic digestion. The idea of utilizing several reactors is to separate the hydrolysis and methanogenesis phases in space and time, with the intention of decreasing the overall retention time and makingthe operation safer. The retention time in multi stage fermentation is approximately seven days, three days for the methanogenesis and between two and four daysfor the hydrolysis phase.
The process scheme of the BTA® multi stage wet fermentation is shown in Figure 4. The processes from waste-to-pulp and biogas-to-energy are similar to the single stage already mentioned before. It is after obtaining the clean homogenous pulp fraction that the two systems differ from each other. In the multi stage the pulp is hygienized and centrifuged, obtaining from it two fractions, one is sent to hydrolysis and the other to methane reactor. A fraction with "a high amount of already dissolved organic material is pumped directly into the methane reactor. The dewatered solids are mixed with process water and fed into the hydrolysis reactor to dissolve the remaining organic solids. After 2-4 days, the suspension is dewatered and the resulting liquid also fed into the methane reactor” (BTA, 2007), while the solids are sent to post-composting. The waste water resulting from the methane reactor is then treated by flocculation and denitrification.
The digesters can be mixed either with lateral or top entry agitators or by injection of compressed biogas with gas lances. The wet medium facilitates a full mixing of the digesters content in order to achieve almost homogeneous conditions over the whole volume. This prevents mass transfer limitation in the digester. Therefore, the good accessibility for the microorganisms allows for a high biogas yield slightly higher than for continuous dry digestion systems and noticeably higher than for discontinuous dry digestion systems.
The further treatment of the produced biogas is a function of the valorization path (conversion to electrical and thermal energy in CHP units, upgrading to biomethane to feed it into the natural gas grid or use as fuel e.g. in boilers). Usually it will consider a desulfurization and a cooling/drying step.
The obtained digestate usually presents a dry matter content below 5%. In case the anaerobic digestion plants are located in agricultural areas (co-digestion with agricultural substrates), the digestate may be used as liquid fertilizer to be applied on the fields –previously undergoing a corresponding sanitation.
In those cases where the anaerobic digestion plants are located in urban areas, usually the digestate undergoes a solid-liquid separation step to obtain a solid phase with a dry matter content of approx. 25 – 30%) and a liquid phase with a dry matter content between 1,5 – 2,5% in function of the dewatering equipment (e.g. centrifuges or screw presses).
The solid phase is characterized by a high homogeneity and is a good raw material, after mixing it with structure material, for a following composting step in order to produce high quality compost.
Sanitation can be achieved either by pasteurizing the organic suspension (e.g. 70°C for 1 hour) or for the solid phase by the high temperatures in the composting step (e.g. more than 55°C for 14 days).
The liquor obtained in the solid-liquid separation usually is widely recirculated for the dry matter content adjustment in the wet mechanical pre-treatment. Some technological concepts furthermore consider the recirculation of further pre-treated process water streams for rinsing purposes, thus minimizing the need of external water and the amounts of surplus water.
The successful operations of wet anaerobic digestion plants for various types of organic residues in different countries worldwide underline their high flexibility for different applications, contributing with a energetic and material valorization of the organic residues to a sustainable waste management and to climate protection.
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