Types of Anaerobic Digesters
Anaerobic digesters are capable of treating insoluble wastes and soluble waste-waters. Insoluble wastes such as particulate and colloidal organics are considered to be high-strength wastes and require lengthy digestion periods for hydrolysis and solubilization. Digester retention times of at least 10–20 days are typical for high-strength wastes. High-rate anaerobic digesters are used for the treatment of soluble wastewaters. Because these wastewaters do not require hydrolysis and solubilization of wastes, much faster rates of treatment are obtained. High-rate anaerobic digesters usually have retention times of less than 8 hours.
High-strength wastes are usually treated in suspended growth systems, whereas soluble wastewaters are usually treated in fixed-film systems. Several anaerobic digester processes and configurations are available for the treatment of insoluble wastes and soluble wastewaters (Table 1). Each configuration impacts solids retention time (SRT) and hydraulic retention time (HRT). Minimal HRT is desired to reduce digester volume and capital costs. Maximal SRT is desired to achieve process stability and minimal sludge production
BACTERIAL GROWTH—SUSPENDED Dome Biodigester
In suspended growth systems, the bacteria are suspended in the digester through intermittent or continuous mixing action (Figure ). The mixing action distributes the bacteria or biomass throughout the digester.
Because completely mixed anaerobic digesters do not incorporate a means for retaining and concentrating the biomass, the SRT is the same as the HRT.
Advantages and Disadvantages of Suspended Growth Anaerobic Digesters
Advantages
Suitable for the treatment of particulate, colloidal, and soluble wastes Toxic wastes may be diluted Uniform distribution of nutrients, pH, substrate, and temperature
Disadvantages
Large digester volume required to provide necessary SRT Treatment efficiency may be reduced due to loss of particulate and colloidal wastes and bacteria in digester effluent
Completely mixed anaerobic digesters are designed for relatively long HRTs. Feed sludge can be added to the digester on a continuous or intermittent basis. Advantages and disadvantages of completely mixed suspended growth digesters are listed in Table
BACTERIAL GROWTH—FIXED Dome Digester
Anaerobic fixed-film (sludge blankets) systems provide a quiescent environment for the growth of an agglutinated mass of bacteria (Figure 2). Because bacterial growth requires relatively long periods of time to develop, the media used in fixed-film systems hold the bacteria in the digester for relatively long periods and provide for long SRTs and short HRTs. The bacteria grow as fixed films of dendritic or “stringlike” masses on the supportive media or as clumps of solids within the openings or voids of the supportive media. Fixed-film systems usually use gravel, plastic, and rock as the supportive media. The openings make up approximately 50% or more of the media. Fixed-film systems operate as flow-through processes, that is, wastewater passes over and through a bed of fixed film of bacteria growth and through entrapped clumps of bacterial growth (Figure 23.3). Soluble organic compounds in the waste-water are absorbed (diffuse into) by the bacteria, whereas insoluble organic compounds are adsorbed (attach) to the surface of the bacteria. The flow of wastewater through fixed-film systems may be from the bottom to the top (upflow) or from the
top to the bottom .Because the bacteria (solids) in fixed-film systems remain in the digester for long
SRTs, the systems allow methane-forming bacteria to acclimate to toxicants such as ammonia, sulfide, and formaldehyde. Therefore, anaerobic fixed-film systems with long SRTs and short HRTs may be used to treat industrial wastewater containing toxicants. Numerous fixed-film systems are available for use in the digestion of municipal and industrial wastewaters and sludges . These systems are capable of treating a variety of wastewaters and sludges, provide good contact
biogas plant
Anaerobic digesters are capable of treating insoluble wastes and soluble waste-waters. Insoluble wastes such as particulate and colloidal organics are considered to be high-strength wastes and require lengthy digestion periods for hydrolysis and solubilization. Digester retention times of at least 10–20 days are typical for high-strength wastes. High-rate anaerobic digesters are used for the treatment of soluble wastewaters. Because these wastewaters do not require hydrolysis and solubilization of wastes, much faster rates of treatment are obtained. High-rate anaerobic digesters usually have retention times of less than 8 hours.
High-strength wastes are usually treated in suspended growth systems, whereas soluble wastewaters are usually treated in fixed-film systems. Several anaerobic digester processes and configurations are available for the treatment of insoluble wastes and soluble wastewaters (Table 1). Each configuration impacts solids retention time (SRT) and hydraulic retention time (HRT). Minimal HRT is desired to reduce digester volume and capital costs. Maximal SRT is desired to achieve process stability and minimal sludge production
BACTERIAL GROWTH—SUSPENDED Dome Biodigester
In suspended growth systems, the bacteria are suspended in the digester through intermittent or continuous mixing action (Figure ). The mixing action distributes the bacteria or biomass throughout the digester.
Because completely mixed anaerobic digesters do not incorporate a means for retaining and concentrating the biomass, the SRT is the same as the HRT.
Advantages and Disadvantages of Suspended Growth Anaerobic Digesters
Advantages
Suitable for the treatment of particulate, colloidal, and soluble wastes Toxic wastes may be diluted Uniform distribution of nutrients, pH, substrate, and temperature
Disadvantages
Large digester volume required to provide necessary SRT Treatment efficiency may be reduced due to loss of particulate and colloidal wastes and bacteria in digester effluent
Completely mixed anaerobic digesters are designed for relatively long HRTs. Feed sludge can be added to the digester on a continuous or intermittent basis. Advantages and disadvantages of completely mixed suspended growth digesters are listed in Table
BACTERIAL GROWTH—FIXED Dome Digester
Anaerobic fixed-film (sludge blankets) systems provide a quiescent environment for the growth of an agglutinated mass of bacteria (Figure 2). Because bacterial growth requires relatively long periods of time to develop, the media used in fixed-film systems hold the bacteria in the digester for relatively long periods and provide for long SRTs and short HRTs. The bacteria grow as fixed films of dendritic or “stringlike” masses on the supportive media or as clumps of solids within the openings or voids of the supportive media. Fixed-film systems usually use gravel, plastic, and rock as the supportive media. The openings make up approximately 50% or more of the media. Fixed-film systems operate as flow-through processes, that is, wastewater passes over and through a bed of fixed film of bacteria growth and through entrapped clumps of bacterial growth (Figure 23.3). Soluble organic compounds in the waste-water are absorbed (diffuse into) by the bacteria, whereas insoluble organic compounds are adsorbed (attach) to the surface of the bacteria. The flow of wastewater through fixed-film systems may be from the bottom to the top (upflow) or from the
top to the bottom .Because the bacteria (solids) in fixed-film systems remain in the digester for long
SRTs, the systems allow methane-forming bacteria to acclimate to toxicants such as ammonia, sulfide, and formaldehyde. Therefore, anaerobic fixed-film systems with long SRTs and short HRTs may be used to treat industrial wastewater containing toxicants. Numerous fixed-film systems are available for use in the digestion of municipal and industrial wastewaters and sludges . These systems are capable of treating a variety of wastewaters and sludges, provide good contact
biogas plant
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| Anaerobic fixed- dome 2 |
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| suspended growth systems 1 |
