The most important factor in sizing the anaerobic digester is that the bacteria be given sufficient time to reproduce and metabolize volatile solids. The key parameters in providing sufficient time are the solids retention time (SRT), which is the average time the solids are held in the digester, and the hydraulic retention time (HRT), which is the average time the liquid sludge is held in the digester. They can be defined operationally as follows:
• SRT, in days, is equal to the mass of solids in the digester (kg) divided by the mass of solids withdrawn daily (kg/d).
• HRT, in days, is equal to the volume of sludge in the digester (m3) divided by the volume of digested sludge withdrawn daily (m3/d).
For digestion systems without recycle, HRT can be calculated based on either the sludge feeding rate or the removal rate. For such a system, SRT and HRT are equal. The three reactions in an anaerobic digestion system are directly related to SRT (or HRT). An increase in SRT increases the extent of reactions. Similarly, a decrease in SRT decreases the extent of reactions. Because a portion of the bacterial population is removed with each withdrawal of digested sludge, the rate of cell growth must at least match cell removal to maintain the system in steady state. Otherwise, the population of bacteria in the digester declines and the process eventually fails. Therefore, a minimum SRT is essential to ensure that bacteria are being produced at the same rate at which they are withdrawn daily.
Figure 5.2 illustrates the relationship between SRT and the performance of a lab-scale anaerobic digester fed with primary sludge. The figure shows how the production of methane and the reduction of degradable proteins, carbohydrates, lipids, COD, and volatile solids are related to SRT. As the SRT is reduced, the concentration of each component in the effluent gradually increases until the SRT reaches a value beyond which the concentration increases rapidly. This is the breakpoint SRT (or critical SRT) at which washout of bacteria begins; that is, the point where the rate at which bacteria leave the system exceeds their rate of reproduction. Calculations based on process kinetics predict a critical SRT of 4.2 days for the anaerobic digestion of wastewater sludge at 35°C (95°F).
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