Figure 13.6 Low rate anaerobic process using a rectangular concrete structure possible. As a result, designs rely on experience with similar systems and wastewaters, and on the replication of HRTs and organic loading rates (expressed as kg BOD, or COD/Xm'-day)) found to be successful. HRTs in the 5 to 15 day range are often appropriate, with longer values required in some cases. Organic loading rates of 1 to 2 kg COD/(m'-day) are often found to be appropriate. Process performance varies, but approaches that achieved by high rate anaerobic processes, discussed below. Further discussion of anaerobic lagoons is presented in Chapter 14.
High-rate anaerobic processes utilize bioreactor configurations that provide significant retention of active biomass, resulting in large differences between the SRT and the HRT.l"~"'f' Three mechanisms are used to retain biomass: (1) the formation of settleable particles that are retained by sedimentation, (2) the use of reactor configurations that retain suspended solids, and (3) the growth of biofilms on surfaces within the bioreactor. In many instances, more than one mechanism is operating in a bioreactor. Consequently, high-rate anaerobic processes represent a spectrum of bioreactor types ranging from suspended growth to attached growth, with hybrid bioreactors, which contain significant quantities of both suspended and attached biomass. in between. Six bioreactor types that span this range are described in this section: (1) anaerobic contact (AC), (2) upflow anaerobic sludge blanket (UASB), (3) anaerobic filters (AF), (4) hybrid UASB and anaerobic filters (UASB/AF), (5) downflow stationary fixed film (DSFF), and (6) fluidized bed/expanded bed (FB/EB).
Hall"" has summarized the typical performance of high rate anaerobic processes, as presented in Table 13.2. A relatively high level of biodegradable organic matter removal can be achieved, as indicated by typical BOD^ removal efficiencies of 80 to 9i)%. Biogas production is about 0.5 m'/kg COD removed, corresponding to a
Table 13.2 Typical High-Rate Anacrobic Process Performance'1
BOD, removal, percent
1.5 X BOD, removed 0.5 m 7kg COD removed 0.35 mVkg COD removed 0.05-0.10 g VSS/g COD removed
COD removal, mass Biogas production Methane production Biomass production
Adapted from Hall.""
methane production of 0.35 nr/kg COD removed. Solids production is low, typically ranging from 0.05 to 0.10 kg VSS/kg COD removed. These performance levels can be achieved by all of the processes discussed in this section if appropriate organic loading rates are used.
Anaerobic Contact. Anaerobic contact systems, illustrated in Figure 13.7, consist of a completely mixed suspended growth bioreactor, a degassificr, and a liquid-solids separation device where the bioreactor effluent is separated into a relatively clear process effluent and a concentrated slurry of biosolids that is recycled to the bioreactor.Therefore, AC is essentially an anaerobic activated sludge system. The degassificr is a device that facilitates removal of carbon dioxide and methane to allow settling of the biosolids in the liquid—solids separator. If the gas is not removed, bubbles attach to the solids, preventing their settling and subsequent recycle to the bioreactor. A variety of devices can be used to degassify the bioreactor effluent,"' as illustrated in Figure 13.8. The bioreactor is often configured like an anaerobic digester, and the heating and gas handling systems provided are similar. Completely mixed conditions are achieved by mechanical mixing systems similar to those used in anaerobic digestion. Conventional clarifiers or plate settlers are often used as the liquid-solids separation device.
The AC process is designed and operated to maintain a desired SRT, which is accomplished by adjusting the solids wastage rate. As indicated in Section 9.3.2, SRTs in the 10 to 20 day range are required. Just as with the activated sludge process,
Figure 13.7 Anaerobic contact process.
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