Compared to anaerobic digestion systems, an aerobic digestion process is relatively simple to operate. Similar to an activated sludge system, the aerobic digestion process is essentially self-sustaining. Operational considerations include pH reduction, foaming problems, supernatant quality, and dewater-ability of digested sludge.
pH Reduction Decreases in pH and alkalinity have been observed in aerobic digesters at increasing detention times. The drop in pH is caused by acid formation that occurs during nitrification; the drop in alkalinity is caused by lowering of the buffering capacity of the sludge due to air stripping. It has been observed that the system will acclimate and perform well at a pH as low as 5.5. If the digester has separate aeration and mixing equipment (or to some extent is provided with mechanical aerators), it is possible to denitrify by operating only the mixing equipment (or turning off the mechanical aerators) during the fill cycle in a batch process. As discussed earlier, denitrification produces alkalinity. Filamentous growth may occur at low pH values. If the feed sludge has very low alkalinity and the pH in the digester continues to drop below 5.5, provisions may have to be made to increase the alkalinity by adding chemicals.
Foaming Problems Foaming may occur in aerobic digesters during warm-weather periods, due primarily to high organic loading rates. Foaming is more prevalent in digesters that use surface aeration devices. Water sprays are typically used to control the problems. Growth of filamentous organisms can also
TABLE 4.2 Characteristics of Supernatant from Aerobic Digesters
cause foaming problems. Control methods that have yielded mixed results include chlorinating the digester feed to destroy filamentous organisms and turning off the aeration equipment to create temporary anaerobic conditions.
Supernatant Quality One of the advantages of aerobic digestion over anaerobic digestion is the better-quality supernatant, especially when the solids-liquid separation is carried out in a gravity thickener or by other mechanical means. Table 4.2 gives characteristics of supernatant from aerobic digesters. As the supernatant is normally returned to the head end of the treatment plant, the true loading to the aeration basins from the supernatant is represented by the soluble BOD, which is typically less than the organic strength of the wastewater. The suspended solids do not exert a high load to the aeration basin because the solids are in the endogenous stage of respiration. Nevertheless, a well-operated aerobic digester can produce a supernatant with less than 150 mg/L of suspended solids.
Dewatering Belt filter press dewatering of aerobically digested sludge produces cake with 14 to 22% solids. Dewatering characteristics show a definite deterioration with increasing SRT. Dewaterability of aerobically digested sludge is also affected by the degree of mixing provided during the digestion process because the high degree of mixing destroys the structure of the solids floc. Unless pilot plant data indicate otherwise, it is recommended that conservative criteria be used for designing mechanical sludge dewatering facilities.
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