Because the predominant application of anaerobic digesters is the degradation of particulate and colloidal wastes, sludge feed or organic loading rates to digesters usually are expressed in terms of volatile solids (VS). Designed and recommended loadings for anaerobic digesters that are mixed and heated are 200-450 lb VS/ 1000ft3/day (3.2-7.2kgVS/m3/day). However, loading rates of 30-50lb VS/ 1000ft3/day (0.5-0.6kgVS/m3/day) are typical. Higher loading rates could be treated if a more concentrated sludge could be fed to the digester.
The volatile solids loadings to anaerobic digesters are controlled in most wastewater treatment plants by the efficiency of the primary and secondary clarifiers in removing and concentrating sludge. Therefore, the thickening of sludge is an important operational factor affecting digester performance.
Typically, raw sludges or feed sludges having low solids content are transferred to municipal anaerobic digesters . These sludges often contain 3-6% solids. These dilute feed sludges adversely impact digester operation. They reduce hydraulic retention time (HRT), reduce volatile solids destruction, and reduce methane production.
The blending of primary and secondary sludges may be helpful in improving anaerobic digester performance (Figure 12.1). Primary sludge may be blended with thickened excess activated sludge, or blended primary and secondary sludges may be thickened.
The percentage of primary sludge in feed sludge may influence VS reduction in the anaerobic digester. Generally, with increasing percent primary sludge in digester feed sludge, an increase in VS reduction can be expected.
The HRT of anaerobic digesters is affected by not only the quantity of feed sludge but also the quantities of digested sludge, supernatant, and grit. Digested
sludge and supernatant must be withdrawn on a routine basis, and grit must be removed as needed to ensure adequate retention time. Common operational problems associated with anaerobic digesters are overpumping of raw sludge and excessive withdrawal of digested sludge.
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