In this example, high-rate, primary digesters (heated and completely mixed) followed by secondary digesters (un-heated and unmixed) provide anaerobic digestion of primary and excess activated sludge from a 10 mgd waste-water treatment plant. Secondary digesters provide a maximum 30-day detention time and are intended mainly for solids-liquid separation, sludge thickening, and sludge storage. The daily loading of solids to the primary digesters is as follows:
1. Primary (raw) sludge: 12,010 lb per day, (65% VSS)
2. Excess activated sludge: 8162 lb per day, (77% VSS)
Total solids = 20,172 lb per day, (70% VSS) Volatile solids = 14,091 lb per day
The daily loading in terms of sludge volume is as follows:
1. Primary (raw sludge):
2. Excess activated sludge:
Total sludge volume = 35,495 gpd (6.8% solids)
VSS reduction in the primary digesters with a 15-day digestion period can be calculated in accordance with Equation 7.45(6) as follows:
The quantity of solids remaining in the primary digesters at the end of the 15-day digestion period can be determined with Equation 7.45(7) as follows:
Equations 7.45(8) and (9) determine the required volume V1 of the primary digesters without the specific gravity correction as follows:
Mean solids concentration =
The digester loading is therefore as follows: 14,091 lb. VSS per day
= 220 lb VSS per 1000 ft3 per day
The amount of solids withdrawn to the secondary digesters on a daily basis is as follows: 243,274 lb.
If the average sludge solids concentration in the secondary digesters (achieved through supernatant decantation) is 8%, Equation 7.45(10) calculates the required digester volume V2 as follows:
This calculation assumes no digestion in the secondary digesters.
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