Control of sludge temperature in autothermal sludge digestion

Claim 1

In a process for controlling the autothermal aerobic digestion of sludge, wherein the sludge is mixed with an oxygen-containing gas in the presence of aerobic micro-organisms in a covered digestion zone to thereby form an aerated mixed liquor which is retained in said digestion zone for sufficient time to partially stabilize the sludge, the improvement comprising: (a) maintaining total volatile solids content of the sludge above at least 20,000 mg/l; (b) maintaining total volatile solids content of the aerated mixed liquor above at least (100-X)% of the total volatile solids content of the slude, wherein X=60 times the ratio of the biodegradable volatile solids content of the sludge to the total volatile solids content of the sludge; (c) introducing said oxygen-containing gas into said covered digestion zone for said mixing therein at an oxygen concentration of at least 20% oxygen (by volume) with said mixing and the rate and quantity of the introduction of oxygen-containing gas being maintained at sufficient levels to dissolve at least 0.03 pounds of oxygen per pound of total volatile solids content in the sludge introduced into said covered digestion zone; (d) sensing temperature of the aerated mixed liquor; and (e) controlling the rate of introducing oxygen-containing gas into the aerated mixed liquor in response to the temperature sensing, by increasing the rate of introducing oxygen-containing gas into the aerated mixed liquor when temperature thereof decreases and by decreasing the rate of introducing oxygen-containing gas into the aerated mixed liquor when temperature thereof increases, to maintain temperature of the aerated mixed liquor within a predetermined range.