Chemical detoxification of sewage sludge

DETAILED DESCRIPTION As shown in FIG.
1, suitable apparatus for practicing the method of the instant invention includes an acid-resistant tank or reaction vessel outfitted with an air or O.
sub.
2 diffuser for aeration of the tank contents and a mechanical mixer.
A pH control unit is interfaced with and acid-base feeding system and an ORP control unit is interfaced with the tank aerators.
The reaction vessel is sized for a hydraulic retention time of 12 to 18 hours and is insulated to maintain the elevated temperature of anaerobically-digested sludge i.
e.
30.
degree.
C.
to 60.
degree.
C.
A quantity of an anaerobically digested sludge is transferred from an anaerobic digester to the reaction vessel, as further shown in FIG.
1.
The sludge is mixed and aerated at a rate sufficient to raise the sludge oxidation reduction potential (ORP) to above +300 mv for a period of 6 to 12 hours.
Immediately following this period of initial aeration, the sludge is acidified to a pH of 1.
0 to 3.
0 for a period of 6 to 12 hours by the addition of a concentrated acid such as sulfuric acid or hydrochloric acid while continuing to agitate the sludge and while maintaining aeration for ORP control above +300 mv.
The acidified sludge is then subjected to conventional dewatering methods to produce dewatered, detoxified acidic sludge and acidic, heavy-metal-containing water.
As a final step, the heavy metals may be recovered from the water by conventional methods, such as ion exchange, electrodeposition or selective precipitation.
If desired, the acidic water may be recycled back to the acid feed system for the reactor vessel, or a buffer may be added to the water to neutralize it for recycling back into the primary settling tanks of the municipal system.
Important features of the ORPA reaction vessel include temperature maintenance and pH and ORP control.
The temperatures of the anaerobically digested sludge are close to the temperatures of the anaerobic digesters which usually operate at 30.
degree.
-35.
degree.
C.
or at 50