BACTERIAL LEACHING OF HEAVY METAL FROM ANAEROBICALLY DIGESTED SLUDGES

Spreading of sewage sludge on land is an attractive sludge management option because it combines disposal and beneficial reuse of the sludge at the same time. However, the heavy metal content of sludge has to be reduced to an acceptable level before it is suitable for use on land.;In the present study, a practical method was developed to remove heavy metals from anaerobically digested sludge. The method consists of four processing steps including--(1) solubilization of heavy metals from the sludge by bacterial leaching, (2) separation of the metal-laden leachate from the leached sludge solids by centrifugation dewatering, (3) removal of heavy metals from the leachate by lime precipitation and (4) neutralization of the leached sludge solids by the lime-treated leachate and/or ammonium hydroxide.;Bacterial leaching involves microbial oxidation of insoluble metal sulfides to soluble metal sulfates. Thiobacillus ferroxidans are the principal micro-organisms associated with this process. The optimum conditions for bacterial leaching of anaerobically digested sludge were found to be--pH 4, aeration rate of 50 cm('3) of air/L of sludge/minute, solids content of sludge < 4%, temperature of 20 to 25 C, leaching period of 8 to 10 days and a Thiobacillus ferroxidans inoculum of 5 x 10('9) cells/L of sludge.;The optimum conditions for the processes downstream from bacterial leaching were centrifugation of the leached sludge at 1100 G for 10 minutes and lime treatment of the leachates to pH 10. The dewatered sludge, with a pH of about 3 to 4, can be neutralized either by the lime treated leachate or ammonimum hydroxide. The required dosage was about 0.022g of OH('-)/g of dry sludge. The final decontaminated sludge slurry had a pH of about 7.5 and the following overall metal removal efficiencies were achieved: Cadmium, nickel and zinc, 80 to 85%; copper 70 to 75%. As a result of the above treatment the decontaminated sludge was made suitable for agricultural use because the heavy metals had been removed to acceptable levels while the soil conditioning and fertilizer potential of the sludge were conserved.;A mathematical model was developed in this study to describe the bacterial leaching process. A leaching system for a 2 x 10('5)m('3)/d conventional activated sludge treatment plant was designed and the total cost estimated.