SIMULTANEOUS SEWAGE-SLUDGE DIGESTION AND METAL LEACHING - EFFECT OF SLUDGE SOLIDS CONCENTRATION

The effect of sludge solids concentration on the simultaneous sewage s ludge (mixture of primary and secondary sludge) digestion and metal le aching (SSDML) process was studied in shake flasks (500 mi capacity) a nd in laboratory bioreactors of 20 liters working volume. The bioreact or studies demonstrated that an increase in sludge solids concentratio n increased the sulfuric acid production rates in the range of solids 8 to 30 g/l. The metal solubilization efficiency (Cu, Mn, Ni and Zn) d uring the SSDML process in the bioreactor was not affected by sludge s olids concentration to a large extent, except Cd, Cr and Pb, which wer e less soluble at increased solids concentration. The sludge solids co ntent, in the range of 16 to 30 g/l did not affect the sludge solids ( total; solids, volatile solids, total suspended solids and volatile su spended solids) reduction during the SSDML process operation at biorea ctor scale. The fertilizer value of the processed sludge remained inta ct. Tests in shake flasks and bioreactors demonstrated that the SSDML process was more efficient than aerobic sludge digestion for the destr uction of total coliforms in the range of solids concentration 10 to 5 0 g/l. Copyright (C) 1996 Elsevier Science Ltd