Effect of biosolids from municipal sewage sludge composted with rice husk on soil functionality

Abstract

Two different biosolids were obtained compost- ing anaerobic (A) and aerobic (B) municipal sewage sludge (SS) with rice husk. Higher amounts of SS (1:1 v/v) could be used in this composting process than in conventional ones. The two biosolids were characterized by chemical analysis and compared with a conventional green manure plus mu- nicipal solid waste and municipal SS compost. The effect of these products on soil functionality was studied in a 14- week incubation experiment by their addition to two different soils (silty clay—Ustic Endoaquert—and sandy loam—Aquic Xeropsamment). The total organic C ranged from 20 to 26 % and total N from 1.6 to 2.5 % in the two biosolids. The most relevant difference was due to dissolved organic C that was lower in the anaerobic biosolid (1 mgCkg −1 ) than in the other prod- ucts (5- 6m gCkg −1 ). The total trace elements (Cd, Cr, Cu, Ni, Pb and Zn) contents were under the limits fixed by the European legislation for soil application of SS (EC Directive 86/278/EEC, 1986). The three biosolids did not show strong negative effects on soil functional- ity during the incubation experiment, although some significant differences were found. The aerobic biosolid B mainly increased cumulative N release, microbial activity, basal respiration rate, microbial biomass-C-to- total organic C ratio, β-glucosidase, alkaline phospho- monoesterase and aryl-sulphatase activities. The anaero- bic one (B) decreased basal respiration rate, microbial biomass-C-to-total organic C ratio and aryl-sulphatase activity. DTPA soil bioavailable heavy metals were not affected by biosolids additions.