RESUMO
Sludge samples from five municipal sewage treatment plants in Liguria (Italy) were examined periodically during a whole year. Four of them are located in Genova and receive the sewage corresponding respectively to 20,000-53,000-125,000 and 285,000 equivalent inhabitants. The last one is in Chiavari and receives the sewage corresponding to 40,000 equivalent inhabitants. All plants included a preliminary screening treatment, primary sedimentation and a secondary activated sludge process. Sludge was thickened by centrifuge. The bacteriological and chemical characteristics of the sludge after centrifuging (detection and estimation of total and fecal coliforms, fecal streptococci and enumeration and isolation of Salmonella, pH, moisture, total and volatile residue, nitrogen, phosphorus, heavy metals) were measured in order to establish the possibility of land disposal. Dry solids contained on average 34 g/kg total nitrogen and 1,4 g/kg phosphorus. Among heavy metals copper and zinc reached high levels: 544 and 1587 mg/kg of dry solids. Other metals were present at lower concentrations (As 6, Cd 2, Cr 70, Hg 2, Ni 53, pb 416 mg/kg). Comparing these results with those of a research carried out contemporarily in another region (Emilia Romagna) it was observed that in Ligury there is a higher content of copper and bacteria (with an average of 69 Salmonella in 1 g of dry solids). The results showed that sludge produced in the five treatment plants in Ligury (Italy) was apt to be used as fertilizer in soil, but that care has to be taken for what concerns microbial contamination. For what regards the differences among the five sewage treatment plants under observation some heavy metals (arsenic and chromium) reached very high average levels (10-14 mg/kg of arsenic and 70-118 mg/kg of chromium) in the sludge of two of them, which treat the sewage from highly industrialized areas. On the contrary lead and cadmium were present at the same levels in all plants (416 mg/kg of lead and 2 mg/kg of cadmium) with the exception of one (Ge-Quinto) where cadmium was practically absent (0,2 mg/kg). The overall average concentrations of heavy metals resulted definitively lower than maximum concentration proposed by CEE (1986) and by some italian region as Lombardia. Some differences among plants for what regards microbiological parameters were noted only for total coliforms.
