Composting sewage sludge with green waste from tree pruning

Sewage sludge (SS) has been widely used as organic fertilizer. However, its continuous use can cause imbalances in soil fertility as well as soil-water-plant system contamination. The study aimed to evaluate possible improvements in the chemical and microbiological characteristics of domestic SS, with low heavy metal contents and pathogens, through the composting process. Two composting piles were set up, based on an initial C/N ratio of 30:1, with successive layers of tree pruning waste and SS. The aeration of piles was performed by mechanical turnover when the temperature rose above 65 degrees C. The piles were irrigated when the water content was less than 50 %. Composting was conducted for 120 days. Temperature, moisture content, pH, electrical conductivity (EC), carbon and nitrogen contents, and fecal coliforms were monitored during the composting. A reduction of 58 % in the EC of the compost (SSC) compared with SS was observed and the pH reduced from 7.8 to 6.6. There was an increase in the value of cation exchange capacity/carbon content (CEC/C) and carbon content. Total nitrogen remained constant and N-NO3- + N-NH4+ were immobilised in organic forms. The C/N ratio decreased from 25:1 to 12:1. Temperatures above 55 degrees C were observed for 20 days. After 60 days of composting, fecal coliforms were reduced from 10(7) Most Probable Number per gram of total solids (MPN g(-1)) to 10(4) MPN g(-1). I one pile the 10(3) MPN g(-1) reached after 90 days in one pile; in another, there was recontamination from 10(5) to 10(6) MPN g(-1). In SSC, helminth eggs were eliminated, making application sustainable for agriculture purposes.