Stability Indexes of Sewage Sludge Compost Obtained with Different Proportions of a Bulking Agent

Abstract

The aim of using wastes in agriculture is to achieve a sustainable cycle in which these wastes will turn into organic sources in agricultural production. The specific objective was to study the composition of compost made with different sewage sludge/bulking agent ratios and to establish the most suitable stability or maturity analytical index for the compost. Three compost piles with different volume proportions (biosolids:sawdust) were performed. Measurements of pH, cationic-exchange capacity (CEC), total carbon (Ct), total nitrogen (Nt), ignition losses (IL), dissolved organic carbon (DOC), humic (HA), and fulvic acids (FA) were conducted. Microbial respiration was measured in the first and last thermophilic stage. Heavy metals [chromium (Cr), nickel (Ni), lead (Pb), zinc (Zn), copper (Cu), mercury (Hg), silver (Ag), arsenic (As), barium (Ba), selenium (Se), and cadmium (Cd)] were determined at the end of composting. The ratios used did not influence the compost properties since no differences were found either in the composition of final product or in the time required to achieve a mature compost. The pH, CEC/Ct, Ct/Nt, and IL% did not indicate degree of stability or maturity of compost. The HA and FA trials did not allow to establish the degree of the humification process. The Ct, Nt, and DOC clearly indicated the end stage in the compost piles maturity. These assessments were achieved after two months of processing, reaching stability with the following values: 3.5 and 5 g kg−1 for the Ct, 0.25 and 0.38 g kg−1 for Nt and 0.03 g kg−1 for DOC. The respiration activity decreased by 70% to 80% at the end of the thermophilic stage. The level of heavy metals, measured when the study was over, did not reach the highest values permitted by current regulations.