Sewage sludge composting: quality assessment for agricultural application

In order to use sewage sludge (SS) composts in agriculture, it is extremely important to estimate the quality of compost products. The aim of this study was to investigate the quality of composted SS as a fertilizer and soil amendment especially in semi-arid areas. To determine the quality and agronomic value of the SS compost products, analyses on pH, electrical conductivity, organic matter content, C/N ratio, phytotoxicity, microbial load, and heavy metal content of composted anaerobically digested SS, with different proportions (1:1, 1:2, and 1:3 v/v) of green and dry plant waste, as bulking agents, were performed. The 1:2 and 1:3 mixtures of SS and green/dry plant waste were the most beneficial for composting, with final composts attaining high organic matter degradation and exhibiting low amounts of heavy metals, a relatively high germination index, and significant reduction of pathogens, suggesting the agricultural relevance of composted SS and green/dry plant waste at 1:2 and 1:3 (v/v) proportions. pH and electrical conductivity were also within the permissible limits. With respect to international standards, it appears that composted SS and green/dry plant waste at 1:2 and 1:3 proportions pose no threat to soil or plant quality if used in agriculture or land restoration.