Toxicity Evaluation of Sewage Sludge Before and After Composting Process

Sewage sludge and the compost were evaluated through chemical characterization and toxicity testing to determine their suitability for land application. The application of sewage sludge/compost, slightly acidic with pH rang of 6.4-6.5, decreased the soil pH. The higher the sewage sludge/compost rates, the higher the mixed soil contents of electrical conductivity (EC), organic matter (OM), nitrogen (N) and phosphorus (P), with N and P in sludge significantly higher than those in compost. Metal speciation of sludge showed that Cu and Ni predominantly existed in organic and residual phases, while Zn did not show any dominant chemical phase. And the available forms of Zn and Ni were significantly decreased after sewage sludge composting. The sludge extracts exerted a stronger inhibition of germination index than compost extracts, indicating the higher adverse effects derived from sludge. In view of its lower EC, the higher NH4-N content and available forms of Zn, Ni in sewage sludge would partly be responsible. The highest biomass of tall fescue was inhibited at a sludge ratio >5% or a compost ratio >20%, suggesting that sewage sludge after composting process was more suitable for land application.