Objective  Waste from pig farms and municipal sewer were mixed and composted to evaluate its utilization as a safe, organic substrate for agricultural applications.

  Method  Manure residue from large pig farms after solid-liquid separation was mixed with municipal sewage sludge for the composting. Two mixing ratios of manure residue:sewage sludge, i.e., a low C/N treatment of 6:10 (ZW1) and a high C/N treatment of 6:5 (ZW2), were applied for the experiment. The changes on temperature, C/N ratio as well as the nutrients (total nitrogen, total phosphorus, total potassium and organic matters) and heavy metals (Cu, Zn, Cd and Pb) contents of the mixtures during the fermentation process were monitored for evaluation.

  Result  The thermophilic phase of ZW2 lasted longer than that of ZW1. After composting, the organic carbon content of ZW2 (C/N=30) decreased by 28.6%, while that of ZW1 (C/N=25) merely 2.1%, indicating that the carbon source in pig manure was easily decomposed by microorganisms. As the fermentation progressed, the total nitrogen, phosphorus and potassium increased, and the heavy metals increased in the mixtures. After 58 d of composting, both resulting materials met the national standards for nutrients content and safety on heavy metals.

  Conclusion  The mass ratio of 6:5 (C/N=30) between the pig manure residue and the sewage sludge was considered appropriate for composting to produce an applicable fertilizer.