Lessons on In-Vessel Sludge Composting

A survey, on behalf of the U.S. EPA, of the operating experience of sewage sludge composting within enclosed reactors is described. The practice, new to the U.S.A., had been adopted at 20 sites, of which 8 were surveyed. None had been in operation for more than 2 years; all divided their composting practice into 2 parts, the initial enclosed composting being followed by curing in the open. Lack of experience by both designers and operators had led to wide departures in attained performance from designed performance, especially in terms of the sludge processing rate. Insufficient attention had been paid to the moisture content of the sludge cake, and in nearly all cases the solids retention time within the reactor to yield an acceptable product in terms of pathogen die-off had been underestimated. Similarly, the degree of outdoor curing needed to give a product acceptability as a commercial item had received insufficient attention. What kind of product would prove saleable to the general public, nurserymen, and operators of landfill sites as cover should have received more consideration, as composters could then have been designed with specific markets in mind; as it was, one works had to pay to have its product taken away. Many operators were unaware of what parameters to monitor to check on their works' performance, or what corrections to make if readings were abnormal. Operator training was scant, and no certification system existed. Although odour control had been foreseen as essential for public acceptance, odour complaints were received at all but one works; many composting units were attached to existing sewage works, and shared with them any unsuitable siting. Odour control should take account of local topographic and climatic factors, and incorporate odour removal technologies capable of dealing with the variety of odorous compounds that might appear from time to time. The volume of admixture of sawdust required to make an acceptable product depended more than expected on sludge composition, and the materials handling and transporting logistics needed to bring the composting materials to, and the finished product from, the site were often inadequate. The control of aeration in the reactor had been better than that of temperature, as the thermal insulating properties of compost often masked actual temperatures. The overall picture was of the emergence of rule-of-thumb practices and self-help by works managers.