Sewage sludge disposal : the composting option : thesis presented in fulfillment of the requirements for the degree of Master of Technology in biotechnology at Massey University

Abstract

The objective of the present studies was to explore the possibility of employing composting as a mean of sewage sludge stabilization. A series of composting experiments were performed using dewatered secondary activated sewage sludge from a domestic wastewater treatment plant in New Plymouth, New Zealand. These trials have been carried out treating the sludge in both open and closed composting systems on a laboratory scale. Two open system methods, one aerated windrow and one static pile, and three closed experiments using a compostumbler were performed. Throughout the whole study woodchips (in varying ratios) were used as a bulking agent. An initial moisture content of nearly 60% in the sludge - woodchips mixture produced the highest degree of composting activity over a three week period. Biological drying during the process was indicated by an increase in total solids up to values between 17% and 27%. Partial stabilization of the organic fraction was indicated by a decrease in volatile solids of 28% - 50%. In two closed system trials a total carbon decrease of 26% - 42% was observed, serving as an additional indication that there had been a reduction in organic matter. Total nitrogen losses were substantial in all experiments. Reductions were in the range of 14% - 58% with the highest losses observed in the static pile experiment. Phosphorus was found to be stable with only minor concentration changes observed. Temperature development in the composting material followed the well known pattern, provided that the factors influencing the composting process were close to optimal. Temperatures approaching 70°C in the initial stage of the process were measured. Bacteriological studies indicated, that the final composted product was not free from microbial hazard. In one closed system trial, however, no entero-streptococci were observed, indicating a complete inactivation of these indicator microorganisms. Ongoing development of the composting systems used, including improvements of methodologies employed is necessary in conducting further investigations.