Compostability of municipal and industrial wastewater sludge : physical, chemical and biochemical transformations and end-product characteristics

Abstract

Can composting save the world from its human generated biological waste problem and give a diversifying organic agriculture a much needed boost at the same time? The age old natural process of composting has been well researched and documented in scientific and other literature. Thermophilic composting transforms biological/organic waste into a stable plant and soil friendly material (compost) that has been sanitised through high temperature and humified at the same time – a true miracle of nature. Mixtures for thermophilic composting can be formulated fairly accurately from their water, C and N content. The pH needs to be within an acceptable range (5-8) but can be adjusted with agricultural lime if needed. The EC and ash content of sludge, together with the C and N content, are fairly reliable indicators of available bio-chemical energy for composting. Monitoring the temperature and water content gives sufficient feedback for proper management of the process. Bio-chemical transformations in composting material tell the tale of the microbial processes at the heart of composting and humification. The quality of compost depends largely on the quality of the raw materials used, even if mixture formulation and process management is effective. The study firstly evaluated the compostability of various types of WWS and eucalyptus sawdust bulking agent with the focus on the known requirements and conditions for thermophilic aerobic composting. This entailed a composting experiment during which the sludge and bulking agent were characterised, feedstock mixtures formulated and certain composting conditions (temperature and water content) and feedstock characteristics (pH, EC and ash content) monitored over time as piles were managed through turning, sampling and watering. Secondly, the study evaluated the quality of compost produced from various types of WWS and eucalyptus sawdust bulking agent as influenced by the raw material and initial feedstock characteristics, microbial dynamics and environmental conditions during composting and as indicated by certain chemical and biochemical transformations (including OM decomposition) during composting and end product (compost) characteristics. Tracking the chemical and biochemical evolution during composting in the four treatments entailed the laboratory analyses of a variety of signature characteristics for samples taken at the start of composting, at the end of the thermophilic phase when pile temperature decreased to below 40ºC and at the end of the curing phase of composting when the temperature of piles did not exceed ambient temperature. Finally, a seedling emergence bioassay was performed as a direct test of compost maturity and quality. The main findings of this study are that dewatered, activated sludge has a high level of thermophilic compostability with Eucalyptus sawdust as bulking agent. Dried, digested sludge has a low level of compostability with Eucalyptus sawdust as bulking agent mainly because of the anaerobic digestion that depleted the C and bio-chemical energy in the sludge. Little value is added to digested sludge through composting with eycalyptus sawdust except that it probably facilitated a further “curing” of the already stable material. Overall, dewatered ACT-MUN sludge and eucalyptus sawdust produced the compost of highest quality and stability/maturity in a volume ratio of 1:4 (sludge:sawdust). Dewatered ACT-IND sludge and eucalyptus sawdust produced an interesting compost of high stability/maturity with a high level of humification and medium fertility status. Increasing the sludge:sawdust ratio closer to 1:4 (volume) will enhance the chemical quality of this good compost. Dried DIG-MUN sludge and eucalyptus sawdust produced a stable compost of very low pH, relatively low fertility, poor physical character and high risk of phytotoxicity. Composting is a great option for WWS treatment that deserves a lot more attention in South Africa.