Composting of high moisture dairy manure using sawdust or mixed paper as amendments and wood chips as the bulking agent : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Environmental Engineering at Massey University
The agricultural industry in New Zealand is a major source of waste generation and about 84% of the country's point source pollution comes from dairy sector alone. Dairy farm effluent in New Zealand is most commonly treated via waste stabilization ponds. Two-pond systems which are frequently used, are not sufficient to make the dairy shed effluent suitable for discharging to surface water, thus there is a need for investigation and development of treatment/disposal technologies, especially where land treatment is not a practical option. Composting is a process whereby the heat that is liberated from the decomposition of organics drives the evaporation of water. By reducing the large amount of water in the slurry, its mass, bulk weight & volume through composting, there is a large potential to reduce associated transportation and handling costs of disposal as well as minimising the area of land required for manure application. Composting can further reduce the risk of pollution from runoff, odour, and nitrate contamination of ground water. A passively aerated composting system was used to treat high moisture (90%) dairy manure slurry. Both sawdust and mixed paper were investigated as amendments with wood chips as the bulking agent. Two identical piles (1,2m× 1.2m× 1,2m) for the sawdust investigation and another two for the mixed paper experiment were established. Passive aeration was achieved with three horizontal aeration pipes in the base of each pile. The piles were monitored for about 70 days for all the experiments. During the active phase of composting, piles reached above 60°C and thermophilic temperatures were sustained for more than three weeks. The importance of pile cooling because of excessive wind flow was demonstrated suggesting the advisability of a wind barrier to protect piles Moisture content in the piles decreased over the period of study. Initial moisture content varied from 67% to 71% but diminished to between 47% and 58% by the conclusions of the experiments. Results of these studies suggest that composting can remove water by virtue of the biologically produced heat. The results of this study also suggest that the amount of heat energy generated from composting depends on the amount of volatile solids degraded. Energy rich feed materials were shown to be converted to energy poor materials due to reduced volatile solids degradation and energy poor feed materials emerged as energy rich due to the greater amount of volatile solids degradation. In this study from 47.2% to 76% of produced heat was lost as latent heat through convective (evaporative drying). From the comparison of results using two different amendments, mixed paper was found better than sawdust as an amendment in terms of biodegradability, heat development, heat accumulation, evaporative drying, moisture removal, volume reduction and weight reduction. The results of this study also indicated that the required extent of total coliforms destruction was not achieved within the period of composting using the materials and method undertaken. A longer maturation or curing phase may be helpful in achieving the recommended level of total coliform inactivation.