Organic waste is an organic resource which can be converted into useful materials by microbes, and can be seen as a resource which can be recycled in a variety of ways. However, until now, organic waste has been disposed of through the inexpensive offshore dumping. However, with the implementation of a ban on offshore dumping set in place since 2014, demand for land-based treatment facilities for the stable and sustained treatment of organic waste has risen, and large budgets are being appropriated to this end. Sewage, wastewater sludge and food waste, which are representative organic wastes, have a high potential for conversion into animal feed or compost, but due to their odor and high moisture content, are difficult to handle. In addition, they have high concentrations of hazardous materials. This study focused on converting wastewater and sewage sludge treated with beneficial microorganism(BM) into compost, using food waste as a supplementary nitrogen source that the sludge lacks.
First, a laboratory-scale reactor innoculated with BM agents was used to produce sludge treated with BM. In testing to compute the optimal amount of BM, the efficiency was highest when the amount of BM inoculation was 1%(w/w). Observation of changes in concentration of proteins and carbohydrates in the sludge resulting from the introduction of BM showed increases of 52% for protein and approximately 32% for carbohydrate. Also, due to introduction of the BM agent into the wastewater, the removal efficiencies of suspended solid (SS), chemical oxygen demand (COD), total nitrogen (T-N) and total phosphorus (T-P) were 94.5%, 91%, 79% and 81%, respectively satisfying domestic standards for the effluent.
Secondly, the composting characteristics of BM sludge and the control sludge were compared. Feasibility of using coffee grounds as a bulking agent was examined, along with sawdust. It was observed that composting of BM sludge had a faster rate of reaction than with the control sludge, and higher temperatures were reached. When using coffee grounds as a bulking agent, the caffeine in the coffee seemed to absorb the odors, allowing a composting with almost no odors. Moreover, when coffee grounds used as the bulking agent, total organic matter content increased by approximately 17% over sawdust, while total nitrogen increased by 49%, and available phosphorus by approximately 3%.
Comparative microbial community analysis based on pyrosequencing analysis was performed for the BM agents used for composting and for the control sludge. Analysis of the diversity of microorganisms showed 559 out of 1580 reads OTU for the control sludge, and 594 OTU out of 2174 reads for BM sludge, indicating a higher diversity control sludge. Pyrosequencing of BM sludge showed that Proteobacteria (41.30%) was the most dominant, followed by Gemmatimonadetes (14.12%). microbial diversity analysis in the compost showed 249 OTU out of 3406 reads in the control sludge compost, and 183 OTU out of 2632 reads for BM sludge compost. Whereas the diversity of microorganisms in the control sludge were observed to be slightly higher than that of BM sludge, the diversity were decreasing as the composting proceeded, leading to a larger proportion of Bacteroidetes, Proteobacteria and Firmicutes occurred during composting for both sludges.
Fourthly, a study was conducted to investigate composting efficiency. In the test for germination rate, the control sludge failed in some cases to satisfy the standard of 70 for matured compost after 28 days, but when using composted BM sludge, all cases were shown to satisfy the requirement. The result for BCL compost was highest, at 101.5. Plant germination rate was shown to increase when using coffee grounds as the bulking agent. Use of BCL compost was observed to have a positive growth effect on leaf weight (g), leaf number (ea), leaf length (cm), root weight (g), root diameter (cm), and root length (cm), etc. Lastly, an economic feasibility analysis on BCL compost, which was shown to have the highest efficiency of composting, showed that approximately 22.2% cost savings relative to current food waste composting were possible, mainly because the caffeine in the coffee grounds absorbed odors, removing the need for odor reducing facilities, and enhanced the value of the compost by boosting the organic matter content and total nitrogen content of the completed compost.