Optimisation of anaerobic digestion of organic solid waste for the production of quality compost for soil amendment
Organic Fractions of Municipal Solid waste (OFMSW) is rich in organic matter, which can be recycled through energy recovery and compost production. The move towards sustainable management of OFMSW is shifting from energy recovery alone to the integration of separation at source, feedstock selection and disposal/reuse of the residuals. The sustainable management of OFMSW presents a challenge due to policy shifts and increasing pressure on landfills. This research was proposed in this background to establish a suitable feedstock combination of source separated wastes for biogas and compost production. Little information is available on factors affecting the combined anaerobic/aerobic treatment of source separated OFMSW and the soil enhancing qualities of the resulting post treated digestate (anaerobic digestate compost). This study investigated suitability of laboratory simulated feedstock for biogas and compost production and the impacts of the compost produced on physical, chemical and biological properties of soil. The study was carried out in five stages. The first stage investigated the natural buffering capacity, biogas production and degradability of various proportions of food and green wastes in anaerobic cultures. The studies showed that a waste mixture of food waste: green waste: inoculum in the ratio 2 :2:1 as optimal for biogas production, volatile solids destruction and pH stability. The study also showed that the choice of waste mixture can be useful for pH control during anaerobic digestion of OFMSW. The second stage, investigated the factors affecting aerobic post-treatment of the anaerobic digestate obtained from the first stage. Results showed increasing solids concentration and decreasing C:N ratio with increasing aerobic treatment. Generally, the results (TS, TVS, C: N) suggest increasing digestate stability with increasing aerobic post treatment. The third stage investigated the quality and stability of the post- anaerobically treated digestate using seed germination and plant growth tests. Results showed that non-digested model feedstock and anaerobic digestates collected at various times during anaerobic digestion showed low seed germination index. On the other hand digestates collected during aerobic post-treatment showed higher seed germination indexes with increasing aeration times. No seed germination inhibition (phytotoxicity) was observed in soils amended with aerobically posttreated digestate. Generally the results indicate increased digestates stability with increasing duration and intensity of anaerobic-aerobic treatments. The fourth stage investigated the soil enhancing qualities of anaerobic digestate and aerobically post-treated digestate. Results obtained showed that anaerobic digestate is not suitable soil for amendment of arable soils. However, post aerobically treated digestate (i.e. anaerobic digestate compost) improved the physical, chemical and biological qualities of amended soils. Soil analyses showed that the anaerobic digestate compost amendment improved water retention, water infiltration, carbon mineralization and assimilation and significantly increased the soil biomass and bioactivity. Plant growth tests showed that the anaerobic digestate compost significantly increased plant heights, number o f leaves and dry matter contents. In the final stage microbial diversity was investigated in anaerobic digestate compost amended soils. The study revealed significant changes in the diversity of soil microbial populations. The results suggest greater variations in fungi indicating that anaerobic digestate compost amendments have greater impacts on soil fungal populations than bacterial communities. In general, this study demonstrated that depending on the raw waste composition and the nature and duration of biological treatment received, OFMSW could be associated with high biogas yield, soil fertility and productivity and can bring about changes in dynamics of soil populations and functions. Whilst the study indicates the suitability of anaerobic digestate compost for soil management applications, the holistic approach employed in this study if applied on a larger scale could lead to sustainability in management of OFMSW.
Networked Digital Library of Theses and Dissertations