Anaerobic degradation of the organic fraction of municipal solid waste in co-digestion with manure. Part 2
The aim of the project was to optimize the biogas process of high ratios of the organic fraction of municipal solid waste (OFMSW) in co-digestion with manure. Optimization was aimed with regard to process stability, degradation efficiency and removal of xenobiotic compounds, specifically the plasticizer DEHP.
The aim of the project is to optimize the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in co-digestion with manure in terms of a high biogas yield and degradation of xenobiotics. The project is the second part of the EFP project 1383/99-0009, which proofed high concentrations of xenobiotics when treating OFMSW in biogas plants. In the present project the digestion process will be investigated regarding the influence of different parameters on the degradation, inhibiting effects and the removal of xenobiotics from OFMSW. The aim is to reduce the content of xenobiotics in such a degree that it will still be possible to use the effluent from biogas plants treating OFMSW as organic fertilizer. It will be focused on the biodegradation of xenobiotics under anaerobic conditions. The optimal ratio of OFMSW and manure, the biodegradation of xenobiotics, the influence by different parameters (e.g. temperature) and inhibiting effects (e.g. from ammonia) will be investigated by batch experiments. Different environment will be screened for bacteria able to degrade xenobiotics and these cultures will be isolated and enriched. The results will be transferred to reactor experiments. The process will then be optimized in terms of sufficient degradation of xenobiotics, stable process performance with a high biogas yield and easy implementation in existing biogas plants
In co-digestion of OFMSW with manure in a ratio of 50% on a VS basis stable process performance was achieved in a conventional thermophilic process (55 deg. C) with a biogas yield of 540-590 ml/g-VS. However, no DEHP degradation was detected. This would lead to high effluent DEHP concentrations per kg dry matter at high co-digestion ratios of OFMSW. Therefore, also the biogas process of solely OFMSW with recirculation of process water was investigated. The present project showed that anaerobic treatment of OFMSW could be significantly improved by combination of a thermophilic (55 deg. C, 15 d retention time) and a hyper-thermophilic (68 deg. C, 1-5 d retention time) treatment. An optimal retention time of 1-2 days in the hyper-thermophilic reactor was identified. In this process configuration a VS reduction and biogas yield from OFMSW of 78-89% and 640-790 ml/g-VS, respectively, was achieved. Between 34-53% of the incoming DEHP was removed and the removal rate was more than 10 times higher than in previous aerobic experiments. Due to the simultaneous VS reduction the DEHP concentration per kg dry matter in the effluent was about the same range as in the influent. Therefore, the influent DEHP concentration should be below the threshold limit, which would still make source sorting of OFMSW with low plastic contamination necessary. The hyper-thermophilic posttreatment has, furthermore, the advantage of improved sanitation and can be used for partial ammonia stripping. Implementation of the hyper-thermophilic post-treatment would only mean a minor modification of the biogas plant since many plants already have a sanitation tank with a process temperature of 70 deg. C. This thermal treatment can, therefore, be replaced fairly easily by the biologically active post-treatment. Addition of higher amounts of OFMSW to the biogas process can, on the other hand, lead to a higher economic efficiency of the centralized biogas plants since biogas yields of 181 - 217 m3/t can be achieved from OFMSW compared to typically less than 30 m3/t from manure
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