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Dry anaerobic digestion of biosolids from microsieve

Vardas Konstantinos

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Summary

The technology of microsieve constitutes a removal method of biosolids from raw wastewater, that utilizes a continuous belt screen filter in order to separate solids from the raw wastewater. An industrial type microsieve has been installed in the municipal wastewater treatment plant (WWTP) in Rethymno, where the primary microsieve sludge was collected. An auger screw fitted in the microsieve dewaters the collected solids, producing a dense cake with total solids (TS) and total volatile solids (TVS) concentration of 39.2±0.9% and 88.6±0.8% (as a percentage of the TS) respectively.The main purpose of this study was the examination and comparison, according to biogas production, of samples of microsieve sludge. In the produced sludge was not added water, so that to be in solid condition, during the anaerobic digestion. Each cycle of stabilization lasted 29 days. In comparison to primary sedimentation, the volume of sludge that is produced during the microsieve is significantly lower. Thus, is achieved the decrease of the total volume of reactor that is required for the stabilization. At the same time, due to solids content in microsieve sludge, the necessity of thickening and dewatering of the sludge were eliminated, before and after the accomplishment of anaerobic digestion, respectively, in comparison with primary sludge. Four different samples were prepared; each of them had different concentration of total solids and different ratio inoculum/ substrate. The inoculum which was used in anaerobic digestion, was collected from the anaerobic reactor of WWTP of Chania, which are being fed with mix of primary and secondary sludge.In the samples were examined, the ratio of inoculum/substrate varied from 1/1 for TS 11%, 1/2 for TS 14%, 1/4 for TS 19% and 1/12 for TS 27%. The results appeared that the higher biogas production was achieved from the samples with concentration of TS 11% (semi-dry), with the maximum daily production of biogas at 100.1±3.5 ml/gr VSdestroyed. Furthermore, samples with 19% TS produced only 1/4 of the total biogas production of the samples with 11% TS, with the maximum daily of biogas was calculated at 41.8±3.8 ml/gVSdestroyed. Samples with 14% TS produced a satisfying amount of biogas with 72.8±3.4ml/gVSdestroyed, almost approaching the peak of 11% TS samples. Finally, the samples with the minimum ratio of inoculum/substrate and highest concentration of total solids 27%, which had really small amount gas production. From the processing of experimental results, is observed that the highest biogas production, was noted from the samples with the highest ratio of inoculum/substrate and highest total solids concentration. However, The increment of biogas production, maintaining the same total solids concentration in the samples, could be achieved with the utilize of an inoculum with higher TS content.

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