Akiko OHYA
Evaluation of the treatment characteristics and the retained sludge properties in an UASB reactor treating municipal sewage
Takashi Yamaguchi
UASB (Upflow Anaerobic Sludge Blanket) is by far the most widely used high rate anaerobic system for anaerobic sewage treatment. Anaerobic sewage treatment is limited until now to regions with constant and relatively warm temperature conditions. In moderate condition, it becomes the problem the water temperature falls in the winter. Under low temperature, hydrolysis significantly low and an accumulation of influent suspended solids in the reactor can occur, leading to a decreasing SRT and deterioration of sludge quality. Hence, the knowledge of sludge property variation (behavior of 1uantity, accumulation of solid, etc.) is important. However, there is little operational report of the full-scale plant at the moderate condition, and the knowledge about the sludge property is not enough.
This study was monitored under moderate climate conditions in order to evaluate the process performance and sludge property. The UASB inoculated with digester sludge was operated at an HRT of 8 hr under the temperature ranging from 11 to 28¡ëC over 1100 days.
Total COD removal efficiency in winter was 58%. The sufficient removal efficiency was maintained in the low temperature conditions (below 15 degree C during 3 months). The cellulose in the influent was virtually removed by means of the entrapment in the sludge bed. It is assumed that the accumulated solids in the winter are subsequently digested in the following the summer.
The sludge increased at the long-term operation and was 26.4 gVSS/L at the end of the experiment. The sludge growth rate was 2.1% in the experimental period. It was very lower values because UASB had high concentration sludge and maintained low organic loading rate. The calculated SRT was approximately 250 days, sufficient for complete mineralization of the sludge. Formation of granule was observed, which increased the settleability of the sludge bringing the SVI value below 50 ml/gSS.
The estimated genus of bacteria in retained sludge was increased at the long-term operation and it was evenness. Bacteria community structure was rich in diversity. Many of constitutive clones belonged to sulfur reducing bacteria (Proteobacteria) and acidogenic bacteria (Furmicutes and Bacteroidete). The archaea structure varied from unevenness to evenness. Because sewage was a substrate to which temperature and composition changed, it influenced microorganism community structure.
This study suggested the sludge management from long-term operation result that sludge quantity and quality determined.
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