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Enhancing the performance of anaerobic digestion of dairy manure through phase-separation

Yılmaz, Vedat
Anaerobic digestion (AD) is an effective way to convert animal manures into profitable byproducts as well as to reduce the pollution of water, air, and soil caused by these wastes. Conventional high-rate anaerobic reactors cannot effectively process high-solids containing animal manures. The two-phase configuration for AD has several advantages over conventional one-phase processes such as increased stability of the process, smaller and cost efficient process configurations, etc. This study investigated the two-phase AD of dairy manure with particular emphasis on the effects of solids retention time (SRT), organic loading rate (OLR) and pH on anaerobic acidification of unscreened dairy manure; the effects of temperature on biogas production and the comparison of one-phase and two-phase system performance of AD. The results revealed that pre-acidification of dairy manure in daily-fed continuously-mixed reactors with no recycle led to substantial volatile fatty acids production. The optimum operational conditions for anaerobic acidification were determined as SRT and OLR of 2 days and 15 g VS/L.day. The pH control at a range of 5.0-5.5 was not found to be necessary for optimum acidification. Molecular analysis indicated that acidogenic bacteria population increased whilst the aerobic bacteria population decreased as time passed in acidogenic phase. The effect of temperature was clearly observed on biogas production efficiency. Two-phase configuration was determined more efficient than one-phase system. The biogas production in two-phase system was calculated to be 41% higher than that of the one-phase for the same OLR of 3.5 g VS/L.day. This translates into significant performance improvement and reduced volume requirement. This finding represents a further step in the achievement of wider use of simple anaerobic reactor configurations in rural areas.