Removal of slowly biodegradable COD in combined thermophilic UASB and MBBRsystems

Starch, cellulose and polyvinyl alcohol (PVA) are common substrates of the slowly biodegradable COD (SBCOD) in industrial wastewaters. Removal of the individual and mixed SBCOD substrates was investigated in a combined system of thermophilic upflow anaerobic sludge blanket (TUASB) reactor (55 degree sC) and aerobic moving bed biofilm. reactor (MBBR). The removal mechanisms of the three SBCOD substrates were quite different. Starch-COD was almost e qually utilized and removed in the two reactors. Cellulose-COD was complete ly (97-98%) removed from water in the TUASB reactor by microbial entrapment and sedimentation of the cellulose fibers. PVA alone was hardly biodegrade d and removed by the combined reactors. However, PVA-COD could be removed t o some extent in a binary solution of starch (77%) plus PVA (23%). The PVA macromolecules in the binary solution actually affected the microbial activ ity in the TUASB reactor resulting accumulation of volatile fatty acids, wh ich shifted the overall COD removal from the TUASB to the MBBR reactor wher e SBCOD including PVA-COD was removed. Since the three SBCOD substrates wer e removed by different mechanisms, the combined reactors showed a better an d more stable performance than individual reactors.