ANAEROBIC TREATMENT OF PARTLY ACIDIFIED WASTE-WATER IN A 2-STAGE EXPANDED GRANULAR SLUDGE BED (EGSB) SYSTEM AT 8-DEGREES-C

Psychrophilic (8 degrees C) anaerobic treatment of partly acidified wa ste water was investigated using a two stage EGSB system with a total volume of 8.6 dm(3). The reactor system was operated at an up-flow vel ocity of 10 m.h(-1) and was fed with a sucrose-VFA mixture of 550-1100 mg COD dm(-3). The average CODsol and VFA-COD removal efficiencies we re 97 and 90 %, respectively, at total organic loading rates (OLR) ran ging between 5.1 - 6.7 g COD dm(-3).day(-1), sucrose loading rates up to 1 g COD dm(-3) day(-1) and a hydraulic retention time (HRT) of 4 h. An increase in the sucrose loading rates resulted in a significant wa sh-out of biomass from the first stage. The second stage satisfactory served as a scavenger of non-degraded VFA from the first stage. Specif ic activity assays showed an increase of 15 % in the specific methanog enic activity of the sludge present in the second stage and a decrease of 9 % in the first stage. Apparently, an enrichment of methanogens a nd acetogens in the anaerobic sludge in the second stage took place at temperatures as low as 8 degrees C. The acidogenic population became much more dominant in the first stage, resulting in a higher acidifyin g activity and a decreased methanogenic activity. 16S rRNA probe-techn iques (dot blot hybridization) showed that the acetate consuming Metha nosaeta (formerly Methanothrix) and the hydrogenotrophic Methanobrevib acter species (or relatives) were the most abundant methanogens presen t in the psychrophilic sludge. The ratio between bacterial and methano bacterial hybridization signal of the first stage was 3 times higher t han that of the second stage. By using NMR techniques, a higher effect ive diffusion coefficient was found for the smaller sized granules in both reactors, which is in congruent with the higher maximum specific acetate degrading activity of the smaller granules. (C) 1997 IAWQ. Pub lished by Elsevier Science Ltd.