Post-mortem examination and analysis of anaerobic filters

This paper presents post-mortem examination and analysis of downflow (DAF) and upflow anaerobic filter (UAF) operated under similar sets of operating and environmental conditions but varying organic loads for more than 20 months. It can be observed from performance studies of these filters that the COD removal, methane gas production and COD methanisation do not differ significantly at 95% confidence level. However, the post-mortem analysis revealed that considerable filter volumes (42% in DAF and 49% in UAF) were occupied by retained biomass leading to significant reduction in operating HRT (hydraulic retention time) towards the end of the study period. Retained biomass (suspended as well as retained in packing media) in the filter was observed to have considerable black, brown-black and brown granular solid fractions. The brown granules were spongy in nature. The deposition of sludge solids in DAF packing media was observed to be uneven as compared to UAF. Solids were simply held-up or retained in the packing media. Solids retained in DAF packing media were mostly composed of black granules (3–5 mm in size) whereas solids retained in UAF packing media were brown-black granules (2–3 mm in size). The retained granular solids in DAF packing media appeared to be more compact as compared to that in UAF. The estimated concentration of retained solids in DAF packing media was 76.39 g TS/L that was significantly higher than that in UAF (69.91 g TS/L).