Abstract

The high process activity of a two-phase UASB (upflow anaerobic sludge blanket) system receiving ethanol wastewater was achieved in conjunction with sludge granulation. The optimal OLR (organic loading rate), as expressed in COD load of feed and overall effective liquid holding capacity, was found to be 32 kg m⁻³ d⁻¹ with a mean granule diameter of 1.12 mm and granule density of 1.23 kg m⁻³. The system yielded a very high process effectiveness, as indicated in a high specific CH<inf>4</inf> production rate (11.83 m³ CH<inf>4</inf>·m<inf>R</inf>⁻³ ·d⁻¹) and a high total COD removal of above 90%. Moreover, the system showed the total energy yield was very high as 15,840 kJ kg⁻¹ COD applied with the energy extraction efficiency of 82.5%. The sludge granulation seemed to begin and was clearly observed at high OLRs of 24 and 28 kg m⁻³ d⁻¹, respectively. Apart from the long time operation (about 2 y) with a slowly increased rate of OLR, causing the accumulation of metal sulfide particles to act as nuclei to facilitate for the formation of microbial granules, the low pH of 5.5 in the primary reactor was responsible for avoiding the micronutrient deficit situation which, in turn, became a main factor in the success of granulation in the investigated two-phase UASB. The presence of lactic acid (HLa) in the secondary reactor can be used to indicate the system overload and process instability. The inhibition levels of total volatile fatty acids (VFA) to acidogens and methanogens were about 9,000 and 400 mg L⁻¹ as acetic acid (HAc), respectively.