Two community on-site UASB-septic tanks were operated in parallel over a six months period under two different hydraulic retention times (HRT) of 2 days for R1 and 4 days for R2 at mean sewage temperature of 24 degrees C. The sewage was characterised by a high CODtot, concentration of 1189 mg/L, with a large fraction of CODsus, viz. 54%. The achieved removal efficiencies in R1 and R2 for CODtot, CODsus, BOD5 and TSS were "56%, 87%, 59% and 81%" and "58%, 90%, 60% and 82%" for both systems, respectively. R2 achieved a marginal but significant (p < 0.05) better removal efficiencies of those parameters as compared to R1. The CODcol and CODdis removals in R I and R2 were respectively 31% and 20%, and 34% and 22%. The sludge accumulation was very low suggesting that the desludging frequency will be of several years. Accordingly, the reactor can be adequately designed at 2 days HRT. (C) 2008 Elsevier Ltd. All rights reserved.