Investigation on dairy wastewater treatment was undertaken at ambient temperature in 11 l effective volume of laboratory--scale upflow anaerobic sludge blanket reactor receiving an average influent chemical oxygen demand of 2100 mg 1(-1) for 3 months of 24 hours, hydraulic retention time. The feeds of the synthetic dairy wastewater operated with HRT of 12 hrs, 16 hrs, 20 hrs and 24 hrs was equivalent to organic loading rates of 1.20 kg COD m(-3) d-7.20 kg COD m(-3) d, 0.9 kg COD m(-3) d-5.40 kg COD m(-3) d, 0.72 kg COD m(-3) d--4.32 kg COD m(-3) d and 0.60 kg COD m(-3) d-3.60 kg COD m(-3) d respectively. After steady state condition was reached, which took about 2 months, the effluent quality parameter were sampled and analysed to quantify treatment efficiencies. The following removal efficiency observed were 73-94.33% COD; 50.04- 56.66% total solids; 45.55-70.63% total dissolved solids; 66-86.67% total nitrogen and 72-94% total phosphorous. Maximum biogas production rate was 383 l kg(-1) COD removed with 260 l of methane gas. Estimation of biogas production was analysed using artificial neural network software model, and the results predicted coincided well with the experimental results.