The effect of organic loading rate on the performance of UASB reactor treating slaughterhouse effluent

Abstract

Organic loading rate (OLR) is an important parameter significantly affecting microbial ecology and characteristics of UASB systems. In this study, UASB performance was evaluated in a 1000 1 reactor receiving feed from a traditional medium-size slaughterhouse. The initial seed for granules formed earlier was from a mesophilic municipal anaerobic digester sludge with a VSS content of 29 g l −1. The temperature of influent was adjusted by an inline thermostat around 33 °C. The reactor was started with an OLR of 5 kg SCOD m −3 d −1 with gradual increase to 10 kg SCOD m −3 d −1 over a 2-week period. Examination of VSS data showed that on the average 89.3±11.3% of bioparticle mass was present at the lower 30% of the reactor height. Under steady state conditions, experiments were conducted at OLRs of between 13 and 39 kg SCOD m −3 d −1 and hydraulic retention times (HRT) of 2–7 h. Removal efficiencies in the range of 75–90% were achieved at feed SCOD concentrations of 3000–4500 mg l −1. A reduction in removal efficiency to as low as 67% could have been related to a combined effect of high OLR and low HRT. Up to 300 l of methane were produced per kilogram of SCOD removed at OLR values of less than 30 kg COD m −3 d −1 but methane production rate seemed to decline to below 200 at higher OLR values. No sign of cell washout was observed at high OLRs and sludge loading rates (SLR) of up to 2.7 kg SCOD kg −1 VSS d −1. Elimination capacity of the reactor consistently increased from 9 to 25 kg SCOD m −3 d −1 corresponding to 1–2 kg SCOD kg −1 VSS d −1. Solids retention time (SRT) calculations for the reactor indicated a range of 3.3 days at high upflow velocity of 1 m h −1 to 60.3 days at low upflow velocity of 0.33 m h −1 s during different phases of the study.