Influence of organic loading rate and hydraulic retention time on the efficiency of a UASB bioreactor treating a canning factory effluent
A mesophylic laboratory-scale upflow anaerobic sludge bed bioreactor design was evaluated for the treatment of a carbohydraterich effluent from the canning industry. The bioreactor was inoculated with 500 g of anaerobic granules and after the system had stabilised the hydraulic retention time (HRT) was set at 24 h and the substrate pH poised at 8.0 to prevent the effect of rapid acidification. In the trust experimental study the chemical oxygen demand (COD) was increased stepwise from 2 300 to a full strength of 4 000 mg·(l)-1. In the second study the organic loading rate was increased by shortening the HRT (24 to 8 h) to give an organic loading rate increase from 3.95 to 10.95 kgCOD·m-3·d-1 with an average COD removal of 90 to 93% and removal rate of 9.8 kgCOD·m-3·d-1. However, the recovery rate of the system at HTR values below 10 h was found to be very slow suggesting that the system had reached its minimum HRT. This was confirmed by the stabilisation of the granule bed. An HRT of 10 h was thus taken as the optimum operational HRT. Since neutralisation costs would influence economic aspects of the process, the influence of lower pH values was investigated in the third study where the pH of the canning effluent was lowered from 8.0 to 5.0. At the lower pH the COD removal dropped drastically, the biogas production decreased and the digester effluent pH dropped to 6.2. It was clear from the slow recovery of the digester and the low COD removal (66.1%) that the lower end of the operational pH had been reached and any further lowering of the substrate pH would lead to system failure. The economic implication of being able to operate at pH 5.5 means that fresh canning effluent can be introduced into the digester without any neutralisation, is considerable.