Influence of retention time and influent pH on the performance of an upflow anaerobic sludge bioreactor treating cannery waste waters

A mesophilic 50-L upflow anaerobic sludge bed bioreactor (UASB) design was evaluated for the treatment of a sugar rich effluent from the canning indus try. The UASB was inoculated with 1670 g anaerobic granules to give a granu le bed height of 141 mm. After the system had stabilized the hydraulic rete ntion time (HRT) was set at 24 h, the bioreactor substrate pH poised at 7.5 to prevent rapid acidification and the chemical oxygen demand (COD) increa sed to a full strength of 4000 mg.L-1. In the first experimental study (I) the organic loading rate (OLR) was increased by shortening the HRT from 24 to 8 h to give an OLR increase from 4.0 to 9.2 kgCOD.m(-3).d(-1) with an av erage COD removal of 81-84% and removal rate of 7.5 kgCOD.m(-3).d(-1). A HR T of 10 h was taken as the optimum operational HRT, as the recovery rate of the system at HRTs lower than 10 h were found to be longer than at longer HRTs. However, the data clearly showed that the bioreactor had not reached its optimal loading capacity, which was confirmed by the constant increase in the granule bed height. Since neutralization costs would influence the e conomic aspects of the process, the influence of lower pH values was invest igated and the pH of the canning effluent was reduced from 7.5 to 5.0. At t he lower pH, the COD removal remained fairly constant between 79 and 82%, w ith an alkalinity level of 700 mg.L-1 and a bioreactor effluent pH value of 6.5. It was clear from the low effluent pH that the lower end of the opera tional pH had been reached and further lowering of the substrate pH would l ead to system failure. An increase of 144% in the granule bed height was fo und over the whole study period and the diameter of the granules obtained v aried from below 1.0 mm to about 4.0 mm. The data from this study showed th at the UASB system could successfully be used to treat canning effluents at short HRTs and without any further neutralization.