Abstract
The performance of three pairs of laboratory-scale upflow anaerobic sludge blanket (UASB) reactors fed with a synthetic medium supplemented with 0.075, 0.25 and 0.5 mM zinc was compared to a pair of control reactors fed with trace amounts of zinc. Analysis of the results by two-way ANOVA showed that the gas production, methane yield and effluent volatile fatty acids (VFA) were not significantly different at the 5% level from the controls. COD removal from the intermediate concentration of 0.25 mM zinc was significantly better than the controls. Zinc also appeared to improve the granulating behaviour of the system, though it was not clear if superior bioreactor performance resulted merely from improved granulation. The importance of maintaining the pH to within normal opearating range was paramount, as low pH resulted in process inhibition and interfered with metal removal by the sludge. At low pH values the sludge could not retain zinc. Under normal operating pH, the level of zinc in the sludge as high as 30 mM did not appear to affect UASB reactor performance.