Effect of the start-up length on the biological nutrient removal process

In this work, a study of the start-up stage of a Biological Phosphorus Removal (BPR) process was carried out. To do this, SBR alternating anaerobic and aerobic conditions were used with conventional activated sludge and fed with synthetic influent at the beginning of each cycle. During the length of the cycles (4 h) the most important parameters were determined. The main results obtained were that during the first days of the start-up stage the phosphorus removal percentage increased more quickly that those of the COD and ammonium-nitrogen. However, it was necessary to extend the start-up stage to at least 30 days in order to obtain the maximum phosphorus removal percentages and a stable Phosphorus Accumulating Organisms (PAO) population, whereas the maximum COD and ammonium-nitrogen removal percentages were obtained in about ten days. Once the start-up period had finished the ratio P released to COD stored was determined, obtaining a value of about 0.14, which is significantly lower than the theoretical value (0.55), suggesting the presence of a significant GAO population.