Environmental impacts of nutrient removal processes: Case study

The influence of primary clarification of sewage on nitrogen and phosphorus removal in a modified University of Cape Town (UCT) type wastewater-treatment plant (WWTP) has been studied on a full scale. Before or after the introduction of raw sewage, the sludge was characterized on (denitrifying) phosphorus-removing activities by batch tests. For both situations, mass and energy balances in the WWTP were made in order to evaluate the effect of presettling on the environmental impact of the treatment process. Extra chemical oxygen demand (COD) input due to the introduction of raw sewage improved the overall nitrogen removal, whereas P-removal remained good. However, the improvement of nitrogen removal resulted mainly from increased assimilation into extra sludge production, not from increased denitrification. In fact, results of the batch tests indicated that the denitrification activity after the introduction of raw sewage was almost similar to the sludge with presettled sewage. The introduction of raw sewage resulted in an increase of energy consumption, mainly due to lower methane production in the digester. This research shows that evaluating treatment processes solely on effluent quality can easily lead to overall adverse effects for the environment. The full environmental impact should also be considered for treatment processes.