Operational optimisation of pilot scale biological nutrient removal at the Ciudad Real (Spain) domestic wastewater treatment plant

The aim of this work is optimising operating conditions for a possible implementation of a Biological Nutrient Removal (BNR) process in the Wastewater Treatment Plant (WWTP) of Ciudad Real (Spain). Several factors (hydraulic retention times, anaerobic nitrate concentration, sludge age and wastewater biodegradability) were tested using a pilot scale VIP (Virginia Initiative Plant) activated sludge process and domestic wastewater from the full scale plant. Hydraulic retention times used did not cause changes in N and P removal. P removal was adversely affected by anaerobic NO3- and improved with higher BOD5/COD ratios in wastewater. Influence of sludge age was very low in P removal, but N removal was mainly affected by this factor. Final operating conditions were selected taking into account their effects over one of both nutrients. COD and SS removal were always successful. N removal was also easily reached and the main difficulty was P removal. P sludge content was very low (2.5-4%) approximately and was also affected by the same factors tested. The main factor to improve P removal was supposed to be the organic wastewater composition. Wastewater characteristics were modified by using different sources from the WWTP. Volatile fatty acids (VFA) addition to the wastewater by using supernatant of the anaerobic sludge digesters seemed to be the best practical solution for a future BNR implementation in the WWTP.