Evaluation of the impact of the effluent from the sewage treatment plant STP-Pirajá over the Parnaíba river, Teresina (Piauí, Brazil)

ABSTRACT The use of water resources for the discharge of effluents for the purpose of dilution, transportation or final disposal requires proper knowledge of the processes to ensure the receiving waterbody quality. This study aimed to assess the impact of the sewage treatment plant Pirajá (STP-Pirajá) effluent discharge on the quality of the Parnaíba river. Field sampling and laboratory analyzes were carried out for a period of 12 months in the treatment system for stabilization ponds and monitoring points upstream and downstream of effluent inputs. The efficiency of the sewage treatment showed typical values for system stabilization ponds, and met the quality requirements for effluent discharge. The dilution capacity of the Parnaíba river was compatible to the polluting loads of effluent from the STP-Pirajá. The Water Quality Index (WQI) ranged from good to regular in the studied area; and diffuse pollution proved to be the main threat in the river water quality. Escherichia coli values exceeded the limits of the quality requirements in almost all the monitoring period, even upstream of the effluent discharge point.