Adjusting wastewater treatment effluent standards to protect the receiving waters: the case of low-flow rivers in central Spain

Freshwater quality is deteriorating worldwide. In populated areas, urban pollution is the main pressure on surface continental waters, but intensive wastewater treatment is costly. Setting standards for treatment of wastewater before discharge is a major policy instrument for water authorities, balancing environmental gains and operational costs. Discharge permits usually define concentration limits at the discharge point of the plant effluent. This approach, however, may not guarantee the good status of the receiving waters. Discharge permits should be directly linked to pollutant concentration in the river. Our paper develops an approach to adaptively adjust discharge permits and applies it to Madrid and the Manzanares river, a city of more than 3 million inhabitants discharging its treated wastewater to a stream having less than 2 m3 s−1 average flow. Stricter limits to 5-day biological oxygen demand (11 mg O2 L−1), ammonium (0.5 mg N-NH4 L−1), nitrate (5.9 mg N-NO3 L−1), and phosphate (0.17 mg P-PO4 L−1) at plant effluent are required to meet the river environmental objectives. The results can be generalized to assess wastewater management decisions in other geographical areas.