Wastewater is still a major problem: a comprehensive evaluation of N and P loads into water bodies of the most populous state in Brazil

Abstract

Nutrient pollution from wastewater systems, particularly nitrogen (N) and phosphorus (P), continues to be a major challenge in Brazil and other low- and middle-income countries (LMICs), driven by rapid population growth, industrial expansion, and urbanization, along with inadequate sanitation infrastructure. This study assessed municipal wastewater systems across all 645 municipalities in the state of São Paulo using the most up-to-date data available (from the Sewage Atlas, 2015–2016). Daily potential nutrient loads were estimated at 327.48 t (N) and 40.93 t (P). The 768 Wastewater Treatment Plants (WWTPs) mainly served small municipalities (≤50,000 inhabitants) in isolated, centralized systems. Stabilization ponds were common in municipalities with populations up to 100,000 inhabitants, while aerobic reactors and activated sludge systems dominated in larger municipalities. Most WWTPs operated below 80 % capacity utilization, with many medium/large plants below 70 %. Secondary treatment was most common, with average expected nutrient removal efficiencies of 56.5 % for N and 37.3 % for P. Due to low efficiencies and limited coverage (64.8 %), modest reductions in residual loads were observed (36.9 % for N and 25.2 % for P). Despite the statistically significant difference between potential and residual loads, substantial discharges into receiving water bodies (206.53 t N and 30.60 t P daily) persist, leading to frequent exceedances of regulatory standards. These findings highlight the need for comprehensive strategies to expand wastewater treatment coverage and efficiency, and mitigate pollution from untreated and non-point sources.