Comprehensive comparative between chemically enhanced primary treatment and high-rate activated sludge for preconcentration of organic carbon in novel wastewater treatment plant configurations

Abstract

Novel wastewater treatment plants (WWTPs) are designed to be more energy efficient than conventional plants. One approach to becoming more energy efficient is the preconcentration of organic carbon through chemically enhanced primary treatment (CEPT) or high-rate activated sludge (HRAS). This study compares these approaches in terms of energy demand, operational costs, organic micropollutants (OMP), and virus removal efficiency. A CEPT pilot-scale plant was operated at a hydraulic retention time (HRT) of 30min, and a lab-scale HRAS reactor was operated at an HRT of 2 h and a solid retention time (SRT) of 1 d in continuous mode. A minimum dose of 150 mg/L ferric chloride (FeCl3) was required to achieve a threshold chemical oxygen demand (COD)-to-ammonium ratio below 2 g COD to 1 g of NH4–N (fulfilling the requirement for a partial nitritation-anammox reactor), reaching high phosphate (PO4 3–)-removal efficiency (>99%). A slightly lower COD recovery was attained in the HRAS reactor, due to the partial oxidation of the influent COD (15%). The lower PO4 3 removal efficiency achieved in the HRAS configuration (13%) was enhanced to a comparable value of that achieved in CEPT by the addition of 30 mg/L FeCl3 at the clarifier. The CEPT configuration was less energy-intensive (0.07 vs 0.13 kWh/m3 of wastewater) but had significantly higher operational costs than the HRAS-based configuration (6.0 vs 3.8 cV/m3 of wastewater). For OMPs with kbiol > 10 L/gVSS$d, considerably higher removal efficiencies were achieved in HRAS (80e90%) than in CEPT (4e55%). For the remaining OMPs, the biotransformation efficiencies were generally higher in HRAS than in CEPT but were below 55% in both configurations. Finally, CEPT was less efficient than HRAS for virus removal. HRAS followed by FeCl3 post-treatment appeared to be a more effective alternative than CEPT for COD pre-concentration in novel WWTPs.