Chemically Enhanced Primary Treatment (CEPT): The Low-Hanging Fruit for Process Optimization, Cost Recovery and Wet Weather Flow Management

At the San Jose/Santa Clara Regional Wastewater Facility (RWF), a full-scale Chemically Enhanced Primary Treatment (CEPT) was conducted for over a year in two phases. During the first phase (8 months) of the study, ferric chloride was dosed at 10 mg/l and anionic emulsion polymer at 0.2 mg/l (active) for the CEPT operations. For the non-CEPT baseline operation, ferric chloride was dosed at 10 mg/l without polymer because the continuous dosing of ferric chloride was required for digester gas H2S control. Similarly, during the second phase (6 months) of the study, ferric chloride was dosed at 15 mg/l ferric chloride and 0.2 mg/l polymer for the CEPT operation. For the non-CEPT operation, ferric chloride was dosed at 15 mg/l without polymer. Although the study was permit driven to stay compliant with 350 ppm digester gas H2S limit from the Bay Area Air Quality Management District, it was also intended to assess and establish the benefits of CEPT in the Biological Nutrient Removal (BNR), biogas generation in the mesophilic anaerobic digestion, residual solids management and struvite control. A cost comparison was made to assess how much of the chemical cost was recouped from the benefits of CEPT. Compared to the 10 mg/l ferric chloride-only non-CEPT baseline operation, the 10 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation achieved additional 11% TSS and BOD removals in the EPCs. The additional TSS and BOD removal in the EPCs with the 15 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation relative to 15 mg/l ferric chloride only non-CEPT operation was 20% for TSS and 23% for BOD. A 6% additional ortho-phosphate removal was achieved in the EPCs during the 10 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation compared to the non-CEPT baseline operation. No phosphorus measurement was taken during the 15 mg/l ferric chloride and 0.2 mg/l CEPT operation. Digester gas production increased by 14% during 10 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation compared to the non-CEPT baseline operation. During the 15 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation, digester gas production increased by an additional 4% relative to the 10 mg/l ferric chloride-only non CEPT baseline operation. Digester gas H2S remained at 200 ppm level throughout the study duration, achieving 43% reduction from the permitted 350 ppm level. A 21% reduction in the bio-p uptake was achieved during the 10 mg/l ferric chloride and 0.2 mg/l polymer CEPT operation relative to the plain sedimentation (no chemical addition) operation, which in turn resulted an 8% reduction on the ortho-phosphate in the digested sludge. A 5% and 13% increase in the primary sludge feed with similar reduction in the Thickened Waste Activated Sludge (TWAS) feed was observed during the 10 mg/l ferric chloride and 0.2 mg/l polymer and 15 mg/l ferric chloride and 0.2 mg/l polymer CEPT operations, respectively relative to the non-CEPT operations. Struvite formation in the digesters and downstream appurtenances was substantially reduced during both CEPT operations. The chemical cost recovery on ferric chloride and polymer usage was 55% and 65% with 10 mg/l ferric chloride and 0.2 mg/l polymer and 15 mg/l ferric chloride and 0.2 mg/l polymer CEPT operations, respectively. Overall, the CEPT operation at 10 mg/l ferric chloride and 0.2 mg/l polymer dosing achieved notable process improvements without impacting the denitrification process and recovering up to 55% of the chemical cost. More process improvements have been achieved with higher CEPT operation at 15 mg/l ferric chloride and 0.2 mg/l polymer dosing.