The Central Contra Costa Sanitary District (Central San), serving nearly 500,000 residents and over 15,000 businesses in Contra Costa County, California, faces a regulatory challenge. The San Francisco Bay Regional Water Quality Control Board introduced a mandate requiring a 40% reduction in total inorganic nitrogen (TIN) for Central San by 2034. The existing conventional activated sludge plant is designed for removal of solids and organics and is not capable of nitrogen removal. Traditional treatment upgrade solutions, such as constructing new aeration basins and clarifiers, were estimated to cost approximately $700 million, a prohibitively expensive option further complicated by the presence of contaminated soil at the proposed expansion site.
To address this challenge, Central San explored Membrane Aerated Biofilm Reactor (MABR) technology as an innovative alternative to conventional treatment processes. Central San partnered with Veolia to pilot test their ZeeLung MABR system, which enables process intensification - achieving more treatment within existing infrastructure. MABR uses gas-permeable membranes to deliver oxygen directly to a biofilm that grows on the surface of the membrane, creating conditions favorable for both nitrification and denitrification. ZeeLung MABR cassettes are immersed into existing tanks to augment the nitrogen removal capacity of a conventional activated sludge system.
From April to November 2024, a ZeeLung MABR pilot system (2.5-6 gallons per minute) was operated at Central San with the following goals:
- Confirm the ability of the MABR biofilm to remove ammonia despite the presence of cyanide from solids incineration that is known to inhibit conventional nitrification processes.
- Determine ammonia removal rate while operating at low solids retention times (SRTs).
- Test biofilm thickness control under varying conditions.
- Identify the optimal location for full-scale ZeeLung MABR implementation.
Résultat
The pilot study successfully demonstrated that ZeeLung MABR technology could provide the necessary TIN reduction while utilizing Central San's existing treatment infrastructure. Key findings included:
- The target of 40% TIN reduction was achieved using MABR, which could be improved with full integration into the plant.
- Consistent biofilm formation and nitrification despite exposure to cyanide, a known biological nitrogen removal inhibitor.
- Ammonia removal rates met the target of 2 grams per square meter of membrane surface area per day.
- Higher soluble carbon loading rates improved TIN reduction efficiency.
- Stable performance was maintained even under the most challenging conditions.
These results are from a seven-month pilot operating at 4 gallons per minute and validated ZeeLung MABR as a viable, cost-effective compliance solution that avoids major construction. Comprehensive full-scale testing, scheduled for 2026, will provide additional performance data.
Figure 1: TIN reduction during pilot phases 2 and 3. Greater than 40% TIN removal was observed in Phase 3, when sufficient carbon was available for denitrification in the biofilm.
Figure 2: MABR nitrification rates across the pilot phases. Phases had varying loading rates and feed sources, where phase 1 had the lowest carbon and cyanide loading and Phase 3 had the highest.
Conclusion:
By leveraging ZeeLung MABR technology, Central San is positioned to meet future nutrient removal regulations in a cost-effective and sustainable manner. This approach not only minimizes capital expenditures but also maximizes the efficiency of existing treatment assets, setting a precedent for innovative wastewater management solutions in the Bay Area and across the industry.