Low DO, High Impact: A sustainable and cost-effective approach to nitrogen management integrating mobile media

Presentation Description: Water resource recovery facilities (WRRFs) are under increasing pressure to meet stringent nutrient limits or reduce effluent total nitrogen (TN) concentrations in support of potable reuse. These challenges are compounded by the need to reduce energy consumption, minimize chemical usage, and accommodate future growth within limited physical footprints.

Low dissolved oxygen (DO) (~0.3-0.5 mg/L) operation is gaining traction as a viable method to reduce aeration energy and support simultaneous nitrification-denitrification (SND). SND is particularly advantageous when supplemental carbon is required for TN removal. SND reduces the need for exogenous carbon, resulting in a lower biomass yield per gallon and higher rated process capacity. Additionally, preliminary modeling suggests SND can enable the integration of swing zones in lieu of dedicated anoxic volume and increase the capacity by 5%-10% without compromising effluent TN.

Low DO operation presents challenges at colder temperatures (10-13˚C), where longer aerobic solids retention times (SRTs) are required to maintain full nitrification. Microbial acclimation to low DO can overcome these constraints, though adaptation is not guaranteed and can pose operational challenges.

To mitigate the challenge of seasonal variability, our research explores the integration of the mobile biofilm technology in a low DO activated sludge process. Unlike conventional biofilm systems such as IFAS, MBBR, or MABR, mobile media are small enough to be fluidized under low DO aeration energies and require minimal retrofit—only a rotary screen for media recovery. Biofilms enhance SND by maintaining anoxic microenvironments and buffer variability in suspended sludge kinetics. Their good settleability results in a significant increase in process capacity.

Our pilot-scale research is implemented using a 400 gallon pilot-scale system at the Boulder Colorado WRRF treating primary effluent. The experimental research is motivated by an extensive Monte Carlo modeling study that shows hybrid systems with mobile media can achieve 3 mg/L greater nitrogen removal and operate at a lower mixed liquor recycle rate than suspended sludge alone. Therefore, the addition of a hybrid mobile media technology significantly improves both process capacity and stability. Our research offers a compelling roadmap for WRRFs seeking to meet stringent nutrient limits while advancing towards sustainability goals.