WCW26

Water Resource Recovery Facilities (WRRFs) face increasing pressure to meet stringent nutrient limits, reduce energy consumption, minimize chemical usage, and accommodate growth within limited footprints. 
 
Low dissolved oxygen (DO~0.3-0.5 mg/L) operation is a promising method to reduce aeration energy and support sustainable biological nutrient removal (BNR) pathways, such as simultaneous nitrification-denitrification (SND).  SND is particularly advantageous for carbon-limited plants, as it can free carbon once used in nitrogen removal and divert it to enhanced biological phosphorus removal (EBPR), which significantly reduces reliance on coagulants (e.g., ferric or alum). Total nitrogen (TN) removal offers benefits such as reduced alkalinity demand (e.g., caustic) and lower biomass yield, especially when SND lowers the use of exogenous carbon.  
 
Biofilm technologies are well known to increase process capacity. Unlike conventional biofilm systems such as integrated fixed film activated sludge (IFAS), moving bed biofilm reactor (MBBR), or membrane aerated biofilm reactor (MABR), mobile media (MM) can be uniquely integrated with low dissolved oxygen (DO) aeration strategies due to their small size and low mixing energy requirements.  They also require minimal retrofitting, limited to rotary screening for media recovery. 
 
In addition to significant capacity improvements, process modeling indicates integration of MM to a low DO process significantly enhances SND, achieving 3-4 mg/L greater TN removal than suspended sludge alone.  With MM, dropping the DO from 2.0 mg/L to 0.5 mg/L resulted in a decrease in effluent phosphorus from 1.2 mg P/L to 0.1 mg P/L, attributed to decrease in nitrate return in the return activated sludge (RAS) stream.  
 
Further, the biofilm potentially buffers seasonal kinetic variability (i.e., degree of acclimation). Low DO operation presents challenges at cold temperatures (10-13˚C), as very long aerobic solids retention times (SRTs) are required for full nitrification. While microbial acclimation to low DO may overcome these limitations, adaptation is not guaranteed and can introduce treatment and operational challenges.  With a biofilm, nitrification activity by the biofilm increases when the suspended sludge performance worsens. 
 
Our study combines process modeling and pilot-scale system operation to evaluate the potential for intensification and the synergistic effects of MM and low DO operation on nutrient removal performance and stability. To test the modeling-informed hypotheses, pilot-scale research is being conducted at the Boulder WRRF (Colorado, USA) using a 400-gallon reactor system treating primary effluent. Currently, a comparative study of activated sludge and hybrid MM systems is being performed using sequencing batch reactors. Next, a plug flow configuration will be evaluated. This research will offer a compelling nutrient removal roadmap for WRRFs needing to meet nutrient limits while advancing towards sustainability goals.