Advanced Biological Nutrient Removal in Secondary Treatment: A Researcher's Guide

Recent Trends in Nutrient Removal Research

Over the past several years, the focus in secondary treatment research has shifted from conventional carbon removal toward integrated biological nutrient removal (BNR) that targets nitrogen and phosphorus simultaneously. Emerging process configurations — such as the use of anaerobic-aerobic-anoxic sequencing, step-feed strategies, and sidestream treatment for return flows — have gained traction in pilot-scale and full-scale studies. Researchers are increasingly examining how microbial community dynamics and operational parameters (e.g., solids retention time, dissolved oxygen setpoints, and carbon-to-nitrogen ratios) influence removal efficiencies for both nutrients.

Background: From Conventional Secondary Treatment to BNR

Secondary treatment historically emphasized biochemical oxygen demand and total suspended solids reduction. Regulatory drivers — particularly effluent limits on total nitrogen and total phosphorus — pushed the development of biological processes that promote nitrification, denitrification, and enhanced biological phosphorus removal (EBPR). Key background concepts include:

• Nitrification/Denitrification: Autotrophic bacteria (e.g., Nitrosomonas, Nitrobacter) oxidize ammonia to nitrate; heterotrophic bacteria then reduce nitrate to nitrogen gas under anoxic conditions.
• EBPR Mechanism: Polyphosphate-accumulating organisms (PAOs) release phosphorus under anaerobic zones and take up excess phosphorus under aerobic zones, creating a net removal via wasting.
• Operational Variables: Solids retention time (SRT) of 8–15 days is common for nitrification; for EBPR, ensuring sufficient volatile fatty acids (VFAs) in the anaerobic zone is critical.

User Concerns Among Researchers

Researchers working with BNR in secondary treatment face several practical and knowledge gaps:

• Process Stability: Seasonal temperature shifts, variable influent loads, and the presence of inhibitory compounds can destabilize PAO or nitrifier populations.
• Carbon Allocation – Competition between denitrifiers and PAOs for limited organic carbon remains a core challenge, especially when influent carbon-to-nitrogen ratios are below 6:1.
• Monitoring Complexity: Real-time measurement of specific microbial groups (e.g., via fluorescence in situ hybridization) is still resource-intensive; surrogate online sensors (e.g., oxidation-reduction potential, ammonia, nitrate) need careful calibration.
• Scaling from Lab to Plant: Bench-scale results often do not fully replicate the hydraulic and mixing patterns of full-scale reactors, leading to overestimation of removal performance.

Likely Impact on the Field

Advancements in BNR for secondary treatment are expected to influence multiple areas:

• Effluent Quality Standards – More facilities will likely meet stringent nutrient limits (e.g., total nitrogen below 5 mg/L, total phosphorus below 0.5 mg/L) using biological processes alone, reducing chemical usage.
• Energy and Chemical Savings – Optimized aerobic/anoxic cycling can lower aeration energy demand by 15–30%, while minimized reliance on metal salts for phosphorus precipitation reduces sludge handling costs.
• Greenhouse Gas Management – Improved control of denitrification can reduce nitrous oxide emissions, though researchers are still refining the mechanistic models linking process conditions to N₂O production.
• New Process Configurations – Examples include the A2O process (anaerobic-anoxic-oxic), the University of Cape Town (UCT) process, and integrated fixed-film activated sludge (IFAS) systems — all under active investigation for reliability and footprint.

What to Watch Next

The following developments merit close observation over the next few years:

• Advanced Control Algorithms: Model predictive control and machine learning methods that adjust aeration and chemical dosing in real time based on online nutrient sensors may become standard at research-oriented facilities.
• Sidestream Treatment Integration: Research on separate treatment of anaerobic digester liquor (high in ammonia and phosphorus) using deammonification or struvite recovery is likely to become more common in secondary process designs.
• Microbiome-Based Optimization: More accessible metagenomic sequencing may allow researchers to identify microbial indicators of process upset before conventional parameters change.
• Low-Temperature BNR: Studies examining anoxic ammonium oxidation (anammox) at temperatures below 15°C are ongoing; success could greatly expand BNR applicability in colder climates.
• Long-Term Reliability Datasets: As pilot trials continue into their third or fourth year, researchers will have better evidence on long-term stability and required maintenance for full-scale retrofits.