Conventional activated sludge plants have long been a standard wastewater treatment, but many don’t remove enough nitrogen to meet effluent standards.
When nitrogen removal must be increased to comply with effluent regulations, Fluence solutions can update conventional activated sludge treatment systems
Excess nitrogen in treated effluent is a key contributor to eutrophication in rivers, lakes, and coastal areas. Ammonia, nitrite, and nitrate from wastewater can disrupt aquatic ecosystems, leading to harmful algal blooms and oxygen depletion.
In response, regulatory frameworks in the United States, Europe, and beyond are imposing tighter effluent nitrogen limits. This includes initiatives such as the U.S. Environmental Protection Agency’s nutrient reduction strategies and the European Union’s Urban Waste Water Treatment Directive, which targets total nitrogen concentrations as low as 10 mg/L or below in sensitive areas.
Around the world, wastewater treatment systems have been evolving in response to these more rigorous environmental regulations. Many facilities were originally built to address organic pollution, removing carbonaceous compounds through the conventional activated sludge (CAS) process. However, the tighter discharge limits are requiring plant operators to adapt.
A shift from carbon-only to full biological nitrogen removal (BNR) presents technical challenges, but also opens opportunities for smarter, more sustainable upgrades, many of which can be implemented using compact, flexible, and modular technologies.
Nitrogen Removal
Conventional activated sludge systems are effective at removing organic matter but are not inherently suited for nitrogen removal. These systems typically operate with a single aerobic zone followed by secondary clarification. While sufficient for carbon degradation, they do not provide the multiple redox environments needed for full nitrification and denitrification. Retrofitting existing CAS infrastructure to support BNR requires creating or simulating anaerobic, anoxic, and aerobic conditions in a controlled and efficient way. Here are some of the methods commonly used for such upgrades:
- MABR: One increasingly adopted solution uses membrane aerated biofilm reactor (MABR) technology. This advanced process uses air-permeable membranes to deliver oxygen directly to biofilms growing on the membrane surface. The resulting biofilm structure supports a layered microbial community that performs simultaneous nitrification and denitrification.
Because the oxygen diffuses inward while nitrate diffuses outward, MABR enables high nitrogen removal efficiency within a single reactor volume. These systems are compact and energy-efficient, and can be deployed in a modular format — like Fluence’s Aspiral™ Flex — making them particularly attractive for decentralized or space-constrained facilities seeking low-impact upgrades.
- In-basin upgrades: Another upgrade pathway involves adding treatment systems to a CAS plant’s existing aeration basin, which is especially valuable when capacity must be increased without expanding physical infrastructure. With Fluence’s SUBRE, towers of MABR modules are submerged in the basin, enabling simultaneous nitrification-denitrification. These energy-efficient, easy-to-install modules improve effluent quality, support biological nutrient removal (reducing total nitrogen and phosphorus), and can boost plant capacity for flows ranging from 528,000 to 26 million GPD (2,000 to 100,000 m³/d). SUBRE retrofits can be completed with minimal disruption — one basin at a time — and results, typically seen within one to three weeks, include enhanced effluent quality and up to a 30% reduction in energy use.
- Containerized treatment: For facilities facing strict compliance deadlines or with limited room for on-site modifications, containerized BNR treatment units offer a streamlined alternative. These systems, including Aspiral™ Flex, are pre-engineered and delivered ready to operate. They can be configured for full BNR, including pre-anoxic, aerobic, and post-anoxic zones, and integrated seamlessly with existing CAS systems. Containerized treatment solutions offer flexibility for temporary installations, phased upgrades, or permanent expansions, and they can be relocated or redeployed as operational needs change.
- Sidestream treatment: For treatment of wastewater with a high nitrogen load, another possibility is sidestream treatment, where high-strength return flows (like digester supernatant) are diverted into compact BNR modules equipped with either MABR, like Fluence Nitro, or IFAS technologies. By targeting ammonia-rich internal streams, these systems can significantly reduce the nitrogen load entering the mainline treatment process, helping plants achieve compliance with minimal disruption to core operations.
In all of these upgrade paths, smart process control and monitoring play an essential role. Real-time sensors for ammonia, nitrate, dissolved oxygen, and redox potential allow operators to dynamically adjust aeration rates, recycle flows, and chemical dosing. These controls not only ensure regulatory compliance but also optimize energy use and reduce operational costs. Systems with integrated automation and remote access are especially useful in smaller or decentralized facilities with limited on-site staffing.
No Need to Start From Scratch
The transition from carbon removal to full biological nitrogen removal does not necessarily require replacing a plant or rebuilding it from the ground up. Today’s technologies offer effective, small-footprint solutions that can be tailored to existing infrastructure. These approaches make it possible to meet modern nutrient discharge standards while extending the useful life of aging treatment assets.
As nutrient regulations continue to tighten and environmental expectations rise, upgrading conventional activated sludge systems with modular, flexible BNR technologies is not just a technical solution, it’s a strategic investment in sustainable water management. Contact Fluence to learn more about optimizing nitrogen removal at your treatment plant.
