Converting Conventional Activated Sludge to Biological Nutrient Removal

A conventional activated sludge plant can often achieve meaningful nitrogen and phosphorus removal without being rebuilt from the ground up. The conversion depends on understanding the existing tank geometry, aeration system, sludge age, return streams and hydraulic constraints before selecting a biological nutrient removal (BNR) configuration. A well-planned retrofit can improve effluent quality while retaining much of the site’s existing civil infrastructure.

This case study describes an illustrative conversion of a 45 ML/day municipal wastewater treatment plant from carbon removal and nitrification to biological nitrogen and phosphorus removal. The example is relevant to Australian utilities managing tighter discharge limits, water recycling ambitions, rising energy prices and limited land in established urban areas.

The Starting Point And The Driver For Change

The plant served a growing coastal catchment with a mixture of residential sewage, food businesses and light industry. Its original process consisted of primary clarification, four parallel aeration basins, secondary clarification and chlorine disinfection. The aeration lanes provided nitrification during warm periods, but there was no dedicated anoxic zone for denitrification and no anaerobic zone to support enhanced biological phosphorus removal.

Average influent flow was 31 ML/day, with peak wet-weather flow reaching 78 ML/day. Typical influent concentrations were 42 mg/L total nitrogen and 7.5 mg/L total phosphorus. Effluent nitrogen generally ranged from 18 to 25 mg/L, while phosphorus varied between 2.5 and 4.5 mg/L. The receiving water had limited assimilative capacity, and the regulator was moving towards annual nutrient loads that the existing process could not reliably meet.

The business case also considered future water recycling. Perth and Adelaide utilities have long treated water security as a central planning issue, while projects in Sydney, Melbourne and Brisbane increasingly examine how treated effluent can support industry, irrigation or environmental flows. Improved nutrient removal would make the plant more suitable for advanced treatment without committing immediately to expensive tertiary infrastructure.

Turning Existing Basins Into BNR Zones

The design team retained the inlet works, primary clarifiers, secondary clarifiers and most of the basin structures. Each aeration lane was divided into three biological zones: an anaerobic selector at the inlet, an anoxic zone in the middle and an aerobic zone at the outlet. New internal mixed-liquor recycle pumps returned nitrate-rich liquor from the aerobic end to the anoxic zone, while modified return activated sludge entered the anaerobic zone.

The anaerobic selector was sized to provide approximately 60 minutes of effective contact at average flow. This area received readily biodegradable carbon and returned sludge with minimal nitrate. Polyphosphate-accumulating organisms could then release phosphorus under anaerobic conditions before taking it up in the aerobic zone. The anoxic zone provided roughly 2.5 hours of hydraulic retention, with an internal recycle ratio initially set at 300 per cent of influent flow.

The conversion required new baffles, mixers, recycle pipework, dissolved oxygen probes, oxidation-reduction potential instruments and variable-speed drives. Existing blowers were retained after testing confirmed adequate turndown and capacity. Fine-bubble diffusers were replaced in the aerobic zones, where oxygen transfer efficiency was more important than simply increasing air volume. A modest ferrous dosing system was also retained as a polishing measure for phosphorus during unusual loading events.

Managing Carbon, Sludge And Seasonal Conditions

Denitrification depends on an available carbon source. Influent characterisation showed that the plant had enough readily biodegradable chemical oxygen demand for average conditions, but the carbon-to-nitrogen ratio fell during some wet-weather events. The team therefore installed a provision for supplemental methanol or acetate dosing, although it was not used during normal operation. This avoided locking the operator into a permanent chemical cost before the biology had been optimised.

The operating strategy began with a sludge retention time of 12 days and a mixed-liquor suspended solids concentration of approximately 3,500 mg/L. Operators adjusted wasting rates to maintain nitrification in winter, when lower temperatures slow ammonia-oxidising organisms. In Australian plants, a process that performs well through a warm Queensland summer can require a different winter sludge age in Tasmania or Victoria, so seasonal operating envelopes were documented rather than relying on a single design value.

The plant also had to account for everyday catchment behaviour. Morning peaks were pronounced because much of the community commuted to work, while weekend flows contained higher loads from hospitality venues. Trade waste agreements were reviewed for high-strength food waste and cleaning chemicals that could inhibit biological activity. These operational details proved as important as the tank modifications because BNR performance depends on a stable and predictable feed.

Commissioning And Performance Results

Commissioning was staged over five months. One lane was converted first, allowing operators to compare the modified process with the remaining conventional lanes. Seeding with return sludge from the existing plant shortened the establishment period, but phosphorus removal lagged behind nitrogen removal because the anaerobic zone initially received too much nitrate and insufficient readily biodegradable carbon.

After the return sludge routing was corrected and primary sludge fermentation was introduced, the biology became more stable. Fermented primary sludge increased the readily biodegradable carbon available to denitrifiers and phosphorus-accumulating organisms. Operators then tuned the internal recycle rate, blower control and wasting schedule using online ammonia and nitrate trends rather than fixed timer settings.

After twelve months, average effluent total nitrogen fell from 21 mg/L to 8.5 mg/L, with monthly results below 10 mg/L during normal hydraulic conditions. Total phosphorus declined to an average of 0.8 mg/L, with occasional peaks during high-flow events. Ammonia remained below 1 mg/L for most samples. Electricity use fell by approximately 9 per cent because dissolved oxygen control replaced constant-speed aeration, although mixer and recycle pumping added a new load.

Compliance, Stakeholders And Long-Term Value

The project team mapped the revised process against the relevant state environmental licence, recycled water objectives and receiving-water requirements. Australian projects may need to consider the NSW EPA licence framework, Victoria’s Environment Protection Act obligations or equivalent requirements in Queensland, South Australia and Western Australia. If effluent is intended for a reuse scheme, the Australian Guidelines for Water Recycling and requirements such as AS/NZS 4020 may also influence treatment barriers, monitoring and validation.

The plant’s location near established housing made construction planning important. Odour control, delivery traffic, noise from new mixers and temporary bypass arrangements were assessed before work began. Guidance on urban siting considerations was useful when the project team explained why process changes could improve environmental outcomes without creating a new industrial footprint.

Engagement also included operators, council staff, local industry, environmental groups and downstream water users. Early workshops identified concerns about chemical storage, wet-weather performance and the reliability of recycled water quality. The project followed practical stakeholder engagement guidance, using clear performance measures rather than promising that the retrofit would eliminate every operational risk.

The final lesson was that conversion is a process-control project as much as a civil works project. The largest gains came from correct zone configuration, reliable instrumentation, disciplined sludge wasting and operator confidence. For Australian utilities, the approach can defer land acquisition, reduce construction disruption and create a platform for filtration, disinfection or advanced reuse treatment later.

The next step is to complete a 12-month influent and effluent characterisation study covering flow, COD fractions, nitrogen, phosphorus, temperature and wet-weather dilution before selecting the retrofit configuration.