A Full-Scale Anammox Case Study for Australian Water Utilities

Nitrogen removal is becoming a sharper priority for Australian water authorities as discharge licences tighten, energy prices rise and population growth places pressure on existing treatment assets. Anammox, short for anaerobic ammonium oxidation, offers a way to remove nitrogen with less oxygen and substantially less external carbon than conventional nitrification–denitrification.

This case study follows a representative coastal New South Wales utility that installed a full-scale sidestream anammox process to treat ammonia-rich liquor from dewatering anaerobically digested sludge. The project reflects conditions familiar to Australian operators: variable wastewater temperatures, limited plant footprints, strict odour expectations and a need to prove value before committing to a major process change.

The Treatment Problem And Business Case

The utility operated a conventional activated sludge plant serving a fast-growing catchment between Sydney and the Central Coast. Ammonia returned from sludge dewatering made up only a small fraction of the plant’s total flow, yet it contributed roughly 20–25 per cent of the incoming nitrogen load. During wet-weather events, dilution changed the liquor strength and hydraulic profile, making the existing treatment process harder to control.

The sidestream was sent back to the head of the works, where its concentrated ammonia load consumed aeration capacity. Operators also needed a dependable supply of methanol for denitrification. Electricity and chemical costs were increasing, while the site’s existing blowers were approaching the end of their service life. A preliminary assessment found that biological nitrogen removal through partial nitritation and anammox could reduce aeration demand, alkalinity consumption and carbon dosing.

Before selecting equipment, the project team prepared a staged financial model covering civil works, instrumentation, commissioning, operator training and whole-of-life maintenance. The utility used a capital improvement plan guide to connect the process proposal with asset risk, renewal timing and broader regulatory obligations rather than treating it as an isolated technology purchase.

Process Selection And Design Basis

The chosen configuration used a separate sidestream reactor with partial nitritation followed by an anammox biomass zone. In the first stage, a controlled amount of ammonia was oxidised to nitrite. In the second, anammox bacteria converted ammonium and nitrite directly into nitrogen gas. The biological pathway avoided the need to oxidise all ammonia to nitrate and then reduce nitrate with added carbon.

The design feed averaged 700–900 cubic metres a day, with ammonia concentrations generally between 700 and 1,200 milligrams per litre as nitrogen. The process was sized for peak loading rather than average flow, with equalisation upstream to smooth the daily return from centrifuges. Temperature was a central design consideration. Sidestream liquor was warmer than the main plant flow, generally ranging from 27 to 32°C in summer and remaining above 22°C through much of winter.

The team selected a granular sludge system because dense granules can retain slow-growing anammox organisms and provide good settling characteristics. The reactor included fine-bubble aeration for the partial nitritation stage, mixers for anoxic contact, dissolved oxygen and oxidation-reduction potential sensors, online ammonium and nitrate analysers, and an automated recycle loop. A bypass allowed the main treatment plant to remain operational during start-up or maintenance.

Building The Biology Safely

Anammox organisms grow slowly, so commissioning focused on protecting the seed biomass and avoiding sudden changes in loading. The utility obtained inoculum from an operating deammonification facility and transported it under controlled conditions. Rather than filling the reactor immediately, operators introduced the seed in stages while maintaining stable temperature, pH and alkalinity.

The first operating target was partial nitritation, with dissolved oxygen kept low enough to suppress nitrite-oxidising bacteria. This required tighter control than the plant’s conventional aeration basins. The operators learned to interpret ammonia, nitrite and nitrate trends together instead of adjusting blowers in response to a single analyser.

During the first months, the system was deliberately run below its design loading. Nitrite accumulation signalled that the anammox population had not yet caught up with the ammonia-oxidising organisms. Later, as nitrogen removal improved, the team increased the sidestream flow and adjusted the recycle ratio. The biology became more resilient after several sludge retention cycles, although the process still required protection from toxic shocks, high suspended solids and sudden changes in centrate composition.

Automation And Operational Control

The control philosophy was built around stable nitrogen ratios rather than maximum instantaneous removal. Online ammonium and nitrite readings adjusted aeration intensity, while flow-paced dosing and equalisation controlled the load entering the reactor. Operators retained the ability to switch to conservative settings during analyser maintenance or unusual centrate conditions.

Automation workshops were particularly useful for bridging the gap between process theory and day-to-day decisions. Shift staff needed to understand why a low dissolved oxygen reading could be beneficial in one zone but dangerous in another, and why a short-term rise in nitrite might require reduced loading rather than extra aeration. This knowledge reduced reliance on vendor support and improved confidence during night shifts.

Electrical reliability was treated as part of process performance. Blowers, variable-speed drives, control panels and analyser cabinets were included in a preventive maintenance programme, with infrared thermography checks scheduled before summer peak demand. In a warm Australian climate, identifying a loose termination or overloaded connection early can prevent a plant trip that would otherwise destabilise the biological process.

Results, Costs And Verification

After a 14-month ramp-up, the reactor consistently removed 80–90 per cent of sidestream ammonia under normal loading. The process reduced the ammonia return to the head of the works by approximately 18 per cent and lowered the plant’s demand for methanol. Aeration energy associated with sidestream treatment fell by about 55 per cent compared with the previous arrangement, although mixers, analysers and pumping introduced new electrical loads.

The project did not eliminate every operating cost. Instrument calibration became more important, specialist biological support was required during the start-up period, and the utility carried spare parts for critical analysers and blower components. These costs were included in the business case. The strongest financial result came from combining energy savings, reduced carbon dosing and deferred expansion of the main aeration system.

Verification included daily laboratory testing during commissioning, independent sampling after stabilisation and a year of performance monitoring across summer and winter. The utility tracked total nitrogen, ammonia, nitrite, nitrate, alkalinity, oxygen demand, sludge settleability and electricity use. This broader data set showed that the process was delivering genuine load reduction rather than shifting nitrogen between forms.

Lessons For Australian Projects

The case demonstrated that anammox is best considered as a process integration project, not a plug-in tank. Reliable upstream sludge digestion, consistent dewatering, equalisation capacity and good instrumentation were prerequisites. Sites with erratic centrate quality may need stronger isolation, blending or feed-forward control before biological treatment can perform consistently.

Local conditions also shape the approval and delivery pathway. A utility in regional Victoria may face different temperature and procurement constraints from one in Brisbane, while a Western Australian site may need to manage long supply chains for specialist equipment. In Melbourne, Sydney or Perth, the market often includes alliance contracting, long lead times for switchboards and a strong expectation that operators will be involved in design reviews.

Stormwater and civil works should be coordinated early, particularly where a new reactor, chemical area or electrical compound changes site drainage. Although written for another regulatory setting, stormwater upgrade guidance offers useful prompts for separating construction controls, permanent drainage design and water-quality risks. Australian projects still need to satisfy their state-based environmental approvals and council requirements.

The practical lesson is straightforward: begin with a well-characterised sidestream, protect the biology, automate the key control loops and measure whole-site benefits. For an Australian water utility considering deammonification, the next concrete step is to complete a 12-month centrate sampling campaign covering flow, ammonia, temperature, alkalinity and suspended solids before selecting a reactor supplier.