Constructed wetlands for tertiary treatment in a small community

A small regional community in southern Australia needed to improve the quality of its treated wastewater before discharge to a nearby creek. Its existing lagoon and biological treatment system could reliably reduce organic matter, yet suspended solids, nutrients and seasonal microbial variation occasionally pushed the final effluent beyond the limits set by its environmental licence. Expanding the conventional plant would have delivered better control, but the capital and operating costs were difficult for a community of fewer than 4,000 people to absorb.

The council selected a constructed wetland as a tertiary polishing stage. The project used planted filtration cells, a shallow flow path and final disinfection to improve effluent quality without adding a complex process that required constant specialist attention. The case is relevant to Australian utilities assessing water recycling, creek protection and lower-energy treatment options for small populations.

The facility also had to suit local working conditions. Summer heat and evaporation, intense rainfall events, variable inflows during holiday periods and a limited operations team all influenced the design. The result was a treatment system that combined engineered hydraulics with natural processes, while retaining enough monitoring and bypass capacity to protect the receiving environment.

Why the project was needed

The community’s wastewater plant treated an average of 120 kilolitres per day, with peak flows reaching about 220 kL/d after storms. The original system consisted of preliminary screening, an aerated lagoon and a maturation pond. It generally achieved acceptable biochemical oxygen demand and total suspended solids results, but nutrient removal was inconsistent. Algal growth in the final pond also created visible water-quality problems during warm weather.

The receiving creek flowed through grazing land before joining a larger river system. During dry periods, its dilution capacity was low, and local residents were concerned about odour, algae and the appearance of the discharge point. The council wanted to protect the creek while keeping treated water available for irrigating public open space. That objective reflected a wider Australian market trend: regional authorities are looking for modest-scale reuse schemes rather than relying solely on costly new water supplies.

A preliminary options assessment compared a membrane bioreactor, tertiary filtration, chemical phosphorus removal and a constructed wetland. Membranes offered a compact footprint but involved higher energy use, membrane replacement and operator training. Chemical treatment required reliable dosing equipment and ongoing storage of reagents. The wetland required more land, yet the available council reserve reduced the property cost and made the passive process attractive.

Designing for local conditions

The design team began with a review of the existing plant rather than treating the wetland as a stand-alone solution. Hydraulic bottlenecks, uneven pond levels and accumulated grit could have undermined any downstream polishing process. The council used a structured headworks evaluation to check screening, bypass arrangements, flow measurement and maintenance access before finalising the wetland layout.

The selected configuration comprised two parallel vertical-flow cells followed by two horizontal subsurface-flow cells. Each cell contained graded gravel, distribution pipework and locally suitable wetland plants, including native rushes and sedges. A balancing basin equalised short-term surges, while adjustable weirs allowed operators to isolate a cell for inspection or media maintenance. The parallel arrangement was important because a small utility cannot afford a full shutdown whenever one treatment zone needs attention.

Australian climate conditions shaped the hydraulic calculations. The design allowed for high evaporation during summer in inland New South Wales, as well as intense rainfall that could rapidly increase inflow. Freeboard was set above normal operating levels, and an overflow route directed exceptional stormwater away from the planted media. The design also considered mosquito management, public safety and the need to prevent children or livestock entering open water areas.

Building the treatment train

The wetland was installed after secondary treatment and before ultraviolet disinfection. Screening and settling remained essential because coarse material, fats and excess solids can block distribution pipes and fill the gravel voids. The treatment train therefore included a fine screen, a small residual-solids settling tank and a flow-splitting chamber upstream of the planted cells.

Within the vertical-flow beds, intermittent dosing spread wastewater across the surface and drew air into the media as the water drained. This supported nitrification and reduced ammonia. The horizontal cells then provided longer contact time for suspended solids capture, denitrification and additional phosphorus uptake. The process was designed for gradual biological improvement rather than a single dramatic removal step.

The capital cost was approximately A$1.6 million, including civil works, pipework, instrumentation, fencing and commissioning. A local earthmoving contractor completed most of the excavation, while specialist suppliers provided the pumps, ultraviolet unit and control panel. Using regional contractors shortened response times and gave council staff a practical local contact for repairs. The wetland’s energy demand was mainly associated with pumping and ultraviolet disinfection, rather than continuous high-rate aeration.

Operating through seasons

During the first six months, the operators established plant growth and adjusted dosing cycles. Early performance was uneven because the gravel media had not yet developed a mature biofilm, and a few distribution laterals became partially blocked by construction fines. Flushing points and removable inspection covers allowed the team to correct these problems without excavating the beds.

By the second summer, average results had stabilised. Biochemical oxygen demand fell from about 18 mg/L after secondary treatment to 6 mg/L at the wetland outlet. Total suspended solids typically remained below 8 mg/L, ammonia was usually below 2 mg/L, and total nitrogen declined by roughly 35 to 45 per cent. Phosphorus removal was more variable, so the council retained a small ferric dosing system for periods when the discharge limit was most demanding.

Wet weather required a different operating response from dry weather. During heavy rain, operators reduced the wetland loading rate where storage allowed and prioritised hydraulic protection of the media. During hot, dry periods, they checked water levels, vegetation health and odour around the inlet zones. The operations team used a simple roster and included wetland inspection in the same routine as pump checks, sampling and the weekly council maintenance schedule.

Measuring performance and value

Monitoring covered flow, pH, electrical conductivity, temperature, turbidity, BOD, TSS, ammonia, total nitrogen, total phosphorus and E. coli. Samples were collected at the secondary-treatment outlet, after the wetland and after ultraviolet disinfection. This made it possible to distinguish problems caused by the upstream plant from issues within the polishing stage.

The council also tracked power consumption, media inspection time, chemical use and irrigation demand. Over the first full year, the wetland reduced downstream chemical consumption and lowered average treatment energy use compared with the proposed membrane option. Its planted appearance helped the council explain the project at community open days, while site fencing and clear signage kept the public away from operational areas. Technical networks such as LABS of CWEA can support this kind of knowledge exchange by connecting Australian practitioners with broader water and wastewater experience.

The scheme did not eliminate all operational risk. Excess solids from the upstream lagoons remained the main threat to hydraulic capacity, and the wetland needed periodic vegetation cutting and inlet-zone maintenance. Monitoring also showed that a wetland should not be credited with reliable pathogen removal unless its design and validation support that claim. Ultraviolet disinfection remained necessary for the community’s reuse objectives.

What the case teaches smaller utilities

The strongest lesson was that constructed wetlands work best as part of a complete treatment strategy. They cannot compensate for poor preliminary treatment, uncontrolled inflow and infiltration or inadequate flow measurement. A modest investment in upstream maintenance protected the wetland more effectively than adding extra planted area after problems appeared.

The project also demonstrated the value of designing for operational simplicity rather than visual appeal alone. Access tracks, isolation valves, sampling points, spare pumps and clear level indicators had a greater effect on reliability than ornamental landscaping. Native vegetation suited the climate, but plant selection still required advice about salinity, waterlogging, frost and local weed risks.

Energy planning was considered separately from the wetland itself. The small plant produced limited biogas, so electricity generation from anaerobic gas was not economically justified. However, broader biogas options can include direct thermal use, vehicle fuel or renewable natural gas where a larger wastewater facility has enough consistent feedstock. This comparison helped the council avoid forcing an energy technology onto a site that could not support it.

For Australian communities, the case also highlights the importance of state-specific approvals. Discharge licences, recycled-water classifications, wetland setbacks and monitoring requirements vary between jurisdictions. A scheme near Melbourne may face different water-reuse controls from one near Brisbane or Perth, while a coastal community may need to account for saline groundwater and rising sea levels. Early consultation with the regulator, health authority, Traditional Owners and nearby landholders reduces redesign risk.

A constructed wetland is therefore a practical tertiary treatment option when land is available, flows are moderate and the community accepts a biological process with seasonal variation. It should be supported by sound headworks, reliable disinfection, clear performance limits and an operating budget for inspection and maintenance.

For a small utility assessing the same path, the practical takeaway is straightforward: confirm the upstream plant’s condition, characterise seasonal flows, design parallel treatment cells, retain disinfection, and measure the whole system before expanding its reuse or discharge claims.