Retrofitting A Trickling Filter Plant With Plastic Media

A regional wastewater treatment plant serving a growing coastal community had reached the practical limit of its original trickling filters. The works reliably removed carbonaceous pollutants, yet seasonal ammonia peaks were becoming harder to manage. Expanding the site with a new biological reactor would have required additional land, major civil construction and a lengthy approval process.

The operator instead investigated a plastic media retrofit. Existing rock-filled filter beds would be emptied, repaired and fitted with high-void structured media designed to provide more protected surface area for biofilm growth. The objective was to increase treatment capacity and improve nitrification while retaining the existing tanks, dosing equipment and much of the downstream process.

For Australian utilities, this approach can be attractive where land is scarce and an established plant must remain operational during construction. The case also illustrates why media selection, hydraulic distribution, ventilation and commissioning need to be considered as one treatment system rather than as separate equipment purchases.

The Plant’s Starting Point

The plant treated an average dry-weather flow of 12 megalitres per day, with wet-weather peaks approaching 24 megalitres per day. It had two parallel trickling filter cells built in the late 1970s, each containing approximately 1,100 cubic metres of quarried rock. Primary effluent was distributed over the beds using rotary distributors, followed by secondary clarification and chlorine disinfection.

The original process was effective for biochemical oxygen demand removal, but its nitrifying performance was inconsistent. During colder months, ammonia in the final effluent rose above the operator’s internal target. The rock media had also accumulated solids and fine material, reducing void space and causing ponding in several zones. Cleaning the beds provided temporary relief without solving the underlying capacity constraint.

The project team included plant operators, a process engineer, a mechanical contractor and the asset owner’s electrical and controls staff. This collaborative model reflects the value of professional networks such as LABS of CWEA, where water and wastewater practitioners exchange practical experience across operations, design and maintenance. Although the plant in this case was in Australia, the operating questions would be familiar to facilities in Sydney, Melbourne, Perth or regional Queensland.

Choosing And Installing The Media

The selected plastic media consisted of modular sheets with a high specific surface area and open vertical channels. Compared with irregular rock, the modules offered greater void volume, lower risk of clogging and more consistent airflow. The design loading was based on the expected organic and ammonia mass rates rather than on volume alone. This distinction prevented the team from treating a catalogue media volume as a guarantee of process capacity.

Each filter was isolated, drained and emptied in stages so that one biological train remained available. The old rock was removed using a combination of excavators, conveyors and vacuum equipment. A portion of the rock was retained for testing and disposal classification, while the remaining material was sent to an approved reuse or waste facility. In Australia, transport distance and disposal fees can materially influence the business case, particularly for inland plants far from specialist contractors.

Before the plastic modules were placed, the concrete walls, underdrain system and distributor arms were inspected. Damaged nozzles were replaced, blocked underdrains were cleared and access platforms were modified for safe inspection. The media support grid was designed to resist buoyancy, uneven loading and hydraulic forces during a sudden flow increase. Leaving the support structure to the final procurement stage would have created a serious programme and safety risk.

Managing Hydraulics And Airflow

The retrofit changed the hydraulic behaviour of the filters. Plastic media creates lower resistance than compacted rock, so the existing distributor could deliver too much flow in some areas and too little in others. The team measured nozzle discharge, checked distributor rotation and modelled the wetting pattern at minimum, average and peak flow. Flow splitting between the two cells was adjusted with calibrated gates rather than relying on visual balance.

Recirculation was also reviewed. Returning clarified effluent to the filter helped maintain wetting during low-flow periods and diluted peak ammonia loads, but excessive recirculation would increase pumping energy and hydraulic loading. The final operating envelope included a variable recirculation rate controlled by influent flow and ammonia trends.

Air movement was equally important. The original plant relied on natural ventilation through the filter walls and roof openings. After the retrofit, the increased biofilm activity created a greater oxygen demand, particularly as nitrification developed. Measurements confirmed that air paths through the underdrains and media remained open. Where wind conditions were unreliable, adjustable vents and low-energy extraction fans were added.

The design team also reviewed downstream hydraulic constraints. A filter upgrade can increase solids production and alter the load reaching secondary clarifiers. The operator used a structured hydraulic study guide to assess the chlorine contact basin and confirm that the revised peak flow would still receive adequate contact time. This avoided improving biological treatment while creating a disinfection bottleneck.

Commissioning The Biological Process

The first filter was commissioned with a conservative flow ramp. Operators began with primary effluent and moderate recirculation, then increased loading over several weeks. Plastic media does not instantly provide mature nitrification; ammonia-oxidising and nitrite-oxidising organisms need time to establish, especially when wastewater temperature falls.

During start-up, the team monitored dissolved oxygen, ammonia, nitrite, nitrate, pH, alkalinity and temperature. Grab samples were combined with online flow and ammonia data to show whether performance changes were caused by biological development or by distribution problems. Operators also inspected the media for uneven wetting, excessive foam and signs of solids accumulation.

The plant retained a limited chemical dosing capability for alkalinity correction, but it was used only when monitoring demonstrated a real need. This reduced the risk of masking hydraulic defects with chemical treatment. The control system was updated to alarm on low distributor speed, abnormal recirculation flow and rising ammonia, allowing operators to intervene before effluent quality deteriorated.

After the second filter was converted, the plant achieved more stable ammonia removal and a measurable reduction in ponding. Organic loading remained within the design range, while the open media structure made visual inspection easier. The process did not eliminate every operational issue: high storm flows still required careful management, and the secondary clarifiers needed closer solids monitoring during wet weather.

Results, Costs And Transferable Lessons

The retrofit increased effective biological capacity without acquiring additional land. Civil construction was limited to repairs, media supports, access modifications and ventilation improvements. The plant also avoided a complete replacement of the primary treatment and disinfection stages. Keeping one filter online at all times reduced the need for temporary bypass arrangements, although it extended the construction schedule.

Energy use increased modestly because of additional recirculation and ventilation, but this was partly offset by improved treatment stability and fewer emergency interventions. The owner’s financial review included media supply, freight, lifting equipment, disposal of old rock, electrical work, laboratory testing and operator training. These often-overlooked items were significant in the Australian market, where specialist media may be imported and freight costs can vary sharply between capital cities and regional sites.

The operating team identified three durable lessons. First, a plastic media conversion is a hydraulic project as much as a biological one. Second, the existing distributor, underdrain and ventilation arrangements should be tested before the media is ordered. Third, commissioning plans must allow for gradual biofilm maturation rather than promising immediate full-load nitrification.

The retrofit also changed maintenance routines. Operators scheduled regular nozzle checks, tracked pressure and flow indicators, and recorded visual observations from access platforms. Spare distributor components and a small quantity of replacement media were added to the stores inventory. These details helped turn a one-off capital project into a maintainable treatment asset.

For Australian water authorities, the case is especially relevant where population growth is occurring around established works, such as outer Melbourne, western Sydney and the fast-developing corridors near Brisbane. Local odour expectations, seasonal rainfall, trade waste characteristics and state-based licence conditions still need to be assessed at each site. Plastic media is a practical option, but it succeeds when matched to the plant’s actual hydraulic and biological constraints.

The next step for an owner considering the same approach is to complete a measured baseline survey covering flow distribution, media condition, underdrain performance, ventilation, ammonia loading and downstream contact time before preparing the retrofit design.