Understanding microplastics in wastewater treatment operations

Microplastics are now a practical wastewater issue rather than a distant environmental concern. Fibres from synthetic clothing, fragments from packaging and particles from tyres enter sewer networks through household wastewater, stormwater inflows and industrial discharges. Once they reach a treatment plant, their behaviour affects screening, biological processes, sludge handling and receiving-water quality.

For Australian utilities, the issue sits within a broader operating environment shaped by water scarcity, recycled-water schemes and strict expectations around coastal and inland waterways. Plants serving Sydney, Melbourne, Brisbane, Perth and smaller regional communities may receive different particle loads depending on catchment land use, rainfall patterns and local sewer design.

Understanding the impact of microplastics on wastewater treatment operations requires more than counting particles in final effluent. Operators need to identify where particles accumulate, how treatment processes redistribute them and which controls reduce release without creating excessive energy, maintenance or disposal costs.

Where microplastics enter the sewer system

Domestic wastewater is a major pathway. Washing polyester, nylon and acrylic garments releases textile fibres, while personal-care products, household dust and degraded plastic goods contribute smaller particles. Commercial laundries, food premises, manufacturing sites and vehicle workshops can add distinctive polymer types and particle shapes to the sewer network.

Stormwater is another important source in combined or poorly separated catchments. Tyre wear, road markings, artificial turf and weathered outdoor plastics can be washed into drains during intense rainfall. In cities such as Melbourne and Sydney, short, heavy storms can produce rapid inflows that carry debris past normal hydraulic assumptions and increase the particle burden reaching preliminary treatment.

Industrial and trade-waste controls can limit some sources, but compliance depends on sampling, communication and enforcement. A catchment assessment should therefore consider population density, textile activity, transport corridors, construction, landfill operations and wet-weather behaviour rather than treating the treatment plant as the only point of control.

How treatment processes redistribute particles

Coarse screens and grit removal capture larger plastic items, films and particles attached to mineral solids. Fine screens and microscreens can improve removal, although performance depends on aperture size, particle shape, flow velocity and the condition of the equipment. Fibres may pass through openings that appear too small for them because they bend, align with the flow or become embedded in organic material.

Primary sedimentation removes many denser particles, while buoyant plastics may float, remain suspended or attach to fats and scum. In activated sludge systems, biofilms and flocs can trap microplastics through adhesion and entanglement. This may reduce the concentration in secondary effluent but transfer the material into waste activated sludge and primary sludge.

Membrane bioreactors generally provide a stronger physical barrier than conventional clarification, especially for larger particles. However, membranes do not eliminate the need for source control. Fibres can accumulate on membrane surfaces, increase fouling and complicate cleaning regimes. Every plant needs site-specific verification because reported removal rates vary with sampling methods and particle size ranges.

Effects on day-to-day plant operations

Microplastics can increase loading on screens, conveyors, compactors and screenings-wash presses. Plastic films and fibrous material may wrap around shafts, foul pumps or interfere with screenings dewatering. These impacts are often mixed with wipes, hair and general debris, making it difficult to assign maintenance events to microplastics alone.

In aeration basins, particles can alter floc structure and settleability when they become incorporated into biomass. Their surfaces may carry metals, hydrocarbons or persistent organic contaminants, creating additional uncertainty for biological health and sludge quality. The effect is usually less dramatic than a toxic industrial shock load, yet chronic exposure can complicate process stability and interpretation of settling problems.

Odour control also deserves attention. Captured plastics mixed with wet organic solids can retain sulfide-producing material and increase handling nuisances. Operators reviewing odour systems may benefit from established guidance on activated carbon filters, while remembering that filtration manages released odours rather than removing the upstream source of contaminated solids.

Implications for sludge and biosolids

Because many microplastics leave the liquid stream through settling, sludge can contain higher concentrations than treated effluent. Dewatering does not destroy the particles; it concentrates them in cake, centrate or filtrate. Centrifuges, belt presses and screw presses can therefore redistribute smaller particles between solids and liquid sidestreams.

The final destination of biosolids is a key consideration. Land application, composting, stockpiling, landfill disposal and thermal treatment each present different risks and regulatory questions. Australia’s biosolids practices vary between states and utilities, so a local risk assessment should consider soil pathways, crop use, transport, storage and community expectations.

Microplastics may also affect corrosion and maintenance indirectly when plastics retain sulfide-rich solids or obstruct flow around wet wells and sewer structures. For background on related sewer asset risks, guidance on hydrogen sulfide corrosion helps place odour, sulfide formation and concrete deterioration within the same asset-management picture.

Measuring performance with useful data

Sampling must define what is being measured. Results can differ greatly depending on whether a programme includes particles below 300 micrometres, counts fibres separately, identifies polymers using spectroscopy or reports only mass. A plant may appear highly effective under one method and less effective under another without any operational change.

Useful monitoring compares influent, primary effluent, secondary effluent, final effluent, return liquors and sludge. Flow-weighted samples are generally more informative than occasional grab samples, particularly where rainfall changes the catchment load. Field blanks, equipment cleaning and non-plastic sampling materials are essential because airborne fibres and contaminated clothing can distort results.

Operators should connect laboratory findings with operational records. Screenings quantities, pump blockages, membrane pressure, sludge settleability, polymer consumption and wet-weather flows can reveal whether particle loads have a measurable plant effect. Australian laboratories and utilities are still developing consistent methods, so transparent quality assurance is as important as the particle count itself.

Practical controls for Australian utilities

Source reduction usually offers better value than relying on a treatment barrier alone. Public messaging about laundry bags, reduced use of disposable plastics and correct disposal of wipes can support broader behaviour change. Councils can also work with laundries, manufacturers and shopping centres on lint capture, trade-waste requirements and improved housekeeping.

At the plant, controls may include finer preliminary screening, improved screenings removal, optimised primary clarification, tertiary filtration or membrane treatment. These options involve capital and energy costs, and finer screens can increase headloss or maintenance. A staged trial is preferable to installing equipment based solely on results from another climate or catchment.

Australian conditions should shape decisions. Water corporations managing drought-sensitive supplies may prioritise advanced treatment where recycled water is produced, while coastal councils may focus on reducing discharge to estuaries and beaches. In Perth, infiltration and inflow patterns differ from those in Brisbane, where intense summer rainfall can produce sharp hydraulic peaks. Procurement should also account for local service capability, imported equipment lead times and the requirements of state environmental regulators.

Building capability across the water sector

Microplastics cross professional boundaries. Laboratory specialists need reliable samples, operators need workable controls, engineers need lifecycle data, and managers need a defensible basis for investment. Training that links polymer science with pumps, screens, biological treatment and sludge logistics is more useful than treating the subject as a standalone environmental topic.

Professional networks can accelerate that learning. Organisations such as LABS of CWEA connect water and wastewater practitioners through technical presentations, facility tours, workshops and professional development. Australian teams can gain value from these exchanges when adapting overseas experience to local sewer configurations, climate conditions and recycled-water objectives.

The immediate goal is not to promise complete removal from every plant. It is to understand the particle pathways, prevent avoidable inputs, protect equipment, manage contaminated solids responsibly and establish consistent monitoring. Over time, shared methods will make comparisons between Australian utilities more meaningful and help regulators set proportionate expectations.

A sound operational approach begins with a catchment source assessment, followed by targeted sampling across liquid and solids streams. The practical takeaway is to treat microplastics as a whole-system management issue: reduce them at the source, track where they move, and select treatment upgrades only where measured risk justifies the cost.