UK works receive somewhere between 5,000 and 50,000 microplastic particles per cubic metre. Most are removed — but removal here means transfer to sludge, not destruction. Between 50 and 80 per cent of what arrives leaves in the solids.
Shape and density decide everything downstream
Microplastics are not a single contaminant with a single behaviour. Their removal is governed by settling velocity, and settling velocity is governed by shape and density — which vary enormously across the categories a works actually receives.
| Type | Typical density | Hydraulic behaviour |
|---|---|---|
| Fibres (polyester, acrylic) | 1.2–1.4 g/cm³ | Align with streamlines; settle very poorly despite being denser than water |
| Fragments (PE, PP, PS) | 0.9–1.1 g/cm³ | Float or settle slowly |
| Tyre wear particles | ≈1.8 g/cm³ | Settle readily |
| Microbeads (PE, PP) | 0.91–0.96 g/cm³ | Float |
Primary settlement
Capture in an ideal horizontal-flow clarifier is the ratio of particle settling velocity to overflow rate:
η = vs / v0 = vsA / Qv0 = overflow rate (m/h); A = surface area; Q = flow
At a typical overflow rate of 1.5 m/h the spread across particle types is stark:
| Particle type | Settling velocity | Theoretical capture |
|---|---|---|
| Tyre wear | ≈10 m/h | ≈100% |
| Dense fragments | ≈0.5 m/h | ≈33% |
| Fibres | ≈0.05 m/h | ≈3% |
Overall primary removal lands at 20–50 per cent, weighted by whatever mix arrives. Ferric dosing at 5–10 mg/L lifts fibre capture to 40–60 per cent by enmeshing them in sweep floc — the single most effective intervention available at the front of the works, and one that costs nothing extra where coagulant is already dosed for phosphorus. See coagulation and flocculation and primary clarifier design.
Secondary treatment: incorporation, not degradation
In the aeration tank microplastics do not degrade. They become floc-associated by two routes: biofilm colonises the particle surface and raises its effective density, and hydrophobic polymers adsorb extracellular proteins that make them wettable and floc-compatible. Both mechanisms move plastic from the water into the solids.
| Configuration | Microplastic removal | Note |
|---|---|---|
| Conventional activated sludge with clarification | 40–70% | Limited by clarifier capture of the finer fraction |
| Membrane bioreactor | >99% above 0.1 µm | Absolute barrier; nanoplastics below the pore size may pass |
The MBR result is the cleanest available, and it also concentrates the entire captured load into the waste sludge — which is where the problem then has to be managed. Configuration detail in membrane bioreactor design.
Tertiary polishing
| Process | Removal | Works best on | Weakness |
|---|---|---|---|
| Dissolved air flotation | 70–90% | Low-density PE and PP | Less effective on dense fragments and tyre particles |
| Rapid sand filtration | 80–95% above 20 µm | Fragments and beads | Fibres pass; nanoplastics pass |
| Disc filter, 10 µm | 90–95% | General polishing | Head loss rises steeply at finer mesh |
| Disc filter, 5 µm | >95% | Sensitive receiving waters | High head loss and backwash demand |
Flotation is the natural fit for the buoyant fraction that settlement cannot touch — the same air-to-solids logic as any other DAF duty, at ratios around 0.05–0.1. See what is a DAF system and media filtration design.
Sludge is the destination
Everything removed from the water is still on site. Between 50 and 80 per cent of the influent microplastic load accumulates in sewage sludge.
- 100,000 × 30 mg/d = 3,000,000 mg/d = 3 kg/d.
- Over a year: ≈1.1 tonnes of microplastic in the sludge stream.
That tonne has to go somewhere, and the sludge route decides the environmental outcome rather than the treatment train does:
- Agricultural application transfers the whole load to soil, where the polymer persists.
- Incineration destroys the polymer matrix, provided flue-gas filtration is adequate to retain fibres.
See biosolids land application and sludge incineration and energy recovery.
Design rules
- Treat fibres as the design case. They are the commonest type in UK wastewater and the hardest to settle; a train sized on fragments will underperform.
- Dose coagulant at the primary stage if microplastics matter. Lifting fibre capture from about 3 per cent to 40–60 per cent is the cheapest intervention available.
- Use multiple barriers with different failure modes. Settlement plus flotation plus fine filtration; no single stage exceeds about 95 per cent.
- Do not describe transfer to sludge as removal in any assessment. The load leaves the works either way; only the sludge route determines the fate.
- Decide the sludge route before selecting an MBR. The highest-capture option also produces the most concentrated microplastic solids.
Frequently asked questions
Why do fibres pass through clarifiers when they are denser than water?
Because settling depends on shape as well as density. A long thin fibre aligns with the flow and follows streamlines rather than crossing them, so its effective settling velocity is around 0.05 m/h — against an overflow rate of about 1.5 m/h, which gives roughly 3 per cent capture. Coagulation solves it by enmeshing fibres in floc.
Does an MBR solve the microplastic problem?
It solves the effluent side, retaining better than 99 per cent above 0.1 µm. It does not reduce the load; it concentrates it into the waste sludge. Whether that counts as a solution depends entirely on the sludge disposal route.
Are microplastics degraded during activated sludge treatment?
No. They are colonised by biofilm and adsorb extracellular proteins, which makes them heavier and more floc-associated, so they transfer to the solids. The polymer itself is unchanged.
Should sludge containing microplastics still go to land?
That is the decision that determines the environmental outcome. Land application transfers roughly a tonne a year, for a 100,000 PE works, directly to soil where it persists. Incineration destroys the polymer, provided the flue-gas filtration retains fibres. There is no current UK microplastic limit for biosolids, so this is a judgement about future risk rather than present compliance.