Tertiary Filtration for Wastewater Treatment
The filtration stage that turns biologically treated effluent into discharge- or reuse-ready water — and how to choose between disc, cloth, sand and membrane filters.
Tertiary filtration is the polishing step that removes the residual suspended solids and particulate phosphorus left in secondary (biologically treated) effluent. It is the single most common building block of a tertiary treatment stage, because meeting the tightening EU limits for total phosphorus and suspended solids almost always requires a filter. The four filtration technologies used in practice — disc filtration, cloth media filtration, sand filtration and membrane filtration — all reduce TSS to around or below 10 mg/L, but they differ sharply in footprint, energy use, achievable quality and cost. The right choice depends on your target effluent, available space, flow and whether you also need phosphorus polishing or reuse-quality water. A short structured plant assessment turns those inputs into a concrete shortlist.
Key Numbers
Where tertiary filtration fits
Tertiary filtration sits between the secondary (biological) stage and final discharge or reuse. Secondary treatment removes most dissolved organic load, but it leaves behind fine suspended solids and the particulate phosphorus bound to them. A tertiary filter captures that residual material, and — where dissolved phosphorus is the constraint — works together with an upstream chemical dosing step that converts dissolved phosphorus into a filterable precipitate.
Because so much of the remaining phosphorus and solids load is particulate, filtration is the workhorse of the tertiary stage. For most plants facing the new EU limits, choosing and sizing the right filter is the central engineering decision of the whole upgrade.
The four filtration technologies compared
Disc Filtration
highRotating disc filters with fine-mesh or woven media — the smallest footprint and lowest energy of any tertiary filter. Ideal where space is tight and influent solids are moderate.
- Smallest footprint of all filters
- Near-zero energy (gravity feed)
- No building required
- Modular and easy to retrofit
- TSS and particulate P only
- Not for high-solids influent
- Chemical dosing needed for TP
Cloth Media Filtration
highPile-cloth (textile) discs that polish suspended solids and particulate phosphorus. Very compact and a frequent fastest-win retrofit, closely related to disc filtration.
- Very compact footprint
- Low energy
- Proven retrofit track record
- Good particulate P capture
- Media replacement over time
- Dissolved P needs dosing
- Sensitive to grease/oil loads
Sand Filtration
mediumRobust depth filtration — including continuous-backwash designs — that reliably removes solids. A proven, forgiving workhorse where footprint is less constrained.
- Very robust and forgiving
- Handles variable influent well
- Long, proven service life
- Larger footprint than disc/cloth
- Higher head loss / energy
- More civil works to retrofit
Membrane Filtration (MF/UF)
mediumMicro- or ultrafiltration membranes that provide a near-complete solids and pathogen barrier, producing reuse-quality effluent where standards go beyond nutrient removal.
- Highest effluent quality
- Pathogen barrier — reuse ready
- Very consistent output
- Highest energy and cost
- Fouling management required
- More operator attention
Engineering Insight: match the filter to the constraint
For Existing Plants
Why filtration is becoming unavoidable
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Last reviewed: September 2026. Regulatory requirements may vary by national implementation. This content is for informational purposes and does not constitute legal or engineering advice.