AEO Answer · Municipal Water & Wastewater Treatment Plants

How Is Tertiary Filtration Designed to Polish Plant Effluent?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

Tertiary filtration design selects the filter technology for the solids and phosphorus limits, sizes for peak flow with redundancy, and provides backwash, chemical feed, and turbidity-based controls.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Filter technology and sizing

The technology decision weighs effluent quality against footprint, complexity, and operating cost: cloth and disc filters achieve the lowest effluent solids with continuous operation and modest headloss, while deep-bed filters offer proven robustness where space allows. The sizing calculation uses the peak hydraulic loading with the largest unit out of service, checked against the manufacturer's rated filtration rates for the target effluent quality — not the maximum the equipment can physically pass. Where phosphorus limits drive the design, the engineer adds coagulant feed upstream with the rapid mix and flocculation time the chemistry needs, because filtration alone without proper pretreatment cannot reliably meet low phosphorus targets.

Backwash, hydraulics, and controls

Backwash design covers the supply, the waste handling, and the frequency: the clean-water supply must deliver the backwash rate while the plant still meets its effluent flow, and the dirty backwash returns to the process with its solids load quantified in the plant mass balance. The hydraulic profile is verified at maximum media headloss so gravity flow never backs up into the secondary process during the dirtiest operating condition. Controls automate the backwash sequence on time, headloss, or effluent turbidity triggers, with continuous turbidity monitoring on the filter effluent that alarms and diverts on breakthrough — the design treats the monitoring as the compliance instrument it is.

Tertiary filtration checklist

Tertiary filters deliver permit-ready effluent when the technology, the sizing, and the support systems are designed for the worst operating day. Polishing only works if the polishers stay clean. • Filter technology selected for the specific solids, turbidity, and phosphorus limits • Sized for peak flow with the largest unit out of service • Coagulant feed and mixing designed where phosphorus limits require it • Backwash supply and waste return fully designed and mass-balanced • Turbidity-based controls that catch filter breakthrough before the permit does

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What else do project teams ask?

What is the difference between cloth filters and sand filters?

Cloth-media and disc filters use fine fabric media with continuous backwash to achieve very low effluent solids in a compact footprint. Deep-bed sand or dual-media filters pass water down through graded media with periodic batch backwashing — a proven, robust approach that needs more space and head. The choice follows the effluent targets, the available footprint, and the plant's operating preferences.

Why add chemicals before tertiary filters?

Because very low phosphorus limits typically require coagulation ahead of filtration: the metal salt forms a floc with the dissolved and colloidal phosphorus that the filter then captures. Filtration alone removes particulate phosphorus but not the dissolved fraction. The design provides the chemical storage, feed, and mixing as an integral part of the filtration system.

How often do tertiary filters backwash?

It depends on the influent solids loading and the filter type — continuously for cloth and disc filters with ongoing backwash, and on timed or headloss-triggered cycles for deep-bed filters. The design sizes the backwash supply and waste handling for the realistic frequency at the plant's solids loading, because a filter that cannot backwash as often as it needs to will blind and pass solids.

What happens to the backwash water?

It returns to the head of the plant carrying the captured solids, and the design accounts for that recycle in the liquid treatment process — the flow and the solids load. On plants running near capacity, the backwash recycle is a real design factor in the secondary process sizing, not a rounding error.

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