The situation is familiar: the permit is renewed, the TSS limit drops from 15 or 10 mg/l to 5 mg/l, and the existing secondary clarifier holds the value on a dry Tuesday but not during a storm event. Building a second clarifier solves it - and consumes a civil works budget that usually is not available.
Tertiary filtration is the pragmatic answer. It is installed after the biological stage, it removes the residual floc that escapes the clarifier, and it can normally be retrofitted into an existing structure.
Typical design case
- Inlet TSS
- 10-25 mg/l
- Target effluent TSS
- < 5 mg/l
- Peak factor to be covered
- 1.5-2.5 x dry weather flow
- Filtration stage
- pile cloth media disc filter
Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.
Why the clarifier fails first at peak flow
A secondary clarifier separates by gravity, so its performance depends on surface loading and on the settling behaviour of the sludge. Two effects usually cause the exceedance:
- Rising surface loading during rain events reduces the effective settling time.
- Filamentous bulking or a raised sludge blanket lets fine floc escape with the overflow.
Both are hydraulic and biological in origin - they cannot be dosed away. A downstream barrier that is independent of settling velocity is what stabilises the value.
What pile cloth media filtration does differently
Pile cloth media filters use textile discs with a raised pile surface. The water passes from the outside to the inside; solids are retained in and on the pile structure, and the filtrate leaves through the hollow shaft. When the differential level rises, only the affected segment is backwashed while the filter keeps running.
< 5 mg/l
achievable effluent TSS in tertiary duty
< 0.01 kWh/m3
typical specific energy demand
< 1 %
backwash water as share of throughput
Why the low backwash share matters
Backwash water is returned to the head of the plant. A filter with a 5 % reject rate quietly increases the internal load on your biology; a filter below 1 % does not.
Sizing: the four figures that decide everything
Filter area follows from hydraulic load and permissible surface loading rate, which in turn depends on the solids load. A supplier cannot size a stage from an average value alone.
Data to collect before asking for a proposal
- Daily flow and peak hourly flow, ideally as a 24 h profile
- Inlet TSS as a range, not only the annual average - state the 85th percentile if you have it
- Required effluent value and the averaging period of the permit
- Available basin geometry, water levels and the position of the outlet structure
- Whether coagulant or polymer is dosed upstream
A single grab sample is not a design basis
Filtration stages are dimensioned for the load peak. If sizing is based on the mean value, the stage will look attractive in the quotation and fail at the first storm event.
Retrofit or new structure?
Disc filters are available as tank-mounted units and as in-basin variants. If a redundant clarifier, an old sand filter chamber or a disused basin exists, the in-basin variant usually avoids civil works completely.
| Situation | Recommended arrangement |
|---|---|
| Existing empty basin available | in-basin disc filter, no new structure |
| No basin, limited footprint | tank-mounted vertical disc filter |
| Very low headroom | horizontal disc filter |
| Phosphorus limit in addition | filter plus upstream coagulant dosing |
Adding phosphorus removal in the same step
If the permit also tightens total phosphorus, the same filtration stage can carry the precipitate. Metal salt is dosed upstream, the precipitated phosphate is bound in the floc and removed with the solids. Filtration therefore serves two limits with one stage - which is normally what makes the investment pay.
Equipment we supply for this
Pile Cloth Media Disc Filter
Vertical and horizontal cloth filters for tertiary filtration
Technical dataVertical Pile Cloth Disc Filter
Compact vertical disc filter for tertiary treatment and water reuse
Technical dataDosing Stations
Precise dosing of coagulants, flocculants and pH correction chemicals
Technical dataCan pile cloth media filters really replace a sand filter?+
In tertiary duty, yes. They need considerably less footprint, use far less energy and return less backwash water. Sand filters remain relevant where deep-bed filtration with long filter runs is genuinely required.
What effluent TSS is realistic?+
With a stable biological stage, values below 5 mg/l are routinely achieved; below 2 mg/l is possible with coagulant support and moderate loading.
How long does a retrofit take?+
The equipment lead time dominates. Where an existing basin is reused, mechanical installation is typically a matter of days rather than months.
Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.