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Retrofitting existing basins with disc filters instead of new civil works
Capacity constraints at wastewater treatment plants rarely arrive with a convenient budget attached. Production volumes grow, discharge permits tighten, or population equivalents increase — and the secondary clarifier that was sized for yesterday's load is suddenly the bottleneck. Building a new basin is the textbook answer, but civil works take time, consume capital, and often require permits that add months to the schedule.
A practical alternative is to install pile cloth media disc filters directly inside an existing basin — reusing the concrete shell, the inlet pipework, and in many cases the existing effluent channel. This approach has been applied successfully in tertiary filtration upgrades and in situations where a lamella separator is added to an existing settling tank to recover hydraulic capacity.
Retrofit at a Glance
- Approach
- Install disc filters or lamella modules inside existing basin
- Typical footprint saving
- 60–80 % versus new-build equivalent
- Civil works required
- Anchor points, inlet baffle, effluent weir adjustment
- Installation time
- 2–6 weeks depending on basin size
- Downtime
- Typically one planned shutdown of 3–10 days
Why the existing basin is often good enough
A secondary clarifier or an old sand filter basin is a reinforced concrete structure designed to hold water under hydraulic load. The walls, floor, and inlet works are already there. What changes when you retrofit is the internal hydraulics: the new filter modules create a defined flow path from the inlet zone through the filter medium to the filtrate channel.
Pile cloth media disc filters operate by drawing water through a textile medium mounted on rotating discs. Solids accumulate on the outside of the cloth and are removed by a backwash system that returns a small reject fraction — typically 2–5 % of throughput — to the head of the plant. The discs sit on a central shaft supported by end frames that are anchored to the basin floor and walls. No new roof, no new drainage, no new access road.
When lamella modules are the better fit
If the goal is to recover settling capacity rather than to add filtration, lamella separator modules can be installed inside an existing rectangular basin. The inclined plates increase the effective settling area by a factor of 5–8 without enlarging the footprint. This is particularly useful when the basin already has a sludge scraper that can be retained.
What to check before committing to a retrofit
Not every basin is a good candidate. A structured assessment covers four areas:
Pre-Retrofit Assessment Checklist
- Basin geometry: minimum water depth ≥ 2.5 m for disc filter installation; rectangular plan preferred
- Structural condition: concrete survey for cracks, spalling, and rebar corrosion; load-bearing capacity for module weight
- Inlet hydraulics: peak flow rate and flow distribution across the basin width; baffling may be needed
- Effluent channel: weir level and capacity to handle filtered flow without backing up
- Backwash return: route and capacity for 2–5 % reject flow back to inlet works
- Electrical supply: 400 V three-phase within reach; control cabinet location
Hydraulic capacity: what the numbers look like
| Basin footprint (m²) | Disc filter modules | Hydraulic capacity (m³/h) | Effluent TSS target |
|---|---|---|---|
| 50 | 2 | 80–120 | ≤ 10 mg/l |
| 100 | 4 | 160–240 | ≤ 10 mg/l |
| 200 | 8 | 320–480 | ≤ 5 mg/l with coagulant |
These figures assume a surface loading rate of 8–12 m/h on the cloth area, which is the normal operating range for pile cloth media. During backwash cycles — which last 30–60 seconds per disc — the effective throughput drops briefly; the control system compensates by adjusting disc rotation speed.
Installation sequence and downtime
- 1Basin survey and anchor design
- 2Prefabrication of module frames and shaft assemblies
- 3Basin dewatering and anchor installation (3–5 days)
- 4Module lifting and assembly inside basin
- 5Pipework connections and electrical commissioning
- 6Wet commissioning and performance verification
The critical path is the basin dewatering phase. Plants that can divert flow to a parallel basin or a temporary bypass keep the shutdown to 3–5 days. Where no bypass exists, a rental disc filter or a temporary cloth filter unit can maintain treatment continuity during installation.
Avoid undersizing the backwash return line
A common retrofit mistake is routing the backwash reject into a downstream channel that cannot handle the additional load. The reject stream is concentrated — TSS can reach 500–2 000 mg/l — and must return to a point upstream of the primary treatment stage. Size the return line for peak backwash flow, not average flow.
Comparing retrofit versus new-build
The economic case for retrofitting rests on three factors: avoided civil works cost, shorter delivery time, and lower permitting risk. Civil works for a new tertiary filtration basin typically account for 40–60 % of total project cost. A retrofit eliminates most of that. Lead time from order to commissioning is typically 3–6 months for a retrofit versus 18–36 months for a new-build including permitting.
The trade-off is that the existing basin constrains the design. If the basin is too shallow, too narrow, or structurally compromised, the retrofit option closes. A thorough survey at the start of the project avoids late-stage surprises.
Can a circular secondary clarifier be retrofitted with disc filters?+
Circular basins can accommodate disc filters, but the installation is more complex than in a rectangular basin. The central column and rotating scraper mechanism must be assessed for compatibility. In many cases a rectangular insert frame is fabricated to create a defined flow zone within the circular shell.
What effluent quality can be expected after retrofitting with pile cloth media?+
Under stable inlet conditions with secondary effluent TSS of 20–40 mg/l, pile cloth disc filters consistently achieve outlet TSS below 10 mg/l. With upstream coagulant dosing, values below 5 mg/l and turbidity below 1 NTU are achievable. Performance depends on inlet load variability and cloth maintenance.
How much energy does a retrofitted disc filter system consume?+
Pile cloth media disc filters are among the lowest-energy tertiary filtration options. Typical specific energy consumption is 0.02–0.05 kWh/m³ treated, covering disc rotation and backwash pump operation. This compares favourably with sand filters at 0.05–0.15 kWh/m³.
Equipment we supply for this
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.