High-Rate Clarification
Coagulation, flocculation, lamella settling and sludge thickening combined in one compact structure — using recirculated thickened sludge instead of an imported ballast medium.
Density from the process itself
High-rate clarification concentrates four unit operations into a single structure. Rather than relying on the natural settling velocity of a freshly formed chemical floc, the process continuously returns thickened sludge from the base of the tank to the flocculation stage.
Those recirculated solids act as nuclei. Incoming colloids attach to an already dense, well-structured floc population, so the material arriving at the settling zone is heavier, more uniform and far less sensitive to changes in raw water quality than a floc grown from a clean start. Solids concentration in the flocculation zone is raised by one to two orders of magnitude compared with a conventional flocculator.
Because the floc is dense, the settling zone can be loaded at rates that would wash out a conventional clarifier. Inclined lamella modules multiply the effective settling area within the same tank plan area, and a scraper and picket-fence assembly at the base thickens the settled solids in place.
The result is a clarifier occupying roughly one eighth to one tenth of the plan area of a conventional circular clarifier of the same duty, discharging sludge concentrated enough to feed a dewatering unit directly — with no separate gravity thickener in between.
Sludge-driven economics, lime softening, combined clarification and thickening — and any plant that would rather not handle a granular ballast.
Process sequence
- Flash mixingCoagulant — ferric or aluminium salt — is dispersed into the raw water at high velocity gradient. Charge neutralisation destabilises the colloidal fraction. 1–3 minutes.
- Flocculation with sludge recirculationPolymer and thickened sludge returned from the base of the settling tank are mixed with the coagulated water. The high solids inventory drives rapid, reproducible floc growth. 8–15 minutes at a controlled velocity gradient.
- Upflow transitionA slow upward-flow zone completes floc maturation and delivers a uniform velocity profile to the settling zone, preventing short-circuiting.
- SettlingDense floc settles out of the rising water. Because the floc carries recirculated mass, its settling velocity is largely independent of the incoming solids load.
- Lamella refiningClarified water rises through inclined modules set at 55–60°. Lighter carry-over is intercepted on the plate surfaces and slides back down. The projected area of the pack, not the tank plan area, sets the hydraulic capacity.
- Thickening and recirculationA scraper and picket fence consolidate the settled sludge. Part is returned to flocculation; the surplus is drawn off at 20–40 g/L.
| Design parameter | Typical range |
|---|---|
| Lamella rise rate — drinking & process water | 15–40 m/h on projected area |
| Lamella rise rate — municipal wastewater | 25–60 m/h |
| Lamella rise rate — wet weather / peak duty | up to 80–100 m/h |
| Plate inclination | 55–60° |
| Plate spacing | 50–80 mm |
| Coagulation time / velocity gradient | 1–3 min at G 300–800 s⁻¹ |
| Flocculation time / velocity gradient | 8–15 min at G 40–120 s⁻¹ |
| Sludge recirculation rate | 2–6 % of feed flow |
| Thickened sludge concentration | 20–40 g/L (2–4 % DS) |
| Footprint vs conventional clarifier | 1/8 to 1/10 |
| Start-up to design performance | 20–40 min |
| Effluent suspended solids | < 10 mg/L typical |
| Effluent total phosphorus | < 0.5 mg/L; < 0.1 mg/L with two-stage dosing |
| Effluent turbidity, drinking water duty | < 2 NTU |
Ranges are typical engineering values for this process type and are given for orientation only. The design basis for a specific plant is fixed from raw water characterisation, jar or pilot testing and the agreed effluent guarantee.
Where it fits
The platform of choice where sludge handling economics, chemical consumption and operating simplicity matter as much as hydraulic rate.
- Chemically enhanced primary treatment ahead of a biological stage
- Tertiary phosphorus removal and suspended solids polishing
- Surface water clarification and lime softening in drinking water plants
- Combined clarification and sludge thickening duty
- Colour, natural organic matter and micro-algae removal
- Flue gas desulphurisation and other industrial effluents with high dissolved solids
- Capacity uprating where tank volume is fixed and cannot be extended
- Plants that prefer not to handle, store and recover a granular ballast
KWS-HRC builds floc density from recirculated sludge rather than from an imported medium. There is no microsand or magnetite to purchase, store, top up or recover, and no abrasive solid circulating through pumps and valves. Where sludge volume and dewatering economics dominate the life-cycle cost, this is usually the right platform of the three.
Sludge leaves the unit at 2–4 % dry solids — roughly an order of magnitude more concentrated than the underflow of a ballasted clarifier. That removes a gravity thickener from the flow sheet and reduces dewatering equipment size, polymer consumption and cake haulage.
Common questions
How is this different from simply adding lamella plates to a clarifier?
A plate pack only adds settling area. It does nothing about the floc arriving at it, and a light, poorly structured floc will carry over between the plates however many are installed. The high-rate process conditions the floc first: external recirculation raises the solids concentration in the flocculation zone by one to two orders of magnitude, which changes floc density and structure. Conditioning the floc is what makes the higher plate loading achievable — the plates alone are not the mechanism.
Do we still need a separate sludge thickener?
Normally not. The integrated thickening zone discharges at approximately 20–40 g/L, which is within the feed range of a belt press, centrifuge, screw press or drying bed. We confirm the achievable concentration against your sludge characteristics before removing a thickener from the flow sheet.
How does it respond to a sudden change in raw water quality or flow?
The solids inventory in the flocculation zone comes mainly from recirculation rather than from the incoming water, so the process is buffered against feed variation. Effluent quality is normally maintained through step changes in load, and full design performance is re-established within 20–40 minutes of a large upset.
Can it be built into existing tanks?
Yes, in rectangular or circular concrete structures, provided depth is sufficient for the lamella pack, the sludge hopper and the required flocculation volume. Send us the tank drawings and the target duty and we will confirm feasibility before any commercial discussion.
What chemicals are required?
A metal salt coagulant — ferric chloride, ferric sulphate, alum or polyaluminium chloride — and an anionic polymer. Lime or soda ash is added for softening duty. Coagulant demand is typically comparable to, or slightly below, a conventional clarifier treating the same water, because the recirculated sludge improves floc formation at a given dose.
Is high-rate clarification right for your duty?
Send the design flow, raw water analysis and target effluent quality. We will confirm whether this platform fits and what footprint it would need.