Stormwater & Wet Weather
Treatment that spends most of the year switched off — online in under fifteen minutes, at rates no conventional clarifier can reach, and stable through the whole event.
An asset that has to earn its keep in a few hundred hours a year
Wet-weather treatment is an unusual duty. The plant may operate for only a few hundred hours a year, always at short notice, always at high flow, and it has to reach compliant quality almost immediately — there is no time to stabilise while the storm passes.
That rules out most conventional processes and it is precisely where ballasted clarification is strongest. Total residence time is 10–15 minutes, and because the ballast inventory is retained between events the process reaches full effluent quality within five to fifteen minutes of start-up. Rates of 60–120 m/h and higher mean a very large flow is handled in a very small structure — which is what makes wet-weather treatment affordable at all.
Combined sewer overflow
A CSO screening and settling chamber removes gross solids and the readily settleable fraction, but leaves suspended solids, bacteria and phosphorus that the receiving water may not tolerate. Adding a high-rate clarification stage on the overflow gives 80–95 % suspended solids removal and a substantial reduction in bacterial load, on a footprint that fits in the space a storm tank already occupies.
Storm tank treatment
Where storm retention tanks already exist, a high-rate unit treating the tank contents on the way out increases the effective capacity of the whole system: the tanks empty faster and are ready again sooner, which matters more than tank volume in a long rainfall sequence.
Peak flow relief at the works
Flow above the biological treatment capacity of a works can be diverted through a high-rate clarifier and returned to the outfall after treatment, protecting the biology from washout while keeping the discharge compliant. The unit can then serve as a tertiary polishing stage in dry weather, so the capital asset works all year instead of a few days of it.
First flush and site runoff
Industrial site runoff, airport and highway drainage and construction dewatering carry a heavy first-flush solids load. Packaged and mobile units handle these duties without permanent civil works, and can be relocated when the site changes.
| Design parameter | Typical range |
|---|---|
| Rise rate | 60–120 m/h, up to 150 m/h short duration |
| Total residence time | 10–15 min |
| Start-up to compliant quality | 5–15 min |
| Suspended solids removal | 80–95 % |
| Total phosphorus removal | 80–95 % |
| Bacterial load reduction | one to two log, coagulation dependent |
| Operating hours per year | typically 100–500 h |
| Automation | Unattended start on level or flow signal |
| Idle-period provisions | Drain-down, flushing sequence, ballast retention |
| Dry-weather alternative duty | Tertiary polishing at reduced rate |
Wet-weather removal efficiency depends heavily on the character of the overflow and on how quickly coagulant dosing responds to the flow ramp.
The failure mode on wet-weather plant is almost never the storm. It is the eleven months in between: dosing lines that block, instruments that drift, pumps that seize. Automated flushing and drain-down sequences, exercised regularly, are part of the design rather than an operational afterthought.
Sizing wet-weather treatment?
Send the overflow hydrograph or the design storm, the receiving water requirement and the space available. We will size the unit and set out how it would be operated.