The dose that just passes consent and the dose that wastes thousands a year can look identical on a flow meter. Strategy is the difference: which reagent, dosed where, to what set-point, paced how — and how hard you push for the minimum effective dose. This page is the decision framework that ties the equations, process and control together.
The first and biggest lever on cost, safety and sludge.
Choosing the reagent fixes most of the lifetime cost before a pump is ever sized. Weigh active cost per treated unit, handling and safety, by-product/sludge production, reaction speed and temperature sensitivity together — the cheapest drum is rarely the cheapest duty.
| Duty | Options | Trade-off |
|---|---|---|
| pH up | NaOH vs lime | NaOH: easy to dose, costlier. Lime: cheap, but scaling, slurry handling, more sludge. |
| pH down | H₂SO₄ vs HCl vs CO₂ | CO₂ self-limits near neutral (over-dose-safe) but weaker; mineral acids stronger, harsher. |
| Coagulant | Ferric vs alum vs PAC | Ferric: wide pH band, more sludge. PAC: less alkalinity demand, costlier. |
| Disinfection | NaOCl vs Cl₂ vs ClO₂ vs UV | By-products, residual, CT credit, storage hazard differ widely. |
Where and how many times you inject changes the dose you need.
Single-point dosing is simplest and right for most duties: one well-mixed injection ahead of adequate contact.
Multi-point / split dosing improves results where a single shock dose is wasteful or harmful:
A coarse stage brings pH near target; a polishing stage trims it. Each loop is gentler and far less prone to over-shoot than a single aggressive dose — the standard answer to a steep titration curve.
Returning treated water or reusing spent reagent (e.g. pickle-liquor as coagulant) can cut net reagent demand — check it against stream chemistry.
Aim for the consent edge with a margin you can defend.
Every unit of unnecessary set-point margin is recurring chemical cost. Set the target as close to the limit as your control precision and consequence-of-failure allow — tighter control buys a smaller margin.
Push the dose down deliberately and measure the result.
Re-establish the true demand and over-dose factor seasonally. The OF you set last year may be 20% too high now.
Plot reagent rate against the controlled variable. Flat regions above target are pure waste — reduce the set-point in steps.
Moving from feedback to compound-loop control narrows the band, letting you safely lower the set-point.
Track £/m³ treated, not just litres dosed. Surfaces drift and rewards optimisation; tie into dosing ROI.
Reduce dose in controlled steps with the analyser and consent watched closely. The goal is the minimum effective dose — stop the moment the controlled variable starts to move toward the limit. Cutting too far is just under-dosing.
We review reagent choice, dosing points, set-points and control mode against your consent and bills, then trial a minimum-effective-dose programme. Typical sites recover the review cost in months of reagent savings.
Our expertise spans multiple industries with sector-specific water treatment solutions.