How to select a coagulant by jar test rather than price per kilogram: ferric chloride, PAC and aluminium sulfate compared, plus cost per cubic metre.
Coagulant is the single largest line in most reagent budgets — and it is usually selected on price per kilogram, which almost always means overpaying. Here is how the choice is made technically: which water parameters drive it, how the main coagulants differ, and why the number that matters is cost per cubic metre treated.
What a coagulant actually does
Suspended particles and colloids carry a negative charge and repel each other, so they never settle on their own. The coagulant introduces a multivalent cation (Fe³⁺ or Al³⁺) that neutralises this charge, and particles begin to agglomerate. A metal hydroxide forms in parallel and sweeps contaminants down mechanically. The flocculant follows, growing the resulting microflocs into something large enough to settle within the clarifier's retention time.
Three parameters that decide the choice
- pH. The dominant one. Every coagulant has a working window; outside it the hydroxide does not form and the reagent is wasted. Iron works over a wider range than aluminium.
- Alkalinity. Coagulation consumes alkalinity. If there is too little, pH collapses after dosing and the process stalls — you need to add caustic soda or lime.
- Temperature. Floc formation slows in cold water. Winter dose is almost always higher than summer dose, and on sites in Uzbekistan with a strong seasonal swing the difference is noticeable.
On top of that you look at the nature of the load: turbidity, colour, phosphate content, presence of hydrogen sulfide.
Comparing the main coagulants
| Coagulant | Working pH | Typical dose | Sludge | Strength |
|---|---|---|---|---|
| Ferric chloride (III) | 4.0–11.0 | 100–600 mg/l | Dense, high volume | Colour, phosphate, H₂S |
| PAC | 5.0–9.0 | 50–400 mg/l | Lower volume | Stability under pH swings |
| Aluminium sulfate | 5.5–7.5 | 100–500 mg/l | Loose | Price, availability |
Practical differences the table does not show: iron tints the water if overdosed and is harder on dosing equipment materials; PAC forgives unstable pH and produces less sludge, which cuts dewatering cost; aluminium sulfate is cheaper but its narrow pH window makes it awkward on variable water.
The jar test: how the choice is made in practice
No table replaces a trial on your own water. A jar test takes a few hours and produces numbers you can actually procure against.
1. Sampling
The sample has to be representative — taken at peak load, not when the plant is idle. For effluent with strong daily variation, take several samples.
2. Dose series
Equal volumes in five or six beakers with an increasing coagulant dose. Rapid mix 1–2 minutes, slow mix 10–15 minutes, settle 30 minutes.
3. Checking the pH corridor
Repeat the best dose at several pH values. It often turns out that correcting pH saves more reagent than switching reagent.
4. Measurement
Residual turbidity, colour, pH, and settled sludge volume. Sludge volume is routinely ignored — yet it drives dewatering and disposal cost.
5. Converting to money
Turn the working dose into cost per cubic metre. That is the only figure on which options compare correctly.
Why price per kilogram misleads
An illustrative example at 1,000 m³/day:
| Reagent A | Reagent B | |
|---|---|---|
| Price | 0.35 /kg | 0.55 /kg |
| Working dose from jar test | 300 mg/l | 150 mg/l |
| Consumption per m³ | 0.3 kg | 0.15 kg |
| Cost per m³ | 0.105 | 0.083 |
| Per year at 1,000 m³/day | ≈ 38,300 | ≈ 30,100 |
A reagent that costs 57 % more per kilogram runs 21 % cheaper. And that is before sludge volume — if B also produces less, the gap widens. The figures are illustrative, but the ratio is typical.
When you need a flocculant
The coagulant makes microflocs; the flocculant makes them settle. If water stays cloudy after coagulation and the sludge will not compact, what is missing is flocculant, not more coagulant. Anionic polyacrylamide normally follows iron salts, while cationic is used for sludge dewatering. Flocculant doses are two to three orders of magnitude lower: 0.5–3 mg/l.
Four common mistakes
- Increasing dose instead of correcting pH. If the coagulant is working outside its window, more dose barely helps — this is the most common source of overspend.
- Not revisiting dose between seasons. Cold water needs a different dose. A setting made once at commissioning holds until the first cold spell.
- Ignoring sludge volume. A cheap coagulant with heavy sludge simply moves the cost to dewatering and disposal.
- Comparing on the datasheet rather than the jar test. Active content differs between suppliers, so an identical mg/l dose can mean a different amount of active metal.
Frequently asked questions
Can coagulants be combined?
Yes, and on difficult effluent a combination often beats a single reagent — PAC for the bulk load plus a small iron dose for colour, for instance. But they are dosed separately at different points, never premixed in one tank.
How do you know the dose is too high?
Signs: water gets cloudier instead of clearer past a certain dose (charge reversal), sludge volume grows without any quality gain, pH drops further than calculated. In a jar test this shows up as the curve turning back.
How often should dose be reviewed?
At season change, when raw water composition or the production programme changes, and on a new reagent batch if the concentration differs. On a stable site, a check jar test once a quarter.
What do you need from us to make a recommendation?
A water analysis (turbidity, colour, pH, alkalinity, COD if available), flow in m³/day, and the discharge point or intended use of the treated water. That is enough for an initial selection and a cost estimate.
Selection against your own water
Aqua Global runs the jar test on a sample of your water, returns the working dose and cost per cubic metre for each option, and supplies the selected reagent from Tashkent stock. The coagulant and flocculant range is in products; send a sample and analysis through the contact form.
