Coagulant, flocculant, antiscalant: what is dosed and where

Three reagents appear in almost every water treatment and wastewater scheme, and they are constantly confused with one another. Their tasks are different, their injection points are different, and the consequences of a mistake are the same: the line works worse than the design intended and chemical consumption goes up. Here is what each of them does, where it is injected and how the dose is set.

Three reagents, three different tasks

A coagulant works on charge. Fine suspended matter does not settle, not because it is light, but because the particles carry an electric charge of the same sign and push each other apart. The coagulant neutralises that charge, the particles stop repelling one another and stick together into microflocs. The reaction is fast and needs intense mixing: the reagent has to be spread through the whole volume before it can react in one spot.

A flocculant works on size. It is a long polymer molecule that catches microflocs at different points along itself and stitches them into a large floc, visible to the eye and mechanically strong. Such a floc can already be removed by flotation or settled in a clarifier. The mixing needed here is the opposite — slow: an intense mixer will tear apart what has already come together.

An antiscalant is a reagent that stops salts precipitating on the membrane. It precipitates nothing and takes no part in phase separation. On the contrary: it holds hardness salts and other sparingly soluble compounds in solution while the concentrate travels along the membrane and grows steadily more concentrated. The task is to get the water out through the concentrate discharge before the salt starts growing on the membrane surface.

Hence a simple rule. The first two reagents are needed so that the contamination drops out of the water and is removed mechanically. The third is needed so that salt does not drop out where there is nothing to remove it with.

The injection point matters no less than the dose

A reagent injected at the correct dose but into the wrong point most often does not work at all.

✓Coagulant — into the rapid mixing chamber at the inlet, before the flocculant and before any phase separation.
✓Flocculant — after the coagulant, into the slow mixing zone immediately ahead of the flotation unit or the clarifier. Injecting flocculant ahead of the coagulant is pointless: there is nothing to stitch together yet.
✓Flocculant into the sludge — a separate point. Ahead of dewatering the sludge is flocculated again: a screw press takes the slurry from the flocculation chamber, and without it the pressing works noticeably worse.
✓Antiscalant — after mechanical pretreatment and before the high-pressure pump of the membrane unit. If it is injected ahead of the cartridge filter, part of the reagent stays on the cartridge.

On packaged equipment some of these points are already provided for. In DAF flotation units the coagulation and flocculation chambers are built into the body together with the clarified water tank — this saves floor space and removes the question of where exactly to tie in. In membrane units a reagent inlet is provided in the pipework, and there is a separate CIP inlet for chemical cleaning.

What a dosing station consists of

A dosing station is a dosing pump, a tank for the working solution and the pipework, assembled into a single unit. Instead of three separate devices that have to be linked together on site, the site gets one ready-made assembly. It corrects pH, reduces the corrosivity of the water, inhibits salt precipitation and handles chemical cleaning of membrane elements.

Power supply is 220 V AC, 50/60 Hz. The station is installed in a dry room away from heat sources: a reagent solution and heating nearby do not go together.

The versions differ in three things, and the station is selected by the same three:

✓Tank volume — 60, 100 or 200 l. What drives it is not reagent consumption directly, but the concentration of the working solution and how often the staff are willing to make that solution up. A 60 l tank that has to be charged every other day is in practice worse than a 200 l tank once a week.
✓Connection size — from dn15 to dn50, according to the pipe diameter at the tie-in point.
✓Pump type — digital or analogue. The HG30 versions carry digital dosing pumps: the dose and the feed mode are set from the panel. The HP30 versions carry analogue ones, adjusted with a regulator.

The difference between a digital and an analogue pump comes down to whether the water flow is constant. With a steady flow an analogue pump, set once, will run for a year. With an uneven flow the dose set for the average flow will be alternately excessive and insufficient, and here a digital pump with an adjustable feed mode is needed.

The stations are not heavy: the HG30 version with a 100 l tank and a dn20 connection weighs 21.65 kg, the same with dn25 weighs 23.3 kg. A unit like this needs no separate foundation, but it does need room to work around and clear access to the tank filler neck.

The dose is found by jar testing

The coagulant dose is not derived from the reagent datasheet. It depends on the composition of the particular water: on the quantity and nature of the suspended matter, on organics, on alkalinity, on temperature and on pH. The same water in winter and in summer needs a different dose, and the wastewater from one workshop changes character when the product changes.

That is why the dose is found by jar testing. In the lab you take several identical samples of the water, give each a different dose of reagent, stir them all to the same programme and look for the jar where the floc comes out large, settles fast and leaves clear water above it. The same series shows the working pH window: coagulants only work within a narrow band, and outside it the reagent you add simply goes into the sludge with no effect.

For the flocculant the test is repeated separately: the grade and the dose are matched to the floc produced by the chosen coagulant. The pair "coagulant plus flocculant" is selected together, not separately.

Jar testing is repeated when the raw water changes noticeably and when the reagent supplier changes. That is cheaper than spending months overpaying for an overdose.

Typical mistakes

✓Coagulant overdose. Beyond a certain dose the particle charge is not neutralised but flipped to the opposite sign — the suspension is stabilised again, the water goes turbid, and the reagent is wasted.
✓Injecting flocculant into a turbulent section. The stitched floc is torn apart by a pump, a tight elbow or an intense mixer, and it does not re-form.
✓One station for two incompatible reagents. Residues of the previous solution in the tank and the line react with the new one, a precipitate forms and the injector clogs.
✓Dosing without pH correction. Acidic coagulants push the pH down; if the water's alkalinity is not enough to compensate, a second station for a correcting reagent is needed.
✓Sizing the tank by reagent volume rather than by solution concentration. The count is in litres of neat product, but what goes into the tank is a diluted solution, and the tank turns out too small.
✓Antiscalant instead of pretreatment. The reagent holds salts in solution but does not remove suspended matter and iron. It will not save a membrane fouled by solids.

What next

Sizing a station needs four things: which reagent you are dosing; at which point and into what pipe (the tie-in diameter); the water flow at that point; and how steady that flow is. If the reagent has not been chosen yet, start with a water analysis and a jar test — until then there is nothing to size.

With this data, look at dosing stations: the HG30 versions with a digital pump are for uneven flow and membrane schemes, the HP30 with an analogue pump for steady flow, and the tank volume and connection are matched to the site.