A water treatment programme fails in one of three ways. The exchanger scales, the mild steel pits, or the tower grows a biofilm that no biocide seems to touch. Each failure is sold as a separate chemical, and the quotation never states how much the system will consume. That omission is where the money is decided.

Begin with the failure, not the drum

The same phosphonate drum is sold as a cooling tower scale inhibitor, a peroxide stabiliser and a bottle-washing sequestrant. It does all three at three dose rates, so price per kilogram across those uses means nothing. Four facts settle the chemistry: system type, the water analysis including silica, chloride and iron, the metallurgy, and the discharge consent. Softening the make-up shows why. It removes calcium but leaves the alkalinity behind the Langelier Saturation Index, so the exchanger still scales after the softener is paid for.

Coagulation and flocculation are two different jobs

Aluminum Sulfate (CAS 10043-01-3, Al₂(SO₄)₃, MW 342.15) remains the reference coagulant: cheap, predictable, understood by every plant chemist. It hydrolyses into aluminium hydroxide floc that sweeps colloids out of suspension, within a narrow band of pH 5.5 to 7.5. The hidden cost is alkalinity. Each milligram per litre of alum consumes close to 0.45 mg/L as calcium carbonate, so on a soft water the pH collapses, the floc never forms, and the plant buys lime to undo the damage.

Ferrous Sulfate Monohydrate (CAS 13463-43-9, FeSO₄·H₂O, MW 169.92) brings more metal per tonne and a denser floc, but the iron must first be oxidised from ferrous to ferric by chlorination or aeration above pH 8.5. Until then it stays soluble, passes the clarifier and reaches the filters as a brown stain.

Polymer dose is where programmes go wrong. Polyacrylamide (CAS 9003-05-8) bridges particles already destabilised by the coagulant, and a chain of several million molecular weight does that at 0.1 to 1 mg/L. Feed more and the floc saturates with loops that repel the next particle, so turbidity climbs with dose. Where turbidity is low and colour high, charge neutralisation is the better mechanism, and Poly(diallyldimethylammonium chloride) (CAS 26062-79-3) is judged on charge density rather than molecular weight. Polyethyleneimine (CAS 9002-98-6) carries the highest cationic density of the group, for oily effluent and dissolved metals.

Threshold inhibition, chelation and the arithmetic in between

HEDP (1-hydroxyethylidene-1,1-diphosphonic acid, CAS 2809-21-4, C₂H₈O₇P₂, MW 206.03) dominates scale control for a reason unrelated to binding calcium. It adsorbs onto the growing lattice and distorts it, holding 2 to 5 mg/L of active against several hundred mg/L of hardness. That is threshold inhibition: the only economic way to protect a system carrying bulk hardness.

Chelation is stoichiometric, and the arithmetic is unforgiving. Tetrasodium EDTA (CAS 64-02-8, C₁₀H₁₂N₂Na₄O₈, MW 380.17) binds one mole of calcium per mole, so removing one kilogram of calcium carbonate takes roughly 9.5 kg of the salt. Nobody chelates bulk hardness for that reason; EDTA earns its place on iron and manganese carry-over and on cleaning circuits, where the load is small and the alternative is an acid wash.

Where phosphorus is capped the substitute is a biodegradable aminocarboxylate. Tetrasodium GLDA (CAS 51981-21-6, MW 287.2) carries four carboxylate groups, passes ready biodegradability tests, and holds calcium at the alkaline end where EDTA weakens. It costs more per kilogram, though a site paying for phosphorus removal often comes out lower overall. Sodium Trimetaphosphate (CAS 7785-84-4, MW 305.88) belongs to the older threshold family still used in low-pressure boiler water, where the ring hydrolyses to tripolyphosphate.

Azoles, pH and the copper alloy

Copper alloys need a different film from steel, and the azoles supply it. 1H-Benzotriazole (CAS 95-14-7, C₆H₅N₃) chemisorbs onto copper and blocks both anodic and cathodic reactions. Its speciation follows pH — protonated below pH 1, neutral through the working range, anionic above pH 8 — and the film is most coherent between pH 6 and 9. Tolyltriazole (CAS 29385-43-1, C₉H₉N₃, a 4- and 5-methyl mixture melting at 80 to 86 °C) is the more oil-soluble member, specified where free chlorine runs high because it tolerates oxidation better. It is poorly soluble in neutral water, so it is fed alkaline.

The azole is also the first casualty of over-chlorination. A loop carrying 2 mg/L free chlorine at the tower and 0.2 mg/L at the exchanger passes a daily test and still strips the film, because oxidant is consumed on the way round. Sodium Hydroxide (CAS 1310-73-2) and Sodium Carbonate (CAS 497-19-8) hold pH, but both raise the saturation index as effectively as they protect the metal, which makes pH and scale control one decision.

The oxidant demand sets the biocide dose

Chlorine dose is set by demand, not by a residual copied from another plant. Ammonia, sulphide, organics and reduced iron consume oxidant before any of it reaches a surface, and 0.5 mg/L of ammonia converts free chlorine to chloramine within seconds — a far weaker biocide that still reads on a DPD test. This is where Potassium Peroxymonosulfate (CAS 70693-62-8, the active component of the Oxone triple salt, MW 168.17, active oxygen 4.5% or more) justifies its price: it is not deactivated by ammonia, works across a wider pH range, and leaves sulphate rather than the halogenated by-products an AOX consent will not tolerate.

No oxidant rescues a biofilm. The sessile population sits under a matrix that consumes biocide before it arrives, so the programme needs a dispersant to penetrate it and a surface method — deposit weight, ATP, a side-stream coupon — rather than a water sample. Activated Carbon (CAS 64365-11-3) illustrates the same principle in reverse: it dechlorinates well, but adsorbs a non-oxidising biocide just as readily, so a carbon filter downstream of the injection point removes what was paid for.

The boiler is not a cooling tower with a lid

Oxygen is the controlling risk, and Sodium Sulfite (CAS 7757-83-7, Na₂SO₃, MW 126.04) is the cheapest way to remove it: 7.9 mg of sulphite consumes 1 mg of dissolved oxygen, and programmes carry 20 to 30 mg/L residual for what enters with the make-up. It adds dissolved solids, and above roughly 900 psi it decomposes to sulphur dioxide and hydrogen sulphide, which is why high-pressure drums use catalysed alternatives. Condensate is the other half, and no scavenger fixes carbonic acid there.

What a comparable quotation contains

• Active content as acid or salt, and in kilograms of active per wet tonne. HEDP at 60% and at 98% are different products, and a 20% polymer solution is not half the price of a 40% one.
• Dose rate calculated on your analysis, including silica, iron and chloride, at your temperature and cycles of concentration.
• Limits for phosphorus, AOX and quaternary ammonium, plus a monitoring plan with response thresholds.

A water treatment programme is a control loop, and a control loop cannot be bought by the drum. The supplier who states the feed rate for your water, and shows the coupon data that followed, offers something a cheaper quotation cannot answer.