Water Treatment Chemistry Concepts: Scaling, Corrosion, Disinfection and Coagulation

Water treatment chemistry comes down to 13 concepts in 4 groups, scaling, corrosion, disinfection and coagulation, plus chemical compatibility, and each one is a reaction that an operator starts, stops or steers by dosing a chemical. Hypochlorous acid is about 50 % of free chlorine at pH 7.5 and only about 10 % at pH 8.5 (25 °C, 77 °F), so which reaction decides the dose in your system?

Scaling covers scale formation, the Langelier Saturation Index, cycles of concentration and threshold inhibition; corrosion covers corrosion measurement and MIC; disinfection covers chlorination, breakpoint, chlorine residual and CT values; coagulation covers coagulation and flocculation and the jar test. Compatibility closes the list, and these reactions explain how each of the 16 families of water treatment chemicals works.

This index gives one governing number per concept, a sheet of 12 formulas, the bases every dose needs (1 gpg = 17.1 mg/L as CaCO3) and the regulatory anchors: an MRDL of 4.0 mg/L as Cl2 (40 CFR 141.65), a CT of 104 mg-min/L for 3-log Giardia at 10 °C (50 °F) and a breakpoint of 7.6:1 Cl2:NH3-N.

Every equation and value on this page is checked against primary sources (eCFR, US EPA, ASTM, Health Canada, HSE and the cited handbooks). See our methodology and fact-checking process.

What Is Water Treatment Chemistry?#

Water treatment chemistry is the set of reactions in industrial, municipal and utility water that treatment chemicals control: precipitation of scale, electrochemical corrosion of metals, oxidation of microbes and contaminants, and charge neutralization of particles. The same equations describe them in a cooling tower, a boiler, an RO train and a filter plant.

US federal rules define two of these reactions in law. Coagulation is the process of using coagulant chemicals and mixing by which colloidal and suspended materials are destabilized and agglomerated into flocs, and flocculation is the gentle stirring that agglomerates those particles into larger, settleable ones (40 CFR 141.2). The same section separates conventional filtration from direct filtration, which omits sedimentation.

Which chemical reactions does water treatment control?#

Water treatment controls 5 reaction classes: precipitation, electrochemical corrosion, oxidation, charge neutralization and the unwanted reactions between incompatible chemicals. The 5 classes are listed below in page order.

• Precipitation: calcium carbonate, calcium phosphate, calcium sulfate and silica leave solution and form scale.
• Electrochemical corrosion: the local pH at cathodic sites runs 1 to 2 pH units above the bulk water (Veolia Water Technologies Handbook, chapter 24).
• Oxidation: sodium hypochlorite hydrolyzes (NaOCl + H2O → HOCl + NaOH), and HOCl oxidizes cell components.
• Charge neutralization: Al3+ is about 700 times as effective as Na+ at destabilizing colloids.
• Incompatible mixtures: acids release chlorine from hypochlorite.

Every class is driven by the same few water properties.

Which water properties drive every reaction?#

Six water properties drive every treatment reaction: pH, alkalinity, hardness, conductivity or TDS, temperature and dissolved oxygen. Alkalinity is titrated to pH 8.3 for P alkalinity and to pH 4.5 for total alkalinity (40 CFR 136.3, Table IB). The US secondary standards set pH 6.5 to 8.5 and TDS 500 mg/L, both non-enforceable (40 CFR 143.3), while the EU indicator for conductivity is 2,500 µS/cm at 20 °C (68 °F) under Directive (EU) 2020/2184. Temperature works in both directions: a rise from 25 to 60 °C (77 to 140 °F) lowers the calcium tolerance of phosphonates by a factor of 1.5 to 2.

The table below compares the 6 properties by the reactions they drive.

13 Water Treatment Chemistry Concepts in 4 Groups#

The 13 water treatment chemistry concepts fall into 4 groups plus compatibility: 4 scaling concepts, 2 corrosion concepts, 4 disinfection concepts, 2 coagulation concepts and 1 page on incompatible mixtures. The directory below gives each concept the question it answers, its key equation or number and the systems where it governs the dose.

1. Scaling concepts (4)#

Scaling concepts explain why dissolved minerals deposit on heat-transfer and membrane surfaces and how far a water can be concentrated before they do; this group has 4 concepts, listed below.

• Scale formation: calcium carbonate, calcium phosphate, calcium sulfate and silica; calcium scale forms on hot surfaces, silica on the coldest parts such as tower fill.
• Langelier Saturation Index: LSI = pH - pHs (W. F. Langelier, Journal AWWA, 1936); per the Veolia Water Technologies Handbook it holds between pH 6.5 and 9.5 and shows a direction, not a rate.
• Cycles of concentration: the ratio of dissolved solids in blowdown to those in makeup, with blowdown = evaporation / (cycles - 1).
• Threshold inhibition: inhibitors adsorb on the growth sites of sub-microscopic crystallites at sub-stoichiometric doses, and they also modify crystals and disperse particles.

2. Corrosion concepts (2)#

Corrosion concepts explain how water attacks metal and how fast; this group has 2 concepts: corrosion mechanisms and measurement, and microbiologically influenced corrosion (MIC). Coupon weight loss under ASTM D2688-23 gives mpy = 534 W / (D A T), and 1 mpy equals 0.0254 mm/y. Carbon steel in open cooling water at 1 mpy or less rates "excellent" on the AWT scale (Boffardi, 2000), while UK HSE guidance HSG274 Part 1 (2nd edition, 2024) targets below 5 mpy (0.13 mm/y), preferably below 2 mpy (0.05 mm/y).

MIC starts under biofilm, where the surface turns anaerobic and anodic (the same handbook, chapter 26). Aerobic plate counts miss it, so sulfate-reducing bacteria are tested on anaerobic media under ASTM D4412-19(2024).

3. Disinfection concepts (4)#

Disinfection concepts explain how chlorine and other oxidants kill microbes and how much is enough; this group has 4 concepts: chlorination chemistry, breakpoint chlorination, chlorine residual and CT values. Chlorination turns on HOCl ⇌ H+ + OCl-, pKa 7.54 at 25 °C (77 °F) (Morris, 1966): free chlorine is about 90 % HOCl at pH 6.5, 50 % at 7.5 and about 10 % at 8.5, and HOCl is about 100 times more effective than OCl-.

Breakpoint chlorination follows dose = demand + residual; the breakpoint sits at 7.6:1 Cl2:NH3-N by weight in theory and 8:1 to 10:1 in practice, while chloramination targets 4.5:1 to 5:1 (Health Canada, 2020). The MRDL is 4.0 mg/L as Cl2 for chlorine and chloramines and 0.8 mg/L as ClO2, a running annual average under 40 CFR 141.65, not an MCL.

CT = C x T10, and 3-log Giardia inactivation with free chlorine at pH 7.0, 10 °C (50 °F) and 0.4 mg/L or less needs 104 mg-min/L, against 1,850 for chloramine, 23 for chlorine dioxide and 1.4 for ozone (40 CFR 141.74).

4. Coagulation concepts (2)#

Coagulation concepts explain how dissolved and colloidal matter is turned into settleable floc; this group has 2 concepts: coagulation and flocculation, and the jar test. Charge neutralization rises steeply with ion charge: Na+, Ca2+ and Al3+ destabilize drinking-water colloids in a ratio of 1 : 60 : 700. Coagulants also consume alkalinity, 0.50 mg/L as CaCO3 per mg/L of dry alum and 0.92 mg/L as CaCO3 per mg/L of ferric chloride (as FeCl3). The jar test is a bench-scale simulation of full-scale coagulation, flocculation and sedimentation, and the US EPA (1999) default runs a rapid mix at 100 rpm for 1 minute, flocculation at 30 rpm for 30 minutes and 60 minutes of settling.

5. Chemical compatibility (1)#

Chemical compatibility is the fifth, cross-cutting concept: it explains which treatment chemicals react dangerously when they meet in a tank, a fill line or a drain. Of 72 unloading accidents from 1953 to 2019 in the ARIA and eMARS records (Chemical Engineering Transactions), 24 % involved hypochlorite with hydrochloric acid and 16 % hypochlorite with sulfuric acid, and 90 % ended in a toxic dispersion. On 2016-10-21 in Atchison, Kansas, a sulfuric acid hose was connected to a hypochlorite fill line; the US Chemical Safety Board found the lines adjacent, identical and unlabeled.

Water Treatment Chemistry Formulas at a Glance#

The formula sheet below gives 12 water treatment chemistry equations with their units, the basis of every input and the concept each one belongs to.

Every saturation index shows a tendency, not a deposition rate, and every CT value applies only to the temperature, pH and residual of its row.

How Is Water Treatment Chemistry Expressed? Units, Bases and Equivalents#

Water treatment chemistry is expressed in 3 layers: a concentration unit (mg/L, ppm or percent), a basis that names what is counted (as CaCO3, as Cl2, as product) and, for ions, an equivalent weight that converts one basis into another. Each layer answers a different question about the same number.

mg/L, ppm and percent#

In dilute water 1 mg/L equals 1 ppm, because ppm times solution density gives mg/L, and the two diverge only in dense liquids such as concentrated chemicals, sludges and glycol loops (Veolia Water Technologies Handbook, chapter 40). Product strength uses percent, where 1 % equals 10,000 mg/L, and 1 ppb equals 0.001 ppm. US hardness uses grains per gallon: 1 gpg equals 17.1 mg/L as CaCO3, while 17.8 belongs to the German degree.

"As CaCO3" and equivalent weights#

Hardness and alkalinity are reported as CaCO3 so that different ions add up on one scale: calcium in mg/L times 2.5 gives calcium hardness as CaCO3, and magnesium times 4.12 gives magnesium hardness as CaCO3. The factor 2.5 is the ratio of the equivalent weights 50.0 for CaCO3 and 20.0 for calcium. Why does the same alum feed read as three different doses? Each basis counts a different part of the product, and the table below compares the conversion factors.

Why every dose states its basis#

A dose without a basis is ambiguous: 100 US gallons per day of liquid alum fed into 10 MGD reads as 13.3 ppm of solution, 6.45 ppm as dry alum or 1.26 ppm as Al2O3, three numbers for one pump rate. The alum has specific gravity 1.33, 48.5 % alum and 9.5 % Al2O3 (NJ AWWA operator example).

Worked example (inputs: flow 10 MGD (37,854 m³/d), dose 2.0 mg/L as Cl2, 12.5 trade % sodium hypochlorite at 125 g/L, 1.04 lb Cl2 per gallon): the plant needs 166.8 lb/day (75.7 kg/d) of available chlorine, or about 160 gal/d (606 L/d) of product. Trade percent is not weight percent: 12.5 trade % is about 11.2 wt % NaOCl.

Which Concept Applies to Which Industrial or Municipal System?#

Each water system leans on a different set of chemistry concepts: cooling towers on scaling, cycles and corrosion, boilers on cycles and dissolved oxygen, membranes on saturation indices and dechlorination, and drinking water on coagulation, disinfection and corrosion control. Engineers should start from the governing number of their own system.

Programs by system are compared under industrial water treatment programs by application. The table below maps each system to its directory rows and governing number.

Cooling water treatment chemistry#

Cooling water treatment chemistry is a chain: evaporation concentrates the makeup water, the rising cycles push the LSI from negative to strongly positive, and the program answers with acid, inhibitors and blowdown. In Table 31-2 of the Veolia Water Technologies Handbook, makeup at LSI -1.2 reaches 0 at 2 cycles, +1.5 at 5, +2.5 at 10 and +3.2 at 15, and acid to pH 8.7 brings the 15-cycle water back to +2.5. At 10 cycles it carries 500 mg/L calcium and 400 mg/L M-alkalinity as CaCO3, 100 mg/L SiO2 and pH 8.9.

The US DOE Federal Energy Management Program calls 2 to 4 cycles common and 6 or more possible; raising a tower from 3 to 6 cycles cuts makeup by 20 % and blowdown by 50 %. Chloride or sulfate tracks cycles, except where chlorine or sulfuric acid is fed. Cooling tower water testing and control ranges sit with the cooling program.

Boiler and steam chemistry#

Boiler chemistry turns on cycles and dissolved oxygen: blowdown as a percent of feedwater equals 100 divided by the boiler cycles, and feedwater oxygen is held below 7 ppb (0.007 mg/L) after deaeration. ASME consensus guidance limits feedwater hardness to 0.300 mg/L as CaCO3 at 0 to 450 psig (0 to 31 bar) and to not detectable above 1,000 psig (69 bar). Blowdown is always stated against feedwater, never against steam.

Membrane chemistry#

Membrane chemistry has 2 hard limits: polyamide RO membranes tolerate less than 0.1 mg/L of free chlorine, and concentrate saturation is judged by the LSI up to 10,000 mg/L TDS (ASTM D3739-19) and by the Stiff-Davis index above it (ASTM D4582-23). The DuPont FilmTec Reverse Osmosis Technical Manual gives dechlorination ratios of 1.34 mg sodium metabisulfite or 1.47 mg sodium bisulfite per mg Cl2 in theory, about 3 mg per mg in practice, and keeps the concentrate LSI below 0 without antiscalant. Seawater adds about 65 mg/L of bromide, which converts HOCl to HOBr.

Drinking water treatment chemistry#

Drinking water treatment chemistry runs in plant order: coagulation brings combined filter effluent to 0.3 NTU or less in 95 % of monthly measurements, chlorination and CT values set the disinfection, and corrosion control protects the distribution system. The turbidity limit (40 CFR 141.173) never allows more than 1 NTU.

Under the Lead and Copper Rule Improvements (89 FR 86418, 2024-10-30), the lead action level drops to 0.010 mg/L at the 90th percentile from 2027-11-01 (0.015 mg/L until then), an action level, not an MCL. Where the State designates corrosion control parameters, distribution pH stays at 7.0 or above (40 CFR 141.82(f)), and the compliance steps are set out under the Lead and Copper Rule and corrosion control.

Disinfection by-products cap that chemistry: TTHM 0.080 mg/L and HAA5 0.060 mg/L as locational running annual averages (40 CFR 141.64).

Wastewater treatment chemistry#

Wastewater treatment chemistry reuses the same concepts at higher loads: coagulation and precipitation remove solids and phosphorus, and dechlorination needs 0.90 mg/L of SO2 per mg/L of Cl2 in theory, about 1:1 in practice. The 0.90 ratio is the molar mass ratio 64.06 / 70.90.

Chemicals and program design per wastewater stream are covered under wastewater treatment chemicals by type and use.

From process stage to chemistry concept#

Each stage of a conventional water treatment process has one governing chemistry concept: coagulation and flocculation ahead of sedimentation and filtration, oxidation and CT during disinfection, and saturation and corrosion chemistry in the finished water.

Direct filtration omits sedimentation (40 CFR 141.2). The table below maps each stage to its concept rows and reaction class.

Definitions follow 40 CFR 141.2. The stages themselves are described under water treatment process stages and the chemicals each one uses.

Which Chemical Families Does Each Chemistry Concept Explain?#

The 16 families of water treatment chemicals map onto the 5 concept groups: scale inhibitors and phosphates onto scaling, corrosion inhibitors, oxygen scavengers and amines onto corrosion, biocides, oxidants and dechlorination chemicals onto disinfection, and coagulants and flocculants onto coagulation. The table below lists the 16 families with their concept rows.

Each chemical, with its CAS number and NSF/ANSI/CAN 60 status, sits in the water treatment chemical database.

Which Water Treatment Calculators Apply These Concepts?#

Six water treatment calculators apply these concepts to your own numbers: an LSI calculator, a cooling tower water balance calculator, a chemical dosage calculator, a CT value calculator, a corrosion rate calculator and a unit converter. Each one uses the equations of the formula sheet above with the units and bases stated there.

Who Applies Water Treatment Chemistry at a Plant?#

Water treatment chemistry is applied at a plant by its operators and by water treatment program providers, who run the tests, set the targets and adjust the doses that these concepts describe.

How to compare providers is explained under water treatment companies and program providers.

What Do Operators Ask About Water Treatment Chemistry?#

Operators ask 5 recurring chemistry questions, about process chemistry, saturation indices, chlorine and pH, units and incompatible chemicals, and each answer points to the concept that owns it.

What chemical process is involved in water treatment?#

Four chemical processes are involved in water treatment, precipitation, corrosion control, oxidation and coagulation, plus keeping incompatible chemicals apart. The 4 processes map onto the concept groups as listed below.

• Precipitation: scale formation, saturation indices, cycles and threshold inhibition.
• Corrosion control: coupon rates in mpy and MIC.
• Oxidation: chlorination, breakpoint, residual and CT.
• Coagulation: charge neutralization, flocculation and the jar test.

Is the LSI still valid for cooling water?#

The LSI is valid as a direction, not as a prediction: US Unified Facilities Criteria state that it "has lost its practical application for cooling water systems". The same criteria cite "serious deficiencies in the accuracy"; an LSI above 0 still marks a water that tends to deposit calcium carbonate.

Why does chlorine disinfect better at lower pH?#

More of it stays as hypochlorous acid: about 90 % at pH 6.5 against about 10 % at pH 8.5, and HOCl is about 100 times more effective than OCl-. The split follows the HOCl pKa of 7.54 at 25 °C (77 °F).

Do ppm and mg/L mean the same thing?#

Yes in dilute water and no in dense liquids: ppm times solution density equals mg/L. A concentrated chemical with a density well above 1 kg/L reads lower in ppm than in mg/L.

Which water treatment chemicals must never be mixed?#

Hypochlorite must never meet an acid, ferric chloride or sodium bisulfite: NaOCl with hydrochloric acid alone accounts for 24 % of 72 documented unloading accidents between 1953 and 2019. Hypochlorite with ferric chloride accounts for 13 % and with sodium bisulfite for 5 %.

Is the chlorine RMP threshold the same as the OSHA PSM threshold?#

No: chlorine triggers EPA RMP at 2,500 lb (1,134 kg) under 40 CFR 68.130 but OSHA PSM at 1,500 lb (680 kg) under 29 CFR 1910.119.

The EPA proposal of 2026-02-24 to amend the RMP rule is not final, and both rules are compared under EPA RMP and OSHA PSM for water treatment chemicals.