The water treatment process is the sequence of physical, chemical and biological stages that turns raw water into water fit for its use; a conventional drinking water plant runs 8 stages, from intake and pre-oxidation through coagulation, flocculation, sedimentation, filtration and disinfection to fluoridation, corrosion control and distribution. Each stage has a chemical and a target, so which chemical is dosed where, and what must the water meet when it leaves?
The process takes 3 forms: drinking water treatment, wastewater treatment and industrial process water treatment. All three share one chemistry, and the coagulants, oxidants, pH adjusters and other chemicals each stage doses form 16 families of water treatment chemicals.
In the US, 66 % of surface-water plants dose a polymer, combined filter effluent must stay at or below 0.3 NTU in 95 % of monthly measurements, and filtration plus disinfection must achieve 3-log Giardia and 4-log virus reduction. Wastewater plants must reach 30 mg/L BOD5 with at least 85 % removal, and membrane plants keep free chlorine below 0.1 mg/L at polyamide membranes.
Table 1 compares the 3 process families by the water treated, the goal and the main chemical families.
| Process family | Water treated | Goal | Main chemical families |
|---|---|---|---|
| Drinking water treatment | surface water, groundwater, seawater | Safe Drinking Water Act limits at the tap | coagulants, flocculants, oxidants, pH adjusters, phosphates |
| Wastewater treatment | municipal and industrial wastewater | discharge permit limits | coagulants, flocculants, pH adjusters, defoamers, dechlorination chemicals |
| Industrial process water treatment | boiler, cooling and membrane makeup | equipment protection | scale inhibitors, oxygen scavengers, amines, biocides, ion exchange resins |
Every value on this page is checked against primary sources (eCFR, US EPA, WHO, Health Canada, NSF listings); see our methodology and fact-checking process.
What Is the Water Treatment Process?#
A water treatment process is a defined sequence of unit processes, such as coagulation, sedimentation, filtration and disinfection, each removing or inactivating one group of contaminants before the water reaches its user or its receiving stream.
US federal rules define 3 of these stages in 40 CFR 141.2. Coagulation destabilizes colloidal and suspended material and agglomerates it into floc, flocculation gently stirs small floc into larger, settleable particles, and sedimentation removes solids by gravity before filtration. Their sequence decides the filtration category and the pathogen credit a plant receives.
Which processes make up conventional and direct filtration?#
Conventional filtration treatment is coagulation, flocculation, sedimentation and filtration in series, while direct filtration is coagulation and filtration without the sedimentation step (40 CFR 141.2). The 2 regulatory trains are defined below.
- Conventional filtration treatment: coagulation, flocculation, sedimentation and filtration resulting in substantial particulate removal.
- Direct filtration: coagulation and filtration only, with no sedimentation basin.
The missing settling step changes the credit. Under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2, 40 CFR 141 Subpart W), a Bin 2 direct filtration plant needs 1.5-log of additional Cryptosporidium treatment, against 1-log for a conventional plant.
How do drinking water, wastewater and process water treatment differ?#
Drinking water treatment produces water that meets the Safe Drinking Water Act limits at the tap, wastewater treatment returns used water to a river at the limits of its discharge permit, and process water treatment conditions makeup water to the purity a boiler, cooling tower or membrane system needs.
The yardsticks differ. A US municipal wastewater plant must meet secondary treatment, 30 mg/L BOD5 and 30 mg/L suspended solids as 30-day averages (40 CFR 133.102), while a process water target is a purity grade, such as ISO 3696 Grade 1 at 0.1 µS/cm (0.01 mS/m) at 25 °C (77 °F).
Drinking Water Treatment Process Steps and the Chemicals Each Stage Uses#
The drinking water treatment process runs through 8 stages in a conventional surface-water plant, and 6 of them dose a chemical: an oxidant at intake, a coagulant at rapid mix, a polymer at flocculation, a disinfectant, fluoride with a pH adjuster, and a corrosion inhibitor before distribution.
Table 2 lists each stage with the chemicals dosed and one key number with its basis or target (mg/L equals ppm).
| # | Stage | Chemicals dosed (examples) | Key number with basis or target |
|---|---|---|---|
| 1 | Intake, screening, pre-oxidation | potassium or sodium permanganate, chlorine dioxide, ozone, chlorine | 1.9 mg KMnO4, 2.45 mg ClO2 or 0.87 mg O3 per mg Mn(II) (stoichiometric; Health Canada, 2019) |
| 2 | Coagulation (rapid mix) | alum, ferric chloride, ferric sulfate, polyaluminum chloride, aluminum chlorohydrate, polyDADMAC | alum 5 to 200 mg/L as Al2(SO4)3·14H2O; 0.50 mg/L alkalinity as CaCO3 consumed per mg/L dry alum |
| 3 | Flocculation | polyacrylamide flocculant aid | 0.05 to 0.3 mg/L (ppm) as product |
| 4 | Sedimentation, clarification | none of its own | presedimentation credit 0.5-log Cryptosporidium (continuous coagulant, ≥ 0.5-log turbidity reduction; 40 CFR 141.717) |
| 5 | Filtration | filter-aid polymer (optional); media under NSF/ANSI/CAN 61 | CFE ≤ 0.3 NTU in 95 % of monthly measurements, never > 1 NTU (40 CFR 141.173) |
| 6 | Disinfection | chlorine, sodium hypochlorite, chloramine, chlorine dioxide, ozone; UV (no chemical) | 3-log Giardia, 4-log virus; CT99.9 Giardia 104 (free chlorine) vs 1,850 (chloramine) mg·min/L at pH 7, 10 °C (50 °F) |
| 7 | Fluoridation, pH adjustment | fluorosilicic acid, sodium fluoride, sodium fluorosilicate; caustic soda, lime, soda ash | 0.7 mg/L F (US optimal, not a limit); fed after filtration |
| 8 | Corrosion control, storage, distribution | orthophosphate, zinc orthophosphate, pH and DIC adjustment; chlorine or chloramine residual | pH 8.8 to 10 and DIC 5 to 10 mg/L as C (EPA OCCT, 2016); entry point ≥ 0.2 mg/L; residual detectable in ≥ 95 % of monthly samples |
Utilities choose and certify these chemicals as one program, and the program view, with doses by chemical, is on drinking water treatment chemicals.
1. Intake, screening and pre-oxidation#
Intake and screening remove debris before any chemical is added, and pre-oxidation then doses permanganate, chlorine dioxide, ozone or chlorine to oxidize iron, manganese and taste and odor compounds ahead of coagulation. WHO calls chlorine dosed here pre-chlorination, the first of 3 application points before primary chlorination after treatment and booster chlorination in distribution.
Oxidant demand starts from stoichiometry: 1.9 mg of potassium permanganate, 2.45 mg of chlorine dioxide or 0.87 mg of ozone per mg of dissolved manganese, Mn(II), in theory (Health Canada, 2019). Practical ozone doses run 2 to 5 times higher, so a jar test sets the plant dose.
2. Coagulation: destabilizing particles at rapid mix#
Coagulation is the stage where a coagulant, such as alum, ferric chloride or polyaluminum chloride (polyaluminium chloride), is dosed at rapid mix to destabilize colloidal and suspended particles so they agglomerate into floc (40 CFR 141.2). Charge neutralization occurs very rapidly, so the coagulant must disperse within the rapid mix.
Alum doses in drinking water span 5 to 200 mg/L (ppm) as Al2(SO4)3·14H2O (NHMRC Australian Drinking Water Guidelines), and each mg/L of dry alum consumes 0.50 mg/L of alkalinity as CaCO3. Destabilization followed by floc growth is a sequence explained on coagulation and flocculation in water treatment.
Coagulation also removes organic carbon: enhanced coagulation must remove 15 to 50 % of source total organic carbon (TOC), by source TOC and alkalinity (40 CFR 141.135).
Step 2 jar testing ends at the point where an additional 10 mg/L of alum removes no more than 0.3 mg/L of TOC, as set out under enhanced coagulation and TOC removal.
3. Flocculation: growing settleable floc#
Flocculation gently stirs coagulated water so that small floc particles collide and grow into larger, settleable floc, often helped by 0.05 to 0.3 mg/L (ppm) of a polyacrylamide flocculant aid (NHMRC Australian Drinking Water Guidelines).
An AWWA WATER:STATS survey cited in the US EPA fourth Six-Year Review (2024) found that 66 % of surface-water plants dose a polymer, mostly cationic, against 13 % of groundwater plants.
The US EPA jar test copies the stage: rapid mix at 100 rpm for 1 minute, flocculation at 30 rpm for 30 minutes and settling for 60 minutes.
4. Sedimentation and clarification#
Sedimentation removes settleable floc by gravity before filtration (40 CFR 141.2); it doses no chemical of its own, so settled-water quality reflects the coagulant and polymer upstream.
Settling also earns a pathogen credit: under LT2 (40 CFR 141.717), presedimentation earns 0.5-log Cryptosporidium credit with continuous coagulant addition and at least 0.5-log mean turbidity reduction. Basin types are compared under clarifiers.
5. Filtration: removing turbidity and pathogens#
Filtration passes settled water through sand, anthracite or granular activated carbon, and a conventional plant must hold its combined filter effluent at or below 0.3 NTU in 95 % of monthly measurements and never above 1 NTU (40 CFR 141.173).
Slow sand and diatomaceous earth filters must meet 1 NTU in 95 % of measurements and never exceed 5 NTU (40 CFR 141.73). Combined filter effluent at or below 0.15 NTU in 95 % earns 0.5-log of extra Cryptosporidium credit (40 CFR 141.718).
A filter-aid polymer runs at 0.01 to 0.1 mg/L (ppm) (NHMRC), and filter media are certified under NSF/ANSI/CAN 61, not NSF/ANSI/CAN 60. NTU and FNU units are explained under turbidity in water.
6. Disinfection: inactivating pathogens and holding a residual#
Disinfection inactivates the pathogens that filtration leaves behind, and together the two stages must remove or inactivate 3-log (99.9 %) of Giardia and 4-log (99.99 %) of viruses in a filtered US surface-water system (40 CFR 141.72).
CT, residual concentration in mg/L times contact time in minutes, sets the dose: 3-log Giardia at pH 7.0 and 10 °C (50 °F) takes 104 mg·min/L with free chlorine and 1,850 mg·min/L with chloramine, about 18 times more (40 CFR 141.74).
UV uses dose instead: 12 mJ/cm² for 3-log Cryptosporidium, 11 for 3-log Giardia and 186 for 4-log virus (40 CFR 141.720), with tables under CT values for disinfection.
WHO targets at least 0.5 mg/L free chlorine after 30 minutes at pH below 8.0, and 0.2 to 0.5 mg/L in distribution.
UV, membranes and chlorine dioxide leave no residual as chlorine does, so plants add chlorine or monochloramine afterward.
Monochloramine is too weak for primary disinfection (WHO) but holds a stable residual; primary and secondary disinfectants are compared on drinking water disinfection.
7. Fluoridation and pH adjustment#
Fluoridation adds fluorosilicic acid, sodium fluoride or sodium fluorosilicate after filtration to reach the US optimal 0.7 mg/L, because about 30 % of the fluoride is lost when it is fed ahead of a 100 mg/L alum rapid mix (CDC, Water Fluoridation Principles and Practices). Fluorosilicic acid is the most widely used US additive.
Caustic soda, lime or soda ash then restore the pH and alkalinity that coagulation consumed.
The 0.7 mg/L value is the US Public Health Service optimal level (2015), not a limit: the US MCL is 4.0 mg/L (40 CFR 141.62), the SMCL 2.0 mg/L (40 CFR 143.3), and WHO, the EU and Health Canada set 1.5 mg/L. Feed rates are covered under water fluoridation chemicals.
8. Corrosion control, storage and distribution#
The last stage protects water on its way to the tap: a corrosion inhibitor such as orthophosphate, or a pH raised to 8.8 to 10, limits lead and copper release, and a chlorine or chloramine residual must stay detectable in at least 95 % of monthly distribution samples (40 CFR 141.72). The entry-point residual must not fall below 0.2 mg/L for more than 4 hours.
US EPA OCCT guidance (March 2016, updated 2019) pairs that pH with 5 to 10 mg/L of dissolved inorganic carbon as C, and uses 1.0 to 3.0 mg/L as PO4 (0.33 to 1.0 mg/L as P) at pH 7.2 to 7.8 as a reference orthophosphate target, not a limit.
The lead action level falls to 0.010 mg/L (90th percentile) from 2027-11-01, 0.015 mg/L until then (LCRI, 89 FR 86418, 2024-10-30); it is not an MCL. More than one in five Americans drink chloraminated water (US EPA). Strategies are compared on corrosion control treatment for lead and copper.
Water Treatment Trains by Source: Surface Water, Groundwater and Seawater#
The source decides the treatment train: turbid surface water runs the full conventional train, clean groundwater may need only oxidation, filtration and disinfection, and seawater needs reverse osmosis or thermal desalination ahead of remineralization and disinfection.
Table 3 compares 4 source waters by train, the chemicals that differ and one key number.
| Source | Typical train | Chemicals that differ | Key number |
|---|---|---|---|
| Turbid surface water | conventional filtration (coagulation, flocculation, sedimentation, filtration, disinfection) | full coagulant and polymer program | CFE ≤ 0.3 NTU (40 CFR 141.173) |
| Low-turbidity surface water | direct filtration (no sedimentation) | lower coagulant dose, filter aid | LT2 Bin 2: 1.5-log additional Cryptosporidium treatment vs 1-log conventional |
| Groundwater with Fe and Mn | aeration or oxidation, filtration, disinfection; softening where hard | oxidants, lime, soda ash | 0.29 mg O2 per mg Mn(II), slow below pH 9.0 to 9.5; US SMCL Fe 0.3, Mn 0.05 mg/L |
| Seawater | intake, pretreatment, RO or thermal, remineralization, disinfection | coagulant, sodium bisulfite, acid, antiscalant | 35,000 mg/L TDS; recovery 40 to 50 % |
Groundwater: aeration, iron and manganese removal and softening#
Groundwater treatment centers on oxidation: aeration oxidizes dissolved iron readily, but manganese oxidizes slowly in air below pH 9.0 to 9.5, so plants add permanganate, chlorine dioxide or ozone before filtration (Health Canada, 2019).
Oxygen demand is small, 0.29 mg of O2 per mg of Mn(II) and, by calculation, 0.14 mg per mg of Fe(II), so kinetics, not oxygen, limit aeration. Oxidant choice is detailed under iron and manganese removal.
Iron and manganese carry non-enforceable US secondary standards of 0.3 and 0.05 mg/L (40 CFR 143.3).
Hard groundwater adds softening. Enhanced softening meets its alternative criterion with at least 10 mg/L magnesium hardness removed as CaCO3 or treated alkalinity below 60 mg/L as CaCO3 (40 CFR 141.135); options are compared under lime softening.
Seawater and brackish water: reverse osmosis and desalination#
Seawater at about 35,000 mg/L TDS is desalinated by reverse osmosis at up to 84 bar (1,200 psi) or by thermal distillation, and a seawater RO train recovers only 40 to 50 % of its feed as permeate. Brackish RO starts at about 5 bar (75 psi) and recovers up to about 90 %.
Polyamide membranes reject 95 to more than 99 % of dissolved salts (a membrane manufacturer's design manual). Plant staging belongs to the reverse osmosis process page in the directory.
Worldwide, 15,906 plants produced about 95 million m³/d, 48 % of it in the Middle East and North Africa (Jones et al., 2019). Intake and antiscalant chemistry is covered under seawater desalination chemicals.
Wastewater Treatment Process: Stages and Chemicals from Screening to Discharge#
The wastewater treatment process runs through 6 stages, preliminary, primary, secondary, tertiary, disinfection and sludge treatment, and US municipal plants must reach secondary treatment: 30 mg/L BOD5 and 30 mg/L suspended solids as 30-day averages with at least 85 % removal (40 CFR 133.102).
About 16,583 US publicly owned treatment works served about 75 % of the population in 2004 (US EPA Nutrient Control Design Manual, 2010).
Table 4 lists the 6 wastewater stages with the chemicals dosed and one key number.
| # | Stage | Chemicals dosed (examples) | Key number |
|---|---|---|---|
| 1 | Preliminary (screening, grit) | calcium nitrate, iron salts for sulfide (collection system) | 40 CFR 403.5(b)(7) toxic-gas prohibition for industrial users |
| 2 | Primary clarification | iron or aluminum salts in chemically assisted settling (optional) | 0.87 mg Al or 1.8 mg Fe per mg P (stoichiometric) |
| 3 | Secondary (activated sludge, biofilm) | alkalinity (lime, caustic, magnesium hydroxide), defoamers | BOD5 30 mg/L (30-day), 45 mg/L (7-day), ≥ 85 % removal; nitrification uses 7.14 g alkalinity as CaCO3 per g NH4-N |
| 4 | Tertiary (nutrients, filtration) | alum, ferric chloride, aluminum chlorohydrate; methanol or other carbon for denitrification | enhanced nutrient removal 3.0 mg/L TN, 0.1 mg/L TP or lower |
| 5 | Disinfection | chlorine or hypochlorite plus sodium bisulfite; UV; peracetic acid | permit-specific (NPDES) |
| 6 | Sludge treatment | polyacrylamide (thickening, dewatering), lime | belt press 1 to 10 g/kg DS (2 to 20 lb/dry ton); lime pH 12 after 2 h |
The coagulants, polymers, carbon sources and odor-control chemicals across these stages are compared on wastewater treatment chemicals.
What are the stages of wastewater treatment?#
Municipal wastewater passes 6 stages: preliminary treatment removes rags and grit, primary clarification settles solids, secondary treatment oxidizes BOD biologically, tertiary treatment removes nutrients and filters, disinfection inactivates pathogens and sludge treatment thickens, stabilizes and dewaters the solids. The 6 stages are listed below in flow order.
- Preliminary treatment: screens and grit chambers.
- Primary clarification: gravity settling, optionally with iron or aluminum salts.
- Secondary treatment: activated sludge at pH 7.0 to 7.5 and a sludge age of 3 to 15 days (US EPA, 1987), with 2.0 mg/L dissolved oxygen for nitrification, which uses 4.57 g of O2 and 7.14 g of alkalinity as CaCO3 per g of NH4-N.
- Tertiary treatment: enhanced nutrient removal to 3.0 mg/L total nitrogen and 0.1 mg/L total phosphorus or lower.
- Disinfection: chlorine with dechlorination, UV or peracetic acid.
- Sludge treatment: thickening, stabilization and dewatering.
Sources that count "four steps" merge disinfection into tertiary treatment and treat sludge separately.
Chemicals in wastewater stages: phosphorus removal, dewatering and disinfection#
Wastewater stages dose 4 chemical groups: alum or ferric salts for phosphorus, polymers for sludge thickening and dewatering, lime for stabilization and chlorine with a dechlorination agent for disinfection.
Phosphorus precipitation needs 0.87 mg of Al or 1.8 mg of Fe per mg of P in theory, but plants dose 1.5 to 2.0 mol of metal per mol of P for 80 to 98 % soluble phosphorus removal (US EPA, 2010); feed points are set out under chemical phosphorus removal.
Conventional clarifiers reach 0.5 to 1.0 mg/L total phosphorus, and below 0.1 mg/L needs tertiary processes.
Lime stabilization meets the 40 CFR 503 pathogen-reduction condition at pH 12 after 2 hours.
Dewatering polymer runs at 1 to 10 g/kg DS (2 to 20 lb/dry ton) on a belt press and 2.5 to 20 kg/t (5 to 40 lb/dry ton) on a centrifuge, as active polymer per dry solids (US EPA fact sheets), covered under sludge thickening and dewatering polymers; chlorine and bisulfite are covered under wastewater disinfection and dechlorination.
Industrial Process Water Treatment: Boilers, Cooling Towers and Membranes#
Industrial process water treatment conditions water for 5 system types, boilers, cooling towers, closed loops, membrane plants and produced water, and its chemistry is set by the equipment rather than by a drinking water standard.
In a cooling tower, blowdown B = E / (C − 1), where E is evaporation and C the cycles of concentration, so every dose follows the cycles.
Each system's chemical program is compared under industrial water treatment.
Table 5 compares the 5 industrial systems by the water treated, the main chemicals and the reason for treatment.
| System | Water treated | Main chemicals | Why |
|---|---|---|---|
| boiler water treatment | softened or demineralized feedwater, condensate | oxygen scavengers, neutralizing amines, phosphates, polymers | scale, corrosion and carryover control |
| cooling tower water treatment | recirculating water concentrated by evaporation | scale inhibitors, corrosion inhibitors, biocides | blowdown = E / (C − 1) |
| closed loop water treatment | chilled, hot water and glycol loops | nitrite, molybdate, azoles | corrosion in sealed systems |
| reverse osmosis and membrane chemicals | RO, NF, UF feed | sodium bisulfite, acid, antiscalant, cleaners | < 0.1 mg/L free chlorine at polyamide membranes |
| produced water treatment chemicals | oil and gas produced water (water-treatment view) | coagulants, oxidants, scale inhibitors | water quality for reuse or disposal |
Makeup and pure water: softening, demineralization and RO#
Makeup water for boilers and high-purity systems is softened, demineralized by ion exchange or desalted by reverse osmosis, and the purest grades approach the 0.055 µS/cm conductivity of theoretically pure water.
Softening removes hardness, while demineralization by ion exchange removes cations and anions together. ISO 3696 Grade 1 water is limited to 0.1 µS/cm at 25 °C (77 °F).
RO pretreatment: the membrane chemical sequence#
An RO plant doses its chemicals in a fixed order: oxidant at the intake, coagulant and flocculant, media filtration, sodium bisulfite to remove chlorine, acid, antiscalant and a 5 µm cartridge filter ahead of the high-pressure pump.
Polyamide membranes tolerate less than 0.1 mg/L (ppm) free chlorine and degrade after about 200 to 1,000 hours at 1 mg/L, so they are not chlorine tolerant. Sodium metabisulfite removes chlorine at 1.34 mg per mg in theory and about 3.0 mg per mg in practice (a membrane manufacturer's design manual).
Coagulant, dechlorination and acid control are covered under RO pretreatment chemistry.
Which Water Treatment Chemicals Does Each Process Stage Use?#
All 16 families of water treatment chemicals map onto process stages: coagulants and flocculants work at rapid mix and flocculation, oxidants and biocides at intake and disinfection, pH adjusters, phosphates and corrosion inhibitors at post-treatment, and the boiler and cooling families in industrial process water. Table 6 lists each family with the stages where it is dosed.
| # | Family | Process stages where dosed |
|---|---|---|
| 1 | coagulants for water treatment | drinking water stage 2; wastewater primary and tertiary (phosphorus); RO pretreatment |
| 2 | flocculants for water treatment | stage 3 and filter aid (stage 5); sludge thickening and dewatering |
| 3 | scale inhibitors | cooling towers, boilers, RO (antiscalant) |
| 4 | phosphates in water treatment | stage 8 corrosion control; boiler internal treatment |
| 5 | corrosion inhibitors for water treatment | cooling towers, closed loops; distribution |
| 6 | oxygen scavengers for boiler water | boiler feedwater |
| 7 | neutralizing amines for condensate treatment | steam and condensate |
| 8 | biocides for water treatment | cooling towers, RO biofouling control |
| 9 | oxidants and oxidizing biocides | stage 1 pre-oxidation, stage 6 disinfection, wastewater disinfection |
| 10 | pH adjustment chemicals | stages 2 and 7; secondary alkalinity; RO acid |
| 11 | dechlorination chemicals | RO feed (sodium bisulfite); wastewater after chlorination |
| 12 | chelating agents in water treatment | boiler programs; membrane and equipment cleaning |
| 13 | defoamers and antifoams | aeration basins, digesters, evaporators |
| 14 | descaling and chemical cleaning agents | RO cleaning, boiler and exchanger descaling |
| 15 | ion exchange resins for water treatment | softening, demineralization, selective removal |
| 16 | adsorbents and filter media | stage 5 filtration; GAC adsorption; iron and arsenic media |
US EPA keeps 47 water treatment chemical supply chain profiles, last updated 2026-06-30, 36 of them for chemicals used in water treatment.
Which Rules Set the Targets at Each Water Treatment Stage?#
Five US rule sets fix the stage targets: the Surface Water Treatment Rules for filtration and log removal, the disinfectant and byproduct rules for residuals, the Lead and Copper Rule for corrosion control, secondary treatment and NPDES permits for wastewater, and NSF/ANSI/CAN 60 for every chemical dosed into drinking water. Table 7 lists each rule with its target, instrument and date.
| Stage | Rule | Target | Instrument and date |
|---|---|---|---|
| All potable chemicals | NSF/ANSI/CAN 60 | product certification; maximum use level per product, as product | NSF listings, 2026-10-21; required in 49 US states (NSF survey of ASDWA members) |
| Filtration, disinfection | Surface Water Treatment Rules | CFE ≤ 0.3 NTU / 1 NTU; 3-log Giardia, 4-log virus; LT2 bins | 40 CFR 141.72, 141.173, Subpart W |
| Disinfection, distribution | Disinfectants and Disinfection Byproducts Rules | MRDL 4.0 mg/L as Cl2 (chlorine, chloramine), 0.8 mg/L as ClO2; TTHM 0.080, HAA5 0.060 mg/L as LRAA; bromate 0.010, chlorite 1.0 mg/L | 40 CFR 141.64, 141.65 (Stage 2 DBPR, 71 FR 388, 2006-01-04) |
| Corrosion control | Lead and Copper Rule | lead action level 0.010 mg/L (90th percentile) from 2027-11-01, 0.015 mg/L until then; copper 1.3 mg/L | 40 CFR 141.80, LCRI 89 FR 86418 (2024-10-30) |
| All drinking water stages | Safe Drinking Water Act | statute behind every national primary drinking water regulation | 42 U.S.C. 300f, P.L. 93-523 (1974-12-16) |
| Wastewater discharge | NPDES permits | secondary treatment BOD5 and SS 30/45 mg/L, ≥ 85 % removal, pH 6.0 to 9.0 | 40 CFR 133.102 |
| Industrial discharge to sewers | National Pretreatment Program | no discharge below pH 5.0; no toxic gases | 40 CFR 403.5(b)(2), (b)(7) |
NSF/ANSI/CAN 60 is required in 49 US states through legislation, regulations or policies (NSF survey of ASDWA members), while filter media and ion exchange resins fall under NSF/ANSI/CAN 61. MRDLs are running annual averages and lead action levels are 90th-percentile triggers; neither is an MCL.
Directory: Every Water Treatment Process Page#
The 10 water treatment process pages below each explain one stage or one complete process, and each row names the water it treats, the question it answers and the chemicals it bridges to.
| # | Process page | Family | Water | What it answers | Chemical bridge |
|---|---|---|---|---|---|
| 1 | sedimentation in water treatment | drinking water stage 4 | surface water | settling types, overflow rates, design | coagulants, flocculants |
| 2 | filtration in water treatment | drinking water stage 5 | surface water, groundwater | rapid, slow and multimedia filters | filter media, filter-aid polymers |
| 3 | aeration in water treatment | drinking water stage 1 (groundwater) and wastewater | groundwater, wastewater | oxygen transfer, Fe and Mn oxidation, stripping | oxidants |
| 4 | UV disinfection in water treatment | drinking water stage 6 | treated water | UV dose, log credit, lamps | oxidants (residual after UV) |
| 5 | wastewater treatment process | wastewater | municipal wastewater | stages from screening to discharge | wastewater chemicals |
| 6 | activated sludge process | wastewater, secondary | municipal wastewater | sludge age, F/M, DO, SVI | pH adjusters, defoamers |
| 7 | how reverse osmosis works | desalting and pure water | brackish, industrial makeup | RO plant operation | membrane chemicals |
| 8 | desalination processes | desalting | seawater | RO, thermal and brine | antiscalants, dechlorination |
| 9 | deionized water | pure water | industrial makeup | production, grades, DI vs distilled | ion exchange resins |
| 10 | zero liquid discharge (ZLD) | wastewater, industrial | industrial wastewater | recovery, evaporation, crystallization | scale inhibitors, defoamers |
How to choose the right water treatment process page#
Choose a process page by the question you need answered: by stage for one step of a drinking water plant, by water source for a complete train, and by system for wastewater, desalting or pure water. The 3 starting points are listed below.
- Start from the stage: sedimentation, filtration, aeration or UV disinfection.
- Start from the source: desalination for seawater, reverse osmosis for brackish and industrial water.
- Start from the discharge: the wastewater process, activated sludge or zero liquid discharge.
Who Designs and Runs Water Treatment Process Programs?#
Water treatment processes are run by utility and plant operators and supported by independent program providers, which set chemical doses by jar test, run online control and report compliance, and by chemical suppliers, which deliver to an NSF/ANSI/CAN 60 and AWWA specification. The operator keeps the permit and the compliance duty.
A provider reviews the whole train, from coagulant dose to residual and corrosion control, and how to compare program providers is covered under water treatment companies.
Why Is Water Treatment Important?#
Water treatment matters because each stage removes a defined risk: filtration and disinfection together remove or inactivate 99.9 % of Giardia and 99.99 % of viruses in US surface-water systems, and secondary wastewater treatment removes at least 85 % of BOD5 before discharge (40 CFR 141.72; 40 CFR 133.102). Each removal is a measured, enforceable target.
What are the 5, 7 or 9 steps of water treatment?#
The step count depends on what a source counts as a step: the 5 core steps are coagulation, flocculation, sedimentation, filtration and disinfection, and this page adds intake and pre-oxidation, fluoridation with pH adjustment, and corrosion control with distribution to reach 8. Counts of 7 or 9 split or merge the same stages.
What is the order of the water treatment process?#
The order of a conventional drinking water process is intake and pre-oxidation, coagulation, flocculation, sedimentation, filtration, disinfection, fluoridation and pH adjustment, then corrosion control and distribution; direct filtration skips sedimentation (40 CFR 141.2).
What is process water?#
Process water is water used inside an industrial system, such as boiler feedwater, cooling-tower makeup or rinse water, and its treatment targets equipment limits like conductivity, hardness and silica instead of drinking water limits.
Which chemical is used in a water treatment plant?#
A drinking water plant uses at least 4 chemicals: a coagulant such as alum or ferric chloride, a disinfectant such as chlorine, a pH adjuster such as caustic soda or lime, and often fluoride and a corrosion inhibitor.
Every chemical is filterable by function and NSF status in the water treatment chemical database.
Which water treatment process chemicals must be stored apart?#
Hypochlorite must be stored apart from acids and acidic coagulants such as ferric chloride and alum, because the mixture releases chlorine gas.
Hypochlorite with ferric chloride caused 13 % of 72 unloading accidents from 1953 to 2019 (Chemical Engineering Transactions); pairings are listed under water treatment chemical compatibility. On 2016-10-21 at MGPI Processing in Atchison, Kansas, sulfuric acid delivered into an unlabeled hypochlorite fill line released a toxic cloud (US Chemical Safety Board).
Chlorine storage triggers 2,500 lb (1,134 kg) under the EPA Risk Management Program (40 CFR 68.130) and 1,500 lb (680 kg) under OSHA Process Safety Management, explained under EPA RMP and OSHA PSM.
What are the different methods used for water filtration?#
Utility filtration uses 4 methods: rapid granular media filtration, slow sand filtration, diatomaceous earth filtration and membrane filtration, and the first three carry their own turbidity limits under 40 CFR 141.73 and 141.173. Rapid filters meet 0.3 NTU in 95 % of measurements, slow sand and diatomaceous earth filters 1 NTU.