Biocides for Water Treatment: Oxidizing vs Non-Oxidizing, 11 Non-Oxidizing Types, Dosage and Selection

Biocides for water treatment are chemicals dosed to kill or control bacteria, algae, fungi and biofilm in cooling towers, closed loops, membranes and raw water, and they fall into 2 classes: oxidizing biocides such as chlorine and bromine, and non-oxidizing biocides such as glutaraldehyde, isothiazolinone and DBNPA. Bacteria can double every 20 to 30 minutes (Veolia Water Technologies Handbook, chapter 26), so which biocide, at which dose and on which schedule, keeps a system under control?

The 2 classes are also spelled oxidising and non-oxidising. Oxidizing biocides cover chlorine and hypochlorites, bromine, chlorine dioxide, ozone and peroxygens. The 11 non-oxidizing biocides are glutaraldehyde, isothiazolinone, DBNPA, THPS, bronopol, methylene bis(thiocyanate), quaternary ammonium compounds, polyquat, TTPC, dodecylguanidine hydrochloride and copper sulfate. Together they form one of the 16 families of water treatment chemicals.

Oxidizers are fed continuously at 0.1 to 0.5 mg/L (ppm) of free halogen as Cl2 in recirculating cooling water, while non-oxidizers are slug fed and held for at least 4 hours (UK HSE HSG274 Part 1, 2024). pH, contaminants, the target organism and the application decide the choice; plate counts and Legionella results prove it, and EPA's final August 2024 guidance asks a remediation claim to show at least a 5.0 log reduction. FIFRA, the EU Biocidal Products Regulation, NSF/ANSI/CAN 60 and discharge permits set the legal limits.

Every value on this page is checked against primary sources (EPA, eCFR, EUR-Lex, NSF listings, CDC, UK HSE); see our methodology and fact-checking process.

What Is a Biocide in Water Treatment?#

A biocide in water treatment is a chemical, such as sodium hypochlorite, glutaraldehyde or a quaternary ammonium compound, that is dosed to kill or inhibit microorganisms that foul, corrode or contaminate an industrial or utility water system. The US Department of Energy names biological inhibitors beside corrosion and scale inhibitors as the 3 chemical parts of a cooling-tower program.

A US product becomes a pesticide once it claims to act as a slimicide, algicide, disinfectant or molluscicide, and it needs EPA registration before sale (40 CFR 152.15). Cooling-tower biocides are non-public-health antimicrobial pesticides, among more than 4,000 registered antimicrobial products (EPA).

What does a biocide control in industrial water systems?#

A biocide controls 5 groups of organisms and deposits: planktonic bacteria, sulfate-reducing bacteria under deposits, algae on sunlit tower surfaces, fungi on wood and fill, and the biofilm that shelters all of them, including Legionella. The 5 targets are listed below.

• Planktonic bacteria: free-floating cells in the bulk water.
• Sulfate-reducing bacteria: anaerobes under deposits.
• Algae: growth on sunlit decks and basins.
• Fungi: growth on wooden fill, which quats do not control.
• Biofilm: the attached layer that shelters Legionella.

Attached bacteria outnumber free-floating bacteria by 3 to 4 log units, and usually less than 1 % of cells grow on a culture plate (FEMS Microbiology Ecology, 2018). Formation, sampling and removal are explained under biofilm in industrial water systems.

Where are biocides used?#

Biocides are used wherever warm or nutrient-rich water stands or recirculates: open cooling towers, evaporative condensers and closed loops, reverse osmosis and ultrafiltration feeds, once-through intakes fouled by mussels, and source-water reservoirs with algae. Legionella grows between 25 and 45 °C (77 and 113 °F), the working range of most cooling water (CDC).

How Do Biocides Work?#

Biocides work in 2 ways: oxidizing biocides oxidize cell walls and enzymes on contact and are consumed by any oxidizable matter, while non-oxidizing biocides react with specific cell components or damage cell membranes and need hours of contact.

Oxidizing biocides: oxidation of cell components#

Oxidizing biocides kill by oxidizing cell walls and enzymes: chlorine and bromine act through hypochlorous and hypobromous acid, while chlorine dioxide, ozone and peracetic acid oxidize cell walls directly without forming chlorinated organics. Sodium bromide added to chlorinated water forms hypobromous acid (NaBr + HOCl → HOBr + NaCl), and chlorine dioxide does not react with ammonia (Veolia Water Technologies Handbook, chapter 27).

Why does an oxidizer residual disappear in dirty water? Every oxidizable substance competes for it: 1 ppm of chlorine oxidizes 1.6 ppm of ferrous iron or 0.77 ppm of manganese before it reaches a cell.

Non-oxidizing biocides: reaction with cell targets and membrane damage#

Non-oxidizing biocides kill by reacting with specific cell targets, such as amine and thiol groups, or by disrupting the cell membrane, which is why they keep working in water that would consume an oxidizer but need hours rather than minutes of contact. The 2 kill modes are listed below.

1. Reactive organics: glutaraldehyde cross-links cell-wall amines, isothiazolinone reacts with thiols, bronopol oxidizes sulfhydryl groups and DBNPA binds to cell walls.
2. Cationic membrane-active biocides: quats, polyquat, TTPC and dodecylguanidine disrupt the cell membrane.

Isothiazolinone stops growth within minutes, but cells die within hours (Silva and co-authors, 2020).

Why does pH change biocide performance?#

pH decides how much of a halogen dose is active: at pH 7.5 and 25 °C (77 °F) only half of free chlorine is hypochlorous acid, and at pH 8.5 about a tenth, while hypobromous acid still makes up 83 % of free bromine at pH 8.0. HOCl has a pKa of 7.54 at 25 °C (Water Research, 2023) and HOBr about 8.7 (WHO).

Non-oxidizers run on their own pH clock. DBNPA hydrolyzes with a half-life of 63 hours at pH 7 but 73 minutes at pH 9, and glutaraldehyde with 394 days versus 63.8 days, both at 25 °C (EPA REDs, 1994 and 2007). The HOCl and OCl- equilibrium is worked through on water chlorination chemistry.

Oxidizing vs Non-Oxidizing Biocides: How Do They Differ?#

Oxidizing and non-oxidizing biocides differ in 5 ways: how they kill, how fast they act, how they are fed and measured, how pH and contaminants affect them, and how they are registered and discharged. Table 2 compares the 2 classes attribute by attribute.

Most open recirculating systems run an oxidizer as the base program and add a non-oxidizer as a periodic slug.

When is an oxidizing biocide the primary biocide?#

An oxidizing biocide is the primary biocide in most open recirculating cooling systems because it can be fed continuously and measured as a residual; New York City requires an approved oxidizing chemical as the primary biocide in every registered cooling tower (24 RCNY 8-05(c)). The same code requires treatment at least once a day by automated feed (24 RCNY 8-05(a)). A continuous oxidizer also controls zebra mussels, because interruptions of about 1 hour let them reoxygenate.

When is a non-oxidizing biocide added?#

A non-oxidizing biocide is added when an oxidizer cannot reach or cannot be tolerated: under deposits and biofilm, against sulfate-reducing bacteria, at high pH, in process-contaminated water, and in polyamide membrane systems that tolerate less than 0.1 mg/L of free chlorine. UK guidance alternates 2 non-oxidizers with different kill mechanisms, each held for at least 4 hours (HSE HSG274 Part 1, 2024).

Types of Biocides: 2 Classes and 11 Non-Oxidizing Chemicals#

Water treatment biocides fall into 2 classes, oxidizing and non-oxidizing, and this site covers 11 non-oxidizing biocides and algaecides in 3 groups: 6 reactive organic biocides, 4 cationic membrane-active biocides and 1 copper algaecide. Table 1 compares the classes and biodispersants by members, kill mode, feed and control test.

1. Oxidizing biocides#

Oxidizing biocides are chlorine, bromine, chlorine dioxide, ozone and peroxygen chemicals that kill by oxidation; they belong to the oxidant family and are compared there. The working chemicals are chlorine gas, sodium and calcium hypochlorite, trichloroisocyanuric acid, BCDMH, activated sodium bromide, chlorine dioxide, monochloramine, ozone, hydrogen peroxide and peracetic acid.

2. Reactive organic non-oxidizing biocides (6)#

Reactive organic biocides are small organic molecules, such as aldehydes, isothiazolinones and brominated amides, that react with cell components; this group has 6 chemicals: glutaraldehyde, isothiazolinone, DBNPA, THPS, bronopol and methylene bis(thiocyanate). Acrolein, dazomet, DCOIT, BIT and terbuthylazine are sections of the non-oxidizing class page.

Glutaraldehyde#

Glutaraldehyde (C5H8O2) is the dialdehyde slug biocide that cross-links cell-wall amines, penetrates biofilm and stays compatible with chlorine, nitrite and molybdate programs; it is sold as 15 % and 50 % solutions. Supplier data show a 50 % loss in 8 hours at 100 ppm of ammonia and pH 9.

Isothiazolinone (CMIT/MIT)#

Isothiazolinone (CMIT/MIT, 3:1) is a broad-spectrum bactericide and algaecide whose sulfur-nitrogen bond reacts with cell thiols, supplied at 1.5 % active and degraded by sulfide, amines and high pH. CMIT degrades about 2,000 times faster at pH 11 than at pH 4.5.

DBNPA#

DBNPA (2,2-dibromo-3-nitrilopropionamide) is the fastest-acting non-oxidizing biocide, sold at 20 % active, and hydrolyzes quickly at neutral and alkaline pH, so it kills fast and leaves little residual. In natural waters its half-life falls below 5 hours.

THPS#

THPS (tetrakis(hydroxymethyl)phosphonium sulfate) is a phosphonium biocide used mainly against sulfate-reducing bacteria and supplied as a 75 % solution. It also controls aerobic bacteria.

Bronopol#

Bronopol (2-bromo-2-nitropropane-1,3-diol) is a brominated nitro alcohol that blocks bacterial enzymes but is a weak algaecide, and it releases formaldehyde as pH and temperature rise. Its decomposition half-life at 20 °C (68 °F) is about 1.5 years at pH 6 but 2 months at pH 8 (EPA RED, 1995).

Methylene bis(thiocyanate) (MBT)#

Methylene bis(thiocyanate) (MBT) is an organic thiocyanate that is believed to bind irreversibly to biomolecules and is one of the strongest algaecides in the group, with an I50 of 1.0 to 1.4 ppm on algae (Veolia Water Technologies Handbook, chapter 26).

3. Cationic membrane-active biocides (4)#

Cationic membrane-active biocides are positively charged quaternary nitrogen, phosphonium and guanidine compounds that disrupt cell membranes; this group has 4 chemicals: quaternary ammonium compounds, polyquat, TTPC and dodecylguanidine hydrochloride.

Quaternary ammonium compounds (ADBAC, DDAC)#

Quaternary ammonium compounds (quats, such as ADBAC and DDAC) are cationic surfactants that damage cell membranes, act as algaecides and biodispersant synergists, and precipitate with anionic dispersant polymers. They can foam.

Polyquat (WSCP)#

Polyquat (WSCP, polyquaternium-42) is a polymeric quaternary algaecide, bactericide and molluscicide sold at 60 % active, stable from pH 6.0 to 10.0 and non-foaming.

TTPC#

TTPC (tributyl tetradecyl phosphonium chloride) is a cationic phosphonium salt, not a quaternary ammonium compound, that acts as both biocide and biodispersant in cooling water.

Dodecylguanidine hydrochloride (DGH)#

Dodecylguanidine hydrochloride (DGH) is a cationic alkylguanidine biocide for cooling towers and closed loops, listed among the metabolic inhibitors in cooling-water handbooks.

4. Copper algaecide (1)#

Copper sulfate is the one inorganic algaecide on this site, dosed to source-water reservoirs rather than to cooling systems.

Copper sulfate#

Copper sulfate (CuSO4·5H2O, 25.5 % Cu) is the reservoir algaecide that controls algae and cyanobacteria at 0.2 to 1 mg/L as Cu, working best near pH 8 in water below 50 mg/L alkalinity (NHMRC, Australian Drinking Water Guidelines). It is applied in the early morning to no more than half of a water body at once.

5. Biodispersants (biocide aids)#

Biodispersants are surfactants, such as sulfonates, quaternary compounds and polyglucosides, that kill nothing themselves but lower surface tension so that a biocide penetrates and lifts biofilm. Enzyme products instead attack the extracellular polymeric substances that bind a biofilm.

Which Biocide for Which System? Selection by Application and Water Chemistry#

Biocide selection depends on 4 factors: system pH, contaminants that consume or deactivate the biocide, the target organism and the application, and a program should combine one oxidizer with alternating non-oxidizers where the system allows it.

By pH and alkalinity#

Above pH 8, bromine or chlorine dioxide replaces plain chlorine: a chlorine residual must rise 3 to 4 times to stay effective, while hypobromous acid and chlorine dioxide keep working (UK HSE HSG274 Part 1, 2024, paras 1.101 to 1.102). Bleed-off, acid dosing or sodium bromide are the listed fixes. Among non-oxidizers, a high-pH slug of DBNPA should rely on its fast kill, because its half-life drops to 73 minutes at pH 9.

By contaminants: ammonia, sulfide, reducing agents and polymers#

Contaminants decide which biocide survives in the water: ammonia consumes chlorine and accelerates glutaraldehyde loss, sulfide deactivates DBNPA and isothiazolinone, residual bisulfite deactivates DBNPA, and anionic dispersant polymers precipitate quats. The 5 contaminants are listed below.

• Ammonia: turns chlorine into chloramines; bromamines remain biocidal.
• Sulfides: deactivate DBNPA and degrade CMIT/MIT.
• Sodium bisulfite: the DuPont FilmTec Reverse Osmosis Membranes Technical Manual (Form 45-D01504) adds 1 ppm of DBNPA as active per ppm of residual reducing agent.
• Anionic dispersant polymers: precipitate quats.
• Nitrite and molybdate: bromine and isothiazolinone may need higher doses (peer-reviewed study, PubMed 19187130).

All incompatible pairs are tabled under water treatment chemical compatibility.

By target organism: bacteria, sulfate-reducing bacteria, algae and mussels#

The target organism narrows the choice: MBT inhibits algae at 1.0 to 1.4 ppm while bronopol needs 80 to 120 ppm, glutaraldehyde and THPS are the sulfate-reducing-bacteria biocides, and mussels need either an uninterrupted oxidizer or 6 to 48 hours of a non-oxidizing molluscicide.

Against Klebsiella pneumoniae, the Veolia Water Technologies Handbook (chapter 26) gives I50 values of 2.7 ppm for MBT, 5.1 ppm for DBNPA and 15 ppm for bronopol.

Sulfate-reducing bacteria live under deposits and drive microbiologically influenced corrosion.

Quats leave fungi untouched, while polyquat covers algae and mussels.

By application: cooling towers, closed loops, membranes and raw water#

Each application has a default biocide pattern: an oxidizer with alternating non-oxidizer slugs in open cooling towers, non-oxidizers in closed loops, DBNPA or isothiazolinone slugs ahead of polyamide membranes, and copper sulfate or polyquat in source-water reservoirs. Table 3 gives the default biocides and one key number for 7 applications.

Cooling towers carry the broadest program, laid out from scale to blowdown under cooling tower water treatment.

Biocide Dosage Ranges by Chemical#

Biocide doses range from 0.1 to 0.5 mg/L (ppm) of free halogen as a continuous residual in recirculating cooling water to 200 mg/L as product for a glutaraldehyde slug, and every range is tied to its own basis: as Cl2, as active ingredient, as product or as Cu. Table 4 lists range, basis, schedule and source per biocide.

The basis changes the number: 100 mg/L of a 50 % glutaraldehyde product is 50 mg/L as active. Label and RED ranges are the legal ceiling, not a recommendation, and a drinking-water use is also capped by the product's NSF/ANSI/CAN 60 maximum use level (Table 5).

Slug dosing vs continuous feed#

Non-oxidizing biocides are slug fed because a high concentration must persist for hours, and in a cooling tower the slug dilutes only through blowdown, with a half-life of 0.693 times system volume divided by blowdown rate (Veolia Water Technologies Handbook, chapter 26). The re-dose falls due at 25 % of the starting level, after 1.385 times volume divided by blowdown; the rule fails for once-through systems and volatile biocides. Oxidizers run the other way: a continuous residual, or 1.0 to 2.0 mg/L as Cl2 held at least 1 hour a day (CTI WTB-148, 2008). Blowdown itself is computed in the cooling tower water balance calculator.

How is a biocide slug dose turned into product quantity?#

Worked example: a 100 m³ (26,417 gal) cooling system dosed with 100 mg/L (ppm) of a 50 % glutaraldehyde product needs 10.0 kg (22.0 lb) of product, or 8.86 L (2.34 gal) at a specific gravity of 1.129, which gives 50 mg/L as active ingredient. Inputs: volume 100 m³, dose 100 mg/L as product, purity 50 %, specific gravity 1.129 (supplier figure).

In US units: pounds of product = 50 mg/L as active × 0.0264 million gallons × 8.34 ÷ 0.50 purity = 22.0 lb.

At 2 m³/h (8.81 gpm) of blowdown, the half-life is 0.693 × 100 ÷ 2 = 34.7 hours, and the level reaches the 25 % re-dose point after 69.3 hours (about 2.9 days); hydrolysis at pH 7 is negligible. The chemical dosage calculator converts any mg/L target into kg, lb and pump settings.

How Is Biocide Performance Controlled and Tested?#

Biocide performance is controlled in 3 layers: a measured oxidizer residual or ORP, a recorded non-oxidizer dose and contact time, and microbial counts that prove the result.

Oxidizer residuals and ORP#

Recirculating cooling water typically holds 0.1 to 0.5 mg/L (ppm) of free halogen as Cl2 continuously, or 1.0 to 2.0 mg/L for at least 1 hour a day when the halogen is fed intermittently (Veolia Water Technologies Handbook, chapter 27; CTI WTB-148, 2008). UK guidance sets 0.5 to 1 mg/L of free chlorine or 1 to 2 mg/L of bromine (HSE HSG274 Part 1, 2024, para 1.103). A DPD bromine reading equals the reading as Cl2 times 2.25.

Oxidation-reduction potential tracks oxidizing power continuously; the response shrinks at higher pH and falls as temperature rises (ASTM D1498-14(2022)e1). DBNPA reads about 400 mV at 0.5 to 3 mg/L, so the DuPont FilmTec manual bypasses the ORP setpoint during dosing. Probe care and setpoints are covered under oxidation-reduction potential (ORP).

Non-oxidizer tests and contact time#

A non-oxidizer is controlled by dose and contact time rather than by a residual: the slug is held for at least 4 hours, measured with a field kit where one exists, and recorded by quantity where none does (UK HSE HSG274 Part 1, 2024). Kits exist for CMIT/MIT, bronopol and THPS, and a glutaraldehyde kit detects 1 ppm active. Legionella samples should be taken immediately before a non-oxidizer shot.

Microbial counts: plate counts, ATP and Legionella#

Microbial control is proven by counts: bulk-water heterotrophic bacteria below 10,000 CFU/mL and surface counts below 100,000 CFU/cm² (CTI WTB-148, 2008), and Legionella below 10 CFU/mL, which CDC treats as well controlled in a cooling tower. The 3 count layers are listed below.

• Bulk water: plate counts, which miss anaerobes.
• Surfaces: coupons or a biofilm monitor at 0.23 to 0.45 m³/h (1 to 2 gpm).
• Legionella: culture in 7 to 14 days, or PCR in 2 to 48 hours.

Dip slides, ATP (ASTM D4012-23a) and plate counts each measure a different fraction of the population, compared under microbiological monitoring.

CDC reads a 10- to 100-fold rise as poorly controlled.

New York City requires Legionella sampling at least monthly from 2026-05-08 (24 RCNY 8-05(f)(3)).

Counts sit inside a written program set out under ASHRAE 188 and Legionella water management programs.

Which Regulations Apply to Water Treatment Biocides?#

Water treatment biocides are regulated in 4 layers: pesticide registration (FIFRA in the US, the Biocidal Products Regulation in the EU), NSF/ANSI/CAN 60 certification for drinking-water uses, discharge permits and the food-contact steam list.

FIFRA registration in the United States#

Every biocide sold in the US for slime, algae or Legionella control is a pesticide that must carry an EPA registration number (40 CFR 152.15), and its label, not a supplier brochure, sets the legal dose range and the systems it may treat. Each active has an EPA PC code, and older actives a RED: glutaraldehyde 2007, DBNPA 1994, bronopol 1995, ADBAC and DDAC 2006, coppers 2009.

EPA's final August 2024 guidance for cooling-tower Legionella claims asks for at least a 5.0 log mean reduction for remediation and 3 log for routine maintenance. Labels and REDs are explained under FIFRA registration of water treatment biocides.

EU Biocidal Products Regulation product types#

The EU Biocidal Products Regulation (EU) 528/2012 sorts water biocides by product type: PT5 for drinking water, PT11 for liquid-cooling and processing systems, PT12 for slimicides and PT2 for disinfectants and algaecides in non-potable water. Chlorine dioxide, ozone and electrolytic chlorine made on site count as biocides when used for a biocidal purpose.

Active chlorine is approved as sodium hypochlorite for PT1 to PT5 (Regulation (EU) 2017/1273), calcium hypochlorite for PT2 to PT5 (2017/1274) and chlorine for PT2 and PT5 (2017/1275), from 2019-01-01 to 2028-12-31, never for PT11 or PT12.

MBT was not approved for PT12 (Decision (EU) 2022/2005). Approvals are tracked under the EU Biocidal Products Regulation.

NSF/ANSI/CAN 60 for drinking-water algaecides and RO biocides#

Only a few biocides carry NSF/ANSI/CAN 60 listings, all product-specific and stated as product: copper sulfate algaecides at 1 to 20 mg/L, polyquat at 0.5 mg/L, and RO membrane biocides at 1 to 100 mg/L depending on the product (NSF listings, 2026-10-20).

Copper sulfate is listed for 21 companies, 93 products and 114 product-function rows, most often at 19 mg/L (46 rows), then 4 mg/L (35 rows) (NSF listings, 2026-10-21). The Reverse Osmosis Membrane Biocide function holds 42 companies, 186 products and 242 rows, 179 of them "NA" because the product is used off-line and flushed out (NSF listings, 2026-10-21). Table 5 gives each biocide's status.

MULs count the total dose across all application points, and DBNPA is not listed under its chemical name. How MULs are set is explained on NSF/ANSI/CAN 60.

Discharge, food-contact steam and banned chemistries#

Biocide discharges are capped by permit: steam electric plants may discharge cooling tower blowdown with at most 0.5 mg/L of free available chlorine, 0.2 mg/L on average, for no more than 2 hours a day per unit (40 CFR 423.13(d)). Once-through cooling at 25 MW or more is capped at 0.20 mg/L of total residual chlorine (40 CFR 423.13(b)).

The 2007 RED cancelled once-through use of glutaraldehyde, and DBNPA is ineligible for once-through systems.

Other dischargers disclose biocides on the application for NPDES permits.

Chromate is banned in comfort cooling towers since 1990-05-18 (40 CFR 749.68).

For food-contact steam, every biocide on this page is not listed in 21 CFR 173.310; the permitted list is under 21 CFR 173.310 boiler water additives.

Complete List of Non-Oxidizing Biocides: All 11 Chemicals in One Table#

The complete list below gives all 11 non-oxidizing biocides and algaecides with CAS number, group, function, systems, NSF/ANSI/CAN 60 status, US FIFRA status and food-contact steam status.

Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.

Who Supplies Biocides and Biocide Programs?#

Biocides are bought in 2 ways: as part of a treatment program, where a provider runs the residuals, slug schedule and microbial testing, or as registered products from a chemical supplier, where the buyer's own staff follow the label.

EPA registration is the first filter, and distributors by region are listed under water treatment chemical suppliers.

The US Department of Energy (FEMP Best Management Practice 10) compares cooling-water vendors on the cost to treat 1,000 gallons (3.79 m³) of make-up water and on their highest recommended cycles.

Proposals also list the FIFRA-registered biocides and an ASHRAE 188 Legionella water management plan.

How to compare providers is covered under water treatment companies.

Are Biocides Safe for People, Equipment and Effluent?#

Biocides are toxic by design, so the risk sits with the concentrated product at the plant, while the dosed water is controlled by label limits, NSF/ANSI/CAN 60 use levels and discharge permits.

Is a biocide toxic to humans?#

Concentrated biocides are toxic to people: glutaraldehyde carries the harmonised hazard statements H330 and H334 and a NIOSH ceiling limit of 0.2 ppm (0.8 mg/m³), so handling follows the product SDS and label PPE. For every other biocide, see the supplier SDS.

What does a biocide do to water?#

A biocide lowers the number of living bacteria, algae and fungi in the water and on wetted surfaces; oxidizers also consume iron, manganese and organic matter and use 1.4 ppm of alkalinity per ppm of chlorine fed (Veolia Water Technologies Handbook, chapter 27).

When should you use a biocide?#

A biocide is used continuously or on a fixed schedule in every open cooling system, at least once a day under New York City rules, and as a high-dose remediation when counts rise, such as at least 20 ppm of free oxidant followed by 10 ppm for 24 hours under CDC emergency guidance. CTI hyperhalogenation uses at least 5 ppm for 6 hours.

Which chemicals must never be mixed with biocides?#

Hypochlorite must never meet an acid or an ammonia product, because acids release chlorine gas and ammonia forms chloramines and explosive nitrogen trichloride, and oxidizer and inhibitor feeds enter the system at separate points.

Can non-chemical devices replace biocides?#

Non-chemical devices do not replace a registered biocide by default: New York City allows it only with approval, and the Cooling Technology Institute found little application data to support them (CTI WTB-148, 2008). FIFRA treats them as unregistered pesticidal devices (40 CFR 152.500).