How to Use CMIT/MIT Isothiazolinone Biocide in Water Treatment

Overview

CMIT/MIT (Chloromethylisothiazolinone/Methylisothiazolinone, CAS 55965-84-9, commonly marketed as Kathon or equivalent) is a 14% active isothiazolinone biocide blend formulated in a 3:1 ratio of CMIT to MIT. It is one of the most cost-effective and widely-used broad-spectrum biocides for industrial water treatment, providing persistent antimicrobial activity against bacteria, algae, and fungi at low continuous dose rates (50–150 ppm on product, equating to 7–21 ppm active ingredient). The mechanism of action involves irreversible reaction with thiol groups in essential microbial enzymes, making CMIT/MIT highly effective against both growing and non-growing organisms, including those embedded in biofilm.

CRITICAL SAFETY WARNING — SVHC SKIN SENSITIZER: CMIT (5-Chloro-2-methylisothiazol-3(2H)-one) is registered as a Substance of Very High Concern (SVHC) under EU REACH regulation due to its proven skin sensitization potential. CMIT/MIT causes allergic contact dermatitis with very low threshold exposures — once sensitized, a person can react to concentrations as low as 1–2 ppm CMIT/MIT in water. Sensitization is cumulative and irreversible. All personnel handling CMIT/MIT must wear: nitrile gloves (minimum), safety goggles (chemical splash rated), chemical-resistant apron, and where splash risk is elevated, a full-face shield. Skin contact must be strictly prevented. Sensitized individuals must not work with CMIT/MIT. This is not a precautionary statement — CMIT/MIT skin sensitization is a documented occupational health issue in water treatment industries.

Regulatory status: CMIT/MIT use in cosmetics, personal care products, and leave-on products is severely restricted or banned in EU markets. It remains permitted in industrial water treatment in many jurisdictions, but users should verify current national regulations. REACH SVHC registration requires that downstream users are informed — this information must be passed through the supply chain.

Preparation & Dissolution

CMIT/MIT 14% is supplied as a yellow-green liquid (pH 2.0–4.0, density 1.02–1.05 g/mL, 14% active CMIT+MIT). It is a ready-to-use liquid and does not require dissolution. Dilute to working concentration by adding the appropriate volume of CMIT/MIT concentrate to system water or makeup water — never add water to the concentrate (splash risk).

PPE MANDATORY during all handling: Nitrile gloves, chemical splash goggles (not safety glasses — full-seal goggles), chemical-resistant apron. If pouring from bulk drums or connecting dosing pump lines, add a full-face shield and chemical-resistant sleeve gauntlets. Sensitized individuals must be excluded from handling CMIT/MIT under any circumstances.

For dosing pump systems: set the pump to deliver CMIT/MIT concentrate directly from the drum to the injection quill. Dilution in a day tank is optional but helps verify dose rate. Maintain good ventilation at the dosing station — CMIT/MIT has a characteristic smell and low-level vapor exposure can be sensitizing. Never work in enclosed spaces with open CMIT/MIT containers.

Do not mix CMIT/MIT with strongly alkaline solutions (pH >9) as this causes rapid hydrolysis and loss of biocidal activity. Avoid mixing with strong oxidizing agents (hypochlorite, hydrogen peroxide) which also degrade isothiazolinones rapidly.

Dosing Guide

Application	Dose (ppm product)	Active CMIT+MIT (ppm)	Dosing Mode
Cooling tower bacteria and algae control	50–100 ppm	7–14 ppm	Shock twice weekly
Cooling tower — continuous bleed-fed systems	20–50 ppm	3–7 ppm	Semi-continuous proportional to makeup
Paper mill white water microbial control	100–150 ppm	14–21 ppm	Semi-continuous or shock
Oilfield injection water — general bacteria	100–200 ppm	14–28 ppm	Continuous or batch
Paint and coating preservation (in-can)	150–300 ppm	21–42 ppm	Add at manufacture
Metalworking fluid preservation	150–200 ppm	21–28 ppm	Add at fluid make-up

Application Procedure

Don all required PPE before opening CMIT/MIT containers: nitrile gloves, full-seal chemical splash goggles, chemical-resistant apron. Add face shield for bulk transfers.
Set metering pump to deliver CMIT/MIT at the calculated dose rate. For a cooling system: Dose volume (mL/hr) = Target ppm × System flow (m³/hr) × 1000 / (140,000 mg/L active). Verify dose by checking pump output into a measuring cylinder.
Inject CMIT/MIT concentrate at the dosing injection quill in the system recirculation line, away from any anionic chemical injection points. Ensure adequate dilution and mixing zone between CMIT/MIT injection and any downstream anionic treatment chemicals.
For shock dosing (cooling water): stop blowdown during the shock period. Add the calculated CMIT/MIT concentrate volume to the cooling tower basin or recirculation line. Allow 4–6 hours of recirculation before resuming blowdown — this ensures adequate contact time for biofilm penetration.
After completing dosing (particularly during drum changes), rinse all exposed equipment and PPE with clean water. Remove gloves last (peel off from wrist without touching the outside of the glove). Wash hands and exposed skin with soap and water even if gloves were worn — verify no skin exposure.
Log dose events with volume used, system volume, calculated ppm dose, and date. Track bacterial counts monthly to verify the dosing program is achieving the intended microbial control levels.

Monitoring & Control

Parameter	Frequency	Target
CMIT/MIT residual in system water	After each shock dose at 1 hr, 4 hr	Detectable residual at 4 hours confirms coverage; zero = underdosing or chemical incompatibility
Bacterial count (HPC) in system water	Monthly	Cooling water: <10,000 CFU/mL; paper mill: <100,000 CFU/mL
Algae visual inspection (cooling tower fill)	Weekly	No visible algae; green or brown growth = increase dose or frequency
System water pH	Daily	7.0–8.5; CMIT/MIT degrades rapidly above pH 9.0
Skin health surveillance for exposed workers	Quarterly	Any skin sensitization symptoms must be immediately referred to occupational health; sensitized personnel must be permanently excluded from CMIT/MIT handling
Regulatory compliance check (jurisdictional)	Annually	Verify CMIT/MIT remains permitted in your jurisdiction for the specific application

Common Mistakes

Inadequate PPE during routine handling: The most serious and common mistake with CMIT/MIT is treating it as a routine chemical that can be handled with standard industrial gloves or safety glasses. The skin sensitization risk is real and documented — partial PPE failures (thin gloves, safety glasses instead of goggles, exposed wrists) during routine tasks like checking pump output or connecting drum fittings can result in sensitization. Once sensitized, a worker can never handle CMIT/MIT again without risk of anaphylactic reaction. Full PPE compliance is non-negotiable.
Applying CMIT/MIT to cosmetics-adjacent or consumer product water systems: Even trace levels of CMIT/MIT in finished products intended for consumer use can cause allergic reactions in already-sensitized consumers (a significant and growing population in industrialized nations). CMIT/MIT must never be used in systems that contact personal care, cosmetic, or food ingredients unless specifically formulated and regulatory-approved versions at verified safe concentrations are used.
Mixing CMIT/MIT with hypochlorite or oxidizing biocides: Oxidizing biocides (NaOCl, H₂O₂, chlorine dioxide) rapidly degrade CMIT/MIT through oxidation of the isothiazolinone ring. Co-application wastes chemical and may generate unknown degradation products. If oxidizing biocide rotation is planned, apply CMIT/MIT and oxidizing biocides on separate days, not simultaneously.
Ignoring pH drift in cooling systems: Many cooling systems experience pH creep above 8.5 due to carbonate buffering of the makeup water and CO₂ stripping in the cooling tower. At pH 9.0, CMIT/MIT half-life drops to hours, making continuous dosing programs inadequate. Monitor system pH daily and maintain below 8.5 for effective CMIT/MIT programs. If pH control is problematic, switch to a pH-stable biocide for that system.
Failing to inform downstream customers in the supply chain of SVHC content: Under EU REACH, suppliers and users of SVHC-containing products have legal obligations to inform downstream recipients. Industrial users of CMIT/MIT in water treatment are considered downstream users of an SVHC substance and must comply with REACH Article 33 communication obligations. Failure to communicate SVHC status is a regulatory violation.

Storage & Handling

Shelf life: 12 months in sealed original container; CMIT/MIT degrades in opened or partially used containers stored at elevated temperatures
Temperature: Store at 5–25°C; avoid freezing (product may separate) and heat above 30°C (accelerates degradation)
Container: Original 25 kg sealed HDPE drums; store upright; keep area well-ventilated; do not store near anionic chemicals or oxidizing agents
Safety (SVHC — MANDATORY REQUIREMENTS): CMIT/MIT is an SVHC skin sensitizer. Mandatory PPE: full-seal chemical splash goggles, nitrile gloves (minimum 0.3mm thickness), chemical-resistant apron/coverall, face shield for bulk transfers. Sensitized individuals must be permanently excluded. In case of skin contact: remove contaminated clothing immediately, flush with water for minimum 20 minutes, seek immediate medical attention and inform physician of CMIT/MIT sensitizer exposure. In case of eye contact: flush with water for minimum 20 minutes and seek immediate emergency medical attention. Report all skin exposure incidents to occupational health for follow-up sensitization screening. Full SDS must be available at point of use and reviewed by all handling personnel.