Q: How do I manage Legionella risk in my cooling tower?

Follow a Water Safety Plan (WSP) per ASHRAE 188, HSE L8 (UK), or equivalent national standard: (1) maintain free chlorine 0.2–0.5 ppm residual continuously, (2) monthly shock with non-oxidizing biocide (DBNPA or BKC), (3) quarterly hyper-chlorination (5–10 ppm free Cl for 2–4 hours), (4) semi-annual tower inspection and cleaning, (5) monthly Legionella culture test of circulating water. Legionella pneumophila proliferates in cooling tower water between 20–45°C (optimum 35–40°C) — the exact temperature range of most open cooling towers in summer. Risk factors: (1) stagnant water in low-flow zones (dead legs, distribution headers with poor circulation); (2) biofilm on tower fill (biofilm provides EPS shelter where Legionella survives oxidizing biocide residuals); (3) aerosol generation from cooling tower drift — Legionella is transmitted by inhaling contaminated droplets, not by drinking. The public health risk is primarily from towers near populated areas. Chemical control strategy: oxidizing biocides (free chlorine, SDIC, TCCA, or bromine-based) are the first line — maintain 0.2–0.5 ppm free Cl at the tower return at pH 7.0–7.8 (chlorine efficacy drops above pH 8.0). Supplement with monthly DBNPA or BKC shock treatment to penetrate biofilm where Legionella hides from chlorine. Physical measures: drift eliminators (<0.001% drift rate), regular cleaning (quarterly minimum), UV irradiation for side-stream treatment. Regulatory requirements vary by country — UK requires Legionella risk assessment and L8 compliance for all cooling towers; France requires annual declaration to health authorities; US ASHRAE 188 requires WSP for any evaporative cooling system.

Q: What is cycles of concentration (COC) and how do I optimize it?

COC = ratio of dissolved solids in circulating water vs makeup water. Higher COC = less blowdown = less makeup water = lower chemical dose per m³ cooling water. Optimal COC is where the marginal water savings equals the marginal scale and corrosion risk. Typical targets: 3–4 COC for high-hardness makeup, 5–8 COC for soft makeup with good treatment program. COC is measured as the ratio of circulating water to makeup water conductivity (or chloride, which doesn't evaporate or precipitate). Water balance in a cooling system: Evaporation (E) ≈ 0.85% of circulating flow per 5°C cooling range; Blowdown (B) = E / (COC - 1); Makeup (M) = E + B = E × COC/(COC-1). At COC = 2: makeup = 2×evaporation; at COC = 4: makeup = 1.33×evaporation; at COC = 8: makeup = 1.14×evaporation — the water savings from 4 to 8 COC are much less than from 2 to 4 COC. The limiting factor for increasing COC is usually: (1) CaCO₃ scale if LSI exceeds treatment program capability (usually LSI > 2.5); (2) SiO₂ scale if SiO₂ concentration exceeds 200 mg/L; (3) Chloride-induced corrosion if Cl⁻ > 500 mg/L in carbon steel systems. Treatment programs with AA-AMPS polymer + PBTCA can push COC to 6–8 even with hard makeup water by keeping scale minerals in dispersed colloidal suspension rather than deposited scale. Always do a water chemistry calculation at the target COC before changing the COC setpoint — use Langelier, Ryznar, and Puckorius indices to predict scale tendency.

Q: What monitoring tests should I run on cooling water?

Daily: pH, conductivity (for COC), free chlorine (DPD test kit). Weekly: total hardness, M-alkalinity, inhibitor residual (HEDP by colorimetry). Monthly: full water analysis (Ca, Mg, Si, Cl, SO₄, Fe, Cu, TDS, bacteria count). Quarterly: corrosion coupon reading (mpy), Legionella culture. Semi-annual: tower inspection and cleaning. The monitoring program is the feedback loop that tells you whether the treatment program is working. Each parameter serves a specific diagnostic role: pH (maintain 7.0–8.5 for balanced scale/corrosion/biocide performance); conductivity in µS/cm divided by makeup conductivity gives real-time COC (run automated blowdown control to maintain target COC); free chlorine by DPD test (Hach test kit, 1 minute) gives immediate biocide efficacy reading. For inhibitor residual monitoring, HEDP can be checked colorimetrically (ferric sulfate method, Hach Method 8021 or equivalent) — target 5–10 ppm HEDP residual means the dosing is correct. Iron and copper in circulating water are direct corrosion indicators: Fe >0.5 mg/L suggests carbon steel pitting; Cu >0.1 mg/L suggests copper tube corrosion or high velocity erosion. Corrosion coupons (carbon steel + copper + 304SS) installed in a bypass coupon rack at system flow and temperature give the most accurate corrosion rate data (target <5 mpy for carbon steel, <0.2 mpy for copper). Bacteria monitoring: heterotrophic plate count (HPC) by standard APHA 9215C, target <10,000 CFU/mL in circulating water; Legionella by culture (ISO 11731) or rapid qPCR, target <100 CFU/L.

Question 1

How do I calculate the right chemical dose for my cooling system?

Accepted Answer

Step 1: Analyze makeup water (Ca hardness, M-alkalinity, pH, TDS, silica, conductivity). Step 2: Determine target cycles of concentration (COC) based on blowdown economics and scale limits. Step 3: Calculate circulating water chemistry at target COC. Step 4: Select chemical program (scale inhibitor + biocide) based on Langelier Saturation Index (LSI) at COC. Step 5: Calculate chemical dose as ppm in circulating water and dosing rate in makeup water. The design calculation starts with makeup water analysis. Key parameters and their effect on treatment: Ca hardness and M-alkalinity determine CaCO₃ scaling potential (LSI = pH_actual - pH_saturation; target LSI at COC ≤ 1.5 with scale inhibitor, ≤ 0.5 without); silica concentration determines maximum COC before SiO₂ precipitation (SiO₂ at COC × makeup SiO₂ must stay <150 mg/L without dispersant, or <200 mg/L with AA-AMPS); TDS determines conductivity and maximum COC before chloride-induced corrosion risk. Chemical dosing calculation: if target circulating water HEDP = 8 ppm and COC = 4, then makeup water HEDP dose = 8 ppm × (1 - 1/4) = 6 ppm added in makeup. For biocides dosed as intermittent shock (e.g., DBNPA 20% at 5 ppm active in circulating water weekly): 5 ppm × circulating water volume (m³) / 1000 = kg DBNPA 20% per week. Most suppliers provide free water analysis and dosage calculation service — send your makeup water lab report for a customized program recommendation.

Question 2

How do I manage Legionella risk in my cooling tower?

Accepted Answer

Follow a Water Safety Plan (WSP) per ASHRAE 188, HSE L8 (UK), or equivalent national standard: (1) maintain free chlorine 0.2–0.5 ppm residual continuously, (2) monthly shock with non-oxidizing biocide (DBNPA or BKC), (3) quarterly hyper-chlorination (5–10 ppm free Cl for 2–4 hours), (4) semi-annual tower inspection and cleaning, (5) monthly Legionella culture test of circulating water. Legionella pneumophila proliferates in cooling tower water between 20–45°C (optimum 35–40°C) — the exact temperature range of most open cooling towers in summer. Risk factors: (1) stagnant water in low-flow zones (dead legs, distribution headers with poor circulation); (2) biofilm on tower fill (biofilm provides EPS shelter where Legionella survives oxidizing biocide residuals); (3) aerosol generation from cooling tower drift — Legionella is transmitted by inhaling contaminated droplets, not by drinking. The public health risk is primarily from towers near populated areas. Chemical control strategy: oxidizing biocides (free chlorine, SDIC, TCCA, or bromine-based) are the first line — maintain 0.2–0.5 ppm free Cl at the tower return at pH 7.0–7.8 (chlorine efficacy drops above pH 8.0). Supplement with monthly DBNPA or BKC shock treatment to penetrate biofilm where Legionella hides from chlorine. Physical measures: drift eliminators (<0.001% drift rate), regular cleaning (quarterly minimum), UV irradiation for side-stream treatment. Regulatory requirements vary by country — UK requires Legionella risk assessment and L8 compliance for all cooling towers; France requires annual declaration to health authorities; US ASHRAE 188 requires WSP for any evaporative cooling system.

Question 3

What is cycles of concentration (COC) and how do I optimize it?

Accepted Answer

COC = ratio of dissolved solids in circulating water vs makeup water. Higher COC = less blowdown = less makeup water = lower chemical dose per m³ cooling water. Optimal COC is where the marginal water savings equals the marginal scale and corrosion risk. Typical targets: 3–4 COC for high-hardness makeup, 5–8 COC for soft makeup with good treatment program. COC is measured as the ratio of circulating water to makeup water conductivity (or chloride, which doesn't evaporate or precipitate). Water balance in a cooling system: Evaporation (E) ≈ 0.85% of circulating flow per 5°C cooling range; Blowdown (B) = E / (COC - 1); Makeup (M) = E + B = E × COC/(COC-1). At COC = 2: makeup = 2×evaporation; at COC = 4: makeup = 1.33×evaporation; at COC = 8: makeup = 1.14×evaporation — the water savings from 4 to 8 COC are much less than from 2 to 4 COC. The limiting factor for increasing COC is usually: (1) CaCO₃ scale if LSI exceeds treatment program capability (usually LSI > 2.5); (2) SiO₂ scale if SiO₂ concentration exceeds 200 mg/L; (3) Chloride-induced corrosion if Cl⁻ > 500 mg/L in carbon steel systems. Treatment programs with AA-AMPS polymer + PBTCA can push COC to 6–8 even with hard makeup water by keeping scale minerals in dispersed colloidal suspension rather than deposited scale. Always do a water chemistry calculation at the target COC before changing the COC setpoint — use Langelier, Ryznar, and Puckorius indices to predict scale tendency.

Question 4

What is the difference between oxidizing and non-oxidizing biocides?

Accepted Answer

Oxidizing biocides (chlorine, SDIC, TCCA, bromine) kill by irreversible oxidation of cell membrane components — fast acting (minutes), broad-spectrum, low cost, but inactivated by high organic load and biofilm EPS. Non-oxidizing biocides (DBNPA, CMIT/MIT, BKC, glutaraldehyde) kill by specific biochemical mechanisms — slower (hours), better biofilm penetration, must be rotated to prevent resistance. Standard practice: continuous oxidizing biocide + monthly non-oxidizing shock. Chlorine-based biocides (free chlorine from sodium hypochlorite, SDIC, TCCA) are the cooling water industry standard for continuous microbiological control. They are fast (kill kinetics at 0.2 ppm free Cl at pH 7.5 in <10 minutes for planktonic Legionella), cheap, and easily monitored with DPD colorimetric test kits. The critical limitation: chlorine is rapidly consumed by organic matter (COD demand), by biofilm EPS, and by ammonia (forming chloramines with lower biocidal activity). In high-organic makeup water systems (river water, recycled water), chlorine demand may exceed practical dosing rates. Also, chlorine efficacy drops sharply above pH 8.0 — at pH 8.0, 75% of total chlorine is as hypochlorite (OCl⁻, less reactive); at pH 7.0, 75% is hypochlorous acid (HOCl, the effective form). Non-oxidizing biocides penetrate biofilm EPS layers that chlorine cannot enter — this is why a monthly DBNPA or glutaraldehyde shock treatment significantly reduces biofilm mass even when continuous chlorination is maintaining zero planktonic bacteria in bulk water. The rotation principle (changing biocide type every 2–4 weeks) prevents microorganisms from developing biochemical resistance pathways — DBNPA inhibits enzyme SH groups (different mechanism from chlorine); BKC disrupts cell membranes (different mechanism from both); glutaraldehyde cross-links proteins. Using only one biocide for months allows enrichment of resistant strains.

Question 5

What monitoring tests should I run on cooling water?

Accepted Answer

Daily: pH, conductivity (for COC), free chlorine (DPD test kit). Weekly: total hardness, M-alkalinity, inhibitor residual (HEDP by colorimetry). Monthly: full water analysis (Ca, Mg, Si, Cl, SO₄, Fe, Cu, TDS, bacteria count). Quarterly: corrosion coupon reading (mpy), Legionella culture. Semi-annual: tower inspection and cleaning. The monitoring program is the feedback loop that tells you whether the treatment program is working. Each parameter serves a specific diagnostic role: pH (maintain 7.0–8.5 for balanced scale/corrosion/biocide performance); conductivity in µS/cm divided by makeup conductivity gives real-time COC (run automated blowdown control to maintain target COC); free chlorine by DPD test (Hach test kit, 1 minute) gives immediate biocide efficacy reading. For inhibitor residual monitoring, HEDP can be checked colorimetrically (ferric sulfate method, Hach Method 8021 or equivalent) — target 5–10 ppm HEDP residual means the dosing is correct. Iron and copper in circulating water are direct corrosion indicators: Fe >0.5 mg/L suggests carbon steel pitting; Cu >0.1 mg/L suggests copper tube corrosion or high velocity erosion. Corrosion coupons (carbon steel + copper + 304SS) installed in a bypass coupon rack at system flow and temperature give the most accurate corrosion rate data (target <5 mpy for carbon steel, <0.2 mpy for copper). Bacteria monitoring: heterotrophic plate count (HPC) by standard APHA 9215C, target <10,000 CFU/mL in circulating water; Legionella by culture (ISO 11731) or rapid qPCR, target <100 CFU/L.

System Parameter	Problem	Treatment Chemical	Typical Dose (ppm)	Note
High calcium hardness (>300 mg/L as CaCO₃)	CaCO₃ scale on HX tubes	HEDP + AA-AMPS copolymer	HEDP 5–10 ppm + AA-AMPS 10–20 ppm	HEDP threshold inhibits CaCO₃; AA-AMPS disperses crystal fines
High LSI (>1.0)	Severe CaCO₃ scaling risk	PBTCA or DTPMP + acid pH control	PBTCA 8–15 ppm + H₂SO₄ to pH 7.0	PBTCA more tolerant of high Ca × HCO₃ product
High silica (>80 mg/L SiO₂ at 4 COC)	SiO₂ scale (sticky, hard to remove)	AA-AMPS high-MW or HPMA	20–40 ppm	Polymer disperses amorphous silica before precipitation
Carbon steel HX, mild corrosion (<10 mpy target)	Carbon steel pitting and general corrosion	HEDP + zinc phosphate or molybdate blend	HEDP 5 ppm + sodium molybdate 10–20 ppm	Molybdate forms passive film on steel
Copper/brass HX (condenser tubes)	Copper dissolution and dezincification	Benzotriazole (BTA) or Tolyltriazole (TTA)	BTA/TTA 1–3 ppm continuous	Azoles form protective complex on Cu surface
Biological control (Legionella risk)	Biofilm formation, Legionella proliferation	DBNPA (fast acting) + CMIT/MIT (residual)	DBNPA 5–10 ppm monthly shock + CMIT/MIT 2–3 ppm weekly	Biocide rotation prevents resistance development
Ongoing oxidizing biocide	General microbiological control	SDIC or TCCA (chlorine release)	Maintain 0.2–0.5 ppm free chlorine	Monitor with DPD test kit; pH 6.5–8.5 for Cl efficacy
Foam in cooling tower (surfactant contamination)	Foaming, loss of tower efficiency	Silicone or polyether defoamer	5–30 ppm continuous or as needed	Investigate foam source; check process contamination
High MIC (microbiologically influenced corrosion)	Sulfate-reducing bacteria (SRB) pitting	Glutaraldehyde 50% (penetrating biocide)	100–500 ppm shock dose quarterly	Glutaraldehyde penetrates biofilm; alternate with oxidizing biocide
High-COC system (>5 COC, zeolith softened makeup)	Multiple scale ions concentrated	DTPMP + HPMA + DBNPA full program	DTPMP 10–15 ppm + HPMA 15–25 ppm + DBNPA 5 ppm	Full multifunctional program for high-stress systems

International Brand Grade	China Equivalent	Major Chinese Producers
NALCO 3DT199 (HEDP scale/corrosion inhibitor)	HEDP 60% technical grade	南京汉德、天津康大、郑州天成
Kurita K-170 (phosphonate cooling water blend)	ATMP 50% + HEDP 60% blend actives	南京汉德、湖南化工研究院
Veolia Hydrex CT-205 (AA-AMPS polymer)	AA-AMPS copolymer 40–50% active	南京汉德、山东天庭、广州立达
Buckman Bulab 6002 (DBNPA biocide)	DBNPA 20% liquid solution	山东绿原、北京日华、山东万科
Dow/Rohm & Haas Kathon 886 (CMIT/MIT)	CMIT/MIT 1.5% (14% active isothiazolinone)	山东绿原、济南金鲁
Lonza Bardac 22 (BKC 50%)	BKC benzalkonium chloride 50% solution	上海和成、山东瑞普
ChemTreat CT1800 (glutaraldehyde 50%)	Glutaraldehyde 50% industrial grade	山东绿原、河北科捷
BASF Irgacor NPA (BTA 100%)	Benzotriazole BTA 99%+ powder	南京汉德、天津诺博
NALCO 8514 (TTA tolyltriazole 50%)	Tolyltriazole TTA 50% liquid	南京汉德、天津诺博
Veolia Hydrex CT-510 (sodium molybdate)	Sodium molybdate 98% technical	湖北振华、山东龙恒

Cooling Water Treatment Chemicals — Scale Inhibitors, Corrosion Inhibitors & Biocides for Industrial Cooling Towers

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All Grades (by chemistry class)

Phosphonate Scale & Corrosion Inhibitors — HEDP, ATMP, DTPMP, PBTCA(4)

HEDP (1-Hydroxyethylidene-1,1-Diphosphonic Acid)

ATMP (Amino Trimethylene Phosphonic Acid)

DTPMP (Diethylenetriamine Penta Methylene Phosphonic Acid)

PBTCA (2-Phosphonobutane-1,2,4-Tricarboxylic Acid)

Polymer Dispersants — AA-AMPS Copolymer, HPMA(3)

AA/AMPS Copolymer Scale Inhibitor

HPMA (Hydrolyzed Polymaleic Anhydride)

Sodium Hexametaphosphate (SHMP)

Non-Oxidizing Biocides — DBNPA, CMIT/MIT, BKC, Glutaraldehyde(4)

DBNPA (2,2-Dibromo-3-Nitrilopropionamide)

CMIT/MIT Isothiazolinone 14%

Benzalkonium Chloride (BKC)

Glutaraldehyde 50%

Azole Corrosion Inhibitors for Copper Systems — BTA, TTA(3)

Benzotriazole (BTA)

Tolyltriazole (TTA)

Sodium Molybdate Corrosion Inhibitor

Cooling Tower Defoamers(2)

Polyether Defoamer

Silicone Defoamer (Emulsion Type)

Imported Brand → China Equivalent

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