Industrial circulating cooling water systems are vital for the stable operation of equipment in petrochemical, power, metallurgical, and manufacturing industries. However, during long-term open operation, cooling towers readily ingest airborne dust and particulates. Meanwhile, suspended solids from makeup water, as well as microorganisms, algae, and biofouling that proliferate within the system, can cause fouling, blockage, and corrosion of heat exchangers, thereby reducing heat exchange efficiency.
Poly(diallyldimethylammonium chloride) (PolyDADMAC), as a water-soluble cationic polymer with high charge density, demonstrates practical "multi-effect" value in circulating cooling water treatment. Its primary function is to act as an efficient coagulant for turbidity control, while its quaternary ammonium chemical structure provides a certain inhibitory and auxiliary biocidal effect against bacteria and algae in the system. This article discusses the two core functions of PolyDADMAC in cooling water systems and their application strategies.
I. Function 1: Efficient Flocculation and Suspended Solids (SS) Control
1. Capture of Wind‑blown Dust and Makeup Water Particulates
Open cooling towers act like large air scrubbers, washing airborne dust, grit, and fine suspended matter into the circulating water. The high‑charge‑density PolyDADMAC rapidly neutralizes the negative charges on these fine suspended particles, destabilizing their dispersion, and through polymer bridging, aggregates them into flocs, significantly reducing water turbidity. This action remains stable over a wide pH range (typically 4–10), showing good adaptability.
2. Prevention of Silt Deposition on Heat Exchanger Surfaces
Fine suspended solids tend to deposit in the tube side of heat exchangers or in plate‑and‑frame heat exchanger channels where flow velocities are low, forming mud deposits. Such deposits not only hinder heat transfer but also cause under‑deposit corrosion. Regular dosing of PolyDADMAC promotes the concentration and removal of these fine particles in side‑stream filtration systems or settling basins, effectively preventing their accumulation on critical heat transfer surfaces.
II. Function 2: Auxiliary Algicidal/Bactericidal Action and Biofouling Control
1. Antimicrobial Mechanism of the Quaternary Ammonium Structure
The main chain of PolyDADMAC is densely populated with cationic quaternary ammonium groups, which form the chemical basis for its auxiliary bactericidal and algistatic effects:
- Disruption of cell membrane structure: The positively charged quaternary ammonium groups strongly adsorb onto the negatively charged cell walls/membranes of bacteria and algae, altering membrane permeability and causing leakage of intracellular contents, thereby inhibiting or killing some microorganisms.
- Interference with metabolic processes: After adsorption on the cell surface, the agent can affect normal material exchange and metabolic activities, thus limiting reproduction.
It should be noted that in cooling water treatment practice, the biocidal effect of PolyDADMAC is generally considered as an auxiliary function; its core role remains as a coagulant/flocculant. To achieve stronger disinfection, it is usually necessary to combine it with dedicated oxidizing or non‑oxidizing biocides.
2. Removal and Inhibition of Biofilm (Biofouling)
Heterotrophic bacteria and algae in circulating water readily secrete extracellular polymeric substances (EPS), which bind with silt to form complex biofouling. The thermal resistance of biofouling is much higher than that of inorganic scale, significantly affecting heat transfer efficiency. PolyDADMAC not only kills active microorganisms in the slime but also weakens the adhesion of the slime to metal surfaces through charge interactions; combined with hydraulic shear forces, it gradually strips and removes the fouling, thereby keeping heat exchange surfaces clean.
III. Comprehensive Operational Advantages of the Dual Functions
| Conventional Single‑Agent Approach | PolyDADMAC Dual‑Function Approach |
|---|---|
| Requires separate dosing of coagulant and biocide, increasing chemical types and management costs. | Combines flocculation with auxiliary algistatic action, reducing the frequency or dosage of dedicated biocides and simplifying daily management. |
| Sediment and biofouling are treated alternately, leading to repeated contamination. | Simultaneously clarifies water and inhibits microorganisms, reducing the formation of combined mud‑biofilm deposits from the source. |
| Long‑term single use may induce resistance, and residues may have environmental impacts. | The polymeric cationic mode of action is less likely to induce resistance, and it is phosphorus‑free and halogen‑free, reducing environmental load while providing auxiliary biocidal effects. |
IV. Dosing Strategies and Precautions in Cooling Water Systems
1. Synergy with Side‑Stream Filtration
The flocs formed by PolyDADMAC need to be removed promptly through the side‑stream filtration units (e.g., sand filters, disc filters) of the circulating system. In engineering practice, it is recommended to place the dosing point in the return water line or the cooling tower basin, so that the flocs fully react in the system before being intercepted by the side‑stream filter. Dosing directly upstream of the side‑stream filter inlet may increase filter loading and shorten backwash cycles. At the same time, regular monitoring of influent and effluent turbidity of the side‑stream filter and timely backwashing are necessary to maintain removal efficiency.
2. Chemical Compatibility Assessment
Circulating cooling water systems usually also contain corrosion and scale inhibitors (e.g., organic phosphonates, polycarboxylates, etc.) which are anionic. Since PolyDADMAC is strongly cationic, direct mixing with high‑concentration anionic chemicals in an undiluted state may cause charge neutralization and precipitation. Therefore, the chemicals should be diluted separately and added at different points or at staggered times to avoid direct contact of the concentrated solutions. In practice, at normal dosing concentrations (active ingredient typically 1–10 mg/L), such interactions are generally manageable, but on‑site compatibility tests are still recommended to determine the appropriate approach.
3. Control of Appropriate Dosing Concentration
During routine maintenance, the dosage should be adjusted dynamically based on the cycle of concentration, influent water quality, and microbiological monitoring data (e.g., heterotrophic plate count, turbidity). Regular jar tests are recommended to determine the economically effective dosage range, avoiding overdosing that wastes chemicals or imposes unnecessary load on the system.
Conclusion
With its high cationic charge density and unique molecular structure, PolyDADMAC achieves the practical dual functions of "efficient flocculation and clarification" and "auxiliary biocidal and biofouling removal" in industrial circulating cooling water systems. Proper application of PolyDADMAC not only helps improve system water quality and enhance heat exchange efficiency but also simplifies chemical management procedures, providing reliable technical support for the stable operation of circulating cooling water systems. In actual practice, a tailored dosing program should be developed based on specific water quality and system conditions to balance treatment effectiveness and operational costs.