As an engineer in a chemical plant’s water treatment workshop, I know drinking water safety is central to public health. Bactericides are key fine chemicals that ensure drinking water is biologically safe. In real-world operations, drinking water treatment must address natural bacteria in water sources, pipeline biofilms, and secondary contamination. The selection, dosage control, and working condition adaptability of bactericides directly determine treatment effectiveness and outlet water safety. This article draws on years of on-site experience to analyze core points of bactericide use in drinking water treatment from a practical application perspective.
1. Core Requirements and Type Selection of Bactericides for Drinking Water Treatment
Bactericides for drinking water treatment must meet three core requirements: high efficiency and broad-spectrum activity (quickly kill bacteria, fungi, algae, and other microorganisms), safety (no residual toxicity, no carcinogenic disinfection by-products), and working condition adaptability (adapt to fluctuations in water source pH, temperature, turbidity, etc.). Currently, mainstream types in industrial applications are divided into oxidizing and non-oxidizing categories. They have distinct application scenarios.
1.1 Oxidizing Bactericides: Mainstream Front-End Disinfectants
Oxidizing bactericides destroy microbial cell structures by releasing reactive oxygen or chlorine free radicals. They disinfect quickly and cost little. They are the “first line of defense” in drinking water treatment. The most widely used ones are chlorine dioxide (ClO₂) and sodium hypochlorite (NaClO). When our plant treats water from the Yellow River, chlorine dioxide gradually replaced traditional liquid chlorine as the first choice. It doesn’t react with organic matter in water to form disinfection by-products like trihalomethanes (THMs).
Key points in practical operation: Chlorine dioxide must be prepared on-site by reacting sodium chlorate with hydrochloric acid. The generator must strictly control the raw material ratio (usually 1:1.05) to avoid residual unreacted raw materials. Dosage must be adjusted based on water source turbidity. If turbidity exceeds 10 NTU, coagulation and sedimentation must be done first. Otherwise, organic matter will consume available chlorine, reducing disinfection efficiency by over 30%.
1.2 Non-Oxidizing Bactericides: Key to Pipeline Biofilm Control
Non-oxidizing bactericides work by inhibiting microbial enzyme activity or destroying cell membranes. They are less affected by organic matter and effective at removing biofilms. They are mostly used for back-end pipe network maintenance or as supplements to oxidizing bactericides. In our plant’s pipe network terminal water maintenance, quaternary ammonium salts (such as dodecyldimethylbenzyl ammonium chloride) and isothiazolinone are commonly used.
Quaternary ammonium salts are suitable for cleaning biofilms on the inner walls of water storage tanks. Soak at a concentration of 100-200 mg/L for 2 hours, then rinse. This effectively removes attached iron bacteria and algae. Isothiazolinone is used for long-term pipe network bacteriostasis. The dosage is controlled at 0.5-1 mg/L. It must be dosed separately from oxidizing bactericides (interval no less than 2 hours) to avoid reduced efficacy due to oxidation.
2. Key Operational Points and Parameter Control for Bactericide Application
In on-site applications, bactericide effectiveness doesn’t just depend on the agent itself. The design of the dosing system, working condition parameter monitoring, and linkage adjustment are the core. Below are three key control links summarized from our plant’s actual cases.
2.1 Optimization of Dosing Points and Mixing Methods
Oxidizing bactericides must be dosed after coagulation and sedimentation but before filtration. This ensures sufficient contact time in the clear water tank (CT value ≥ 60 mg·min/L, where C is disinfectant concentration and T is contact time). Non-oxidizing bactericides are dosed at multiple points on the outlet water pipeline to avoid uneven agent distribution from single-point dosing.
In terms of mixing equipment selection, our plant upgraded traditional static mixers to pipeline ultrasonic mixers. This increased the mixing uniformity of the agent and water from 85% to 98%. It effectively solved the problem of insufficient residual chlorine in terminal water.
2.2 Dynamic Parameter Monitoring and Automatic Adjustment
Establishing a linked monitoring system for “water source quality – bactericide dosage – outlet water residual chlorine” is key. Our plant installed online pH and turbidity monitors at the raw water inlet. An online residual chlorine analyzer is set at the clear water tank outlet. Data is transmitted to the PLC control system in real-time.
When the raw water pH rises from 7.5 to 8.5, the system automatically increases the chlorine dioxide dosage from 1.2 mg/L to 1.5 mg/L. When residual chlorine is below 0.3 mg/L (national standard limit), an alarm is triggered immediately and the dosage is increased. This system raised the qualified rate of outlet water from 92% to 99.5% and reduced the lag of manual adjustment.
2.3 Agent Compatibility and Safety Control
Mixing different agents may pose safety risks. For example, chlorine dioxide and ammonia nitrogen coexisting will form chloramines, reducing disinfection effectiveness. Quaternary ammonium salts mixed with anionic coagulants will produce flocculent precipitation.
Therefore, on-site operations must strictly follow the principle of “separate storage and dosing of agents”. Dosing pipelines use corrosion-resistant UPVC materials to avoid chemical reactions with metal pipelines. At the same time, operators must be equipped with gas masks and corrosion-resistant gloves. A ventilation system must be installed near the chlorine dioxide generator to prevent respiratory irritation from gas leakage.
3. Common Problems and On-Site Solution Cases
In actual operation, bactericide application often faces working condition fluctuations and effectiveness attenuation. Below are typical cases and solutions from our plant.
Case 1: Disinfection Failure Due to Sudden Increase in Water Source Turbidity During Rainy Season
After heavy rain in July one year, the raw water turbidity rose from 5 NTU to 35 NTU. Even when the chlorine dioxide dosage was increased to 2 mg/L, the water still failed to meet standards. On-site analysis found that suspended particles in high-turbidity water adsorbed a large amount of available chlorine, leading to insufficient microbial killing.
Solution: Increase the dosage of polyaluminum chloride (PAC) in the coagulation stage (from 20 mg/L to 40 mg/L). Extend sedimentation time to 2 hours. Dosing chlorine dioxide only after turbidity drops below 15 NTU. At the same time, add 0.2 mg/L of isothiazolinone at the clear water tank outlet to ensure residual chlorine in terminal water meets standards.
Case 2: Water Odor in Terminal Pipe Network Due to Biofilm Growth
Some old pipe network terminals had fishy-smelling water. Testing showed excessive iron bacteria content. The reason was long-term use of only oxidizing bactericides, which could not penetrate the interior of biofilms.
Solution: Conduct “shock dosing” once a month. Increase the concentration of quaternary ammonium salt bactericide to 500 mg/L. Circulate and rinse through the drainage valve at the end of the pipe network for 2 hours. At the same time, adjust the daily dosing plan to a compound use of “chlorine dioxide (1 mg/L) + isothiazolinone (0.3 mg/L)”. The biofilm problem was basically solved after 3 months.
4. Application Summary and Future Trends
Bactericide use in drinking water treatment must follow the principles of “select based on needs, precise dosing, and dynamic regulation”. For front-end disinfection, prioritize oxidizing bactericides with low by-products like chlorine dioxide. For back-end pipe network maintenance, use non-oxidizing bactericides as supplements. On-site operations must strengthen parameter monitoring and equipment linkage to avoid effectiveness impact from working condition fluctuations. At the same time, pay attention to agent safety and compatibility to prevent operational risks.
In the future, with the improvement of water quality standards, bactericide application will move towards “greenization and compounding”. Our plant has begun piloting the use of hydrogen peroxide and silver ion compound bactericides. Their disinfection by-products are only water and oxygen. Their biofilm removal effect is 40% better than single agents. It is believed that with the progress of fine chemical technology, bactericides for drinking water treatment will achieve a better balance between efficiency and safety, providing more reliable protection for water supply safety.