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Recently, operators at a small-town drinking water treatment plant noticed an unusual fluctuation in finished-water quality: disinfection performance declined, noticeable odors appeared, and routine monitoring showed abnormal increases in organic matter and ammonia nitrogen.
An investigation found that wastewater from a small medical facility upstream had accidentally entered the source water, introducing refractory organic matter, ammonia nitrogen, and pharmaceutical residues. This placed a significant shock load on the conventional treatment process and affected the performance of the existing chlorine dioxide disinfection system.
For some small and medium-sized water plants, limitations in scale, investment, and operating conditions mean that relatively simple treatment trains such as coagulation–sedimentation–filtration–chlorine dioxide disinfection are commonly used. These systems are comparatively economical and easy to manage. However, sudden complex organic pollution, ammonia nitrogen shocks, or pharmaceutical residues can pose serious challenges to such conventional processes.
Chlorine dioxide (ClO₂) is a broad-spectrum, strong oxidizing disinfectant that provides effective inactivation of many bacteria and viruses. Compared with conventional chlorination, chlorine dioxide has a lower tendency to form certain halogenated organic disinfection by-products. However, chlorite, chlorate, and other related transformation products still require careful control.
Chlorine dioxide is not a universal solution.
An increase in ammonia nitrogen does not necessarily mean that chlorine dioxide will be directly consumed in large quantities in the same way as free chlorine. However, sudden pollution events are often accompanied by increases in organic matter, nitrite, and other reducing substances. These compounds can increase oxidant demand and reduce the amount of effective chlorine dioxide available for disinfection.
Some reducing organic and inorganic substances introduced during a pollution event may also increase chlorine dioxide demand and reduce the residual concentration. The actual effect depends on the type and concentration of the pollutants.
Different pharmaceutical micropollutants vary considerably in their reactivity with chlorine dioxide. Some may not be sufficiently removed under conventional disinfection conditions, and their oxidation transformation products should also be considered.
Once the source water is affected by this type of sudden contamination, continuing to operate the plant using normal treatment parameters may result in reduced disinfection performance, unpleasant odors, deterioration of sensory water-quality indicators, and potentially increased health risks.
If abnormal source-water conditions are detected, particularly unusual increases in ammonia nitrogen, permanganate index, TOC, or pharmaceutical and other characteristic pollutants, the plant should promptly activate its emergency response procedures.
The source of the contamination should be traced upstream as quickly as possible, and relevant authorities should be coordinated with to stop further pollutant input.
At the same time, the monitoring frequency for critical parameters should be increased according to pollutant transport speed, water intake volume, and on-site analytical capacity. Parameters suitable for online monitoring should be monitored continuously or at high frequency.
Optimize coagulation: Based on changes in source-water quality, jar testing can be used to optimize the type and dosage of coagulants such as PAC (polyaluminum chloride), improving the removal of particulate matter and certain organic compounds.
Activate powdered activated carbon treatment if available: Where a powdered activated carbon dosing system is installed, the emergency dosage should be determined according to the target pollutant, source-water concentration, and adsorption-test results. This can help reduce certain organic micropollutants, pharmaceutical residues, and odor-causing substances.
Reassess the chlorine dioxide dose: The required dose should be recalculated according to source-water oxidant demand, target microorganisms, actual chlorine dioxide residual, and by-product control requirements. It should not simply be increased by a fixed multiple of the normal dose.
Control the disinfectant residual: Chlorine dioxide residual should be controlled in accordance with applicable drinking-water standards and the specific treatment process, while chlorite, chlorate, and other relevant parameters should also be monitored.
Optimize contact conditions: The required contact time should be reassessed according to the effective chlorine dioxide concentration, target microorganisms, water temperature, and actual hydraulic conditions. A fixed contact time alone should not be used as the basis for determining disinfection effectiveness.
Where necessary, treatment flow or operating conditions should be adjusted in accordance with the plant’s emergency response plan and water-supply dispatch procedures to provide sufficient time for abnormal-water treatment.
Consider secondary or backup disinfection: If a backup disinfection process needs to be activated, its compatibility with the existing process, potential by-product risks, and actual disinfection performance should first be evaluated under a validated emergency plan.
If validated UV disinfection facilities are available, they may be used as an additional treatment barrier provided that turbidity, UV transmittance, and delivered UV dose meet the required conditions. However, UV cannot replace the need for a persistent disinfectant residual where one is required in the distribution system.
Targeted finished-water testing: Immediately conduct targeted water-quality testing, focusing on key routine parameters, microbiological indicators, characteristic pollutants identified through source investigation, and relevant disinfection by-products.
Distribution-system monitoring: Increase sampling frequency at distal points in the distribution network to verify that distributed water remains safe.
Timely public communication: Information on the water-quality incident, affected areas, and recommended public actions should be communicated accurately and promptly through the established emergency information system. Updates should be provided as new monitoring results become available.
If a chemical contamination risk in finished water has not yet been ruled out, drinking-water advice issued by the competent authorities should be followed. Where necessary, affected water supplies should be suspended or safe alternative drinking-water sources should be provided. Boiling alone should not be treated as a solution to chemical contamination.
Although this is a hypothetical scenario, it highlights an important reality: source-water contamination risks can occur even in small cities.
Small and medium-sized water plants should therefore strengthen their resilience.
Improve the monitoring system: According to source-water risks, analytical capability, and available investment, appropriate online or rapid-monitoring methods for ammonia nitrogen, TOC, permanganate index, and other relevant indicators can be installed to establish an early-warning system for abnormal source-water conditions.
Maintain emergency chemicals and equipment: Emergency treatment materials such as powdered activated carbon should be kept available where appropriate, and dosing equipment should remain ready for use.
Develop practical emergency response plans: Specific process-adjustment strategies and operating checklists should be prepared for different types of contamination, including organic pollutants, ammonia nitrogen, pesticides, and pharmaceutical residues.
Conduct regular training and emergency drills: Routine emergency exercises can improve operators’ ability to quickly identify abnormal conditions and implement appropriate responses.
Some small and medium-sized drinking water plants may have relatively simple treatment processes, but their responsibility for water safety is no less important.
When faced with sudden complex contamination, calm decision-making, rapid response, and scientifically justified process adjustments are essential to protecting drinking-water quality.
More importantly, sudden chemical contamination of source water is not simply a case of “chlorine dioxide failure.” It is a shock to the entire drinking-water treatment process caused by an abnormal pollutant load.
The core emergency strategy should therefore be to control the pollution source, identify the contaminants, and assess whether continued water abstraction is safe before adjusting treatment. Targeted coagulation, adsorption, and other enhanced treatment measures can then be used to reduce the pollutant load, followed by appropriate adjustment of the disinfection process while keeping by-product risks under control.
By combining immediate enhanced treatment with long-term emergency preparedness, even relatively simple small and medium-sized water treatment plants can strengthen their ability to respond to sudden source-water contamination and maintain reliable water-supply protection.
Chlorine dioxide can efficiently disinfect drinking water, is not easy to generate carcinogenic disinfection by-products, and will produce chlorite and other residues, which must comply with the national standard limits. It shall be accurately added according to the water quality and equipped with qualified equipment and process. The safety of compliant use is high. Water supply enterprises need to do a good job in control and monitoring to ensure the safety of water quality.
Chlorine dioxide is a who Certified grade A1 safe disinfectant with high sterilization efficiency, few harmful by-products, strong water quality adaptability and multiple water purification functions; Compared with traditional chlorine disinfection, it is safer and more environmental friendly, and can be widely used in drinking water, sewage treatment, food and beverage disinfection and other fields.
Chlorine dioxide can sterilize and remove odors in water treatment plants and is unlikely to generate trihalomethanes. However, in practical applications, it can fail to remove odors or even produce new unpleasant smells. Organic matter in the water source, bromide ions, and low temperatures can affect treatment effectiveness. Furthermore, its removal capacity for geosmin and 2-methylisoborneol is limited, and improper dosage can also cause odor problems. Water treatment plants need to strengthen water quality monitoring, dynamically adjust dosages, and, when necessary, combine ozone-activated carbon processes to ensure that the odor of the treated drinking water meets standards.
When adding chlorine dioxide to the water pumping station to maintain residual chlorine in the pipeline network, it is necessary to avoid pipeline corrosion caused by local high concentrations. The dosage should be accurately controlled by flow-linked dosing, pipeline mixing, and online residual chlorine monitoring, while also paying attention to under-scale corrosion, thus balancing disinfection and pipeline protection.
Pipeline network renovations can easily alter the hydraulic conditions of the network, increasing flow velocity and creating stagnant zones that consume chlorine dioxide, leading to fluctuations in water quality. Water plants can ensure the quality of water supplied to the end points through monitoring and prediction, dynamic chemical dosing, pipeline flushing, and the activation of contingency plans.
Traditional chlorine disinfection is prone to producing carcinogenic halogenated by-products; chlorine dioxide oxidizes and kills bacteria, with almost no such by-products, good inactivation effect, and the ability to remove odor and inhibit algae, but it generates by-products such as chlorite, requires higher raw material and operation and maintenance costs, and is suitable for water plants that can be upgraded and managed in a coordinated manner, rather than being simply and directly replaced.
This study simulates a scenario where a small town's water plant experiences a sudden pollution outbreak caused by medical wastewater and the failure of chlorine dioxide disinfection. It analyzes the reasons why pollutants consume disinfectant and provides four-step emergency response measures. Furthermore, it proposes long-term improvement suggestions in terms of monitoring, supplies, contingency plans, and drills to enhance the emergency response capabilities of small and medium-sized water plants.
Switching from sodium hypochlorite to chlorine dioxide disinfection in water plants is not just about replacing equipment; it's a complete system upgrade. Chlorine dioxide disinfection is more effective, produces fewer byproducts, and has lower overall costs. However, many water plants only replace the hardware, neglecting personnel training and adapting management systems. The core of this transformation is shifting from experience-based to scientific operation.
The actual dosage of chlorine dioxide at water treatment plants often exceeds the theoretical value, influenced by factors such as reducing agents in the water, sediment in the pipe network, water quality fluctuations, and byproduct consumption. By leveraging monitoring and local operational data to optimize dosage, we can provide water quality assessments and customized dosage solutions.
A water plant in a county in Qinghai Province with a capacity of 30,000 cubic meters per day was found to have excessive byproducts from sodium hypochlorite disinfection. It was upgraded to a Xiuba chlorine dioxide dosing system. Four on-site tests showed that residual chlorine dioxide in the effluent was within acceptable limits, achieving stable compliance. Water treatment upgrades need to be tailored to specific on-site conditions to create a complete closed-loop disinfection process.
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