In drinking-water plants that use chlorine dioxide for disinfection, the generation, transfer, and dosing system is an important part of stable disinfection operation.

During long-term operation, some plants may encounter white, yellowish-white, or other deposits in dosing lines, valves, flowmeters, and injection assemblies. As the deposits accumulate, flow can decrease, metering can become unstable, and valves may begin to malfunction. In severe cases, stable chlorine dioxide dosing may be affected.

What are these deposits? Why do they form, and how should they be prevented and removed?

The most reliable solution is not simply to “unblock the line” or increase pressure. The entire system—including generator performance, chemical quality, piping design, injection-point conditions, and maintenance practices—should be evaluated.

I. Identify the Deposit Before Deciding How to Remove It

It is too simplistic to assume that every white or yellowish deposit in a chlorine dioxide dosing line is crystallized chlorite or chlorate formed by disproportionation of chlorine dioxide.

Chlorine dioxide can undergo decomposition or transformation depending on light exposure, temperature, pH, and water chemistry, producing chlorite, chlorate, and other species. However, whether these compounds form visible hard deposits depends on solution composition, concentration, evaporation, temperature, and the presence of other ions.

Deposits in a chlorine dioxide system may have several sources. Incomplete generator reactions can leave residual feed chemicals or salts. High local concentrations and long stagnant periods may cause salts to concentrate and crystallize. Hard water may contribute calcium- or magnesium-based scale, while iron and manganese can produce oxide deposits.

When the same line repeatedly blocks, a more reliable approach is to collect a small sample of the deposit and analyze its composition while also reviewing chemical-feed conditions, generator performance, line flow, temperature, and injection-point design.

Only after determining whether the deposit is salt crystallization, hardness scale, iron/manganese oxide, or another material should a specific cleaning method be selected.

Blockage should also not automatically be blamed on poor maintenance. In many systems, it results from a combination of operating conditions, reaction efficiency, water quality, piping geometry, and maintenance practices.

II. Preventing Blockage: A Systematic Strategy from Generation to Injection

Preventing blockage requires more than a single measure. It should be managed throughout chlorine dioxide generation, transfer, injection, flushing, and maintenance.

1. Optimize Generator Operation

Blockage prevention begins with the generator itself.

Whether the system uses a chlorate-based or chlorite-based process, feed chemicals should meet the specifications required by the equipment manufacturer, and concentration, dosing ratio, and flow should remain within the validated operating range.

The objective is not simply to achieve the “highest possible chlorine dioxide purity,” but to maintain stable conversion efficiency and minimize abnormal residual feed chemicals and by-products.

Where appropriate analytical capability exists, operators may trend chlorine dioxide concentration, chlorite, chlorate, raw-material consumption, and other indicators relevant to the specific generator chemistry.

Stable generation, qualified feed chemicals, and reliable metering provide the first line of defense against abnormal deposits.

2. Design Dosing Lines to Minimize Dead Zones and Stagnation

The main objective of the dosing line is to transfer chlorine dioxide solution to the injection point through a stable and reasonably short path while minimizing dead legs, stagnant pockets, and backflow.

The line should not be made unnecessarily long, but it should still allow safe pressure control, maintenance, and inspection.

Valve selection should not be reduced to a rule that ball and globe valves are always unacceptable while diaphragm and butterfly valves are always preferred.

In small chemical dosing systems, diaphragm valves, compatible check valves, and back-pressure valves are commonly used. Full-port ball valves may also be suitable in some applications.

The main objective is to avoid configurations that trap chemical solution or cannot be completely drained and flushed.

Flow velocity should also not be universally fixed above 1.5 m/s. Whether flow is laminar or turbulent depends on pipe diameter, flow rate, fluid properties, and Reynolds number. Excessive velocity can also increase pressure loss and equipment loading.

An appropriate velocity should therefore be established from the actual design flow and pipe diameter, with particular attention to avoiding long periods of stagnation or very low flow.

Material compatibility must also be evaluated under the actual chlorine dioxide concentration, temperature, pH, chloride level, and exposure time.

PTFE and some grades of CPVC are chemically resistant in many chlorine dioxide applications, but manufacturer compatibility data should still be checked. 316L stainless steel should not automatically be described as universally suitable, particularly at higher oxidant concentrations, higher temperatures, or in chloride-containing environments.

3. Use a Proper Flushing Program

For intermittently operated lines or injection points where local concentration can increase, an appropriate flushing program is often valuable.

After dosing stops, clean water or suitably treated water may be used—where the system design allows—to displace residual chemical solution and reduce the risk of concentration, decomposition, or crystallization during extended stagnation.

Continuous low-flow carrier water, however, is not suitable for every system. Poorly designed carrier-water systems can affect actual dosing concentration, metering accuracy, or create backflow risks.

The flushing arrangement should therefore include appropriate check protection, interlocks, and a defined discharge route and should be commissioned to ensure that it does not interfere with normal dosing.

4. Temperature Management Means Avoiding Extremes, Not Heating Every Line Above 15°C

The original recommendation to keep every dosing line above 15°C requires correction.

Low temperature may affect the solubility of certain salts, but chlorine dioxide itself can decompose and off-gas more rapidly when exposed to excessive heat.

In cold climates, the priority should be freeze protection in accordance with equipment-manufacturer requirements.

At the same time, product solution and piping should be protected from direct sunlight and excessive heating.

Whether insulation or heat tracing is needed, and what temperature range should be maintained, should be determined according to the generator chemistry, equipment design, and manufacturer specifications rather than a universal 15°C rule.

5. Establish Proactive Monitoring and Maintenance

Effective blockage prevention should shift from reactive cleaning to trend-based maintenance.

Routine inspection should include changes in dosing flow, metering-pump discharge pressure, flowmeter stability, valve performance, and visible deposits around injection points or transparent sections.

Where appropriate, flow, pressure, or differential-pressure trends can help identify increasing hydraulic resistance. However, rising pressure alone does not prove blockage because valve position, back pressure, and downstream water pressure can also affect readings.

Preventive cleaning frequency should not be fixed at “every quarter” or “every six months.” It should be based on actual fouling rate, pressure and flow trends, shutdown frequency, and equipment-manufacturer guidance.

Most importantly, a chemical cleaner should not be selected until the nature of the deposit is understood.

Sodium bisulfate is an acidic salt rather than a general reducing agent. If sodium bisulfite was intended, that is a different chemical altogether. Hydrogen peroxide can also behave differently depending on reaction conditions and should not be treated as a universal cleaner for chlorine dioxide systems.

A safer maintenance approach is to stop generation and dosing, isolate the affected section, thoroughly flush and displace residual chemicals, confirm that hazardous chlorine dioxide concentrations and active feedstocks are no longer present, and then select an appropriate cleaning method based on deposit analysis and manufacturer recommendations.

If chlorite, chlorate, or other oxidizing residues may still be present, strong acids such as hydrochloric acid should never be introduced without a formal risk assessment because acidification of certain chlorine-oxygen systems can generate hazardous chlorine dioxide or chlorine gas.

Chemical cleaning should therefore follow a simple principle: identify the deposit first, confirm material and chemical compatibility second, and only then develop the cleaning procedure.

Critical components such as flowmeters, check valves, diaphragms, and injection valves may be stocked as appropriate spare parts, and the plant should establish a safe shutdown, flushing, inspection, and restart procedure.

Preventing blockage in a chlorine dioxide dosing system is a system-engineering task. It begins with generator conditions, raw-material quality, and conversion efficiency and continues through piping geometry, valves, flushing design, temperature management, and routine maintenance.

The most effective approach is not to memorize fixed numbers such as “1.5 m/s flow velocity,” “15°C minimum temperature,” or “clean every six months.” It is to identify the actual deposit mechanism in the specific plant and establish control parameters that reflect real system behavior.

Every water treatment plant has different water quality, generator chemistry, chemical concentration, pipe diameter, flow, and operating pattern. Continuous observation, analysis, and optimization are therefore necessary to develop a truly effective anti-blockage strategy.