Future-proofing Water Disinfection for Chlorate Regulation

Chlorate (ClO3-) continues to be a much-watched contaminant by both regulatory bodies and the water industry. With potential chlorate-producing processes in food preparation, agriculture, or within the water treatment process, scientific studies into this contaminant have increased in number and priority. 

What is Chlorate?

Chlorate is a highly oxidized form of chlorine that can be introduced to a water source as an industrial or agricultural contaminant or into finished water as a disinfection by-product (DBP). As a DBP, Chlorate can result from water disinfected with bulk sodium hypochlorite, chlorine dioxide, or hypochlorite formed through on-site electrolytic generation (OSG) systems.

Chlorate was added to the Third Chemical Contaminant List (CCL3) in 2010, indicating that the Environmental Protection Agency (EPA) is reviewing chlorate as a potential candidate for regulation under the Safe Drinking Water Act. 

In 2014, chlorate was evaluated as a candidate for regulation under the Regulatory Determinations 3 (RD 3) program, which indicates the EPA’s growing attention to chlorate. While there is no indication that chlorate is a potential carcinogen to humans, adverse health impacts such as thyroid issues, reduced hemoglobin production and reduced weight gain have been observed in laboratory animals subjected to prolonged exposure to chlorate.

The current regulatory status

The World Health Organization (WHO) recommends a chlorate limit of 0.7 mg/L (700 µg/L). In Canada, chlorate is regulated with a limited concentration level of 1.0 mg/L (1000 µg/L). 

Currently, the EPA does not regulate chlorate in drinking water, meaning there is no enforceable Maximum Contaminant Limit (MCL) at this time. 

While no final recommendations have been promulgated, recent literature on the topic indicates that new regulations may fall within the range of 0.21 mg/L (210 µg/L) to 0.8 mg/L (800 µg/L) in the US. 

Though the EPA has established 210 µg/L as a health reference level (HRL), it is speculated that it will not set regulation at such a low level as this could severely impact the viability of using delivered bulk hypochlorite for disinfection.

What is the impact on water treatment plants using delivered sodium hypochlorite?

Several studies have been conducted by researchers regarding the occurrences of chlorate in drinking water and the factors that influence chlorate introduction. Concerning delivered hypochlorite, chlorate mainly arises as a degradation product of hypochlorite ions. Hypochlorite degradation is exacerbated by several factors, including:

• The freshness of the solution — older hypochlorite solutions will have higher relative chlorate concentrations
• Concentration of delivered solution — higher concentration hypochlorite solutions degrade more rapidly than lower concentration solutions
• Storage temperature — higher temperatures increase the rates of hypochlorite degradation and chlorate production
• Solution pH-bulk sodium hypochlorite is typically formulated to have a pH in the range of 12 — 13 to minimize chlorate production upon storage, making it a highly caustic chemical requiring safety oversight

Because of these factors, some water treatment plants will struggle with using bulk hypochlorite depending on how fresh they can purchase the product from chemical manufacturers and how quickly they can use the solution. If chlorate is regulated as a disinfectant by-product (DBP), most utilities that use bulk hypochlorite will have to significantly alter their use of this chemical to avoid exceeding the regulatory limits. These changes will also result in a substantial increase in operational costs. Operational changes may include:

• Cooling of the room where the hypochlorite is stored to slow hypochlorite degradation
• Requiring “born-on dating” labeling from the manufacturers
• Limiting storage volumes, which may not be feasible in locations that require long term supplies of water treatment to be stored at the treatment plant
• Purchasing lower concentrations to slow hypochlorite degradation
• Dilution of the concentrated hypochlorite once it has been delivered to the treatment plant

Since they would involve increased logistical complexities as well as higher operating costs, any of these operational adaptations can be expected to create significant hardship for water treatment plant operators. These impacts would be felt the most for the lowest chlorate MCLs that are being contemplated.

On-site generation

On-site generation (OSG) uses basic, simple chemicals to generate oxidant at the point of use and is often used for safety, environmental, and cost benefits. In this case, the hypochlorite is generated using salt. Softened water is mixed with salt in a brine tank to form saturated brine. The saturated brine then enters the electrolytic cell, and an electrical current is passed through the electrolytic cell, producing the oxidant:

• Anode Primary Reaction (+ Side): 2 Cl- → Cl2 + 2 e-
• Cathode Reaction (- Side): 2 H2O + 2 e- → H2↑ + 2 OH-
• Chlorine Hydrolysis Reaction: Cl2 + H2O → HOCl + Cl- + H+
• HOCl Equilibrium Reaction: HOCl ↔ OCl- + H+ (depends on pH)

Hydrogen gas produced during the electrolysis process is vented outside, and the oxidant solution leaves the electrolytic cell and is stored in an oxidant tank. The oxidant solution is dosed into the treatment process by a metering pump. The on-site generator is turned on and off from a signal located inside the oxidant tank. During this electrochemical process, chlorate ions are produced as an undesirable side reaction. In the process, a portion of the hypochlorous acid or hypochlorite ions are oxidized at the anode to produce chlorate ions.

De Nora on-site generation

De Nora has a deep understanding of many of the factors involved in the production of chlorate using sodium chloride brine electrolysis, including storage time, storage temperature and hypochlorite concentration. As a result, our labs have developed technology to control and influence these factors and currently offer multiple on-site generation systems that address some of the biggest concerns surrounding chlorate formation, including:

• Chlorate formation from hypochlorite solutions generated by De Nora electrolysis is well below the proposed regulatory levels. Chlorate is expected to be regulated by the EPA, likely at or near the international WHO guideline of 0.7 mg/L (700 µg/L). Extensive testing on the chlorate production by De Nora OSG systems has been undertaken both internally and in conjunction with third-party studies. Typically, the De Nora systems produce chlorate at a rate of fewer than 40 micrograms (0.04 mg) per milligram of free available chlorine (FAC).
   
• Since the hypochlorite solution is generated on-site and as needed, storage time is typically 24 hours or less, minimizing hypochlorite degradation.
   
• The hypochlorite solution is generated at less than 1% chlorine concentration, which also reduces the chlorine degradation rate.

Therefore, even at a high FAC dose of 5 mg/L, the expected chlorate concentration in the treated water will be less than 200 µg/L (0.2 mg/L), well below the likely 700 µg/L limit and is even below the EPA health reference limit of 210 µg/L. Typically, water treatment plants only dose 2 – 3 mg/L as free available chlorine. At these dose points, chlorate content in the finished water will likely be half of the lowest contemplated chlorate regulatory limit.

De Nora — including MIOX, which they acquired in early 2019 — has been working independently for several years to gain a better understanding of how chlorate is produced during electrolysis. As part of these studies, scientists have utilized several laboratories to actively research the mechanisms resulting in the production of chlorate during electrolysis. Through these ongoing studies, they hope to ensure that their products will be on track to meet future chlorate regulatory requirements. 

Several new patented technologies are expected to result from these research efforts, ensuring that De Nora electrochlorination systems will be able to meet even the strictest chlorate limitations as regulations continue to evolve.

While it’s impossible to know if, when, and how long EPA might regulate the production of chlorates in drinking water, De Nora — a company committed to electrolysis for more than 95 years — is sure to be at the forefront of new technologies to meet and exceed these future regulations.

Source: DNWT – Chlorate DBPs: Future-proofing Water Disinfection for Chlorate Regulation, 8/2019