Taking control of chlorate – your questions answered

Reducing chlorate formation is rising in importance as water companies work to meet new water quality regulations under the Drinking Water Directive. Evoqua recently held a webinar exploring the ways in which on-site electrochlorination solutions can help operators confidently meet new chlorate targets and received some interesting questions. Here, Ryan Flood, global product manager and Nigel Knight, regional sales manager at Evoqua Water Technologies respond to some of the most frequently asked questions.   

Firstly, for anyone not too familiar with chlorate, it is a highly oxidized form of chlorine and disinfection by-product (DBP) that can form where bulk sodium hypochlorite, chlorine dioxide, or on-site electrochlorination systems are used for disinfection. Today, the World Health Organization (WHO) recommends a chlorate limit of 0.7 mg/L (700 µg/L). The new Drinking Water Directive aims to address concerns about the high concentrations on human health.  


Q. Any indication andevidence why EFSA (European Food Safety Authority) sets more stringent standards for chlorate than WHO (World Health Organization)? 
A. It is based on the availability of new scientific research that a number of groups have been working towards which has gone further to evaluate chlorate values. As is the case with other areas of water quality, research is ongoing and groups like the EFSA are driving this forward.  


Q. Are reliable online chlorate monitors readily available?

A. The key here is reliability. There is a lot of talk of analysers and monitors, but the best methodology now would be ion chromatography, and that’s not typically something that’s done to monitor online or even available on-site at a municipality. There are some municipalities that invest in this on-site but the majority, in our experience, will still send out samples to an accredited laboratory to conduct the analysis. But as more is learned about chlorates, I’m sure we will see technologies come to market.


Q. In what form is salt delivered to site typically? 

A. It all depends on the size of the water treatment unit. Some of the larger units bring it in by tanker and it’s blown into salt saturator tanks. On the much smaller plants, it is brought in by bags and then installed into the brine solution or softened water system.  The volume of salt deliver is typically 1/4th of the volume of bulk hypo deliveries, reducing your caron footprint.


Q. As part of the optimisation of chlorate, does this include the storage conditions of chlorate produced for UK water companies?

A. This is down to how the municipality is handling their storage. What is notable with onsite electrochlorination, as opposed to bulk hypo deliveries, is that we recommend one to two days max storage of sodium hypochlorite. What we’ve seen is the delta formation of chlorate in that time period being pretty close to negligible. When it comes to your bulk hypochlorite deliveries, you would see higher rises of chlorates and more notable perchlorates as it degrades, which can be just as dangerous, if not more, for health. That’s where it can be an issue, as I know some municipalities want to keep a store on site for 30-60 days to treat their water. So, it will depend on the operational aspects of each site.


Q. I’ve heard that on-site electrochlorination can be challenging to manage, how often do you need to carry out servicing?

A. This is a common misconception. Typically,our OSEC® units need one annual service.  

Q.  When looking at an electrochlorination system for my project, what capacity of system should be selected? 

A. The system operates in a batch mode to maintain a 12–36-hour supply of dilute <1% sodium hypochlorite solution available for dosing. These generators are sized based on average and peak chlorine dosage and flow rates of the plant (to determine daily chlorine demand). To reduce rapid cycling and extend life of the equipment, the capacity should be sized 20-30% greater than the peak chlorine demand. If the usage levels fluctuate heavily throughout the day, we recommend using peak hourly rate for sizing, and run during low electrical tariff supply. Our Evoqua team can easily help size the right equipment for you!


At Evoqua, we have carried out pioneering laboratory and field testing into how to reduce chlorate formation with the OSEC® On-Site Electrochlorination system. You can watch the webinar here for more information. Or contact Nigel or Ryan directly to discuss. 


*Evoqua OSEC® systems are designed to produce 0.65-1% sodium hypochlorite under normal operating conditions. With less than one minute of contact time, the OSEC®-produced concentration range (6,500-10,000ppm) is significantly higher than the concentration requirements to meet industry standards for microorganism inactivation and disinfection. Specific disinfection rates depends on dose, concentration and time (CT value), pH, and water temperature. Performance limitations depend on feed conditions, overall installed system design, and operation and maintenance processes; please refer to Operations Manuals. For more information: Contactus@evoqua.com