Update 5 – Monitoring chlorine demand - November 2016
The purpose of these updates is to create a knowledge environment where all members can access the best information to plan, operate and maintain drinking water networks and generally accepted best practices.
There is more to chlorine dosing than just adjusting the chlorinator once to maintain at least 0.2 mg/L as it leaves the treatment plant, there are many factors which can influence the dose of chlorine required. By maintaining the optimum chlorine dose there is the opportunity to make your work easier by potentially making savings in the amount of chlorine required; reducing the number of chlorine odour complaints, as well as potentially get an early warning of changing conditions within the water supply system.
Chlorine is one of the more reactive chemicals used in water supplies and is consumed by reaction with inorganic and organic contaminants in water. The amount of chlorine destroyed by these substances is known as the chlorine demand. Most reactions with chlorine will occur within the first few minutes of contact, which is why the FACe (Free Available Chlorine equivalent) residual is measured after 30 minutes from being dosed, i.e. the outlet of the contact tank or treated water reservoir.
The chlorine chemical reaction rate increases with increasing temperature. If the chlorine dosing is operated to achieve a chlorine residual at the ends of the distribution system, then during cold wintery conditions the chlorine residual leaving the treatment plant can be lower than in warm summer conditions. Winter also tends to have longer retention times in distribution reservoirs, so the desired residual leaving the plant can only be determined by trial and error through FAC tests carried out at many parts of the distribution system.
Some of the factors that can reduce the amount of free available chlorine are:
- UV light, water exposed to sunlight, or the UV plant was installed downstream of the chlorine dosing point.
- Aeration, water cascading over a weir, top feed reservoirs. This effect will vary with flow rate.
- Changing raw water conditions, dissolved metals which the coagulation process isn’t removing
- Non-optimum coagulation and flocculation processes where dissolved natural organic molecules, such as humic and fulvic acids (colour) are passing through the processes. The oxidation of these compounds by chlorine is often slow can lead to their partial disintegration and the production of compounds which micro-organisms are able to use as a food source. The presence or production of these compounds in water entering distribution systems can enhance the regrowth of micro-organisms, particularly if there is little or no FAC. See DPB’s further down.
- A possum climbed up past the grill of reservoir overflow pipe and fell in.
- A leaky concrete seal on reservoir roof, effect of this is typically seen in the first rain event after a dry spell.
Where 3 and 30 minute online chlorine analysers have been installed, the difference between these two readings is a good indicator of what oxidisation is occurring after dosing hence how well the treatment process is removing contaminants. This difference can also be identified by calculating the dose rate of chlorine being applied and comparing to the chlorine leaving the treatment plant.
Where chlorine residuals are monitored in the distribution system a similar comparison can be made between the water leaving the treatment plant and the water passing through the distribution network. That possum might be there consuming some of the chlorine but still leaving enough of a chlorine residual to mask any potential E.coli detection.
Keeping a record of manual chlorine readings from the distribution system and being aware of the typical seasonal differences from source to distribution system, provides additional useful information. Where a larger than normal difference is detected it should be a prompt to have a closer look before it escalates into an issue.
Chlorine dosing control
A number of factors need to be taken into account when establishing a chlorine dose rate at the treatment plant. Generally, the first and most basic requirement is to achieve a minimum of 0.2 mg/L FACe after 30 minutes contact time. In many cases, the chlorinator is set to match this.
If the water has a low turbidity and colour, and a free chlorine residual is maintained, consumers can be reasonably confident that most (if not all) pathogenic organisms will have been destroyed after a 30-minute period. Some pathogens (eg, oocysts of the protozoa Cryptosporidium) are more resistant to chlorine and require removal by filtration or inactivation by another disinfection system, but chlorination is still regarded as the most appropriate key defence against contamination by bacteria and viruses.
The characteristics of the raw water source needs to be taken into account when the method of chlorine dose control is being selected. Waters in which the chlorine demand and the flow through the plant are almost constant can be chlorinated adequately by manual control. Where the water quality is fairly constant, but where the flow rates change, a proportional flow controller is necessary.
Waters in which both the chlorine demand and the flow rate change, require an automated system with the dosing controlled by measurement of the FAC residual in the water. If unattended, neither manual nor proportional flow controls can alter their dose rates to match changes in raw water quality, hence this is the more failsafe option but is typically also the more expensive and complicated.
Storage issues with hypochlorite that are not widely known
The storage of high concentration hypochlorite solutions (sodium and calcium hypochlorite solutions, “hypo and HTH”) for extended periods of time should be avoided. At high concentrations, these chlorine solutions decompose with the production of chlorate and perchlorate. Sufficient chlorate can be produced for it to be detectable in the treated water. Chlorates are likely to be looked at as part of a national study next year; onsite hypochlorite production where the product is used within a few days is typically ok when a salt with a low bromide content is used.
Dry calcium hypochlorite is a powerful oxidant and is dangerous if mishandled. It should not be allowed to come into contact with heat, combustible materials, oils or reducing agents, and spillages should be washed away with large amounts of water. Follow the instructions on the containers.
Disinfection by-products (DPBs)
New Zealand’s foremost concern, like other countries, is to provide microbiologically safe water. The microbiological quality of the water must never be sacrificed just to minimise disinfection by-product formation. This is not to say that efforts should not be made to keep disinfection by-product concentrations to a minimum.
The effects from a microbiological contaminated water supply are more immediate, typically 1 – 10 days, whereas the effects from disinfection by-products from drinking water are long term at an excess cancer risk of one in a 100,000 over a 70 year period at the maximum allowable values specified in the Drinking Water Standards New Zealand - hence the statement in the paragraph above.
DPB concentrations increase with increasing disinfectant concentration. The best-characterised relationship is between Trihalomethane (THM) production and chlorine dose. There is a moderately steep increase in THM production as the chlorine dose is increased, until sufficient chlorine has been added to meet the full chlorine demand of the water. At doses beyond this value there is little increase in THM concentration as the chlorine concentration is increased.
The influence of pH on the concentration of DPBs depends upon the category of DPBs in question. Within the pH range of typical drinking-water, increasing the pH (up to pH 9.5) increases the concentrations of THMs; whereas the concentrations of trihaloacetic acids increase as the pH is decreased (maximum dichloroacetic acid production occurs at pH 7.0–7.5.
The production of DPMs from organic matter is not instantaneous. The production of THMs, for example, may continue for weeks, although, at typical pH and temperature values, greater than 80 percent of the final concentration may be formed within 48 hours. Concentrations of THMs in a distribution system are therefore expected to be greater than the concentrations in the water leaving the treatment plant.
The holding times in service reservoirs before the drinking-water enters the distribution system will have an influence on the DPB concentrations in the reticulated water; the longer the holding time in the reservoir, the higher the disinfection by-product concentrations entering the distribution system. However, it has been observed that haloacetic acids tend to exhibit higher concentrations near the treatment plant
There is a lot more to chlorine dosing than just ensuring it’s being dosed!
Other useful related documents are:
Water New Zealand – Good Practice Guidance Note on the Supply of Chlorine for use in Drinking-Water Treatment.
Ministry of Health - Guidelines for Drinking-Water Quality Management for New Zealand, which was the source for some of the information in this update.
A Water NZ document on Chlorine Emergency Response Plans will be on our website in the near future.
Future technical updates are planned for:
- Online Monitoring and compliance
- GNS explanation of age dating process
- Educating senior staff and Councillors Community education
- The national inquiry information as it unfolds
Interaction with other agencies to ensure we have safe water supplies and the roles of respective agencies; who takes a lead in these situations?
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