Trihalomethanes (THM) are a bi product of the water treatment process and are formed when chlorine is used as the disinfectant. They are produced whenever chlorine reacts with natural organic matter in the water. If the levels are not controlled then they may become a hazard to health. The level of THM in NZ supplies is restricted by the DWNZ regulations. The maximum allowable value is relatively high (0.2mg/l chloroform compared to 0.1mg/l Total THM in the EU and 0.08mg/l Total THM in the US). In step with international trends this MAV is likely to be reduced in the future and will present an increased challenge to water suppliers and distributors.
The conventional methods of reducing THM levels are to either take measures to reduce the levels of organic matter before chlorination, limit the levels of chlorination as much as possible or to use alternate disinfection methods. All these methods can be effective but also can be complex and costly. Biologically active carbon filters combined with ozone dosing is now an established method used to reduce organic matter. This is effective but has both high capital and operating cost implications. Replacing chlorine with alternate disinfectants (e.g. ozone or UV treatment) is also an option but all other technologies incur an increase in cost and complexity.
In this presentation, an alternate approach that has been used to supplement these is discussed. This is where the contact time with the chlorine is reduced to a minimum and constantly monitored and controlled. The amount of decay of the chlorine is directly proportional to the amount of THM formed and therefore can be predicted and controlled automatically. Work in the UK has shown that the correlation is very reliable and systems have been implemented whereby THM is controlled automatically and a maximum of 0.05mg/l (Total THM) has been achieved with no additional dosing or asset replacement.
The basis of the control is to compare historical contact time (the length of time the water has been subjected to a given chlorine concentration) with the historical inlet and outlet chlorine levels. By working out the decay, the THM level can be calculated. The chlorine levels can then be set to achieve the minimum Ct required and the storage time can be adjusted in a dynamically rather than utilising a fixed set point. For example when flows are low, lower chlorine doses would be applied and less storage might be held.
An operator interface at the SCADA presents the control data clearly and provides both historical information as well as the live THM prediction