Challenges, Lessons and Methods for Leaving a Residual Impact

Jessica Hamilton (Jacobs) and Ian Baker (Christchurch City Council)

What happens when you combine earthquakes causing significant and widespread damage to water supply infrastructure, and a water reform resulting from the contamination event in Havelock North? The answer is retrofitted chlorination infrastructure. Retrofitting of chlorine dosing is a task that faces water suppliers around New Zealand. It is a requirement under the Drinking Water Quality Assurance Rules 2022 (released 25 July 2022) to provide residual disinfection unless a specific residual exemption has been granted.

The Christchurch drinking water supply scheme contains earthquake damaged and aging infrastructure and changes have been observed in source water quality. The Water Reform process has enabled a risk-based approach and has resulted in a change in the standards and rules for drinking water treatment, while maintaining a continued focus on the safety of consumers. Numerous operational issues have been identified across the Christchurch scheme, which have been assessed by Christchurch City Council (CCC) as presenting an unacceptable residual risk to consumers. As a result, CCC have embarked on a programme of works to upgrade temporary chlorine infrastructure to permanent standby chlorination across the scheme. Taumata Arowai (Water Services Regulator for New Zealand) have signalled that standby chlorination is required to gain a chlorine exemption. 

Chlorine dosing for both primary and secondary disinfection is commonly used around New Zealand and throughout the world. There are well established processes for the installation of chlorination and the related equipment and infrastructure as part of new water treatment plants (WTP) or upgrades of existing schemes which are already chlorinated. However, the instruction manual onconsiderations around retrofitting chlorination is considerably thinner, especially retrofitting permanent installations.

This paper discusses the challenges associated with the addition of chlorine (an oxidising agent) into an existing treatment and reticulation system which has been operational for some time and was not originally designed for chemical dosing.

These challenges include:

• Pitting and corrosion in relation to pipe materials.
• Solubilising legacy deposits of dissolved constituents resulting in a change in drinking water composition and characteristics.
• Sloughing of biofilm leading to colour and taste changes for consumers.

This paper aims to provide practical insights into the considerations necessary when completing this retrofitting exercise. Chlorine system retrofitting considerations also present valuable lessons learned which may be applicable to installations of fluoridation equipment, a topic numerous water suppliers across New Zealand are presently considering.


565 KB
27 Oct 2022

1400 Jess Hamilton and Ian Baker Tue 18th 2pm B3 Residual Impact.pptx

11 MB
09 Nov 2022