UV disinfection - Case study for Rotarua district spring water resources

In this project, the credentials of Ultraviolet (UV) Disinfection as a means of reducing protozoa contamination risk in all the water supplies of Rotorua District is presented.

The Drinking Water Standards for New Zealand 2005, DWSNZ (2005), which became effective in October 2005, formally recognised the ability of UV disinfection to deal effectively with protozoan contamination, prescribing minimum raw water quality and operational conditions, in line with European and US Standards. Over recent years several UV disinfection systems have been installed across drinking water supplies in NZ, raising the profile of the technology as one of the most cost-effective means of meeting protozoa removal requirements set by the DWSNZ (2005). In addition to pathogenic protozoa, pathogenic bacteria and viruses are also effectively inactivated by UV irradiation. UV disinfection is based on the destruction of the DNA of micro-organisms by photochemical reactions.

This paper covers the experiences of the authors’ in the recent implementation of eight UV systems for all water supplies in the Rotorua District, ranging in scale from 2 to 40 ML/d, and details up-to-date information on UV technologies and important design parameters and essential design factors considered for all these sites.

Rotorua District Council sources water mainly from natural springs for almost all its supplies, with chlorination as the only treatment practiced on these supplies. Springs´ raw water quality present ideal conditions (turbidity less than 0.5 NTU, UV transmittance over 95%) for the use of UV disinfection with minimal or no pre-treatment. It was proven that UV reactor validation to overseas reference standards was the most cost effective system to apply in this case, while innovative plant arrangements was necessary to accommodate the UV reactors in all supplies, matching diverse operational conditions and space availability. In many sites, high water pressure (over 10 bar) and availability of services proved to be the most critical site constraints. One of the key advantages of the use of UV disinfection is that it does not generate any known harmful by-products, such as those associated with chlorination and other disinfection methods.

For all Rotorua Water Supplies’ upgrades, low-pressure high output (LPHO) UV lamps were selected, with primary emission at 254 nm in order to provide 3-log (99.9%) protozoan inactivation. Different UV reactors arrangements were selected to suit individual site conditions, serviceability, level of service associated with population served, and failure risk minimisation. Dose control was based on maintaining a minimum UV Intensity set by each UV reactor model validation against German Standards DVGW Technical Standard W294, which explicitly stipulate a verified fluence of 40 mJ/cm2 is required for UV systems.