Blending and Slow Sand Filtration at Little River WTP

Christchurch City Council (CCC), with assistance from CH2M Beca, has carried out an upgrade to the Little River Water Treatment Plant (WTP) to meet the Drinking Water Standards for New Zealand (DWSNZ). The original plant treatment process consisted of slow sand filtration and chlorination for a single surface water source. Turbidity spikes during periods of wet weather caused non-compliance with the DWSNZ.

To provide a balance between increasing the security of supply, reliability of treatment, and cost, it was decided to upgrade the plant with the addition of a new groundwater source (which required softening), refurbishment of the slow sand filters including new media and underdrains, and UV disinfection.

Having both a groundwater and surface water source provided greater security of supply and also allowed the sources to be blended to assist with hardness in the groundwater source. Taste testing was used to develop the process philosophy for blending that best suited the local community.

The slow sand filter process was retained as they provide cost effective treatment. They also produce no residual waste stream, and only require periodic scraping of the sand. Although there are no other known slow sand filters in New Zealand, there are numerous slow sand filter installations operating successfully overseas. Therefore the design was carried out in consultation with slow sand filter experts from CH2M in the US.

This paper outlines the drivers behind key decisions and covers key design, commissioning and operational requirements, as well as a summary of the lessons learned. The performance of the upgraded filters is also reviewed. The applicability and cost-effectiveness of slow sand filtration technology to other small communities is discussed.