Two Identical Water Treatment Plant Process Upgrades: Two Different Sludge Management Solutions?

In 2009, Dunedin City Council upgraded two rural water treatment plants at opposite ends of the city’s boundary. The primary driver of each upgrade was to improve treated water quality, with one plant also requiring an increase in capacity, however to gain efficiencies, they were procured jointly. Each plant utilised an identical membrane filtration process technology, installed by the same contractor but each project involved a different engineering consultant. Whilst the end products of treated water met the required parameters, each project also resulted in the production of an increased volume of sludge as a by-product from the treatment process, which stressed the existing traditional drying bed configurations.

This gave rise to two additional capital projects to address the treatment and disposal of the sludge. The original two consultants were engaged to investigate options and two different sludge management solutions have now been trialled at the two water treatment plants: one utilising reed beds and the other using geotextile dewatering bags, with the latter leading to construction of a full scale system. This paper examines the two approaches taken on the two projects, their similarities and differences.