Waikouaiti Membrane Filtration Upgrade

Background
Waikouaiti Water Treatment Plant was experiencing filter break through and other difficulties so the Dunedin City Council, along with MWH engineers, decided to upgrade the treatment plant to a 3,000 m³/day membrane filtration plant by way of a design/build contract.

Challenge
In order to keep within budget the DCC wanted to keep the existing treatment building, however this meant removing the existing plant and putting only disinfected water to reticulation. In order to minimise this downtime, strict contract conditions were placed on the time frame that the plant could be offline, before the new plant came online. From site establishment to putting water into reticulation was a period of only 3 months.

Process
The water is supplied from the local river and pumped to a 3000 m³ raw water reservoir, from here the water is gravity fed through a 1 mm strainer to a flocculation tank.
The water flows from the flocculation tank and into the membrane tanks where a light vacuum is applied to the submerged membrane fibres in order to produce permeate. This water is then pH corrected and disinfected with Cl₂ before sending to reticulation.
Filtec faced tight constraints on the recovery rates even during times of high demand and fresh river conditions. To overcome this a lamella clarifier was installed to recycle waste water back to the front of the plant allowing for a recovery greater than 96%.
Cleaning of the membranes takes place approximately every 6 months. Due to constraints on discharge consents, all spent chemicals are neutralised in a buried tank before being discharged to the ponds.

Specification