Discoloured Water and Swabbing Information
Council are trying to minimise the occurrences of discoloured water. One of the strategies is to swab the truck mains from Turitea to the Square.
The City of Palmerston North water supply comes from Turitea and four bores (Keith Street, Roberts Line, Papaioea Park and Takaro Park).
On a typical day we supply approximately 29,000 m3 of water to the residents of PN. Of this, typically 60% is supplied from Turitea with the remaining being supplied from the four bores to boost the supply during times of peak demand.
The Turitea water source is surface water with the catchment being the Turitea Reserve. The source of discolouration is due to the presence of Iron and Manganese in the source water.
The water is piped from the Water Treatment Plant (WTP) to two storage reservoirs at Ngahere Park. Prior to 2002 town was fed one trunk main (the 525) from the reservoirs to the Fitzherbert Bridge, where several 'smaller' mains distribute the water to the City. In 2002 a second (duplicate) main was commissioned to boost the supply during peak demand periods (the 600 main was constructed). The 600 feeds the City from the Ngahere Park reservoirs to the Ftizherbert Bridge also.
Over the years Iron and Manganese deposits have formed within the mains and during periods of peak demand (especially during summer) the increased flow and velocities in the network effectively scour (pick up) these deposits and distribute them through the network and ultimately they appear as discoloured water.
The deposits occur when the residual chlorine in the water oxidises the Iron and Manganese and small deposits form. The water is chlorinated to meet the requirements of the New Zealand Drinking Water Standards.
In order to improve the water quality City Networks commissioned a project to build the swab chamber at Centennial Drive Reserve. The purpose of this was to place a pig (swab) into the mains at the reservoirs and using water pressure drive the swab down through the mains and discharge the resultant discoloured water into the Manawatu River. The chamber was designed to capture the swab.
Only one main can be swabbed per event. In order to swab we need to shut down one main from supply and rely solely on the other main and the bore supplies. In order to discharge the discoloured water from the swabbing chamber into the Manawatu River the PNCC was required to get a Resource Consent from Horizons Regional Council. This consent sets certain conditions that must occur in the Manawatu River before the discharge can occur.
Condition 2 of our resource consent states that "The Permit shall ensure that the discharge to the Manawatu River only occurs when the flow in the Manawatu River, as measured at the Teachers College flow recording station is equal to or greater than 100 cubic metres per second (100m3/s)"
Condition 2A further states that "The Permit shall ensure that the discharge to the Manawatu River only occurs when the water turbidity (colouration) of the Manawatu River, as measured at the Teachers College flow recording station is equal to or greater than 20 Nephelometric Turbidity Units (NTU)"
We are also further restricted by condition 3 which states that "The permit holder shall ensure that the discharge to the Manawatu River is restricted to during night time, defined as the time between one hour after sunset and one hour before sunrise"
These three conditions make it very difficult to predict and plan for this discharge to occur.