The City of Palmerston North water supply comes from Turitea dam 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 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 Fitzherbert 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 foam 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 Manawatū River. The chamber was designed to capture the swab.
Only one main can be swabbed per event. To swab we need to shut down one main from supply and rely on the other main and the bore supplies. In order to discharge the discoloured water from the swabbing chamber into the Manawatū River the PNCC was required to get a Resource Consent from Horizons Regional Council. This consent sets certain conditions that must occur in the Manawatū River before the discharge can occur.
Conditions 3 to 5 dictate the river conditions and time of night the swabbing must occur at:
"The discharge to the Manawatū River must only occur when the flow in the Manawatū River, measured at the Teachers College flow recording station at map reference NZMS T24:331-892 and NZTopo BM34:230-274 is equal to or greater than 100 cubic metres per second (100 m3/s)."
"The discharge to the Manawatū River must only occur when the water turbidity of the Manawatū River, as measured at the Teacher's College flow recording station at map reference NZMS T24:331-892 and NZTopo BM34:230-274, is equal to or greater than 20 Formazin Turbidity Units (FTU)."
"The discharge to the Manawatū River is restricted to during night time, defined as the time between one hour after sunset and one hour before sunrise."