NCC’s mains cleaning programme flushes out the city's water pipes, removing (harmless) biofilm and preventing buildup. This is done with the ‘pig’, a foam swob forced through mains pipes, scouring them clean as water pressure propels the pig along to its exit point.
Drawn from the Heretaunga Plains aquifer, Napier's water has been filtered by natural processes associated with an artesian system shaped over millions of years.
Our water is then reticulated through some 481kms of pipe.
There are two main methods we clean the pipes: 'pigging' and flushing. The increased rate of brown water issues that some customers in Napier are experiencing lately have raised our cleaning goals. The amount of mains cleaned in winter/spring 2018 was around 110km (out of 130km of mains that are currently 'piggable'). This is the number we intend to exceed in winter/spring of 2019.
In the week ahead of a scheduled cleaning run, we write to affected householders, warning them of the disruption to supply and likely water discolouration.
Weekly radio adverts will be broadcast Friday to Monday providing information about what suburbs and areas our water team will be operating in.
The council also posts the streets that are going to be affected the day before on their website, these can be found in the public notices section. If there is any mains cleaning taking place at present then this is where you will find the streets been affected.
Supply isn't affected before 9.00am each day and no cleaning is carried out on Mondays and weekends.
This straightforward procedure involves minimal disruption to the water supply. A foam swab, called a "pig", is forced through the main, scouring it clean as water pressure propels it along to a predetermined exit point - another hydrant generally located several thousand metres further along the road.
Staff shut valves to feeder mains to control the direction taken by the pig. Travelling through the main at roughly the pace of a brisk walk, it emerges covered in sludge. The pig is then discarded.
Sixty to seventy percent of Napier and Bay View's pipes are asbestos cement, and diameter ranges from 75mm to 200mm. Pigs come in a variety of sizes. The diameter selected for cleaning a main is always larger than the pipe diameter - compressing the pig helps the cleaning action.
No cleaning agent is involved in the process. Then it is coated in a bactericidal grease to ease its entry to the hydrant. Before the water is discharged into the stormwater network, it is dechlorinated (using dechlorination mats) to ensure minimum environmental impact.
The CBD and Napier Hill are not included in the programme - the water pressure is not great enough for the cleaning method used, and cleaning the older cast iron, smaller diameter pipes could result in more problems than benefits. Generally the flow in these pipes is good, and dead-ends are flushed out regularly. Pigging is generally undertaken during low water demand periods (late autumn, winter, early spring) to allow minimum interference to the supply.
Flushing is usually undertaken as a reactive measure upon a customer complaint on discoloured water and as a planned mains cleaning activity where pigging is not an option due to various reasons (older cast iron pipes, diameter of the pipes, industrial areas, etc). The operator discharges water through a fire hydrant or a scour valve at high flow, which picks up the sediments and biofilm in the pipe and removes them from the system. There is an all-year-round flushing programme in place throughout Napier and Bay View focusing on problematic mains and cul-de-sacs (mains dead ends).
During every planned mains cleaning activity all discharged water is dechlorinated before entering storm water drains (using dechlorination mats and/or fire hydrant diffusers) in order to protect receiving environments.
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