Backflow is the undesirable reversal of the flow of water from its intended direction. Backflow at a property boundary is dangerous because it can allow contaminants from activities within a property to be drawn into the public drinking water supply.
Boundary backflow can occur as a result of:
- Back-siphon. This is when the pressure drops in the water supply main creating a vacuum in the main and water is sucked from private property into the water supply main. Examples of this include situations where large volumes of water are being drawn off for fire-fighting, when a water main has broken, or when a water main is shutdown for repair work.
- Back-pressure. This is when the water supply pressure from a private premises exceeds the pressure in the water supply main and water is forced from private property into the water supply main. Examples of this include situations where a private water storage tank is installed at a higher level than the water supply network, or when a private pump system is pumping at a pressure which is greater than that of the water supply network.
There are three levels of hazards within a non-residential site:
- High - any condition, device or practice which, in connection with the potable water supply system, has the potential to cause death.
- Medium - any condition, device or practice which, in connection with the potable water supply system, has the potential to injure or endanger health.
- Low - any condition, device or practice which, in connection with the potable water supply system, would constitute a nuisance, by colour, odour or taste, but not injure or endanger health.
A fourth level of Very Low applies to most residences, however there may be activities within a residential property that increase the hazard level.
To protect a potable water supply from the hazards within a property connected to that water supply a backflow prevention device is fitted to the water connection at the point of supply to the property, usually the front boundary.
There are three types of devices used for boundary backflow prevention:
- Reduced Pressure Zone (RPZ) - a testable device used for High level hazards
- Double Check Valve - a testable device used for Medium level hazards
- Dual Check Valve - a non-testable device used for Low level hazards
The boundary backflow prevention device is intended to protect the public drinking water supply from being contaminated from an individual property. It does not protect people working or living within that property from being affected by the contamination, which is the responsibility of the property owner under the Building Act 2004 and subsequent amendments.
Council is required to protect the public supply from the risk of a backflow event as described above. Relevant legislation includes, but is not limited to:
- Health (Drinking Water) Amendment Act 2007
- Building Act
- Local Government Act 2002
- Health Act 1956
Council adopted the Boundary Backflow Prevention for Water Supplies Code of Practice 2006 in 2007, and it was updated in 2013. This document sets out the guidelines for assessing the level of risk from a property and selecting the appropriate level of boundary backflow protection. A copy of the Code can be found on the Water New Zealand website. (See 'Useful Information' below)
Hazard Levels for Common Activities
Some common examples of activities within properties that require boundary backflow protection are:
|Pumped sewer systems|
On-site sewage treatment systems
Oil and grit interceptors
Air conditioning systems
Swimming pools and spas
Auxiliary water supplies (bores, wells, untreated water supply network)
Will a boundary backflow prevention device be needed for my connection?
Council can be made aware of hazardous activities within a property independently or through the application process, such as Building Consent, Resource Consent or Licensed Premises applications. This may trigger a requirement to install or upgrade the level of protection at the property owner's cost.
All new and replaced water connections have a minimum level of backflow protection installed by way of a non-testable dual check valve. The level of protection is increased if any higher level hazardous activity is identified within a property.
Any commercial property that has a Tradewaste Consent with a Passive Grease Trap and/or an Oil & Grit Interceptor will at renewal stage be required to have an RPZ device fitted by MDC at the property owners cost if the property does not already have appropriate boundary backflow prevention installed.
Fees and Charges
The supply and installation of all boundary backflow prevention devices is assessed and priced on a case-by-case and site specific basis. An application for this assessment can be made by completing a water connection application form. This form should then be emailed as an attachment to the assets and services department of the Marlborough District Council.
All testable devices will be inspected annually. There is an annual fee for this.
View Connection and Disconnection fees