Source, monitoring and treatment

Backflow is when water or other liquids are sucked back into the water network from a tap, hose, trough or other place where water usually flows out. This can cause drinking water to become contaminated and unsafe to drink. A backflow prevention device is designed to stop this process (see below). For example, if a hose is sitting in a container of water and detergent that is being used for cleaning and the pressure within the distribution network drops, this cleaning solution could be pulled into the distribution system. The purpose of backflow is to stop this and protect the network. 

A backflow prevention device stops water or other liquids from flowing the wrong way through pipes and potentially contaminating the water that flows from consumers’ taps. 

Backflow can also be prevented by creating an airgap so that liquids cannot be pulled into the distribution system. 

Back-washable media filters use a medium such as sand to remove particles from water as it passes through. They are generally used in larger supplies to remove particles from water and reduce turbidity levels. Media filters gradually become clogged with particles and can be cleaned, by running treated water back through the sand (or other media). This frees the particles which are removed in the wastewater.  

Bore water is groundwater obtained by drilling into an underground water source (called an aquifer). The water is normally extracted using a pump but in some cases the water will be forced out by natural pressure. Groundwater is generally of more stable quality than surface water and may be protected from some types of contamination. However, even deep bores can contain bacteria or elevated levels of chemicals so suitable treatment and monitoring is required.  

Calcium hypochlorite is a widely used chemical compound for disinfecting water. It is important that calcium hypochlorite is stored and used in accordance with the manufacturer’s instructions. This is because poor storage practices can lead to the formation of disinfection by-products.  

Cartridge filters remove suspended particles from water as it passes through them. They are an effective way to reduce turbidity and some other contaminants from water but they cannot be relied on solely to remove microorganisms.

A centralised treatment system makes water safe to drink at a single place, often a large water treatment plant, before the water is sent out through a network of pipes to the community.

Chlorine is a safe, simple and inexpensive way to disinfect drinking water. It kills bacteria and viruses in drinking water and continues to protect the water as it travels through to the people who drink it. 

However, chlorine is not effective against protozoa at the levels used in drinking water treatment. Most supplies require another form of treatment for protozoa, such as a suitable type of filtration or UV treatment.

The maximum acceptable value (MAV) for chlorine is 5mg/L. 

Read more about chlorine.

Chlorine dioxide is a powerful disinfectant that inactivates bacteria and viruses, preventing them from replicating and thereby rendering them harmless for people to drink. Chlorine dioxide is rarely used by drinking water suppliers in New Zealand. 

Coagulation is a process by which a chemical, often aluminium- or iron-based, is added to raw water to cause particles to clump into groups that can then be removed more easily from the water. 

C.t. value is a measure of the concentration of a disinfectant and the ‘contact time’ required to cause a specified level of inactivation in a microorganism. The C.t. value is a measure of the exposure of the disinfectant. 

A determinand is a substance or characteristic that is measured in drinking water. For example, iron is a determinand because it can be measured when a water sample is analysed. 

End-point treatment is a method of treating water to ensure it is safe to drink that occurs at the end of a drinking water network, often at the house(s) or building(s) where the water will be consumed. An end-point treatment system typically consists of cartridge filters and a UV treatment system.  

FAC stands for Free Available Chlorine. This is the chlorine in water that has not yet reacted with contaminants such as bacteria, viruses and organic material. The FAC is particularly important in distribution systems as it needs to be high enough to kill or inactivate bacteria that could contaminate the water as it travels to consumers. The FAC level should be maintained above 0.2 mg/L throughout the distribution system of most supplies. 

Added after a coagulant is used, flocculants are used to bring particles in water together to form larger and stronger flakes or ‘flocs’. This process is called flocculation. Flocculation causes the flocs to further clump together and get bigger so they are easier to remove. Flocs settle out of the water and are removed in a process called clarification or sedimentation. 

Fluoride is found naturally in water in some parts of the world. However, levels in New Zealand are low. In areas with naturally low fluoride levels, fluoride is often added to water sources for its oral health benefits, helping prevent cavities in people’s teeth. It is almost completely absorbed during digestion and has no adverse health effects at low doses. Where fluoride is added to drinking water, suppliers are required to monitor the levels of fluoride continuously.

The maximum acceptable value (MAV) for fluoride is 1.5mg per litre of drinking water. This is a long-term MAV and set at a level to ensure fluoride levels in drinking water won’t pose a significant health risk over a lifetime (70 years). For this reason, short-term exposure to water with fluoride levels above the MAV is not generally a risk to health. 

Groundwater is water that occurs naturally underground. It can typically be accessed through a spring or the drilling of a bore. Groundwater is generally of more stable quality than surface water and may be protected from some types of contamination. However, even deep bores can contain bacteria and elevated levels of chemicals and require suitable treatment and monitoring. 

Instantaneous peak demand is the highest amount of water that is expected to be drawn from a water supply at any given time. A water treatment system must be able to meet this demand, or the supplier must ensure there is already enough treated water available to meet the demand. For example, water storage tanks can enable a reserve of treated water to be created in off-peak times, for use during peak times.

Log credits measure how effective a treatment process is against microorganisms. The more effective the process, the more log credits it is given. One log credit means that the process will remove 90% of the target organism, a two log credit process will remove 99% etc. If more than one treatment process is used on water, the log credits of each process are added together. Each water source has different log credit requirements for protozoa, which must be met to ensure the water is safe to drink. 

A MAV is the maximum amount of a determinand that is allowed in drinking water. MAVs for chemical, physical and microbiological determinands are set out in the Water Services (Drinking Water Standards for New Zealand) Regulations 2022outbound. 

Membrane filters are made of fine tubes of membranes with very small pores that can filter out very fine particles. There are four common types of membrane filtration which remove different sizes of particles. Microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Reverse osmosis is the tightest membrane and removes everything except water molecules.  

Drinking water comes from a range of places, including lakes, rivers, streams, and groundwater. But if you drink water straight from these sources there’s a risk that it could be contaminated with bacteria, viruses, or other things and make you sick.

So, drinking water suppliers must use a range of processes, procedures and tools to protect and treat water, to make it safe and keep it safe as it travels to the people who drink it. This is called a multi-barrier approach.

This approach means that there’s always a range of safeguards (or multiple barriers) in place to make sure hazards and risks that might affect drinking water are addressed appropriately and the water stays safe.

Read more about multi-barrier approach.

Ozone treatment is a chemical that disinfects water and can help in the removal of some metals. Unlike chlorine, however, it doesn’t provide ongoing protection for the water while it is distributed to consumers. 

Protozoa are single-celled parasites that can make people sick. They can be removed from water with fine filters or be inactivated using ultraviolet light and other disinfectants. 

Roof supply is a drinking water supply that uses rainwater that is collected from a roof for use as a water supply. Roof water will contain contaminants it has picked up from the roof and plumbing used to collect it. It must therefore go through treatment to disinfect it and be tested for the presence of certain chemicals.  

A slow sand filter is a water treatment system that consists of a layer of fine sand with a layer of microbes on top of it. 

The microbes in slow sand filters remove or breakdown the organic material in the water. In New Zealand, slow sand filters are normally only used to reduce turbidity in water. 

Sodium hypochlorite is used as a form of chlorination in some supplies. It is important that sodium hypochlorite is stored and used in accordance with the manufacturer’s instructions as poor storage practices can lead to the formation of disinfection by-products.

Sodium hypochlorite is also the key ingredient in bleach. Plain, unscented bleach with no additives can be used for disinfection of water and equipment during incidents and emergencies. However, it is important to follow use guidelines properly as it can be harmful. 

Source water refers to the water body from which water is abstracted for use in a drinking water supply, e.g. lakes, rivers, streams, open-air artificial watercourses and aquifers, and rainwater.

Spring water is groundwater that naturally pushes towards the surface. Groundwater is generally of more stable quality than surface water and may be protected from some types of contamination. However, even deep bores can contain bacteria and chemicals at elevated levels and requires suitable treatment and monitoring. 

Surface water is water that can be found above ground. Examples of surface water are rivers, streams and lakes. Surface waters are more vulnerable to contamination, e.g. from agricultural runoff and wastewater discharges. The quality of source water can also change quickly, e.g. due to heavy rainfall or harmful algal blooms (cyanobacteria). Suppliers need to understand the risks and manage the risks to their source water. 

Tank storage can be used to hold a reserve of raw or treated water. Treated water reserves help suppliers manage periods of peak demand and ensure water is available if there is an incident or emergency that affects water production. 

T₁₀ is used to help ensure the correct contact time is achieved. T₁₀  is the time it takes for the fastest 10% of water to pass through a treatment process . This time varies depending on the type of disinfectant being used.