Environmentally friendly options - using rainwater

People have relied on rainwater for household, landscape and agricultural water uses for centuries. As communities have become larger and more centralised, community water treatment and distribution systems, have gradually replaced the collection of rainwater as our primary water supply.

As we have begun to understand the need for sustainable use of water worldwide there has been a renewed interest in collecting rainwater.

Using rainwater:

• Reduces demand for mains water supply and you save on your water rates
• Reduces flooding by providing temporary storage for rainwater
• Reduces wet weather sewage overflows
• Reduce the amount of pollutants entering water bodies
• Prevents rainwater from becoming stormwater as the water is used within the house.

Options for using rainwater

Option 1: Water your garden - building consent not required

Option 2: Water your garden and supply your laundry and toilet (no mains top-up) - building consent required

Option 3: Gardens, laundry and toilet gravity system with mains top-up (backflow preventer required) - building consent required

Option 4: Garden, toilet and laundry boosted (pump) system with mains top-up (backflow preventer required) - building consent required

Health warning

The city's public water supply should still be used for drinking, food preparation, kitchen and bathroom use. For rainwater to be safe for drinking it must be appropriately treated.

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How much rainwater can a typical household use?

Approximately 65 per centof household water non-potable needs can be met by collecting rainwater from the roof.

How much rainfall can you collect?

The amount of rainfall you can collect is dependent on the size of the roof area and the amount of rainfall.

To do this multiply the length by the width of the collection area, e.g. your roof, to give you the area then multiply the area by the annual rainfall. The annual rainwater for North Shore City is 1200 mm.

Area of roof catchment - example

Example: amount of rainwater falling on a 150 m2 roof in a location with average annual 1200mm rainfall

= 150 m2 x 1200 mm/year
=180m3/year
=180,000 litres/year
= average of 500 litres /day.

Typically, 80 to 90 percent of this volume is collected, with 10 to 20 percent lost through evaporation and spillage.

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Components of a rainwater system

All rainwater harvesting systems have six basic components:

1. The roof - the surface we collect the rainwater from

Rainwater can be collected from iron roofing, clay tiles and slates. It is important that no lead is used as roof flashing or gutter solder, as the slightly acidic quality of rain can dissolve the lead and contaminate the water supply. Be aware that some composite asphalt, asbestos, shingles, concrete tiles and lead based paints can leach pollutants and affect the quality, colour and taste.

2. Gutters and downpipes - the transport channels from the collection surface to storage

Seamless extruded aluminum, galvanised steel or PVC are commonly recommended for use as gutters and downpipes. Gutters and downpipes must be properly sized, sloped and installed to maximise the quantity of rainwater collected. The connection between the downpipes and the storage tank is generally made of PVC pipe.

3. Primary screening and first flush diverters - to remove debris, and divert the first part of rainfall

These are primary screening devices to prevent leaves and other debris entering the rainwater collection system. Some typical devices are gutter filters, rain heads and first flush diverters (shown in the diagram). The first flush devices are designed to divert the first part of the rainfall that picks up most of the dirt and debris from entering the rain tank.

Primary screening devices may have a 6mm wire mesh leaf screen in a metal or plastic frame installed near the downpipe. If there are trees nearby and leaves pose a problem, a leaf screen may be installed along the entire length of the gutter.

The first flush picks up most of the dirt, debris, and contaminants, e.g. bird droppings that have collected on the roof prior to the rain, commonly by diverting the first water into a separate small chamber. Typically 40 litres for every 100 m2 of roof area is diverted. Once the chamber has filled, the rest of the water flows to the downpipe connected to the rainwater tank. The small chamber has a small tube in the base that allows it to empty before the next rain event.

4. Storage tanks - above ground, below ground, or wall mounted (just under the gutter).

There are a lot of options available in tank materials (e.g. plastic, steel, concrete and fiberglass). The tank should have a durable, watertight, opaque exterior and a clean, smooth interior. A tight fitting top cover is to prevent evaporation, mosquito breeding and keep insects, rodents, birds, and children out of the tank. It is best to locate the tank in a cool place, out of sunlight so that algae do not grow. A suitable overflow outlet/(s) and access for cleaning is also important. The tank should be placed high enough for gravity to convey the water, or be fitted with a pump.

The rainwater tank size will depend on:

• The volume of water needed
• The amount and pattern of rainfall
• The area of the collection surface
• The security of the supply needed.

5. The pipes - the delivery system for the treated rainwater

Effective plumbing is important for efficient rainwater collection and ensuring water and to protect your household or mains water supply from contamination. Debris needs to be diverted and backflow preventers may need to be installed. We recommend that all plumbing be carried out by a qualified plumber. You are required to meet recognised plumbing standards.

6. Water treatment for non-potable (non-drinking) use

Additives for settling sediments or buffering pH and simple filters are optional treatments for non-potable use of rainwater. Fine filters and microbiological disinfection are only necessary for potable use, i.e. for drinking water.

It is important to note that water quality may be unsatisfactory even if it is safe for non-drinking use e.g. dirt, rust, scale, bird and rodent droppings and airborne bacteria may still enter the tank even when primary screener and first flush diverters are in place. Even for non-drinking uses, sedimentation of suspended solids inside the tank and further filtration is a good idea. Fine filters may be installed prior to the end use e.g. at the washing machine and toilet cistern. Simple cartridge filters similar to those used for domestic swimming pools or hot tubs are suitable (e.g. 80 micron washable filters). For best results follow the instructions for operation, maintenance and replacement of the filters.

Have your water tested by an approved laboratory and consult with Environmental Services or your public health service before making a decision about what water treatment methods to use.

A closer look at ground tanks

The installation of a 4,500 litre tank for collecting rainwater from a typical 150 m2 roof (as shown below) can reduce peak stormwater flows by 20 to 35%.

Rain tank capacity (in litres)
	Water use (litres)	200	1,000	3,000	4,500	9,000
	125	50%	80%	95%	100%	100%
	225	40%	65%	85%	90%	100%	Average yearly % of water supplied (for 150m² roof area)
	325	35%	50%	70%	80%	90%

Types of barrels and tanks

• Rainwater barrels - small roof areas 30m2 or less can easily be adapted to collect rainwater in barrels. These are typically 240 litres and are reasonably priced.

• Larger rain tanks are also available which have a 'silo' shape. Relatively small tank volumes can provide significant quantities of water for use around the house. A typical house roof and 200 litres of tank storage can provide up to a half of the average toilet water use.

Please ensure that rainwater tanks are clearly labelled:

NOT Suitable for Drinking signage is fixed next to all rainwater system faucets.

Backflow prevention devices are installed to stop possible mains water contamination.

North Shore City backflow prevention guidelines

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Do you need a building consent?

A building consent is not generally required for a garden irrigation rain tank. A building consent is required for all household plumbing. This includes rainwater collection systems that:

• Will connect to the mains water system as backup and therefore require a backflow prevention device  
• Exceed 25,000 litres capacity and are supported directly by the ground 
• Exceed 2,000 litres capacity and are supported more than 2 metres above the supporting ground
• Exceed 500 litres capacity and are more than 4 metres above the supporting ground

Tanks larger than 6,000 litres may require a resource consent to ensure that they meet certain criteria, such as heights in relation to boundaries etc. Please check with North Shore City Council Environmental Services.

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Do you need a back flow prevention device?

A backflow prevention device is required to protect the mains (public) water system from contamination. If your rainwater system is connected in any way to the mains water system you will require a backflow prevention ("double check valve" type) device.

Plumbing

All plumbing is required to be carried out by a registered plumber and must comply with the New Zealand Building Code. This is covered by your building consent

Using rainwater - checklist:

Roof:

• Use safe, lead-free roof paint (check your old paint for lead through your public health service)

Tank:

• Regularly check your tank and ensure the lid is secured to prevent insects, rodents, animals and children getting in.
• Periodically clean your tank and gutters to ensure they are clean of debris, leaves and dirt.
• Ensure light is excluded from penetrating your tank to prevent growth of algae.
• Consider installing primary screening and first flush diverters to improve your rainwater quality.
• Consider chlorinating the tank water annually to reduce contamination.
• Ensure you obtain a building consent where necessary and that plumbing conforms to the New Zealand Building Code and North Shore City Council requirements for backflow prevention.

Warning

If there are any changes in the colour or odour of your rainwater contact your local public health service and avoid using the water for watering vegetable gardens.

What you can do

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