Rainwater harvesting
PRESERVING OUR MOST PRECIOUS RESOURCE PRESENTS SOME DISTINCT CHALLENGES
A guide to setting up and good practice to collect water. Do it before you are made to.
Water is becoming political; it won’t be long till we are taxed for it or paying for how much we use. Climate change will only exacerbate this as droughts become more frequent.
It will soon be incumbent on all of us to preserve and reuse water. That is already the case in Australia where many states insist that new homes have water tanks, and grey-water tanks are often mandatory too. In New Zealand around 10 per cent of the population relies on tank water, mostly rural or coastal properties. Others use tank water to supplement their town supply for gardening and other purposes especially where usage is monitored and charged.
Mandatory harvesting
New Zealand is blessed with ample supplies of water, in the main, but even we can suffer from seasonal shortages in regions according to the weather. The east coast of both islands has quite significantly reduced rainfall compared with the west coast or central regions. In areas where the water supply is problematic or will require large investment in infrastructure to secure in the future, some district councils such as Kapiti have passed by-laws so that all new dwellings from 2008 on must have a 10,000-litre rain water tank or a 4000-litre rain water tank and a grey-water diversion system.
In parts of Australia it is mandatory to have water-storage tanks in any new building consent and grey-water diversion is encouraged too.
The case for mandatory water harvesting is gathering momentum. In an age when we are urged to conserve resources as much as possible it seems fairly ludicrous to be using treated potable water for flushing the toilet. In fact only a small proportion of our water supply is used for purposes that require potable water: drinking and personal washing — we drink or cook with only five per cent of our water. The rest goes on showering, washing clothes, flushing toilets, watering the garden, and cleaning the car, etc. — tasks which could just as easily be accomplished with harvested water. In fact flushing the toilet could reuse grey water from the shower or washing machine, but that’s another story.
Gather 500 litres a day
Depending on where you live in New Zealand you can collect around 180,000 litres per year or an average of 500 litres a day for an average 150m2 house, where rainfall is around 1200mm annually as it is in Auckland. Around 80–90 per cent of this can be collected, with the rest being lost to evaporation or spillage. In an area that charges for water consumption — both supply and waste, as in Auckland — it makes good sense to take advantage of the free stuff and use it for things that can offset the metered usage such as watering the garden or cleaning the car.
Installing a 35,000-litre tank that is sited on the ground on a suitable substrate does not generally require a permit or resource consent although it pays to check with your local authority. Bear in mind that that 35,000-litre tank
The case for mandatory water harvesting is gathering momentum
holds 35,000kg (35 tonnes) of water so try to position it on a dry substrate in an area where it will not sink into the ground with the first decent rain in winter.
Cleanliness
Where water is reticulated in regional towns and indeed in some cities, its purity is not always guaranteed, as the residents of Havelock North recently discovered. The same can be said of water harvested from your roof. But if the cleanliness of reticulated supply is suspect that stuff washing off your roof is even worse. Too often we are simply creating a tank of pathogens that we then blithely consume. A survey of 560 tanks taken across the country over five years up to 2006 by Massey University showed at least half of the samples exceeded minimal acceptable levels for contamination, and more than 40 per cent of samples showed evidence of heavy faecal contamination. That study was conducted by the Roof Water Research Centre at Massey University in Wellington headed
by microbiologist Stan Abbott. He is an enthusiastic supporter of urban water harvesting, both to offset the waste of potable supplies and for emergency situations.
Of course those who have lived with tank water for years will claim that “it never hurt me” and that may well be true but it is unlikely be true for your visitors. You may indeed be immune to some of the pathogens in your tank, but not all — a recent Food Safety Authority study showed that there is significant under-reporting of illness related to tank water. In fact less than a third of people who became ill due to contaminated water were reported to health authorities.
Most of the disease-causing pathogens come from the roof and are delivered in the first flush of rain that washes all the recent bird and small-mammal faeces and decaying plant material in the gutters and microorganisms that are present in road dust on the roof into the tank. Bird faeces carry a variety of microorganisms including the ubiquitous E. coli but they can also host Salmonella, Campylobacter, and Cryptosporidium. Most of these thrive in warm tank water but can survive even in cold weather.
Primary treatment
Of course you can filter and treat the water after harvest but most of these pathogens can be simply avoided with some simple precautions. The most obvious should be a first-flush diverter, a tank that collects or diverts the first few litres from the roof, and allows this to drain away. The first flush from rain will contain all the accumulated road dust and dried bird faeces that have settled on your roof since the last rain. In fact the Massey University study found the first-flush diverter to be the single most effective method of maintaining good water quality.
Most first-flush diverters work by having a cylinder that can contain between 50 and 100 litres of water. There is a ball in the unit that rises as the diverter fills until it eventually blocks the intake and lets the remaining water run to the tank. Prior to the first-flush diverter it is wise to include a leaf screen that ensures that the diverter does not get clogged with leaves or other rubbish. The diverter will slowly release the water or it can be drawn off for watering the garden.
Tank fittings
Within the tank, pathogens and microorganisms will gradually fall to the bottom, as will any algae and other impurities. The layer at the bottom of the tank is typically an anaerobic layer that is often low in dissolved oxygen. Water entering the tank should do so
Of course those who have lived with tank water for years will claim that “it never hurt me” and that may well be true but it is unlikely be true for your visitors
through a calmed inlet set above the base that prevents the incoming stream from stirring up this material and making the water turbid. The output of the tank is ideally taken from the water at the top of the tank where it should be relatively clear and clean. This is often achieved with a floating intake that ensures the output is taken a controlled distance from the top of the water column.
A siphon is generally used to spill water when the tank is over-full. This is a pipe with a bend that runs from the base of the tank’s interior to the outside. The top of the siphon bend has a hole. When the water level covers the hole, the siphon starts automatically and will tend to suck up debris off the base and from the anaerobic layer first rather than spilling the cleaner water at the top. When the water level drops and uncovers the hole again and air enters the siphon it will automatically stop to avoid the entire tank being emptied.
Getting started
If you are setting up your rainwater-harvesting system for the first time or thinking of modifying your present arrangement in light of what we’ve just discussed, here’s how to do it.
( This is a revision of material we published in 2008 in a similar article. It utilizes mainly Marley products. David Oliver, the business development manager with Marley at the time, presented it for us. Marley makes a comprehensive collection of rainwater-harvesting accessories that make the business of securing your water supply straightforward.)
Rainhead
Assuming your guttering is installed correctly with adequate fall, first calculate the amount of water that needs to be diverted to the first-flush diverter (see First-Flush Calculations panel). There are two options — you can either flush into a downpipe collector from the gutter or into a separate freestanding cylinder, ideally near the tank. The latter is useful for larger roof spaces and larger flushes, especially for areas near the coast or where birds might be a problem — do you have a pigeon fancier as a neighbour?
You must install a rainhead with a leaf screen; this needs to be installed as high
In fact the Massey University study found the first-flush diverter to be the single most effective method of maintaining good water quality
as possible under the eaves. It’s the point where the gutters meet. The mesh on the leaf screen is less than 1mm to stop mosquitos and insects getting though into the tank. There is also a wider secondary mesh to collect leaves and other larger debris before the fine filter of the rainhead. The mouth of the rainhead is intentionally wide to cope with a sudden dump of rain in a storm. There are other leaf screens, including the Marley curve, but the rainhead has the most comprehensive filtering. It pays to have some kind of debris-filtering system in your gutter too, either one of the many gutter screens or bristle filters to eliminate most of the debris at source.
First-flush diverter
If you wish to make your first-flush diverter in the downpipe from the rainhead, fix a T-shaped connector at the outlet of the rainhead to the diverter. The diverter chamber can run off this if it is a small diverter. Alternatively, run the outlet of the rainhead to the tank and install the diverter to a separate post closer to the tank. The diverter itself contains a plastic ball that floats upwards as the diverter fills, until it eventually blocks the inlet. As the diverter slowly leaks it will, from time to time, need to be refilled with the run-off from the roof but this will only be a very small part of the flow.
The diverter tank empties via a drip filter attached to the end of a secondary mesh filter in the tank that makes sure the drip hole is not blocked by debris. Marley supplies a variety of filters with differentsized holes that will cause the stored water to drip at different rates. There are usually two filters: one larger one made of stainless-steel mesh to clear the larger material, and a finer one to trap anything that got through the first one. It’s a good idea to clean the filter at least once or twice a year. Larger diverter tanks can be made using 300mm pipe cut to size.
Marley makes a kit that only requires you to supply the 300mm pipe of your choosing. A galvanized bracket is even included in the kit to secure the tank to a post or a wall — the tank will have to hold upwards of 70–150kg depending on how big it is.
Cut the 300mm tube with a fine-toothed saw or an angle grinder with a thin blade and bevel the edge of the cut end so it makes a better seat for the solvent cement to mate to the caps at either end of the pipe. You can attach a hose to the end of the diverter and use it to empty the contents later on the garden.
Sterilization
These measures will ensure that what is going into your tank is as clean as it realistically can be. It is worth considering adding other filtering and cleaning devices after the output too. The
It is worth considering adding other filtering and cleaning devices after the output too
gold standard is ultraviolet tank-water filtration, in which the water is exposed to high levels of ultraviolet light that will kill any parasites of microorganisms and even some viruses.
But there are alternatives, such as carbon filters and under-sink filters, that are very effective at removing contaminants. The drawback with these is they must be replaced periodically, but the cleaner the water supply to them is, the longer the filters will last.
Cleaning or painting
If you are doing roof improvements, cleaning your gutters, or painting the roof, do remember to disconnect the roof connection to the tank first.