Your options, and products to collect your own rainwater
In Issue No. 77 of The Shed, March/ April 2018, Jude Woodside wrote a definitive article about rainwater harvesting. It was timely coverage of an important topic that we aren’t discussing as a society.
Looking at the big picture, increases in population, changes in land use practices, and climate trends are all affecting water availability. We are
seeing increasing arid zones, depletion of major underground aquifers, and an increasing number of rivers no longer making it to the sea.
In some states of the US, you don’t even have rights to the water that lands on your roof — someone else owns it downstream from you. Water is also traded ‘virtually’. We are seeing more and more attempts to own or export our water and ultimately this issue is set to be one of the major triggers of global conflict. In light of that, we can expect water to become more expensive — all the more reason to store our own.
A water-harvesting system is money in the bank. It goes further than that too, as food is water dependent and it follows that food production is going to be part of the increasing global contention over water. So it makes sense to also be food resilient.
All of this aligns with being resilient to natural disasters, regardless of man-made ones. Since the recent earthquakes in New Zealand (not to mention droughts, fires, leakages, and rain events) councils have become a lot more supportive of water harvesting. Wellington has a scheme to supply small tanks at cost, Auckland offers rebates, and some councils are beginning to make tanks mandatory for new buildings. It’s in their own interest, as private tanks act as a buffer in the event of a deluge and relieve demand in peak dry periods.
In some states of the US, you don’t even have rights to the water that lands on your roof
According to the Ministry of Business, Innovation and Employment (MBIE), you can flush your toilet, wash your clothes, and water your garden with untreated water. Drinking and “other household uses” require treatment. Also, if you’re wanting to run tank water as an optional feed to a mains-connected house, you’ll need a consent and a plumber to install an isolating valve, which may need annual checks. Water is heavy stuff and tank stands will need a permit if they exceed various litreages at various heights — check with your local council.
The typical system is a ground-level tank catching roof water from a singlestorey house. Height and pressure are potential problems with this. The eaves of a single-storey house will be not much higher than the top of a 25,000-litre tank, yet ideally a drop is needed for leaf catching. Lowering the height of the tank by digging the tank into the ground or by using a lower (smaller) tank reduces the pressure available for things like garden sprinklers. This can be overcome by pumps and header tanks but that adds to cost and complexity. It’s better to plan a garden downhill of the tank if you have the choice. If there is an option to site your tank in the shade, take it.
The cleaner the water we harvest, the better, starting with the roof. One guideline suggests that if there’s lead, chromium, or cadmium in the roof materials, soldering, flashings, paint, or any other part of the roof, you shouldn’t collect rainwater at all.
Overhanging foliage should be kept cut back and an in-gutter mesh can help keep coarse material out of your system. This will need regular inspection and cleaning. A regular roof washdown is recommended — say, yearly — and the quality of what we send up our flues is worth contemplating too. Remember to bypass the tank during washdowns.
More convenient to access (especially as we get older) is an in-line leaf-catcher situated below the spouting. The proprietary ones take a lot of beating, and the best ones sport two gauges of mesh. Keep an eye on the mesh, as it’s surprising how often it needs to be cleaned. Mosquito mesh somewhere in the input line (and over any tank vents) is a good idea too.
Following that, a first-flush system is a useful (and highly recommended) adjunct. Essentially, it is a small tank ranging from 50 to 100 litres, which fills first when a rain event starts. What happens is that the dust, bird poo, and debris that has landed on your roof in the previous dry period washes off in the first few minutes. First flush catches this dirty water, fills up, then overflows to the storage tank. The dirt settles out, and between rain events the first-flush tank slowly drains (or you can manually drain it) ready for next time. There are proprietary units, but the concept is easily fabricated on site too. First flush is the single most effective move we can make, cleanliness-wise.
Tanks are usually plastic (polyethylene), but concrete, coated steel, and fiberglass are other options. They come in all sizes and shapes and even in tight urban situations there will be something that will work for you. Usually they are dark coloured because internal darkness reduces the chance of algal growth. With guarantees ranging from 10 to 20 years, and being made from UV-stabilized plastic, we have yet to see sunlight degradation being a factor with the current crop of tanks — but it will happen. A coating of acrylic paint is one way of delaying this process.
Remember that the overflow is stormwater and has to be disposed of as such
In the tank
Sludge will end up at the bottom of the tank, above which will be a layer of less oxygenated water, known as the ‘anaerobic’ layer. Oxygenated is what we want, so the inlet pipe should feed its well-aerated water to the bottom of the tank. A fitting is available for the end of the entry pipe (called a ‘calmed inlet’), which limits stirring up of the sludge. ‘Quiescence’ (the opposite of turbulence) is a function of tank volume — the bigger the better. Given that we want to reduce the amount of anaerobic water and solid particles, our overflow should be a syphon from near the bottom of the tank too. Remember that the overflow is stormwater and has to be disposed of as such.
Water is best taken from near — but not at — the surface. A flexible intake hanging beneath a float sorts this problem nicely. It’s worth keeping an eye on your tank and de-sludging it periodically (good leaf interception, first flushing, stilling, and syphoning will lengthen those periods). Most have a sludge tap at the bottom, or you can use suction from the top. It is also a good idea to drain and clean your tank periodically. Every two to five years is recommended, depending on what gets into your tank, and how often you remove sludge and sediment.
If you have an underground pipe feeding your tank, it should have a cleaning eye fitted and the water trapped in the pipes should be drained every few months.
Usually they are dark coloured because internal darkness reduces the chance of algal growth
As noted, as well as a pump controller on the pump you need a changeover device in your plumbing where the tank water and mains supply meet. Electric changeover devices are easier to understand — a float switch in the tank controls a solenoid switch, but mechanical devices can also work. They simply favour the side with the most pressure. If the pump is running it will favour the tank. If there’s no flow because the water tank is empty, the pump will switch off and the changeover will open the mains side. But the sensors in the pump controller will keep checking and, as soon as there is water available from the tank again, it will use this water source. Given the cost of retrofitting a system like this, Garth says someone would have to be pretty committed to the ethics of water harvesting to do it. But they make perfect sense for new builds — certainly wherever councils require homeowners to install detention tanks. They are designed to capture stormwater and release it slowly. That avoids the massive impact all the extra run-off from new housing has on public stormwater systems, which were designed for much lower density neighbourhoods.
Naturally homeowners think about making use of that water. There is a solution. “If you have to store 5000 litres for detention, why not dig a bigger hole and store 10,000 litres?” says Garth. While the top half of the tank has to drain to preserve the 5000-litre capacity for stormwater, the other 5000 litres can be retained to flush toilets, water the garden, or for laundry. Some new developments, such as Hobsonville Estate in Auckland, have made rain harvesting mandatory. On the cost of a new build, a bit of extra plumbing, the larger tank and the $900–$1700 cost of a pump with a Rainsaver smart switch device is a small incremental cost, but it will make a massive difference in making the best use of your water-harvesting system and the rain we get for free.
If you are piping water into the house for potable purposes, treatment is needed. The options are adding chlorine, using a very fine in-line filter or purifier, boiling the water for one minute, or ultraviolet light treatment. One manufacturer recommends two-stage filtration; the first to remove sediment, and the second (one micron) to polish the water ready for UV treatment. This is because UV is a light source and the fewer particles casting shadows, the more effective it is. Check with your council, as some have different requirements and some require regular testing of your water.
You can have water tested by a specialist water-testing laboratory. Search for ‘laboratories’ when using an online search tool or ask your council. You should have drinking water tested annually.
Beyond the tank
Bigger diameter pipes will reduce friction, improving the flow to your garden. If you live in a frost-prone area you can bury them, lag them, or drain them during winter. Burial has the advantage in that it is less prone to damage, including UV (sunlight) degradation. Given that resilience equates to retarding degradation generally, it is worth shading other fittings from UV where possible too. Covering hose reels and hanging hoses in the shade are practical examples. If you need to add pressure to your system, in-line pumps and small header tanks are options. And in-line timers are an easy way to control your garden watering — just set and forget.
There is no downside to private water harvesting and no time like the present to set it up. The components are readily available, the technology is well within the capabilities of a sheddie, and local authorities are increasingly supportive. Whether you see it in terms of summer gardening, fire-fighting, household consumption, civic-load reduction or event resilience, it just makes sense. Call it money in the bank.
Water is best taken from near — but not at — the surface
Do what you have to collect the most rainwater. If there is no room for one large tank, two smaller ones may do nicely
Above: An easy-to-clean filtering set-up
Many schools and businesses now collect their own rainwater
Left: Keeping gutters constantly clean and clear of debris avoids a multitude of issues
An older-style corrugated iron tank
Mechanical rain-harvesting valve: switches between tank and mains. Used with surface or submersible pump and mains supply Pump controller for submersible pump with weather cover fitted Surface-mounted pump and pump controller (needs harvesting valve)
Attaching a first-flush diverter
Below: Smaller tanks really make sense for garden water supplies
Above: Hose to a waste-water outlet — the waste water can be used on the garden