Research station

Rainwater harvesting, part 2: Clean and safe

posted in: Magazine Articles, Water | 0
Organic NZ Magazine: January/February 2015
Section: Building and technology
Author: Rachel Rose


Part 2: Clean and safe


Research station
Stan Abbott, head of the Roof Water Harvesting Centre, at its windblown rooftop research station at Massey University’s Wellington campus. Photo: Rachel Rose


Typically, how clean is the rainwater being drunk in rural New Zealand? Often, not very. Stan Abbott heads up the Roof Water Harvesting Centre at Massey University. In 2006 Stan and his colleagues tested roof water samples from 560 homes for total coliforms (an indicator of environmental contamination e.g. soil and vegetation) and E. coli (an indicator of faecal contamination). Seventy percent of the sample failed the drinking water standard for New Zealand, which requires zero of either indicator in the mains supply. The study considered a level of 60 organisms per 100 ml to be ‘still contaminated but acceptable’, yet 53 percent of samples contained total coliforms in excess of this level and 30 percent had E. coli in excess of this somewhat arbitrary cut-off.

There’s something in the water

So what’s in the water? Faecal contamination sounds like … shit? Yes, it is: typically from birds, rodents and animals like possums that have defecated on the roof and gutters. But research shows animal and bird droppings can also be wind-blown from nearby trees. This brings the risk of E.coli, Salmonella, Campylobacter and others, all capable of making humans very sick.

Other sources of contamination include lead, leaves, soil and other organic debris, ash, chemicals, and the bodies of dead animals or insects in the gutters – or worst, in the tank itself.

Yet let’s remember humans have been collecting and drinking rainwater for thousands of years and vast numbers of the world’s current population still do. And the research also shows that people are not getting sick nearly as often as the data may lead us to expect.

Stan describes most rural rainwater systems as very rudimentary. “But a lot of people [drinking that water] will swear blind they never get sick – and I believe them,” he says.

The explanation for that is likely twofold, starting with under-reporting. “If people get sick, they’re more likely to think it’s food poisoning and blame last night’s takeaways,” Stan says. And unless a whole household gets very ill, the health authorities are unlikely to hear of it.

The other reason is human adaption: if people have been drinking the water for their whole life, they’ve built up immunity to the germs in their water supply.

Sean Lynch of Clearflow Contracting installs a lot of rainwater systems on large lifestyle blocks in the Manawatu and he’s often called in when townies move to the country onto a block with old tanks. “They’re used to town water, and they get sick on the tank water,” Sean observes. He sees far fewer issues with new tanks.

Getting clean

So how to make your tank water safe to drink? There is a bewildering array of accessories available and it’s hard to know where to start – and when to stop. Stan recommends these steps in this order.

1.      Make sure your roof is a suitable material for collecting water. That means no lead paint, nails or flashings, no treated timber shingles, asphalt or tar roofing materials, and preferably not roofing materials containing asbestos. What’s suitable? Steel; concrete, clay or factory-painted tiles; slate; glass; PVC (without lead stabiliser added); fibreglass; polycarbonate; and untreated timber shingles.

2.      Make sure the roof is clean and the gutters are clean and unblocked. Blocked gutters mean a lot of water will be spilled and won’t reach your tank. If gutters regularly clog or foul, Stan recommends installing gutter mesh. And be careful up that ladder when you’re clambering up onto the roof!

3.      Install screened rain heads (ones with a single screen) on each downpipe. These are sold under various trade names but all work as a physical barrier that stops debris like leaves entering the pipes connected to your tank.

4.      Consider a first-flush diverter. This is a chamber installed before the tank that collects the first amount of each rainfall, which in theory carries away any contamination that has built up on the roof since it last rained. Once the chamber is filled, a floating ball seals the chamber and subsequent water runs straight to the tank. The diversion chamber empties to the ground in a slow trickle via a nozzle. Small first-flush diverters can be wall-mounted or post-mounted beside the tank. You can make one yourself or buy a kit that fits onto a length of pipe, usually 90 mm or 300 mm for bigger systems. A diverter can also be fitted onto each downpipe. They can be buried, but this is officially recommended only on a sloping site where the contour allows the end of the chamber to be above ground (so the nozzle can be periodically removed and cleaned).

First-flush diverters

First-flush diverters introduce a lot of complexity into a rainwater harvesting system. You’ll collect less water – something to consider in drought-prone areas. It also makes for complex equations when estimating how much water you can capture, because you must consider not just total rainfall but the distribution of rain. How many rainfall events occur in a month or season? Remember, the larger the tank, the more diluted any incoming contaminants – many rural tanks are 15,000 litres or more.

There is another benefit to first-flush diverters: they can convert the typical ‘wet system’ to a dry system. In a wet system, there is always water in the pipes, to the height of the top of the tank. That water will become stagnant when days or weeks pass between rain falling, but it is pushed into the tank when it next rains. That fouled water would be sent to the first flush diverter if it is appropriately sized and properly sited within the system.

That’s a big if! First-flush diverters are not common; Sean has installed hundreds of tanks in the past five years, but only a handful of systems included a first-flush diverter and only because the client specified it. Stan’s met plumbers who didn’t even know what they were and the most common mistake he finds when consulting is incorrectly installed first-flush diverters (see case study on the Organic NZ website).

“Why is the message not getting through? Some tradespeople seem unwilling to read even the most basic technical information,” says Stan. He believes suppliers need to do more to educate the industry. Meanwhile, he suggests asking a lot of questions of prospective tradies: “How many rainwater tanks have you put in? How many first flush diverters? How often do you fit screened rain heads? Keep asking questions, and you’ll soon get a feel for what they know.”

It’s common to find sales material suggesting first flush diverters that hold a mere 15 or 20 litres of water. Stan advises 50 litres as a minimum. If you connect a first-flush diverter at the top of the vertical riser entering the tank, that won’t drain a wet system.

There are in-between solutions. Sean builds flushing points into the systems he installs, that can be uncapped on a rainy day to flush out physical debris. He typically installs two tanks on rural blocks. The tanks are level at the bottom; water collects into one tank and is drawn off the other. That makes for cleaner water because sediment settles at the bottom of the first tank and physical debris like leaves float on the top. Sean connects the tanks in series about 200–300 mm from the bottom via a fat connector (40–50 mm). That means the water level in each tank will always be equal, and water will move freely between the tanks even in heavy rain. Remember two tanks will always cost more than one large tank of equal capacity.

Filters and treatments

There is a whole other suite of filtering and water cleansing treatments that can be applied after the tank, before the water is pumped back into the house for consumption: coarse and fine filters, ultra-violet light, ozonation, charcoal under-bench filters … much is possible. Without doubt these all produce very clean water but professionals we spoke to aren’t convinced of the need and haven’t installed these systems in their own houses.

‘Going the whole hog’ gets expensive and requires ongoing maintenance. Sean has encountered clients who are willing to outlay the capital expense of sophisticated filtering but then baulk at the cost of replacing UV bulbs and physical filters. These might need replacing every year or removing and cleaning every few months.

Even a simple system requires maintenance. It’s a good idea to make a habit of regularly opening the access hatch. Is the water clear? Does it smell OK? These are basic and reliable indicators of water quality. Make sure the hatch is always secured. The writer has lived in remote communities reliant on rainwater where dead possums fouled large tanks for weeks before detection. More tragic are some cases in Australia of children drowning in water tanks left unsecured.

Cleaning and maintenance

Gutters should be checked and cleared when necessary. Stan emphasises designing the rainwater harvesting system so the tank can be easily and swiftly disconnected from the pipes. This is crucial in an emergency: the last time there was a volcanic eruption on the Central Plateau and ash was falling from the sky, some people had to cut their downpipes to isolate their tanks. It’s also necessary if you need to wash down your roof. Tip: don’t glue in your screened rainheads. They may need securing; do this with a single screw. When necessary, remove them and block the open downpipe. A tennis ball does an adequate job in 90 mm pipes! Otherwise, design fittings at the tank that can be disconnected but consider where the water will discharge.

Some authorities are saying tanks must be drained and cleaned every year but Stan considers this unnecessary and expensive. “Rather, concentrate on preventing stuff from getting into the tank in the first place. Draining and cleaning is only called for if someone has got very sick or you have ongoing problems.”

Most councils require backflow prevention devices to be fitted when tank water is plumbed into the house. That will likely require a building consent: always check with your local and regional authority because regulations vary considerably throughout the country. All plumbing work must be carried out by a qualified plumber.

Beware spraydrift

One final and important warning for rural areas: beware spraydrift of agrichemicals. There’s a code of practice that should be followed but there are still real risks of roof contamination. See Appendix 3 in this document for guidelines:


Rachel Rose is establishing an urban permaculture property in Whanganui.

Rainwater butts revisited

In Part I, we looked at small rainwater butts that provide some emergency drinking water supply in urban areas. The open wooden barrel that illustrated the story looks beautiful, in a rustic, traditional way, but an open container of water is an invitation to massive contamination from bird and animal droppings (and dead animals and insects: rotting possum, anyone?) and the sunlight will produce algae. You’ll also create a mozzie breeding paradise, right next to your house.

Do consider the plastic butts – with lids! – of which many types are available.

You can also recycle a food-grade plastic barrel with a tight fitting lid: ensure it was only ever used to transport food, not chemicals.

Remember you must treat this water if you have to drink it in an emergency. Boil it; or use additives like Acquasafe or Pour N Go, which are preferable to chlorine bleach. They break down without residuals.


A cautionary tale

Poor design and installation is costly and inefficient. Consider this litany of problems that arose in a recent installation of a small suburban rainwater harvesting system. Its primary function was to buffer the release of stormwater into a block that is waterlogged in winter. Physical and geological constraints required siting a small (5400-litre) tank close to the house. Plumbing into the laundry etc. was ruled out, after a $10K quote.

The system was put in by an experienced drainlayer who claimed to have installed lots of rainwater tanks. Turns out he had done a few: basic systems on farms, with no additional fittings. Screened rainheads were new to him and he was flummoxed by the first-flush diverter. The homeowner knew how she wanted the system to function and specified the components but did not supervise the work closely, relying on the tradie’s supposed expertise. Mistake #1: Tradesperson’s expertise not established, references not checked.

The homeowner wanted to use the water on the garden and as an emergency drinking supply, so was concerned about water quality. Mistake #2: the roof wasn’t inspected or cleaned prior to plumbing into the tank. Screened rainheads were cut into the four downpipes. Three are easily accessible; the fourth is cut in 3.3 m above ground (mistake #3), requiring an extension ladder and assistance to remove the screen and clean it. Result: six months later, it hasn’t been cleaned. (One plus: the rainheads were not glued into the pipes.)

It was with the first-flush diverter that things really went awry. The supplier calculated that about 230 litres of water needed to be diverted given the size of the roof and the relatively small tank. That required a three-metre length of 300 mm pipe that needed to run underground: not cheap, and a lot of digging too. Fittings were bodged to save costs and the bottom of the system was sealed with a rubber gasket fitted to a trickle-fed nozzle that discharged the diversion chamber. The diversion chamber was at the bottom of the whole system, about 1.2 metres lower than the bottom of the tank. The wrong components were installed and the ball didn’t block the diversion chamber once it was full. The water pressure from heavy rain blew the rubber gasket clear off the pipe: water gushed out and nothing reached the tank. How to count the mistakes here? (#4) Well-meaning and helpful suppliers lacked experience with first-flush diverters and (#5) the tradesperson was completely out of his depth. And out of patience too, which made for a tense environment.

Site visits ensued from the local agent and the supplier – and by Massey University’s Stan Abbott, with two French PhD students in tow. Upshot? The problems outlined above were identified. The first-flush diverter was dug up and the fittings scrapped. A Marley first-flush diverter kit was fitted to the 300 mm pipe, a new ditch dug, the correct components installed and buried at the exact 12 degree angle specified by Stan … and breath was held until the next rain, at which point the system operated correctly.

But it wasn’t entirely over. Months later, problems emerged with the positioning of the tank. The drainlayer didn’t use fittings to connect the inlet and outlet pipes into the tank (#6); rather he’d made a rough hole with a jigsaw (rather than a hole saw)(#7), and used a cheap silicone substitute to patch around the holes (#8). Exposed to UV, this compound quickly broke down and the tank started leaking around the inlet pipe. These pipes should have been plumbed in beside the access hatch. Instead they were on the opposite side of the tank (#9) – so in order to repair the pipe fittings, someone had to climb into the tank, two-thirds full of water. That someone was the slender homeowner, because the plumber called in to fix up the shoddy work was too burly to fit through the hatch!