Rain­wa­ter har­vest­ing


The Shed - - Contents - By Jude Wood­side Photograph­s: Jude Wood­side

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.


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.)


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.


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.

At­tach­ing the first-flush di­verter

Fit­ting the bracket to hold the di­verter to a post

Above: Adding the first stain­less-steel fil­ter … Left: … and the sec­ond plas­tic fil­ter

Left: Fit­ting the ball into the di­verter

Above: Show­ing how the ball valve works block­ing the di­verter

Top: (Left) Drip­per wash­ers. These reg­u­late how quickly the di­verter will empty. (Right) An in­ter­nal fil­ter to pre­vent the drip­per from get­ting blocked Above: The washer fit­ted to the end cap

Above: At­tach­ing a rain­head Far right: Fit­ting a down­pipe first-flush di­verter

Left: The rain­head fit­ting with both screens fit­ted Be­low left: The stuff you don’t re­ally want in your tank

A float­ing out­let. Note how the ball keeps the out­let be­low the sur­face but close to the top of the wa­ter col­umn

At­tach­ing a hose to the out­let — the waste­water can be used on the gar­den

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