Wa­ter har­vest­ing

Your op­tions, and prod­ucts to col­lect your own rain­wa­ter

The Shed - - Contents - By Mur­ray Grim­wood and Ian Parkes

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.

Be­ing self-suf­fi­cient

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.

First flush

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

Smart switch­ing

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.

Split sup­plies

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

Pho­to­graphs: The Shed archive

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

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