Rainwater harvesting model

The completed rainwater reservoir.

A rainwater harvesting model for your home, garden or even small farm is an imperative before you turn the first sod.

Without water nothing thrives; not us, nor our pets or the gardens. In an underground reservoir like this it's much colder and we find it perfectly safe to drink; you may want to filter and sterilise it.

This page was last updated by Dr Bernard Preston on 25th April, 2019.

Why might you want to do build such a thing? Well, perhaps you are a greenie and want to save the planet. I don't have a problem with that; in fact it's a large part of my own motivation.

"We do not have to pay for wind and sun - the feedstock will come free."

- Michael Power

By Bernard Preston

Perhaps you don't have piped water available where you live; collect and store it, or perish.

My own main motivation is that the municipal supply in our village is using very old and decaying asbestos pipes; that means a deadly cancer called mesothelioma.

Perhaps your municipal supply is not well treated, and you are regularly getting diarrhoea; many towns in South Africa now have a very high bacterial count in their reticulated water; likewise plastic microbeads.

And if you live in the Third World as I do, the water supply is probably unreliable, and cut off periodically without notice for one reason or another; theft of the copper wire supplying the pumps is a common source of irritation in our lawless land.

Large parts of Johannesburg were recently without water for a whole week. Edendale in my own neck of the woods has been without for months. Eco friendly homes are coming into vogue to cope with problems like these.

Official SA statistics confirm that in 2017 less than half of South Africans have piped water where they live. If they have a roof over their heads with gutters, our rainwater harvesting model could change their lives.

"With the world's population expected to grow by one third to more than nine billion by the year 2050, the world will need

  • 70 percent more energy, and
  • 55 percent more water."
- United Nations

Rainwater Harvesting Model

This is my rainwater harvesting model; it's a main feature of our backyard permaculture philosophy. Yours may be slightly different but it will need to include at least

  • Rainwater collection
  • Pipework
  • Storage
  • Pump and
  • Filtration and perhaps sterilization.

Cost obviously is also a factor; that needn't be excessive if you give due consideration to your rainwater harvesting model before making a start.

It took, incidentally, four strong men a couple of days to dig the hole; the alternative is a far more expensive backhoe.

Water remains the bedrock of permaculture; without it no garden, or society for that matter, can thrive.

The size of your reservoir is obviously dependent on the weather patterns. Countries with a very short, wet season will need larger storage capacity which comes at a greater cost.

We decided on a roughly 25,000 litre tank; it's four metres in diameter, by two deep. That's enough to last about three months, if conservatively used during the dry periods, assuming there is zero precipitation; usually we have some rainfall in winter.

Safety is a feature too; our reservoir is immediately adjacent to the children's swings. You don't want to have a tragedy. Heavy concrete weights or a locking system is vital to keep inquisitive kids firmly out.

We built into our model a trap for trapping solids from the roof; the pipework runs directly into a small recess, from which it overflows into the reservoir.

Make it large enough, and not too deep so you can easily climb in and clean out the debris and leaves; a square container like a five litre ice cream container is perfect for scraping the bottom.

One can spend a lot of time and money on filtering the water; we gave consideration to various sand traps but, at the end of the day, settled on something inexpensive and easy to clean.

At least every two months I siphon the water out of the trap and clean out the muck; it stinks if you leave it longer.

This trap needs to be about 700mm deep and large enough so you can physically get in to clean it.

Ideally about 1000 x 850mm. The point where the rainwater pipes enter tends to leak; seal it properly.

Depending on the lie of the land, you may also want to consider an elevated fiberglass tank from which you can gravity feed to your home and garden. It's much easier on the pump which won't need to recycle every time a tap is switched on or the toilet flushed.

A double story house, like ours, means your rainwater tank would have to be elevated very high above the ground to give your shower sufficient pressure; I've decided against it for the present.

Rainwater collection

WE have concrete tiles on the roof and it's not ideal for rainwater harvesting model; a fine grit collects at the bottom of the trap. Clay tiles, iron or a synthetic cladding would be better suited to cashing in on the benefits of rainwater.

PVC gutters are useless and asbestos dangerous; aluminium gutters and downpipes are more costly but can be expected to last indefinitely. Ours are 35 years old and still in perfect condition.

Pipework rainwater

Pipework for rainwater trenches.

Dig trenches 250mm deep for your rainwater harvests, and then connect the downpipes to your reservoir. Standard 110mm UG PVC sewerage piping is perfect. At the final connection point, as we have a large roof, I wanted to have two pipes in parallel to carry heavy rain. Draw up a plan for your pipework rainwater, and you may need a plumber to connect it up for you. Since it's not under pressure small leaks are not critical except in that final piping; there the water is under nearly a metre of pressure.

There are so many advantages of using a rainwater harvesting model; better for drinking, perfect for the vegetable garden, and good for the eco environment.

Rainwater storage

My advice is to get a civil engineer to draw up plans for you. At the depth of two or three metres the pressure is considerable and you don't want cracks developing.

Remember to put a sealant in the plaster, or you may have to fibreglass the inside of the tank; that is ideal, but expensive.

A roof is vital for your rainwater harvesting model, firstly to keep it dark, stopping algae growth, but also for safety. It also keeps leaves, etc out. It must have some form of trap, so that you can get into the reservoir; it needs to be cleaned annually.


World wide losses in the reticulation are a huge problem, constituting what is known as non-revenue water. In our city, Pietermaritzburg, it accounts for more than half the water drawn from the Umngeni water system.

On average around the world, losses amount to between 30-40%. In the mother city of South Africa, in desperate need in 2017, it's only 15% due to good government.

The joy of this rainwater harvesting model is that losses amount to zero. The only loss is when the underground reservoir needs to be cleaned. Wasting 1-3kl annually is minute. Water scarcity in the world is set to increase and it's up to us to individually become more resilient in the face of these challenges.

With only about 8% of rainfall reaching rivers and dams even if every single home built a one of these rainwater harvesting models, it would still have almost no effect, but certainly would reduce storm water damage.

Self priming pumps

Only use self priming pumps; much fewer headaches if it runs dry for any reason.

A submersible pump is workable, but isn't necessary. The arguments go back and forth whether to use a bladder; we found it unnecessary with the modern pump, where you can set the pressure at which it switches on. But that's utterly dependent on no leaks in your rainwater harvesting model.

Rainwater filter

A rainwater filter may be necessary after harvesting and storing in the reservoir.

A variety of rainwater filters are available; remember they must carry a minimum of about 5 bars of pressure provided by your pump. The standard swimming pool sand filter is not suitable.

I settled on this inexpensive model; it must be firmly attached to the wall so that the bowl can be easily removed without stressing all the piping.

These filters will remove fine particulate matter but not bacteria; for that you will need some sort of ozone or UV sterilization.

Modern thinking is that exposure to a certain amount of bugs, bacteria and worms is actually healthy; without their stimulation, our immune systems do not prepare us for the real world. Disease of the immune system makes for an interesting read.

Note well; before unscrewing the filter cover, you must first relieve the pressure. Turn off the pump, and open a tap to drain off some water.

Also: before screwing the filter cover back onto its fitting, make sure you have filled it with water; otherwise that air will be pumped into your pipes causing airlocks and pipe hammer. Any rainwater harvesting model comes with its pitfalls for the unwary; first talk to those who are knowledgeable.

Rainwater tanks

If you are going to use an above ground rainwater tank, spend the extra and buy a fibreglass tank; they last indefinitely, even if standing in the full sun. PVC tanks don't last.

Just how many suburban rainwater tanks would you need; it depends on the length of your dry season, and whether you want to be entirely independent of the local utility.

But then, if you are planning to pipe the rainwater into your home, you will need to sterilize the water in the above ground tank; the temperature of the water is much higher and bacteria can multiply. An underground rainwater harvesting model keeps it much colder. We have no problems.

Indulgent shower

An indulgent shower using the largest rose you can find is one of the joys of those who have invested in a rainwater harvesting model.

The bath vs shower debate has no merit in our home; we can have our cake and eat it.

An indulgent shower is one of the rewards of rainwater harvesting.

There are few greater pleasures than enjoying your indulgent, full-stream shower, using your own rainwater, heated by the sun, with a pump driven by your own solar generator.

Your rainwater harvesting model is a central feature of any backyard permaculture philosophy. It's needed for your veggies and fruit trees, to supply the chooks and worms, and your home.

Solar tubes and a geyser are necessary if you want to turn your harvested rainwater into an indulgent shower or bath.

Water, harvesting rainwater and then heating it; sunshine, harvesting sunshine and then turning it into electricity for your rainwater pump equals the totally indulgent shower!

Solar generator

A solar generator collecting electricity from PV panels, and then turning it into alternating current using an inverter is the next step, if you want to be a total greenie; this rainwater harvesting model may lead you on progressively on the green journey; your own personalised one.

For me it was vice versa; I started with collecting sunshine, and only then moved on to rainwater.

Plan big and don't start with small panels as I did. I wouldn't use anything less than a 300 watt panel today, if I was starting from scratch. A stud diode has to be used if you use different voltages; try to use a uniform size and that means doing your planning right from the beginning.

For me our solar generator is another central feature of our backyard permaculture, driving the rainwater harvesting model and providing power for our home; working with and not against nature is the name of the game.

Residential solar panels complete the task of going green after your rainwater harvesting model.


Having spent a considerable sum on your rainwater harvesting model, you are now in a position to save money every month. In the minds of some strange people, you are robbing your municipality and they make rainwater harvesting illegal. Or insist you put a meter on it, and pay for the rainwater that falls on your roof.

We don't use the water; instead of passing straight from your gutters and downpipes, and thence back to the earth, we simply divert it through our toilets, showers and garden, and thence return it to the soil.

So, storing and using rainwater isn't such a big deal, is it?

Electricity and water

The cost and provision of electricity and water are irretrievably linked. Generation of power using traditional turbines requires a great amount of water.

Likewise pumping water consumes massive amounts of electricity.

Society appears unwilling to cut back on the use of either. That can only mean new infrastructure costing a great deal of capital, which of course is passed on to the consumer.

South Africa has just built with much fanfare a R200 million facility to extract potable water from the sea; what the minister didn't say is that it uses R30,000 of electricity every day, and will provide only 90 litres for every family dependent on it; a mere drop in the ocean of need.

The solution for the individual is a rainwater harvesting model and generating solar power right in your own backyard. Neither are rocket science; there's plenty falling on the roofs of most homes. All you have to do is collect and store it. Purchase or build your own eco friendly homes.

A panel of international and local experts in South Africa recommend that Eskom be allowed to buy power back from independent producers, presumably including home owners, and educating the public about cutting back on water and electricity.

Are we willing? By sourcing one's own power and water from the skies one can use both generously and in a relatively unlimited fashion. Every homeowner should be looking into a rainwater harvesting model that suits their budget and needs.

Rainwater harvesting model

Rainwater harvesting model could be a reality in your home and garden. Ours is enormously successful. We now use virtually no mains water; it's possible we'll need some utility water in the future during an extended drought, so we have not gone off the reticulation system.

But right now SA is in the midst of the worst drought in 40 years, and our reservoir is full; it's amazing just how much water can be harvested from just 5mm on your roof.

600mm of annual rainfall falling on a 200m2 roof will provide 1200 cubes of water; at least four times our needs, for example.

Before setting off on this really quite major project, it's vital to talk to specialised folk, get advice and make sure your rainwater harvesting model is properly constituted before turning the first sod.

The right specs for your reservoir otherwise it will crack and leak; a self priming pump and what filter? How are you going to cope with having the option to draw either municipal or your own rainwater? How is the water going to be efficiently collected from your downpipes? There's plenty of planning to be done.

Oh, and a non-return valve so that your water cannot return to the mains supply.

Rainwater reticulation

Rainwater reticulation from your reservoir to the home and garden is the next step. Don't be tempted to use half inch piping; once inside the house, yes, half inch is fine perhaps, but getting the water any distance requires a thicker pipe.

In fact, it made such a difference, I'd put three quarter in the house too, if I was to do it again.

Loss of pressure is considerable in a half inch pipe if it's over any distance. It's 30m to our garden, plus the length of the hosepipe; I ended up pulling up all the half inch and replacing it. There's a huge difference now in the pressure at the end of the hosepipe; in fact we can run two hosepipes at once now.

For small amounts, a small fibreglass watertank in the vegetable garden is useful, for filling a watering can. Place a tap near the tank so you can fill it easily from the main reservoir. Remember, once switched off, the hose running into the tank acts as a siphon; if there are any leaks the water will slowly drain from the tank.

A good valve at the pump for the water running to the garden is essential. Any tiny leaks in your rainwater harvesting model will mean that the pump keeps recycling.

Rainwater harvests

Rainwater harvests not only the gift from heaven but also a lot of crud. A vital part of your rainwater harvesting model is how to clean out your tanks and reservoir.

Getting siphon started on sump makes your life a lot easier.

There are certainly a few tiny creatures swimming about in your reservoir; what do tadpoles eat you may be thinking?

Plant the rain

If you've never heard of Brad Lancaster, an American ecologist, then enjoy this inspiring TED video. He points out that even in drought stricken cities, more rain falls on the area than they can possibly consume. All we have to do is find ways to store it.

He does it by building sponges in his garden and down the street. They too use no municipal water. Inspired by a black Zimbabwean farmer, he calls it planting the rain.

Here is more about harvesting rainwater.

Population growth

Despite many doubts about long the world's population growth can continue as it is without calamity, a 33% growth in the population means a 70% increase in food, according to the UN, remembering that one third that is produced is never actually eaten.

Both the race to end waste, new strategies to deal with waste water, and use of a rainwater harvesting model must be prime strategies.

Swimming pools

South Africans love their pools but homes with pools consume far more water. Evaporation is high in the summer months.

The beauty of harvesting your own rainwater is that you have more than enough for the pool so you can keep fit without guilt.

Find out more about our solar swimming pool.


When water threatens to run out, people get stressed; seeing water leaking makes them mad. They get angry because they can't water the vegetables, their favourite plants and the pools; they may not wash their cars and have to take showers standing in buckets with a tiny trickle from the rose.

A long generous shower is one of my most potent ways of destressing; breaking water releases negative ions into the air helping us relax.

Build you own rainwater harvesting model and you can de-stress. You will have more than enough water for every eventuality; the garden, the pool and generous showers. Make it larger than you think you will need; an extra metre deep adds very little to the overall cost.

So, what's your water innovation?

Useful links

Drinking water

There may be tadpoles in your rainwater harvesting reservoir.
ยป Rainwater harvesting model

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56 Groenekloof Rd,

Hilton, KZN

South Africa