Rain Water Tank and Supply System – Case Study


Although there are only two of us in the house, a few years ago, we found that our potable water consumption numbers were creeping up significantly. Combining our various water uses (drinking, cooking, washing up, showering, toilet flushing, laundry, gardening, car washing and sundry cleaning), we found that we were using approximately 12 kilolitres (12,000 litres) of potable, treated water per month.

Analysis – Disposal

Figure 1 – rainwater tank (Tank 3), pump in-line filter, and level
indicator (bottom R)

We took a good look at the situation and decided that toilet flushing, laundry, cleaning and car washing was taking over half (+/- 6 kilolitres) of our consumption per month. We decided that although we were on a septic tank system (two tanks on ether side of the house with their own dedicated French drain systems draining into the one acre sloping garden property), we could not, in the short to medium term, re-align to a waterless dry flush toilet system. We also decided that we would not design a new grey water management system, preferring to continue to drain grey water to the septic tank system.

Analysis – Non-Potable Water

A review of the house (4 bedrooms, 3 bathrooms and a 2 room, home office arrangement) suggested that we had sufficient roof area (+/- 250 M2,) for rainwater harvesting  although only part of the roof area was covered by gutters connected to the tanks. We have a distinct wet and dry season (the months of September – March and April – August, respectively) and there are occasional storms in between the seasons to “top up” the tanks. This ensures that our tank capacity can last from one wet season to the next, thus avoiding the need to fall back on piped, potable, water supply when the tanks are empty. We are also located on high ground in a “mist belt” area so our incidence of rain and “cloud moisture” is higher and therefore creating greater likelihood of additional precipitation.

Final Tank and Pump Configuration

Figure 2 – Tank 2 pump, in-line filter
and tap to right of tank. First flush
unit obscured

The final rainwater tank and supporting infrastructure in our installation series was installed recently so we have our final configuration. Tank 1 (2,500 litres) is located at the front of the house, fed from the front main roof, and pumps rainwater to the two flush toilets located in the house and to a yard tap for garden and car washing purposes. (There are switching valves which allow the system to be switched back to potable mains water supply if the tanks should run dry.) Tank 1 is linked to another 2,200 litre tank (Tank 3), fed from the roof at the rear of the house. Tank 3 is linked to Tank 1 by a pump system so that Tank 3 can top up Tank 1, as its level drops.

Figure 3 – Tank 1 with first flush unit
on the right

Tank 2 (2,500 litres) is located in the kitchen yard at the back of the house and can top up tank 4 (950 litres) and supplies a yard tap for cleaning and garden use. It is supplied by rainwater from the side roof over the kitchen and dining room area. A pump tops up Tank 4 (which supplies the outside toilet and the office toilet).

Rainwater Quality

As the rainwater was not required for potable or semi-potable purposes, the

Figure 4 – pipe connection showing level indicator,
in-line filter, and valve (L to R) on Tank 2

main means of “cleaning” the rainwater was through the use of first flush systems and filters. First flush units were installed on the three main tanks (Tanks 1, 2 and 3). Tank 4, the smaller tank, uses a swimming pool basket filter for large debris and a fine grid filter to reduce particles going into the tank. All rainwater pipes between the tanks and pumps, have a fine mesh filter to prevent particulate damage to the pumps and reduce the quantities of particles getting into the pipes and cisterns of the flush toilets.

Figure 5 – fine mesh filter for
rainwater tank (Tank 4)


Our potable water consumption has gone down from 12 Kilolitres (12,000 litres) per month to approximately 5 kilolitres (5,000 litres) for the two of us and that includes clothes washing, showering, cooking, and drinking. We believe that under drought stress conditions, and applying conservation practices, we could reduce this further to a maximum of 3.1 kilolitres (3,100 litres) per month.

It is possible that we could link the washing machine to the rainwater system, but we feel that this would require additional filtering and cleansing techniques. That, however, is a project which will have to wait until other current priorities are dealt with!

Arend Hoogervorst  

Figure 6 – swimming pool basket filter
used to catch large debris on Tank 3
and Tank 4