Chapter 12 Rainwater Tanks, includes the following figure that is relates:
- number of people living in a house
- roof area
- reliability of supply
- tank size.
The water demand is for toilet flushing, assumed to be 20 L/day per occupant. Rainfall is based on that of Melbourne, 660 mm per year (graphs are also provided for other sites in Victoria).
Example: What size tank do we need to provide 70% reliability for toilet flushing when there are three occupants of a Melbourne house with a roof area of 200 m2.
Solution: Three people in a house with 200 m2 means we need the pink line of the graph (1.5 persons/100 m2) (see the figure below).
Read along from 70% reliability and down to 0.3 on the x-axis. This represents tank size as a % of roof area.
0.3% of 200 m2 = 0.6. The tank size is assumed to be 1 m deep so the tank volume is 0.6 m 3 = 600 L.
The procedure is not particularly straightforward, and this was borne out when I asked the example problem of 14 students. Seven got the correct answer, although one was for the wrong reasons. The seven wrong answers ranged from tanks sizes of 720 L to 120,000 L (mean tank size = 12,895 L; median = 960 L). People had problems scaling occupancy to to number of people per area of roof and then converting the percentage from the x-axis to a tank volume. The upshot is, take care when using this figure. Perhaps it could be improved in the next edition of the book.
Some other approaches to approximate tanks sizing.
- ‘4 weeks demand’. A rule of thumb is that tanks should be the equivalent volume of 4 weeks demand. In this example, the demand is 20 L per person per day, so the demand for three people for 4 weeks is 3 x 20 x 4 x 7 = 1680 L. This is obviously very rough but has has the advantage of being quick.
- Get Tanked website. This offers a sophisticated approach to tank sizing and includes the ability to measure roof area from a google earth image. Trying it out for the example problem, a 600 L tank is shown to be 81% reliabile, so results are similar to those from the analysis above.