# Approximate sizing of rainwater tanks – III

I’ve discussed approximate sizing of rainwater tanks in earlier posts, here wanted to highlight the method recommended the guideline WSUD: Engineering Procedures Stormwater.

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).

Relationship between toilet flushing water supply reliability and rainwater tank size for Melbourne

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.

1. ‘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.
2. 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.