Resource 14

Saving water ¹

Australia is the driest inhabited continent in the world and water supplies are limited. Even in areas with good rainfall, we need to conserve our precious water. Flushing toilets uses a lot of water and Australian companies have been trying to design more efficient ones which use less.

The water used to flush a toilet is stored in the cistern just above the bowl. Older designs used about 11 litres of water per flush. Caroma, an Australian company, realised that it was not necessary to use as much water to flush urine as to flush solid waste. They were the first in the world to design a dual-flush toilet in which the cistern has two buttons, one which uses 11 litres to flush and the other which uses only 5.5 litres.

More recent designs use a 6-litre full-flush and a 3-litre half-flush. It sounds a fairly simple idea! Reduce the size of the cistern and you will reduce the amount of water you use. The trouble with this is that sometimes not all of the wastes in the toilet bowl will be washed away. To overcome this problem, the design of the bowl had to be changed to increase the strength of the water jet. Caroma estimates that the new designs use up to 67% less water than the older 11-litre flush toilets.

Reducing the amount of water used in each flush can have other benefits. There would be less to pay in water rates and there would be a slowing down in Melbourne's demand for both water and sewerage services

Measurement

The units used to measure the volume of a liquid depend on how much liquid is being measured. At the small end of the scale, some biology experiments use tiny amounts of liquid measured in microlitres, or millionths of a litre. At the larger end of the scale Melbourne Water measures the amount of wastewater produced each year in megalitres. A megalitre is one hundred million litres! What are the units of volume and how are they related?

Unit Symbol Compared with a litre
gigalitre GL 1,000,000,000 litres
megalitre ML 1,000,000 litres
litre L 1 litre
millilitre mL 1 / 1,000 litre
microlitre µL 1 / 1,000,000 litre

The kilolitre, or one thousand litres, is not usually used as a unit. Sometimes volume is measured using cubic centimetres (cm3). One cubic centimetre is the same as one millilitre.

Questions

  1. Dan Davis does not have a new Dinkum Dunny. He has one of the old 11-L single-flush models.
    (a) How much water will this use each flush?
    (b) How much water will Dan Davis use to flush his toilet each day? (You will need to estimate how many times Dan uses the toilet each day.)
  2. Dan Davis has two neighbours, Mr Trinh and Ms MacSporran. Mr Trinh has a 11/5.5 L dual-flush toilet and Ms MacSporran, the canny lass, has a 6/3 L flush model.
    Estimate how much water each will use in a day. (You will need to estimate the number of times they flush the toilet each day, as well as which flushes they use.)
  3. Compare your estimates of the amount of water used each day in flushing the toilet by Dan, Mr Trinh and Ms MacSporran. Show your estimates on a bar chart.
  4. Find out how often the toilet in your home is flushed each day. How much water does this use? Add this information to your bar chart.

¹ Based on an activity from: Keith McTaggart and Paul Saddler 1993, Flushing Dunnies, Melbourne Water and Science Teachers Association of Victoria.