Stop and wonder

DISCUSS IN GROUPS

1. Until recent years in various Mediterranean islands, many houses had their own rainwater harvesting cisterns. Why so? Why did they stop using them?
2. Today, in several cities around the world, even in those where water is abundant, many new buildings are equipped with rainwater harvesting systems. Why so? Where do you think this water can be used ?
3. All around the world, infrastructures like sport fields and airports are equipped with rainwater collection systems. How do they use the collected water?

Water-readings

The parts of a rainwater harvesting system

A rainwater harvesting system includes the following basic parts:

• A catchment surface, on which the rain falls. This can be a roof, a terrace, etc.
• A gutter or drainage pipe that is an open horizontal or closed vertical pipe through which the rainwater flows. A sieve (net) is placed at the opening of the gutter to retain any debris, leaves, pebbles etc.
• A control valve to control the flow of water from the gutter either to the tank or the yard.
• A filter to further clean water from small sized solids, dust, etc.
• A tank (cistern) to store the water. This can be made of cement, plastic or metal. It may stand freely on ground or be carved underground. In any case the users need to have access to it.
• A pump and pipes to transfer the water.
• An overflowing siphon that leads the excess water out of the tank  (and helps out the aeration of the system as well).

Stop and wonder

DISCUSS IN GROUPS

1. Even when the tank is underground, why do we need to have access to its opening?
2. During which season we need to clean the filter?
3. The control valve gives us the option to choose whether to direct the rainwater inside the tank or not. In which cases the rainwater is not wanted?

Solo activity

How much rainwater can you collect at school?

If your school has a water tank, you can easily calculate how much rainwater it collects approximately within a year. Even if there is no tank, you can still calculate how much rainwater could have been collected if you know:

• The area of the catchment surface (in square meters, m²). With help of your teacher calculate it after measuring the building’s perimeter.
• The annual average rainfall of your area (in millimeters per year, mm/ yr).  You can find this in the website of the Meteorological Agency.

Then, the amount of rainwater that can be collected in a year (in cubic meters, m³) is:

Area (m²) x Average annual precipitation (mm) x [runoff coefficient] *** = .... m3)

What is a run-off coefficient? During the rainwater system operation there are some losses e.g. due to evaporation or spills at the catchment surface.  So, we use a “run-off coefficient”, a number that expresses these losses and it depends on the material of the catchment surface. For instance, it is about 08-0.9 for metal; 0.75 to 0.9 for concrete surfaces; 0.8 – 0.85 for gravel; 0.5  for clay tiles, etc.

Way forward

Surfing around the internet

Watch here a short animation about the rainwater harvesting system that has been installed in a Primary School in Malta.

Dive in…

The water cycle if you want to  make rain through a model of the water cycle!

Rainwater Harvesting Systems in practice if you want to read more about a real installation of a rainwater harvesting system!

Go back to the homepage!