More than half of the rain evapotranspirates. The rest percolates into the ground and becomes groundwater, which flows very slowly underground until it resurfaces as wells. Evaporation, transpiration by plants and filtration are all mechanisms to clean water. When water becomes dirty from particles and dissolved substances, evaporation and transpiration separate solid and dissolved compounds from the water. The same is true for a healthy soil that also cleans water. The soil is made up of many different layers, such as sand, gravel and clay. On its way through the ground, the water runs through all these layers and particles contained in the water are held back. The soil filters the water. Other substances are «eaten» by bacteria that live in the soil.

Soil filter: Carefully cut out the bottom of the plastic bottles. Fill the bottles with the filter materials you think will best clean the water. Wash the filters with clear water until the outlet is clear. Take 500 mL of artificial wastewater and pour it onto the filter. Collect the effluent and compare it with the wastewater or use the set of methods for water analysis (see experiment «Wastewater analysis»).

Evapotranspiration box: Fill the box with the mixture of sand and soil (10 cm high). Add the plants and place the test tube with the funnel in the center of the box. Fill in 1 L of artificial wastewater. Close the box hermetically with plastic foil and adhesive tape. Gently push the plastic film towards the funnel and and weigh it down with the one or two stones. Place the evapotranspiration box close to the window and observe the processes in the box.