Oil and fats hardens and block the sewage pipes. In an open drain also, fats block porosity of soil which affects its water filtering efficiency. Therefore, it should not be released in the drain.

The steps involved in getting clarified water from wastewater are as follows:
(i) Use of an aerator to bubble air through wastewater. A mechanical stirrer or a mixer can also be used in place of the aerator. It helps in reducing bad odour of the wastewater.
(ii) Then, the water is filtered through the layers of sand, find grains and medium gravel. This filtration makes the wastewater clean from various types of pollutants. The water is filtered continuously until it becomes clear.
(iii) Then, disinfectant such as chlorine tablet is added to the filtrate and stirred to obtain completely clear and purified water. The water in a river is cleaned naturally by processes that are similar to those adopted in a wastewater treatment plant.

Collected semi-solid wastes such as faeces that settle down during wastewater treatment are called sludge.
The sludge is removed by using a scraper and then transferred to a tank where it is decomposed by anaerobic bacteria to produce biogas. This biogas is used as a low cost fuel for heating, cooking, etc. Activated sludge produced by decomposition of bacteria is used as manure.

Human excreta may cause water pollution and soil pollution which can lead to a lot of health related problems. Water polluted with it contains disease causing bacteria which can spread epidemics and becomes the most common route for water-borne diseases like cholera, dysentery, typhoid, etc.

Two chemicals used to disinfect water are chlorine and ozone.

Vermi-processing toilets.

Chemical toilets.

Biogas and manure.

The Eucalyptus trees are planted along sewage ponds because these trees absorb all surplus wastewater rapidly and release pure water vapour into the atmosphere.

By following proper sanitation and house-keeping practices, by creating less waste at an individual level, we can avoid the addition of the load in wastewater treatment plant.

Inorganic impurities present in sewage are nitrates and phosphates

It is used as manure, thus returning organic matter and nutrients to the soil.

In the vermi-processing toilet, the human excreta is treated by earthworms in a pit. The earthworms gradually eat up all the organic matter present in human excreta, decompose it and pass out from their body in the form of warm casting (also called vermi cakes).
The useful product obtained from a vermi-processing toilet is vermi cakes which is kind of high quality manure.

(a) Typhoid and cholera.
(b) In vermi-composting toilets, human excreta is treated by earthworms in a pit.
(c) He is intelligent and concious about health

We can minimise waste and pollutants entering the water and create less wastewater by taking following few steps at home:
(i) By not throwing used tea leaves, solid food remains, etc., in the drain. We should throw it in the dustbin.
(ii) By not throwing chemicals like medicines, paints, insecticides, etc., in the drain as they increase the pollution load of the sewage