(i) In solar cells, the energy is obtained only during the day when sun shines.
(ii) Solar battery provides only direct current. But all the appliances are working by alternating current. So to run various devices DC should be converted as AC
(iii) These can be installed in remote and inaccessible areas (forest and hilly region) where setting up of power plant is expensive.
So, the solar cells are not used to meet all our energy needs.

Deforestation gives rise to ecological problems like floods, drought, soil erosion, loss of wild life, extinction of species, imbalance of biogeochemical cycles, alternation of climatic conditions and desertification.

(i) Untreated sewage or wastewater generated from domestic and industrial process is the leading polluter of water sources in India.
(ii) Sewage water results in agricultural contamination and environmental degradation.

(i) It burns without smoke and therefore causes less pollution.
(ii) An excellent way to get rid of organic wastes like bio-waste and sewage material.
(iii) Left over slurry is good manure rich in nitrogen and phosphorus.
(iv) It is safe and convenient to use.
(v) It can reduce the amount of green house gases emitted.

(i) Overcome the rapid depletion of ground water levels.
(ii) To meet the increase demand of water.
(iii) Reduce flood and soil erosion.
(iv) Used for drinking purpose.