Freshwater molluscs--snails and bivalves--have been used frequently as bioindicator organisms. With increasing needs for research on contaminant effects in freshwater ecosystems, this kind of biomonitoring is likely to develop further in the future. Molluscs can be used effectively for studies of both organic and inorganic contaminants; this review focuses on studies involving bioaccumulation and toxicity of metals. Two important advantages of snails and bivalves over most other freshwater organisms for biomonitoring research are their large size and limited mobility.

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen required by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. It is used to measure of the amount of organic waste in water.

Water pollution is the increase in the concentration of the undesirable harmful compounds in the water resources. The water pollution can be controlled by many ways. The sewage and the factory wastes which contain the pollutants and high number of microbes can be treated before letting the water discharged into water bodies. Biodegradable form of fertilizers and pesticides can be used for the reduction of the soil and water pollution, and eutrophication. 

Biological treatment of the waste water can be performed with the use of microbes. The bacteria and fungal cells are commonly used in the treatment of the water through activated sludge. Activated sludge refers to floc of microorganisms including bacterial cells and fungal filaments which are used in the process to break down organic matter into carbon dioxide, water, and other inorganic compounds and help in the clearing of the water. 

The methods that can be used to control water pollution are:
(i) Avoiding the use of chemical substances freely.
(ii) Educating society about the dangers of water pollution.
(iii) Using biological control in agricultural areas instead of using pesticides.

Drinking arsenic-rich water over a long period results in various health effects including skin problems (such as color changes on the skin, and hard patches on the palms and soles of the feet), skin cancer, cancers of the bladder, kidney and lung, and diseases of the blood vessels of the legs and feet, and possibly also diabetes, high blood pressure and reproductive disorders.

• Water conservation is a very important practice which helps to maintain water balance in a natural resource. 
• The water which has been used for one type of work can be used for varied purposes. 
• The water which is used for washing the fruits and vegetables should not be thrown away. 
• The water can be used for watering the plants or for other purposes. 
• Thus, the correct answer is option D. 

Disinfectants are the antimicrobial agents which is mainly used to stop the growth of pathogen on the non-living organism. It may affect the substances on which it is applied. Phenyl is a disinfectant which cannot be used for purification of drinking water because it will kill the pathogens of the bacteria but the quality of the water will  degrade as it is poisonous for the living organisms. It has a pungent odour. All the other options are used as disinfectant because they do not have an adverse effect on the quality of water.

The Ministry of Environment and Forests has. initiated Ganga Action Plan and Yamuna Action Plan, to save these major rivers of our country from pollution. Under these plans, it is proposed to build a large number of sewage treatment plants so that only treated water may be discharged in the rivers.

Eichhornia crassipes, commonly known as common water hyacinth, is an aquatic plant native to the Amazon basin, and is often a highly problematic invasive species outside its native range. Water hyacinth is a free-floating perennial aquatic plant (or hydrophyte) native to tropical and subtropical South America. With broad, thick, glossy, ovate leaves, water hyacinth may rise above the surface of the water as much as 1 meter in height. One of the fastest growing plants known, water hyacinth reproduces primarily by way of runners or stolons, which eventually form daughter plants. Each plant additionally can produce thousands of seeds each year, and these seeds can remain viable for more than 28 years. It was introduced in Bengal in India because of its beautiful flowers and shapes of leaves, but turned out to be an invasive weed draining oxygen from the water bodies and resulted in death of many fish. Fish is a supplement food in Bengal, and because of the fish scarcity in Bengal caused by Eichhornia, the water hyacinth is also called "Terror of Bengal".

BOD is a chemical procedure for determining the amount of dissolved $O _2$, needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at a certain temperature over a specific time period.

A bioindicator is a living organism that gives us an idea of the health of an ecosystem. Some organisms are very sensitive to pollution in their environment, so if pollutants are present, the organism may change its behaviour, or it could even die. Sewage fungus is a thick growth of fungi that develops on stagnant water contaminated with sewage effluents. The filamentous fungus indicates that the sewage has not had secondary treatment. Adult stoneflies are always found near freshwater only as aquatic larvae. As water temperature increases, the larvae dies. As BOD increases in polluted water, there occurs an increase in haemoglobin possessing blood and sludge worms. Hence, the correct answer is option E.

Water hyacinth( Eichhornia crassipes) is known as the terror of bengal because it is an exotic shrub which strongly competes with the native species and has also eliminated many of them. It is a water-based plant that flourishes in the hot and humid climate of Bengal. Fish is a supplement food in Bengal. Water hyacinth is efficient in utilizing aquatic nutrients and solar energy for profuse biomass production and results in decline in number of fishes and because of the fish scarcity in Bengal caused by Eichhornia, the water hyacinth is also called "Terror of Bengal". 

Autotrophs of greatest significance in sanitary engineering are the nitrifying, sulfur, and iron. Because these bacteria use CO$ _2$ as the carbon source and oxidize inorganic compounds for energy. 

Indicator organisms are organisms used as a sign of quality or hygienic status in food, dietary supplements, water or the environment for example algal bloom is an indicator of eutrophication. The presence of E.coli is an indicator for water pollution.

The main purpose of FOAM is to treat wastewater around a place called Arcata and the surrounding bay through a natural system.