Membrane Filter Method
Gridded MCE membrane filter
When the number of indicator organisms in water is very low, direct inoculation on solid media is not practicable and other methods must be used by which large volumes can be examined. Other, methods of examination include membrane filtration and multiple tube test.
Membrane filter techniques are widely used for the enumeration of bacteria from water samples. In this method, the bacterial cells are filtered through a membrane , like gridded MCE, as the sample passes through it. The cells, along with the membrane, are placed on a suitable solid medium. On incubation these cells produce visible colonies which can be counted.
Nutrients
The major nutrients required for the growth of algae include carbon, hydrogen, nitrogen, oxygen and phosphorus. Of these, nitrogen and phosphorus are the ones which occur in very low concentration in water in relation to algal nutrients. Thus they act as limiting factors for algal productivity. Nitrogen is available in the form of ammonium or nitrate and phosphorus is available in the form of phosphates. The increased concentrations of nitrogen and phosphorus in eutrophic waters are responsible for the excessive algal growth.
Source of Nutrients
Membrane filter
Various natural sources contribute to the increased nutrient level in a water body. Contamination by excreta of birds and animals especially of live stocks is the major source. Gross fire accident of vegetation in an area near to the water body can also provide nutrients in the form of burned ashes. Addition of both treated and untreated sewage into water bodies is another main cause of eutrophication. Detergents which are rich in phosphate may enter natural water bodies. Human excreta and industrial effluents can also add to the nutrient status. Fertilizers applied to agricultural lands are often leached out by irrigation water and rainfall into aquatic environment which provide a direct source of nutrients.
Source for Water Borne Pathogens
Waterborne pathogens make their entry into the water bodies through a number of sources. Recycling of treated/inadequately treated wastewater by mixing them with natural water bodies adds microorganisms. When septic tanks are built near the water bodies mixing or seeping of excreta may occur and this may act as a source of waterborne pathogens.