TREATMENT OF WATER A.Sateesh Chemist State Level Laboratory
Treatment The aim of water treatment is to convert raw water from it s contaminate laden state to as aesthetically acceptable and hygienically safe end product
Treatment technology options In ground water, generally, colloidal and suspended solids are absent; whereas, dissolved solids content are more than that in surface water. In surface water, where dissolve solids are lesser and the colloidal & suspended solids are very high. The method of treatment to be employed depends on the nature of raw water constituents and desired standard of water quality. The unit of operation in water treatment include aeration, flocculation, sedimentation, filtration, disinfection and defluoridation.
Aeration Aeration is required for transfer of gas in water or from water. Aeration may be necessary in water treatment due to following reasons. To impart freshness to water. To reduce corrosiveness of water (specially ground water) by driving off carbon dioxide and raising the ph. To oxidize iron and manganese. To strip out carbondioxide, hydrogen sulphide, ammonia and other organic matters including volatile substances causing taste and odour.
Coagulation Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. Cationic coagulants provide positive electric charges to reduce the negative charge (zeta potential) of the colloids. As a result, the particles collide to form larger particles (flocks).coagulation, thus, implies formation of smaller compact aggregates. Rapid mixing is required to disperse the coagulant throughout the liquid.
Coagulants Aluminum sulphate{alum } Poly aluminum chloride {PAC} Ferrous sulphate Sodium Aluminate Silicon Derivatives Lime Synthetic Organic Polymers
Flocculation/Sedimentation Flocculation refers to water treatment processes that combine or coagulate small particles into larger particles, which settle out of the water as sediment. Alum and iron salts or synthetic organic polymers (used alone or in combination with metal salts) are generally used to promote coagulation. Settling or sedimentation occurs naturally as flocculated particles settle out of the water.
Filtration Many water treatment facilities use filtration to remove all particles from the water. Those particles include clays and silts, natural organic matter, precipitates from other treatment processes in the facility, iron and manganese, and microorganisms. Filtration clarifies water and enhances the effectiveness of disinfection.
Water Filtration methods There are simple and cheap ways to filter contaminants out of water. Sand, Gravel and Charcoal Filtration
Slow Sand Filtration Slow sand filtration improves the physical, chemical and microbiological quality of water; it is reliable and inexpensive, and useful small-community water supplies. On the surface of the sand a gelatinous layer develops which is rich in microorganisms ( for example bacteria, protozoa and algae). It is the layer that is largely responsible for the treatment of pollutants and by the biological treatment of dissolved organic matter and nutrients. While water percolating downwards, the fine particles are trapped on the sand grains, where microorganisms consume organic material. The overall effect is a substantial reduction in the number of coliform organisms and pathogenic micro-organisms in the water. In well operated SSF, the efficiency of pathogen removal may be exceed 99.5%.
Flow chart of SSF
Rapid Sand Filtration The Rapid Sand Filter comprises a bed of sand serving as a single medium granular matrix supported on gravel overlying an under drainage system. The distinctive features of rapid sand filtration as compared to slow sand filtration include careful pre-treatment of water to effectively flocculate the colloidal particles, use of higher filtration rates and coarser but more uniform filter media to utilize greater depths of filter media to trap influent solids without excessive head loss. Backwashing of filter bed by reversing the flow direction is carried out to clean the entire depth of filter.
Flow chart of RSF
Membrane Technology I A membrane is a thin material that has pores (Holes) of a specific size. Membranes trap larger particles that won t fit through the pores of the membrane, letting water and other smaller substances through the other side.
Micro-filtration Typical pore size: 0.1 microns (10-7m) Very low pressure Removes bacteria, some large viruses Does not filter small viruses, protein molecules, sugar, and salts
Ultra-filtration Typical pore size: 0.01 microns (10-8m) Moderately low pressure Removes viruses, protein, and other organic molecules Does not filter ionic particles like lead, iron, chloride ions; nitrates, nitrites; other charged particles
Ultra Filtration
Nano filtration Typical pore size: 0.001 micron (10-9m) Moderate pressure Removes toxic or unwanted bivalent ions (ions with 2 or more charges), such as Lead Iron Nickel Mercury (II)
Reverse Osmosis Typical pore size: 0.0001 micron (10-10m) Very high pressure Only economically feasible large scale method to remove salt from water Salty water cannot support life People can t drink it and plants can t use it to grow
Osmosis is a natural process that moves water across a semipermeable membrane, from an area of greater concentration to an area of lesser concentration until the concentrations are equal
To move water from a more concentrated area to a less concentrated area requires high pressure to push the water in the opposite direction that it flows naturally.
A Series of Filtrations Increases Efficiency
Defluoridation Presence of fluoride more than the permissible limit in drinking water may cause mottling of teeth or dental fluorosis. Excessive fluoride may Pr cause skeletal fluorosis. Removal of fluoride in the water is called as defluoridation. There are various methodologies by which fluoride can be removed. These methodologies are;
Co precipitation (Nalgonda technique) Filter alum (ferric alum) can be added in the water for removal of fluoride. During the process, lime or sodium carbonate, bleaching powder solution and alum (aluminium sulphate) are added in the water. Initially, lime or sodium carbonate solution is added and mixed. Thereafter, alum is added and mixed rapidly for at least 60 seconds. Slow mixing for at least 5 to 7 minutes will help in forming the floc. Fluoride will be adsorbed out on sludge. Settlement is allowed for 2 hrs. The supernatant is then filtered to obtain fluoride free water. The method is known as Nalgonda technique. The quantity of chemical required is dependent on alkalinity and fluoride content in the water.
Adsorption by activated carbon and activated alumina: Activated carbon and activated alumina can be used as an adsorbing media for removal of fluoride. Different granuled activated carbon/alumina materials have been developed commercially for different uses including removal of fluoride.
Reverse Osmosis (Membrane technology): Fluoride from water can be removed by the action of reverse osmosis by using semi permeable membrane.
Exchange method: Anion exchange resins can be used for removal of fluoride from ground water.
W a t e r W arrants A dequate T esting for E ffective R egulation of quality