The following points highlight the four main processes employed in a municipal water purification system to ensure the supply of non-polluted drinking water. The processes are: 1. Flocculation 2. Sedimentation, 3. Filtration 4. Disinfection
Process # 1. Flocculation:
Water after pumping from raw water reservoir (natural sources) is collected in large tanks/basins for a sufficient time period to permit large particulate matter to settle down at the bottom.
This material is removed and then the water is treated with flocculants such as aluminium sulphate, allum, sodium alluminate, colloidal silicate, calcium, or bentonite which form a floe that precipitates and carries with it microorganisms on the surface, and suspended organic matter settle onto the bottom of the tanks/basins. In this way, most substances that impart turbidity to water get coagulated.
Process # 2. Sedimentation:
The water, after coagulation, is left in settling basin further for sufficient period to allow sedimentation of remaining materials. Sedimentation, however, considerably reduces microbial population of the water aside from removing most of the suspended particles.
Process # 3. Filtration:
After sedimentation, the water is subjected to sand-filters to remove flocks of living organisms. The process of filtration is highly critical and important as it can remove protozoan cysts and also about 98-99% of bacteria from water. The water may also be filtered through activated charcoal to remove potentially toxic organic compounds and organic compounds that impart undesirable colour and/or taste to the water.
(i) Slow Sand Filters:
These sand filters comprise of layers of fine sand, coarse sand, fine and coarse gravel. The floor at the base is made of tiles provided with perforations for intake of filtered water. The uppermost fine sand layer acts as a biological filter as its interspaces are clogged by colloidal flocules in water, encapsulated microorganisms, and algae, and retains bacteria making the water free from them.
The efficiency of filter is further enhanced as the negatively charged bacteria are held by the positively charged colloidal material in the layer. This filter normally removes 98% of the bacterial population present in water. Nearly 5 to 7 million gallons of water per acre area of filter may be purified by this method but this method proves slow and generally requires several acres of land.
(ii) Rapid Sand Filters:
This method involves essentially the same equipment’s as the slow sand filters but the sand filter area is much smaller with provision for frequent back washing. This makes possible the rapid filtering of water. Rapid sand filters may deliver as much as 150 million gallons of water per acre/day.
Process # 4. Disinfection:
Disinfection is the final step in municipal water purification and it ensures that no pathogenic microorganisms are carried through water. For water supplies of small towns and localities sodium or calcium hypochlorite (NaOCl or CaOCl2), respectively may be used to disinfect water, but for larger cities, however, chlorination (use of chlorine) has been the traditional method for disinfection.
Chlorination often removes bacterial contamination since chlorine reacts with water to yield hypochlorous and hypo-chloric acids which are potent microbicides. Chlorine is obtained as liquid under pressure but releases into water as gas. It easily dissolves in water and, in addition to killing the microbes, it is also effective in oxidizing organic matter.
However, the greater the organic matter in water, the higher the chlorine demand for a given amount of water, i.e., the chlorine demand for different waters varies depending upon the organic matter of the water.
In any case, the residual amount of chlorine present in drinking water after the process of disinfection should not exceed 0.2 ppm because the disadvantage is the incidental production of trace amounts of organochlorine compounds particularly trihalomethane (THM), a suspected carcinogen.
Fortunately, it has been found that disinfection by chloroamination is the least expensive way to reduce the formation of trihalomethane (THM) and is spreading rapidly as an alternative to chlorination.
In this process monochloramine is generated directly in the water to be disinfected by adding ammonium prior to or simultaneously with chlorine or hypochlorite. Mono-chloramine is quite effective and produces much lower amounts of THMs.
After disinfection the water is, finally, stored in large reservoirs and is supplied for domestic consumption through gravity taps.