Steps that should be followed to make Surface water potable are listed below:
(i) Screening:
Parallel steel bars followed by wire mesh screen with small openings, are generally used to remove relatively large floating and suspended materials.
(ii) Coagulation:
The screened waste water might contain suspended particles, including bacteria which are too small to be screened and to settle in a reasonable time period and if the particles are of colloidal dimensions they will never settle down.
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Under these circumstances, the water is mixed with chemicals which help small suspended materials to coagulate into larger particles, thereby facilitating precipitation. Colloidal particles carry the same charge and thus repel each other.
The repulsion of the similar charged particles does not allow them to come closer. The particles hence, remain suspended or scattered throughout the solution. If by any means the charges are removed the particles will come together to form bigger aggregates and will settle down under the influence of gravity.
The intended action of coagulant is to neutralize the charges so that the particles can come together to form bigger aggregates to settle down.
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The coagulation is thus, defined as the process by which colloidal particles come closer resulting into precipitation. Coagulation can be brought about in many different ways such as:
(a) Adding electrolytes
(b) Mixing oppositely charged sols
(c) Through boiling
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(d) Through electrophoresis, etc.
In water treatment process, usually the electrolytes are added for coagulation. The electrolytes used for coagulation are called flocculating agent as they make the bigger particles to flocculate.
The coagulating capacity depends upon the valence of the added ion, i.e., the more the valence of the ion the more is the capacity and nature of colloid. Lyophobic colloids are much easily coagulated compared to lyophilic colloid.
The use of such coagulant is based on the strong power of to coagulate negatively charged colloidal impurities present in natural water.
When alum is added, the positively charged Aliens are taken upon the surface of the colloidal particles, as a result the charge of colloidal particles is neutralized and thus the particles get coagulated.
The coagulating power of trivalent ion is much higher than monovalent ion and is comparable to that of bivalent ion but still higher. The other electrolytes which can be used are FeCl3, FeS04, etc.
The coagulating or flocculating agent is added to the raw water into a rapid mix chamber having rapidly rotating paddles, with a detention time of approximately 90 seconds. In this chamber the raw water and the flocculating agent are mixed well.
After proper mixing, the mixture is allowed to pass through another chamber for gentle agitation for about 30 minutes.
In this time period, the surface charge neutralization of colloidal particles takes place and they form bigger flocculent. The time of agitation must not be so high that the flow particles break apart.
(iii) Sedimentation :
After flocculation, the water is allowed to flow through a sedimentation basin or clarifier. Sedimentation basin is a concrete tank and is circular or rectangular in shape. The concrete tank can hold water for a longer time without any damage.
The usual detention time for water varies from one to ten hours. Obviously, with greater detention time settlement of suspended materials is much efficient. The solids settled at the bottom of the tank can be removed manually or mechanically by a scrapper.
(iv) Filtration :
The filtration unit is sand filter. It consists of multiple layers of graded gravel. The gravels at the uppermost layer are bigger in size and those at the lowest layer are finer.
The pore openings between the sand grains are smaller in size compared to the size of flow particles, so that they cannot pass out, allowing water to be filtered.
(v) Disinfection:
The surface water is now free from suspended solids and small amount of bacteria but contains a lot of pathogenic organisms. To make water of drinkable quality, these pathogens should be disinfected as well as fluoridation has to be done for preventing dental caries.
The most commonly used chemicals for disinfection are either chlorine gas (Cl2), sodium hypochlorite (NaOCl) or may be calcium hypochlorite [Ca (OCl) 2] and the method is known as chlorination.
Chlorine reacts with water to form hypochlorous acid and is known to be responsible for disinfection.
The strong oxidizing power, present due to liberation of nascent oxygen, destroys enzymatic processes in the pathogenic cell and thus kills them.
The advantage of chlorine as disinfectant is mainly due to its slow reaction with water and thus it can remain in water for a longer time and pathogenic microorganism cannot grow.
Though chlorine is very effective against bacteria but it is very negligible against viruses and protozoan like giardia, lamblia and Cryptosporidium.
The greatest disadvantage of chlorine is formation of trihalomethanes (THM), for example chloroform (CHC13) which is a carcinogen.
The formation of THM is due to the reaction of chlorine with organic matters present in the surface water or due to the decaying of vegetation, dead animals, insects, etc. Concentration of THM can be greatly reduced if the organic wastes are removed first, followed by chlorination.
Chlorine dioxide (C102) can be used to disinfect both bacteria and virus but it produces toxic chlorate and chlorite substances and involves a very costly process.
Disinfection can be carried out using ozone (03). It is both effective against bacteria and viruses. It does not produce toxic products and causes no problems in tastes and odors. It is, however, very expensive and is mainly used in European water treatment.