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For clearer, healthier water
In a report published in Dawn earlier this month, the Karachi Water and Sewerage Board (KWSB) claimed that in the rainy season it was normal for mud to get mixed with municipal water because it was taken to pumping stations in open canals. But there was no cause for concern, added the statement, because water was chlorinated properly at the KWSB’s filter plants and, thus, safe for drinking.
Well, the second part of the above statement is utterly wrong. It is important that the record is set right.
Turbidity is caused by a variety of suspended materials that range in size from colloidal to coarse dispersions, depending on the degree of turbulence. In turbulent water (for instance, floodwaters), turbidity occurs due to relatively coarse dispersions. Under relatively quiescent water conditions (such as in lakes), most of the turbidity will be due to colloidal and extremely or ultra-fine dispersions.
The size of the colloidal particles ranges from 1 to 100 millimicrons and they do not settle under quiescent conditions. This is because the mass of colloidal particles is so small as compared to their surface area that the gravitational forces are rendered ineffective.
All colloidal particles are normally negatively charged. Since similar charges repel, the like-charged colloids cannot come close in order to form larger-sized (coarse) particles. The action of agglomeration is brought about by chemical coagulation.
The objective of chemical coagulation is to reduce the magnitude of repulsive forces so that it is less than the forces that attract. The coalescence of the colloidal particles thus occurs and agglomeration takes place.
Since the colloids are negatively charged, the salts of trivalent metals are used to coagulate them. For effective coagulation, control of the hydrogen ion concentration is important.
Various coagulants have different ranges of pH. If aluminium sulphate is used, then the water pH must be within the range of six to eight. If ferrous sulphate is used as a coagulant, then the pH should be between eight and 11.
Typically, sulphuric acid and carbon dioxide are used to lower the pH of water. Soda ash, lime and caustic soda are normally used to raise the pH value of water. Some water treatment plants find it useful to apply coagulants at several points. This is called double coagulation.
It is important that when a coagulant is applied to water, it is distributed properly in the liquid so that complete contact with colloidal particles is made quickly. This is because a reaction starts immediately once a coagulant is added, leading to floc formation.
The reactions involved are chemical, physical-chemical and ionic in nature. A quick distribution of the coagulant is achieved through the rapid mix or flash mix procedure which is usually accomplished in less than ten seconds.
Coagulation is followed by flocculation, wherein slow moving flocculator paddles promote intimate and gentle contact among the flocculating particles. Typically, the paddles are between 5 and 8 feet in diameter and revolve at a speed of 1 to 1.5 revolutions per minute. The water retention time in the flocculation unit is 30 to 60 minutes.
Flocculation is followed by sedimentation and then by filtration. The whole process, typically, ends with disinfection.
It must be kept in mind that all well-designed treatment plants are able to handle turbid water. Technically, thus, it is incorrect to say that increased turbidity in the finished water is simply due to high turbidity in raw water. While the finished water turbidity is dependent to an extent on raw water turbidity, the operating parameters available now allow production of clear water even if influent water turbidity is high.
In the case of Karachi, turbidity greater than 10 Jackson Turbidity Units (JTU) in water could be due to the hydrogen ion concentration of incoming liquid in the coagulation unit, which may not be in accordance with the type of coagulant used.
Similarly, if water temperature is below 5o Celsius, difficulty will be encountered in floc formation. Floc formation is done satisfactorily at moderate temperatures. Then there are issues in flash mixing and maintenance of sedimentation tanks and flocculator paddles.
When the rapid sand filter system is used for water treatment — as in the case of Karachi — proper pre-treatment of raw water with coagulants is an important factor. In fact, the ability of such filters to handle almost any quality of raw water is mainly due to the proper pre-treatment of raw water by way of coagulation.
The local treatment plant operators must understand that their plant will never give clear finished water if proper and adequate coagulation — consistent with the requirements as laid down by the plant’s design — is not carried out. This will hold true even if the rest of the treatment units (flocculation, sedimentation and filtration) are working properly.
The above contention can be proved through a simple experiment in which household alum is mixed with municipal water. After about 45 minutes of the mixing, you will see that all turbidity has settled down and the water is relatively clear.
Experience has shown also that, with some exception, no bacteria and viruses are removed if un-coagulated water is passed through the rest of the treatment units. If the finished water has high turbidity (greater than 10JTU), it will escape disinfection since micro-organisms encase themselves in turbid particles, where the disinfectant cannot reach.
Since it has been proved conclusively that turbid water is unsafe, harmful and unfit for drinking purposes, various international organisations dealing with public health have recommended that finished water turbidity should not be more than 5JTU on two consecutive days.
The writer is a freelance contributor
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