Shock Chlorination

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Chlorination is widely used for the disinfection of raw water in the pre-treatment stage in desalination plant. Some technicians believe that continuous chlorination in the range below 1 ppm makes bacteria tolerant to the disinfectant. They suggest shock chlorination of a few ppm chlorine on the upstream as a more effective remedy. To what extent do you agree or disagree with this view? Does anybody have any experience in this regard?

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2 Answers

  1. Fully agree, as per my desalination plant experience over the past 38 years. Continuous, low dosage chlorination of raw seawater feed breaks down the humic and fulvic acid organic compounds and makes them digestible as food by bacteria, which otherwise they are not, thus it actually promotes favorable conditions to biological activity and membrane biofouling. Besides, effective seawater disinfection requires a 3-log reduction of bacterial colonies, which only shock chlorination, with 5 ppm at frequencies and durations adjusted seasonally and according to site conditions, can achieve. 

  2. Shock chlorination is the process by which home water systems such as wells, springs, and cisterns are disinfected using household liquid bleach (or chlorine). Shock chlorination is the most widely recommended means of treating bacterial contamination in home water systems.Shock chlorination is a process used in many swimming pools, water wells, springs and other water sources to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount of sodium hypochlorite, which can be in the form of a powder or a liquid such as chlorine bleach, into the water. The common advice is that the amount added must raise the level of chlorine to 10X the level (in parts per million) of chloramines present in the pool water; this is "shocking". A lesser ratio is termed superchlorinating.[Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three ppm or less. Commercial sodium hypochlorite should not be mixed with commercial calcium hypochlorite, as there is a risk of explosion. Although a verb for superchlorination,shock" is often misunderstood (through marketing and sales language) to be a unique type of product.

    While "shocking" pools to reduce the buildup of chloramines works with inorganic, ammonia-based chloramines, in two studies it was found ineffective with the organic chloramines present in all pool water e.g. with creatinine, an organic component in human sweat. Indeed, superchlorination produces free chlorine that reacts with organic contaminants to form a variety of disinfection byproducts (DBPs) which are hazardous to swimmer health e.g. one of the worst DBPs is the noxious and volatile trichloramine (NCl3), well known for irritating the eyes nearby a pool. It has been pointed out that ozone is an excellent alternative, a much more effective oxidizer than chlorine shock.