Free chlorine versus total chlorine

There are three types of chlorine, each with a different makeup. Following is an overview of what constitutes each type. Free chlorine Free chlorine is defined as the concentration of residual chlorine in water which is present as dissolved gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl-). The three forms of free chlorine exist together in equilibrium and their relative proportions are determined by the pH value and temperature. The effect of pH value on the composition of free chlorine Between a pH of 2 to 7.4, the predominant form of chlorine is HOCl. When the pH is between 2 and 7, the equilibrium is in favor of HOCl. As the pH falls below 2, the predominant form of the chlorine is Cl2. At a pH of 7.4, HOCl and OCl – are about equal, and as the pH goes above 7.4, increasing proportions of OCl – are present. A test kit that measures free chlorine will indicate the combined concentrations of HOCl, OCl–, and Cl2. Combined chlorine Combined chlorine is defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines. Organic amines can be found in natural or polluted waters. Ammonia is sometimes deliberately added to chlorinated public water supplies to provide inorganic chloramines. Total chlorine Total chlorine is the sum of free and combined chlorine. When chlorinating most potable water supplies, total chlorine is essentially equal to free chlorine, since the concentration of ammonia or organic nitrogen compounds needed to form combined chlorine will be very low. When chloramines are present in the municipal water supply, then the total chlorine...

Floods and water treatment plants – no small matter

When flooding occurs the greatest concern for a Water Treatment Plant (WTP) is contamination. Most WTPs would be able to handle the additional volume of water. The influx of floodwater could result in some damage to the infrastructure, but its impact on the operational capacity of the plant would only be short-term. The quality of the floodwater however, is another matter. Floodwater usually has high levels of nitrates, phosphates, industrial solvents, iron and manganese as well as pesticides and algal toxins. These usually impact the plant’s operational ability to produce clean, safe water as eliminating them is both complex and time consuming. Typically, WTP operators process high levels of contaminants via effluent disinfection by chlorination and tertiary flocculation settling in using clarification and filtration processes. A resin ion exchange process during secondary treatment removes phosphates and nitrates. Pesticides and pathogens (industrial solvents) are eliminated by Ozone and GAC (granulated activated carbon) processes along with the use of coagulation flocculation and DAFF (dissolved air floatation-filtration). Chlorine is used to control all infectious micro-organisms (except Guardia and Cryptosporidium). However, chlorine can react with natural organic matter present in all surface water to cause potentially harmful disinfection by-products to form. To partially control this, coagulation or absorption is instigated. UV lighting is also used to monitor and eliminate any by-products. The above procedures together with the use of SBS (Sodiumbisulphite) dosing after disinfection and the UV process ensure that the fouling of reverse osmosis membranes is kept to a minimum. This complex process of eliminating contaminants from floodwater is as far as we know, the greatest headache and poses the biggest risk...