turns into chloric acid and hydrochloric acid. Chloric acid is the compound that sanitises and oxidises. When people swim in a pool, they introduce contaminants, usually in the form of ammonia and creatine from urine and sweat. These combine with chloric acid to form chloramines or combined chlorine. The chloric acid that does not initially combine is free residual chlorine. If the free chlorine level is high enough to both burn up the chloramines and combine with body fluids, the pool will remain clean and clear of odour. However, when combined chlorine levels get too high, the pool will begin to smell of chlorine. In recent times, chloramines like trichloramine have come under the radar of public health authorities as key water quality parameter. Trichloramines have been blamed for stress corrosion cracking of the pool buildings and corrosion of the ventilation systems. They are also primarily responsible for the odour and skin, eye, throat or nose irritation reported by swimmers, lifeguards and non-swimmer companions in pool environments,
Table 1 Disinfectant
Disinfection by-products
Chlorine/hypochlorite
trihalomethanes haloacetic acids haloacetonitriles haloketones chloral hydrate (trichloroacetaldehyde) chloropicrin (trichloronitromethane) cyanogen chloride chlorate chloramines
Ozone
bromate aldehydes ketones ketoacids carboxylic acids bromoform brominated acetic acids
Chlorine dioxide
chlorite chlorate
Bromine/hypochlorite BCDMH
trihalomethanes, mainly bromoform bromal hydrate bromate bromamines
COURTESY: Guidelines for Safe Recreational Water Environments by WHO
Optimise filter performance
A
FM filtration will reduce the bacterial growth within the filter bed; however, bacteria can also develop on the solids removed by the filter. It is important to backwash the filters, even although the pressure differential across the filters does not warrant a backwash. The growth of bacteria on the collected solids is so rapid that it will start to impact water quality after just one week. Ideally, the filters should be back-washed once each week, with two week cycles as the absolute limit. The per formance of any media bed filter works inversely propor tionally to the flow of water through the filter. In essence, the slower you run the filters the better the per formance. Ideally the flow rates for swimming pools should be limited to a rate less than 15 m3/hr per square metre of filter bed sur face area. It is just as impor tant to ensure that all of the collected solids are removed during the backwash. If solids remain in the filter, they simply act as a food source for bacteria, and trichloramine levels will star t to increase. In order to ensure thorough cleaning, the filters should be air-scoured at a rate between 70 and 90 m3/hr per square metre for a period of five minutes before a backwash. The backwash rate should also be at a flow that expands the bed by at least 15%. In order to achieve this expansion using AFM or sand, a water flow of between 40 and 45 m3/hr per square metre is required. If this strategy is followed with AFM, the chemical demand will be reduced by at least 80%, THM levels will be reduced by 80% and trichloramine production will be eliminated.
UV is a physical system and is generally not considered to produce by-products
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