The core treatment process of many water purification systems is reverse osmosis. Spirally wrapped, polyamide membranes are used in the vast majority of today’s RO plant. These membranes have a limited exposure life to ‘free Chlorine (approximately 1,000 ppm hours).
Chlorine is added to drinking water supplies by the water authorities to kill any harmful bacteria. The by-product of free Chlorine is Chloramines, which then provide protection to the water supply distribution system as they are more stable and take longer to break down compared with free Chlorine.
For renal dialysis patients, Chloramines, if present in excess, can give rise to conditions such as haemolysis and haemolytic anemia, which cause the destruction of red blood cells. Therefore, within the designs of many water treatment systems, dechlorination forms an integral part of the pre-treatment programme.
One of the most effective media for the removal of both free Chlorine and its by-products is “activated carbon”. Raw materials such as bituminous coal, peat and coconut shells can all be used to produce activated carbon.
Many stages of treatment are required to turn the original raw material into the final product. Firstly, the selected material is ground into small granules, then, to remove any residual organic impurities, the granules are heated to extremely high temperatures and thermally activated with super-heated steam under extreme pressure.
To remove any inorganic impurities the granules are washed in a solution of diluted acid and then neutralised with an alkaline rinse. The resulting product resembles that of a sponge made from pure Carbon, with a surface area of 800-1500m² /gm. The higher the surface area, the higher the activity the carbon will have for adsorption.
Apart from activated Carbon’s ability to dechlorinate water, its porous structure and high surface area are ideal for the removal of dissolved organic compounds, which, in water supplies, can be both man-made or natural in their origins. Compounds such as pesticides, herbicides or micropollutants such as polychlorinated bi-phenyls (PCBs) or trihalomethanes (THMs) would fall under the category of man-made organics, while humic and fulvic acids, derived from decaying vegetation, would be classed as naturally occurring organics.
Activated Carbon (AC) units are typically sited in the pre-treatment train to remove free Chlorine and organics, therefore providing protection for downstream RO plant. For free Chlorine removal, a contact time of six minutes is necessary, but for the more stable Chloramine compounds a contact time in excess of 10 minutes is generally required.
AC units can also be installed in the purified water distribution system to reduce the Total Organic Carbon (TOC) levels. TOC in purified water can be attributed to low molecular weight organics leached from the plastic pipe and solvent adhesives commonly used to construct distribution systems. TOC is generally expressed in µg/l (ppb, parts per billion).