4.8.2 Removal of trace organic compounds
After conventional treatment, water may still contain trace concentrations of synthetic organic compounds, which, if left in the water, can lead to taste and odour problems. The problem is most likely to arise where the raw water source has been badly polluted. The problem can be solved by including the process of granular activated carbon adsorption after the filtration process. Activated carbon is carbon which has been activated by heating in the absence of oxygen. This results in the formation of many pores within each carbon particle. Charcoal is a form of activated carbon but with fewer pores. Granular activated carbon (GAC) can be obtained from roasting vegetable or animal matter at 800–900°C in a vacuum furnace. It can have a surface area of up to 1000 m2 g−1. GAC is therefore an effective adsorbent of organic compounds. Its effectiveness can be measured by the reduction in the chemical oxygen demand (the oxygen needed to chemically oxidize all carbonaceous material present, COD) and the total organic carbon of the water. GAC can be used for the removal of soluble phenols which would produce strongly smelling and tasting chlorophenols upon reaction with chlorine in the disinfection stage. In the event that trihalomethanes are formed after disinfection by chlorine, GAC can be used to eliminate these toxic compounds. GAC, once exhausted, can be regenerated by heat treatment.
Another method of removing trace organics is to oxidize them to harmless products such as CO2 by using ozone. Ozone and activated carbon are capable of removing trace quantities of organics present in water. These substances are used for the reduction of pesticide levels in water supplies to comply with the limit specified by the European Union.
Powdered activated carbon is also an option. It can be added to water for the adsorption of trace organics. It has been used to eliminate tastes and odours in drinking water brought about by algae, actinomycetes and fungi. It is usually added in the coagulation stage prior to sand filtration. Unlike GAC, the regeneration of powdered activated carbon is not practicable, so it is only used when intermittent water quality problems occur.
Membrane filtration is becoming popular in water treatment and nanofiltration (see next section) has been employed for trace organics (pesticides) removal. Nanofiltration is similar to reverse osmosis, which you have already come across. Pesticides, which are often carcinogenic, are not at all desirable in potable water. The limit of each individual pesticide is 0.1 μg l−1, and for the sum of all the individual species, the limit is 0.5 μg l−1. The latter is still a very low concentration. It's the sort of concentration of sugar you'd get if you dissolved 1250 sugar cubes in Loch Ness!