Formation and Removal of Off-Flavour Compounds during Ozonation and Filtration through Biologically Active Sand Filters

Abstract

Formation and removal of off-flavour compounds during ozonation and during filtration through biologically active sand filters have been evaluated by gas chromatography and column sniffing. A well aerated sand filter (0.5 m deep) with a water flow rate of 25 mm/h was able to produce an almost odourless water from a raw water with a strong muddy and earthy off-flavour. Frequently occurring off-flavour compounds, such as geosmin, 2-methylisobomeol, 2,4,6-trichloroanisole, 1-octen-3-one and dimethyl trisulphide, were all effectively removed, provided the oxygen supply was sufficiently high. The removal of colour, however, was not satisfactory. Treatment with ozone removed the colour of the water and caused a dramatic change in odour quality. Certain off-flavour compounds, such as geosmin and 2-methylisobomeol, decreased in concentration, while several other off-flavour compounds, such as octanal, increased in concentration. The ozonation by-products were only partly removed during sand filtration, and the filter effluent had a weak, but noticeable, off-flavour. Chromatographic sniffing indicated that some of the ozonation by-products were, in fact, less amenable to biodegradation than the off-flavour compounds that were removed by ozonation.