Drinking Water Quality for the 21 st Century
279
levels of many heavy metals, including arsenic, into the waters they are treating. In
this case, not only is drinking water quality at risk but any attempt to recycle
contaminated sludge resulting from alum usage has the potential to reintroduce heavy
metals at elevated concentrations into the treatment regime. Also, if metal
contamination is too great, the sludge may be considered too hazardous to dispose of
by conventional means.
The most commonly employed distribution disinfectant, chlorine, is frequently
supplied in the form of hypochlorite solutions which are unstable and need to be
stored and used under a strict set of guidelines. A 15% commercial solution, for
example, has a half-life of 100 days when stored in a darkened area at 77 OF, but
most manufacturers indicate a shelf-life of between 60 and 90 days. The stability
of these solutions is often dependent on the manufacturing process and, in
particular, on the solution pH, the storage conditions (particularly exposure to heat
and light), and heavy metal cation impurities. The major degradation products of
sodium hypochlorite are sodium chlorate and sodium chloride (Gordon et al.
1995) and although the toxicity of chlorate is not well established there is
evidence (Delcomyn et al. 2001) that additional oxyhalogenated species such as
bromate and iodate, due to bromide and iodide contamination, respectively, of the
hypochlorite, may be produced during the disinfection process. Bromate is,
therefore, found in most chlorinated water supplies across the globe. It is also
present in the poorer or unregulated bottled water supplies often at higher
concentrations (Weinberg et al. 1993). This is just one example where the
treatment process is contributing to compromised chemical quality of the finished
product while protecting the consumer from microbial contamination. It is this
balance which is being addressed at the beginning of the new century using
technological advances that were developed in the closing stages of the last
millennium.
3.2 Natural Organic Matter (NOM)
NOM, a mixture of humic and nonhumic substances, contributes to the DBP
precursor levels in drinking water (Owen et al. 1993). The total organic carbon
(TOC) concentration of a water or its UV absorbance is generally a good
indication of the amount of THMs and other DBP precursors present (Singer and
Chang 1989).
NOM is sufficiently diverse in its character that two natural waters with the
same TOC or UV absorbance from which NOM is extracted will undergo different
disinfection and oxidation mechanisms. Ozone, for example, has been shown to
cause a shift to smaller molecular size and hydrophilic fractions of NOM,
indicating the breakdown of larger molecules and the creation of more polar
compounds (Koechling et al. 1996). Consequently, ozonation enhances
biodegradability, which results in increased DOC removal by subsequent
biofiltration (Schechter and Singer 1995) and in tum this results in some control of
halogenated DBP formation by subsequent chlorination.
279
levels of many heavy metals, including arsenic, into the waters they are treating. In
this case, not only is drinking water quality at risk but any attempt to recycle
contaminated sludge resulting from alum usage has the potential to reintroduce heavy
metals at elevated concentrations into the treatment regime. Also, if metal
contamination is too great, the sludge may be considered too hazardous to dispose of
by conventional means.
The most commonly employed distribution disinfectant, chlorine, is frequently
supplied in the form of hypochlorite solutions which are unstable and need to be
stored and used under a strict set of guidelines. A 15% commercial solution, for
example, has a half-life of 100 days when stored in a darkened area at 77 OF, but
most manufacturers indicate a shelf-life of between 60 and 90 days. The stability
of these solutions is often dependent on the manufacturing process and, in
particular, on the solution pH, the storage conditions (particularly exposure to heat
and light), and heavy metal cation impurities. The major degradation products of
sodium hypochlorite are sodium chlorate and sodium chloride (Gordon et al.
1995) and although the toxicity of chlorate is not well established there is
evidence (Delcomyn et al. 2001) that additional oxyhalogenated species such as
bromate and iodate, due to bromide and iodide contamination, respectively, of the
hypochlorite, may be produced during the disinfection process. Bromate is,
therefore, found in most chlorinated water supplies across the globe. It is also
present in the poorer or unregulated bottled water supplies often at higher
concentrations (Weinberg et al. 1993). This is just one example where the
treatment process is contributing to compromised chemical quality of the finished
product while protecting the consumer from microbial contamination. It is this
balance which is being addressed at the beginning of the new century using
technological advances that were developed in the closing stages of the last
millennium.
3.2 Natural Organic Matter (NOM)
NOM, a mixture of humic and nonhumic substances, contributes to the DBP
precursor levels in drinking water (Owen et al. 1993). The total organic carbon
(TOC) concentration of a water or its UV absorbance is generally a good
indication of the amount of THMs and other DBP precursors present (Singer and
Chang 1989).
NOM is sufficiently diverse in its character that two natural waters with the
same TOC or UV absorbance from which NOM is extracted will undergo different
disinfection and oxidation mechanisms. Ozone, for example, has been shown to
cause a shift to smaller molecular size and hydrophilic fractions of NOM,
indicating the breakdown of larger molecules and the creation of more polar
compounds (Koechling et al. 1996). Consequently, ozonation enhances
biodegradability, which results in increased DOC removal by subsequent
biofiltration (Schechter and Singer 1995) and in tum this results in some control of
halogenated DBP formation by subsequent chlorination.