Report No DWI0776
INFLUENCE OF UV DISINFECTION ON BY-PRODUCT FORMATION AND THE OPERATIONAL IMPLICATIONS
Final Report to the Drinking Water Inspectorate
DWI0776
Sept 1997
SUMMARY
EXECUTIVE SUMMARY
This report details the findings of investigations
into the influence of UV disinfection on the formation of disinfection
by-products. These investigations were carried out on behalf of the
Drinking Water Inspectorate as part of the DoE contract 'Disinfection
by-products from drinking water treatment' (DWI 7(A2\32).
The study did not highlight any widespread problems
likely to be associated with the use of UV disinfection. The detailed
findings can be summarised as follows:
- the irradiation of solutions of nitrate with high UV doses (high
relative to the levels recommended for achieving adequate disinfection,
16 mJ cm-2) produces concentrations of nitrite well in
excess of the 100 µg/l regulatory limit. The concentration formed
appears to be directly dependent on the UV dose applied and the nitrate
concentration present. At low UV doses, pH values typical of UK water
sources, and nitrate concentrations within the regulatory limit, the
formation of nitrite is rarely, if ever, likely to exceed 100
µg/l;
- aldehydes, in particular formaldehyde and acetaldehyde, are
formed as a result of the UV irradiation of solutions of humic acid and
of Thames river water. However, they do not appear to be formed at
levels that would be of concern;
- UV irradiation may reduce the chlorination by-product formation
potential (THMs and TCA) of humic acid solutions and Thames river
water, but the changes observed are too small to be categorical. Under
some conditions there may be a slight increase in formation potential;
- UV irradiation will remove brominated THMs, but has little, or
no, effect on chloroform. There is insufficient information as to
whether an analogous observation would be true for haloacetic acids;
- the use of UV irradiation to treat water contaminated with PCE
can give rise to significant concentrations of DCA. TCA can also be
formed but generally at lower concentrations. This may also be a
consideration where UV-based advanced oxidation processes are used to
remove PCE;
- UV irradiation of nitrate-containing river water results in the
apparent formation of nitrogen- or phosphorus-containing compounds.
However, GC-MS experiments failed to confirm the formation of any
compounds of interest. Some unsaturated long-chain amides were
tentatively identified in some samples but these were not confirmed;
- GC-MS analysis of river water spiked with DMA and nitrite did not
detect the presence of any identifiable nitrosamine under the limited
set of conditions used (the estimated detection limit being of the
order of a few µg/l).
While the purpose of paramount importance in the use
of UV disinfection is to achieve adequate disinfection, the formation
of by-products may, in some circumstances, influence the mode of
operation. A number of potential implications for the operation of UV
disinfection systems are identified and these can be summarised as
follows:
- the prime controlling factor in by-product formation will be the
overall energy input. However, the formation of by-products will be
affected by the wavelength (i.e. photon energy) of the W radiation to
some degree, but the extent is hard to quantify and will be compound
specific. In general, the longer wavelengths (i.e. those with lower
photon energy) will be less likely to produce by-products. Thus, the
conservative approach would be to base any controls on overall energy
input using results obtained with the narrow-band, low pressure lamps,
since this would tend to overestimate the extent of by-product
formation for the same energy input from the broad-band, medium
pressure lamp.
- if UV units are operated at a dose of around 100 mJ cm-2,
then at high nitrate levels the concentration of nitrite may approach
or exceed the regulatory limit of 100 µg/l. In this case some
monitoring for nitrite should be undertaken, and if necessary,
adjustments made to the UV dose applied to allow an operational safety
margin. If the UV units are operated in the range 25 - 30 mJ cm-2
then, even for waters with high nitrate levels, the concentration of
nitrite formed is unlikely to approach the regulatory limit.
- if UV units are operated at a dose of around 100 mJ cm-2,
then if PCE is present in high concentrations, i.e. at, or above, 1
mg/l, then the concentrations of DCA produced may approach the WHO
Drinking Water Guideline value of 50 µg/l. Therefore, if PCE is
found to be present at high concentrations then monitoring should be
applied to ensure that significant concentrations of DCA are not being
formed.
- before a unit designed to inactivate cryptosporidium is
installed then the dose received by the water passing through the unit,
as well as by any cysts, should be established for all modes of
operation. In the light of the comments above, appropriate monitoring
and control schemes should then be drawn up before the unit is
commissioned.
- this study found no firm evidence, within the scope of the GC-NPD
and GC-MS methods applied, that the UV irradiation of water resulted in
the formation of any nitrogen-containing organic compounds.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.