Report No DWI0776

Sept 1997

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 the report are available from FWR, price £15.00, less 20% to FWR Members.