Background

In 1993, when I was Chief Scientist, Environment for Welsh Water the company adopted a policy of providing full treatment and disinfection to all permanent outfalls to sea and estuary. The preferred treatment process was ultra-violet light (UV) but other techniques were considered.

One immediate effect of this announcement was an improvement in relations with coastal communities who had frequently opposed planning permission for schemes which had involved long outfalls and only preliminary treatment or primary settlement. The issue was now, not "what treatment level", but "when". Environmental groups, some of which had previously been critical of company policy, now became supportive.

To gain first-hand operational experience 5 Vanguard full-scale schemes were planned, at Aberystwyth, Criccieth, Tywyn and Newgale (UV), and at Aberporth (micro-filtration).

Implementation

In 1995 the five Vanguard schemes were commissioned. A variety of manufacturers and scheme layouts were deliberately chosen to evaluate their relative merits and enable future decisions on standardisation.

The three in-channel UV works Aberystwyth, Criccieth and Tywyn were monitored for the three years 1995-97 to compare their performance. At Aberystwyth a reduction in the mean number of faecal coliforms and faecal streptococci across the works as a whole was greater than log10 4, and mean numbers in the final effluent were less than 100/100 ml. F specific coliphage showed a broadly similar pattern. However removal rates of enteroviruses appeared to be less, possibly because full recovery of enteroviruses in crude sewage was not achieved due to build up of toxic components following the concentration of the required to achieve a measurable number of plaque-forming units.

Mean removal rates at Criccieth and Tywyn were generally lower reflecting less reliable performance, especially in the early part of the trial. At Tywyn the problems were overcome and kills reliably reached log10 4. At Criccieth the equipment did not provide the UV intensity expected and was replaced with a unit similar to that at Tywyn, after which a comparable performance resulted. At both sites the provision of a single UV unit lead to a risk of total process failure in the event of unit breakdown and it was considered preferable to instal two units in series.

The results were sufficiently good to encourage the use of UV at a further 10 sites in Welsh Water's area including major works at Tenby, Llanelli, Treborth and Swansea.

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