Medium pressure UV for Wastewater Treatment plant

A paper presented at the International UV Association describing the importance of an accurate dose when designing UV disinfection for wastewater treatment, received a very positive response

A paper presented at the recent International UV Association Congress in Vienna, describing the importance of an accurate dose when designing UV disinfection systems for wastewater treatment, received a very positive response from delegates.

The paper, entitled 'The impact of minimum UV fluence on Microbiological reduction in Wastewater disinfection', was presented by Ben F Kalisvaart of Netherlands-based Berson UV-techniek.

It outlined the findings from a number of independent studies and trials comparing UV dose (fluence) with microbial survival rates in open-channel, low pressure UV treatment reactors and medium pressure, in-pipe UV systems.

"The response from delegates was very encouraging," commented Mr Kalisvaart.

"Our findings proved to be a real eye opener and I'm sure they will be the subject of widespread discussion in the UV wastewater treatment community".

The most striking conclusion of the findings was that designing UV disinfection systems based on average UV dose is actually very inefficient.

It is much more effective (and, in the long run, cheaper) to design UV systems based on the minimum required UV dose - that is, the absolute minimum dose of UV required to reduce micro-organism levels in wastewater by a factor 1000 or higher.

This minimum UV dose will vary depending on local conditions such as flow volume and the amount of organic and inorganic material in the wastewater.

The optimum dose will therefore be unique to each site.

Each UV installation therefore needs to be designed individually, with lamp distances set according to specific local conditions, to ensure effective disinfection at all times.

Another important conclusion was that micro-organisms receiving the lowest UV fluence in open-channel, low pressure UV systems (ie those microorganisms furthest away from the UV lamps) are actually capable of repairing their UV-damaged DNA.

The findings showed that in low pressure, open-channel UV reactors a small but significant percentage of wastewater - that receiving a UV dose below about 10 mJ/cm2 (but with higher UV doses not completely excluded) - experiences a surprisingly low rate of microbial reduction, and in some cases none at all.

This is partly due to the UV dose being too low but, more importantly, it is down to DNA repair.

Through CFD (computational fluid dynamics) modelling it was also demonstrated that the laminar flow patterns common to open-channel UV systems often produce wide variations in UV dose within the treatment channels, with those micro-organisms receiving the lowest UV doses being more likely to undergo DNA repair.

Those operators of open-channel, low pressure UV disinfection systems who are concerned about these findings may want to consider in-pipe, medium pressure UV systems, such as the bersonInLine, as a more reliable alternative.

The bersonInLine range is designed to produce a turbulent flow through the treatment chamber, resulting in a more even spread of UV fluence.

The systems utilise the innovative bersonMultiWave medium pressure UV lamp.

This lamp emits a very broad spectrum of UV at wavelengths which act not only on the microorganism DNA, but also on other molecules like DNA repair enzymes.

This results in permanent, non-repairable cellular damage.

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