Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | OCTOBER 2015 | 31 In the know Digging deeper: UV disinfection T he benefits of ultraviolet disinfection (UV) are widely known amongst both water companies using UV in wastewater applications, and the wider water treatment industry such as those involved in food processing. Systems have been widely implemented across the sector over the past two decades. Traditionally, the priority has been to stay in compliance with the appropriate regulations under all conditions. The result has been that UV system sizing is usually based on conservative estimations for peak flow conditions, water quality and design UV dose. Avoiding the overdose It is time the industry ended the flawed 'one size fits all' approach to UV systems, and used more accurate dosing for the job in hand BeAtrice MArtin UV systems specialist Xylem Water solUtions More o•en than not this has led to a 'one size fits all' approach where the same milliJoule per square centimetre dose is applied across all sites and applications. The common view is that a large size of dose, which is o•en significantly in excess of what is required, will kill any bugs, such as fecal coliforms, in its path. However, this one-size-fits- all approach inevitably leads to overdosing in conditions that are not worst case, and moves are already underway from the regulatory authorities to abandon the "worst case" dosing method in favour of a more application specific dose. This inevitably means that UV system sizes are going to get smaller and more tailored to the size of the site. The sizing of a UV system is a theoretical calculation based upon a number of factors, which include flow rate, UV transmittance, total suspended solids, the lamp ageing factor and the lamp fouling factor. One way of reducing the dose is to turn down the UV system by powering off individual banks of UV lights. This reduction in lamp power, in addition to lowering energy costs, can also be an effective way of reducing consumables, labour and maintenance costs. Nevertheless, any reduction in lamp power means that an accurate measurement of the UV dose is going to be much more important as, without accurate measurement, the delivered UV dose is essentially guesswork. The reason for this is that the assumption is o•en made that if power is turned down by 50 per cent then 50 per cent of the UV intensity will be lost. This is not necessarily the case, because the condition of the lamps will significantly affect the UV dose being delivered and, potentially, result in under-dosing. For example, a critical factor in assessing lamp condition is fouling. Lamp sleeves which are fouled will potentially only let through 35 per cent of the UV being generated, which is in theory insufficient to deal with the target organisms. For companies dealing with potential public health issues, this is a very serious matter and could lead to those businesses being in breach of their disinfection permits. The answer is to measure, in real time, the dose being delivered. Any calculation must take into account three key parameters: flow, which defines the residence time of the effluent in the system; UVT, which defines the water quality; and UV intensity, which measures the actual delivered UV energy. If we consider the various conditions that can affect the UV dose, it is clear that accurate measurement of the delivered UV energy can avert over or under- dosing. For example, excessive sleeve fouling or an ageing lamp can result in lower intensity, but with measured intensity the lamp power or number of banks would be increased in order to maintain the required dose. Similarly, with water quality, using worst case UVT, the delivered dose

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