UV technology lights the way
Mike Shaw from Hanovia details the impact of minimum UV dose in wastewater disinfection
Ultraviolet (UV) disinfection technology is becoming an increasingly popular method of destroying pathogenic micro-organisms in wastewater. A major problem UV systems have to cope with in WwTWs, however, is the regular fluctuation in wastewater quantity and quality caused by variations in the levels of suspended solids and organic loads.
These fluctuations affect UV transmittance and can, if plant designers do not take them into account, result in variable microbial killing rates. It is therefore essential any UV disinfection system is designed so the UV lamps always provide the minimum required UV dose at those points in the system where UV intensity may reach its lowest point. This will ensure all the micro-organisms in the effluent receive sufficient UV dose at all times and killing rates will be more predictable.
For this reason, optimal lamp positioning is very important when designing a UV system for a WwTW. To reduce headloss in gravity-driven wastewater systems, UV disinfection equipment is usually installed in open channels, with the UV lamps positioned parallel to the low-velocity fluid flow. This design means the flow through the UV unit will be virtually laminar. Although the distance between the individual UV lamps may be small, and the velocity of the wastewater low, there will always be a centre-point between lamps that receives less UV dose than the rest of the wastewater. This is due both to the centre-point's relative distance from the two nearest lamps, and because the wastewater at this point will have a slightly higher velocity than the rest of the liquid.
The UV dose received at the centre-point will also vary depending on organic loads and the levels of suspended solids in the wastewater. When there are many UV lamps, and considering the volume of wastewater passing through the system, the number of centre-points increases and the volume of liquid receiving less than the minimum required UV dose quickly becomes significant. Many UV treatment systems are designed on average parameters and do not consider the lower UV doses received in the centre-points. In addition, they do not generally take into account fluctuations in the levels of suspended solids or periodic increases in wastewater volumes caused, for example, by heavy rainfall.
The result is a large proportion of the wastewater does not receive an adequate UV dose and many micro-organisms pass through the system unharmed. UV dose is usually based on total UV energy emitted into a liquid flowing at a particular rate. The average UV dose can be increased either by increasing the UV output of the lamps or by reducing the flow rate. This works well enough when the water being treated is clear and UV transmission is high, such as with drinking water. When UV transmission is low, however, as in the case of most wastewater, increasing the UV output of the lamps or reducing the flow rate may not always achieve the required rates of microbial kill.
With low-pressure lamps in particular, the beneficial effects of this approach are very limited and there remains a central point that receives less than the minimum required dose. A more effective method is to position the lamps so they are closer together and perpendicular on the flow. This will ensure all points in the system receive the minimum required dose. As Figure 1 shows, at a certain UV transmission the UV dose at the centre-point between two lamps is such that if, for instance, 10,000 faecal coliforms/100ml enter the UV system:
It is therefore more effective to create a constant, minimum UV dose at the centre-point. This minimum UV dose ensures all the wastewater passing through the system receives an adequate dose of UV, even at the weakest point in the system. The minimum centre-point UV dose therefore gives a much better indication of the effectiveness of a wastewater UV system than average UV dose, which is used in most clean water applications.
To demonstrate the importance of minimum UV dose, a field test was carried out recently at a WwTW in the Netherlands. The purpose of the test was threefold:
A UV treatment chamber fitted with four UV lamps was installed to treat 200m3/h of unfiltered secondary wastewater (Figure 2). By varying the distances between two of the lamps (Figure 3), the minimum centre-point UV dose was varied, but the average UV dose remained the same. Before UV treatment the number of faecal coliforms was about 10,000cfu/100ml (cfu = colony forming units). After UV treatment the surviving quantities of faecal coliforms at the same flow were as shown in Table 1. Figure 4 shows the logarithmic reduction of faecal coliforms in the trials with minimum UV dose at the centre-point, as well as average dose. What Table 1 and Figure 4 clearly show is that increasing the minimum centre-point dose increases the killing rate of faecal coliforms.
To obtain a 99.9% (three-log) reduction, a minimum centre-point UV dose of 200J/m2 was necessary for the particular secondary-treated effluent used in the trials. An increased average UV dose (over 360J/m2) and a minimum centre-point dose (above 200J/m2) did not immediately result in higher microbial reductions - this was probably due to the levels of suspended solids in the unfiltered effluent. In addition to ensuring open-channel UV systems are designed with minimum UV dose in mind, there is another important factor to consider.
Recent research has shown that, following exposure to low-pressure (monochromatic) UV, the DNA of micro-organisms (for example E.Coli) is capable of repairing itself if subsequently exposed to sunlight (a process known as 'photoreactivation'). Following exposure to medium-pressure (polychromatic) UV, however, photoreactivation does not occur1,2. These findings have significant implications for open-channel UV WwTWs, in particular, they show medium-pressure systems are the most effective method of UV wastewater disinfection. This article has shown the minimum centre-point UV dose is more important in ensuring effective microbial kill rates that average UV dose. A minimum UV dose can only be ensured by optimal UV lamp positioning within the UV system. This positioning needs to be site-specific and must take into account a range of factors such as anticipated variations in organic loads and levels of suspended solids. Hanovia believes permanent deactivation of micro-organisms can also only be effectively ensured by using medium-pressure UV lamps in place of low-pressure versions.References: 1 Oguma K, Katayama H and Ohgaki S, Applied & Environmental Microbiology, Vol.68, No.12, 6029-6035, (2002). Photoreactivation of Escherichia Coli after low or medium-pressure UV disinfection determined by an endonuclease sensitive site assay. 2 Zimmer JL and Slawson, RM, Applied & Environmental Microbiology, Vol.68, No.7, 3293-3299, (2002). Potential repair of Escherichia Coli DNA following exposure to UV radiation from both medium and low-pressure UV sources used in drinking water treatment.