INDUSTRY FOCUS WATER & WASTE TREATMENT BROADENING THE APPEAL OF UV IRRADIATION
Barry Hopton, atg UV Technology, looks at how process control has opened the way for ultraviolet irradiation to become more accepted
U
ltraviolet irradiation is now an established process unit operation in a
wide range of industries. It’s early uses included non-chemical disinfection for high purity water in pharmaceutical and semiconductor manufacturing, and has, more recently, found new applications as diverse as process water and sugar syrup disinfection, health care and fish farming, sewage treatment and drinking water supply, where it has become the process of choice for dealing with chlorine resistant organisms like Cryptosporidium. Ultraviolet radiation in the UV-C band
has a wavelength around 250nm which is very close to the absorbance wavelength of the amino acid groups which form the “rungs” of the DNA double helix. UV radiation fuses adjacent amino acid groups making it impossible for the molecule to divide and replicate. This means that bacteria exposed to UV radiation cannot reproduce. UV is a broad spectrum disinfectant, inactivating a range of micro-organisms, is chemical-free with no health and safety or environmental issues and, unlike some chemical
The atg UV Technology AOP Skid Package for use on Municipal Drinking Water and Industrial Effluent Water to breakdown and remove problem micro- pollutants such as ETDA, MTBA, PAH’s and taste and odour compounds such as MIB and Geosmin
disinfectants, it produces no by-products. But UV is more than just a disinfectant:
in combination with hydrogen peroxide or a catalyst like titanium dioxide, it can generate free hydroxyl radicals which are capable of oxidising recalcitrant organic species like the “emergent pollutants” – for example, endocrine disrupting chemicals – that are of increasing concern in our rivers. UV can also destroy free chlorine in water by photolysis, making it an excellent pre-treatment for chlorine- sensitive processes like reverse osmosis and ion exchange. With this kind of versatility it is, perhaps,
Computational Fluid Dynamics (CFD) example, used to ensure and optimise performance for a variety of applications and installations
surprising that it is only relatively recently that UV has become so widely accepted. The reason is one of process control. UV depends for its efficacy on the effective dose (or “fluence”) reaching the target micro-organisms or organic species. This depends on the amount of radiation absorbed by the liquid (the UV transmissivity or UVT). In high purity water the UVT is typically more than 99% so this is not a problem: the UV lamp can be set to give a fixed output giving 4 log
HELP TO PUT A STOP TO FOG BUILD-UP IN SEWERS
At a municipal wastewater treatment plant in Brazil, Next Filtration’s Next FOG STOP has improved overall effluent water quality by reducing residual effluent biochemical oxygen demand (BOD) by more than 50%. Combined with an increase in dissolved oxygen (DO) levels of 45.5%, use of Next FOG STOP could allow the facility to operate under increased flow, and/or organic loading of up to 25% or more. The 40-day trial of FOG Stop, which alters the core metabolism of indigenous bacteria, also improved
the rate of ammoniacal nitrogen removal from 31.9% removal to 57.7%. In the test by Sabesp, the water and waste management
company owned by São Paulo state, volatile suspended solids were reduced from 20% to just 13.3%. Furthermore, sludge production dropped from 11,089 mg/L to
7,546 mg/L, for an overall reduction of 31.95%. Again, FOG STOP showed that the operating cost of handling and disposing of sludge at a wastewater treatment plant can be greatly reduced. In England, Next Filtration recently secured a contract with
Severn Trent to deploy its Next FOG STOP technology to cut down the build-up of fats, oils and grease (FOG) in sewers. The agreement followed an extensive 18-month trial period. Next Filtration
or greater reductions. However, if the liquid is turbid, coloured or contains suspended solids then the UVT will be much lower and may be variable. It can also cause deposition fouling of the quartz tubes that house the lamps, effectively reducing the lamp output. Add to this the natural decay in UV output as the lamp ages and it becomes clear that a fairly sophisticated control system is required, and this is only a recent development. The UV lamp output needs to be modulated to ensure that the correct fluence is delivered, and this is normally achieved by an algorithm which accounts for liquid flow rate, UVT, turbidity and colour, all of which are monitored by on-line instrumentation. Whilst this open loop, feed forward control gives a consistent UV lamp fluence, process engineers prefer a feedback loop to ensure that the control action has been effective. A UV intensity monitor located in the reaction chamber goes some way to achieving this, but it is only a “spot” sample and does not necessarily mean that the UV fluence has reached all the liquid in the reactor. atg UV Technology uses computational
fluid dynamics (CFD) – these days an affordable tool – to determine the hydraulic design of the reaction chamber that will ensure that all the liquid flow is exposed to equal intensity of radiation with no short circuiting. This is validated by a third party to a recognised test protocol, such as that set out in the US EPA 2006 Ultraviolet Disinfection Guidance Manual. With this assurance, a UV disinfection system is simple to install and has low operating and maintenance costs – only a few pence per cubic metre of water depending on the bacteria reduction required – and it can give operational and economic benefits.
www.nextfiltration.co.uk
atg UV Technology
www.atguv.com
24 NOVEMBER 2018 | PROCESS & CONTROL / PROCESS&CONTROL
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