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WATER TREATMENT


Diane White of atg UV Technology discusses how chemicalfree disinfecon can improve swimming pool water management


candidates from folliculitis to cryptosporidiosis and legionnaires’ disease. So water hygiene is top of the priorities, and disinfection by halogenation (chlorination or bromination) is the traditional weapon against these water-borne pathogens. However, high levels of chlorine or bromine can result in complaints from bathers and even cause skin itching and rashes. A bad bather experience is not good for business, especially if the pool is part of a hotel or spa, nobody wants to come home from holiday feeling unwell!


P Contamination origins


Before we look at the problems of disinfection we need to understand where pathogen contamination comes from and what it is. Assuming that make- up water is of drinking water quality, the main source, and in indoor pools the only source, is from the bathers themselves. Skin, mucus and faeces are the principle sources of microbiological contamination, but bathers also contribute sweat, organic matter and urine to the water and these have an impact on chemical disinfection. If disinfection is inadequate, film forming bacteria, like Pseudomonas aeruginosa, can form adherent biofilms on pipework and infest activated carbon filters providing ongoing contamination that is difficult to remove.


Halogens, chlorine and bromine, exist in water as an equilibrium between hypochlorite or hypobromite ions and the corresponding hypochlorous or hypobromous acids. The equilibrium is pH sensitive with the acid form being favoured at lower pH (below 6.5 for hypochlorous acid and below 8.0 for hypobromous acid) so pH control is critical. Hypochlorous acid is very effective against bacteria like E Coli, Pseudomonas and Legionella, but protozoans like Cryptosporidium and Giardia, common causal agents of sickness and diarrhoea, are notably resistant to it. The hypochlorite ion is less effective as a biocide but is a strong oxidising agent which reacts with ammonia (the breakdown product of uric acid, urea, histidine and creatinine in sweat and urine) to form monochloramine and, at higher concentrations, dichloramine, trichloramine and eventually, nitrogen gas.


These chemical reactions have to be completed before there is a sufficient ‘residual chlorine’ concentration to kill bacteria. This process is called ‘breakpoint chlorination’ and it consumes a significant amount of chlorine. Chloramines are responsible for the chlorous smell and eye irritation that so many bathers complain about, but have also been identified as a contributory cause of asthma and of corrosion of structural steelwork. Further, hypochlorite and hypobromite react with organic matter to form carcinogenic tri-halo methanes. But halogen disinfectants have the overriding advantage that they provide a stable ‘residual disinfectant’ concentration in the pool water. To overcome some of the problems of chlorination, many pools opted to change to ozone as a primary disinfectant with just a low dose of chlorine as a residual. Ozone is a powerful oxidising agent, generating free hydroxyl radicals which oxidise ammonia and organic matter without producing the objectionable by- products. However, ozone is highly toxic and a significant fire hazard. Also it does not provide a residual, so chlorine dosing is required in addition. Ultraviolet radiation, on the other hand, offers a chemical-free alternative to ozone. The process works by exposing the water, after filtration, to electromagnetic radiation in the UV-C range (200-280nm wavelength). This range coincides with the absorption spectrum of the four bases that make up the DNA molecule (around 254nm).


UV radiation treatment


uBarr + Wray is currently refurbishing four of its Glasgow Life Leisure Centres at Scotstoun, Gorbals (pictured), Easterhouse and Springburn


Read the latest at: www.bsee.co.uk


Exposing micro-organisms to UV radiation acts on the DNA molecule to prevent replication, so the organism cannot reproduce. By taking most of the microbiological loading, UV disinfection reduces the chlorine dose needed to maintain the required residual chlorine concentration in the pool water. The UV-C range also covers the bond energies in chloramines: monochloramine (NH2Cl) 245nm, dichloramine (NHCl2) 297nm and trichloramine (NCl3) 260nm. So UV disinfection has the added advantage of destroying chloramines by photolysis. Like ozone, UV generates free hydroxyl radicals which harmlessly oxidise organic matter to produce carbon dioxide and water.


Lorne Kennedy, operations director, Barr + Wray, adds: “We are currently refurbishing four of Glasgow Life Leisure Centres at Scotstoun, Gorbals (pictured), Easterhouse and Springburn. At each site we are removing existing ozonation systems and replacing them with WF medium pressure UV systems handling flows up to 440m3/h.” atg UV Technology has also supplied 16 medium pressure WF systems to FT Leisure for the Subtropical Swimming Paradise project at Center Parcs, Longford Forest in Ireland. Medium pressure lamps, which produce polychromatic radiation, are preferred to monochromatic low pressure lamps for this application since low pressure lamps produce only 254nm radiation and do not, therefore, have the capability of chloramine destruction. atg UV Technology’s WF range of medium pressure UV systems is designed specifically for the swimming pool industry and is the most compact currently available. All the models are fully third party validated to achieve a 3-log reduction of Cryptosporidium parvum oocysts in accordance with standard international protocols.


Crystal clear water


One of the benefits of using UV technology or microbiological control in swimming pool water, and to break down and remove chloramines from the swimming pool environment, is crystal clear water. This contributes to safety and maintenance because clear water allows easier observation of what is going on below the surface, whether it is an emergency, like a bather in trouble or a faecal pollution incident, or simply better maintenance. It also means fresh, clean air with chloramine concentrations well below the 0.5mg/m3 discomfort threshold and a significant reduction in skin and eye irritations.


In addition, the requirement for ‘shock dosing’, and the use of additional chlorine is significantly reduced. In most cases, combined chlorine levels are reduced to as little as 0.2mg/l, representing up to 75 per cent reduction on conventional chlorination systems. By minimising the use of chlorine in swimming pools many of the chemical hazards are removed and the reduced chemical consumption has clear economic benefits.


www.atguv.com BUILDING SERVICES & ENVIRONMENTAL ENGINEER JUNE 2019 31


uOne of the benefits of using UV technology or microbiological control in a swimming pool is crystal clear water


robably the greatest fear of a swimming pool operator is that of a bather contracting a disease from the water, and there are many


BSEE Shining a light on swimming pool water





Exposing


microorganisms to UV radiaon acts on the DNA molecule to prevent replicaon, so the organism cannot reproduce





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