Dricksvatten – vattnets beredning inför desinfektion med UV Koagulationsfiltrering i snabbfilter / Drinking water – preparation for disinfection with UV-light Coagulation filtration in rapid sand filters
In the early 70ies, water in lakes and streams in Sweden was generally clean enough for minimal water treatment to be considered sufficient. The situation was altered by the introduction of novel organisms, e.g., Cryptosporidium and Giardia, into Swedish waters. Since the year 2000, these unicellular parasites are inherent to several surface waters in Sweden. Existing waterworks, however, are not built to handle such infectious agents, and recent years have seen outbreaks in Lidingö, Östersund and Skellefteå affect thousands. In addition, there is a constant risk of both new and recurrent infections, ranging from bed bugs in apartment complexes to tuberculosis or measles. As wastewater is a major source of infectious agents, prolonged treatment of sewage is essential. Parasites such as Cryptosporidium are chlorine-resistant; however, UV radiation has proved highly efficient as a means of water disinfection. Previous studies show that prior to radiation treatment, water turbidity and colour must be low. With this method, more than 90 % of all microorganisms are removed, and the overall water quality improved. Ideally, waterworks should be equipped with a two-step purification barrier, which efficiently decreases microbial contamination compared to single-filtration methodology, even though it can never be 100 % impermeable. Pre-treatment prior to UV radiation should utilise modern membrane filtration technology, where adjustable filter and pressure settings allow for control of a broad spectrum of water pollutants at a select level of efficiency. Alternatively, the established method of contact-coagulation-filtration offers water purification in a single step, discarding humic matter that affects the colour, taste and smell of untreated water. More than 98 % of colloidal particles, even up to quantities of more than 1000 g/m3, can thus be removed.