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CASE STORY Denitrification Denitrification – the solution for municipal Waste Water Treatment Plant of Alingsås How Purac provide a maximum flexible process, with optimum use of existing volumes, at a minimum investment cost! Background Europe has recently set up rather stringent requirement for the outlet of nitrogen compounds from municipal waste water treatment plants in order to minimize the eutrophication of our common water environment. In 2012, eight municipalities in Sweden did still not reach the new standards, one of them being Alingsås. The industrial city Alingsås is situated close to Gothenburg. The city is famous for the honorary Jonas Ahlströmmer, the man whom introduced potatoes as stable food to Sweden, built up the woolen based industry in the western part of Sweden and established tanneries. The Alingås municipal waste water treatment plant is situated at the Säveån river and has the lake Mjörn as it’s recipient. The www.purac.se lake is used for fishing, sailing, and other leisure activities and it is therefore vital to reach as low nitrogen outlets as possible to prevent algea bloom. The Ahlströmmer castle Nolhaga is neighbour to the municipal Nolhaga waste water treatment plant. The plant has a traditional municipal treatment process with pre-treatment, biological treatment and chemical sedimentation. The load is 30.000 pe including industries. The present total Nitrogen outlet is 26 mg/l on a yearly average. For year 2014, the yearly average must be lowered to 15 mg/l. The Nolhaga Castle Park is used for the public nowadays as a recreation area and thus the amount of trucks to and from the plant must be minimized as the road to the plant passes straight through. Post-denitrification is thus not considered to be the main process solution as this required Methanol transports. Solution As the nitrogen outlet is measured on a yearly basis, the existing aerated stone filters can be used for nitrification in the traditional biological system as the sole nitrogen removal stage part of the year. Two of the existing sludge oxidation tanks where converted to pre- and post-denitrification tanks by filling them with suspended carriers on which the nitrification organisms thrive. Depending on how much additional denitrification is required to reach the stipulated outlet on a yearly basis, the process can be run either with predenitrification or with both pre- and postdenitrification. Carbon source may be used in both the pre- and the postdenitrification tanks, and thus the operator has full control of the denitrification process on a daily basis no matter what levels of Carbon and Nitrogen that comes into the denitrification plant. If the Nitrogen load to the waste water treatment plant will increase, or the outlet permit become even more stringent, more carriers can be filled into the tanks to meet up the new conditions. Sieves must be used to prevent the carriers to leave the plant. New creative design for these where adopted to incorporate the existing sloping walls into the plant design. Result The plant is able to bring the total Nitrogen outlet down to below 13 mg/l on a yearly basis. Facts One of the obstacles where to accommodate the new pipelines in the existing, already crowded, pump gallery in the basement of the sludge oxidation plant. • Plant load: 30.000 peq incl. industries More or less the entire reconstruction of the existing plant could be made without disturbance to the daily operation of the existing process. The entire project time was 10 month. • Total Nitrogen inlet: 490 kg/day As existing volumes where reused, the price of the project was only half the cost of build a new plant for the same purpose. • Mechanical treatment; Max 4000 m3/h • Biological/chemical treatment; Max 2.700 m3/h • Organic Nitrogen inlet: 230 kg/day • Total Nitrogen outlet before: 26 mg total Nitrogen /l • Total Nitrogen outlet after: 13 mg total Nitrogen /l • Pre-denitrification volume: 600m3 (1.5 basins) • Post-denitrification volume: 200m3 (0.5 basin) Purac AB Emdalavägen 10, SE-223 69 Lund, Sweden Box 1146, SE-221 05 Lund, Sweden Phone: +46 46 19 19 00, Fax: +46 46 19 19 19 E-mail: [email protected] www.purac.se