The project demonstrates an innovative technical solution for sewage treatment, based on heat exchange, to improve efficiency and effectiveness in regions with a cold winter climate. With a pilot plant at the R&D facility Hammarby Sjöstadsverket, Stockholm, the effect of increased constant temperature in the influent wastewater treatment is demonstrated. The Directive 98/15/EEC stipulates that the total Nitrogen (N) effluent concentration from municipal sewage treatment plants (STP) must be limited to 10 mg/l for plants larger than 100 000 person equivalents (p.e.) as an annual average. In the Scandinavian countries and in the Baltic Region in general the low winter temperature of the sewage (10°C or less) is an obstacle to fulfil this requirement especially if more stringent national or regional requirements with quarterly or monthly effluent averages apply. It is further well known that problems in STP are common in spring and autumn with fast changes in temperature. The common technology to remove nitrogen from sewage is by oxidation of organic nitrogen compounds and of ammonia (NH4+) to nitrate (NO3-) which by denitrification is converted to nitrogen gas. To some extent the denitrification process also leads to the formation of Nitrous oxide (N2O). Both, the nitrification and denitrification processes are strongly temperature dependent and often considered to be inactive below 5°C. Within the project, a control system maintains water temperatures at 19-20°C throughout the process regardless of flow and temperature fluctuations at the inflow. This is achieved by heat exchange of the incoming water with the treated water and with return flow from district heating. By implementation of 2 parallel lines, of which one serves as reference, the effect of heating is demonstrated. Life Cycle Assessment (LCA) methodology is applied to assess the overall environmental impact of the demonstrated solution. The measurements of treatment with and without heating show the increased capacity for nitrogen removal from the wastewater in the heated line. The results also indicate a reduced need for aeration in the aerobic treatment. On the downside of the treatment are the increased risk for floating sludge in the sedimentation, and the increased need for pump energy to overcome the pressure drop in the heat exchanger. When using return heat from district heating, the reduced temperature after heat exchange can be used at the Combined Heat and Power plant for increased electricity production.
Nyckelord: Wastewater treatment, efficiency, preheating, nitrogen removal
Författare: Uwe Fortkamp, Christian Baresel, Klara Westling, Mats Ek, Christian Junestedt, Johansson C., Levlin L
Publicerad i: Presented at the NordIWA 2013, Malmö