Sludge handling process at Lubeck WWTP, Germany
Solution to which problem
Sludge management is a critical part of modern, economically efficient and environmentally sustainable waste water treatment. Research shows that sludge management costs can be as high as half of the total costs in wastewater treatment. Challenges in sludge management in the Baltic Sea region abound: in worst cases, nutrients that have once been removed from the waste water return from inadequately handled sludge to watercourses. Improving sludge management had been identified as part of the PURE project, Lübeck WWTP has been active in sludge activities of the project.
Lübeck Central wastewater treatment plant is operated by the organisation Entsorgnungsbetriebe Lübeck, responsible for wastewater and waste management as well as street cleaning in the city. Lübeck central wastewater treatment plant treats 420 000 PE, 3 Communities (39 300 inhabitants) are connected to the plant. There are 80 pumping stations, 1262 km of sewers, 2 wastewater treatment plants (one was connected in 2008) and all the sludge is treated at the Central WWTP in Lübeck.
The plant is equipped with a modern 2-step filtration enhancing the wastewater purification results. The sludge handling process in Lübeck consists of large primary sedimentation units, belt thickeners for excess sludge, digesters for stabilisation and chamber filter presses for sludge dewatering. The sludge fed to dewatering is conditioned with lime and after dewatering the sludge was completely disposed to agriculture. In 2015 a new legislation entered into force in Germany and, for that reason, the sludge can not be dispose to agriculture anymore, at least for the next 10 years.
The sludge is equipped with a large primary treatment unit. Primary sludge is fed directly to the digester. The excess sludge from the biological treatment (activated sludge) is thickened mechanically with belt thickeners to approximately 5.5 % at low polymer (1–3 g/kg DS) and energy consumption rate. Sludge is stabilised in mesophilic digesters with a retention time of at least 18 days at 36–40 °C and a dry matter content of 2.5 %. Between 2013/2014 the digesters were equipped with new vertical tube mixers.
The produced biogas contains approximately 62% methane, it is dried and desulfurised before storing in a gas tank of 4 000 m³. There are three CHPs (combined heat and power plants) with an electrical power rate of 844 kW produce electricity and heat totalling 10 GWh in total. The energy production (thermal and electric) is equal to consumption. A DS content of 36–39 % is achieved in the sewage sludge. A DS content of 36–39 % is achieved in the sewage sludge.
HELCOM recommendations of nutrient removal are achieved constantly: in 2011, the average nutrient concentrations in outgoing waste waters were 0.18 mg/l for phosphorus and 6.95 mg/l for nitrogen.
The disposal of sewage sludge at landfills has been banned in Germany since 2005, so that the only remaining alternative left after agricultural use is incineration. After changes in Germany legislation, in aprox. 10 years sludge incineration incl. phosphorus recovery will be the only way of disposing sludge.
Constant development work is done at the wastewater treatment plant to improve the purification results as well as to increase the sustainability and energy efficiency of the processes of wastewater treatment and sludge handling.