Danish
Sand filtration has been used to treat water for over 150 years, yet understanding of the processes involved remains very limited. Anna Demming speaks to Professor Hans-Jørgen Albrechtsen about his group’s work to bring a more strategic approach to optimising the performance of sand filtration water plants
Sustainable water treatment plants get a strategic makeover
Sand filtration is the favoured technique for treating water in many countries across the world. The benefits of
the approach
include a lower energy cost and less use of harsh chemicals compared with other treatments such as membrane processes. However, as Hans-Jørgen Albrechtsen, professor in the Department of Environmental Engineering at the Technical University of Denmark (DTU) explains, the industry still lacks a strategic diagnostic approach to resolve problems in water plants that are not performing as they should. “We have cases where we have two parallel lines — one is ok and the other is not working. Same water, same filter, so why is one not working?” To address these issues, Professor Albrechtsen formed and shaped the sustainable
drinking water treatment
biological filters project. Generally speaking, sand filters work by
using a metre or two of sand with the finer sand at the top and coarser sand at the bottom. The water then filters through the sand over the course of 20-30 minutes, during which time a number of processes come into play to clean the water. This contact time is an important parameter as it limits the range of processes relevant for cleaning the water to those with quite a fast reaction rate. The filter also undergoes a regular ‘back wash’ process, where the flow of water is reversed to clean rust-like precipitates and other impurities that have built up in the system over time. Among the impurities that need to be
removed are manganese and iron. As Professor Albrechtsen explains, the water that comes out of our taps contains
PROJECT PARTNERS
• Department of Environmental Engineering, Technical University of Denmark (DTU Environment)
• Department ofManagement Engineering, Technical University of Denmark (DTUManagement Engineering)
• Ghent University, Belgium • Krüger, Denmark
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• Weber – Saint-Gobain, Norway • DanishWater andWastewater Association, Denmark
• KompetenzZentrumWasser Berlin, Germany
• Copenhagen Energy, Denmark • AarhusWater, Denmark • NordVand, Denmark • Esbjerg Supply, Denmark
Insight Publishers | Projects
oxygen. Anaerobic water—without oxygen—would taste strange. However this means any iron and manganese in the water oxidises giving it a distinct metal taste and turning the water reddish or black, which stains. “You certainly don’t want that,” he adds. Chemical treatments can reduce the iron and manganese to insoluble states so that they precipitate out. However, interactions with microorganisms can also be used to remove these metal impurities, and part of the project is dedicated to investigating these processes. Another important constituent to remove
is ammonia, which is oxidised to form nitrate. “That is certainly a biological process,” Professor Albrechtsen points out. If the oxidation of ammonia is allowed to occur freely throughout the water distribution system, the water from the tap will be anaerobic. Finding ways to control this biological process is another objective of the project. In fact, much of the cleaning process in
sand filtration is due to the presence of microorganisms. Professor Albrechtsen stresses that these microorganisms are not added but are naturally present
in the
water system. The project aims to identify this complex microbial community in terms of the quantity and diversity of the microorganisms. A potential future development may even be to add the right microbes if those required are not found to be present in the right quantities. “That is one of the visions,” says Professor Albrechtsen, “Today you don’t do that but it could be a tool in the future.”
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