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Wastewater treatment plants produce more than just clean water

Drinking water is essential for sustaining human life. However, only 0.0001 per cent of the 1.4 billion cubic kilometres of water on the earth is easy accessible for use as drinking water. It is therefore a precious commodity, and wastewater needs to be purified before it re-enters the water cycle. The German city of Stuttgart purifies 27 million litres of wastewater every hour, thus eliminating up to 95 per cent of the organic compounds. Scientists are now trying to find ways to use wastewater treatment plants for purposes other than the purification of wastewater. Besides making the purification of water more effective and complete, the scientists are investigating whether fertilisers and hydrogen can be produced during the reclamation process.

The average German uses around 130 litres of drinking water a day. Before used water ("wastewater") re-enters the water cycle, it needs to be chemically, physically or biologically purified in wastewater treatment plants. Big wastewater treatment plants normally use a combination of all three methods, whereas small wastewater treatment plants apply either biological or physical-biological methods.

Chemical treatment involves using chemicals that precipitate contaminants. The solid particles are then physically removed using filters or microstrainers. Biological purification involves microorganisms and plants that decompose nutrients and pollutants. It is important to reduce the organic load of the dissolved organic compounds before the reclaimed water is returned to the water cycle as the algae and plants would otherwise use them in combination with oxygen for growth, thereby depleting the water of oxygen, causing other organisms to die.

Small wastewater treatment plants are used in peripheral wastewater purification

Biological SBRs (sequencing batch reactors) involve four steps. © BIOPRO/cm

Small wastewater treatment plants are used if a connection to the public wastewater system is not economically feasible in the case, for example, of individual houses or small, rural towns where wastewater is then dealt with locally. Most small treatment plants are based on SBRs (sequencing batch reactors), membrane filtering and fixed-bed systems, all multi-chamber systems that separate solid particles in a process known as mechanical primary treatment (1). The sewage is subsequently pumped into a second chamber where the pollutants are absorbed by bacteria. Air is pumped into the chamber to provide the bacteria with the oxygen they require for growth (2). In SBR systems, the incoming air also leads to the dispersal of the bacteria at the bottom of the chamber and supplies them with oxygen which they need for the degradation of nutrients. In fixed-bed systems, the second chamber is equipped with a matrix to retain the bacteria. In both systems, the microorganisms convert the pollutants and nutrients into sludge that is allowed to settle at the bottom of the chamber (3), and the purified water is then allowed to flow off (effluent) (4). In addition, membrane filters with tiny pores are used to remove pathogens. Water that has been purified by this method qualifies as process water and can be re-used for instance to irrigate gardens.

Different regulations apply to wastewater treatment plants supplying reclaimed water and also to the sites where the water is used. Since the year 2005, in accordance with valid approval regulations, the German Institute for Structural Engineering (DBIt) has classified the performance of small wastewater treatment plants into five classes of effluent. All classes (C, N, D, +P, +H) eliminate carbon (chemical symbol C). In addition, class N indicates the removal of nitrogen via oxidation (nitrification) and class D, the removal of nitrogen by microorganisms (denitrification). Class +P plants remove carbon, nitrogen and phosphorous. Class +H plants indicate the elimination of carbon and nitrogen as well as the disinfection of the water in order to remove pathogens. While class +H is mandatory when the reclaimed water is to be discharged into water reserves, class C requirements are usually sufficient for small wastewater treatment plants used for domestic purposes.

The correct bacteria form the basis for successful clarification

The performance of a wastewater purification plant depends decisively on the biological purification method used, and hence on the quality and effectiveness of the bacterial culture. It is therefore crucial that the bacteria are taken care of correctly and that dead bacteria are always replaced with new bacteria. The risk of destroying the bacterial culture is much higher in small wastewater treatment plants than in bigger systems that combine all three aforementioned methods and where toxic elements are eliminated by way of chemical purification. In addition, large-scale plants treat much larger quantities of water, which leads to the pollutants being diluted. However in small wastewater treatment plants, drugs that are flushed down toilets or otherwise disposed of can have devastating effects on the bacteria used to purify wastewater. The plant's bacterial culture consists of bacteria that arrive with the sewage. That is why each plant has its own unique bacterial profile, which needs to be regularly checked in order to ensure that the plant continues to conform with the stipulations of the effluent class for which it was approved. Attempts to operate wastewater treatment plants with foreign bacterial strains have proven unsuccessful, which is why researchers are focusing, amongst other things, on optimising the purification process rather than interfering with the bacterial profile.

Wastewater treatment with an extra benefit

Recycled products that can be directly applied in agriculture. © Fraunhofer IGB

Scientists at the Fraunhofer Institute for Interfacial Engineering and Biotechnology in Stuttgart are currently investigating the potential of wastewater nutrients for use as fertilisers. In the field of nutrient management, the IGB's Department of Physical Process Technology concentrates on the recovery of phosphorous for use in various sectors of agriculture. Another of the institute's projects focuses on the elimination of chemicals from hospital wastewater. Already low concentrations of such contaminants, for example drugs or their degradation products, are toxic and can accumulate in the food chain once they enter the water cycle. Therefore, the IGB and its collaboration partners have developed a membrane bioreactor that enables the removal of such contaminants. However, the scientists found that some compounds, including the psychopharmaceutical carbamazepine, were not significantly degraded or converted into pharmaceutically inactive metabolites and are now developing a bioreactor that is able to effectively eliminate carbamazepine and other non-degradable drugs. Chemists from the University of Jena are working on a purification process that avoids the use of chemicals altogether. The process they are working on is based on an effect known as cavitation, which relates to the formation of small bubbles as a consequence of forces exerted by moving parts on liquids, for example a spinning ship propeller. If the bubbles burst, high pressures and temperatures occur at the interfaces, which causes radicals to be generated from water. These radicals are then able to degrade toxic substances such as carbamazepine.

The Institute of Sanitary Engineering, Water Quality and Solid Waste Management (ISWA) in Stuttgart is pursuing a completely different approach: researchers there hope to use the sewage sludge that accumulates during wastewater purification for energy production. The ISWA is developing methods that use the energy from the digestion of sewage sludge for the generation of hydrogen, a process that is usually associated with energetic hurdles. This becomes possible through the constant influx of carbon and would be another step towards finding alternatives to fossil energy. Standard wastewater treatment plants already have in place the technology required; only the general conditions need to be adjusted, for example temperature or pH. The amount of hydrogen, which is a normal by-product of sewage sludge digestion, could then be increased by way of gas separation and ion exchange and thus contribute to achieving virtually emission-free energy generation.

Sustainable water management as long-term goal

Already in September 2006, the German Federal Ministry of Education and Research (BMBF) launched the project "Wasser 2050" (Water 2050) designed to identify lines of innovation which will enable a clear improvement in the sustainability level for water supply and sanitary systems by 2050. Many countries in the world have launched similar incentives to counteract the risk of water supplies becoming unsustainable by 2050 and Germany would benefit from this scenario as a major technology provider. Future generations will have a decisive role in implementing the goals described in the book entitled "Wasser2050: Chancen für die deutsche Wasserwirtschaft" (Water 2050: Opportunities for German Water Management). Education and training programmes have already been put in place to prepare young people for their future work in this field. For example, numerous Baden-Württemberg universities offer study programmes that deal with the treatment of wastewater, including the University of Stuttgart, which offers a course in environmental protection technology and "WASTE", an international master of science programme on air quality control, solid waste and wastewater process engineering.

Website address: https://www.biooekonomie-bw.de/en/articles/news/wastewater-treatment-plants-produce-more-than-just-clean-water