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Constructed wetlands applications Sludge dewatering |
(Fonte: AKUT Gmbh)
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SLUDGE DEWATERING THROUGH CONSTRUCTED WETLANDS: EUROPEAN EXPERIENCES Sludge dewatering through constructed wetlands has a very important application in treating sludges produced by activated sludge plants, replacing technological solutions. At the European level, countries like Denmark, Germany and France have adopted this kind of solution since more than a decade, with many realized plants for the management of sludges, both in small scale and in large scale plants, these giving the best advantages fromthe technical and economical points of view. In France, for example, about 150 plants are working, and about 20 new plants are realized every year; in Denmark, the 70% of the large scale plants (>100.000 PE) have replaced technological sludge cycle with phytodisidratation. In the construction of a phytodisidratation system for the treatment of sludges, the most adopted solutions consist in the realization of a filtering system, mostly comparable to vertical submerged flow constructed wetlands used for the treatment of waste water. So, phytodisidratation beds consist in a drainage layer made with inerts (sand, gravel, little stone) of a depth of 50 cm, accomplished with a bottom drainage system collecting leachate and ensuring aeration of the bed, that is fundamental for the maintenance of aerobic conditions required for the obtainance of the best results of the system. Phragmites Australis (reeds) are planted in the beds: these macrophytes, largely used in constructed wetlands, catalyze and help the processes of dewatering and mineralization of the sludges, as it is shown by many scientific works. Most part of water is lost through evapotranspiration, the rest is drained and then recycled in the head of the plant. Macrophytes in the beds give the needed conditions for dewatering and avoid the clogging of the bed and the presence of anaerobic conditions in the bottom layers, that could produce bad odours and a lower mineralization of organic matter. Also, leachate presents a better chemical quality compared to unvegetated beds, so it is more easy to recycle it in the system. The height of the banks of the beds determines the length of the period of work of the system: usually, it is between 1 to 2 meters, so to obtain a 10 years lasting complete cycle. The cycle of utilization of dewatering beds starts with a slow initial phase, of about 1 year, after which begins the filling of the beds with sludges to be treated (6 to 9 years); distibution of sludges is discontinuous (1-3 times a week, better with resting periods of some weeks (3-7), so to make preferrable a configuration of the system with 6-8 parallel beds); this is made to allow the sludge to gradually settle and dewater; after this phase, heavy mineralization takes place (1 year), while no sludge is distributed; then beds are partially emptied, becoming ready for a new cycle. If beds are emptied correctly, without damaging the inerts layer, that is filled with rhyzomes, reeds will newly develop in a few weeks without any intervention. Sludges are dewatered more than 60% and so they are easily manageable, and can be reused as fertilizers, according to limits imposed by laws. A great advantage, specially in economic terms, is the good performance in volume reduction of the matter that has to be transported to the final destination (preferrably on fields spreading) without using "non-renewable" energy, excluding the one needed for pumping sludges. In the design of beds, in Europe an aeral load of 30-60 kg of Total Solids for year is used, that is about 1-1.5 liters/mq per day of liquid sludges coming from secondary sedimentation. Using phytodisidratation plants brings many advantages:
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