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This review discusses the problem of management of digested sludge with the use of various methods for the management, including extrusion cooking. Extrusion cooking as a method of management of digestate can be an innovative approach to this topic. Until now there have been no studies on the use of the extrusion process to convert anaerobic digestion sludge. The extrusion process plays an important role in the transformation of materials on an industrial scale. An agricultural biogas plant can produce up to several tons of digested sludge per year (depending on the size of the installation). The most common method for utilisation of this kind of material is the use thereof as a fertiliser. However, this solution requires large areas of farmland. The best methods for conversion of digested sludge are those allowing the separation of the solid part from the liquid part. One of these methods consists in obtaining pellets in the extrusion process.
The feasibility of magnetic field was examined as a factor affecting sludge conditioning intensification. In the experiment sludge after preliminary anaerobic digestion was used, coming from a domestic wastewa­ter treatment plant. Digested sludge was taken directly from a fermentation tank. The experiment was run in three phases. They were performed on a laboratory scale, at various experimental stands. Different dosages of iron chloride, hydrogen peroxide and Fenton's reagent were applied to determine their influence on the sludge properties as well as the effect of constant magnetic field on the conditioning parameters was deter­mined. Straight dependence was found between dosage of the reagents and the way of sludge introduction in the magnetic activity zone, as well as physical and chemical parameters of the prepared sludge.
Over three successive experimental years total of 183 samples of sewage sludges from 9 biggest sewage treatment plants of the Cracow province were analysed as to the possibility of biological sludges management. A vast majority of the sludges revealed great value as fertilizers; they contained usually more than 40% of organic matter, over 3% of nitrogen and 1% of phosphorus in dry mass. However, only sludges from three sewage treatment plants may be unconditionally used as fertilizers. Sludges from the other sewage treatment plants contained too much heavy metals (mostly Cr and Zn), which excludes their application on arable lands. A decrease in the contents of two most dangerous metals, i.e. cadmium and lead in the sludges was observed in most sewage treatment plants throughout the experimental period. It may be a result of generally decreased pollution of natural environment with these metals in the Cracow province.
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