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MANAGEMENT OF SEWAGE SLUDGE - NEW INSIGHTS INTO VALORIZATION AS SUPPLEMENTARY MATERIAL IN THE CONSTRUCTION INDUSTRY

What is the main reason why are we participating at the conferences related with sewage sludge, why are we dealing with sewage sludge in our professional and scientific carriers. Is it possible to see the beauty in sludge? Would you rather visit sludge disposal site or any other place on earth? Would you rather hold a sludge in your hands or any kind of master piece of the nature? Does sludge make us laugh? Maybe it does, sometimes! So, the question still is… why are we here talking about sludge, why are we dealing with sewage sludge? The most correct and the simplest answer is because sludge is money, and sludge is a lot of money, and of course poses certain threat to the environment and human health. So, sewage sludge treatment and disposal are costly and environmentally and socially highly sensitive processes and sewage sludge quantities worldwide are increasing on a daily basis. Sewage sludge disposal has become a significant burden to many utility companies, as well as to certain aspects of the operation of local governments. Based on a review and description of many technical and technological solutions for treatment and disposal of sewage sludge used worldwide, it is obvious nowadays that this problem is certainly not only a technical issue, but is a result of different approaches that depend on the natural and social givens of each state or region. There are many options not only for sludge treatment but for its disposal as well. You can also imagine how many different combinations of treatment processes with final use or disposal route can be generated for each treatment plant. There exists no single strategy or unified guidelines for the disposal of sludge at the global level. Even at the EU level, there are significant differences in the method of disposal of sewage sludge between countries. EU Directives promote that sustainable sludge handling may be defined as a method that meets requirements of efficient recycling of resources without supply of harmful substances to humans or the environment. On a worldwide level sewage sludge has so far been used in agriculture, in construction (for soil improvement, rod construction, in production of bricks, in concrete industry) and for phosphorus extraction which is very popular nowadays. At EU level disposal of sludge in agriculture is most widespread in Ireland, United Kingdom, Spain, and Denmark with a share of over 50%, while the beneficial use of sludge on non-agricultural areas (horticulture and landscaping) is the dominant method in Luxemburg, Hungary and Estonia. On the other hand, incineration is the primary method of sludge disposal in the Netherlands and Switzerland, but with the greatest share in Belgium, Germany and Austria and sludge disposal in landfills, although limited and increasingly abandoned in line with EU directives, is still the dominant method for sludge disposal in Romania and Malta where essentially represents the only solution. In certain countries there are even different strategies and guidelines for sludge disposal between individual regional areas or provinces. Due to different practices, experiences and policies in different countries, a unique conclusion about the general trend of final sludge disposal cannot be drawn, which makes the job for many countries worldwide quite difficult. Based on that, my colleagues and me came into idea to give this presentation here, on the topic related to probably the most important aspects of sludge disposal, and that is financial aspect and final use of sewage sludge for example in brick industry and it will be presented on a Case study in Croatia. Estimated sludge production in Croatia by regions by the 2031. Is given on the slide in tonnes of dry matter per annum. Herewith the case study of central region related the country’s capital the City of Zagreb will be presented. I must say at the beginning that during the preparation of the complete study analysis, 48 alternative solutions were generated as potential combinations of additional treatment of previously dewatered sludge and final use or disposal of sludge, or by-products of its treatment such as ash or char. For the needs of this presentation, for better visibility, only 5 alternative solutions have been selected. On the slide you can see the initial result of economic analyses for the selected five alternatives with the unit net present values in €/t of dewatered sludge of the total cost over a period of 30 years. The economic advantage of the first alternative is observed with a slightly worse result of 4th alternative, while the remaining alternatives prove to be a more expensive solution. However, it is important to understand the overall economic balance and the applied mathematical algorithm, and to take into account that a large number of input parameters are stochastic such as construction costs of mono-incineration plant, unit electricity prices, fees for farmers to take sludge to their agricultural land, and whether farmers are interested in sludge collection, what will be the fee to the construction industry for ash collection, whether all compost produced will be used and at what price, what will be the price of sludge delivered to other countries, and many other parameters. Therefore, it is extremely important to know how to assess these economic risks in order to avoid making bad decisions. Taking into account all the above risks and using the Monte Carlo method, the results of the economic risk analysis shown in the slide were obtained. Let's take a closer look at the result for the first alternative, which was assessed as the most favourable on the basis of initial analyses. Risks associated with this alternative are among the highest, ie it is characterized by a high range of potential unit costs, which certainly affects its final value range, because for example a price of over 100 €/t is certainly considered as not acceptable comparing with some other alternatives. On the other hand some alternatives are characterized by a small range of values of potential unit cost, which makes them more acceptable solutions. Thus, for example, the second alternative emerges as the most favourable solution with the lowest economic risks. But this alternative generates sewage sludge ash that needs to be properly disposed. As previously mentioned, use of sewage sludge ash in construction industry is one of the routes. From 2014-2017 at the University of Zagreb, Faculty of civil engineering we did a research in cooperation with one of the biggest concrete producers in Croatia. We produced hexagonal concrete prisms that are usually used in practice as pavers in watercourses or parking lots. Tests were performed with 10 to 20% replacement of cement with ash. The results of compressive and flexural strength tests of the produced elements are shown on the slide where it can be seen that samples with 20% of cement replacement with ash have even higher compressive and flexural strengths as well as the reference mixture without added ash. The leaching of heavy metals was also tested on the same samples with 20% replacement of cement with ash and a comparison was made with the leaching from the ash itself as well as the limit values according to the legislation. It is observed that heavy metals remain encapsulated in concrete elements and that the leaching of heavy metals is within the permitted limits. The second research we are still conducting at the Faculty of Civil Engineering, University of Zagreb is related to the use of sewage sludge ash in the production of bricks. The share of ash as a substitute for clay in the amounts of 5%, 10% and 20% was examined. The results of compressive strength of the produced brick are shown on the slide and a comparison with the reference mixture is given. It can be seen that with up to 10% replacement of clay with ash, the same compressive strengths of the brick as with the reference mixture are achieved. Conclusion Scientific and professional community, including politicians, should reach a consensus at the state or regional level about which solutions are acceptable, and based on that consensus develop a strategy for sewage sludge management. Detailed economic analysis of different alternatives should present foundation for any other analysis in the decision making process, including the risk analysis. It is also necessary to make a full survey of the issue, including other factors, such as the environmental impact, by carrying out, for example, the life-cycle analysis. Analyze the regulations and adopt necessary improvements. Think about sewage sludge as a resource (e.g. in construction industry) not as a waste!

WWTP, sewage sludge, econimic analysis, risk analysis, recycling, use, concrete, brick

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