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The potiential for using sewage sludge ash (SSA) as a supplemenatry cementitious material (SCM) has been investigated. Controlled combustion of sewage sludge collected in Croatia from two wastewater treatment plants (WWTP) produced SSA with different characteristics. These were used to substitute for cement in mortar samples. The chemical composition and physical properties of SSA depends on wastewater composition, the sludge treatment process and the combustion temperature. These factors influence the suitability of SSA to be used as a SCM. For three different combustion temperatures (800°C, 900°C and 1000°C), it was concluded that properties of fresh mortar were not affected while in the hardened state, the most favorable combustion temperature is 900°C regarding mechanical properties. Regardless of combustion temperature, for all types of SSA used in mortars as cement replacement (up to 30%), the average decrease in both, compressive and flexural strength values was less than 8% for every 10% of added SSA. The results presented indicate that using up to 20% replacement of cement by SSA produces mortars that meet the specific technical requirements analyzed in this work.

and concrete samples. However, replacement of cement by MID-MIX ® ash did not significantly affect the strength, water penetration or gas permeability compared to control samples. Leaching tests on MID-MIX ® powder, MID-MIX ® ash and mortar samples produced low levels of heavy metal release. The research shows that thermally processed MID-MIX ® treated sewage sludge produces an ash product that has potential to be beneficially used in cementitious materials for selected applications.

Final disposal of sewage sludge is important not only in terms of satisfying the regulations, but the aspect of choosing the optimal wastewater treatment technology, including the sludge treatment. In most EU countries, significant amounts of stabilized and dewatered sludge are incinerated, and sewage sludge ash (SSA) is generated as a by product. At the same time, lime is one of the commonly used additives in the sewage sludge treatment primarily to stabilize the sludge. In doing so, the question arose how desirable is such addition of lime if the sludge is subsequently incinerated, and the generated ash is further used in the production of cementitious materials. A series of mortars were prepared where 10-20% of the cement fraction was replaced by SSA. Since all three types of analyzed SSA (without lime, with lime added during sludge stabilization and with extra lime added during sludge incineration) yielded nearly same results, it can be concluded that if sludge incineration is accepted solution, lime addition during sludge treatment is unnecessary even from the standpoint of preserving the pozzolanic properties of the resulting SSA. Results of the research carried out on cement mortars point to the great possibilities of using SSA in concrete industry.

Management of sewage sludge-new possibilities involving partial cement replacement An increasing trend of sludge generation at wastewater treatment plants (WWTP) has been observed in developing countries like Croatia. Thermal processing of sewage sludge facilitates its further management, although ash is generated as new waste in the process. The proposed approach, while eliminating the need to dispose ash at non-hazardous waste disposal sites, directly reduces not only the sludge and ash disposal costs, but also the raw cement production costs. All analysed technical and environmental requirements are met when 20 % of cement is replaced with ash.

Quantities of sewage sludge that are thermally treated (mostly incinerated) are increasing, resulting in growing quantities of sewage sludge ash (SSA) which also requires further management. Despite its potential as a resource, it is still largely landfilled. Considering the presence of potentially toxic and hazardous heavy metals in SSA, this paper analyzes how the change in the leaching concentrations of the selected heavy metals from landfilled SSA impacts the environment (air, water, and soil) by the means of LCA. When considering human toxicity potential as impact category, dominant impacts were due to emissions into the air, primarily caused by leaching of selenium and somewhat less cadmium, mercury and nickel. Mercury had a dominant impact when considering the terrestrial ecotoxicity potential impact. In the SSAs obtained from Croatian sludge, molybdenum leaching, along with selenium and mercury, showed a dominant impact. Therefore, due to the high variability of trace elements, detailed analysis of different SSAs is needed.

Due to the complexity of calculating the minimum required volume of water tanks and the associated regime of pumping water into the tank depending on the consumption pattern in the water supply systems, finding the functional dependence of these variables is a complex process. The main idea of this paper was to provide a methodology for the calculation of the minimum water tank volume considering all input variables, which could be used in a simple and applicable way in everyday water supply management and engineering. As a final product, a desktop application TankOPT was developed that is easy to run and use on a PC with a user-friendly interface for data entry (data on maximum daily consumption and the pattern of daily water consumption). A software solution was created based on a numerical model that simplifies the usual manual calculations using known spreadsheet software and solves this problem. The solution was determined with combinations of the start and duration of water pumping in the water tank, for which the minimum required volume of the tank is obtained. JavaScript programming language was used to create the app. The use and operation of the application are shown through two hypothetical examples.

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