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A new simulation method for investigating mixed aqueous electrolyte systems at thermodynamic equilibrium is presented. Based on the commercial simulation package IPSEpro, a graphical procedure for assembling equilibrium systems followed by simultaneous solution of the underlying equation system was developed. At graphical setup, species and reactions objects are connected by the information of activity and stoichiometry in a network-like manner. Consequently, the resulting equations of mass action, the electro-neutrality and balance conditions are solved incorporating the Pitzer activity coefficient model and equilibrium constants from thermodynamic standard data. The method developed allows fast and flexible application of the complex thermodynamic principles to engineering tasks. The modeling approach and all basic models for graphical input and simultaneous solution of coupled equilibrium conditions are explained. For demonstration of the simulation method, example simulations for the systems of H2SO4-H2O, CaSO4-H2SO4-H2O and NaCl-CaSO4-CO2-H2O are described and evaluated successfully.

The global shortage of resources and the intention to reduce CO2 emissions from fossil sources has triggered new approaches in the wastewater industry. Better ways are sought for to manage sewage sludge and to exploit its value as a secondary resource. However, we see a lack of tools to assess the appropriate treatment strategy which leads to imperfect process choices and designs. Here we present (a) a modeling approach and a simulation tool that allows the systematic evaluation of different sewage sludge treatment options and (b) a test case study for a mid-sized waste water treatment plant. The sludge model is discussed and tested for consistency with alternative approaches. In the case study, the direct conversion to electricity in a Rankine cycle is shown to be superior to biogas production or dried sludge export based on an energy cost analysis. The development presented is believed to enable more efficient waste-to-energy conversion and to contribute to sustainable resource management.