Irreversible impact of temperature on rheological behaviour of anaerobic digested sludge

In 2050, more than 65% of the world population will live in cities (or in urban areas). Consequently, the global wastewater generation is increasing at an exponential rate, as a result of rapid population growth and urbanization. It is said that the current global wastewater production is about 1,500 km3 per day but a huge volume of untreated wastewater is dumped directly into our water resources, threatening human health, ecosystems, biodiversity, food security and the sustainability of our water resources.In future cities and for sustainable urban design, biosolid has to be seen as a ‘valuable resource’ with a better integration of treatment plant in the whole urban water cycle in which treated water has to be reused while excess sludge has to become a second-life raw matter for agriculture or energy production.Renewable energy is one of the cornerstones of sustainable energy. Biogas from the anaerobic digestion of organic waste materials can provide a clean, easily controlled source of renewable energy, replacing firewood and/or fossil fuels. In order to maintain the requisite constant homogeneous conditions within digesters, operating conditions must be regulated according to the rheological characteristics of the sludge. An accurate estimate of sludge rheological properties is required for the design and efficient operation of sludge pumping and digester mixing.In this paper, we have determined the rheological behaviour of digested sludge at different concentrations and different temperatures, and highlighted common features. We showed that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations and temperatures, and depends only on the yield stress and Bingham viscosity and modelled it with modified Herschel-Buckley model.A master curve was obtained independent of both temperature and concentration. These two parameters (Yield stress and Bingham Viscosity) increase when the solid concentration increases but decrease when the temperature increases.Although the results showed that by changing the temperature, the sludge becomes more fluid; however, the rheological characteristics of sludge including yield stress and apparent viscosity do not come back to their original values after cooling the heated sludge. The solubilised COD showed a composition change after cooling the heated sludge which confirmed the irreversible change in the rheological characteristics of digested sludge. Dissolution of some of the solids may cause a decrease of the yield stress and an increase of the infinite viscosity or Bingham viscosity