An integrated system for sewage sludge drying through solar energy and a combined heat and power unit fuelled by biogas

The main challenges of wastewater treatment plants are high-energy demand and sludge disposal. A hybrid system for sewage sludge drying is proposed in this work to face both these issues. The developed layout is based on the integration of two renewable energy sources, biogas from anaerobic digestion of sludge and solar energy in order to increase sustainability and energy self-sufficiency of the plant. Biogas is used to fuel a combined heat and power system and solar energy is exploited through a parabolic trough collector field. Electrical energy produced by the combined heat and power is used to cover the plant demand, whereas thermal energy coming from the combined heat and power unit and solar field is employed to run an anaerobic digestion process and for thermal drying of the residual sludge. The considered case study is based on data collected for an existing wastewater treatment plant. The proposed system is studied through data obtained from the scientific literature, suppliers of the considered equipment and properly developed sub-models of the various components, to assess its feasibility and the overall energy, environmental and economic performance. The system is dynamically simulated and daily, weekly and yearly results are presented. For the considered desiccant flow temperatures, the solar collector field achieves a thermal efficiency of about 56.0%, while for the combined heat and power unit it is about 44.0%, which is slightly higher than the nominal one, due to the preheating of combined heat and power unit intake air. The integration of solar energy with the combined heat and power system leads to a primary energy saving of about 14.6% with respect to the existing plant, where the sludge is not digested and neither thermally dried. The economic analysis shows that the Simple Pay Back period of the system is less than 3.40 years.