The present work provides an overview of technological measures to increase the self-sufficiency of wastewater treatment plants (WWTPs), in particular for the largely diffused activated sludge-based WWTP. The operation of WWTPs requires a large amount of electricity. Thermal energy is also required for heating the sludge and sometimes exsiccation of the digested sludge. On the other hand, the entering organic matter contained in the wastewater is a source of energy. Organic matter is recovered as sludge, which is digested in large stirred tanks (anaerobic digester) to produce biogas. The onsite availability of biogas represents a great opportunity to cover a significant share of WWTP electricity and thermal demands. Especially, biogas can be efficiently converted into electricity (and heat) via high temperature fuel cell generators. The final part of this work will report a case study based on the use of sewage biogas into a solid oxide fuel cell. However, the efficient biogas conversion in combined heat and power (CHP) devices is not sufficient. Self-sufficiency requires a combination of efficient biogas conversion, the maximization the yield of biogas from the organic substrate, and the minimization of the thermal duty connected to the preheating of the sludge feeding the anaerobic digester (generally achieved with pre-thickeners). Finally, the co-digestion of the organic fraction of municipal solid waste (OFMSW) into digesters treating sludge from WWTPs represent an additional opportunity for increasing the biogas production of existing WWTPs, thus helping the transition toward self-sufficient plants.

A unique cogeneration system integrating a biogas fed Solid Oxide Fuel Cell (SOFC) and a Concentrating Solar Thermal (CST) system for a reference Waste Water Treatment Plant (WWTP) in Italy is proposed. Biogas - which is locally in the WWTP from the anaerobic digestion (AD) of the collected sludge - can be used to produce electricity using SOFC power modules. The thermal power recovered from the SOFC exhaust stream is used to meet part of the digester thermal load. However, the rest heat loads are provided by using the integration with the CST system and an auxiliary boiler. Energy analysis is performed to determine the effect of using the solar heating system on the system performance. Also, the economic performance is evaluated through a cash-flow analysis and the calculation of the Levelized cost of electricity (LCOE). It is observed that installing 300 m2, 700 m2, 1100 m2 of solar collectors could cover 8%, 18% and 30% of total digester heat load, respectively. Results show an overall beneficial effect of the solar installation, both from an energy and economic standpoint of view. For all the scenarios analyzed, the LCOE is lower than the grid electricity price and, with increasing solar integration, the value is further reduced showing that, despite the investment return time, the electricity production during the entire system lifetime is competitive against grid electricity prices