OTTIMIZZAZIONE ENERGETICA DI UN ESSICCATORE FANGHI IN UN IMPIANTO DI DEPURAZIONE DI ACQUE DI RIFIUTO URBANE

Abstract

Wastewater treatment involves high management costs linked mainly to energy consumption and sewage sludge disposal. Cogeneration of electricity and thermal energy applied to a wastewater treatment plant (WWTP) is one of the most suitable solution to cut off the costs and to improve environmental sustainability. WWTPs with complex sludge treatments (anaerobic digestion, sludge dryer, incineration, etc) are often good examples to enhance energy requirements and financial statements by a CHP installation. Cafc Spa, the company that manages the water network and the WWTPs of the Province of Udine, have decided to invest on the most energy-consuming plants in all water integrated cycle (both for the wastewater treatment and the drinking water plants). This article describes San Giorgio di Nogaro WWTP case (700.000 equivalent inhabitants capacity but treats wastewater for 120.000 EI) where a sludge drier is used before landfill disposal. The drying process is one of the most energy-consuming unit inside WWTP. Sewage sludge drying is used to decrease the water content of sludge after mechanical dewatering treatment. Thanks to drying process, the sludge is suitable for recovery or disposal. In this case sewage sludge after mechanical dewatering has an average content of 18% total solids (TS) with production of 672 kg/h. Sludges are dried to 96% TS and then send to landfill. The plant uses a conductive dryer, equipped with discs fed by diathermic oil. Thermal energy is produced by a boiler of 2.528kW with a methane consume of 265 Sm3/h (yield 91%) but the line is oversized and used at 30% of its capacity. The plant revamping consists in a new trigenerative system to dry the sludge in substitution of diathermic oil boiler. The electricity produced by the new system can be used to cover WWTP electric energy demands while the heat is used for the sludge dryer and to heat and cool offices and laboratory. Three different scenarios were analysed to identify the best CHP solution: a 200kW gas turbine, a 400 kW gas turbine and 200kW gas engine. The best solution was identified in the gas engine, it allows to cover WWTP electric energy request with a pay back of 3,5 years, an internal rate of return of 19,75% and a net present value of 394.950 €. The economic savings were estimated in 185.208 € per year. To understand real cost savings, one year monitoring was performed after CHP installation. During this monitoring period some operative conditions changed respect business plan hypothesis, respectively: 1) due to a change in wastewater flow and an increase in sludge production, a back-up boiler was used to cover the thermal energy drier demand; 2) methane and electricity tariffs were significantly reduced; 3) some improvements were performed in the management of water line process determining minor electric energy consumption. The real savings for the first year were about 133.603 €. Nevertheless, the CHP installation allowed to optimize the sludge drier unit and the energy plant balance.