Blowers energy consumption decrease in an extended aeration activated sludge process by cooling the suctioned air

• Aspirated air cooling by WWTP blowers using a cold room or EAHE has been analysed. • The EAHE system cool the air more efficiently than a cold room during summer. • Cold room or EAHE are ineffective at cooling the air during the early morning hours. • At 1.5 depth EAHE, decrease blowers instantaneous work up to 2 % two-thirds of the year. • Pipe length and ground temperature are the most decisive variables in the EAHE. In order to reduce energy consumption of the blowers in extended aeration wastewater treatment plants, decreasing suctioned air temperature is planned. For this purpose, in a full-scale facility, a 16.0 m 3 cold room and an earth-air heat exchanger (EAHE) at a depth of 1.5 m (four parallel sections of 10.5 m followed by one of 9.1 m of PE DN 315 pipe) have been built. Temperatures of suctioned and ambient air and ground at four depths have been continuously controlled. The EAHE stands out in summer, decreasing air aspirated temperature an average of 4.0 °C, with point values up to 6.0 °C. This system enables a decrease in the instantaneous work of the blowers on 63.0 % of the days of the year, with average decreases up to 2.0 % and maximum of up 4.0 %. By the cold room instantaneous work is decreased 39.8 % of the days of the year, with average decreases up to 1.2 % and maximum of up to 3.3 %. Both systems are effective during the central hours of the day, with a wider range of hours for the EAHE, and are inefficient during the early morning. According to statistical analysis, the EAHE should be located at a depth where the earth´s undisturbed temperature is reached, using several pipe segments to improve heat exchange and avoid excessive pressure loss in the air aspiration. Soil material round the EAHE with high thermal conductivity is essential to achieve a high heat transfer and avoid temporal performance degradation by thermal saturation of the ground