Mixers at Italian wastewater treatment plant can reduce energy use by up to 50%

Mixers at Italian wastewater treatment plant can reduce energy use by up to 50%

When a wastewater treatment plant near Florence, Italy, needed to replace a set of mixers, it consulted Xylem’s experts to find the optimal mixing solution. The Publiacqua treatment plant selected the new low-speed, high-efficiency Flygt 4530 mixers, which can reduce energy consumption by up to 50%.

The Publiacqua water treatment plant, located in San Colombano, handles wastewater for more than 600,000 people. Serving the city of Florence and eight regional municipalities, the plant plays a crucial role in removing pollutants from wastewater and ensuring effluent concentrations remain within defined limits.

The plant, which treats more than 65 million cubic meters of wastewater every year, has three lots of 24 sludge tanks. Each tank requires a submersible mixer to keep activated sludge in suspension. The mixers are crucial in reducing deposits that slowly diminish process efficiency, and in reducing the number of times the tanks need to be cleaned.

When Publiacqua identified that one lot of tanks required new mixers, they compared and tested different brands and models in the sludge tanks, including the Flygt 4530 mixer from Xylem.

Efficient mixing with a lower speed and larger propeller

Xylem’s approach to mixer projects is to assist customers in evaluating their tank geometry, liquid characteristics, installation restrictions and mixing duty in order to find the correct mixer for the application.

“The technical support and relationship with Xylem have been very good,” says Simone Caffaz, Process Engineer and Wastewater Treatment Plant Manager at San Colombano. “Publiacqua is also interested in all new technologies, such as the Flygt 4530 mixer.”

The Flygt 4530 was the clear winner in operational efficiency in all 24 tanks tested, thanks to its larger propeller and lower speed of rotation. The mixer is a recent addition to the Flygt range of innovative mixers and agitators. It is designed to operate in small and medium-sized tanks in activated sludge processes, and in sludge holding tanks and digesters where there is adequate depth for a larger propeller.

“Flygt 4530 midsize mixers are an ideal solution for medium to small-sized tanks, such as at the San Colombano plant,” says Clara Rondinini, Mixing Product Manager for Xylem. “Instead of using high-speed mixers, which are often used for this tank size, we proposed the Flygt 4530. This low-speed mixer combines high efficiency with high reliability, which were important factors for the customer.”

Reduced energy use and easy installation

The Flygt 4530 mixer has a three-bladed stainless-steel propeller, 1.2 meters in diameter, with a propeller speed of just 108 revolutions per minute. It also has a dependable IE3 motor with Class H insulation, and a heavy duty two-stage gearbox designed for more than 50,000 hours of operation.

The mixer provides high thrust relative to power consumption, which is essential for keeping deposits in suspension. Its uninterrupted and clog-free mixing operation reduces both operational and lifecycle costs. Compared to a direct-drive, compact high-speed mixer, Flygt low-speed mixers can reduce energy use by up to 50%.

Xylem initially installed 24 Flygt 4530 midsize mixers at the treatment plant. The mixers can often be installed on the same guide bar system as a legacy mixer, which was the case in San Colombano. This made the installation quick and simple and kept downtime to a minimum. The first 24 mixers were so successful that an additional 24 mixers were later installed in other tanks.