The Baltic Sea, the largest body of low-salinity water in the world, has a coastline shared by nine countries. As the drainage basin to 85 million people, the environmental strain on the sea has been considerable for many years.
Saint Petersburg, the former capital of Russia, stands on the eastern reaches of the sea and has a population of five million people. To reduce the environmental pressure on the Baltic, a multilateral initiative aims to reduce levels of phosphorous and nitrogen by deep biogenic removal from the treated discharge.
Treating 400 mgd
An important stage in meeting the stringent requirements has focused on the Central Wastewater Treatment Plant, situated just five kilometers from the historic heart of Saint Petersburg. The facility services 2.2 million of the city’s inhabitants, processing 400 mgd.
The plant has six pairs of primary and secondary basins for mechanical treatment, as well as 12 aeration basins for biological treatment. Wastewater enters the inlet chamber, continues through the main pump station and proceeds to mechanical, biological and chemical treatment processes.
“The agitators, installed in aeration basins, have three specific roles: One, to maintain uniform concentration of the biomass and substrate in the reactor volume; two, to prevent sludge sedimentation in the aeration basins; and three, to check the formation of floating sludge,” said Per Selenius, product manager, water solutions at Xylem.
The agitators, 44 in all, are installed in the pre-denitrification, anaerobic, anoxic and gas removal zones. The SY4850 units have a rated motor rating of 2.2 kW and are equipped with Flygt self-cleaning banana blades.
The plant is operated by SUE Vodokanal of Saint Petersburg, which set a number of criteria that needed to be met by the winning tender, including suspension of both top and bottom sludge, low energy consumption and high operational reliability.
With experience from over 200,000 mixer installations around the world and over 50 years of experience designing and commissioning mixer applications, Xylem was well placed to meet these criteria.
Solids suspension
“One of the reasons Xylem won the competitively fought tender was because we could demonstrate the ability not only to keep the bottom free from sediment, but also to create sufficient flow to prevent the buildup of floating sludge,” said Karl Tawaststjerna, technical director at Petroplan, a Xylem distributor in Russia, which also supplied engineering consultancy for this project.
With over half a century of mixing experience and hands-on knowledge of various applications, Xylem pioneered the use of thrust as the main performance parameter, which is now established by the ISO 21630:2007 standard. Today, Xylem meticulously engineers each installation to capitalize on the tank’s natural hydraulic characteristics.
“To ensure that the entire volume of a basin is kept moving, you need to create sufficient thrust,” said Tawaststjerna. “With the help of tools such as computational fluid dynamics, we were able to determine the right technology, mixer size and layout while minimizing power consumption.”
Low energy consumption
National legislation stipulates the use of energy-efficient equipment in water treatment facilities. Even though the agitators are in constant operation, their overall power consumption is extremely low. The agitators themselves have an installed power rating of just 2.2 kW and a very low rotational speed of 30 rpm or less.
The self-cleaning characteristics of the banana blades also help account for the low power consumption, since they reduce the amount of fibers and rags that can get caught on the propellers. This results in sustained energy efficiency over time.
“All the hard work paid off in the end,” said Selenius. “By optimizing thrust and bulk flow, the agitators consume just 5.9 kW per mgd, while generating sufficient flow in the basins. In other words, we have delivered the maximum bulk flow for the minimum of power input.”
High operational reliability
The construction of the agitators is very robust, with an operational lifetime of over 100,000 hours. High-quality materials have been used throughout—the shaft, for example, is made from high-alloy steel.
But perhaps most important of all is the gearbox’s high service factor. “With Flygt top-entry agitators, we have, as standard, a cast-iron bearing housing positioned beneath the gearbox,” said Selenius. “This is because it allows the bearings to take most of the radial forces created by the propeller. We prefer this arrangement because bearings are less sensitive than gearboxes, and, of course, less expensive to replace.”
As the fight to preserve the Baltic continues, Flygt aerators are helping Saint Petersburg reduce emissions of phosphorous and nitrogen while keeping associated energy costs low.
