Coagulation and flocculation in the Helsinki drinking water treatment plant

The raw water for Finland’s capital Helsinki is pumped from Lake Päijänne, located approximately 120 km away, via a tunnel to two water treatment plants, where it is treated and purified. This secures sufficient drinking water of excellent quality for around one million people. HSY, Helsinki’s Environmental Services Authority, is the communal treatment plant operator; it is responsible for securing the entire water supply. In the drinking water treatment plant in Vanhakaupunki 18 HYPERCLASSIC® Evolution7 mixers and three CYBERPITCH® mixers made by INVENT have been in operation since July 2017.

The current plant was reconstructed in order to increase its capacity and to realize a more energy-efficient configuration. As early as during the design phase of the reconstruction INVENT’s distribution partner Puwimex Oy was able to lay-out and recommend suitable INVENT mixers for plant operator HSY to optimize coagulation and flocculation. In addition and with the help of high-resolution, realistic flow simulation processing steps were modeled and optimized.

One challenge during the layout was the significant improvement of the mixing result in the supply tank of the drinking water plant. The goal is to achieve a spontaneous equalizing of the charge on the particles’ surface so that colloidally present particles can grow into flakes. This is where CYBERPITCH® mixers come into operation, as they mix with high velocity and energy. The mixer consists of a refined angled-blade mixer, which accelerates the water not only in an axial but also in a radial direction. The CYBERPITCH® mixers also evenly stir in the flocculation agent.

The HYPERCLASSIC® Evolution7 mixers are employed in the flocculation tanks in order to remove unwanted ingredients in the drinking water, such as particles causing turbidity or solute organic matter; this is achieved through agglomeration of suspended or colloidal particles. Subsequently the water passes through a sedimentation, flotation and filtration phase.

The design of both mixers very much meets the requirements of drinking water treatment. The power units are dry installed above the water’s surface and therefore do not come into contact with water. The mixers are equipped with food-grade oil in power units and oil trays in accordance with the customer’s specific requirements to prevent impurities in drinking water.

With the help of the HYPERCLASSIC® mixers all prerequisites for the optimal formation of flakes can be fulfilled and undesirable impurities can be separated. By rotating close to the bottom of the tank the HYPERCLASSIC® Evolution7 hyperboloid mixer’s eight integrated and specifically optimized transport ribs generate a radially external bottom flow. This flow is particularly turbulent close to the bottom of the tank and efficiently stirs up sediments, so that sedimentation of particles in the flocculation tank does not occur. Additionally, incidences of short circuit flows are securely eliminated. Due to the fact that the flow on the water’s surface is directed towards the drive shaft all particles are evenly mixed throughout the tank, so that almost all of the particles come into contact with the flocculation agent, agglomerate and form flakes.

Due to the mixer’s large diameter, its optimal geometry and its low rotational speed the HYPERCLASSIC® mixer’s shear forces are minimized. This enhances the agglomeration of the flakes and avoids loading the flakes. The bigger and more stable these flakes are, the more easily they can be separated in the subsequent purification steps. Impurities are almost entirely removed from the drinking water. These facts contribute to the significant improvement of the plant’s purification capacity.

The reconstruction and retrofitting of the plant in the summer of 2017 with INVENT mixers enabled the operator HSY not only to meet a higher demand, but also to deliver the best and a more energy-efficiently produced drinking water to approximately one million people in and around Helsinki. Kippis!

Questions? Please send us an email