ARA Thunersee uses HYPERCLASSIC®-Mixers for the elimination of micro-pollutants
The increasing pollution of our waterways by micro-contaminants from medicines, cleaning agents, and personal care products in waste water is a growing problem for humans and animals.
EXCURSUS ON THE FOURTH STAGE OF TREATMENT
Due to the incomplete metabolism of pharmaceuticals and the improper disposal of personal care products and chemicals, ever-increasing amounts of micropollutants are entering the wastewater requiring treatment. Conventional three-stage wastewater treatment plants, with mechanical, biological, and chemical treatment stages, are not designed to remove these micropollutants. If it cannot be ensured with measures at the source, i.e., avoidance or behavioral change, that these substances do not enter the wastewater in the first place, the existing wastewater treatment plant must be expanded to include a fourth treatment stage. This expansion step has been underway for many years in some regions in Germany, and also in Switzerland.
Within the fourth treatment stage, two different types of processes have been established so far:
- ozonation with the aim of oxidizing trace substances
- the use of activated carbon with the aim of adsorbing the contaminants.
In particular, the powdered activated carbon (PAC) process in combination with coagulation and flocculation is a proven and effective solution.
OPTIMIZATION OF PROCESSES
For the elimination of micropollutants using the INVENT HYPERCLASSIC®-Mixer, the focus is on the process variant with PAC. One example is Thunersee wastewater treatment plant in Switzerland, where the so-called “Ulmer Verfahren” has been implemented since 2018. In this process PAC is added to and mixed with the treated wastewater from the secondary clarification stage in a contact reactor. By adding coagulants and flocculants, loaded PAC flocs grow, which can subsequently be separated in 2 sedimentation step downstream.
The effectiveness of the process depends on the contact reactor design and the mixing technology used. Optimizing the individual stages enables the plant operator to ensure safe and efficient operation in terms of both energy and chemical consumption. INVENT offers a service for this purpose with its own department for computational fluid dynamics simulations, THINK Fluid Dynamix®, which determines the optimal positioning of all inlets, chemical dosing points, and agitators in order to achieve maximum mixing and stable flocculation with minimal energy input. With the HYPERCLASSIC®-Mixer, INVENT also offers the optimal technology for mixing a contact reactors. The hyperboloid mixer is positioned close to the bottom, generating high radial bottom velocities for efficient suspension. The mixer’s high pumping capacity enables an overall strong volume flow for reliable homogenization of the reactor contents. The HYPERCLASSIC®-Mixer is also ideal for flocculation, as the acceleration of the flocs generated along the large surface of the hyperboloid mixer body is gentle. The low-shear yet intensive mixing creates optimal conditions for floc growth, hence, the desired cleaning process.
The flocs, containing micropollutants, can then be separated in the downstream sedimentation stage and recirculated. Good settling properties of the flakes are extremely important here in order to prevent activated carbon carry-over with the treated effluent. At the end of the process, the loaded activated carbon is discharged with the waste sludge, dried and incinerated.
INVENT AND ARA THUNERSEE – A SUCCESS STORY SINCE 1996
The Thunersee municipal wastewater treatment plant in Switzerland has been in operation since 1972 and treats the wastewater of around 120,000 connected residents.
Between 1994 and 1998, as part of a complete overhaul of the biology and sludge treatment, twelve fourth-generation INVENT HYPERCLASSIC®-Mixers were installed in the anoxic tank sections for denitrification on a total of four lines. These have now been running continuously for over 25 years, with low maintenance and high energy efficiency. As of 2024 old mixing units are being gradually exchanged for the new HYPERCLASSIC®-Mixers Evolution 7 to update the technology and adapt to the now more challenging situation with heavier PAC-loaded activated sludge on the biology.
INVENT provided support during the planning phase for the fourth treatment stage by optimizing the 4-stage contact reactors, while THINK Fluid Dynamix® answered related questions based on preliminary plans using flow simulations:
- determination of the optimal inflow position of PAC suspension
- flocculants and flocculating agents in the first and last tank sections,
- determination of residence times and degree of homogenization of the tank contents.
Suspending the heavier PAC sludge with a sludge volume index of 80 to 40 ml/g requires more mixing power than, for example, simple denitrification. In addition, the requirements for homogenization and floc growth pose a challenge for the mixing technology used.
Since the fourth generation of HYPERCLASSIC®-Mixers was installed in the aeration tank in 1996, INVENT has continuously worked on improving its mixer, so that the completely redesigned version of the HYPERCLASSIC®-Mixer Evolution 7 was now available for use in the fourth treatment stage of the Thunersee wwtp. The HYPERCLASSIC®-Mixer Evolution 7 efficiently generates a high bottom velocity in the reactor with low shear force, and the optimized design of the transport fins on the hyperboloid mixer body increases the homogenization performance.
The expansion of the plant has been in operation since July 2018, and the positive results show, that not only is the fourth treatment stage a complete success, but also that the placement of the inlets and mixers recommended by INVENT based on fluid mechanics was correctly analyzed. The officially published report on the results of the wastewater treatment plant’s first year of operation describes the chosen designs as follows: “After secondary clarification of the biological stage, the wastewater is lifted by two pumping stations and conveyed to two contact basins with a capacity of 1,100 m3. The wastewater passes through four zones equipped with INVENT mixers, with no partition between zones 2 and 3.”1 This effect of a ‘virtual wall’ between two counter-rotating HYPERCLASSIC®-Mixers is a unique effect which, combined with good retention time behavior, can save on construction costs for a partition between the zones. The report goes on to describe the process: “The powdered activated carbon suspension and Fe3+ solution are dosed into the first zone. […] An anionic flocculant is added in the fourth mixing zone at the bottom of the tank near the mixer. The PAC sludge is settled in four clarifiers, each with a capacity of 1,944 m3 […], and conveyed to the first mixing zone of the contact tanks by return sludge pumps.”1
In order to test the effectiveness of the plant, intensive water comparison tests were carried out before and after the fourth purification stage during the first year. In the report mentioned above, 22 of a total of 24 48-hour samples were reported to have elimination rates of 81% to 95%, which is above the legally required 80% for the specified indicator substances. The two poorer values were attributed to heavy rainfall, as in this case 10% to 20% of the water bypassed the fourth treatment stage, consisting of a contact reactor, sedimentation basin, and filter system.
1 Report on the results of the wastewater treatment plant’s first year of operation
