The challenge

Processing of vegetables requires large quantities of water, mostly for washing and blanching. The highly-polluted (COD values up to 10’000 mg/l) effluents are discharged to the wastewater treatment plant. This WWTP has an average flow of 4’000 m3/day, but the flow and the load can vary greatly depending on the season and the type of vegetables processed. Consequently, it is important to have versatile equipment, which can adapt quickly to changing conditions.

For the aerobic activated sludge process, the three biological basins (4-6 meters water height each) were initially equipped with high-speed surface aerators that did not perform in a satisfactory way. Below a depth of three meters, the oxygen transfer in the mixed liquor became insufficient. Furthermore, some sediments were accumulating at the bottom of the basins. Other problems occurred above the surface, such as splashing noises and aerosols polluting the walls of nearby buildings. Therefore, Ardo started to look for an alternative aeration system in 2016.

The solution

Sulzer, together with local contractor Vewaco, delivered four OKI aerator mixers fed by one HST turbocompressor for basin 1. Ardo also received offers from other suppliers, but finally chose Sulzer’s equipment – not only based on technical considerations, but also relying on the strong trust in our products and people. Prior to the order, Sulzer organized a visit with Ardo to a French municipal wastewater treatment plant using OKI and HST. This was a good opportunity for the client to see the equipment in operation, and to hear the very positive feedback from the French operators.

After the order, the collaboration with the client remained strong, including product training and advice given on how to optimize the biological process by using OKI alone (mixing mode) or with HST (mixing and aeration mode). As soon as the first four OKI 2000 aerator mixers had been installed at the bottom of the main biological basin, the improvement was remarkable. Total power consumption was reduced by 35% and all previous problems (uneven oxygenation, sedimentation, bad odors, aerosols) were eliminated.

After one year of smooth operation for basin 1, a second set of four OKIs + one HST replaced the surface aerators in basin 2. A third replacement set for basin 3 followed afterwards.

We discovered the winning combination for our aeration process: OKI aerator mixers and HST turbocompressors.

Technical Service Ardo Group

Customer benefit

The OKI aerator mixers are liftable and self-standing. They were installed easily and quickly at the bottom of the tanks without emptying them. The OKIs have an adjustable standard oxygen transfer rate (SOTR) with remarkable maximum values, which makes them ideal for aeration and mixing of wastewater with high and changing organic loads. Their performance remains constant with time. Ever since the first four machines were started in the main aeration basin, they have been running smoothly with low maintenance needs thanks to their robust design and reliability.

The HST high-speed turbocompressors offer an outstanding wire-to-air efficiency. They use digitally controlled magnetic bearings, which cause no mechanical friction or wear. This means that regular maintenance is reduced to changing the air filters only. In the compressor room the working environment is optimal as the HSTs are extremely silent (61 dB).

Product specification
Three rectangular aeration basins with 4-6 meters water height each:

Basin 1: 4 OKI 2000 C-15AM fed by 1 HST 20-4500-1-125

Basin 2: 4 OKI 2000 C-15AM fed by 1 HST 20-4500-1-125

Basin 3: 4 OKI 2000 C-15AM fed by 1 HST 20-4500-1-125


OKI 2000 C-15AM

Power 15 kW
Protection type IP68
Voltage 400 V / 50 Hz
Temperature control Thermal switch 130°C
Dimensions (D x H) 2’045 x 2’075 mm
Unit weight 1’930 kg
SOTR 0 to 120 kg O2/h


HST 20-4500-1-125

Airflow 2’500 to 5’200 Nm3/h
Pressure rise 40 to 60 kPa
Input power 114 kW max.
Current (400 V) 198 A max.
Power supply 400 V
Input frequency 50 Hz
Thermal protection 2 x PT100
  • OKI aerator mixer
    The submersible OKI aerator mixer is a heavy-duty, non-clogging, gentle mixing unit which can operate both as an aerator and as a mixer.

  • HST™ turbocompressor
    The modern and silent HST™ turbocompressor features an advanced design with proven magnetic bearing technology and a high-speed motor driven through a built-in frequency converter.
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