Energy optimization of active sludge process at Kakolanmäki WWTP
Solution to which problem
Aeration process may take up to 50 % of the WWTP's total energy consumption. Most of the energy is needed for production of compressed air which is necessary for nitrification in the aeration tank. Optimizing the control mode of aeration and compressors remarkable savings may be reached.
Technical conditions
Kakolanmäki wastewater treatment plant treats the water of 325 000 PE, with primary sedimentation, activated sludge process for enhanced biological phosphorus and nitrogen removal and sand filtration as a tertiary treatment.
There are four biological treatment lines. Each line consists of 15 meters deep rectangle basin which is divided into six sections. First, there is two anaerobic zones followed by a switch zone, which can be operated either anoxically or aerobically. Last three sections are aerobic. MLSS concentration of the aeration is 4-5 g/l.
After aeration wastewater is led to a secondary sedimentation where clarification is enhanced with ferrous sulphate and kationic polymer. Concentrations of nitrogen, phosphorus and solids are measured after the sedimentation process. The purified wastewaters are discharged to the Baltic Sea.
Implementation
Wastewater samples from aeration were taken and analysed to sort out ammonium and nirtate concentrations in each section of the aeration tank.
In the test period one aeration line was equipped with an ion-selective electrode to measure ammonium and nitrate directly from the aeration tank. Ammonium ion content was then programmed to adjust the setpoint of dissolved oxygen level in the aeration. The setpoint changes linearly as the ammonium ion concentration in the aeration changes. The wanted level of dissolved oxygen is then produced by screw compressors.
Six screw compressors were tested with different frequencies and combinations to find the most energy efficient range of use for production of compressed air.
Result
Optimization of the frequency range for production of compressed air has given 5 % savings of electricity consumption of the screw compressors.
All four biological treatment lines were equipped with ion-selective electrodes by year 2017. Purification efficiency of nitrogen has increased by 15 % and resilience of the process has increased significantly in addition to energy savings. Higher energy savings could be achieved by adjusting the dissolved oxygen setpoints limits.