Power Quality in Water and Wastewater Treatment
ZASE – the joint waste water management authority Solothurn-Emme – is a public-law special purpose association in the sense of the Solothurn municipal code with seat in Zuchwil. The purpose of ZASE is the construction, operation and maintenance of a wastewater treatment plant with inlets and outlets and special structures.
Industry: Water and Wastewater Treatment
Local partner: Atlas Umwelttechnik
Timeline: February 2019
Background / Facts and Figures
With the water and wastewater treatment plant Emmenspitz in Zuchwil ZASE operates the biggest water treatment plant in the canton Solothurn. It has an urban catchment area of an impressive 47 municipalities and it reliably purifies waste water from industry, agriculture and private households:
- 27,500,000 m3 - That is the amount of wastewater flowing into the water treatment plant every year. This equates to 75'300 m3 per day or 3'150 m3 per hour.
- 93% - Over 93% of the impurities in the water are reliably eliminated by the treatment plant.
- 92'235 inhabitants dispose their wastewater in the water treatment plant in Emmenspitz (as of 2017).
- 45,000 tons of sewage sludge from other sewage treatment plants in the region are dewatered at ZASE and then disposed of at KEBAG.
The required steps and methods for water treatment depend on the nature of the raw water itself and is driven by the desired water quality required at the end of the process. It consists in a very complex process with up to 3 main treatment stages including hundreds of steps in between. Primary treatment includes the holding of the sewage in a quiescent basin to separate the water from mainly solids, oil and grease. Material that settles or floats can be removed, and the remaining liquid is transferred into secondary treatment. Within secondary treatment dissolved and suspended biological matters are removed. A typical biological stage uses indigenous, water-borne micro-organisms in a managed habitat. The goal of biological wastewater treatment is to create a system in which the results of decomposition are easily collected for proper disposal. Because biological wastewater treatment uses natural processes to support the decomposition of organic substances, it is of utmost importance that the complex biological and biochemical levels are controlled and refined. This is valid for both aerobic and anaerobic processes to achieve the optimal removal of organic substances from wastewater.
The described biological technology offers several operational and performance advantages over conventional processes. On the other hand, working with living microorganisms requires a reliable contribution of oxygen into the basins. For that purpose, high investments have been made to build a state-of-the-art biological stage with modern VSD turbo compressors, pumps and fans. Beside all the positive effects of this very modern equipment, severe side effects in terms of power quality issues arise. As in any business, energy losses, low efficiency, unplanned stops or unreliable functionality of equipment have devastating consequences. If processes involving biological stages living organisms are affected, then such issues become even more severe. On top of the critical functionality aspects of the water treatment, the fulfillment of power quality standards as well as the highest possible energy efficiency are required.
Therefore, ZASE focusses on those concerns from the very first planning and included Schaffner and its partner Atlas Umwelttechnik to design-in a suitable and reliable solution in order to mitigate power quality disturbances. After the plant construction, electrical measurements were performed according to the power quality standards D.A.CH.CZ and EN 50160, which highlighted the need for mitigation measures. Besides the need to lower the overall THDi and THDv to fulfill the power qualitiy standard limits, several other aspects needed to be considered. Measurements showed the specific need for mitigation of the 5th harmonics, as well as some other higher frequencies above the 40th. The perceived goal is to prevent premature aging of the plant and ensure its reliable functionality. Ultimately, all these measures are taken in order to avoid the worst-case scenario consisting in a shutdown or failure of the biological stage, garnering significant costs and plant outages.
Figure 1: Individual harmonics before the installation of the Schaffner active harmonic filters
Figure 2: Voltage and Current before the installation of the Schaffner harmonic filter
Two active harmonic filters from Schaffner’s new ecosine active sync series have been installed to solve the given issues and ensure an energy efficient and reliable water and wastewater treatment. Beside the well-known active harmonic filters with the brand new and market unique sync module, the cooperation between ZASE engineers and the main stakeholders for power quality was the key to a successful solution.
When asked why the project became a success over competing solutions, Atlas Umwelttechnik stated, “As Schaffner partner and power quality engineers, we acted out of one hand with customer, Schaffner engineers and the engineering consultants, solving every aspect of the given and acknowledged demanding issues and required targets. As a team, we showed expertise, professionalism and flexibility so that the customer needs are addressed with our solution.”
Harmonic measurements were carried out by engineering consultants to verify the performance of the installed active harmonic filters. Figures 1 to 4 compare the voltage and current harmonics and waveforms under the operating conditions with- and without active harmonic filters.
Observations from the summarized measurement results reveal that the active harmonic filters successfully reduced the distortion level complying with the established standards and ensuring the application reliability. Through the implementation of the active harmonic filters solution, both the total harmonic distortion of voltage and current as well as individual harmonic amplitudes at respective switchboards are below given limits. Thus, the water treatment plant is safeguarded in terms of power quality.
Figure 3: Individual harmonics after the installation of the Schaffner active harmonic filters
Figure 4: Voltage and Current after the installation of the Schaffner active harmonic filter