Meters based on microwave transmission are used in the food & beverage, pulp & paper, and water & wastewater industries to measure a mixture’s total solid content, for example in sludge treatment. Applications in wastewater treatment plants are clarification, digestion, and dewatering. While total solids are often measured through lab samples, there is a growing preference for continuous inline measurements, which is more efficient and does not result in delays.

While other sensor technologies for measuring total solids are dependent on the mixture’s turbidity, limited to a certain total solids range, or require high maintenance, measuring devices using microwave transmission are independent of color or turbidity, maintenance-free, and cover a range of 0 to 50% total solids.

Microwave transmission: measurement principle

Microwaves passing through a medium are electromagnetic waves that interact with matter at the molecular and atomic levels. These interactions result in absorption and changes to the speed of wave propagation. From these physical properties, the permittivity of the medium can be calculated, which in turn allows the determination of certain quantities in the medium, like the total solids content.

Permittivity is a measure of the electric polarizability of a dielectric. A medium with high permittivity polarizes more in response to an applied electric field than a material with low permittivity.

In media with high total solids content, microwaves travel faster than in pure water. Their time-of-flight is correspondingly shorter, and their attenuation is less. This is because the polarization properties of its molecules give water a high permittivity. This differs from the total solids commonly found in wastewater sludge, which exhibits a relatively low permittivity.

Benefits

• Reliable and permanent process control through real-time and continuous inline measurement
• Independent of color/turbidity or other characteristic of sludge mixture
• Low in maintenance and timesaving, ensuring higher productivity
• Covering a broad total solids range, from primary clarification to dewatering