What is Lamella?
Lamella, or lamellae, is a term frequently used when discussing sedimentation and clarifier design. Lamella clarifiers can be used in applications ranging from municipal water and wastewater treatment to various industrial applications. The primary benefit of utilizing lamella clarifiers over conventional clarifiers is the reduced footprint. However, the term “lamella” is often used too broadly in the industry without clear intention.
According to Merriam-Webster, lamella is defined as “a thin flat scale, membrane, or layer”. This is anatomically equated to the gills on the underside of a mushroom, for example. But how and why is this term used when referring to tube settlers and plate settlers? Let’s provide some clarity (pun intended!) to the term.
Lamella in the Sedimentation Process
In 1904, American engineer Allen Hazen defined some rules for sedimentation. He observed that a particle in water can only settle down if its settling velocity is higher than the result of flow divided by tank area. This is also known as surface loading. By increasing tank area, we decrease surface loading and therefore increase the settling performance of the solids. Aside from building larger tanks to increase tank area, more horizontal surfaces could be installed to create a larger settling area within a smaller footprint:
The addition of these horizontal surfaces, or flat layers, are where we derive the term lamella in the context of clarification and the sedimentation process. While this concept is important, horizontal layers are not very practical. To create an increased effective settling area, these surfaces are placed on an angle:
There are two main technologies for enhanced clarification that are based on Hazen’s law: tube settlers and plate settlers. Both technologies adopt the same principle of solids settlings. This may cause a bit of confusion when discussing clarifiers and the implementation of lamella. Simply requiring lamella for a clarification process leads to ambiguity. Tube settlers and plate settlers can both be viable options in some applications, but not all. It’s important to specify whether tube settlers (lamella tubes) or plate settlers (lamella plates) are to be used.
To understand more about the respective characteristics and design parameters of each technology, check out our blog: Tube Settlers vs. Plate Settlers: Comparing Lamella Technologies.