Grit-filled aeration basin. |
What is Advanced Grit Management?
Collaboration between experienced plant operators, consulting and design engineers, as well as important academic research centers have pinpointed some of the most important factors and led to a range of AGM solutions. The term refers to an approach to grit removal whereby systems are designed to achieve 95 percent or more removal efficiency while targeting grit particles of 75 to 150 microns. Grit removal systems that have been based on AGM typically remove 85 to 95 percent of the total amount of grit entering the plant.
Slow to be adopted initially, the underlying scientific principles have gradually become accepted. Installation of AGM techniques is accelerating with close to 500 installations based on this approach operating in North America.
The Essential Starting Point
The first premise of AGM is that incoming grit has a wide variability in particle size, shape and SG - each impacting the all-important settling velocity. This modifies the concept of an 'ideal,' or uniform, grit size by utilizing more realistic assumptions and is based on extensive sampling and observation at a wide range of plants across North America. The distribution of these particle variables in the incoming load at different plants serving different areas and in different seasons has major implications for the effectiveness of removal.
For example, as much as 70 percent or more of the grit load can be smaller than the standard 212 microns diameter presumed by conventional design. Conventional removal would, therefore, miss most of it. This reality means that profiling the incoming grit at a specific plant, and designing the grit removal system for fine particle removal while considering the settling velocity, can give a true measure of expected removal efficiency.
Settling, Size and Specific Gravity
Recent industry research projects have compared wastewater grit and clean sand settling characteristics. Combined with experience in the field, the findings indicated that actual grit particle settling velocities are significantly less than physical size alone would indicate. For example, municipal grit settles more slowly than would be conventionally expected based on its physical size, providing strong evidence that simple physical particle size analysis is not sufficient to accurately characterize municipal grit.
The importance of this lies in the fact that settlement time is a basic process design parameter, and if settlement is slower than predicted, more particles will be left in suspension to cause problems downstream.