Is My Grit Really Being Removed? - Measuring the Elusive Grit Particle
In the article “Going the Extra Mile – Grit Washing Saves Money”, we explored several ways handling and cleaning captured grit makes a difference to the bottom line. This is even more the case with retaining the grit through the cleaning and classification process. Now we will look at techniques to identify what kind of grit is present in a given system. Knowing grit qualities and quantities will help determine how to select the right technology.
Much has been written and studied on most processes used in treatment of wastewater. The article Simple Steps to Save: The Economics of Clean explored how, as more was being asked of a WWTP, it transformed into a WRRF. A closer examination of the dynamics within the system was needed as process performance expectations increased. The same can be applied to the grit process specifically.
Current Status of Grit Knowledge
Up to very recent times, comparatively little was known about the true makeup and behavior of sedimentation commonly known as grit. Early on, the settling characteristics of grit were documented as Type 1 Discrete Particle Settling. In Metcalf & Eddy 2nd Edition 1979, grit was described as “…consisting of sand, gravel, cinders, or other heavy solid materials that have subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids,… Grit also includes eggshells, bone chips, seeds, coffee grounds, and large organic particles, such as food waste.”
As attention moved towards removing finer sizes of grit it became critical to understand and quantify, not only the particle fraction distribution of the raw incoming grit load, but also the particle behavior itself. Early designs of grit capture technologies used settling rates in the designs that were similar to that of a clean sand particle: a specific gravity of 2.65.
Real-World Conditions Skews Conclusions
While it greatly simplifies the theoretical design process of a grit system to view the targeted grit as a simple sand particle, this does not reflect the actual conditions occurring within a given wastewater treatment process.
There are a number of variations that can occur related to the particle targeted to settle out of the waste stream. One example would be the shape of the particle. A flat or flake shaped particle can affect and change the rate of settling within the chamber.
The 2010 WEF Publication “Design of Municipal Wastewater Treatment Plants” pointed towards an average bulk density of 1.44 of dewatered grit. This data seemingly challenges the assumption of using a 2.65 specific gravity as basis for sizing a given grit removal device. One justification for this change is the presence of Fats, Oils, and Grease.
In many cases, the Fats, Oils, and Grease (FOG) in wastewater come in contact with the grit particle. As the particle gets coated by the FOG, it can change the buoyancy of the actual particle. This in turn changes the behavior of the particle resulting in changing and slowing its real-world settling rate.
SES - Changing Settling Rates Due to Particle Coating
The implications of a FOG coated particle are: A larger particle can behave like a smaller particle by affecting the velocity at which the particle settles out. This can affect what a given grit chamber design is actually capable of capturing and extracting. In this case it would be possible to miss a larger particle falling below a defined design range of the grit chamber because the particle is actually settling out at a rate similar to a finer particle that it is not designed to remove.
In order to better understand and characterize the real-world particle behavior, a method has been developed that assigns a value to the real-world settling rates of grit particles. By comparing observed settling rates as it relates to the theoretical settling rate of an equivalent sized clean sand particle, a more accurate distribution can be understood and designed for. This method is identified as Sand Equivalent Size (SES).
Knowing the SES of the particle distribution as it approaches the finer range of removal (200 micron to 75 micron) becomes critical to meeting performance expectations of a grit chamber design. The smaller the FOG coated particle is, the more of an effect there is on the settling velocity. This method is described in Chapter 3 part 5.2 in Guidelines for Grit Sampling and Characterization.
Importance of a Measuring Standard
By accurately understanding the nature and makeup of the grit entering a wastewater treatment facility, it is possible to more finely tune high performance grit chamber designs that can target reliably down to very fine grit particle removal (>75 micron).
Timing is critical when a sample is drawn. The grit loading will vary throughout the day. An initial flushing event is when grit can enter the plant in slug loads. It is important to characterize the sampling to provide a composite of the day. This can also be affected seasonally. This slug-loading situation can be a more critical concern with gravity systems as opposed to pumped systems where the act of pumping provides predictable flows and velocities.
A full size WWTP installation can be a challenge to properly identify and quantify the makeup of the grit entering the facility. Hydraulics and channel configuration can radically affect the ability to properly extract representative samples. Ideally, a turbulent homogenously mixed sample site is desired.
Currently there are two basic types of testing techniques most commonly employed for a large plant sampling and analysis. There are different schools of thought regarding effectiveness and accuracy of one testing method over the other. Selection of these approaches will strongly depend on the hydraulics of the site, channel geometries, and the specific technology being employed in the design. Consensus on method among all stakeholders will be essential.
- Cross Channel Sampling and Dry Sieving:
This method uses a small probe with a minimized profile that can be positioned at multiple points within the water column. This approach is best used in sample locations where hydraulics allows stratification of the grit in the flow.
- Vertical Integrated Slot Sampling and Wet Sieving:
This method uses a vertically oriented slotted tube allowing for multiple elevation extraction at a single position. This approach is best used in a sample location that is well mixed.
It should be noted that whatever method is used in gathering of the sample and subsequent analysis, identical methods and conditions should be used for comparing performances of technologies being considered for a given site.
Knowledge of Grit Allows for Good Design
A team of thirty-five environmental engineers and scientists formed a Grit Sampling and Characterization Task Force to develop the publication Guidelines for Grit Sampling and Characterization published in 2017. This work presents a solid discussion of industry best practices for grit sampling and characterization. They also identified key areas needing further research and development.
As these approaches and methods outlined in this publication are adopted and applied in the early stages of design, it will create a solid basis by which highly effective and efficient fine grit removal technologies can be properly designed, implemented and evaluated.
By researching and testing, knowledge becomes a powerful tool for solving downstream grit accumulation issues. Additionally, these considerations become more relevant as the quest to remove a smaller size grit particle unfolds.