How does wastewater grit behave?
Grit removal systems are effective only when their design is based on a scientific understanding of how grit actually behaves in wastewater.
Grit in wastewater is a complex phenomenon. The way grit behaves is influenced by a wide variability in particle size, shape and specific gravity each of which impacts the all-important settling velocity.
Recent research projects, both in the field and the laboratory, suggest that grit particles behave very differently to how many designers and operators expect – they settle more slowly than their physical size alone would indicate.
Conventional grit removal design methods therefore frequently overestimate settling velocity by wrongly targeting grit particles larger than 212 micron. They are based on accepted industry conventions that assume that grit behaves like perfect spheres of silica sand with a specific gravity of 2.65, settling quickly through clean water.
Because of this assumption, many systems may only be removing 30-50% of incoming grit. In many cases, operators may simply be unaware of how much grit they are missing.
Real Life Conditions
In the real world, conditions are very different to ‘ideal’ assumptions and vary significantly between plants. Grit gets into incoming raw sewage from a variety of sources, e.g. wind-blown sands and dust, surface water run-off over soil and hard surfaces, and collection system degradation.
Local geography, weather patterns and winter degritting activity can all impact on how grit behaves. Certain locations may be known for fine grit, ‘sugar sand’ or loess soils (a result of wind erosion in regions characterised by high winds) and may therefore require more attention to finer grit removal. The age and design of the collection system will also influence how grit arrives at the treatment plant.
Wet weather results in peak flow and peak collection of entrained grit. In locations where there is a wide variation in flows, grit can be expected to settle throughout the collection system during low flow periods. During peak events, this settled grit will be re-suspended in the flow and will arrive at the treatment works in concentrated amounts that can overwhelm grit removal systems that have not been designed to cope with these conditions.
Grit that passes through the removal system compromises effective wastewater treatment downstream through the combined effects of abrasion of mechanical equipment and accumulation in channels and basins. Ineffective grit removal leads to process inefficiencies and disrupts biological processes. This increases energy demand and can even threaten final effluent quality and discharge limits.
The following graphics demonstrate the measured discrepancies between the settling velocities of clean sand and grit sampled from municipal wastewater treatment plants.
What is wastewater grit?
Conventional grit removal equipment is designed based on the mistaken assumption that all grit particles are perfect spheres of silica sand greater than 212 microns in diameter and with a specific gravity of 2.65.
In reality, wastewater grit is constituted not only of silica sand, but also of other materials with different specific gravities such asphalt, limestone and concrete, as well as eggshells, coffee grounds, seeds, bone fragments and other organic food waste particles of all sizes.
Grit particles vary in shape and are not all spherical; some are flat and some are angular, so they will settle differently than a sphere. While in the sewer collection system, fats, oils, greases, soaps and scum attach to the grit particles and alter their settling characteristics.
Real grit behaves very differently compared to clean sand, and accurately predicting how it will settle in wastewater requires a scientific approach. It is not uncommon for large grit particles – i.e. 300 micron and larger – to settle in the same way as a much smaller particle, as their shape, size and specific gravity all affect the settlement rate.
The differences in the characteristics of grit particles and sand particles are clearly seen under digital particle analysis.
Delivering an effective grit removal system begins with an analysis of both the physical size gradation and settling velocity distribution in the inflow.
A grit characterisation study is highly advisable to fully understand a plant’s specific design requirements and to achieve grit collection engineered to meet that site's needs. The distribution of physical particle size and settling velocity should both be measured. Care must be taken in collecting the sample to ensure that it is representative of all sizes of incoming grit and flow conditions such as weather, seasons, inflow channel shape and depth.
To collect fully representative grit samples, the Advanced Grit Management process uses a whole-channel sampling method known as VIS (Vertically Integrated Sampling) to give a true profile of grit particles in the inlet channel across all heights and flow rates.
Samples are then sieved through different pore size sieves, and their size distribution is assessed by weighing the collected particles in each sieve size. Once sampling is complete, the size gradation and settling velocity distribution provide accurate data upon which to base a system design.
There are no standardised methods to sample or characterize grit, which is why the Advanced Grit Management philosophy has been developed and refined by Hydro International during more than 35 years of laboratory and real-world research and operational experience.
We have developed a better understanding of grit behaviour using a variety of measuring and observational techniques. To complement real-world observations Computational Fluid Dynamics techniques have been refined to accurate simulate realistic particle behaviour. Our collaboration with academic institutions and use of independent third-party field testing has also contributed to a robust and proven approach.
Hard-earned knowledge of grit characteristics and best practices gives Advanced Grit Management practitioners a matrix of unit processes and a wide range of tools capable of matching and solving site-specific requirements.
Operators also have the option to install pilot units to sample, test and refine the treatment approach in situ prior to the design and installation of a full system.
This graph shows the size distribution of grit measured in a number of wastewater treatment plants in the US; in many plants 50% of the incoming grit is smaller than the conventional 212 micron design cut point. To remove 90% of the incoming grit, the design cut point needs to be changed to somewhere between 75 and 150 micron, depending on the actual size distribution.
Every plant's grit removal should be approached differently, based on the understanding that a grit removal system should be designed to target the range of particle sizes and flows specific to that site.
The design intent of grit removal systems all over the world has been based for many years on Metcalf & Eddy, Water Environment Federation Method of Practice #8 and similar guidance such as the WIMES committee in the UK. This sets a minimum grit separation efficiency of 95% at the specified flow rate of grit particles greater than 212 micron in diameter with a specific gravity of 2.65. These standards all go back to an outdated industry design practice established 70 years ago.
Advanced Grit Management challenges this convention: once the size and settling velocity distributions are established through grit sampling and characterisation, the designer can set a percentage removal efficiency target based on the minimum particle size to be removed.
Sand Equivalent Size
Because the grit size, shape and specific gravity are all so varied and affect settling rates, it is important to convert the grit characterisation results into data that can be easily be compared. The Advanced Grit Management philosophy uses a simple process that converts all data to a Sand Equivalent Size (SES).
So, for example, where a large, light grit particle settles at the equivalent rate of a small sand particle, it has the SES of the small sand particle. An allowance is also made for the difference between the design flow rate and particle size and the actual flow rate when the sample is taken.
Most conventional grit removal technologies target a particle size of between 212 and 300 micron. On average, however, roughly 56% of grit entering wastewater treatment plants is smaller than 300 micron, while almost 40% of grit entering the wastewater treatment plant is smaller than 212 micron. (Figure 1)
Based on SES, even less of the grit settles as though it is a 300 or 212 micron particle. Nearly 60% of grit entering wastewater treatment plants settles as though it is a smaller particle than 212 micron.
Figure 2 shows that the physical size and settling velocity curves converge in the 75 to 150 micron range, often near 106 micron. This, along with the settling velocity gradations, indicates this is the optimal design range for effective grit removal. Grit systems designed to remove grit particles 106 micron and larger generally remove 85-95% of all grit entering the plant.
With Advanced Grit Management, systems are generally designed to achieve >95% removal efficiency while targeting grit particles with a minimum size between 75 and 150 microns. Grit removal systems that have been based on Advanced Grit Management typically remove 85-95% of the total amount of grit entering the plant.