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Grit causes abrasive wear in mechanical equipment such as pumps and dewatering screws, and components like bearings, chain links and seals. Reduced performance or even failure may lead to expensive shutdowns and early equipment replacement.
Grit can cast a long shadow over wastewater treatment. It can be a stalking menace which, if not removed effectively at the inlet works, interferes with plant and equipment downstream causing significant and costly damage.
Yet, while operators the world over naturally expect their plant to treat incoming effluent-charged water to consistently high standards of engineering performance, until more recently many have accepted much less robust approaches to removing grit.
Now a new philosophy of grit removal is providing a scientific foundation for reviewing grit system design and pioneering the concept of ‘Advanced Grit Management’. This new understanding is prompting operators to review their assumptions about the way grit behaves in their treatment plants.
Settling out at low flow points downstream, grit can cause pipe blockages and depth reduction in channels and tanks. Carrying with it organic matter, the grit also restricts oxygen penetration leading to a reduction in aerobic treatment efficiency.
Depending on the local climate and conditions, grit can be present in substantial amounts. Operators are often dissatisfied with their grit removal systems, but may not be clear as to why they are not performing as required.
To achieve effective grit removal it is important to understand the nature and characteristics of grit arriving at a treatment plant and how it actually behaves in wastewater. An effective system design will address both size and settleability, produce a clean, dry product for disposal and minimize deposits and accumulations in the plant.
Grit particle complexity
Grit behavior in wastewater flows is a complex phenomenon and governed by a number of factors such as size, specific gravity, shape, tendency for agglomeration with other wastewater constituents and by environmental conditions such as wet weather.
Settling velocity is frequently overestimated as conventional design guidelines target removal of grit larger than 210 micron and grit particles have been assumed to be perfect spheres of silica sand with a 2.65 specific gravity (SG) settling fast through laminar conditions in clean water. Such idealized assumptions do not represent the true behavior of influent grit in a wastewater treatment plant.
To deliver an effective grit removal system it is important to first understand the size and composition of incoming grit, as both the size and the settling velocity of the grit particles will determine an effective design. Utilizing both size distribution and settling velocity distribution enables the designer to establish a removal efficiency target and determine the minimum particle size to be removed.
Minimum grit particle size
Where possible, a set of grit characteristics should be developed specific to the wastewater flow being treated. Where not available, for example in new treatment plant construction, a cut point designed at 75-106 micron will generally remove 85-95% of the grit entering the treatment plant.
Certain locations may be known for fine grit, ‘sugar sand’ or loess soils (a result of wind erosion in regions characterized by high winds) and the finer cut point of 75 micron should be used. Many other areas may achieve adequate results with a 106 micron cut point.
Meanwhile, the larger, heavier grit that is transported to the treatment plant during higher flows seen during diurnal flow cycles, seasonal variations and spikes in flow from wet weather events, is easily captured.
To show that the conventional design criterion for removal of 210 micron has allowed passage of large quantities of grit into wastewater treatment plants. Many US plants, 50% of the incoming grit is smaller than the conventional design cut point of 210 micron. To modify design criteria to remove 90% of the incoming grit, the design cut point needs to be changed to somewhere between 75-150 micron, depending on the actual size distribution.
The conventional design assumption that grit settles like silica sand with a specific gravity of 2.65 is too simplistic. Wastewater grit may consist of asphalt, limestone, concrete, slowly degrading organics and various other materials that rarely have a specific gravity of 2.65.
Grit particles vary in shape, are rarely spheres and many plants have noted that much of their larger grit is flat. An angular particle will settle more slowly than a sphere as its drag coefficient is higher so designs allowing for such settling are desirable.
While in the collection system, the grit particles are exposed to a variety of common materials including fats, oils and greases (FOG), soaps, scum and some chemical constituents that can attach to the grit particles and alter the particles’ settling characteristics.
Grit washing and dewatering
Once collected, the grit must be washed and dewatered in order to produce a clean, dry product for landfill. Washing and dewatering must each be as effective as the collection device otherwise the overall system efficiency will suffer. A recent study at a plant showed that while the aerated grit basin removed 58% of total entering grit volume, the cyclone/screw classifier washing and dewatering equipment only retained 17% of what it received. The loss of grit reduced the system’s overall removal efficiency to only 10%.
Each grit removal system developed for a wastewater treatment plant is different and will require its own design solution. To achieve grit collection engineered for a specific treatment site, it is preferable to first measure the grit size distribution and settling velocity. Care must be taken in collecting the sample to ensure it is representative of all sizes of incoming grit and flow conditions such as weather, seasons, inflow channel shape and depth. Once sampling is complete, the size distribution and effective settling velocity or effective specific gravity of the grit sample must be determined. Both characteristics are needed in order to have accurate data upon which to base a system design.
In the absence of site specific information, a conservative approach is to base the design on the smallest practicable particle size which would typically be in the 75-106 micron size range.
Recent surveys over a range of treatment plants and localities in the US have confirmed the variation in particle specific gravity, and the effect of organic material on specific gravity through agglomeration with grit particles. They confirm that bulk density and specific gravity of wastewater grit are lower than for clean silica sand, indicating a shift in design considerations.
Newer editions of design manuals such as the United States’ Water Environment Federation Manual of Practice are recommending targeting particles smaller than 212 micron in grit system design and many grit removal systems being designed today are targeting removal of particles in the 75-106-150 micron size range.
In North America, Hydro International is pioneering the philosophy of Advanced Grit Management with a stated objective of 95%+ removal efficiency of particles in the 75-150 micron range, and 85-95% efficiency of the total grit load entering a treatment plant.
A new website www.AdvancedGritManagement.com is providing a clearinghouse for science-based information on grit removal design which can be used by engineers, operators and owners to select the best solution for their project's needs.
Key to this approach has been the intensive study of grit parameters and behavior, and the design of grit removal solutions with a small footprint, low headloss, low power use and low operating costs.
The HeadCell from Hydro International is a modular, multiple-tray settleable solids concentrator that achieves high-performance removal of fine grit as small as 75 micron.
The HeadCell eliminates the inefficiencies of conventional forced vortex grit removal systems. Its stacked tray design optimizes the capacity for grit separation within a small footprint – making it ideal for both new and retrofit applications.
The Hydro Grit King is an advanced hydrodynamic vortex separator that augments gravitational forces to separate grit from water with minimal headloss. Its small footprint design uses no power and has no moving parts, achieving low operating and maintenance costs.