By Rich MacGregor
The rising demand for high-efficiency grit removal systems is driven by the need to protect plant infrastructure. This trend is particularly noticeable with the increasing adoption of technologies such as membrane bioreactors (MBRs), specialized pumping equipment, and fine bubble diffusers.
Effectively removing ultra-fine grit can yield a substantial return on investment by preventing line blockages, as well as deposition in downstream channels, process basins, while reducing the fouling of diffusers and membranes. This, in turn, extends the lifespan of downstream equipment, significantly lowers maintenance costs, and enhances overall operational efficiency.
Ultra-fine grit can also present significant challenges to aeration systems. Accumulated grit disrupts oxygen circulation, decreases the efficiency of biological treatment, and increases compliance costs associated with effluent regulations. This buildup limits the volume necessary for optimal retention times in biological treatment and sludge digestion.
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The necessity to tackle ultra-fine grit differs by region and season. Geographic location and environmental factors significantly affect the grit size distribution encountered by wastewater treatment facilities. Coastal regions and agricultural areas face elevated levels of ultra-fine particulate matter, and seasonal factors such as winter road salt and sanding also influence particle size distribution.
As the diameter of grit decreases, the challenge of removal increases. Currently, there are two approaches to ultra-fine grit removal available in the market, each employing a distinct method.
Stacked tray systems
Around 15 years ago, the stacked tray system was developed, raising expectations for the removal of ultra-fine grit through gravity-based separation. This method utilizes a series of vertically-mounted trays to expand the surface area in a square settling tank, allowing grit particles to settle gradually from wastewater.
The design features a notable head loss throughout the system, primarily due to the inlet distributor, which evenly disperses the incoming flow across the multiple trays (up to 300 mm). Typically, a continuously operating remote pump is used to remove the grit that accumulates at the bottom of the tank.
While high removal efficiencies are touted for particles as small as 75 microns, efficiencies are typically derated down to 85% during peak flow conditions in many of the installations. At such peak flow rates, manufacturers of stacked-tray systems will often only guarantee performance specifications for the removal of particles as small as 212 microns.
Stacked-tray systems are sized and designed based on surface area and particle settling rates, which determine the number of trays and tank depth required. Increased flow demands more surface area to sustain a surface overflow rate (SOR) that effectively removes fine and ultra-fine grit.
Practical limitations on available surface area restrict system capacity. The largest stacked tray systems can accommodate a maximum of approximately 50 million litres per day (MLD) for 75-micron removal. To manage larger flows, multiple systems are necessary: 75 MLD requires two systems, while 190 MLD requires four systems. This not only increases the footprint significantly, but also necessitates the duplication of ancillary equipment, such as pumps and grit washing and dewatering systems.
Integrating forced vortex technology with inclined plates
As the market began to recognize the importance of ultra-fine grit removal while grappling with the capacity limitations of stacked tray systems, engineers at Smith & Loveless Inc. (S&L) developed a scalable system that could provide consistent and efficient removal of ultra-fine grit across all flow rates, without any performance degradation at low, average, or peak flows.
They developed the PISTA® INVORSOR® system, which can achieve 95% grit removal at 75 microns across all flow conditions, from low to peak flows, accommodating flow up to 190 MLD per chamber.
It builds on the features of S&L’s PISTA® grit removal system, which can capture 95% of particles down to 105 microns.
Forced hydraulic systems aren’t settling devices like most other grit systems. Instead, the chamber geometry with patented baffles regulates and optimizes flow velocities to generate a true forced hydraulic vortex path that effectively removes grit.
To accomplish this hydraulic action, the systems combine a specifically inclined inlet flume, a circular chamber with a unique flat-floor upper chamber, and a lower collection hopper for intermittent pumping.
The INVORSOR features an additional stage to remove ultra-fine particles by augmenting the hydraulic forced vortex with integrated inclined parallel plates. Flow transitions from the hydraulic vortex to an upper hopper, optimized through computational fluid dynamics to effectively remove particles as small as 75 microns. Inclined plates in the upper chamber disrupt the vortex, facilitating a sedimentation process under quiescent conditions.
By augmenting the hydraulic forced vortex with the upper chamber of the inclined parallel plates, the INVORSOR design can achieve a single system capacity up to 190 MLD while maintaining a grit removal efficiency of 95% at 75 microns.
Flow and level control
Scaling up to accommodate higher flow capacity is one key reason why the INVORSOR can effectively replace multiple stacked tray systems. Another crucial factor is its level control, which enables the automatic maintenance of optimal velocity to consistent high-efficiency grit removal across a range of conditions, from minimal flow to peak flow.
Maintaining the optimal velocity is essential for effective grit removal. If the flow rate falls below this threshold, grit will settle excessively, leading to accumulation in the approaching channel (i.e., grit trapped prior to the grit chamber).
No additional downstream control devices are required to keep the velocity in the preferred range during both peak flow and minimum flow, as the system possesses up to 10:1 turndown.
Rich MacGregor is a technical writer for Smith & Loveless Inc.
For more information, visit: www.smithandloveless.com
This article appeared in the February 2025 edition of ES&E Magazine.
