Headworks Equipment for Efficient Screening and Vortex Grit Removal

Grit is heavy mineral matter consisting of a variety of particles including sand, gravel, cinder, and other heavy, discrete inorganic materials typically ranging from 100 to 300 microns in size that can cause problems for a variety of equipment and process basins in municipal wastewater treatment plants and water resource recovery facilities.

Grit removal systems are designed to remove grit particles from wastewater streams at municipal wastewater treatment plants and water resource recovery facilities. Not every grit removal system is the same nor achieves the same removal efficiency. The most effective systems will remove 95 percent of grit particles sized down to 75 microns (mostly for coastal regions) and 100 microns for most other locations. The systems are typically advanced vortex grit chambers with complimentary grit dewatering or washing systems that can maintain the removal efficiencies through the entire connected system.

In recent years, the significant budgetary pressures felt by designers and operators have resulted in an increased emphasis on protecting equipment infrastructure and improving operational facilities, two of the primary functions of any facility. The escalating application of membrane bioreactors (MBRs), specialized pumping equipment, and fine bubble diffusers drives investment in grit removal systems and other types of protective headworks’ equipment, which maximize the performance of these newer technologies.

If not effectively removed by a grit removal system, grit accumulates in downstream aeration basins, clarifiers, and digesters, requiring manual removal by staff. Therefore, investment in grit removal is an effective way for sites to minimize long-term operation costs. Abrasive grit can scour and plug lines and foul specific types of equipment like diffusers and membranes. In general, the effects of accumulated grit will reduce the intended service life of most types of wastewater equipment. A two percent increase in grit removal efficiency translates to tons more grit removed per year and, therefore, significant savings in handling and disposal costs.

Grit removal system approaches include a basin, channel, and/or chamber that reduces flow velocity, allowing inert grit particles to be hydraulically removed or settled out. From a technological standpoint, various approaches range in sophistication, from rudimentary settling basins and channels to aerated chambers to advanced forced vortex systems.

These methods continue to evolve because time and experience are the best teachers. Likewise, ongoing research and development by market leaders, the emergence of computation fluid dynamics, and intensifying industry competition drive system advancements. Today, the “vortex” grit chamber concept dominates the installation landscape, occurring in different forms: sloped-floor settling basins, stacked-tray settling basins, and flat-floor grit chambers (also known as hydraulic forced vortex or “PISTA®”).

Conventional grit removal systems tend to offer removal efficiency of 95 percent for particles larger than 300 microns, 85 percent for particles between 300 and 210 microns, and 65 percent for grit smaller than 210 microns. But recent developments in hydraulic control technology have paved the way for advanced vortex systems to provide the finest grit removal, including a 95 percent removal efficiency across all flows for grit down to 75 microns. Offering high turndown ratios, these systems do not experience diminished efficiencies during low or peak flow conditions.