​Grit and other solid materials are a costly component of both process water and wastewater, clogging systems, reducing efficiencies and causing abrasion damage and wear that lead to increased cleaning, maintenance and repair.

What is Grit Removal?

Grit removal is the process used to remove sand, silt and grit from water.

Grit (and sand) removal is often found in the headworks or inlet works of wastewater treatment plants (WWTPs). Grit removal can also be used to remove sand from river water intakes prior to processing for potable water, used in industrial applications to remove fine abrasives, as well as being used to remove grit entrained in sludge.

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 consists not only of irregularly shaped 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.

Why Remove Grit?

Sand in treatment plants reduces process capacity and increases maintenance costs.  

In WWTPs grit and other solid materials such as sugar sands and silt are a costly component of both process water and wastewater, clogging treatment systems, reducing efficiencies and causing abrasion damage and wear that leads to increased cleaning, maintenance and repair. 

Conventional assumptions about the nature and behavior of grit mean that many grit removal systems may only be removing 30-50% of total suspended solids (TSS). That material is passing downstream, abrading critical systems and processes and gradually reducing the overall effectiveness of the facility. In many cases, operators may simply be unaware of how much grit they are missing. 

Effective grit removal removes the abrasive solids and sand before they have the chance to enter other processes, erode expensive equipment, and deposit throughout a treatment plant. 

What is the Cost of Grit?

Grit that passes through a WWTP or industrial process facility has a cost. If not removed, grit compromises downstream processes through the combined effects of abrasion of mechanical equipment and accumulation in channels and basins.

Ineffective solids removal leads to clogging, operational inefficiencies and disrupted processes such as biological treatment. This in turn increases energy demand, increases the maintenance and repair burden and can even threaten final effluent quality and discharge limits.

Learn more: Do you know how much inefficient grit removal is really costing you?

Real-World Conditions

While laboratory testing is useful as an indicator of performance, it should never be taken in isolation. After all, WWTPs and industrial facilities operate in the real world, not in a lab.

In the real world, conditions are very different from the conventional 'ideal' assumptions and vary significantly between plants. Grit and other solids make their way into influent from a range of sources, ranging from wind-blown sands and dust to rainwater run-off over soil and hard surfaces such as concrete and bitumen.

Local geographical and climatic patterns can have impact on the nature of influent grit. Certain locations may contain higher levels of fine grit, 'sugar sand' or loess soils, and may require greater attention to finer grit removal.

In locations where there is a wide variation in flows, grit can be expected to settle during low flow periods. During later peak events, 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.

Owners and operators should question performance claims, should insist on independent testing, and should be prepared to conduct real-world performance evaluation.