Liquid Stream Fundamentals: Grit Removal

A common challenge for all wastewater resource recovery facilities (WRRFs) is the removal of grit from the wastewater early in the treatment process. Grit is generally composed of sand, gravel, cinders, and other heavy solid material with specific gravities substantially greater than the organic matter found in wastewater. It is important to remove grit from the wastewater to protect moving...

The quantity and characteristics of grit entering a WRRF varies based on a number of factors, including the type of collection system (combined or separate), characteristics of the drainage area, use of household garbage grinders, condition of the sewer system, sewer grades, and types of industrial waste. In most cases, grit accumulates in the collection system due to its faster settling...

The most common types of grit removal technologies are summarized in Table 1 below.

One of the earliest types of grit removal systems, horizontal flow (also known as velocity controlled) grit chambers use physical controls and chamber geometry, such as proportional weirs and rectangular control sections, to vary the depth of flow and maintain the velocity within the chamber at a constant 1 foot per second (fps). Operational experience has shown that this velocity allows heavier...

Aerated grit removal tanks are constant liquid level, short detention time rectangular settling chambers. Air is introduced along one side of the tank through submerged diffusers which creates a spiral flow pattern that moves perpendicular to flow through the tank. The velocity of the spiral flow pattern controls the size and specific gravity of the particles that will be removed. If the velocity...

Vortex grit removal tanks are circular in shape and rely on a mechanically induced vortex to capture grit solids in a center hopper. The hydraulic flow pattern created by this shape forces the grit to the chamber floor within one revolution inside the chamber. Incoming flow straightens in the inlet flume to minimize turbulence at the inlet of the chamber. At the end of the inlet flume, a ramp...

Detritus grit removal tanks are constant liquid level, short detention time settling chambers. They are typically square in shape and relatively shallow, approximately 18 to 36 inches deep. Influent wastewater is distributed along one edge of the tank and typically flows over an effluent weir along the opposite side of the tank, creating a cross tank flow pattern. Grit settles along the tank floor...

Multi-tray vortex technology was introduced in 1999 as a means to target fine grit in an effort to increase overall system capture efficiency. Conically shaped trays are stacked vertically in a stainless steel support frame. An inlet duct delivers influent onto the trays tangentially while evenly distributing flow to each tray. Low surface overflow rate and short settling distances allow capture...

Hydraulic vortex units are centrifugal separators relying on accelerated gravity to remove grit from influent. Capture of grit particles ≥ 106 micron are retained. A cylindrical above ground tank with conical bottom receives a tangential feed where energy of the influent flow creates a vortex with no moving parts. Grit moves to the tank wall and settles by gravity in the bottom cone where it...

As liquid stream grit removal processes were established, the removal of grit from primary sludge became less common. Today, sludge degritting is mainly used in cases of poorly performing liquid stream processes. Devices used for washing and dewatering grit slurry are used to remove grit from sludge for less cost than installing new liquid stream grit removal processes. Conventional...

Grit pumping is the most commonly used method for transporting collected grit. The following items should be considered when designing a grit pumping system:

Cyclone systems use centrifugal forces to separate grit from organic material in grit slurries. Heavier grit and suspended solids concentrate along the sides and bottom of the cyclones, while lighter material exits through the top. Cyclones operate best at constant flow and pressure. Sizing is based on feed flow rate and a target grit slurry solids concentration, ideally less than 1%. The cyclonic...

Conical grit washers utilize stainless-steel, conical-shaped vessels to capture grit slurry. Rotating arms within the vessel slowly mix the settled grit and a washing jet at the bottom of the unit vigorously washes it. Lighter organic material continuously overflows the unit and heavier organic material is blown off at regular intervals from a midlevel overflow. Washing provided by these units is...

Grit can be conveyed directly to trucks, dumpsters, or storage hoppers. Containers should be covered to prevent odors during storage and hauling. Conveyors frequently are used for transporting grit from handling facilities to containers. Overhead storage hoppers that discharge to truck containers avoid the need to keep a truck at the facility.

WEF Municipal Resource Recovery Design Committee