Grit collector for waste water treatment facility

A grit chamber for removing grit and other dense inorganic materials from a waste water stream in a waste water treatment facility. The grit chamber includes an upper settling chamber and a lower grit collection chamber, separated by a divider with one or more holes therethrough near the center portion to allow grit to pass. The settling chamber is substantially cylindrical, but with a transition curve connecting the inlet to the cylindrical portion, to assist in the production of a forced vortex with a minimum of turbulence.

FIELD OF THE INVENTION 
The present invention relates to the field of grit collectors for use in 
waste water treatment facilities. In particular, the invention relates to 
a grit collector utilizing an approximately cylindrical settling chamber 
with and involute curved inlet portion to produce a flow pattern such as a 
forced vortex. 
BACKGROUND OF THE INVENTION 
Waste water entering a waste water treatment facility normally includes a 
substantial quantity of sand, grit and plastic, in addition to suspended 
organic matter. One of the first steps in treating the waste water is to 
remove these non-organic solids so that the waste water can then be 
treated by oxidation and biodegradation techniques without the non-organic 
solids fouling or wearing out the mechanical components of the facility. 
Frequently, the plant design requires that inorganic solids having a 
specific gravity of 2.65 and higher and a size of 65 mesh or larger should 
be removed before further treatment of the waste water. 
Although it is important that the non-organic solids be removed early in 
the treatment process, it is equally important that the organic solids 
such as sewage sludge not be removed at that point. This is because, at 
that point, the waste water has not been subjected to the oxidation and 
biodegradation process that clean the water by consuming the sewage. 
Therefore, if the organic solids are removed along with the inorganic 
solids before the treatment process, then the removed solids will contain 
raw sewage which presents difficult problems with the storage and disposal 
of a possibly hazardous waste. 
Fortunately, the inorganic solids are generally distinguishable from the 
organic solids on the basis of their specific gravities. Inorganic solids 
such as sand and grit tend to be much denser than organic solids such as 
sludge. Therefore, inorganic solids tend to settle to the bottom of the 
fluid stream quicker than organic solids. 
In most of the existing waste water treatment facilities, it is this 
difference in density between inorganic solids and organic solids that it 
is relied upon to remove the inorganic solids. Typically, the incoming 
waste water flows through an inorganic removal chamber that may be 
referred to as "grit chamber". The grit chamber normally operates with a 
continuous flow through it rather than as a batch at a time. Waste water 
flows into the chamber through an inlet and out of the chamber through an 
outlet. At the bottom of the chamber is generally a collection space. 
Existing grit chambers include several designs by Weis. In U.S. Pat. No. 
3,941,698 by Weis, the grit chamber includes an upper settling chamber and 
a lower grit storage chamber, with a transition surface dividing the two. 
Rotating paddles within the settling chamber positioned above the 
transition surface, cause liquid entering the settling chamber at the 
outer periphery to rotate about a vertical axis within the chamber to 
produce a forced vortex. The forced vortex results in an upward spiral 
flow, so that the flow enters at the settling chamber periphery through an 
inlet, and flows spirally upward and exits the settling chamber from an 
outlet, also at the periphery. Dense particles fall through an opening in 
the transition surface between the settling chamber and the storage 
chamber coinciding with the vertical axis of the forced vortex. 
Variations of the device disclosed in U.S. Pat. No. 3,941,698, discussed 
above, include those disclosed in U.S. Pat. Nos. 4,107,038 and 4,767,532. 
Such patents disclose modifications in the settling chamber configuration 
to enhance the liquid flow. Other devices in the prior art include those 
disclosed in U.S. Pat. No. 4,519,907 by Rooney. 
It has now been discovered that further improvement is possible in the grit 
chamber efficiency by further modifying the settling chamber 
configuration. This is important, because the prior art devices result in 
fairly high turbulence, which interferes with effective and reliable 
settling and also consumes additional energy. It is also desirable to 
employ a design which facilitates the removal of finer grits. 
SUMMARY OF THE INVENTION 
The present invention is an improvement over existing grit chambers, by 
employing a novel grit chamber configuration which results in more 
complete grit removal, removal of finer grits, and a reduction in power 
requirements. Specifically, the grit chamber is a nearly cylindrically 
cylindrically shaped element having an inlet which introduces in-flowing 
liquid in a flow path which is not tangential to the axis of the cylinder. 
Instead, the inlet is an involuted or curved channel which introduces a 
radial component to the incoming liquid velocity vector to contribute to 
vortex formation. By so contributing to the vortex formation, the 
turbulence in the settling chamber is reduced and the power requirements 
for the mixer drive are lessened. 
The lessening of mixer drive power requirements results in lower energy 
consumption, and lower torsional and bending loads on the mechanical 
components which allows for the use of smaller motors, speed reducers, 
gears and shafts. These advantages translate into lower costs of 
manufacture and also lower maintenance and operational costs. In addition, 
it is believed that the novel design disclosed herein simplifies the 
internal configuration of the grit chamber by lessening the need for 
baffling and the like.

DETAILED DESCRIPTION OF THE INVENTION 
A schematic view of the grit chamber 10 in accordance with the present 
invention is shown in FIG. 1. The principal elements of the grit chamber 
10 include a settling chamber wall 16, an inlet channel 18, an outlet 
channel 20, and a transition surface divider 22. As shown by the arrows in 
FIG. 1, incoming waste water moves through the inlet channel 18 and enters 
the grit chamber 10, specifically the settling chamber 16. The settling 
chamber 16 is cylindrically-shaped, except in the region receiving the 
incoming flow from the inlet channel 18. In that region, the outer wall 26 
of the inlet channel 18, together with the settling chamber 16 form a 
transition curve which imparts a radial movement to the incoming waste 
water. As a result of this, the outer wall 26 at the point where the inlet 
channel 18 first joins the settling chamber 16, (point 28), is radially 
more distant from the center 30 of the settling chamber 16, than is the 
cylindrical portion 32 of the settling chamber 16; thus, the incoming 
waste water must move radially inward as it flows from the inlet channel 
18 into the settling chamber 16. This involute curve as the inlet channel 
18 joins the settling chamber 16, and the consequential radial movement of 
the waste water, results in significantly less turbulence as compared to 
conventional designs wherein the waste water enters the settling chamber 
tangentially to the cylinder of the settling chamber. 
The outlet 20 for the waste water receives the waste water tangentially to 
the cylinder of the settling chamber 16. In contrast with the case of the 
incoming flow through the inlet channel 18, the outgoing flow through the 
outlet 20 need not be received through an involute curve. For example, a 
simple outlet that receives the flow tangentially to the cylinder of the 
settling chamber 16 is acceptable. 
FIGS. 2 and 3 show side sectional views of the grit chamber 10 through the 
center 30 (see FIG. 1) of the settling chamber 16. FIG. 2 looks directly 
toward the outgoing flow, while FIG. 3 looks directly toward the incoming 
flow. As shown in both FIG. 2 and FIG. 3, the grit chamber 10 includes a 
settling chamber 16 positioned over a grit collection chamber 17. A 
transition surface divider or plate 22 may separate the settling chamber 
16 from the grit collection chamber 17. A paddle wheel 38 or some other 
propeller or impeller or flow producing or directing device may be 
positioned in the settling chamber 16 above the transition surface plate 
22 to assist in generating a vortex within the settling chamber 16. The 
paddle wheel 38 is driven by a shaft 40, which in turn is driven by 
suitable drive means (not shown). 
FIG. 3 shows the inlet 44 for the incoming flow, positioned at the bottom 
of the settling chamber 16 at the radially outer edge thereof which joins 
the inlet channel 18 (see FIG. 1). FIG. 2 shows the flow outlet 48, 
positioned at an elevation higher than the inlet 44 (see FIG. 3) and 
radially inward from the inlet 44 (see FIG. 3). It can be appreciated that 
the configuration of the settling chamber 16, having a substantially 
cylindrical inner surface with a transition portion gradually extending 
radially outwardly along a curve such as an involute, and the relative 
positions of the inlet 44 and outlet 48, result in the vortex flow shown 
by the arrows of FIG. 1. 
FIG. 4 shows an enlarged top view of the transition surface plate 22. In 
the preferred embodiment shown, the transition surface plate 22 is 
substantially round, to cover the round upper boundary of the grit 
collection chamber 17 (see FIGS. 2 and 3). In plate 22 are annular holes 
54 made up of a plurality of slots 56 separate by tabs 58, to define a 
central portion 60. Thus, the plate 22 includes an outer portion 62 and a 
central portion 60 separated by the annular holes 54, wherein the central 
portion 60 is supported by the attachment to the outer portion 62 through 
the tabs 58. Alternately, for example, the transition surface may include 
a main plate with a hole in the center region, and a center plate that is 
smaller than the hole and suspended in the hole by a set of clips or other 
supports to define an annular hole between the center plate on the main 
plate. 
The annular hole 54 allows grit settling from the settling chamber 16 (see 
FIGS. 2 and 3) to pass through the surface transition plate 22 which 
divides the settling chamber 16 from the grit collection chamber 17, and 
into the grit collection chamber 17. Once the grit has settled through the 
annular hole 54 in the transition surface plate 22 and into the grit 
collection chamber 17, it can be removed and discarded by techniques known 
in the art. 
In operation, waste water is directed through the inlet channel 18 (see 
FIG. 1) and passed into the settling chamber 16 through the inlet 44 (see 
FIG. 3). At that point, the incoming waste water is moving along the 
involute curve defined by the radially outer edge of the inlet 44 and the 
radially outer boundary of the settling chamber 16 where it merges with 
the inlet 44, so that the waste water has both a radial velocity and a 
tangential velocity. This combined radial and tangential velocity vectors 
translate the linear movement of the waste water into a circumferential 
toroidal movement within the cylinder of the settling chamber 16, while 
imparting a minimum of turbulence to the liquid as compared with prior art 
systems wherein the incoming flow is directly tangential to the cylinder 
of the settling chamber. 
By means of the insertion of angular momentum, the incoming flow, either 
with or without the additional movements produced by the paddle wheel 38, 
generate a forced vortex typical flow pattern in which the liquid flows 
spirally around and upward through the settling chamber 16. The liquid 
exits the settling chamber 16 via the outlet 48 (see FIG. 2). The forced 
vortex causes dense materials such as grit to settle downward and toward 
the vortex axis which coincides with the cylinder axis of the settling 
chamber 16. As the grit moves across the upper surface of the transition 
surface plate 22 toward the center axis, it encounters the annular hole 54 
(see FIG. 4) in the transition surface plate 22, and drops through the 
annular hole 54 into the grit collection chamber 17 (see FIG. 2). 
It can be appreciated that variations of the invention are possible without 
departing from the spirit of the invention. For example, the surface 
transition plate 22 (see FIG. 4) may have openings therethrough for the 
passage of grit in a configuration other than what is shown, or may also 
include a central hole through which gas, sludge and water can pass upward 
from the grit collection chamber 17 to the settling chamber 16 (see FIGS. 
2 and 3). The transition surface plate 22, may be of a non-flat 
configuration, such as a cone or truncated cone, and may include a large 
central opening so that it does not substantially cover the grit 
collection chamber. The outlet may be configured such that waste water 
exits in a direction other than tangential with respect to the vortex 
flow.