CE 330 Design Project
Screening and Grit Removal
Group 1 Members:
Jesse Clark
Sharon Strom
John Swidecki
DECEMBER 6, 2013
HEWLETT-PACKARD
Table of Contents
Section 1: Background and Introduction ..................................................................................................... 1
Section 2: Process/Technology Description ................................................................................................ 1
Section 3: Design Alternatives ..................................................................................................................... 7
Section 4: Recommendations .................................................................................................................... 12
References: ............................................................................................................................................... - 1 -
Section 1: Background and Introduction
The Water and Wastewater Division of Moscow, Idaho is in the process of designing a
new water resource recovery facility (WRRF). Municipal WRRFs are designed to treat raw
wastewater influent in order to prevent biological and chemical contaminants from effecting
water bodies downstream. Standard practice in WRRFs is to process the raw wastewater
influent by screening the water for harmful solids or objects, and then to remove all fine
material via a grit basin. The purpose of screening is to protect the rest of the system from
potential damage caused by large solids of an inorganic nature which can obstruct the flow of
water, cause clogging in the pipes and pumps, and cause undue wear in the WRRF’s mechanical
equipment. The purpose of grit removal is to remove fine-grained materials that will settle
throughout the WRRF and damage the mechanical equipment. The objective of this report will
be to explain in detail the particulars of screening and grit removal, to describe current
alternatives and process limitations, and to conclude with a recommended design to be
implemented in a new WRRF in Moscow, ID.
Section 2: Process/Technology Description
Screening
Screening is the first step in preliminary wastewater treatment and consists of the
removal of inorganics such as plastics, cloth, paper products, and metal materials from the
influent wastewater stream in order to avoid blockage of and damage to pipelines, and other
equipment downstream. The removal of these inorganic materials is achieved by passing the
influent wastewater stream through a screen which consists of either regularly spaced bars or a
1
Figure 1 Coarse, bar screen6
metal sheet with regularly spaced perforations which allow the water to pass through but
prevent large debris from being carried further into the treatment facility. Screens are classified
based on the size of these openings into coarse, fine, and very fine screens. Coarse screens, as
can be seen in Figure 1, “typically have openings of 6 millimeters (0.25 inches) or larger”1. Fine
screens typically have openings between 1.5 and 6 millimeters (0.06 and 0.25 inches) while very
fine screens typically have openings ranging from 0.2 to 1.5 millimeters (0.01 to 0.06 inches).
Fine screens are used to remove small material that may build up in and block pipes as well as
wear down equipment, such as pumps, and are increasingly replacing primary sedimentation
basins2. Very fine screens are used to reduce the level of suspended solids. In addition, “fine
screens are capable of removing 20 to 35 percent suspended solids and BOD 5”1. BOD5 is the
biochemical oxygen demand of the wastewater measured over a period of five days and is a
relative measurement of the organic matter present in the wastewater.
2
Screens are also classified into two categories based on their method of maintenance.
Manually cleaned screens are generally only used in small or old treatment facilities and will
not be further discussed herein. Mechanically cleaned screens have many advantages
compared to manually cleaned screens and therefore are always used in new installations. In
general, mechanically cleaned screens require less manual labor and facility personnel since the
screens are raked automatically. However these rakes should be checked regularly for breaking
and bending1. Pressurized water-jets can also be used to mechanically clean screens3.
A final classification can be made with regards to screen movement. Screens can be
classified as either fixed or moving. Fixed screens, such as racks and bar screens, remain
stationary and are generally manually cleaned, however they can be mechanically cleaned.
Moving screens, such as belt and drum screens as shown in Figure 2, generally require
mechanical cleaning.
Figure 2: Drum screen7
Material that is removed from the screens can be disposed of in one of the four
following ways: 1) direct burial in a hole or trench at a depth sufficient to prevent disruption by
rodents4, 2) pressing, bagging and burying in a landfill, 3) pressing and incineration5, or 4) direct
3
deposit into a dumpster/landfill1. Pressing consists of the removal of water from the solid
material in order to facilitate incineration or decrease the space required for burial.
Grit Removal
Grit removal is the next treatment step in a WRRF. The purpose of grit removal is to
remove any inorganic material that was not caught by the screening process. This is a very
important step in water resource recovery. Without it, mechanical systems would undergo
excessive wear and ultimately fail. Grit also occupies valuable space, clogs pipes and solids
dewatering equipment, as well as produces harsh odors8. It is dense material that is
transported by domestic wastewater or street runoff, and can include sands, silts, ash, broken
glass, small pieces of metal, bone chips, coffee grounds, and other similar items9. Grit typically
exhibits a specific gravity of 2-2.5.
The theory of grit settlement is modeled by Stoke’s Law, which will provide the
maximum settling velocity for a spherical particle10. The relationship is:
𝑔 ∗ (𝜌𝑠 − 𝜌) ∗ 𝑑 2
𝑉𝑠 =
18 ∗ 𝜇
Where:

vs is the settling velocity in m/s

g is the gravitational constant in m/s2

d is the diameter of the spherical particles in meters

ρ is the density of the fluid in kg/m3
4

ρs is the density of the particle in kg/m3

µ is the dynamic viscosity in Pa-s
There are a number of processes for removing grit from wastewater. All process options
rely on the assumption that the inorganic material is more dense than water and will therefore
settle out. Organic particles are kept suspended in the water so they can be treated at
subsequent stations of the WRRF. The types of grit removal systems that exist include the
following: aerated grit chambers, vortex-type, detritus tanks which are short-term
sedimentation basins, velocity controlled horizontal flow grit chambers, and hydro-cyclones1.
In an aerated chamber, grit is removed by forcing the wastewater into a spiral motion
by blowing air through one side of the chamber causing a perpendicular spiral flow pattern.
This spiral motion causes the heavier particles to accelerate at which point they drop out of the
velocity streamlines and settle to the tank bottom. The main advantages of this grit removal
system include the following: constant removal efficiency over a wide flow range, improved
performance of downstream equipment due to the use of pre-aeration which reduces septic
conditions in incoming wastewater, and versatility which allows for chemical additions, mixing,
pre-aeration, and flocculation. Disadvantages may include leakage of potentially harmful and
volatile organics and odors, an increased power demand, additional manual labor, and
potential problems with downstream biological treatment processes due to excessive aeration1.
In a vortex-type grit chamber, influent wastewater is pumped into the top of the
chamber tangentially, which causes a vortex motion causing the heavier particles to settle to
the bottom of the chamber. The effluent is then discharged at the top of the chamber. Vortex-
5
type grit chambers also have a constant removal efficiency over a wide flow range, can remove
large percentage of very fine particles, do not have submerged parts that require maintenance,
and they have a relatively small footprint when compared to other grit removal systems. Due
to a manufactured design, modifications are difficult. In addition installation requires deep
excavation and the paddles within the chamber may collect rags1.
The detritus tank is a constant level, short-detention time tank that requires a grit
washing step to remove organic material, which is done after the grit has settled in the tank.
The grit is scraped from the bottom and this motion causes lighter organic materials to become
re-suspended in the tank. Detritus tanks do not require flow controls since all of the mechanical
parts are above the water surface. Head loss is also minimal within these systems. However,
detritus tanks may remove a large quantity of organic material1.
Horizontal flow grit chambers are large tanks, where the velocity is controlled by weirs
or rectangular control sections. These systems, which are simple to construct, are the oldest
type of grit removal systems and have proven quite effective. Larger, heavier particles settle to
the bottom and are removed by a conveyor system. Performance of these chambers is flexible
and may be changed by adjusting the outlet flow. However the mechanical parts undergo
significant wear and head loss within the system is excessive1.
Hydro-cyclones are generally used to remove grit from organics in grit slurries or grit
from primary sludge. Flow is pumped from a certain head range which induces a cyclone in the
chamber. The heavier grit particles collect on the sides and bottom of the cyclone, while the
organic material is removed from the center at the top of the cyclone. Hydro-cyclones may
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remove as much grit and suspended solids as a primary clarifier, however they require
significant energy to operate the pump that removes the sludge1.
Grit is usually disposed of by burial in a landfill as it is generally inoffensive, and many of
the particles are inert. Care needs to be taken however, when handling grit because all
materials associated with wastewater may contain pathogenic and parasitic organisms10. The
nature and quantity of the grit influences the method of ultimate disposal. If solids separation is
very efficient and less than 15% volatile solids are included in the grit, it can be disposed of as
fill. Well-washed grit has been used on sludge drying beds, as a cover for screenings, and as a
surfacing material for walks and roadways. A few sewage treatment plants have incinerated
grit along with dewatered sludge. Being largely inorganic, most of the grit solids are ultimately
discharged with the incinerator ash9.
Section 3: Design Alternatives
When looking at potential screening and grit removal options, it is important to consider
whether or not each option will positively affect the effluent in order to meet the design
criteria. Mechanical screens and grit removal systems specifically reduce the effluent flow rate
as well as the effluent total suspended solids, which for this specific application are to be ten
million gallons per day (MGD) and less than or equal to five milligrams per liter, respectively.
Other potential design considerations include, but are not limited to: influent approach
velocity, redundancy, head loss, and size. Cost will not be considered for this application.
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A mechanical, perforated plate screen, as shown in Figure 3, is a viable option for the
removal of inorganic solids from the wastewater influent. As the perforated plates that
constitute the screen pass through the channel, inorganics that are larger than the perforations
are trapped on the screen and carried out of the wastewater until they reach the top of the
screen where they are removed and deposited in a dumpster for future disposal as previously
described.
Figure 3: Mechanically cleaned, perforated plate screen 12
A fine, perforated screen that is commercially available is the Screenmaster® SP which
can be purchased through Franklin Miller, Inc. Once installed directly into the influent channel,
the Screenmaster® SP can process flows up to 95 MGD, which accommodates the specified flow
rate of 10 MGD. This fine screen can be ordered in various heights, ranging from 98.4 to 374
inches, and various widths, ranging from 19.7 to 78.7 inches, with perforations ranging in size
from 0.125 to 0.25 inches. The screen can be installed at various inclinations between 50 and
80 degrees from the horizontal. Also, the perforated plates are automatically cleaned by a
8
rotating brush once the head loss reaches a preset value13. To prevent a potential halt in
wastewater treatment, two perforated plate screens should be placed in parallel so that the
influent can continue to be screened if one of the units is out of order.
Rotary drum screens, as shown in Figure 4, can also be used to remove solids from the
wastewater. The influent wastewater flows directly into the perforated drum where the solids
that are larger than the perforations collect, preventing some of the influent from escaping
until the wastewater reaches a predetermined level. At this point the drum rotates so that the
solids are removed from the influent channel and by the force of gravity are dropped into a
screw conveyer/compactor positioned at the center of the drum which dewaters the solids and
transports them to a dumpster for future disposal. A spray wash system or brush removes any
solid material still attached to the drum. While the solids are being removed, the trapped
influent wastewater in the drum escapes through the perforations now at the bottom of the
drum, and the fresh influent begins to pass through and fill the drum as the perforations start
to clog, beginning the process again14.
Figure 4: Rotating Drum Screen14
9
One commercially available rotating drum screen is the Lakeside Raptor® which can be
purchased from Lakeside Equipment Corporation. The Raptor® can handle up to 50 MGD, which
accommodates the specified flow rate of 10 MGD. The perforated openings in the basket come
in a range of sizes, the smallest of which is 0.08 inches (2 millimeters), which may reduce the
amount of total suspended solids in the influent by as much as 35% as previously mentioned,
which will help to achieve the specified effluent total suspended solids of five milligrams per
liter or less. In addition, the solids collected from the screen are dewatered by up to 40% by the
screw conveyor/compactor, which will reduce the amount of space required for their
containment and disposal. An optional bagging attachment can also be purchased in order to
reduce odors from the screenings. Finally, the Raptor® has a seal assembly at the base of the
drum which prevents influent wastewater from bypassing the drum screen and contaminating
or clogging the rest of the system14.
For the removal of grit, the vortex-type grit chambers are a widely popular type of grit
removal system. Figure 5 shows a typical vortex grit removal system. The vortex chamber is set
Figure 5: Vortex-type grit chamber11
10
into an excavated hole, with the top near ground level. This allows the operator to see inside
the hydro-chamber to determine if there is any buildup from large materials that were not
removed by screening. When the vortex grit chamber is in operation, sludge settles in the
bottom cone and is then pumped to the grit classifier, which returns organic material to the
WRRF and inorganic particles to a dumpster for disposal.
Smith and Loveless Inc. offer a vortex-type grit removal system called the PISTA® 360™
with V-FORCE BAFFLE™. The V-FORCE BAFFLE™ is designed to direct the influent into the
chamber in a way that ensures the proper vortex flow and prevents short-circuiting. This allows
for a full 360° rotation through the inlet and outlet, providing maximum travel for the grit
particles. The V-FORCE BAFFLE™ on the outlet directs the flow out of the unit and acts as a
“slice weir” to control the water level in the main chamber and in the inlet channel. No
additional downstream flow control device is required to keep the velocity between 3.5 feet
per second (fps) at peak flow and 1.6 fps at minimum flow. By increasing the chamber velocity
during low flow periods, the baffle extends the grit removal path within the vortex chamber.
This is important because a longer grit path within the flow pattern increases the effectiveness
of grit being captured on the chamber’s floor.
The PISTA® 360™ with V-FORCE BAFFLE™ also permits design flexibility so that water
elevations can be controlled. Water level control is important because it maintains the proper
velocities of wastewater as it approaches the grit chamber. Previously, the most common way
to accomplish water level control was to back up the flow with a downstream, submerged weir.
The PISTA® 360™ has preset inlet and outlet openings which negate the need for a submerged
weir. By integrating the water elevation settings with the baffle, the overall outlet footprint
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requirements decrease by as much as half of typical distances. The resulting smaller footprint
provides significant reduction in construction costs11. Additional product data is provided in
Table 1.
PRODUCT DATA
Unit Capacities
0.5 mgd – 100 mgd (1,892 cmd – 378,500 cmd)
Arrangements
360° with V-FORCE BAFFLE™
Chamber Construction
Concrete or steel
Removal Efficiency 95% grit removal efficiency down to 140 mesh particle size (105 microns)
Pump Sizes
4” (≈ 10 cm) & 6” (≈ 15 cm)
Pump Arrangements
Top-Mounted & Remote-Mounted
Table 1: Product Data for the Pista 360 with V-Force Baffle11
Section 4: Recommendations
The objective of this report was to develop a design for a screening and grit removal
system for the new WRRF that is to be built in Moscow, ID. This report has presented and
discussed the design objectives made for the city of Moscow’s new WRRF, and based on this
analysis three alternative design options have been presented, specifically that of two different
screen options, and one grit removal option that will meet the requirements of the system. The
design requirements that pertain to screening and grit removal include an effluent flow rate of
10 MGD with a TSS concentration of 5 mg/L or less.
With these objectives in mind, the recommended screen design for this system is the
rotary drum screen, which can be procured in a size that would accommodate our design flow
12
of 10 MGD. This is based on the fact that while all the screen alternatives are viable options for
the removal of inorganics that are larger than the perforations, the drum screen is
recommended because it compacts and dewaters the screened solids, which will then require
less room for containment and burial later. In addition, the rotary drum screen has the
potential to reduce the total suspended solids and BOD5 of the effluent by 20-35% as previously
mentioned. For the grit removal option, the vortex-type is recommended, as it is a widely
popular and quite an efficient system in the industry based off of its performance for providing
high removal rates and having a small footprint within the system, both of which are very
important aspects of design. This grit removal system would be ordered in a size that would
accommodate the design flow of 10 MGD.
13
References:
1United
States. Environmental Protection Agency. Wastewater Technology Fact Sheet.
Municipal Branch Technology, 2003. Web.
<http://water.epa.gov/aboutow/owm/upload/2004_07_07_septics_final_sgrit_removal
.pdf>.
2"Fine
Screens." www.infobarscreens.com. N.p., n.d. Web. 5 Nov 2013.
<http://www.infobarscreens.com/fine-screens.htm>.
3"Ovivo."
Municipal Wastewater Band Screening. GLV, n.d. Web. 8 Oct 2013.
<http://www.ovivowater.us/content/files/data/cs_126_534d438aa9274ea487425185ec
dee694.pdf>.
4Cooke,
Rosa-Lee. "Solids Handling and Disposal." Lesson 13: Advanced Wastewaer
Treatment. Mountain Empire Community College. Web. 5 Nov 2013.
<http://water.me.vccs.edu/courses/ENV149/disposal.htm>.
5"Preliminary
and Primary Treatment.” WEDC. Web. 8 Oct 2013.
<http://wedc.lboro.ac.uk/resources/units/WWT_Unit_3_Preliminary_and_Primary_Trea
tment.pdf>.
6"Manual
Bar Screens." Benefit the Environment. Benenv, n.d. Web. 12 Nov 2013.
<http://www.benenv.com/en/prod7_sdgs.html>.
7"Drum
screens for wastewater treatment." Headworks Bio Inc.. Direct Industry. Web. 12
Nov 2013. <http://www.directindustry.com/prod/headworks-bio-inc/drum-screenswastewater-treatment-90397-1097255.html>.
8
"Lesson 13: Racks, Screens, Comminutors and Grit Removal ."water.me.vccs.edu. N.p.,
n.d. Web. 19 Nov 2013. <http://water.me.vccs.edu/courses/env110/lesson13.htm>.
9"Solids
Handling and Disposal ." water.me.vccs.edu. N.p., n.d. Web. 19 Nov 2013.
<http://water.me.vccs.edu/courses/ENV149/disposal.htm>.
10
"Preliminary and Primary Treatment." wedc.lboro.ac.uk. N.p., n.d. Web. 19 Nov 2013.
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11"PISTA®
360™ with V-FORCE BAFFLE™." smithandloveless.com. N.p., n.d. Web. 24 Nov
2013.<http://wedc.lboro.ac.uk/resources/units/WWT_Unit_3_Preliminary_and_Primary
_Treatment.pdf>.
12
"Huber Belt Screen EscaMax." Huber Technology Waste Water Solutions. Huber SE. Web.
19 Nov 2013. <http://www.huber.de/products/screens-and-fine-screens/perforatedplate-and-bar-screens/huber-belt-screen-escamaxr.html>.
13
"Screenmaster SP." Franklin Miller. Franklin Miller, Inc.. Web. 19 Nov 2013.
<http://www.franklinmiller.com/screenmaster-sp.html>.
14"Lakeside
Equipment Corporation, Cleaner Water for a Brighter Future." Rotating Drum
Screen. Lakeside Equipment Corporation. Web. 21 Nov 2013. <http://www.lakesideequipment.com/products/rotating_drum_screen/rotating_drum_screen.asp&xgt;>
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