A
Technical Memorandum
Date:
August 2007 (Updated January 2008)
Project:
Norwalk, CT Water Pollution Control Facility (WPCF)
CSO Capacity and Treatment Evaluation
Subject: Task 3 – Existing Grit Removal System Assessment
This memorandum presents an assessment of the existing grit removal system at the Norwalk
Water Pollution Control Facility (WPCF) Supplemental Treatment Building and considers
alternative technologies and design modifications to the existing facilities to improve
operations.
Existing Grit Removal Systems
The following is a description of the grit removal systems in the Supplemental Treatment
Building and for the primary sludge. The focus of this memorandum is the process in the
Supplemental Treatment Building. However, plant improvements being investigated under
the plant-wide facilities planning study could impact and/or enhance primary sludge
degritting operations.
Supplemental Treatment Building
The WPCF currently removes grit using horizontal
flow grit settling chambers as the primary grit
removal system for flow up to 30 million gallons per
day (mgd). This system is located in the
Supplemental Treatment Building. Each of the two
horizontal flow chambers is 46 feet long with a depth
of approximately 5.8 feet assuming a 9 foot operating
depth in the wet well at the Main Pumping Station
The grit channel width is 4 feet at the top and 3 feet at
the bottom.
The settled grit is removed from the bottom of the
grit channels with buckets attached to two countercurrent grit elevators. The buckets lift grit from the
grit channels on the lower level of the Supplemental
Treatment Building to the grit conveyors on the main
floor. Transverse screw conveyors then transfer the
Horizontal Flow Grit Chambers
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 2
grit to the grit washers for dewatering and then dewatered grit is discharged into a container
for off-site disposal.
Preliminary calculations show that at 30 mgd,
with both chambers operating and a channel
depth of 5.8 feet, the velocity through the
chambers is about 1.08 feet per second (fps) and
the detention time is 43 seconds. This velocity is
within the typical design range of 0.8 to 1.3 fps
for horizontal flow grit chambers, but the
detention time is lower than the generally
accepted design criteria of 45-90 seconds (M&E
2003). If one channel is down for repair, the flow
velocity through the remaining channel is
doubled and the detention time is cut in half,
Grit Processing Equiment in Supplemental
Treatment Building
which further reduces grit removal rates.
Flows in excess of 30 mgd are diverted to the wet weather treatment train in the
Supplemental Treatment Building. Two mechanical flow regulating gates are located
upstream of the horizontal grit chambers and are intended to provide a means to control the
amount of flow entering the plant for secondary treatment. Excess wet weather flow passes
over a weir (just upstream of the regulating gates) and enters the wet well of the high flow
pumping station.
Wet weather flow (up to about 65 mgd) is pumped
up to rotary drum screens, also located in the
Supplemental Treatment Building. There are six
drum screens, with two drums in each of three bays.
The east bay contains screens with 74-micron
openings and the central bay contains screens with
149-micron openings. The west bay is out-ofservice, but previously contained screens with 256micron openings. After the flow is screened, it is
disinfected using chlorine in a contact tank.
The supplemental flow system was not designed
with any grit handling but typically grit that does
Wet Weather Treatment Drum Screens
overflow into the wet weather treatment train
collects in the drum screens, caught in the smaller micron screens and/or settles out in the
wet well, the screen tanks or the chlorine contact tank, and is removed by a vacuum-truck
after the storm. Most grit during wet weather events is conveyed along with the 30 mgd dry
weather flow, is pumped through the Main Pumping Station and settles in the primary tanks.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 3
Primary Sludge Degritting
Primary sludge is pumped to three Wemco cyclone degritters to remove grit from the sludge
prior to thickening. The grit passes through a classifier and is dewatered and discharged on
conveyors before being deposited into containers for offsite disposal. Each cyclone/ classifier
pair is designed to handle 270 gallons per minute (gpm) of primary sludge. The maximum
input solids concentration is about 1 percent.
It is important to note that, when the total influent flow to the primary clarifiers increases to
approximately 20-25 mgd during storm events, the
primary sludge degritters are typically taken off-line
as they cannot process the higher grit load. At these
higher flow rates and higher grit loading conditions,
the cyclones are less effective. First, grit removed
from the cyclones under these conditions has higher,
and unacceptable, amounts of organic material.
Second, the cyclone centrate discharge line backs up,
reportedly due to more friction because of the
increased grit loading that is being discharged in the
centrate. Accordingly, under high flow conditions,
the primary sludge pumps discharge directly to the
gravity thickeners where excess grit settles in the tank
for later removal.
Grit that is not captured in the primary clarifiers
collects in the secondary aeration basins and is
removed when the tanks are drained for diffuser
maintenance.
Existing Facility Assessment
The city reports challenges in successfully operating the grit removal systems, which are the a
result of a number of plant operating and plant and collection system design conditions.
Under current conditions, the wet well of the Main Pumping Station operates at a fairly
consistent and high water elevation. The current wet well operating level of about -10.83 feet
(USGS NGVD 1929) is about a 9 foot operating depth and was established based on a wet well
depth that provided the best measurement control using the existing ultrasonic device
(without creating a turbulent surface that caused inconsistent and inaccurate depth
indication). The wet well stays fairly constant because the pumps use variable speed drives
with a level set point control and the wet well is no longer operated in a draw-fill mode.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 4
The high water depth in the wet well normally results in a backwater elevation upstream of
the Supplemental Building bar screens that surcharges the 72-inch influent interceptor, lowers
pipeline and channel velocities, and causes sediment deposition to occur in the interceptors
directly upstream of the building. During rain events, the high wet weather flow (and higher
pipeline velocities) re-suspend the grit and solids that have settled on the bottom of the pipes
and conveys the material into the Supplemental Treatment Facility. This slug of solids and
grit tends to overwhelm the grit removal system, interfere with screening operations, and grit
not captured by the grit system is carried through the channels into the Main Pumping
Station wet well where it is pumped to the primaries. The marginal design of the horizontal
flow grit chamber renders it ineffective during by this type of grit loading as grit is passing
through to the primaries without much removal.
Some operating conditions could be modified (such as installing the ultrasonic wet well level
device within a stilling well to allow lowering the wet well level) to help improve the grit
deposition/re-suspension condition. However, the level cannot be lowered sufficiently to
impact velocities in upstream pipelines because of the existing interceptor invert elevations.
For example, the 72-in pipe between Junction Box No. 1 and the Supplemental Building has a
flat slope resulting in low velocities under all dry weather flow conditions. Variable grit
loadings such as those experienced at Norwalk are also a typical problem in a combined
sewer system. Thus, a grit removal system for the Norwalk WPCF needs to be sized and
designed with adequate flexibility to handle these slug flow events.
Grit capture is also reduced because of the design of the existing grit channels and flow
control gates. The flow control gates include an orifice-type channel restriction that appears
to have been used in conjunction with the gate mechanism to control flows. The exact
dimensions of the opening are uncertain as the drawings do not provide sufficient detail.
Field measurements require isolating the entire Supplemental Building. However exact
measurement is not necessary as with the addition of VFDs at the Main Pumping Station,
flow to the remainder of the plant is controlled by pump speed paced off a flow signal from
the pump discharge flow meter. Thus the flow control gates are reportedly fixed in their most
open position and are not adjusted.
The channel restrictions even with no throttling may still be creating higher velocities into the
grit chambers during higher flow events. There is also a concrete block installed in each grit
channel, just upstream of where the grit collector buckets leave the channel and travel
vertically up to the floor above. The blocks were installed to help dissipate channel entrance
velocities created by the control gates. But the concrete blocks occupy 2 feet of a 4 foot wide
channel leaving a one foot wide passage around each side of the block. The impact of these
structures on channel velocities and grit bucket operation has not been fully investigated but
it is theorized that vortexes may be created just downstream of the blocks that are causing the
sediment within the grit buckets to be washed out as the buckets are lifted above the water
surface.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 5
The existing grit chambers could theoretically be improved if they could be modified to
increase the length of the channels to improve detention time and/or to lower velocities.
However, the channels cannot feasibly be lengthened or widened enough to improve either
design parameter dramatically, with significant confidence that the improvement would
handle the widely variable grit loadings. The entrance to the grit channels along with the
concrete block would need to be modeled to determine the impact of this design and whether
modifications to improve performance would be feasible. In addition, generally, this
technology is considered antiquated by today’s standards primarily due to the problems
experienced at Norwalk which are typical for this type of system.
Accordingly, CDM determined that trying to reuse or retrofit the existing horizontal grit
chambers would not be a viable option because of the historically poor performance, the lack
of system redundancy, and the limited ability to substantially expand the existing chambers
within the Supplemental Treatment Building. A retrofit of the existing grit system will also
not help to address the problem of heavy grit loads in the wet weather flow that is treated by
the supplemental treatment system as those problems are inherent in the existing collection
system design.
Grit Removal Strategies
Since the existing grit removal system could not be reasonably modified as is to meet project
objectives, several options were developed and considered utilizing areas within the
Supplemental Building. Grit removal improvements would have to be developed in
coordination with future wet weather treatment options as available space within the
building will only be created if/when the existing drum screens are abandoned. CDM
considered various grit removal technologies/strategies to remove grit from the combined
dry and wet weather flow and assessed the potential installation of these technologies within
the existing building.
Each technology was generally assessed for its capital and operating costs, space
requirements, grit removal capability, ease of operation, and feasibility for use in Norwalk. In
addition, another important consideration was maintenance of existing operations. The use
of the technologies was discussed with CTDEP in a series of workshops. During a July 2007
meeting with Norwalk and CDM, CTDEP officials noted that the state would require
Norwalk to maintain a primary level of treatment along with disinfection for the wet weather
flow even during construction.
The initial design flow for the grit removal system was set to about 95 mgd in order to treat
the combined dry and wet weather flows, which reflected the approximate current capacity of
the wet weather treatment system. In addition, a higher flow rate of 120 mgd was also
considered in the event that additional wet weather treatment was necessary to minimize or
eliminate CSO discharges from the Ann Street Siphon CSO. This design flow target may
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 6
change as the future treatment plant flows are estimated as part of the larger facilities
planning process.
Grit Quantities and Characteristics
Limited information was available regarding the quantities of grit captured at the plant and
no information is available about its physical properties. As such, CDM engaged Grit
Solutions to perform two days of continuous grit sampling in January 2008 of the plant
influent so that a grit characterization could be performed. The main goals of the analysis
were to determine the particle sizes, the actual settling velocity, and the relationship of grit
size to settling velocity.
The grit characterization study is finished and being analyzed at the time when this
memorandum was being prepared. The results of the analysis will be used in the final
evaluation of the various grit systems and in the design of the selected process.
Grit Removal Process Performance Requirements
The goal of a new grit removal system should be to remove about 95 percent of all grit
particles with a specific gravity of 2.65 that are greater than or equal to 150 microns (100
mesh) in size. However, this design point may be refined after the grit characterization report
is issued and reviewed.
Potential Technologies
CDM considered a variety of grit removal technologies for installation within the
Supplemental Building at the Norwalk WPCF including horizontal flow grit chambers,
aerated grit chambers, settleable solids concentrators, and vortex-type grit chambers.
Horizontal Flow Grit Chambers
Horizontal flow grit chambers are operated by maintaining a velocity of approximately 1 ft/s
through a horizontal settling channel. The grit settles out in the channel and equipment such
as a chain and flight system are used to remove the settled grit from the bottom of the
channel. In general, the main advantages for this technology include its simple design and the
flexibility allowed by controlling the outlet flow. The disadvantages include potential high
head loss at the flow control structure, difficulty maintaining constant velocity during varying
flow conditions, high organics removal if flow is too low, low grit removal efficiency if flow is
too high, and large space requirements, as compared to other technologies. The existing
horizontal flow grit chambers at the Norwalk plant are not effective at removing grit and
there does not appear to be a feasible way to improve their capacity and efficiency. CDM
therefore does not recommend horizontal flow grit chambers for further consideration
because of their performance limitations, their inability to handle varying flows, and the
problems with the existing chambers at the WPCF as described in this memo.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
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Aerated Grit Chambers
Aerated grit chambers have a long history of use at major wastewater treatment facilities with
well-established design criteria. The aerated grit process creates a spiral rolling motion with
the addition of air through coarse diffusers located along one side of the chamber. The
diffused air addition is used as a method of controlling particle velocities within the chamber,
which in turn controls of the size of grit particle allowed to settle and be removed. Because of
the spiral air flow pattern, grit particles make several passes across the tank bottom thus
improving the chances for removal. The addition of diffused air keeps the lighter organic
particles in suspension and they are discharged over the effluent weir. This is a proven
process with numerous installations nationwide.
A typical section through
an aerated grit chamber is
shown in Figure 1.
The aerated grit chamber
also provides additional
storage within the process,
which enables further
flexibility for varied flow
and load conditions as
experienced in a combined
sewer system.
Typical process design
Figure 1: Typical Section Through Aerated Grit Chamber
criteria for aerated grit
(Source: M&E 2003)
tanks are listed in Table 1
and Table 2 below along
with the proposed dimensions for a new aerated grit removal tanks. CDM established deeper
basins to minimize the footprint of the aerated grit tanks. Aerated grit chambers also create
odors and these tanks would have to be covered and have odor control/mitigation.
A
Table 1: Aerated Grit Design Criteria 90 mgd
Typical Criteria
Proposed for Norwalk
Number of Tanks for 90 mgd
As Required - Min. of 2
3
Hydraulic detention time
2 to 5 minutes at peak flow
3 minutes
Overflow Rate for 100 Mesh
42,000 gal/day/sq ft
42,000 gal/day/sq ft
Side Water Depth
7 to 16 ft
12 ft
Width (each tank)
Varies to Suit
20 ft
Length (each tank)
Varies to Suit
40 ft
Length to Width Ratio
1:1 to 5:1
2:1
Air Supply
3 to 8 cfm/ft of length
6 cfm/ft
Table 2: Aerated Grit Design Criteria 120 mgd
Typical Criteria
Proposed for Norwalk
Number of Tanks for 120 mgd
As Required - Min. of 2
4
Hydraulic detention time
2 to 5 minutes at peak flow
3 minutes
Overflow Rate for 100 Mesh
42,000 gal/day/sq ft
42,000 gal/day/sq ft
Side Water Depth
7 to 16 ft
12 ft
Width (each tank)
Varies to Suit
22 ft
Length (each tank)
Varies to Suit
44 ft
Length to Width Ratio
1:1 to 5:1
2:1
Air Supply
3 to 8 cfm/ft of length
6 cfm/ft
Historically, grit has typically been removed from aerated grit chambers using grit screws at
the tank invert in conjunction with bucket elevators or in conjunction with recessed impeller
centrifugal pumps in a drywell adjacent to the grit tank. A small number of plants utilize a
clamshell bucket mounted on a two way bridge crane to lift grit from the bottom of the tanks.
The advantage to this is that the grit is somewhat dewatered as the clamshell is lifted out of
the flow stream.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
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The traveling bridge grit removal system is
a newer technology to the market that is
well suited to handle the high peaks
associated with wet weather flow and is
suitable for use in aerated grit chambers.
Submersible pumps are mounted on a
bridge system that moves along the
chamber in both directions and
continuously removes grit collecting in the
troughs at the bottom of the tanks. A
typical section through an aerated grit
chamber with traveling bridge pumps is
shown in Figure 2 below.
Vortex Type Separators
Vortex type separators include low energy,
medium energy and high-energy
variations. The difference in variation is the
method used to induce and maintain a
rotary vortex motion over the range of design
flows for the unit. The typical range of flow
for an individual low and medium energy
induced vortex type unit is about 4:1.
Figure 2: Typical Section Through Aerated Grit
Chamber with a Traveling-Bridge-Type Grit
Removal System (Source: M&E 2003)
High-energy units rely solely on velocity to maintain the induced vortex and have limited
flexibility in design flow rates. High-energy units such as a cyclone separator are typically
used as a secondary classification device to separate water from collected grit and are not
applicable as a primary grit removal system for Norwalk.
Low energy vortex grit collectors use a mechanical paddle in the center of the tank to
maintain rotary motion and induced vortex action. Vortex units are widely known by the
proprietary names of Pista Grit by Smith & Loveless or JETA by Jones & Attwood/Eimco, but
are also manufactured by WesTech, Waste Tech, Lakeside, and others. These units have the
lowest head loss of the induced vortex processes, typically a few inches or less. Medium
energy units rely on a velocity control device instead of a mechanical paddle to maintain
rotary motion over the 4:1 flow range and thus have higher headloss, typically in the 9 to 12in range.
Grit is collected in a lower hopper section of the tank, and removed with pumps suitable for
handling a grit-water slurry. The pumps may be top-mounted suction-lift style, or can have a
flooded suction if space is available to construct a below-grade gallery or basement.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 10
Figure 3 shows a typical section through a vortex grit chamber. Table 3 contains model
information and planning level sizing for 30-mgd and 100-mgd Pista Units for conceptual
level design.
Figure 3: Typical Section Through Vortex Unit (Source: M&E 2003)
Table 3: Vortex Unit Data
Model 100.0A
Flow (mgd)
Upper Chamber Diameter
Upper Chamber Depth
Lower Chamber Diameter (min)
Lower Chamber Depth (min)
Inlet Channel Width
Outlet Channel Width
100
32'-0"
12'-8"
8'-0"
12'-0"
8'-0"
8'-0"
Model 30.0A
Flow (mgd)
Upper Chamber Diameter
Upper Chamber Depth
Lower Chamber Diameter (min)
Lower Chamber Depth (min)
Inlet & Outlet Channel Widths
30
18’-0”
9’-2”
5’-0”
7’-0”
4’-6”
Most vortex units require a pre-screening size no larger than ¾- inches. The proposed influent
screens will be finer than this spacing.
Because of the variable flows during a storm and for operational flexibility, the better design
approach is to use multiple 30 mgd units to meet the treatment capacity required for dry and
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 11
wet weather flows. Each unit must have the appropriate turn down ratio to meet low
nighttime flow requirements. In addition, multiple units provide some flexibility to
maximize redundancy. However, the disadvantage of multiple units is that space
requirements increase for the additional units, connecting piping, and workspace/clearances.
Headcell Settleable Solids Separators
The Eutek Headcell Settleable Solids Separator is a type of vortex unit that operates under
differential pressure (as shown in Figure 4). The modular, multi-tray concentrator has a small
footprint and can be designed to fit into existing grit chambers or basins. Flow feeds into the
concentrator tangentially, establishing the vortex flow pattern. Solids settle into the boundary
layer of each try and are drawn down the center to the collection chamber. The Headcell
provides fine grit removal down to 50 microns. The headloss in 30-mgd units is 12 inches.
The Headcell requires a considerable amount of service water; however if the Norwalk WPCF
plant water meets the reuse water requirements, then this requirement could be incorporated
into the system to avoid using city water.
Figure 4: Headcell Concentrator
Diagram (source: Eutek
brochure)
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
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The Headcell has the advantages of efficient grit removal, a small footprint, and a modular
design. The disadvantages include high capital cost, higher headloss than other technologies,
and potential siting issues due to the equipment’s height and weight. This is also a newer
technology to the market with limited installations at the design flows for Norwalk to judge
its performance, maintenance, and reliability.
Typical design parameters are shown in Table 5. The grit washing and dewatering units are
typically installed paired with each Headcell Concentrator.
Table 5: Headcell Unit Data
Parameter
Flow per Unit
Number of Units Required
for 30 mgd
Number of Units Required
for 60 mgd
Number of Units Required
for 90 mgd
Number of Units Required
for 120 mgd
Footprint
Height
Headloss
Headcell
Concentrators
30 mgd
Slurrycup Grit
Washing Units
30 mgd
Grit Snail
Dewatering
Units
60 mgd
1
1
1
2
2
1
3
3
2
4
17'x17' each
21'
12"
4
2
2 washers, 1 dewaterer: 12'x17'
12'
16.8'
-
As discussed with the vortex grit removal technology, multiple smaller units would probably
be recommended to meet the variable flows and to maximize operational flexibility and
redundancy. However, the disadvantage of multiple units is that space requirements increase
for the additional units, connecting piping, and workspace/clearances. Space is also required
for various accessory equipment associated with grit removal technologies. These include
recessed impeller pumps and bucket elevators, odor control measures, and grit treatment
systems such as cyclones, grit classifiers, and grit washers.
Conclusions
Based on the preliminary design criteria and equipment dimensions discussed above, efforts
were made to fit the new grit removal systems into the existing Supplemental Treatment
Building (assuming that the existing rotary drum screens for wet weather treatment would be
removed).
Because of existing unstable soil conditions at the site, all of the major piping and facilities at
the Norwalk WPCF are supported on piles. This means that lowering existing floors or
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
Page 13
adding substantial weight with new equipment in the existing building is problematic.
Adding piles under the existing floor to support new weight may not be practical or cost
effective. In addition, space within the bays of the rotary drum screens is marginal at best to
be able to fit in the multiple vortex or Headcell type grit removal units. The footprint for an
aerated grit removal facility also exceeds the available dimensions of the bays. Substantial
removal of concrete to increase space within the existing bays may not be practical or costeffective considering the structural issues. Accordingly, most of the units would have to be
installed at the main floor levels.
Finally, the hydraulics of adding grit removal within the Supplemental Treatment Building is
also difficult to address. Dry and wet weather flow would have to be pumped into the grit
removal facilities in order to maintain downstream hydraulics. Increasing the capacity of the
existing wet weather flow pumps meet peak wet weather flow conditions has several
disadvantages including:
„
The existing wet well for the wet weather pumping station may not meet hydraulic
standards if flow is increased by at least 30 percent.
„
The existing wet weather flow turbine type pumps may not be the appropriate
pumping application for dry weather pumping conditions (as opposed to intermittent
wet weather conditions), especially for the combined sewer flow with higher grit
loadings.
„
The issue of screening adequacy for all flows still remains.
These alternatives and the benefits and disadvantages of each were discussed in a series of
workshops with CTDEP and the city during 2007. Based on the discussions, there was
agreement that it was not practical to install a new grit removal system within the existing
Supplemental Treatment Building.
Primary Sludge Solids Degritting
One other alternative would be to eliminate grit removal operations entirely before the
primary clarifiers. By this approach, all degritting of dry weather flow would be
accomplished using sludge degritting equipment. Also, grit removal for wet weather flow
would not be performed.
The Norwalk WPCF currently utilizes Wemco cyclone degritters and grit washers to remove
grit from primary sludge during dry weather. The degritters can handle flows up to
approximately 20-25 mgd. The existing degritting operation could potentially be upgraded to
handle the peak dry weather flow (30 MGD) during normal operations.
Task 3 - Grit Removal System Assessment
August 2007 (Updated January 2008)
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This approach eliminates the cost of building, operating, and maintaining separate grit units
for preliminary treatment. The disadvantages include increased wear on the primary clarifier
equipment, increased pumping costs, and increased primary sludge handling. In addition,
many communities that practice this approach have had continued difficulties in consistently
removing grit from the primary sludge.
Recommendation
Accordingly, based on the evaluation of the existing grit removal system limitations in the
Supplemental Building and the performance limitations associated with the existing primary
sludge degritting operations, CDM recommends that new appropriately sized grit removal
facilities in a new wet weather preliminary treatment building be installed at the Norwalk
WPCF to remove the grit as a standard preliminary treatment process. This approach will
mitigate existing grit system performance problems in the Supplemental Building, provide a
means to remove and process high wet weather grit loads which will continue to arrive at the
plant due to the design limitations of the existing upstream conveyance system, eliminate
pumping grit into the primary clarifiers and associated primary sludge degritting problems,
and eliminate grit carryover into the aeration tanks.