1. art and entertainment
2. visual art and design
3. design

Headworks Upgrades at the Back River Wastewater Treatment
Plant to Improve Collection System Performance
Ben Asavakarin1, Paul Deardorff1*, Kelly Baxter1, David Cox1
1
Johnson, Mirmiran and Thompson, 72 Loveton Circle, Sparks, MD 21152
Email: [email protected]
ABSTRACT
Following improvements in the collection system to increase hydraulic capacity and reduce
infiltration and inflow (I/I) influences, the peak day influent flow to the City of Baltimore’s Back
River Wastewater Treatment Plant (BRWWTP) is expected to increase from the historical peak
of 420 million gallons per day (mgd) to more than 700 mgd. New headworks and wet weather
storage facilities are proposed to provide treatment and storage of flow in excess of the hydraulic
capacity of the downstream plant processes. This paper provides a discussion of the evaluation
and preliminary design of the new headworks facilities. Criteria defined by project stakeholders
including the BRWWTP operations and maintenance staff, the City of Baltimore sanitary sewer
collection system operations and maintenance staff, and the Wet Weather Consent Decree
Program are discussed. The discussion focuses on the challenge of providing a design solution
that satisfies the criteria defined by the various project stakeholders.
KEYWORDS: Headworks, wet weather storage, screens, grit removal, pump station
BACKGROUND
Back River Wastewater Treatment Plant
The BRWWTP was originally constructed in 1907. The Plant is situated on the west shore of the
Back River, a tributary of the Chesapeake Bay, and is owned and operated by the City of
Baltimore. The plant occupies a 466-acre site and has a 35-foot elevation difference from
influent to outfall, allowing wastewater to flow through the plant entirely by gravity. An
estimated 1.3 million residents in a 140 square mile area of Baltimore City and County are
served by this plant. The Plant has received various upgrades over the years and is currently
designed and permitted to treat 180 mgd of wastewater. The historical peak influent flow to the
BRWWTP is approximately 420 mgd. Based on a review of the design flows for the various
process units at the BRWWTP, hydraulic capacity calculations and analysis, and conversations
with consultants for the various improvements being implemented at the BRWWTP, the design
hydraulic capacity of the BRWWTP processes downstream of the existing headworks facilities
has been identified as 469 mgd. An aerial view of the BRWWTP is provided in Figure 1.
Figure 1. Aerial view of the BRWWTP
The wastewater at the BRWWTP receives four levels of treatment: preliminary, primary,
secondary and tertiary. Wastewater from both Baltimore City and County enters the Back River
plant through two large conduits – the Outfall Sewer and the Outfall Relief Sewer. The existing
Headworks Facility includes fine screens and grit removal basins.
Following preliminary treatment, the wastewater is distributed among 11 primary settling tanks
(PSTs) (nine, 170-ft. diameters and two, 200-ft. diameters). The primary effluent flows to two
Activated Sludge Process Plants (2 and 3) where a culture of microorganisms is maintained to
absorb and metabolize organic matter. Both Activated Sludge Process Plants are retrofitted for
Biological Nutrient Removal, which allows removal of nitrogen to a level of 8 mg/L in the plant
effluent. Ferric chloride and polymer are added prior to the secondary clarification to further
reduce phosphorus.
In 2003, in response to a continued decline in the health of the Chesapeake Bay and recognition
that additional effort was needed to achieve the goals of the Bay Program, the State of Maryland
initiated the Enhanced Nutrient Removal (ENR) Program. The ENR Program set goals for total
nitrogen and total phosphorus discharges of 3 and 0.3 mg/L, respectively. The City has two
Contracts in the design stage to implement ENR at the BRWWTP.
Effluent from secondary clarifiers is treated by Sand Filters (forty-eight, 16 ft x 116 ft sand
beds), which entrap the remaining solid particles on the filter beds. The effluent from sand filters
is disinfected by chlorination using sodium hypochlorite. After chlorination, the effluent is
dechlorinated by addition of sodium bisulfate. The effluent then receives post aeration in a stepdam cascade system where it is re-aerated to achieve a dissolved oxygen concentration in excess
of 5 mg/L before being discharged into an outfall in the Back River.
Existing Headworks Facilities
The existing BRWWTP headworks include influent metering, fine screen and grit removal
facilities. Plant influent flow is metered by two, 78-inch magnetic flow meters located in a
below-grade meter chamber.
There are a total of six mechanical fine screens with quarter-inch openings in the existing screen
facility. These screens were installed in the Screen House in 1989. The design hydraulic
capacity of the existing screening facility is approximately 300 mgd (50 mgd per screen). The
Plant’s current channel configuration includes a passive bypass which is intended to bypass
flows in excess of 300 mgd. The bypass diverts flows on the influent side of the screen around
to the screen discharge. A single belt conveyor carries the screenings to two piston-type
compactors prior to discharging into a dumpster for disposal. The existing fine screens at the
BRWWTP are shown in Figure 2.
Figure 2. Existing Screen Facility at BRWWTP
There are a total of four existing square horizontal flow grit tanks with associated grit
conveyance and washing equipment. Grit Tanks 1, 2, and 3 were constructed in 1937. Grit Tank
4 was added in 1983. Each tank is 50’-0” x 50’-0” x 9’-0” deep, and is provided with a rotating
rake mechanism that scrapes settled grit to a hopper on the west side of each tank. The settled
grit slurry is pumped from the grit hoppers to a conveyor via submersible grit pumps which
discharges into an at-grade dumpster. Grit cyclones and classifiers were added for grit
washing/dewatering in 2000. These units are currently operational but may only be used when
both the grit collector mechanisms and grit pumps are functioning. The existing grit removal
facility is shown in Figure 3.
Figure 3. Existing Grit Removal Facility at BRWWTP
Wet Weather Consent Decree Program
On September 30, 2002, the City of Baltimore (City) entered into a Consent Decree with the
United States Environmental Protection Agency (EPA), the State of Maryland Department of the
Environment (MDE), and the Department of Justice. The Wet Weather Consent Decree Program
includes elimination of SSOs, elimination of combined sewer overflows (CSOs), and elimination
of engineered overflow structures which currently discharge to storm drains or streams. The
Consent Decree requires upgrades to the wastewater collection system as follows:
•
•
•
•
Elimination of existing sanitary sewer overflows (SSOs) and combined sewer overflows
(CSOs);
A comprehensive sewer evaluation program including infiltration/inflow (“I/I”) analysis
and development of a dynamic hydraulic model;
A comprehensive sewer rehabilitation program to alleviate excessive I/I and remedy
structural defects;
Continuous upgrades to operations and maintenance.
As part of the Wet Weather Consent Decree Program, the City has engaged a Technical Program
Management Consultant, identified in this paper as the Wet Weather Program Consultant
(WWPC). The WWPC has developed a system wide hydraulic model, known as the macro
model, which predicts wastewater flows into the BRWWTP. The flow predictions consider
future growth and system aging, proposed improvements in the collection system to increase
hydraulic capacity and reduce I/I influence.
During the modeling analysis, the WWPC identified the existing grit tank effluent weirs at the
BRWWTP as a hydraulic control point or constraint on the Outfall Sewer collection system. The
grit tank weirs are 5 feet higher than the invert elevation of the Outfall Sewer and Outfall Relief
Sewer. Figure 4 provides an illustration of the hydraulic constraint at the BRWWTP. The
WWPC noted that the elevation of the grit tank weirs creates a backwater condition that
negatively influences the hydraulic behavior of the Outfall Sewer. This backwater condition
extends thousands of feet into the collection system. The backwater condition impacts the City
of Baltimore wastewater collection system in two ways:
•
The backwater condition combined with the relatively flat slope (0.025 percent)
contribute to grit/sediment deposition in the Outfall Interceptor System
•
The elevated hydraulic gradient, coupled with the grit/sediment deposition, reduces
conveyance capacity during wet weather events and increases the potential of sanitary
sewer overflows (SSOs).
Figure 4. BRWWTP Headworks Hydraulic Profile.
It was determined that if the hydraulic restriction were to be alleviated, the incoming Outfall
Sewers would be able to develop their full flow capacity. As such, the WWPC has identified the
removal of the hydraulic constraint at the BRWWTP as one of the key improvements for
compliance with the City’s Wet Weather Consent Decree Program requirements. As one
component of the larger effort to eliminate SSOs, the WWPC has defined a requirement for a
free discharge for the Outfall Sewer flow at the BRWWTP influent. The “free discharge”
condition is defined as maintaining the water surface elevation at the BRWWTP boundary below
the invert of the Outfall Sewer. In order to meet this requirement, it is necessary to construct an
influent pump station at the BRWWTP to pump all the influent flow to the plant during wet
weather conditions.
The WWPC builds the proposed hydraulic improvements into the macro model and using
historical data for storm intensities and durations, model-predicted influent hydrographs for the
BRWWTP are developed. In order to meet the schedule for improvements to the existing
headworks facilities at the BRWWTP, the evaluation and design of the proposed headworks
facilities at the BRWWTP has been progressing based on the most up-to-date model-predicted
influent flow data to the BRWWTP. Table 1 provides a summary of the top 10 model-predicted
peak influent flows to the BRWWTP following the proposed hydraulic improvements in the City
of Baltimore collection system. These data are based on the provision of a free discharge
condition at the BRWWTP boundary.
Table 1. Model-predicted peak influent flow to the BRWWTP.
Storm Event
Predicted Peak Flow (mgd)
1
725
2
671
3
635
4
624
5
600
6
566
7
546
8
526
9
519
10
507
The design hydraulic capacity of the BRWWTP has been identified as 469 mgd. The information
presented in Table 1 shows that following improvements in the collection system to eliminate
SSOs, the peak weather flows are anticipated to exceed the design hydraulic capacity of the
BRWWTP by more than 50%.
EVALUATION AND RECOMMENDED IMPROVEMENTS
Goals and Objectives
The capacity and treatment capability of the existing screening and grit removal facilities at the
BRWWTP is inadequate at current peak wet weather flows. The proposed rehabilitation and
hydraulic capacity improvements in the City of Baltimore collection system to eliminate SSOs
are anticipated to increase the influent flows to the BRWWTP during wet weather conditions.
The intent of the project is to perform an assessment of the hydraulic and treatment capabilities
of the BRWWTP influent headworks, relative to predicted wet weather flow events. Specific
project objectives include:
• Assess the hydraulic and treatment capabilities of the existing headworks and evaluate
options for improvements to increase the capacity of the headworks facilities to handle
wet weather flows as predicted by the macro model.
• Evaluate alternatives at the BRWWTP headworks to improve the hydraulic conditions of
the Outfall Interceptor System, specifically in relation to the current backwater condition
caused by the elevation of the existing grit tank weirs
• Evaluate options for storage of flows in excess of the hydraulic and treatment capacities
of the headworks facilities and downstream plant processes.
The objectives of this evaluation are representative of the interests of the City of Baltimore
collection system operations and maintenance personnel, as well as the City of Baltimore
BRWWTP staff. The evaluation must consider the needs of both groups and provide solutions
that are mutually beneficial.
Design Criteria
The evaluation of the existing headworks facility was completed based on the following design
criteria:
•
•
Design average day flow: 180 mgd
Peak day flow: 725 mgd
The peak day flow is based on the model-predicted peak day flow provided by the WWPC. The
preliminary design evaluation was completed assuming an ultimate design capacity for the
headworks facilities of 700 mgd. One redundant process unit will be provided for a firm
capacity of 700 mgd. With the redundant process unit, the headworks will be able to process the
model-predicted peak flow of 725 mgd.
Fine Screen Facility
The combined design capacity of the six existing screens is 300 mgd. This is well below the
historical peak flow to the BRWWTP of approximately 420 mgd, and is less than half of the
model-predicted peak wet weather flow of 725 mgd following the collection system
improvements to eliminate SSOs. The screening capacity limitation may have negative impacts
on the downstream plant processes as large objects and rags may be allowed to enter the primary
and secondary plant processes. Due to the insufficient hydraulic capacity of the existing screens,
a new screening facility and/or extension to the existing screening facility is required. The
screening facility would be sized to accommodate the peak day design flow of 725 mgd.
Based on the existing design of the facility, in order to increase the hydraulic capacity of the
existing screening facility to 725 mgd, a minimum of 5 new channels with two screens per
channel will be required. The addition of the additional channels would require expanding the
existing fine screen facility to more than twice its current length. The existing screens were
installed in 1989 and are reaching the end of their expected life. Replacement of the existing
screens would likely be required if expansion of the existing facility was chosen as the preferred
alternative to increase screening capacity.
An expansion of the fine screen facility is only feasible in one direction and there are several
underground utilities in the area, such as electrical duct-banks, that would be impacted with a
proposed building expansion. Additionally, the BRWWTP staff is concerned with challenges
associated with maintaining operation of the existing facility during construction if expansion of
the existing facility is chosen.
Because of the requirements to significantly increase the size of the existing facility, concerns
with impacts to existing utilities and maintenance of flow during construction, expansion of the
existing screen facility was not selected as the preferred alternative. A new screening facility is
proposed.
An evaluation of various mechanical screen technologies was conducted. The following
mechanical screen alternatives were discussed with the City of Baltimore engineering and
BRWWTP operations staff. A brief summary of each alternative is provided.
Continuous Belt Screen
This type of screen is currently in servcie at the existing BRWWTP screening facility. The
screen typically uses a belt of UHMWPE (ultra-high molecular weight polyethylene) or stainless
steel filter elements “weaved” together to capture screenings. Typical available element spacing
is 1-30mm. A “mat” or “blanket” forms on the filter elements increasing screenings removal.
Once the screenings mat increases the head loss across the screen to a pre-determined setpoint,
the filter belt rotates to lift the screenings out of the channel and deposit them onto a conveyor,
compactor or dumpster. A perforated stainless steel plated belt may be provided in lieu of the
aforementioned elements. Some manufacturers of this type screen offer standard and severe duty
models.
Center Flow Continuous Belt Screen
The center flow continuous belt design is similar to the continuous belt design, except
wastewater is funneled into the center of a continuous belt screen instead of directly onto the
filter media. Typical available element spacing is 1-25mm. The center flow design directs water
into the screen and exits the screen through the filtering media/elements on the sides. A stainless
steel or UHMWPE grid media separates the solids from the flow stream and lifts screenings out
of the channel. Similar to the continuous belt design, a “mat” or “blanket” builds on the
screening elements and improves removal efficiency. At the top of the unit a spray header
knocks the screenings onto a conveyor, compactor or dumpster. The center flow screen has a
feature where the back wall of the unit contains a manual gate that, when opened by an operator,
allows flow to pass without screening.
Chain and Rake Bar Screen
These units are similar to the continuous belt screen, except that it utilizes a fixed rack installed
in the flow channel to trap debris. Multiple rakes are typically provided to continuously pull
screened material up the rack to the collection point. A wiper located at the top of the screen
knocks debris onto a conveyor, compactor or dumpster. This type of screen has an automatic
reversing feature that attempts to clear obstructions lodged in the rake flights or screen field. The
intent of operating the screen in reverse and cycling back in the forward direction repeatedly is to
successfully dislodge any object from the rake flights or screen field. If the obstruction remains
after several attempts, an alarm is initiated for the operator and the machine shuts down,
protecting the equipment.
Step Screen
Step screens are comprised of two packs of vertical plates called lamella with a stair step shape
cut into the upstream face. The stationary and movable packs are woven together to form the
screen surface. Screenings will build up on the unit to form a “mat” causing the upstream level to
rise and raise the screenings out of the channel for discharge. At the discharge point, the screen
‘self cleans’ by the rotating motion of the lamella plates.
The evaluation of the screen alternatives considered removal efficiency at the specified design
flows, access to mechanical systems for maintenance, operations staff familiarity and the extent
to which moving parts are exposed directly to the waste stream. Table 2 provides a summary of
key advantages and disadvantages of the screen types considered.
Following the evaluation and discussions with the City engineering and BRWWTP operations
staff, the continuous belt type screens were selected. The BRWWTP staff is familiar with the
continuous belt screen; it has no moving parts within the waste stream, and allows rotation of the
screen to provide maintenance access making it the most desirable screen type.
Table 2. Summary comparison of screen type alternatives.
Screen Type
Key Advantages
Key Disadvantages
• Personnel are familiar with
screen type
• Potential screenings carry over if filter
Continuous Belt
elements are insufficiently cleaned
• Moving parts are out of flow
channel, allowing for easy
• Brush replacement
maintenance
• Screenings may have a problem with
accumulating debris in the center and
Center Flow
bottom of the unit
• Minimizes solids carryover
Continuous Belt
• Built in bypass
• Spray water used for debris removal
typically requires a compactor or washer
compactor on discharge.
• Screen is fixed within channel and cannot
• Capable of moving large
Chain and Rake
be rotated out for maintenance
objects (tree limbs, lumber,
Bar Screen
• Removal efficiencies can be lower due to
etc.)
bar configurations
• Difficulty in removing larger solids (relies
on screenings mat to remove items such as
• Self cleaning (no spray water
Step Screen
bottles, tennis balls, etc.)
or brushes)
• Wear parts below the water surface require
replacement.
Grit Removal Facility
Discussions with the BRWWTP operations staff and a review of several previous studies
indicate that the performance of the existing grit removal process is unacceptable. Final Design
Memorandum for SC 9535: Grit Removal Facilities Rehabilitation (Sverdrup, 1999) documented
that during the period from July 1997 to June 1998, an average of 3,773 pounds per day of grit
was removed, which is only 10% of the grit tanks’ design capacity. Flow distribution into the
four grit tanks is unequal, with greatest grit load removed at Grit Tank 3, and Grit Tanks 1, 2,
and 4 providing minimal grit removal. This was documented in a memorandum, An Evaluation
and Proposed Solution for Maximizing Daily Grit Removal at the Back River Wastewater
Treatment Plant Grit Chamber Process (McClinton, 1987) which stated that Grit Tank 3
removes approximately 75% of all grit removed in the grit tanks.
During wet weather flow events, the grit tanks are overloaded, and large quantities of grit carry
over to settle out in the PSTs, resulting in PST drive over-torque shutdowns and requiring
manual cleaning of the PST’s to remove accumulated grit. Additionally, the grit pumps are
unreliable, and in order to remove grit from the tanks, operations personnel are forced to remove
the tanks from service and use a rented crane with clamshell to clean out the tanks.
Based upon the flow distribution non-uniformity and inherent problems with the existing grit
facilities, complete replacement of the grit facility was recommended. An evaluation of various
grit removal technologies was conducted. Grit removal processes, as well as the mechanism for
collected grit removal were evaluated. The following grit removal process alternatives were
discussed with the City of Baltimore engineering and BRWWTP operations staff.
Horizontal Grit Removal
This technology is currently employed by the existing grit removal tanks at BRWWTP.
Horizontal grit removal tanks may be square or rectangular in configuration, and rely exclusively
on controlling flow velocity through the tank in order to settle out grit. These tanks may be open
to the atmosphere as the lack of aeration or turbulent flow through the tanks reduces the potential
for odor release.
Aerated Grit Removal
This technology utilizes rectangular tanks with diffused air along one tank side to create a spiral
flow pattern lengthwise down the tank. Grit removal can be affected by increasing or decreasing
air flow to settle out particles of different size. Proper air adjustment allows for grit particles to
settle out while maintaining organic material in suspension. Aerated grit tanks are shown in
Figure 5.
Figure 5. Aerated grit tanks at City of Baltimore Patapsco Wastewater Treatment Plant
Vortex Grit Removal
This technology utilizes centrifugal and gravitational forces in a cylindrical tank which creates a
vortex flow pattern whereby flow enters the grit tank tangentially at the top of the unit and
settled grit is collected in a hopper in the bottom center of the tank. These units may include a
paddle mixer to promote grit separation, may include stacked trays for increased grit removal, or
may only rely on tank size and approach velocity for grit removal. Typically grit is removed
from the tanks by a grit pump or air lift system to a washing/dewatering system before ultimate
disposal.
Table 3 provides a summary of the advantages and disadvantages for the grit removal processes
considered. Following discussions with the City engineering and BRWWTP operations staff, as
well as site visits and phone interviews to evaluate the technologies considered, aerated grit
removal was ultimately selected as the preferred alternative. The BRWWTP operations staff was
very dissatisfied with the existing horizontal grit removal tanks, so this alternative was
eliminated from further consideration. Both aerated grit removal and vortex grit removal
received serious consideration, but the aerated grit removal technology was ultimately selected
because the operations staff preferred the ability of the aerated grit removal process to
accommodate variations in flow. There was some concern with the vortex grit removal
processes ability to accommodate large variations in flow which are anticipated at the BRWWTP
during wet weather conditions.
Table 3. Summary comparison of grit removal process alternatives.
Grit
Removal
Advantages
Disadvantages
Process
• Limited ability to adjust grit removal
performance or accommodate flow
variations
• Simple operation
Horizontal
•
Rake mechanism can fail if overloaded
Grit Removal • Tanks can be uncovered
• Pump required for grit removal from tank
odor generation unlikely
• Largest footprint required
• Existing grit system does not function well
• Ability to adjust air flow to
• Energy consumption by air supply blowers.
improve grit removal over
• Aeration releases hydrogen sulfide; creates
varying flow range
Aerated Grit
hazardous, corrosive, odorous
• Settled grit is washed and
Removal
environment.
ready for disposal
• Larger footprint than vortex grit removal
• No mechanical equipment
technology.
within the grit tank
• Smallest footprint
• Requires grit pump or air lift system for
• Capable of high grit
grit removal.
Vortex Grit
removal efficiency
• Limited ability to adjust grit removal
Removal
• Grit tanks do not require
performance or accommodate flow
building or cover - odor
variations.
generation unlikely.
The design intent of the aerated grit system is to settle grit at the bottom of the tanks and
maintain organic material in suspension through the tanks. This necessitates having a means to
remove the settled grit from the tanks to avoid grit carryover back into the system. Batch process
and continuous grit removal options were considered as part of the evaluation. A summary of
the alternatives considered is provided.
Batch Grit Removal
Grit removal would only occur when a tank was taken out of service and drained. Typically, an
overhead bridge crane is provided with a clamshell capable of removing grit from the bottom of
the tank into a truck or dumpster located adjacent to the grit tanks. This option requires the
operations staff to be diligent to schedule the batch process grit removal regularly to prevent
carryover of collected grit back into the system.
Pumped Grit Removal
In this option, grit is typically captured in a hopper at the bottom or at one end of each tank.
Pumps may be either submersible, suction lift, or dry pit submersible and may be either
stationary, or mounted on a traveling bridge system which conveys the pumps the length of the
grit tanks. With any pumped grit option, grit is pumped from the tank to either a separate grit
washing/dewatering system, or directly to a dumpster.
Chain and Bucket Grit Removal
This option involves using a chain and bucket elevator system to remove captured grit from the
tanks. In this option, grit is typically captured in a hopper at the bottom or at one end of each grit
tank. The chain and bucket elevator system lifts captured grit out of the hopper to the ground
floor level where additional means of conveyance is required to transfer the grit to its ultimate
disposal point.
Screw Conveyor
A screw conveyor is used to transport captured grit from the tank hopper to ultimate disposal.
Screw conveyors on small systems may be inclined to provide some grit washing/dewatering
during transport, however for a facility such as the BRWWTP, space limitations will likely
require a horizontal screw conveyor. This will require the disposal site to have a substantial
drain/sump pump system to send excess water collected back into the main process stream.
Table 4 provides a summary of the advantages and disadvantages of the collected grit removal
systems. Manual grit removal was eliminated because there is limited automation and requires
significant effort from the operations staff. The chain and bucket elevator and screw conveyor
were both eliminated from consideration because the submerged mechanical systems and the
requirement to drain the tanks to perform maintenance on the mechanical grit removal systems.
A pumped grit system was selected to remove collected grit. The large tanks would require
multiple sumps or some other mechanism to convey the grit to a central pump unit. A pump
mounted on a traveling bridge provides coverage of the entire tank. The traveling bridge
provides the benefit of removal all mechanical systems from the wastewater process flow. A
traveling bridge pumped grit removal system was ultimately selected as the preferred design
option.
Table 4. Summary of collected grit removal alternatives.
Collected
Grit
Advantages
Removal
Mechanism
Batch
Process Grit
Removal
• No submerged mechanical
equipment
• No pumps/airlift equipment which
is prone to clogging/wear in grit
applications
Pumped Grit
Removal
• Minimal grit stored in tank;
minimizes grit carryover to
downstream processes
• Pump maintenance can be done
without dewatering tank
• Grit can be dewatered/washed.
• Tank may be covered to minimize
ventilation and odor control
Chain and
Bucket
Elevator Grit
Removal
• Minimal grit stored in tank;
minimizes grit carryover to
downstream processes
• No lift height limitations
Screw
Conveyor
Grit Removal
• Minimal grit stored in tank;
minimizes grit carryover to
downstream processes
• Tank covers may be used to
minimize ventilation and odor
control.
Disadvantages
• Tanks must be taken out of service for
grit removal and maintenance
• Requires operations personnel to be
inside grit building to operate
• Requires bridge crane capable of
spanning entire length of grit tanks
• Grit builds up in between tank cleaning
– can lead to reduced removal efficiency
and/or grit carryover
• Pumps and piping prone to
clogging/wear from pumping grit
• Additional maintenance for traveling
bridge system where used
• Additional equipment required to
transport grit to ultimate disposal
• Submerged mechanical equipment subject to wear, jamming
• Tank must be dewatered for
maintenance
• Separate drain/pump system to convey
excess water back to main process
stream.
• Submerged mechanical equipment.
• Tank must be dewatered for
maintenance.
• Large screw conveyor to handle peak
grit loading and prevent over-torqueing.
Collection System Hydraulic Impacts
As part of the Wet Weather Consent Decree Program evaluation efforts, it was determined that
the existing grit tank weirs at the BRWWTP create a hydraulic restriction in the Outfall Sewers.
The hydraulic restriction creates a backwater condition of 4 – 6 feet in the Outfall Sewer which
results in low velocities and grit deposition in the Outfall Sewer. Previous grit sampling efforts
and grit sampling efforts completed as part of this Contract confirmed the presence of significant
grit accumulation in the Outfall Sewer system. It is estimated that multiple feet of grit have
accumulated in the Outfall Sewer, significantly reducing the hydraulic capacity of the system. In
addition to the backwater condition and reduced velocities, the hydraulic restriction at the
BRWWTP reduces the capacity of the Outfall Sewer during wet weather conditions and
contributes to the occurrence of SSOs in the Outfall Sewershed.
Following an evaluation of the impacts of the existing grit tank weirs, the WWPC determined
that the elimination of the hydraulic control point at the BRWWTP could provide significant
benefits in the elimination of SSOs. However, eliminating the grit tank weir does not eliminate
the hydraulic influence on the Outfall Sewer because the PST weir elevation is only one foot
lower than the grit tank weirs and would still create a hydraulic control point impacting the
hydraulic grade line in the Outfall Sewer.
Based on discussions with the WWPC, it was agreed that in order to eliminate the hydraulic
influence of the BRWWTP on the Outfall Sewer, the water surface elevation at the treatment
plant boundary would need to be maintained below the invert of the Outfall Sewers. In order to
meet this requirement, an influent pump station is required.
The alternatives evaluation considered the wet well configuration, pump type, flow control and
other supporting systems that will be required as part of the influent pump station facilities.
During the evaluation, relevant design standards, information from pump manufacturers,
information from other WWTP facilities and the preferences of the BRWWTP operations and
maintenance staff were considered when developing recommendations.
Pump Station Wet Well
Alternative configurations considered for the influent pump station wet well include the trenchtype wet well design and the confined wet well design as defined in the American National
Standard for Rotodynamic Pumps for Pump Intake Design (2012). These intake structure
designs are intended specifically for solids-bearing liquids.
Wet wells for solids bearing liquids require consideration of design features to minimize settling
of solids in the wet well. The design should also allow for removal of solids that do settle in the
wet well, as well as floatables. The main design principle for wet wells for solids bearing liquids
is to minimize horizontal surfaces in the wet well anywhere but directly within the influence of
the pump inlets. Sloped surfaces are provided to direct solids to a location where they can be
removed by the pumping equipment.
Summary descriptions of the characteristics of the trench-type and confined inlet wet well
designs are provided below. A summary of the advantages and disadvantages associated with
each design is provided in Table 5.
Trench-Type Wet Well
The trench-type wet well configuration and summary design criteria are shown Figure 6. The
trench-type wet well design includes an ogee (i.e. curved) spillway transition at the inlet to the
wet well trench. The ogee spillway is designed to promote a scouring flush across the trench
floor which mobilizes settled solids and allows them to be removed from the wet well by the
pumps.
Figure 6. Trench-type wet well design criteria (Hydraulic Institute, 2012)
The trench-type wet well is inherently small and is not easily adapted to provide large storage
volume in the wet well. The trench-type wet well is very narrow and requirements for
significant storage volume result in a wet well which is very deep or very long. Neither are
desirable in terms of construction cost. The storage volume requirements in the wet well can be
reduced with the use of pump flow control to match flows into and out of the wet well.
During the cleaning procedure, as outlined by the Hydraulic Institute Standards, the last pump in
the trench should be operated at full speed when the influent flow to the wet well is about half
the capacity of the last pump. If the low influent flow is too high, multiple pumps can be used.
The pump(s) are operated to lower the liquid level in the wet well as rapidly as possible. As the
liquid level decreases to a minimum, a hydraulic jump is created at the toe of the ogee spillway.
The jump moves across the bottom of the wet well scouring settled solids that are then removed
by the pump(s). As the hydraulic jump passes each pump intake, the pump loses prime and
should be stopped. The cleaning procedure requires the pump to be operated at undesirable
hydraulic conditions.
The trench-type wet well design limits the size of the wet well if variable speed pumps can be
used to match flow into and out of the pump station and limit storage volume requirements. The
design of the wet well provides optimum geometry for efficient cleaning procedures. However,
the cleaning procedures cause the pumps to operate at an undesirable hydraulic condition.
Confined Inlet Wet Well
The confined wet well configuration and design criteria are shown in Figure 7. The suction bell
is located in a confined pocket to isolate the pump from disturbances created by adjacent pumps.
The confined pockets also limit the area that solids can settle and maintain higher velocities at
the suction inlet to facilitate solids removal from the wet well. Horizontal floor space in the wet
well is limited to the area in the immediate influence of pump suction. A minimum slope of 45
degrees is provided for all other wet well surfaces.
Figure 7. Confined inlet wet well design criteria (Hydraulic Institute, 2012)
There is flexibility in the confined wet well design to provide operating storage in the wet well,
but it is preferable to limit storage requirements by using flow control on the pumps in order to
match flows into and out of the pump station.
Based on the cleaning procedures defined in the Hydraulic Institute Standards, solids removal
from the wet well can be achieved by operating the pumps one at a time at full speed for the
duration of a few minutes. Like the trench-type wet well, the solids removal cleaning procedure
for the confined wet well design temporarily subjects the pumps to vibration, dry running and
other undesirable hydraulic conditions. The detailed design of this wet well would need to
consider design options and operating procedures to limit settling in the pockets of the pumps
that are not in use.
Table 5. Summary of influent pump station wet well design alternatives.
Type of Wet Well
Advantages
Disadvantages
• Inherently small - reduced
• Not easily adaptable for large
construction costs
storage volumes
Trench-type
• Facilitates effective cleaning
• Cleaning procedure results in
to remove solids and
undesirable hydraulic conditions
floatables
for pumps
• Minimal horizontal surfaces
to avoid solids settling
• Increases velocities at pump
suction to facilitate solids
• Cleaning procedure results in
removal
Confined Inlet
undesirable hydraulic conditions
• Flexible design for varying
for pumps
storage volume requirements
• Does not require operation of
pumps under cavitation
conditions for solids removal
The wet well alternatives considered are similar in that the intention of both designs is to limit
settlement of solids in the wet well and provide a means to remove solids that do settle from the
wet well. Both wet well design alternatives considered require the pumps to operate at
undesirable hydraulic conditions during the recommended cleaning procedures. Because of the
large design capacity of the influent pump station and the requirement to maintain the water
surface elevation in the wet well below the invert of the Outfall Sewer, there is concern with the
size, resultant depth and construction complexity of the ogee spillway associated with the trenchtype wet well design.
With these considerations in mind, the preliminary design recommendation for the influent pump
station wet well at the BRWWTP is the confined inlet wet well. It is recommended to advise the
pump manufacturers of the intention to utilize a pump in a trench-type or confined inlet wet well
design and temporarily operate the pump under cavitation conditions during the cleaning
process. The owner, engineer and the pump manufacturer will collaborate during the design
process to understand the design intent and make provisions in the design to mitigate, to the
extent possible, the impact of temporarily operating the pumps under cavitation conditions.
Pump Type
The pumps selected for implementation at the BRWWTP will need to be able to handle large
solids and heavy grit loads, particularly during wet weather events when these items are often
mobilized. The proposed influent pump station is a high flow (100 mgd per pump) and low head
(40-50 feet) application. This evaluation considered horizontal and vertical centrifugal end
suction non-clog pumps and dry-pit submersible pumps. A brief description of the alternatives
considered is provided. A summary of the advantages and disadvantages is provided in Table 6.
Centrifugal end suction non-clog pumps are made to handle raw sewage containing solids and
stringy material. They are designed with large impeller eyes and a straight inlet design to
facilitate solids handling. This pump requires a dry-pit installation and can be configured in a
vertical or horizontal orientation. These pumps can pass solids up to 8 inches in diameter. Based
on the information provided by pump manufacturers contacted during the Study and Preliminary
Design phases, the maximum individual pumping capacity of a standard non-clog pump is
approximately 100 mgd. There is some flexibility for the maximum capacity of depending on
head conditions and impeller characteristics.
Vertical Non-Clog
Vertical Non-Clog pumps are installed with an extended shaft connecting the elevated motor to
the pump volute which is located in the drywell. With this particular orientation, a smaller
footprint is occupied as compared to the horizontal installation. The pump and motor are not
visible in a common line of sight. The motor is easily accessible for maintenance, but the pump
volute is more difficult to access. Extended shafting may require multiple guides and additional
bearings that require greasing. Figure 8 provides a representation of a typical vertical installation.
Figure 8. Vertical non-clog pump installation.
Horizontal Non-Clog
Horizontal Non-Clog pumps are installed with the pump motor and volute in the drywell in a
common line of sight. The footprint associated with this orientation is larger than that of a
vertical orientation. No extended shafting is necessary; however, in the event that significant
flooding occurs in the dry well, the motor will be damaged and the pump cannot be operated.
Figure 9 provides a representation of a typical horizontal installation.
Figure 9. Horizontal non-clog pump installation.
Dry-Pit Submersible
Dry-Pit Submersible Pumps are built with the pump and motor as an integral unit and is typically
installed in a drywell. This pump unit can be temporarily submerged and continue to operate.
These pumps can pass solids up to 3 inches in diameter. Based on the information provided by
pump manufacturers, the maximum individual capacity for the dry-pit submersible pump is
approximately 60 mgd. Figure 10 provides a representation of a typical dry-pit submersible
pump installation.
Figure 10. Dry-pit submersible pump installation.
Because of the limited capacity (maximum 60 mgd) of the dry-pit submersible pumps, this style
was eliminated from consideration.
Based on their large solids handling capacity and high flow capacity, centrifugal, end suction
non-clog pumps are preferable for the influent pump station. The BRWWTP operations and
maintenance staff has experience with both the horizontal and vertical type pump installations
and is comfortable with operating and maintain both types of pumps. The preliminary design
recommendation is to install vertical non-clog pumps at the proposed BRWWTP influent pump
station. The primary drivers for this selection are the reduced footprint associated with vertical
mounted pumps and the design characteristic of having the motor elevated out of the dry well in
the event of dry well flooding.
Table 6. Summary of pump type alternatives.
Pump Type
Non-Clog
Vertical
Non-Clog
Horizontal
Dry-Pit
Submersible
Advantages
• 8" diameter solids pass
• Maximum capacity = 100 mgd+
• Smaller footprint (10’ x 10’)
• Motor out of drywell in the event of
a flood
• 8" diameter solids pass
• Maximum capacity = 100 mgd+
• Pump volute and motor in common
line of sight
• No extended shaft
• Smaller footprint (12.5’ x 7’)
• Can be temporarily submerged
• Pump volute and motor in common
line of sight
Disadvantages
• Extended shaft required
• Multilevel installation
• Motor is susceptible to damage
from flooding
• Larger footprint (21’ x 10’)
• Maximum flow capacity = 60 mgd
• 3” diameter solids pass
• Reduced pump efficiency
Storage of Wet Weather Flows
An evaluation of the BRWWTP processes downstream of the proposed headworks facilities
determined that the hydraulic capacity is 469 mgd. Based on the information provided by the
WWPC, when the rehabilitation and hydraulic improvements to eliminate SSOs in the collection
are completed, the peak wet weather flows to the BRWWTP will increase significantly from the
current historical peak of 420 mgd. The model predicted influent hydrographs predict that the
peak influent flow to the BRWWTP will be in excess of 700 mgd. The proposed headworks
facilities will be designed to treat these peak weather flows, but the downstream processes at the
BRWWTP do not have adequate capacity. As such, storage for wet weather flow in excess of
the hydraulic capacity of the BRWWTP will be required.
To provide some measure of conservatism, for the purposes of calculating the required wet
weather equalization storage volume, the design hydraulic capacity of the BRWWTP has been
defined as 400 mgd. This means that influent flow in excess of 400 mgd will be diverted to the
proposed wet weather storage tanks following screening and grit removal. Based on the current
information from the WWPC’s macro model, it is estimated that 1 to 2 events per year will
exceed the design hydraulic capacity of 400 mgd. The duration and intensity of each wet
weather event determines the required storage volume. In order to meet the requirements of the
Wet Weather Program, 36 million gallons (MG) of wet weather storage is proposed at the
BRWWTP.
The preliminary design includes storage tanks downstream of the pump station, fine screen and
grit removal facilities. The tanks were placed downstream of the headworks processes to limit
floatables and grit accumulation in the storage tanks during wet weather events. The preliminary
design includes three, uncovered concrete storage tanks. It is anticipated that the tanks will be
used one or two times per year. The cost of providing covers, ventilation and odor control
equipment is difficult to justify with such infrequent use. A hydrogen peroxide system will be
provided for odor control.
The preference of the operations staff is to have the storage tanks placed at an elevation that
allows the tanks to fill completely by gravity. This will require significant excavation. The
preliminary soil boring data indicates significant groundwater presence. Excavation and
dewatering for construction and the foundation design for large tanks with a deep foundation will
be very expensive. The geotechnical investigation planned during the design phase will help
determine the elevation of the tanks and the requirement for pumping into or out of the tanks.
Proposed Headworks and Wet Weather Facilities
Figure 11 shows the proposed site plan for the Headworks and Wet Weather Facility at the
BRWWTP. A summary of the process is provided.
Figure 11. Proposed Headworks and Wet Weather Facility Site Plan.
Influent Pump Station
A diversion structure will deliver flow from the Outfall Sewers to the trench-type wet well.
Eight, 100 mgd horizontal, centrifugal end-suction pumps will lift the influent flow into a
common screen influent channel. Variable frequency drives will be provided for pump speed
control and influent flow matching. Seven duty, and one spare pump will be provided for a total
pumping capacity of 800 mgd.
Fine Screen Facility
Eight (8), 100 mgd 6 mm, continuous belt-filter element type screens (7 active, 1 spare) are
proposed. One washer/compactor will be provided for each screen. Conveyors will be provided
to deliver the screenings to a central screenings handling location.
Grit Removal Facility
The proposed grit removal facility includes sixteen, 50 mgd capacity aerated grit removal tanks.
The total capacity of the proposed facility is 800 mgd. One traveling bridge is proposed for
collected grit removal from each basin. A rack and pinion drive mechanism is proposed to
facilitate bridge movement along the basin. The grit slurry will be pumped to elevated troughs
which discharge to grit classifiers. The classifiers will discharge to conveyors which deliver grit
to a central solids handling location.
Wet Weather Storage Facility
Influent flow in excess of 400 mgd will be diverted to the wet weather facility from the grit
facility common effluent channel. Flow will be collected and stored here until the wet weather
event ends. A total of 36 MG of storage is proposed. The preliminary site plan shows three, 12
MG tanks with a side water depth of 40 feet and a diameter of approximately 230 feet. A pump
station is proposed to drain or fill the tanks, depending on the final selection for the hydraulic
profile. The pump station is sized at 110 mgd to drain the tanks in 8 hours.
The grit removal facility/wet weather facility effluent will be sent to a proposed PST distribution
chamber for primary treatment.
Ancillary Facilities
The following ancillary and support facilities will be provided for the proposed Headworks and
Wet Weather Facility.
•
•
•
•
•
•
Odor control facility(ies)
Electrical power supply and distribution facilities
Emergency power generation facilities for critical processes
Controls systems for the proposed equipment
Building and foundation structures
Roadway, site grading, drainage and stormwater management for the proposed facilities
SUMMARY AND CONCLUSIONS
The existing headworks facilities at the BRWWTP are inadequate in terms of hydraulic and
treatment capacity. The ineffective performance of these unit processes during peak wet weather
flows can allow large solids and grit carryover into the downstream processes that negatively
impacts performance.
The City of Baltimore is currently in the process of implementing rehabilitation and hydraulic
improvements in the collection system to eliminate SSOs in response to EPA and MDE Consent
Decree requirements. The City of Baltimore WWPC has identified the removal of a hydraulic
restriction (existing grit tank weirs) at the BRWWTP as a key hydraulic improvement to
eliminate SSOs in the Outfall Sewer system. It has also been suggested that the removal of the
hydraulic restriction and elimination of the backwater condition in the Outfall Sewer system will
improve wastewater transport velocities and reduce grit deposition in the Outfall Sewer.
Following the improvements in the collection system, including the removal of the hydraulic
restriction at the BRWWTP, peak influent flows to the BRWWTP during wet weather events are
predicted to increase from the current peak of 420 mgd to more than 700 mgd. The increased
influent flows will further exacerbate the issues associated with the inadequate hydraulic
capacity of the existing headworks facilities at the BRWWTP. In order to meet the requirements
of the Wet Weather Program, and meet the needs of the BRWWTP operations staff,
improvements to the headworks facilities at the BRWWTP are proposed.
The proposed improvements will include a new influent pump station to provide a “free
discharge” condition at the BRWWTP boundary. The proposed influent pump station will left
flow into a new screening facility and downstream grit removal facility. The maximum day
design capacity of the proposed influent pump station, screening facility and grit removal facility
is 700 mgd. 36 MG of wet weather storage is proposed. Influent flows in excess of 400 mgd
will be diverted to the wet weather storage tanks. The tanks will be drained following the wet
weather event and the stored wastewater volume will be returned to the BRWWTP process.
The headworks and wet weather facility upgrades at the BRWWTP provide a mutually beneficial
solution to problems that have been identified with the existing treatment plant and collection
system performance. The information and decisions presented are representative of the
preliminary design of the proposed facilities. The requirements of the Wet Weather Program
will continue to be refined and the design of the proposed headworks facilities at the BRWWTP
will be modified in the early stages of the design to respond to those changes. Project
stakeholders including the City of Baltimore Wastewater Facilities Engineering Section, City of
Baltimore Wet Weather Program and the BRWWTP Operations and Maintenance Staff will
continue coordination to define the final design characteristics of the proposed BRWWTP
Headworks and Wet Weather Storage Facility.
REFERENCES
Hydraulic Institute (2012) Pump Intake Design; ANSI/HI 9.8; Parsippany, New Jersey
McClinton, Jack. (1987) An Evaluation and Proposed Solution for Maximizing Daily Grit Removal At
the Back River Wastewater Treatment Plant Grit Chamber Process
Sverdrup Civil Inc. (1999) Final Design Memorandum for SC 9535: Grit Removal Facilities Rehabilitation