Rick Fuller
Senior Project Manager
Swinerton Management & Consulting
4055 Nelson Avenue
Concord, CA 94520
Cell: 925.270.7920
rfuller@swinerton.com
To:
Rich Davidson, City of Richmond
Mary Phelps, City of Richmond
Jim Good, Veolia Water
Jon Whitfield, Veolia Water
Steve MacLennan, Swinerton M&C
Date:
March 26, 2007
Re:
Wastewater Treatment Plant Capital Improvement Projects
Introduction
The key unit processes at the Richmond wastewater treatment plant (WWTP) are shown in Figure 1.
Capital improvement projects are recommended for several of these unit processes. In all, nine projects
have been identified with a total estimated project cost of $17,430,000 (see Table 1 on the following
page). A schedule for implementing these projects is provided in the Appendix.
The cogeneration project, in this case using a fuel cell, has a cost of $5.5 million. Grant money from
PG&E may be available to finance as much as one-half of this project. And it is the only project that will
provide a direct benefit to Richmond in that it will reduce power consumption at the treatment plant by
converting excess methane gas from the anaerobic digesters to electricity. Implementation of
cogeneration at the Richmond WWTP needs to be a top priority for the City during 2007/2008.
Figure 1: Richmond WWTP
INNOVATION
INTEGRITY
EXPERIENCE
Rev. A
The capital projects listed in Table 1 are not listed by priority. Rather, they follow the flow of wastewater
through the treatment plant. But changing regulations may well result in one or more of these capital
projects becoming a priority. Given that Richmond’s National Pollutant Discharge Elimination System
(NPDES) permit is up for renewal and that there is increasing pressure from the Environmental Protection
Agency to minimize or eliminate blending events, there is a particular need at this time to focus on the
performance of the primary clarifiers. In the sections that follow each of the nine recommended capital
projects will be discussed in more detail.
Table 1: WWTP Capital Projects Summary
1
2
3
4
5
6
7
8
9
Capital Project
Estimated Cost
Influent pumps
$1,200,000
Enhanced primary clarification
$375,000
Inclined plates for the primary clarifiers
$425,000
Redirect primary clarifier surface skimmings from digester to dumpster
$360,000
Boilers and heat exchangers for the anaerobic digesters
$1,700,000
Cogeneration to utilize digester methane for energy savings
$5,500,000
Gravity belt thickeners
$750,000
Sludge dewatering facility
$7,000,000
Stamford baffle for secondary clarifier no. 3
$120,000
Total:
$17,430,000
Influent Pumps
There are four Fairbanks-Morse influent pumps at the Richmond WWTP. Two are 125-HP units and two
are 100-HP units. Combined, these pumps can produce 40 million gallons per day (MGD). Though the
average daily flow to the treatment plant is approximately 8.0 MGD, during periods of precipitation, and
a significant contribution from infiltration and inflow, the flow to the treatment plant can quickly increase
to 40 MGD. When all four pumps are in service experience has shown that they are prone to “tripping
out.” In addition, these pumps were installed in the 1950’s. All four pumps need to be replaced with new
pumps. With the installation of new pumps we will also need to install new variable frequency drives
(VFDs). Further, there may be a need to replace some of the wiring for these pumps. The installation of
four new influent pumps, VFDs, and electrical improvements is estimated to cost $1,200,000.
Enhanced Primary Clarification
We don’t know at this time how Richmond’s NPDES permit will be modified upon its renewal later this
year or next. But we can, and should, anticipate more stringent discharge requirements. And we can
anticipate the need to reduce or eliminate blending events. Enhancing the performance of the primary
clarifiers can be done through chemical addition. For this capital project we would need to install a
concrete pad, chemical storage tanks, chemical feed pumps, and the associated piping. We would most
likely feed ferric chloride at the head of the primary clarifiers to achieve chemical precipitation. The
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addition of a polymer, in addition to an iron salt, will also need to be evaluated and has been shown to be
very effective in combination with an iron salt.
Chemically enhanced primary treatment, whereby wastewater is chemically coagulated before
clarification, is the simplest enhancement that can be made to conventional primary clarification to
increase treatment capacity. The use of chemicals allows a higher peak overflow rate during peak flow
events while maintaining or increasing primary clarifier performance. With chemical precipitation, it is
possible to remove 80 to 90 percent of the total suspended solids (TSS) including some colloidal particles,
50 to 80 percent of the biochemical oxygen demand (BOD), and 80 to 90 percent of the bacteria.
Comparable removal values for well-designed and well-operated primary clarifiers without the addition
of chemicals are 50 to 70 percent of the TSS, 25 to 40 percent of the BOD, and 25 to 75 percent of the
bacteria. Enhanced primary clarification is estimated to cost $375,000 not including the annual cost of
chemicals. Implementation of this option will require an amendment to the Veolia Water contract in order
to compensate them for the unanticipated chemical use.
Inclined Plates for the Primary Clarifiers
Plates installed at an angle in a clarifier will significantly increase the settling area available within a
given footprint (see Figure 2). It may be worthwhile to retrofit one primary clarifier with inclined plates
and compare its performance under a variety of flow conditions to the “standard” clarifier. This retrofit
could prove very beneficial in reducing the number of blending events. The estimated cost of this capital
project, for both primary clarifiers, is $425,000.
Figure 2: Inclined Plate Clarifier
Remove Primary Clarifier Surface Skimming’s from Digesters
The Richmond WWTP has a mechanical barscreen to remove coarse material from the wastewater. The
spacing between the bars is one inch. This spacing allows the passage of many “floatable” nonbiodegradable materials (plastics, for example) to the primary clarifiers. Currently, all skimming’s from
the primary clarifiers are pumped to the anaerobic digesters. This does not represent good engineering
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practice. The non-biodegradable materials take up space in the digester and reduce detention time. Instead
of sending these materials to the digesters they need to be discharged to a container for disposal at a
landfill. The estimated capital cost for this project is $360,000.
Digester Boilers/Heat Exchangers
The boilers and heat exchangers at the Richmond WWTP have reached the end of their useful life. They
need to be replaced. Currently, the one in-service digester is operated at a temperature of approximately
75ºF rather than at the recommended temperature of 95ºF. In addition to replacing the boilers and heat
exchangers there will likely be a need to replace some of the associated piping and valving. In order to
maximize methane gas production and position the City to benefit from cogeneration, we need to make
this a high-priority capital project. The estimated capital cost for replacing the boilers, heat exchangers,
piping, and valving is $1,700,000.
As stated above, the in-service digester is currently operating at approximately 75° F. This low
temperature results in an improperly digested sludge that will be very odorous upon being pumped to the
West County sludge drying beds where it is exposed to the atmosphere. Given the problems associated
with cogeneration, including the need to reduce seawater intrusion and hydrogen sulfide concentrations in
the digester, we need to proceed with the installation of new boilers and heat exchangers. This is a valid
capital project and it needs to be at the top of our list for improvements needed at the treatment plant.
Regardless of whether the boilers and heat exchangers are financed through a cogeneration project or
through a direct capital improvement, the City will be using its bond money to implement this project so
we should proceed with this immediately. In addition to the need for new boilers and heat exchangers we
also need to replace the waste gas burners and the associated equipment which would include pressure
regulators, gas meters, check valves, drip traps, and pressure relief and flame trap assemblies.
Figure 3: Fuel Cell
Cogeneration
After completing the installation of new
boilers and heat exchangers the City should
proceed to implement a cogeneration project.
Cogeneration can be accomplished using
engines, microturbines, or fuel cells (see
Figure 3). Veolia Water gave a proposal to
Richmond in 2006 that recommended the use
of an engine. The DER group also gave a
proposal to the City in 2006 that
recommended the use of a fuel cell. Richmond
needs to implement some type of cogeneration
at the WWTP. The City needs to turn its
excess methane gas into electricity to reduce power consumption costs at the treatment plant. The
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estimated cost of installing a cogeneration project is $5,500,000 (for the fuel cell, the project cost for an
engine would be much less). Up to one-half of this project would qualify for funding through PG&E.
Gravity Belt Thickeners
The use of dissolved air flotation (DAF) to thicken waste activated sludge (WAS) is inefficient given the
technologies that exist today. WAS should be thickened on a gravity belt thickener (GBT) before being
pumped to the anaerobic digester. The concentration of WAS is typically about ½ to 1% solids. A DAF
unit will increase this concentration to about 3 percent. A GBT will increase the concentration of WAS to
5 or 6% prior to being pumped to the anaerobic digester. This reduces the volume of water that must be
heated in the digester and increases the detention time in the digester which in turn maximizes methane
gas production. The estimated cost of installing GBTs is $750,000.
Figure 4: Totally Enclosed Gravity Belt Thickener for Outdoor Installation
Stamford Baffle
With the installation of a “Stamford Baffle,” which is also known as a “Submerged Flow Deflection
Baffle” or an “Upflow Baffle,” in a clarifier, the clarifier’s performance can be improved by up to 38%.
Stamford Baffles eliminate the “wall effect,” whereby density currents flow up the wall of the clarifier,
causing suspended solids to flow directly into the effluent trough. The baffles are attached to the wall of
the clarifier and slope downward at a 45° angle, extending into the tank approximately 3 feet. The baffle
re-directs the density currents away from the launder, toward the center of the tank, which increases
efficiency by settling more solids. This is shown graphically on the following page in Figure 4.
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Figure 5: Stamford Baffle
High solids carryover.
For a variety of reasons, some clarifiers exhibit
poor hydraulics. This is certainly the case with
secondary clarifier no. 3. In such cases, a
secondary clarifier without a Stamford Baffle
allows solids to travel along the tank bottom, up
the sidewall, and over the weirs.
A secondary clarifier with a Stamford Baffle
(portrayed as the red lines in the diagram)
intercepts the solids and directs them back to
the bottom of the tank where they are
removed.
Conclusion
It should be understood that this document was prepared without direct input and participation from
Veolia Water. But I have had many conversations with Jon Whitfield, Veolia Water Plant Manager, and
know well his needs and concerns. So I believe I have identified those capital projects that will most
beneficial to both Richmond and Veolia Water. Nonetheless, acceptance of this document by Richmond
requires acceptance by Veolia Water. Therefore, before adopting the capital projects identified in this
report, and their associated costs, Veolia Water needs to comment.
Finally, the City needs to work with Veolia Water to implement a cogeneration system at the wastewater
treatment plant. This is the single most important capital project the City can do that will directly result in
lowering its cost of operation at the treatment plant. Excess methane gas is currently burned (flared) to the
atmosphere. Methane is a valuable by-product of anaerobic digestion. The City needs to capture the
excess methane and convert it to electricity.
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Appendix
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Figure 6: Fuel Cell Control Panel
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Figure 7: Fuel Cell Gas Cleaning System
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ID
Task Name
1
Richmond WWTP Capital Projects
2
Influent pumps
Duration
Start
Actual Start
Finish
Cost
990 days
Mon 3/26/07
Mon 3/26/07
Fri 1/7/11
$17,430,000.00
210 days
Mon 3/26/07
NA
Fri 1/11/08
$1,200,000.00
120 days
Mon 3/26/07
NA
Fri 9/7/07
$300,000.00
Predecessors
3
Electrical
4
VFDs
45 days
Mon 9/10/07
NA
Fri 11/9/07
$200,000.00 3
5
Pumps & motors
45 days
Mon 11/12/07
NA
Fri 1/11/08
$700,000.00 4
255 days
Mon 3/26/07
NA
Fri 3/14/08
$1,700,000.00
120 days
Mon 3/26/07
NA
Fri 9/7/07
$1,000,000.00
6
Digesters
7
Digester boilers & heat exchangers
8
Digester piping
60 days
Mon 9/10/07
NA
Fri 11/30/07
$250,000.00 7
9
Digester valving
30 days
Mon 12/3/07
NA
Fri 1/11/08
$400,000.00 8
10
Digester automation/SCADA
45 days
Mon 1/14/08
NA
Fri 3/14/08
$50,000.00 9
11
Cogeneration
450 days
Mon 9/10/07
Mon 9/10/07
Fri 5/29/09
$5,500,000.00 7
12
Enhanced primary clarification
150 days
Mon 5/4/09
Mon 5/4/09
Fri 11/27/09
$375,000.00
13
Primary clarifier inclined plates
150 days
Mon 5/3/10
Mon 5/3/10
Fri 11/26/10
$425,000.00
14
Primay skimmings removal
160 days
Mon 4/6/09
Mon 4/6/09
Fri 11/13/09
$360,000.00
15
Gravity belt thickeners
180 days
Mon 5/3/10
Mon 5/3/10
Fri 1/7/11
$750,000.00
16
Sludge dewatering
730 days
Mon 3/26/07
NA
Fri 1/8/10
$7,000,000.00
17
Stamford baffle
60 days
Mon 3/26/07
Mon 3/26/07
Fri 6/15/07
$120,000.00
Page 1
ID
Task Name
1
Richmond WWTP Capital Projects
2
Influent pumps
3
Electrical
4
VFDs
5
Pumps & motors
6
'07
Q2 '07
Q3 '07
Q4 '07
Q1 '08
Q2 '08
Q3 '08
Q4 '08
Q1 '09
Q2 '09
Q3 '09
Q4 '09
Q1 '10
Q2 '10
Q3 '10
Q4 '10
Q1 '11
e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e
Digesters
7
Digester boilers & heat exchangers
8
Digester piping
9
Digester valving
10
Digester automation/SCADA
11
Cogeneration
12
Enhanced primary clarification
13
Primary clarifier inclined plates
14
Primay skimmings removal
15
Gravity belt thickeners
16
Sludge dewatering
17
Stamford baffle
Project: WWTP_CIP
Date: Mon 3/26/07
Task
Milestone
External Tasks
Split
Summary
External Milestone
Progress
Project Summary
Deadline
Page 1
USFilter to Supply Primary Wastewater Treatment Technology for Alberta's Capital City
WARRENDALE, Pa., August 30, 2005 – The City of Edmonton, Alberta, Canada, has chosen USFilter, a part of Siemens Water
Technologies, to provide primary wastewater treatment technology for an upgrade and expansion of its Gold Bar Wastewater Treatment
Plant (WWTP). USFilter will supply Zimpro® inclined plate separators and Envirex® HS730 Loop Chain and scraper systems for the
plant’s new, enhanced primary treatment process.
As part of the Enhanced Primary Treatment project, USFilter provided 32 Envirex® longitudinal loop chain and scraper collector systems
with four cross collectors. The collectors will be installed into a new rectangular clarification tank, providing the city with additional
treatment capacity.
Typically, fiberglass flights mounted on two parallel strands of collector chain scrape the settled solids along the tank floor to sludge
hoppers. On the return run, the flights can skim the surface and concentrate the floating material at a scum removal device.
“Faced with more than 100 mechanisms to monitor and adjust, reduction of maintenance was an important issue for the wastewater
treatment staff,” said Bill Selle, product manager for USFilter. “With the collector systems’ light weight components, such as the nonmetallic chain and fiberglass flights, they are designed to make maintenance tasks as easy and convenient as possible.”
In addition, within four new basins, 192 Zimpro® incline plate separators will provide dual use either as primary clarifiers prior to
biological treatment or as stormwater clarification. The 158-mgd system is the first installation by USFilter of inclined plate separators for
primary clarification of wastewater.
Typically a drinking water treatment product with 2-inch-spaced inclined plates, the Gold Bar WWTP separators will use inclined plates
with increased spacing to accommodate a higher concentration of wastewater solids. During normal flow conditions, the separators can be
used as primary clarification prior to the biological treatment, helping improve the efficiency of existing equipment. Secondarily, the
separators can be used to treat stormwater peaks, which are then blended with effluent from the conventional treatment train before
discharge.
“This is an interesting application for our plate separators,” said Mike Mayer, technical director for USFilter. “For the same amount of
treatment, conventional clarifiers would take up to 10 times the space of our separators. This is critical because without the use of inclined
plate separators, there would not have been enough space available at the site to accomplish the desired treatment. The separator system will
provide the customer with flexibility and major space savings.”
USFilter partnered with mechanical subcontractor Schendel Mechanical Contracting, Ltd.; prime contractor Sure-Form Construction Ltd.;
and project consultant Stantec Consulting, Ltd., all of Edmonton, Alberta, Canada. Funding for the project was provided under the terms of
the Infrastructure Canada-Alberta Program Agreement.
USFilter is represented locally by Mequipco Sales, Ltd., Calgary, Alberta, and Canada.
As part of the ongoing process upgrade at the Gold Bar WWTP, USFilter will also provide chain and scraper components to retrofit existing
clarification basins at the plant. Components and hardware for 45 longitudinal collectors and 10 cross collectors will be supplied.
BAFFLE BASICS ... A BRIEF TUTORIAL
"Density currents" and "density current baffles" have become common
terms in the lexicon of clarifier design over the past several years, with the
baffle becoming a standard element in clarifier design. Here are some
"Baffle Basics" to keep in mind when designing the baffle into the next
project.
1. Density currents form in all activated sludge clarifiers and cause
short-circuiting of the main clarification volume of the tank. This
increases effluent suspended solids and reduces the tanks actual hydraulic
capacity. In fact, the hydraulic performance of all center-fed clarifiers is the
same, regardless of the process type, i.e. activated sludge, trickling filter,
etc. In each case the magnitude of the density currents will vary depending
upon flow, the configuration of the tank, the configuration of the feedwell
and the operation of the clarifier.
2. The principal cause of density currents in center-feed tanks is the
downward flow of the dense input mixed liquor at the feedwell,
although temperature differentials and internal tank hydraulics may also
introduce currents which disrupt settling. Because the hydraulic flow within
the tank tends to spiral slowly from the center outward toward the walls,
the currents acting within the tank contribute to the movement of solids
toward the weirs.
3. Density currents affect deep tanks as well as shallow tanks. In deep
tanks, the input liquor cascades downward over a greater distance and
may reach significant velocity. Tanks with 12 to 15 foot SWD commonly
exhibit short-circuiting, as do those with less SWD. Density currents also
occur irrespective of tank diameter, with short-circuiting common in both 25
foot and 200 foot diameter clarifiers.
4. Density currents tend to move along the top of the blanket, where
MLSS are in the 2000 range, and carry the lighter solids to the outer wall
and upward toward the weir. If there is insufficient distance between the
bottom of the inlet feedwell and the top of the blanket, the currents may be
sufficient to cause scouring of the tank.
5. The action of the Density Current Baffle is two-fold: 1) it directly
impedes the current (and solids) flowing up the tank wall and diverts it
back into the center of the tank, and 2) it produces a hydraulic barrier
around the tip of the baffle which causes those currents in the vicinity of
the baffle to flow around and toward the tank center without ever reaching
the wall.
6. The Stamford, or Inclined, Baffle offers several advantages over
the McKinney, or Horizontal, Baffle: 1) because it is inclined away from
the weir, the Stamford Baffle keeps the lighter solids at a greater
distance from the weir overflow current, 2) the incline allows solids which
build up on the upper surface of the baffle to slide off periodically ... the
horizontal baffle is known to collect solids and cause maintenance
problems, 3) the inclined design is easier to retrofit in an existing tank than
is a horizontal design.
7. Tanks with internal launders also require density current baffles.
Although the bottom of the launder does act as part of the baffle, solids
emerging from beneath the launder are easily drawn into the localized
current of the overflow weir. A baffle cantilevered out and downward from
the lower inboard comer of the launder will deflect solids away from the
weir and back into the main clarification volume of the tank, just as in the
case of a wall mounted baffle.
8. Baffle size varies as a function of tank diameter, depth and
configuration. For inboard launders, we recommend a 24" to 36” long
baffle mounted at the lower inboard corner of the launder and inclined at a
45° angle. Larger baffles may be required in larger tanks. For outboard
launders and dual weir configurations, we recommend a baffle mounted to
the tank wall at a 45° angle. The size of this baffle can range from 24" to as
large as 68".
9. The proper depth of the baffle in the tank is a function of tank
depth and the height of the blanket. Ideally, the baffle should be
positioned such that the lowest point of the baffle will be in the clear zone
above the blanket. This usually means that the baffle should be mounted
three to four feet below the weir. This height can vary somewhat to avoid
pipes, supports and other obstructions in the tank, but the bottom of the
baffle should be positioned low enough to be effective, yet as far above the
blanket as possible. It may be necessary to reduce the inclination angle of
the baffle to obtain the desired horizontal projection and keep the baffle
above the blanket.
10. At a minimum, the baffle must be designed to withstand the
upward buoyancy force of the liquid in the tank. This force varies as a
function of the density of the mixed liquor. Assuming that the liquid has the
density of water, this force can be 3000 pounds or more under an eightfoot baffle section. Some form of venting is required to allow gas that can
collect beneath the baffle to escape. These conditions require a rugged
baffle that is firmly attached to the tank wall. Thin baffles, and those that
simply cantilever from the wall without benefit of a triangular support
bracket, cannot withstand the forces at work in the tank.
11. Density current baffles are most effective when the clarifier is
operating in the average to peak flow ranges. TSS reductions in the
range of 25% to 35% are routinely reported under average flow conditions,
and 40% to 50% improvements are not uncommon at peak flow. Clarifiers
that are operated below design flow may not benefit from the use of a
Density Current Baffle.
Density Currents In Activated Sludge Clarifiers
It’s Better To Be Baffled!
Earle Schaller
here are 22,000 waste treatment plants in the United
States, each processing over 1 MGD. Two-thirds—
approximately 15,000—are municipal plants, while
the remainder are industrial. It has been estimated that some
40 percent of these plants are operating at or beyond permit
levels. There are numerous reasons for this, including design
problems, operating problems, and increased loading.
Density Currents
In activated sludge plants, we can attribute some of the
problem to the presence of density currents in the clarifiers.
We know that density currents form in all activated sludge
clarifiers, regardless of configuration. We also know that these
currents have a significant negative impact on clarifier performance. Density currents create short circuits, increase effluent solids and reduce hydraulic capacity. Figure 1 depicts a
typical centerfeed well and external launders. Density currents form in this tank when the denser input from the
feedwell plunges to the bottom of the tank and then moves
across the bottom just above the blanket until it reaches the
outer wall. It then travels up the tank wall toward the effluent
launder. As it courses along the bottom of the tank, the density
currents pick up lighter solids which have been deposited and
carry them up the wall to the effluent, in effect short-circuiting
the main clarification volume of the tank. The net effect is an
increase in TSS and a decrease in the apparent hydraulic
capacity of the tank.
Density currents do not move in waves. Rather, their action
is jet-like and random. They have been known to bounce off the
bottom of the tank, and ricochet off the sides. Potential solutions to the density current problem include modifications to
the feedwell where the problem appears to begin, placement of
the weirs as far from these currents as possible, and the use of
in-tank baffles as a physical barrier to these currents.
Virtually every form of feedwell modification has been
attempted over the years. While several have been successful
in specific configurations, no one modification has proven
transportable for general use in clarifier design.
Numerous design standards recognize the effect of density
currents and suggest that weirs be placed as far from them as
possible. Here again, numerous configurations have been
examined, including the use of duel inboard weirs located
several feet from the tank wall. No single design has proven
effective in and of itself in eliminating the effect of density
currents, and some have even exacerbated the problem.
In-Tank Baffles
In-tank baffles have proven to be the most effective defense
against density currents. Baffles have reduced TSS by as
much as 50 percent, restored hydraulic capacity, provided a
significant reduction in BOD, and reduced the downstream
costs of treatment in terms of chlorine and other chemicals.
Three types of baffles have been employed and each has been
successful to almost the same degree in managing the impact
of these currents on clarifier performance.
The ring baffle is a cylinder that sits within the clarifier and
has a diameter approximately half that of the tank itself. If the
OCTOBER 1995
Figure 1. Density Currents in an
Activated Sludge Clarifier
Figure 2. Clarifier with Inclined
“Stamford” Baffle
tank has a 100-foot diameter, the ring baffle has a 50-foot
diameter. The baffle cylinder reaches from just above the
blanket to just below the surface of liquid in the tank. The
baffle is suspended from the clarifier mechanism and rotates
with it. While ring baffles have proven effective in dissipating
density currents, they are more complex and more expensive
to implement than the other baffle types. Moreover, the ring
baffle places additional load on the clarifier mechanism and
imposes added maintenance complications. Since it is no more
effective than the other baffles, it is not generally found in use.
It should also be noted that the use of the ring baffle causes
some problems with sludge removal balance where riser pipes
are used.
The horizontal baffle, sometimes known as the McKinney or
Lincoln baffle, is formed by extending the bottom of the
inboard launder eighteen to twenty-four inches into the tank.
This baffle has proven effective in stopping the solids carried
by the density currents from reaching the weir. It is generally
poured in place during the initial construction of the tank and
is not easily retrofitted. The negative aspect of the horizontal
baffle is the fact that sludge collects on the upper horizontal
surface and requires considerable maintenance. Note that the
baffle extends inboard of the weir and does not stop at the
inboard edge of the launder trough. This is important in
ensuring that the solids carried by the density currents are
redirected towards the central clarification volume of the tank
and are not allowed to simply drift upward toward the weir.
FLORIDA WATER RESOURCES JOURNAL
33
The inclined, or Stamford, baffle is simply the horizontal
baffle inclined at an angle sufficient to allow built-up sludge
to fall away. The Stamford baffle has proven extremely
effective and is the most popular baffle in use today. The
baffle can be fabricated from numerous materials and is
ideally suited to both new and retrofit installations. In Figure
2, the inclined baffle is mounted directly to the tank wall and
inclined at an angle of 30 to 60 degrees. The baffle is normally
located approximately three feet below the level of the scum
baffle. In tanks with internal launders, the baffle is cantilevered from the lower inboard corner of the launder trough.
The Connecticut Experience
The Stamford baffle was first implemented at the Stamford, CT WWTP. The Stamford plant supports a large urban
population and is located on the shores of the Long Island
Sound. Stamford had two 130-foot secondary clarifiers. The
plant had never operated within permit levels and had a
significant odor problem. Dye tests and other studies indicated that density currents were a significant contributing
factor to the plant’s problems. A prototype baffle was fabricated of plywood and was installed in one of the two tanks.
This resulted in a 38 percent improvement in TSS. Within 60
days the plant was operating within permit levels. NEFCO,
Inc. later replaced the wooden prototype with the first full
scale fiberglass density current baffles which were installed
in each of the two 130-foot clarifiers. Another baffle was
installed in the new third clarifier, which was constructed in
1991. All the work carried out at the Stamford plant was done
in conjunction with the Connecticut DEP, which called the
baffle “the most cost-effective improvement in clarifier performance today.”
Connecticut DEP lobbied the state legislature to institute
a municipal grant program enabling municipalities to improve clarifier performance by installing baffles and other
devices. Connecticut now requires that a baffle be installed as
part of all new and upgrade projects. Other states are expected to follow this lead and mandate the installation of
baffles. Connecticut now has more than fifteen plants equipped
with density current baffles. According to the State DEP the
baffles improve plant performance by 35 to 40 percent under
average flow conditions and as much as 50 percent during
peak flow.
The Greater Lawrence plant in Lawrence, Massachusetts,
installed density current baffles in all four of its 165-foot
diameter secondary clarifiers in 1993 and experienced even
greater improvements than those at Stamford. In mid-1994,
the city of Phoenix, Arizona, installed baffles in four of ten
140-foot diameter primary clarifiers. This underscores the
fact that density currents form in primary as well as secondary clarifiers and that the baffle is equally effective in dealing
with these currents at the primary treatment level. Baffles
are currently being installed in four of the 80-foot square
tanks at the Calumet WWTP, which serves much of Chicago.
Here in Florida, density current baffles are currently being
installed at the Northside WWTP in Lakeland and are planned
for several other locations.
horizontal distance from the wall to the outer edge of the
baffle. D generally ranges from 18 to 48 inches. The actual
length of the baffle, of course, depends upon the inclination
angle. In the case of an inboard launder configuration, the
bottom of the launder trough contributes to the baffle action.
Here, a smaller baffle can be used. Note, however, that this
baffle extends out beyond the weir to ensure that solids are
not simply allowed to drift upward. In the case of dual weirs,
solids are often deposited in the space between the outer weir
and the wall and then flow over the outer weir. Here the baffle
is fastened directly to the tank wall as though in an external
launder configuration.
The baffle is typically described as a series of brackets
attached to the wall at two-foot intervals and then covered
with panels of aluminum, pvc, stainless steel, or fiberglass.
The brackets themselves may be made of any of these materials. While this is a typical baffle configuration, it is expensive to fabricate and labor intensive to install. It involves
considerable material handling, and alignment can be tedious and time-consuming. Proper fastening of the baffle is
critical, however. The force of gasses trapped beneath the
baffle, and the hydraulic pressure on the baffle when the tank
is being filled can be sufficient to tear the baffle from the tank
if it is not adequately secured.
The Stamford Baffle is comprised of eight foot baffle
“modules.” Each module is a one-piece unit which incorporates the baffle panel, bracket, and mounting and stiffening
flanges. The module is molded of corrosion-resistant, uv
protected fiberglass. The modules interlock to form a rigid
structure, once installed.
There are a number of criteria to consider when developing
specifications for the baffle.
• It must fit the curvature of the tank with no large gaps
between the baffle and the tank wall.
• The width of the baffle should be scaled to the tank
diameter and range from 18 to 48 inches. Too small a
baffle would be ineffective. Too large a baffle would interfere with the clarification action of the tank.
• The baffle should always extend beyond the weir.
The baffle should be inclined at an angle of 30 to 60 degrees
to facilitate self-cleaning of sludge.
• The baffle should be supported adequately to ensure that
it is held rigidly in place and has a professional, wellengineered appearance.
Provision should be made for venting gases and air that
can become trapped beneath the baffle. To accomplish this,
NEFCO molds vents directly into the mounting flanges.
There are three materials generally considered appropriate for baffle construction: stainless steel, aluminum and
fiberglass. Stainless steel is an expensive material for use in
this application. Aluminum is subject to corrosion in many
waste environments. Fiberglass is the ideal material: it can
be molded to shape; it can withstand years of being submerged; and finally, it can be formulated to withstand the
most corrosive environments, a factor which is often encountered in industrial waste.
Design Considerations
There is a relationship between tank diameter and the
optimum width, D, of the baffle, where D is measured as the
34
Earle Schaller is president of NEFCO, Incorporated, Palm Beach Gardens.
FLORIDA WATER RESOURCES JOURNAL
OCTOBER 1995