High-Rate Solids Classifier for Combined Sewer Overflow treatment

1. EDITOR’SFOCUS
By K. Scott Henderson
16 August 2007 • WWDmag.com • WATER & WASTES DIGEST
I
n December 2002, the city of Toledo, Ohio, signed
a consent decree with the U.S. Environmental
Protection Agency (EPA) that required the city to update
its sewer and wastewater treatment facilities to stop wet
weather bypassing of untreated wastewater into the
Maumee River. To achieve regulatory compliance associated
with combined sewer overflow (CSO) events, it was
determined that the Bay View Wastewater Treatment Plant
(WWTP) must increase its total treatment capacity from
200 to 400 million gal per day (mgd), or approximately
5.3 times the average dry weather flow rate.1
Prior to the final design, pilot testing of high-rate clar-
ifier technologies was conducted from December 2002
to February 2003 to determine the feasibility of using
this technology to achieve the required effluent criteria
for both primary and CSO wastewaters. Based on the
results of the pilot study, the DensaDeg high-rate solids
contact clarifier, manufactured by Infilco Degremont,
Inc., was chosen for the full-scale installation.
Based on the design parameters, it was determined that
the facility would need six of these clarifiers. In November
2006, the construction of the largest high-rate clarification
system for CSO treatment was completed. The overall layout
can be seen in Figure 1, and a picture of the constructed
facility is shown in Figure 2. The maximum design capacity
for the facility is 232 mgd with each of the six high-rate
clarifiers having the capacity to treat 38.7 mgd at a surface
overflow rate of 47 gpm/ft2
.
The clarifier
The DensaDeg clarifier is a high-rate solids contact clarifier
that combines optimized flocculation, internal and external
sludge recirculation and lamellar settling to achieve very high
hydraulic loadings and treatment efficiencies. It is well proven
in the field of physical and chemical treatment of wastewater
and drinking water and has been employed for a wide range
of municipal and industrial applications.
The DensaDeg 2D-100 for physical and chemical treatment
of raw municipal wastewater, CSO and sanitary sewer over-
flows (SSO) combines coagulation, flocculation, static and
lamellar clarification, scum removal and sludge thickening in a
single treatment unit. Optimized flocculation and solids con-
tact through internal and external sludge recirculation pro-
duces dense floc with an extremely high settling rate (typical
design rates are 40 gpm/ft2
with the hydraulic capacity to
accommodate rise rates up to 60 gpm/ft2
, with a moderate
reduction in treatment efficiencies) without adding ballast
materials such as sand.
The process
Rapid mix. The first step in the clarifier process, coagula-
tion, takes place in a single vessel. A coagulant, either alu-
minum or iron salt, is injected directly into the inlet pipe, or if
possible, upstream in the feed channel. Mechanical mixers
cause an intense bulk fluid motion for coagulant dispersion
and mixing. The flow then passes to the reactor vessel.
Reactor vessel. The reactor vessel is where particle floc-
culation and densification occurs. The flocculation reaction is
optimized in the reactor vessel through the combined effects
of internal sludge recirculation, high solids concentration and
efficient mixing. A high concentration of solids is maintained
in the reactor through external recirculation of settled solids
from the clarifier/thickener vessel. Internal recirculation is
achieved through an axial flow impeller/draft tube arrange-
ment. Polymer, usually a high molecular weight anionic type,
is injected either directly beneath the impeller blades or in the
sludge recirculation line. Polymer aids in the flocculation and
densification of the coagulated particles.
Clarifier/thickener vessel. Quiescent conditions exist in
the clarifier/thickener vessel, where the dense floc particles
formed in the reactor vessel are allowed to settle and thick-
en. A circular, picket fence-type scraper mechanism collects
A look at the largest
high-rate clarification
system for CSO treatment
in North America
A Compact Solution
for CSO Treatment
FIGURE 1: Overall Layout – Bay View WWTP Wet Weather Facilities
FIGURE 2: Bay View WWTP – DensaDeg High Rate Clarifier Facility
16_EF_0807.qxd 8/3/07 5:43 PM Page 16

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3. 18 August 2007 • WWDmag.com • WATER & WASTES DIGEST
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settled sludge into a center bottom hopper.
Thickened sludge (typically 20 to 40 g/L)
is pumped to a holding tank and then trans-
ferred directly to digesters or dewatering
facilities. Additional sludge thickening is
generally not required. A separate set of
recycle pumps return a portion of the
sludge to the reactor vessel.
Settled water then flows upward through
settling tubes where stray floc is removed.
The majority of the solids are removed from
the stream prior to entering the lamellar mod-
ules. The lamellar tubes act as polishers only
and are therefore not subject to clogging.
Final clarified effluent collects in a trough for
further downstream treatment or discharge.
Operational strategy
One of the unique features that the
DensaDeg facility provides is the ability to
treat both primary and CSO wastewaters.
In order to incorporate this flexibility of
operation, an operational guideline was
created for the Bay View WWTP. Based on
the incoming flow and status of the equal-
ization basin volume, multiple modes of
operation are available. The excess flows
can be directed to the high-rate clarifiers,
and effluent produced can then be sent to
the activated sludge system. Or, during
high flow events, HRC effluent can be
directed to the equalization basin, and
when the basin reaches design capacity,
the flow is then directed to the dedicated
wet weather disinfection system and
blended with the plant effluent from
the dry weather facilities before it is
discharged into the Maumee River. The
four main operational scenarios are
summarized in Table 1.
Performance testing
Based on the required operational flexibil-
ity of the Bay View WWTP, the performance
testing was formulated to simulate the vari-
ous operational scenarios. Each clarifier was
tested individually to assess performance.
Scenario Operational Strategies
Q = 0 to 130 mgd • All flow is treated by the dry
weather Bay View WWTP facilities.
Q = 130 to 190 mgd • Up to 160 mgd is treated by the dry
weather Bay View WWTP facilities;
• 30 mgd is directed to the high-
rate clarifier (HRC) facility—HRC
effluent is routed to the activated
sludge system;
Q > 190 mgd • Up to 160 mgd is treated by the dry
& Veq < Vtot weather Bay View WWTP facilities;
• Excess flow are directed to the
HRC facility;
• 30 mgd of HRC effluent is routed
to the activated sludge system;
• Remainder of effluent is dis-
charged to the equalization basin.
Q > 195 mgd • Up to 160 mgd is treated by the dry
& Veq = Vtot weather Bay View WWTP facilities;
• Excess flows are directed to the
HRC facility;
• 30 mgd of HRC effluent is routed
to the activated sludge system;
• Remainder of HRC effluent is
routed to disinfection and
blending with effluent from dry
weather Bay View WWTP.
Vtot = Total volume of equalization basin (25 million gal)
Veq = Volume of wastewater in the equalization basin at a given time
TABLE 1: Operational Strategy Summary
– Bay View WWTP
16_EF_0807.qxd 8/3/07 5:44 PM Page 18

4. WATER & WASTES DIGEST • WWDmag.com • August 2007 19
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FIGURE 3: Typical Performance Test Run – TSS Removal Efficiency FIGURE 4: Typical Performance Test – Particulate CBOD Removal Efficiency
TSS CBOD Total P Influent TSS Effluent TSS Influent CBOD Effluent CBOD Influent Total P Effluent Total P
Primary treatment performance level 83% 54% 89% 121 17 116 50 2.9 0.29
Wet weather treatment performance level 1 79% 53% 86% 102 21 105 46 2.9 0.41
Wet weather treatment performance level 2 70% 46% 72% 96 29 90 52 2.5 0.53
*Concentrations are in mg/L
TABLE 2: Performance Test Summary - Average Removal Efficiencies and Average Concentrations*
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5. 20 August 2007 • WWDmag.com • WATER & WASTES DIGEST
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The first level of performance testing
required the units to operate at a flow rate
of < 27 mgd (SOR = 32.9 gpm/ft2
), the sec-
ond level of testing required a flow rate of
31 mgd (SOR = 37.5 gpm/ft2
), and the third
level a flow rate of 38.7 mgd (SOR
= 47 gpm/ft2
).
During the testing of each performance
level, samples were taken during each hour
of operation to measure TSS, CBOD and
total phosphorus. Each clarifier was tested
for a six-hour period and figures 3 and 4 (
page 19) show representative runs for an
individual DensaDeg unit. Table 2 (page 19)
provides a summary of the average removal
efficiencies and average concentrations
achieved during of the full-scale perfor-
mance testing.
Conclusion
The system recently constructed at the
Bay View WWTP is the largest high-rate clar-
ification system for CSO treatment in North
America. As seen in the performance test
data, the DensaDeg can achieve removal
efficiencies in the range of 70 to 95% for
TSS, 55 to 70% for total CBOD and 70 to
95% for total phosphorus, while operating
at high surface overflow rates.
These full-scale results confirm those
achieved during the pilot testing and meet
the design requirements of the facility.
This demonstrates that based on the high-
rate design of the process, large CSO treat-
ment capacity can be achieved in a rela-
tively moderate site area (232 mgd within
a 43,500 ft2
area), thereby providing
municipalities with a compact solution
for CSO treatment.
References
Nitz, D. et al. (2004) Session 23: Wet
Weather Treatment Issues “Wet Weather
Treatment Pilot Testing: High Rate
Clarification and UV Disinfection in a Cold
Climate.” WEFTEC 2004, New Orleans, La.
K. Scott Henderson, P.E., is application
engineer at Infilco Degremont. He can be
reached at 804/756-7623 or by e-mail at
scott.henderson@infilcodegremont.com.
For more information, write in 1101 on this
issue’s Reader Service Card.
WWD
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FIGURE 5: Sectional View of DendaDeg 2D-100
16_EF_0807.qxd 8/3/07 5:44 PM Page 20