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 processRapid 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

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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 StrategiesQ = 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

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*

16_EF_0807.qxd 8/3/07 5:44 PM Page 19

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.

ReferencesNitz, 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 atscott.henderson@infilcodegremont.com.

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FIGURE 5: Sectional View of DendaDeg 2D-100

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