where

where k1, k2, ..., kn are the reaction rates in cells 1 through n (all usually assumed equal for lack of better information) and t1, t2, ..., tn are the hydraulic residence times in the respective cells.

Mara (1975) has shown that a number of equal volume reactors in series is more efficient than unequal volumes; however, due to site topography or other factors, in some cases it may be necessary to construct cells of unequal volume.

The selection of the k value is the critical decision in the design of any pond system. The Ten-States Standards (Great Lakes-Upper Mississippi River Board of State Sanitary Engineers, 1990) recommended a design value of 0.276 d-1 at 20°C and 0.138 d-1 at 1°C. Using these values to calculate the temperature coefficient yields a value of 1.036. Boulier and Atchinson (1975) recommended values of k of 0.2 to 0.3 at 20°C and 0.1 to 0.15 at 0.5°C. A temperature coefficient of 1.036 results when the two lower or higher values of k are used in the calculation. Reid (1970) suggested a k value of 0.28 at 20°C and 0.14 at 0.5°C based on research with partial-mix ponds aerated with perforated tubing in central Alaska. These values are essentially identical to the recommendations of the Ten-States Standards.

When using the partial-mix design model, the number of cells in series has a pronounced effect on the size of the pond system required to achieve the specified degree of treatment. The effect can be demonstrated by rearranging Equation 4.11 and solving for t:

All terms in this equation have been defined previously. Example 4.2

Compare detention times for the same BOD removal levels in partial-mix aerated ponds having one to five cells. Assume C0 = 200 mg/L, k = 0.28 d-1, and Tw = 20°C.

Solution

1. Solve Equation 4.13 for a single-cell system:

0.28

2. Similarly, when:

n = 2, then t = 11 d. n = 3, then t = 9.4 d. n = 4, then t = 8.7 d. n = 5, then t = 8.2 d.

3. Continuing to increase n will result in the detention time being equal to the detention time in a plug-flow reactor. It can be seen from the tabulation above that the advantages diminish after the third or fourth cell.

4.3.1.3 Temperature Effects

The influence of temperature on the reaction rate is defined by Equation 4.14:

where kT = Reaction rate at temperature T (d-1).

0 = Temperature coefficient = 1.036.

The pond water temperature (Tw) can be estimated using the following equation developed by Mancini and Barnhart (1976):

f = Proportionality factor = 0.5.

Q = Wastewater flow rate (m3/day).

An estimate of the surface area is made based on Equation 4.13, corrected for temperature, and then the temperature is calculated using Equation 4.15. After several iterations, when the water temperature used to correct the reaction rate coefficient agrees with the value calculated with Equation 4.15, the selection of the detention time in the system is completed.

The ideal configuration of a pond designed on the basis of complete-mix hydraulics is a circular or a square pond; however, even though partial-mix ponds are designed using the complete mix model, it is recommended that the cells be configured with a length-to-width ratio of 3:1 or 4:1. This is done because it is recognized that the hydraulic flow pattern in partial-mix systems more closely kT — k2o9Tw 20

Tw Af+Q

where resembles the plug-flow condition. The dimensions of the cells can be calculated using Equation 4.16:

V = [LW + (L - 2sd)(W - 2sd) + 4(L - sd)(W - sd)]] (4.16)

where

V = Volume of pond or cell (m3). L = Length of pond or cell at water surface (m). W = Width of pond or cell at water surface (m). s = Slope factor (e.g., for 3:1 slope, s = 3). d = Depth of pond (m).

The oxygen requirements control the power input required for partial-mix pond systems. Several rational equations are available to estimate the oxygen requirements for pond systems (Benefield and Randall, 1980; Gloyna, 1971, 1976; Met-calf & Eddy, 1991, 2003). In most cases, partial-mix system design is based on the BOD entering the system to estimate the biological oxygen requirements. After the required rate of oxygen transfer has been calculated, equipment manufacturers' catalogs should be used to determine the zone of complete oxygen dispersion by surface, helical, or air gun aerators or by the proper spacing of perforated tubing. Schematic sketches of several of the various types of aerators used in pond systems are shown in Figure 4.2. Photographs of some of the aeration equipment illustrated in Figure 4.2 plus additional photographs of installed aeration equipment are shown in Figure 4.3. Equation 4.17 is used to estimate oxygen transfer rates:

where N

Na a

Equivalent oxygen transfer to tapwater at standard conditions (kg/hr). Oxygen required to treat the wastewater (kg/hr) (usually taken as 1.5x the organic loading entering the cell).

(Oxygen transfer in wastewater)/(oxygen transfer in tapwater) = 0.9. P(Css)P = oxygen saturation value of the waste (mg/L), where P = (wastewater saturation value)/tapwater oxygen saturation value) = 0.9; Css is the tapwater oxygen saturation value at temperature Tw; and P is the ratio of barometric pressure at the pond site to barometric pressure at sea level (assume 1.0 for an elevation of 100 m). Minimum DO concentration to be maintained in the wastewater (assume 2 mg/L).

\\tit Water

ain5

SHED

Diffuser Membrane

tennd

Header

Concrete Anchor Static Tube Aerator

Air Intake

Air Intake

Paddles Attached _ A to Rotating Drum

Brush Aerator

FIGURE 4.2 Schematics of aeration equipment used in wastewater ponds.

Cs = Oxygen saturation value of tap water at 20 °C and one atmosphere pressure = 9.17 mg/L. Tw = Wastewater temperature (°C).

Equation 4.15 can be used to estimate the water temperature in the pond during the summer months that will be the critical period for design. The partial-mix design procedure is illustrated in Example 4.3. The four-cell system can be obtained by using floating plastic partitions such as those shown in Figure 4.4.

Design a four-cell partial-mix aerated pond with two trains to remove BOD5 for the following environmental conditions and wastewater characteristics: Q = 1136 m3/d (0.3 mgd), C0 = 220 mg/L, Ce from the fourth cell is 30 mg/L, k20 = 2.5 d-1, winter air temperature = 8°C, summer air temperature = 25°C, elevation = 50 m (164 ft), pond depth = 4 m (13.1 ft).

Solution

Flow rate = Q = 568.00 m3/d Influent BOD = 220.00 mg/L Influent TSS = 200.00 mg/L Total nitrogen = 30.00 mg/L

Electric Motor m

Float

Pier ni

Platform for Gear Box and Motor

Pier

(b) Pier-Mounted Impeller with Draft Tube -r

Pier

(b) Pier-Mounted Impeller with Draft Tube -r

Platform

(c) Pier-Mounted Impeller ws

FIGURE 4.2 (cont.) (From Reynolds, T.D. and Richards, P.A., Unit Operations and Processes in Environmental Engineering, 2nd ed., PWS Publishing, New York, 1996. With permission.)

Total phosphorus = 10.00 mg/L Reaction rate at 20°C = 0.276 d-1 Influent temperature = 15.00°C Winter air temperature = Ta = 8.00°C Summer air temperature = Ta = 25.00°C f = units conversion factor = 0.50 Temperature correction coefficient = 1.09 Surface elevation = 50.00 m Minimum DO concentration = 2.00 mg/L Depth = 4.00 m Length-to-width ratio = 2.00 Side slope = 3.00

1. Begin solution by assuming a winter pond temperature and determine the volume of cell 1 in the pond system.

Assumed water temperature = 12.06°C.

Correct reaction rate for temperature: kT = k20(1.036)(T-20) = 0.210 d-1. Hydraulic residence time in cell 1 is 3.60 d. Effluent BOD in cell 1 = C0/(1 + kt) = 125.69 mg/L. Volume in cell 1 = 2044.80 m3.

2. Calculate the dimensions of cell 1 at the water surface and the surface area:

3. Check pond temperature using cell area calculated above and equation shown below:

Floating Aerators During Winter Operation

Floating Aerators During Winter Operation

If the calculated Tw differs from the assumed water temperature, another iteration is necessary. Add a freeboard = 0.90 m. Dimensions at top of dike in cell 1: W top of dike = 29.91 m. L top of dike = 54.42 m.

Entering water temperature = 11.40°C

Correct reaction rate for temperature: kT = k20(1.09)(T-20) = 0.20 d-1. Influent BOD in cell 2 = 125.69 mg/L. Hydraulic residence time in cell 2 = 3.50 d. Effluent BOD cell 2 = 73.39 mg/L

Mixers

Mixers

Volume in cell 2 = 1988.00 m3.

Calculate dimensions of cell 2 at water surface and the surface area: Depth = 4.00 m. Width = 24.28 m. Length = 48.56 m. Area = 1179.11 m2 = 0.134 ac.

POLVFOAM FLOTATION STANDARD END CONNECTOR

POLVFOAM FLOTATION STANDARD END CONNECTOR

BOTTOM TYPICAL

ANCHOR POINT

FIGURE 4.4 Floating baffle. (Courtesy of Environetics, Inc.; Lockport, IL.)

BOTTOM TYPICAL

ANCHOR POINT

FIGURE 4.4 Floating baffle. (Courtesy of Environetics, Inc.; Lockport, IL.)

Add a freeboard = 0.90 m. Dimensions at top of dike in cell 2: W top of dike = 29.68 m. L top of dike = 53.96 m. 5. For cell 3:

Entering water temperature = 9.67°C. kT = 0.19 d1.

Influent BOD to cell 3 = 73.39 mg/L. Hydraulic residence time in cell 3 = 3.00 d. Effluent BOD in cell 3 = 46.61 mg/L. Volume in cell 3 = 1704.00 m3.

Calculate dimensions of cell 3 at the water surface and the surface area: Depth = 4.00 m. Width = 23.07 m. Length = 46.14 m. Area = 1064.56 m2 = 0.134 ac.

Add a freeboard = 0.90 m. Dimensions at top of dike in cell 3: W top of dike = 28.47 m. L top of dike = 51.54 m.

Entering water temperature = 8.86°C. kT = 0.19 d1.

Influent BOD to cell 4 = 46.61 mg/L. Hydraulic residence time in cell 4 = 3.00 d. Effluent BOD in cell 4 = 29.91 mg/L. Volume in cell 4 = 1704.00 m3.

Calculate dimensions of cell 4 at the water surface and the surface area: Depth = 4.00 m. Width = 23.07 m. Length = 46.14 m. Area = 1064.56 m2 = 0.263 ac.

Add a freeboard = 0.90 m. Dimensions at top of dike: W top of dike = 28.47 m. L top of dike = 51.54 m.

7. Determine the oxygen requirements for pond system based on organic loading and water temperature. Maximum oxygen requirements will occur during the summer months:

Tw for summer cell 1 = (AfTa + QT)/(Af + Q) = 20.14°C. Tw for summer cell 2 = (AfTa + QT)/(Af + Q) = 22.62°C. Tw for summer cell 3 = (AfTa + QT)/(Af + Q) = 23.77°C. Tw for summer cell 4 = (AfTa + QT)/(Af + Q) = 24.36°C. Organic load (OL) in the influent wastewater: OL on cell 1 = C0 x Q = 5.21 kg/hr.

Calculate effluent BOD from first cell using equations below at Tw for summer:

kTw = k20 x (temperature coefficient)(Tw-20) = 0.28 d-1. C1 = C0/[(kt) + 1] = 110.08 mg/L. Winter = 125.69 mg/L. OL on cell 2 = C1 x Q = 2.61 kg/hr.

kTw = k20 x (temperature coefficient)(Tw-20) = 0.30 d-1. C2 = C1/[(kt) + 1] = 53.45 mg/L. Winter = 73.39 mg/L. OL on cell 3 = C2 x Q = 1.26 kg/hr.

kTw = k20 x (temperature coefficient)(Tw-20) = 0.32 d-1. C3 = C2/[(kt) + 1] = 27.46 mg/L. Winter = 46.61 mg/L.

kTw = k20 x (temperature coefficient)(Tw-20) = 0.32 d-1. C4 = C3/[(kt) + 1] = 13.97 mg/L. Winter = 29.91 mg/L. Oxygen demand (OD) is assumed to be a multiple of organic loading (OL) (with a multiplying factor of 1.50):

OD in cell 1 = OL1 x multiplying factor = 7.81 kg/hr. OD in cell 2 = OL2 x multiplying factor = 3.91 kg/hr. OD in cell 3 = OL3 x multiplying factor = 1.90 kg/hr. OD in cell 4 = OL4 x multiplying factor = 0.97 kg/hr.

8. Use the following equation to calculate equivalent oxygen transfer:

N = NOD/(a[(Csw - Cj)/Cs](temperature factor)(TW-20))

where NOD = oxygen demand in various cells; Csw = b x Css x P; b = 0.90; P = ratio of barometric pressure at pond site to pressure at sea level = 0.80.

Cell 1 tapwater oxygen saturation value Cell 2 tapwater oxygen saturation value Cell 3 tapwater oxygen saturation value Cell 4 tapwater oxygen saturation value Cell 1 Csw = 6.59 mg/L. Cell 2 Csw = 6.29 mg/L. Cell 3 Csw = 6.16 mg/L. Cell 4 Csw = 6.09 mg/L. a = (oxygen transfer in wastewater)/(oxygen transfer in tapwater) = 0.90.

Cj = minimum oxygen concentration to be maintained in wastewater

(usually assumed to be 2 mg/L) = 2.00 mg/L Cs = oxygen saturation value of tapwater at 20°C and 1 atm = 9.17 mg/L.

Temperature factor (normally 1.025) = 1.025. N1 = 17.29 kg/hr. N3 = 4.23 kg/hr. N2 = 8.70 kg/hr. N4 = 2.18 kg/hr.

9. Evaluate surface and diffused air aeration equipment to satisfy oxygen requirement only:

Power requirement for surface aerators is approximately 1.9 kg O2 per kWh, or 1.40 kg O2 per hp per hr. Power requirement for diffused air is approximately 2.70 kg O2 per kWh, or 2.00 kg O2 per hp per hr. Total power for surface aeration in one train: Cell 1: 9.10 kW or 12.35 hp. Cell 2: 4.58 kW or 6.21 hp. Cell 3: 2.23 kW or 2.99 hp. Cell 4: 1.15 kW or 1.54 hp.

Css = 9.15 mg/L. Css = 8.74 mg/L. Css = 8.56 mg/L. Css = 8.46 mg/L.

Total power for diffused aeration in one train: Cell 1: 6.40 kW or 8.64 hp. Cell 2: 3.22 kW or 4.35 hp. Cell 3: 1.57 kW or 2.12 hp. Cell 4: 0.81 kW or 1.09 hp. These surface and diffused aerator power requirements must be corrected for gearing and blower efficiency: Gearing efficiency = 0.90. Blower efficiency = 0.90. Total power requirements corrected for efficiency for one train are: Cell 1 surface aerators: 10.11 kW or 3.56 hp. Cell 2 surface aerators: 5.09 kW or 6.83 hp. Cell 3 surface aerators: 2.48 kW or 3.33 hp. Cell 4 surface aerators :1.27 kW or 1.70 hp. Total power for surface aerators: 18.95 kW or 25.41 hp. Power cost per kilowatt-hour = $0.06/kWh. Total power costs for surface aerators per year for one train = $9958.02/yr.

Cell 1 diffused aeration: 7.11 kW or 9.53 hp. Cell 2 diffused aeration: 3.58 kW or 4.80 hp. Cell 3 diffused aeration: 1.74 kW or 2.33 hp. Cell 4 diffused aeration: 0.90 kW or 1.21 hp. Total power diffused aeration: 13.33 kW or 17.87 hp. Power cost per kilowatt-hour = $0.06/kWh. Total power costs for diffused aerators per year for one train = $7007.49/yr.

These power requirements are approximate values and are used for the preliminary selection of equipment. These values are used in conjunction with equipment manufacturers' catalogs to select the proper equipment.

Surface aeration equipment is subject to potential icing problems in cold climates, but many options are available to avoid this problem (see Figure 4.2 and Figure 4.3). Improvements have been made in fine bubble perforated tubing, but a diligent maintenance program is still a good policy. In the past, a number of communities experienced clogging of the perforations, particularly in hardwa-ter areas, and corrective action required purging with HCl gas. The final element recommended in this partial-mix aerated pond system is a settling cell with a 2-day detention time.

There are many configurations of complete-mix pond systems, but most are similar in design. Examples of several types that utilize the complete-mix concept are discussed in the following sections: high-performance aerated pond systems, nitrogen removal in pond systems, modified high-performance aerated lagoon systems for nitrification and denitrification, the BIOLAC® process, and Lemna systems. Most complete-mix systems are designed using the equations that are presented in the following paragraphs with minor modifications. As noted previously, the trend toward omitting redundancy in design of aerated lagoon systems has been alarming. It appears that little thought is given to the need for maintenance in the future, as operating costs associated with aerated lagoon systems frequently have been ignored or overlooked when comparing options available to a community. The initial costs of systems without redundancy obviously are lower than those for systems that include flexibility in operation, but the cost to the environment and the owner will be far greater when maintenance is required.

An examination of Example 4.5 reveals the similarity between the design for the high-performance aerated pond system and the complete-mix design presented below when the final three cells of the complete-mix design (Example 4.5) are supplied only enough dissolved oxygen to satisfy the BOD. This is not to imply that both are the same but only demonstrates the similarity between the two design methods.

The design model using first-order kinetics and operating n number of equal-sized cells in series is given by Equation 4.18 (Middlebrooks et al. 1982; Great

Lakes-Upper Mississippi River Board of State Sanitary Engineers, 1990; WEF/ASCE, 1991).

Cn = Effluent BOD concentration in cell n (mg/L). C0 = Influent BOD concentration (mg/L).

k = First-order reaction rate constant (d-1) = 2.5 d-1 at 20°C (assumed to be constant in all cells). t = Total hydraulic residence time in pond system (d). n = Number of cells in the series.

If other than a series of equal volume ponds are to be employed or it is desired to use varying reaction rates, it is necessary to use the following general equation:

where kl, k2, ..., kn are the reaction rates in cells 1 through n (all usually assumed equal for lack of better information) and t1, t2, ..., tn are the hydraulic residence times in the respective cells.

where

Mara (1975) has shown that a number of equal-volume reactors in series is more efficient than unequal volumes; however, due to site topography or other factors in some cases it may be necessary to construct cells of unequal volume.

Selection of the k value is one of the critical decisions in the design of any pond system. A design value of 2.5 d-1 at 20°C (68°F) is recommended by Reynolds and Middlebrooks (1990) based on a study of a complete-mix aerated lagoon system located in Colorado. Higher values are recommended by others, but designs based on this value of kC have worked well at full design load, whereas designs with higher kC values have not functioned as well at design flow. In most cases, the designer will be constrained by state design standards, and in many states the prescribed reaction rate will exceed the value of 2.5 d-1.

When using the complete-mix design model, the number of cells in series has a pronounced effect on the size of the pond system required to achieve the specified degree of treatment. Rearranging Equation 4.18 and solving for t can demonstrate the effect:

All terms in this equation have been defined previously. Example 4.4

Compare detention times for the same BOD removal levels in complete-mix aerated ponds having one to five cells. Assume C0 = 200 mg/L, k = 2.5 d-1, Tw =

Solution

1. Solve Equation 4.20 for a single-cell system:

2. Similarly, when:

n = 2, then t = 1.03 d. n = 3, then t = 0.75 d. n = 4, then t = 0.64 d. n = 5, then t = 0.58 d.

3. Continuing to increase n will result in the detention time being equal to the detention time in a plug-flow reactor. It can be seen from the tabulation above that the advantages diminish after the third or fourth cell.

4.4.1.3 Temperature Effects

The influence of temperature on the reaction rate is defined by Equation 4.21:

where kT = Reaction rate at temperature T (d-1).

0 = Temperature coefficient = 1.036.

The pond water temperature (Tw) can be estimated using the following equation developed by Mancini and Barnhart (1976):

f = Proportionality factor = 0.5.

An estimate of the surface area is made based on Equation 4.20 and corrected for temperature, then the temperature is calculated using Equation 4.22. After several iterations, when the water temperature used to correct the reaction rate coefficient agrees with the value calculated with Equation 4.22, the selection of the detention time in the system is completed.

The ideal configuration of a pond designed on the basis of complete-mix hydraulics is a circular or a square pond; however, even though complete-mix ponds are kT — k20QTw 20

where designed using the complete-mix model, it is recommended that the cells be configured with a length-to-width ratio of 3:1 or 4:1. This is done because it is recognized that the hydraulic flow pattern in complete-mix designed systems more closely resembles the plug-flow condition. The dimensions of the cells can be calculated using Equation 4.23:

where | |

V |
= Volume of pond or cell (m3). |

L |
= Length of pond or cell at water surface (m) |

W |
= Width of pond or cell at water surface (m). |

s |
= Slope factor (e.g., for a 3:1 slope, s = 3). |

d |
= Depth of pond (m). |

4.4.3 Mixing and Aeration

The mixing requirements usually control the power input required for complete-mix pond systems. Several rational equations are available to estimate the oxygen requirements for pond systems (Benefield and Randall, 1980; Gloyna, 1971, 1976; Metcalf & Eddy, 1991, 2003). Complete-mix systems are designed by estimating the BOD entering the system to estimate the biological oxygen requirements and then checked to ensure that adequate power is available to provide complete mixing. After calculating the required rate of oxygen transfer, equipment manufacturers' catalogs should be used to determine the zone of complete mixing and oxygen dispersion by surface, helical, aeration chain, or air gun aerators or by the proper spacing of perforated tubing. Schematic sketches of several of the various types of aerators used in pond systems are shown in Figure 4.2. Photographs and drawings of some of the aeration equipment illustrated in Figure 4.2 plus additional photographs of installed aeration equipment are shown in Figure 4.3.

Equation 4.24 is used to estimate the oxygen transfer rates:

Na a

where

N = Equivalent oxygen transfer to tapwater at standard conditions (kg/hr). Na = Oxygen required to treat the wastewater (kg/hr) (usually taken as 1.5

x the organic loading entering the cell). a = (Oxygen transfer in wastewater)/(oxygen transfer in tapwater) = 0.9.

Csw = P(Css)P = oxygen saturation value of the waste (mg/L), where P = (wastewater saturation value)/(tapwater oxygen saturation value); Css is the tapwater oxygen saturation value at temperature Tw; and P is the ratio of barometric pressure at the pond site to barometric pressure at sea level (assume 1.0 for an elevation of 100 m).

CL = Minimum DO concentration to be maintained in the wastewater, assume 2 mg/L.

Cs = Oxygen saturation value of tapwater at 20°C and one atmosphere pressure = 9.17 mg/L.

Tw = Wastewater temperature (°C).

Equation 4.22 can be used to estimate the water temperature in the pond during the summer months that will be the critical period for design for biological activity; however, with power to provide complete mixing, adequate dissolved oxygen is normally readily available. The complete mix design procedure is illustrated in Example 4.5. The four-cell system can be obtained by using floating plastic partitions such as those shown in Figure 4.4, and the aeration equipment can be selected from the types shown in Figures 4.2 and 4.3.

Design a four-cell complete-mix aerated pond with two trains to remove BOD5 for the following environmental conditions and wastewater characteristics: Q = 1136 m3/d (0.3 mgd), C0 = 220 mg/L, Ce from the fourth cell is 30 mg/L, k20 = 2.5 d-1, winter air temperature = 8°C, summer air temperature = 25°C, elevation = 50 m (164 ft), pond depth = 4 m (13.1 ft); maintain a minimum DO concentration of 2 mg/L in all cells.

Flow rate = Q = 568.00 m3/d Influent BOD = 220.00 mg/L Influent TSS = 200.00 mg/L Total nitrogen = 30.00 mg/L Total phosphorus = 10.00 mg/L Reaction rate at 20°C = 2.500 d-1 Influent temperature = 15.00°C Winter air temperature = Ta = 8.00°C Summer air temperature = Ta = 25.00°C f = units conversion factor = 0.50 Temperature correction coefficient = 1.09 Surface elevation = 50.00 m Minimum DO concentration = 2.00 mg/L Depth = 4.00 m Length-to-width ratio = 2.00 Side slope = 3.00

1. Begin solution by assuming a winter pond temperature and determine the volume of cell 1 in the pond system.

Assumed water temperature = 12.74°C.

Correct reaction rate for temperature: kT = k20(1.09)(T-20) = 1.34 d-1. Hydraulic residence time in cell 1 is 1.00 d. Effluent BOD in cell 1 = 94.13 mg/L. Volume in cell 1 = 568.00 m3.

2. Calculate the dimensions of cell 1 at the water surface and the surface area:

3. Check pond temperature using cell area calculated above and equation shown below:

If the calculated Tw differs from the assumed water temperature, another iteration is necessary. Add a freeboard = 0.90 m. Dimensions at top of dike in cell 1: W top of dike = 21.88 m. L top of dike = 38.37 m.

Entering water temperature = 12.74°C

Correct reaction rate for temperature: kT = k20(1.09)(T-20) = 1.34 d-1.

Hydraulic residence time in cell 2 = 1.00 d.

Effluent BOD cell 2 = 40.28 mg/L

Calculate dimensions of cell 2 at water surface and the surface area: Depth = 4.00 m. Width = 16.48 m. Length = 32.97 m. Area = 543.40 m2 = 0.134 ac.

Add a freeboard = 0.90 m. Dimensions at top of dike in cell 2: W top of dike = 21.88 m. L top of dike = 38.37 m.

Entering water temperature = 11.20°C. kT = 1.17 d-1.

Influent BOD to cell 3 = 40.28 mg/L. Hydraulic residence time in cell 3 = 1.00 d. Effluent BOD in cell 3 = 18.55 mg/L. Volume in cell 3 = 568.00 m3.

Calculate dimensions of cell 3 at the water surface and the surface area: Depth = 4.00 m. Width = 16.48 m. Length = 32.97 m. Area = 543.40 m2 = 0.134 ac.

Add a freeboard = 0.90 m. Dimensions at top of dike in cell 3: W top of dike = 21.88 m. L top of dike = 38.37 m.

Entering water temperature = 10.17°C. kT = 1.07 d1.

InTfluent BOD to cell 4 = 18.55 mg/L. Hydraulic residence time in cell 4 = 1.00 d. Effluent BOD in cell 4 = 8.96 mg/L. Volume in cell 4 = 568.00 m3.

Calculate dimensions of cell 4 at the water surface and the surface area: Depth = 4.00 m. Width = 16.48 m. Length = 32.97 m. Area = 543.40 m2 = 0.134 ac.

Add a freeboard = 0.90 m. Dimensions at top of dike: W top of dike = 21.88 m. L top of dike = 38.37 m.

7. Determine the oxygen requirements for pond system based on organic loading and water temperature. Maximum oxygen requirements will occur during the summer months:

Tw for summer cell 1 = (AfTa + QT)/(Af + Q) = 18.24°C. Tw for summer cell 2 = (AfTa + QT)/(Af + Q) = 20.42°C. Tw for summer cell 3 = (AfTa + QT)/(Af + Q) = 21.90°C. Tw for summer cell 4 = (AfTa + QT)/(Af + Q) = 22.91°C. Organic load (OL) in the influent wastewater: OL on cell 1 = C0 x Q = 5.21 kg/hr.

Calculate effluent BOD from first cell using equations below at Tw for summer:

kTw = k20 x (temperature coefficient)(Tw-20) = 2.15 d-1.

C1 = C0/[(kt) + 1] = 69.90 mg/L. Winter = 94.13 mg/L. OL on cell 2 = C1 x Q = 1.65 kg/hr.

kTw = k20 x (temperature coefficient)(Tw-20) = 2.59 d-1. C2 = C1/[(kt) + 1] = 19.45 mg/L. Winter = 40.28 mg/L. OL on cell 3 = C2 x Q = 0.46 kg/hr.

kTw = k20 x (temperature coefficient)(Tw-20) = 2.95 d-1. C3 = C2/[(kt) + 1] = 4.93 mg/L. Winter = 18.55 mg/L. OL on cell 4 = C3 x Q = 0.12 kg/hr.

kTw = k20 x (temperature coefficient)(Tw-20) = 3.21 d-1. C4 = C3/[(kt) + 1] = 1.17 mg/L. Winter = 8.96 mg/L. Oxygen demand (OD) is assumed to be a multiple of organic loading (OL) (with a multiplying factor of 1.50):

OD in cell 1 = OL1 x multiplying factor = 7.81 kg/hr. OD in cell 2 = OL2 x multiplying factor = 2.48 kg/hr. OD in cell 3 = OL3 x multiplying factor = 0.69 kg/hr. OD in cell 4 = OL4 x multiplying factor = 0.18 kg/hr. 8. Use the following equation to calculate equivalent oxygen transfer:

N = NOD/(a[(Csw - CL)/Cs](temperature factor)(Tw-20))

where NOD = oxygen demand in various cells; Csw = b x Css x P; b = 0.90; P = ratio of barometric pressure at pond site to pressure at sea level = 0.80.

Cell 1 tapwater oxygen saturation value Css = 9.49 mg/L. Cell 2 tapwater oxygen saturation value Css = 9.10 mg/L. Cell 3 tapwater oxygen saturation value Css = 8.85 mg/L. Cell 4 tapwater oxygen saturation value Css = 8.69 mg/L. Cell 1 Csw = 6.84 mg/L. Cell 2 Cww = 6.55 mg/L. Cell 3 Cww = 6.37 mg/L. Cell 4 Cww = 6.26 mg/L.

a = (oxygen transfer in wastewater)/(oxygen transfer in tapwater) = 0.90.

CL = minimum oxygen concentration to be maintained in wastewater

(usually assumed to be 2 mg/L) = 2.00 mg/L. Cs = oxygen saturation value of tapwater at 20°C and 1 atm = 9.17 mg/L.

Temperature factor (normally 1.025) = 1.025. N1 = 17.19 kg/hr. N2 = 5.50 kg/hr. N3 = 1.54 kg/hr. N4 = 0.39 kg/hr.

9. Evaluate surface and diffused air aeration equipment to satisfy oxygen requirement only:

Power requirement for surface aerators is approximately 1.9 kg O2 per kWh, or 1.40 kg O2 per hp per hr. Power requirement for diffused air is approximately 2.70 kg O2 per kWh, or 2.00 kg O2 per hp per hr. Total power for surface aeration: Cell 1: 9.05 kW or 12.28 hp. Cell 2: 2.89 kW or 3.93 hp. Cell 3: 0.81 kW or 1.10 hp. Cell 4: 0.21 kW or 0.28 hp. Total power for diffused aeration: Cell 1: 6.37 kW or 8.60 hp. Cell 2: 2.04 kW or 2.75 hp. Cell 3: 0.57 kW or 0.77 hp. Cell 4: 0.14 kW or 0.19 hp. These surface and diffused aerator power requirements must be corrected for gearing and blower efficiency: Gearing efficiency = 0.90. Blower efficiency = 0.90. Total power requirements corrected for efficiency are: Cell 1 surface aerators: 10.05 kW or 13.48 hp. Cell 2 surface aerators: 3.21 kW or 4.31 hp. Cell 3 surface aerators: 0.90 kW or 1.20 hp. Cell 4 surface aerators :0.23 kW or 0.31 hp. Total power for surface aerators: 14.39 kW or 19.30 hp. Power cost per kilowatt-hour = $0.06/kWh. Total power costs for surface aerators per year = $7564.74/yr. Cell 1 diffused aeration: 7.07 kW or 9.49 hp. Cell 2 diffused aeration: 2.26 kW or 3.03 hp. Cell 3 diffused aeration: 0.63 kW or 0.85 hp. Cell 4 diffused aeration: 0.16 kW or 0.22 hp. Total power diffused aeration: 10.13 kW or 13.58 hp. Power cost per kilowatt-hour = $0.06/kWh. Total power costs for diffused aerators per year are $5323.33/yr. These power requirements are approximate values and are used for the preliminary selection of equipment. These values are used in conjunction with equipment manufacturers' catalogs to select the proper equipment.

10. Evaluation of power requirements for maintaining a complete-mix reactor:

Power required to maintain solids suspension = 6.00 kW/1000 m3, or

30.48 hp/MG. Total power required in cell 1 = 3.41 kW or 4.57 hp. Total power required in cell 2 = 3.41 kW or 4.57 hp.

Total power required in cell 3 = 3.41 kW or 4.57 hp. Total power required in cell 4 = 3.41 kW or 4.57 hp. 11. Total power required in the system will be the sum of the maximum power required in each cell as measured above. Assuming that complete mixing is to occur in all cells, use the first set shown below. Another alternative is to use the power calculated for each cell to satisfy oxygen demand or a mixture of complete-mix and oxygen requirements.

Power required for complete mix in all cells in one train: Cell 1 = 3.41 kW. Cell 2 = 3.41 kW. Cell 3 = 3.41 kW. Cell 4 = 3.41 kW. Total = 13.63 kW. Power costs = $7164.98/yr. Power requirements for each cell based on BOD removal in one train: Cell 1 = 10.05 kW or 13.48 hp. Cell 2 = 3.21 kW or 4.31 hp. Cell 3 = 0.90 kW or 1.20 hp. Cell 4 = 0.23 kW or 0.31 hp. Total = 14.39 kW or 19.30 hp. Power costs = $7564.74/yr.

The system is over-designed so it is necessary to make another iteration and change some of the reactors. Another possibility is to reduce the number of cells in series. Many combinations will yield a satisfactory solution. It is not advisable to reduce the hydraulic residence time below 1 day, and 1.5 days is preferable. Because of the small size of the reactors, more aeration horsepower is required for BOD reduction than is required to maintain complete-mix conditions; normally, the opposite would be true.

Anaerobic lagoons or ponds have been used for treatment of municipal, agricultural, and industrial wastewaters. The primary function of anaerobic lagoons is to stabilize large concentrations of organic solids contained in wastewater and not necessarily to produce a high-quality effluent. Most often anaerobic lagoons are operated in series with aerated or facultative lagoons. A three-cell lagoon system can produce a stable, high-quality effluent throughout its design life. Proper design and operation of an anaerobic lagoon should consider the biological reactions that stabilize organic waste material.

In the absence of oxygen, insoluble organics are hydrolyzed by extracellular enzymes to form soluble organics (i.e., carbohydrates such as glucose, cellobiose, xylose). The soluble carbohydrates are biologically converted to volatile acids. These organic (volatile) acids are predominantly acetic, proprionic, and butyric. The group of facultative organisms that transforms soluble organic molecules to short-chain organic acids is known as acid formers or acid producers. The next sequential biochemical reaction that occurs is the conversion of the organic acid to methane and carbon dioxide by a group of strict, anaerobic bacteria know as methane formers or methane producers.

Anaerobic decomposition of carbohydrate to bacterial cells with the formation of organic acids can be illustrated as:

Bicarbonate buffer present in solution neutralizes the acid formed in the above reaction:

2CH3COOH + 2NH4HCO3 ^ 2CH3COONH4 + 2H2O + 2CO2

During the growth of methane bacteria, ammonia acetate (CH3COONH4) is decomposed to methane and regeneration of the bicarbonate buffer, NH4HCO3:

2CH3COONH4 + 2H2O ^ 2CH4 + 2NH4HCO3

If sufficient buffer is not available, the pH will decrease, which will inhibit the third reaction.

The facultative acid formers are not as sensitive to ambient environmental factors such as pH value, heavy metals, and sulfides. Acid formers are normally very plentiful in the system and are not the rate-limiting step. The rate-limiting step in anaerobic digestion is the methane fermentation process. Methane-producing bacteria are quite sensitive to such factors as pH changes, heavy metals, detergents, alterations in alkalinity, ammonia nitrogen concentration, temperature, and sulfides. Furthermore, methane-fermenting bacteria have a slow growth rate.

Environmental factors that affect methane fermentation are shown in Table 4.6. In addition, work by Kotze et al. (1968), Chan and Pearson (1970), Hobson et al. (1974), Ghosh et al. (1974), and Ghosh and Klass (1974) provides some evidence that the hydrolysis step may become rate limiting in the digestion of particulates and cellulosic feeds. Design and operation of anaerobic lagoons should be founded on the fundamental biochemical and kinetic principles that govern the process. Most anaerobic lagoons, however, have been empirically designed.

A major problem associated with anaerobic lagoons is the production of odors. Odors can be controlled by providing an aerobic zone at the surface to oxidize the volatile organic compounds that cause odors. Recirculation from an aerobic pond to the primary anaerobic pond can alleviate odors by providing dissolved oxygen from the aerobic pond effluent that overlays the anaerobic pond

TABLE 4.6

Environmental Factors Influencing Methane Fermentation

Variable Optimal

Oxidation/reduction -520 to -530 Potential (MV):

Volatile acids (mg/L as acetic) 50-500

Alkalinity (mg/L as CaCO3) 2000-3000

Extreme

2000 1000-5000

and oxidizes sulfide odors (Oswald, 1968). To avoid contact of anaerobic processes with oxygen, influent wastewater can be introduced to the anaerobic pond at the center into a chamber in which the sludge accumulates to some depth as shown in Figure 4.5 (Oswald, 1968). Mixing of the influent with the active anaerobic sludge will enhance BOD removal efficiency and reduce odors (Parker et al., 1968).

As stated earlier, the purpose of anaerobic lagoons is the decomposition and stabilization of organic matter. Water purification is not the primary function of anaerobic lagoons. Anaerobic lagoons are used as sedimentation basins to reduce organic loads on subsequent treatment units. A general compilation of information about the design of municipal anaerobic lagoons is presented in the following text.

There is no agreement on the best approach to the design of anaerobic stabilization ponds. Systems are designed on the basis of surface loading rate, volumetric loading rate, and hydraulic detention time. Although done frequently, design on the basis of surface loading rate probably is inaccurate. Proper design should be based on the volumetric loading rate, temperature of the liquid, and hydraulic detention time. Areal loading rates that have been used around the world are shown in Table 4.7. It is possible to approximate the volumetric loading rates by dividing by the average depth of the ponds and converting to the proper set of units. Based on these loading rates, it is obvious that there has been little consistency in the design loading rates for anaerobic ponds. In climates where the temperature exceeds 22°C, the following design criteria should yield a BOD5 removal of 50% or better (WHO, 1987):

• Volumetric loading up to 300 g BOD5 per m3 per d

• Hydraulic detention time of approximately 5 d

In cold climates, detention times as great as 50 d and volumetric loading rates as low as 40 g BOD5 per m3 per d may be required to achieve 50% reduction in BOD5. The relationships among temperature, detention time, and BOD reduction are shown in Table 4.8 and Table 4.9.

One of the best approaches to the design of anaerobic lagoons has been presented by Oswald (1996). In his advanced facultative pond design, Oswald incorporates a deep anaerobic pond within the facultative pond. The anaerobic pond design is based on organic loading rates that vary with water temperature in the pond, and the design is checked by determining the volume of anaerobic pond provided per capita, which is one of the methods used for the design of separate anaerobic digesters. An example of this design approach is presented in Example 4.6.

Example 4.6

Design flow rate = 947 m3/d.

Influent ultimate BOD = 400 mg/L.

Effluent ultimate BOD = 50 mg/L.

Sewered population = 6000 people.

Maximum bottom water temperature in local bodies of water = 20°C.

Temperature of pond water at bottom of pond = 10°C.

TABLE 4.7

Design and Operational Parameters for Anaerobic Lagoons Treating Municipal Wastewater

Areal BOD5 Mass Loading Rates (lb/acd)

Estimated Volumetric Loading Rates (lb/1000 ft3d)

BOD5 Removal

Depth

Hydration Detention Time

Depth

Hydration Detention Time

Summer |
Winter |
Summer |
Winter |
Summer |
Winter |
(ft) |
(d) |
Ref. |

360 |
— |
2.34 |
— |
75 |
— |
3-4 |
— |
Parker (1970) |

280 |
— |
1.84 |
— |
65 |
— |
3-4 |
— |
Parker (1970) |

100 |
— |
0.66 |
— |
86 |
— |
3-4 |
— |
Parker (1970) |

170 |
— |
1.11 |
— |
52 |
— |
3-4 |
— |
Parker (1970) |

560 |
400 |
3.67 |
2.62 |
89 |
60 |
3-4 |
— |
Parker (1970) |

400 |
100 |
— |
— |
70 |
— |
— |
— |
Oswald (1968) |

900-1200 |
675 |
5.17-6.89 |
3.88 |
60-70 |
— |
3-5 |
2-5 |
Parker et al. (1959) |

— |
— |
— |
— |
— |
— |
8-10 |
30-50 |
Eckenfelder (1961) |

220-600 |
— |
— |
0.51-1.38 |
— |
— |
— |
15-160 |
Cooper (1968) |

500 |
— |
— |
1.15 |
70 |
— |
8-12 |
5 |
Oswald et al. (1967) |

— |
— |
— |
— |
— |
— |
8-12 |
2 (summer) |
Malina and Rios (1976) |

— |
— |
— |
— |
— |
— |
— |
5 (winter) |
Malina and Rios (1976) |

TABLE 4.8

Five-Day BOD5 Reduction as a Function of Detention Time for Temperatures Greater Than 20°C

Detention Time (d) BOD5 Reduction (%)

1 50

5 70

Source: WHO, Wastewater Stabilization Ponds: Principles of Planning and Practice, WHO Tech. Publ. 10, Regional Office for the Eastern Mediterranean, World Health Organization, Alexandria, 1987.

Solution

1. Calculate the BOD loading:

BOD loading = Influent BOD x flow rate/1000 = 378.8 kg/d

2. Design the anaerobic pond (fermentation pits). Except for systems with flows less than 200 m3/day, always use two ponds, so one will be available for desludging when the pond is filled. The surface area of the anaerobic pond should be limited to 1000 m2, and it should be made as deep as possible to avoid turnover with oxygen intrusion. Minimum pit depth should be 4 m.

Number of anaerobic ponds in parallel = minimum of two ponds = 2.

BOD loading on single pond = 189.4 kg/d. First, size pond on basis of load per unit volume:

Load per unit volume (varies with temperature of water) = 0.189 kg/m3/d.

Volume in one pond = 1002.7 m3.

Hydraulic residence time in ponds = 2.12 d.

Pond surface area (assuming vertical walls) = 250.7 m2.

Maximum pond surface area = 1000 m2; number of ponds = 0.25 (round to next largest number of ponds = 1.00).

Overflow rate in ponds = (total surface area)/(total flow rate) = 1.89 m/d.

Overflow rates of less than 1.5 m/d should retain parasite eggs and other particles as small as 20 |im, which includes all but the smallest parasite eggs (ova). The size of the pond should be increased to reduce the overflow rate to 1.5 m/d.

TABLE 4.9

Five-Day BOD5 Reduction as a Function of Detention Time and Temperature

Temperature Detention Time BOD Reduction

Source: WHO, Wastewater Stabilization Ponds: Principles ofPlan-ning and Practice, WHO Tech. Publ. 10, Regional Office for the Eastern Mediterranean, World Health Organization, Alexandria, 1987.

Check pond volume per capita:

Total volume in ponds = (total BOD loading)/(loading rate) = 2005 m3.

Pond volume/capita = (total volume)/(population) = 0.33 m3/capita. Pond volume/capita should be greater than 0.0566 m3/person as used in conventional separate digesters. When pit volume/capita exceeds 0.0566 m3/person, fermentation can go to completion with only grit and refractory organics left to accumulate

Designs of anaerobic ponds based on information in Table 4.8 and Table 4.9

are presented in Example 4.7 (Reed et al., 1995).

Example 4.7

Temperature = 10°C, detention time (d) = 5, BOD reduction (%) = 0-10.

Temperature = 10-15°C, detention time (d) = 4-5, BOD reduction (%) = 30-40.

Temperature = 15-20°C, detention time (d) = 2-3, BOD reduction (%) = 40-50.

Temperature = 20-25°C, detention time (d) = 1-2, BOD reduction (%) = 40-60.

Temperature = 25-30°C, detention time (d) = 1-2, BOD reduction (%) = 60-80.

Climates with temperatures exceeding 22°C:

Volumetric loading — up to 300 g BOD5 per m3 per d Hydraulic detention time — approximately 5 d Depth — 2.5 to 5 m

Cold climates (50% estimated reduction in BOD5):

Volumetric loading — as low as 40 g BOD5 per m3 per d Hydraulic detention time — approximately 50 d

Design input:

Influent BOD5 = 250 mg/L.

Temperature = 10°C.

Length-to-width ratio = 1

Volumetric loading = 60 g BOD5 per m3 per d.

Detention time = 5 d.

Output (volumetric loading): Volume = 78854 m3 Length = 71 mL. Width = 171 m.

Output (detention time): Volume = 94,625 m3. Length = 187 m. Width = 187 m. Detention time = 5 d.

Oswald's design procedure is semirational, whereas the other approaches are empirical. It is possible that some of the newer approaches to anaerobic reactor design may be applicable to the design of anaerobic ponds; however, it is likely that the controls required in the newer approaches will be impractical for pond design and operation.

4.6 CONTROLLED DISCHARGE POND SYSTEM

See Chapter 5 for details.

4.7 COMPLETE RETENTION POND SYSTEM

See Chapter 5 for details.

4.8 HYDROGRAPH CONTROLLED RELEASE

See Chapter 5 for details.

Reactor Basin Settling Basin

Reactor Basin Settling Basin

Reactor Cell

Settling Cells

FIGURE 4.6 Flow diagram of dual-power, multicellular (DPMC) aerated lagoon system: (a) two basins in series utilizing floating baffles in the settling cells; (b) a single basin using floating baffles to divide various unit processes. (From Rich, L.G., High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999. With permission.)

4.9 HIGH-PERFORMANCE AERATED POND SYSTEMS (RICH DESIGN)

The high-performance aerated pond system (HPAPS) described by Rich (1999) has frequently been referred to in the literature as a dual-power, multicellular (DPMC) system. The system consists of two aerated basins in series. Screens to remove large solids precede the system. A reactor basin for bioconversion and flocculation is followed by a settling basin dedicated to sedimentation, solids stabilization, and sludge storage. Algae growth is controlled by limited hydraulic retention time and dividing the settling basin into cells in series. Disinfection facilities follow the settling basin (Figure 4.6).

Aeration is provided in both the reactor portion and the settling basin. Aeration in the reactor is provided at a level of approximately 6 W/m3 to keep the solids suspended, and a minimum hydraulic detention time of 1.5 days is required. In small systems, the reactor and the settling basin can be placed in the same earthen basin; however, in large systems, it is best to put the reactor in a separate basin. Using a separate basin makes it easier to modify the system for upgrading to include nitrification and denitrification. (Nitrification and denitrification will be discussed in another section.)

Reactor basins generally are designed using Monod kinetics but with a minimum hydraulic retention time of 1.5 days. Rich (1999) strongly discourages the use of a safety factor when designing the reactor, because the settling basin provides adequate retention time to compensate for any errors that may be made in estimating the time required in the reactor basin.

Aeration in the settling basin should not exceed 1.8 W/m3 and should be evenly distributed between the cells established with floating plastic dividers. Aeration in the settling basin is important because it maintains an aerobic water column and an aerobic layer at the top of the sludge deposit, thus minimizing feedback of reduced compounds from the sludge to the water column, eliminating odors, and reducing the resuspension of bottom solids. Aeration provides mixing that reduces dead spaces where algae can become established and grow. Large quantities of respiratory carbon dioxide that accumulate during night hours are exhausted to the atmosphere and are not available for the algae to utilize when light becomes available. Aeration must be at a level that will allow settleable solids to settle.

Problems with aerated lagoon systems may occur when treating wastewaters with carbonaceous biological oxygen demand (CBOD5) concentrations of less than 100 mg/L because few settleable solids may be produced. This is particularly a problem when the wastewater has been presettled. Application of HPAPS at schools and seasonal recreational areas should be avoided. At these operations, lagoon volumes are often too small to provide adequate depth; with side slopes of 3:1, commercially available aerators are too large to be used in the settling basin, and flow is intermittent, leading to long hydraulic retention times and excessive algae growth. Design procedures are available for the HPAPS system (Rich, 1999).

Several sets of performance data for the HPAPS systems are available, but all are for locations in mild climates such as South Carolina and Georgia. It is likely that the process has been introduced in areas with more severe climates, and these data should used to design in more severe climates. Performance data for the DPMC system in Berkeley County, South Carolina, are presented in Figure 4.7. Data in the figure are for 6 years of operation, but Rich (2000) presented an additional 3 years of data on the Internet showing similar results. The system has functioned as designed for over 9 years. The performance has been exceptional for several years, but sludge removal data are not available.

Continuous operation of the aeration system is essential to obtain maximum efficiency, as illustrated by Figure 4.7 and Figure 4.8. The performance data for Berkeley County shown in Figure 4.7 were obtained with continuous aeration, while performance data for a similar system also located in South Carolina were obtained under conditions of intermittent aeration (operation 50% of the time). Results with continuous aeration were improved by about 50%.

FIGURE 4.7 Performance of dual-power, multicellular (DPMC) aerated lagoon system in Berkley County, South Carolina, with aerators operating continuously. (From Rich, L.G., High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999. With permission.)

FIGURE 4.7 Performance of dual-power, multicellular (DPMC) aerated lagoon system in Berkley County, South Carolina, with aerators operating continuously. (From Rich, L.G., High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999. With permission.)

Data

FIGURE 4.9 Monthly average BOD5 and TSS from Ocean Drive plant. (From Rich, L.G., High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999. With permission.)

Data

FIGURE 4.9 Monthly average BOD5 and TSS from Ocean Drive plant. (From Rich, L.G., High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999. With permission.)

A DPMC system (design flow = 3.4 mgd or 12,870 m3/d) followed by an intermittent sand filter at the Ocean Drive plant located in North Myrtle Beach, South Carolina, has been

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