Conditions for Facultative Design Comparisons

Ce = required effluent BOD = 30 mg/L

T = water temperature at critical part of year = 10°C

Ta = average winter air temperature = 5°C

Adequate light intensity

Suspended solids (SS) = 250 mg/L

4.2.6 Comparison of Facultative Pond Design Models

Because of the many approaches to the design of facultative ponds and the lack of adequate performance data for the latest designs, it is not possible to recommend the "best" procedure. An evaluation of the design methods presented above, with operational data referenced in Table 4.2, failed to show that any of the models are superior to the others in terms of predicting the performance of facultative pond systems (USEPA, 1983; Middlebrooks, 1987). Many other studies of facultative pond systems with limited data have been conducted and reached much the same conclusions (Pearson and Green, 1995). Each of the design models presented above in detail was used to design a facultative pond for the conditions presented in Table 4.4, and the results are summarized in Table 4.5.

The limitations on the various design methods make it difficult to make direct comparisons; however, an examination of the hydraulic detention times and total volume requirements calculated by all of the methods show considerable consistency if the Marais-Shaw method is excluded and a value of 1.0 is selected for the dispersion factor in the Wehner-Wilhelm method. The major limitation of all these methods is the selection of a reaction rate constant or other factors in the equations. Even with this limitation, if the pond hydraulic system is designed and constructed so the theoretical hydraulic detention time is approached, reasonable success can be assured with all of the design methods. Short-circuiting is the greatest deterrent to successful pond performance, barring any toxic effects. The importance of the hydraulic design of a pond system cannot be overemphasized.

The surface loading rate approach to design requires a minimum of input data and is based on operational experiences in various geographical areas of the United States. It is probably the most conservative of the design methods, but the hydraulic design still cannot be neglected.

The Gloyna method is applicable only for 80 to 90% BOD removal efficiency, and it assumes that solar energy for photosynthesis is above the saturation level.

TABLE 4.5

Results From Facultative Pond Design Methods

TABLE 4.5

Results From Facultative Pond Design Methods

Detention Time (d)

Volume (m3)

Surface Area (ha)

Primary Cell Depth

(m)

Number

Organic Loading

Primary Cell

Total

Primary Cell

Total

Primary Cell

Total

of Cells

(kg BOD ha-

, d-')

Method

System

System

System

in Series

Primary

Total

Areal loading rate

53a

71

82,900a

135,300

6.3

11.5

1.7 (1.4)c

4

60

3

Gloyna

65

82,900a

123,000

12.3

1.5 (1.0)c

31

Marais and Shaw

17b

34

32,000b

64,000

1.3

2.6

2.4

2d

290

145

Plug flow

53a

53

82,900a

123,000

6.3

6.3

1.7 (1.4)c

1d

60

60

Wehner and Wilhelm

53a

36-58

82,900a

68,100-109,800

6.3

4.8-7.8

1.7 (1.4)c

4

80-50

a Controlled by state standards and equal to value calculated for an areal loading rate of 60 kg/ha-d and an effective depth of 1.4 m.

b Also would be controlled by state standards for areal loading rate; however, the method includes a provision for calculating a value, and this calculated value is shown. c Effective depth.

d Baffling recommended to improve hydraulic characteristics.

Source: Reed, S.C. et al., Natural Systems for Waste Management and Treatment, 2nd ed., McGraw-Hill, New York, 1995. With permission.

Provisions for removals outside this range are not made; however, an adjustment for other solar conditions can be made as described previously. Mara (1975) should be consulted if a detailed critique of the Gloyna method is needed.

The Marais-Shaw method is based on complete-mix hydraulics, which is not approached in facultative ponds, but the greatest weakness in the approach may lie in the requirement that the primary cell must not turn anaerobic. Mara (1975, 1976) provides a detailed discussion of this model.

Plug flow hydraulics and first-order reaction kinetics have been found to adequately describe the performance of many facultative pond systems (Neel et al., 1961; Thirumurthi, 1974; Middlebrooks et al., 1982; Middlebrooks, 1987; Pearson and Green, 1995). A plug-flow model was found to best describe the performance of the four pond systems evaluated in an EPA study as well as several others (Middlebrooks et al., 1982; USEPA, 1983). Because of the arrangement of most facultative ponds into a series of three or more cells, logically it would be expected that the hydraulic regime could be approximated by a plug-flow model. Reaction rates calculated from the USEPA (1983) data are very low primarily because of the long hydraulic detention times in the pond systems (70 to 231 days) and will yield designs that are too conservative.

Use of the Wehner-Wilhelm equation requires knowledge of both the reaction rate and the dispersion factor, further complicating the design procedure. If knowledge of the hydraulic characteristics of a proposed pond configuration are known or can be determined (Equation 4.9), the Wehner-Wilhelm equation will yield satisfactory results; however, because of the difficulty of selecting both parameters, a design using one of the simpler equations is likely to be as good as one using this model. The Wehner-Wilhelm equation is used in many countries around the world to design facultative ponds and apparently has been used successfully. In summary, all of the design methods discussed can provide a valid design if the proper parameters are selected and the hydraulic characteristics of the system are controlled.

4.3 PARTIAL-MIX AERATED PONDS

Changes in the basic approach to the design of partial-mix aerated ponds since the publication of USEPA's 1983 design manual have been limited primarily to the introduction of floating plastic partitions to improve the hydraulic characteristics of the pond system and the development of a wider selection of more efficient aeration equipment (WEF/ASCE, 1991). The importance of hydraulic characteristics was emphasized in the 1983 design manual and has been restated numerous times in many publications; however, based on the number of pond systems constructed over the past 20 years with poor hydraulic characteristics, one would assume that many designers have not read the literature or have ignored what they read.

The trend toward omitting redundancy in the design of aerated lagoon systems has been alarming. It appears that little thought is given to the need for maintenance in the future. 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 are obviously 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.

In the partial-mix aerated pond system, the aeration serves only to provide an adequate oxygen supply, and no attempt is made to keep all of the solids in suspension in the pond as is done with complete-mix and activated sludge systems. Some mixing obviously occurs and keeps portions of the solids suspended; however, an anaerobic degradation of the organic matter that settles does occur. The system is sometimes referred to as a facultative aerated pond system.

Even though the pond is only partially mixed, it is conventional to estimate the BOD removal using a complete mix model and first-order reaction kinetics. Studies by Middlebrooks et al. (1982) have shown that a plug-flow model and first-order kinetics more closely predict the performance of these ponds when either surface or diffused aeration is used. However, most of the ponds evaluated in this study were lightly loaded, and the reaction rates calculated are very conservative because it appears that the rate decreases as the organic loading decreases (Neel et al., 1961). Because of the lack of better design reaction rates, it is still necessary to design partial-mix ponds using complete-mix kinetics.

4.3.1 Partial-Mix Design Model

The design model using first-order kinetics and operating n number of equal-sized cells in series is given by Equation 4.11 (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) = 0.276 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 and it is desired to use varying reaction rates, it is necessary to use the following general equation:

0 0

Post a comment