Facultative pond design is based on biological oxygen demand (BOD) removal; however, the majority of the suspended solids will be removed in the primary cell of a facultative pond system. Sludge fermentation feedback of organic compounds to the water in a pond system is significant and has an effect on the performance. During the spring and fall, the thermal overturn of the pond contents can result in significant quantities of benthic solids being resuspended. The rate of sludge accumulation is affected by the liquid temperature, and additional volume is added for sludge accumulation in cold climates. Although total suspended solids (TSS) have a profound influence on the performance of pond systems, most design equations simplify the incorporation of the influence of TSS by using an overall reaction rate constant. Effluent TSS generally consist of suspended organism biomass and do not include suspended waste organic matter.
Several empirical and rational models for the design of these ponds have been developed. These include the ideal plug flow and complete mix models, as well as models proposed by Fritz et al. (1979), Gloyna (1971), Larson (1974), Marais
(1970), McGarry and Pescod (1970), Oswald et al. (1970), and Thirumurthi (1974). Middlebrooks (1987) presented a summary of many models, including the ones referenced in the preceding sentence, that have been developed to evaluate and design facultative pond systems (Table 4.1). This is not an exhaustive list, and most of these models are variations of the ones in the references listed above. Several produce satisfactory results, but the use of some may be limited because of the difficulty in evaluating coefficients or by the complexity of the model. The methods and equations used most are discussed in the following paragraphs.
A cursory review of state design standards since Canter and Englande (1970) reported that most states have design criteria for organic loading and hydraulic detention times for facultative ponds shows that little has changed since 1970; however, individual states should be contacted to obtain the latest information. These criteria are assumed to ensure satisfactory performance; however, repeated violations of effluent standards by pond systems that meet state design criteria indicate the inadequacy of the criteria. A summary of the state design criteria for each location and actual design values for organic loading and hydraulic detention time for four facultative pond systems evaluated by the U.S. Environmental Protection Agency (EPA) (Middlebrooks et al., 1982; USEPA, 1983) are shown in Table 4.2. Also included is a list of the months the federal effluent standards for BOD5 were exceeded. The actual organic loading for the four systems is nearly equal, but the system in Corinne, Utah, consistently satisfied the federal effluent standard. This may be a function of the larger number of cells in the Corinne system — seven as compared to three for the others. More hydraulic short-circuiting is likely to occur in the three-cell systems, resulting in actual detention times shorter than those for the Corinne system. The detention time may also be affected by the location of the pond cell inlet and outlet structures. Many of the design faults in the systems referenced in Table 4.2 have been corrected since 1983.
Based on many years of experience, the following loading rates for various climatic conditions are recommended for use in designing facultative pond systems. For average winter air temperatures above 59°F (15°C), a BOD5 loading rate range of 40 to 80 lb/ac-d (45 to 90 kg/ha-d) is recommended. When the average winter air temperature ranges between 32 and 59°F (0 and 15°C), the organic loading rate should range between 20 and 40 lb/ac-d (22 and 45 kg/ha-d). For average winter temperatures below 32°F (0°C), the organic loading rates should range from 10 to 20 lb/ac-d (11 to 22 kg/ha-d).
The BOD loading rate in the first cell is usually limited to 35 lb/ac-d (40 kg/ha-d) or less, and the total hydraulic detention time in the system is 120 to 180 days in climates where the average air temperature is below 32°F (0°C). In mild climates where the air temperature is greater than 59°F (15°C), loadings on the primary cell can be 89 lb/ac-d (100 kg/ha-d).
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