Rich's (1999) method provides a way to design for nitrification in an aerated lagoon. The equations in Table 4.22 are empirical and may or may not apply to a general design; however, these equations will serve as an estimate of what might be expected in terms of nitrogen removal. Designing a lagoon system to nitrify a wastewater is not difficult if the water temperature and detention time are adequate to support nitrifiers and adequate dissolved oxygen is supplied.
Obviously, providing recycle of the mixed liquor is a significant benefit. As with all treatment methods, an economic analysis should be performed to determine the choice of a system.
4.11.4 Pump Systems, Inc., Batch Study
In 1998 a solar-powered circulator (equivalent to the SolarBee® Model SB2500) was installed in a 29-acre pond with a depth of 15 feet at Dickinson, North Dakota, with no incoming wastewater. The circulator flow rate was 2500 gpm. The ammonia nitrogen concentration at the beginning of the experiment was approximately 20 mg/L. Dissolved oxygen, pH, BOD, TSS, ammonia nitrogen, water temperature, and various other parameters were measured over a 90-day period at various locations and depths. Over 1500 samples were collected over the 90-day testing period. Average data for the various locations and depths are shown in Table 4.23. The average water temperature during the 90 days of testing was 20.5°C. Dissolved oxygen was present throughout the pond at all depths but on occasion dropped to 0.4 mg/L at the bottom. These occasional low DO concentrations may have had an adverse effect on the results presented below, but the results do provide some guidance as to how to estimate the expected conversion of ammonia nitrogen in a partial-mix aerated lagoon system.
A plot of the data for complete-mix and plug-flow models was prepared, but little difference in the fit of the data was observed. The plug-flow plot is shown in Figure 4.31. The plug-flow model and the model for a batch test are the same and should fit the data best. The reduction in NH4-N with time was directly related to the variation in pH value (Figure 4.31). When the pH exceeded 8.0, the reduction in NH4-N increased, resulting in a greater loss of the ammonia gas to the atmosphere.
The results of this study are useful for revealing the very low reaction rate for nitrification that occurs in partial mix aerated lagoons. The reaction rate of 0.0107 d1 obtained at an average temperature of 20.5°C in the Dickinson experiments agrees with results obtained with data collected in an aerated lagoon located in Wisconsin (Middlebrooks, 1982). At 1°C, the ammonia nitrogen conversion reaction rate for the Wisconsin partial-mix lagoon ranged between 0.0035 and 0.0070 d-1. Using an average value of 0.005 d-1 at 1°C and the value of 0.0107 d-1 obtained at Dickinson at 20.51°C, an approximate value of 1.04 results for 0 in the classical temperature correction equation; thus, kT = k20(1.04)(T-20). Example 4.8 illustrates the effects of reaction rates and temperature on the performance of partial-mix lagoon systems.
Estimate the expected NH4-N conversion in a partial mix aerated lagoon receiving adequate DO and alkalinity to nitrify an ammonia-N concentration of 20 mg/L at a water temperature of 10°C. Determine the effluent concentration at a detention time of 30 days and at a desired effluent concentration of 10 mg/L.
Temperature correction factor = 9 = 1.04.
Known detention time = 30 d.
Desired effluent NH4-N = 10 mg/L.
1. Correct reaction rate for temperature: kT = k20(9)(T-20) = 0.005079 d-1
2. Determine the effluent concentration with the detention time known:
3. Determine the detention time required to achieve the desired effluent concentration:
4.11.5 Commercial Products
Numerous products and processes are available to improve lagoon performance and remove nitrogen. Several of these options are presented below; information about them was extracted from Burnett et al. (2004).
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