Storage of wastewater may be required for cold weather, wet weather, or crop harvesting. Cold weather storage is the most common with operations ceasing when temperatures fall below 32°F (0°C). Design storage days should be estimated from climatic records for the site. Storage for wet weather is generally not necessary. Rainfall effects on BOD removal are minimal, and storage is not required during normal rainfall events. If storage is not necessary for cold or wet weather, it is usually advisable to provide a week of storage to accommodate emergencies such as equipment problems, crop harvest, or maintenance. The storage pond should be located offline so it contains pretreated effluent for a minimum time and is drained as soon as application is possible.
Calculate the field area for an overland flow system to treat 0.5 mgd of septic tank effluent. Preapplication is by a community septic tank, and the discharge BOD limit is 30 mg/L. The climate is moderately warm with 25 days of winter storage required.
1. Select an application rate appropriate for the degree of preapplication treatment, climate, and BOD removal requirement. Select 0.30 gal/ft-min from Table 8.14.
2. Select a slope length and application period. Select 150 ft for slope length and 8 hr/d for the application period.
3. Calculate the hydraulic loading rate using Equation 8.8: Lw = (q)(P)(96.3)/Z
4. Calculate the number of operating days per year: D = 365 - 25 = 340 d/yr
5. Use a net gain from precipitation on the storage pond of 2 Mgal. Calculate the field area using Equation 8.12:
A = [(365)(0 + Vs] 36.8/DLw A = [365(0.5) + 2]36.8/(340)(1/5) A = 13.3 ac
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