These wastewaters from feed lots, dairy barns, swine barns, poultry operations, and similar activities tend to have high strength, high solids, and high ammonia and organic nitrogen concentrations. It is necessary to reduce the concentration of these materials in a preliminary treatment step, and an anaerobic pond is typically the most cost-effective choice. Procedures in Chapter 4 of this book can be used for design of that system component. In most cases, the FWS wetland will be the cost-effective choice for treatment of these wastewaters, as the smaller land area and other potential advantages of the SSF concept are not usually essential in an agricultural setting. The SSF concept may be at a disadvantage if spills occur in the preliminary treatment step and high solids concentrations are allowed to enter the wetland. The SSF concept may still be desirable for year-round operations in cold climates due to the enhanced thermal protection provided by this system.
Design of a wetland component for this application should follow the same procedures described in Section 6.5 to Section 6.9 of this chapter. A summary of performance data from a two-cell FWS wetland system treating wastewater from swine barns is presented in Table 6.13. An anaerobic lagoon was used as the preliminary treatment step, and that effluent was mixed with periodic discharge from a stormwater retention pond prior to introduction to the wetland component. Because flow rates were not measured, it is not possible to determine the HRT in this system. The volume of flow from the stormwater pond was about 1.5 times the volume from the anaerobic lagoon.
The 500-animal swine operation is estimated to produce 90 kg BOD d1 which is reduced to 36 kg/d in the diluted wetland influent. The organic loading rate on the 3600 m2 of wetland surface area is 89 lb/ac-d (100 kg/ha-d), and this is identical to the value recommended in Section 6.6 of this chapter.
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