Performance of Constructed Wetlands Treating Swine Waste

Ammonia

Total

BOD

TSS

TKN

Nitrogen

Phosphorus

Fecal Coliform

Fecal Streptococcus

Location

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(number/100 mL)

(number/l00 mL)

Anaerobic lagoon

111

346

116

84

49

817,500

118,750

Stormwater pond

32

51

4

1

3

1022

679

Wetland influent

64

105

26

55

26

175,164

76,727

Wetland effluent cell 1

14

25

18

13

11

2733

3927

Wetland effluent cell 2

10

31

9

5

7

2732

1523

Note: BOD, biological oxygen demand; TSS, total suspended solids; TKN, total Kjeldahl nitrogen.

Source: Hammer, D.A. et al., in Constructed Wetlands for Water Quality Improvement, Moshiri, G. et al., Eds., Lewis Publishers, Chelsea, MI, 1993, 343-348. With permission.

to an equalization pond from which it is transferred to the wetland unit. The pond at the Escambia County landfill in Florida is aerated, because septage is also added to the pond (Martin et al., 1993).

Characterization of the leachate is essential for proper wetland design as it can contain high concentrations of BOD, ammonia, and metals, can have a high or low pH, and can possibly include priority pollutants of concern. In addition, the nutrient balance in the leachate may not be adequate to support vigorous plant growth in the wetland, and supplemental potassium, phosphorus, and other micro-nutrients may be necessary. Because leachate composition will depend on the type and quantity of materials placed in the landfill and on time, a generic definition of characteristics is not possible and data must be collected for each system design.

Examples of leachate water quality from several landfill operations in the Midwest are presented in Table 6.14. These data confirm the earlier statement that BOD, chemical oxygen demand (COD), ammonia, and iron can exist in relatively high concentrations. Some of the volatile organic compounds such as acetone, methyl isobutyl ketone, and phenols can also be present in significant concentrations.

The design of the wetland for leachate treatment will follow the same procedures described in Sections 6.5 to 6.9 of this chapter. The removal of metals and priority pollutants will be as described in Section 6.3. Typically, the wetland will be sized to achieve a specific level of ammonia or total nitrogen in the final effluent. This can be achieved with only a wetland bed or with a wetland bed combined with either a nitrification filter bed (see Section 7.9) or a vertical-flow cell (see Section 7.11). The atmospheric exposure and relatively long HRT provided by any of these options will result in very effective removal of the volatile priority pollutants. If the leachate BOD is consistently above 500 mg/L, then the use of a preliminary anaerobic pond or cell should be considered. Many of the advantages of the SSF wetland concept are not necessary at most landfill locations, so a FWS wetland may be the more cost effective choice even though more land will be required. The exception may be in cold climates where the thermal protection provided by the SSF concept is an operational advantage. The performance of a FWS constructed wetland is shown in Table 6.15.

The nutrient and micronutrient requirements for biological oxidation are presented in Table 6.16. Landfill leachates, industrial and commercial wastewaters, and similar unique discharges should be tested for these components prior to design of a wetland system. If nutrients or micronutrients are deficient in these landfill leachates, the rate constants for BOD and nitrogen removal may be an order of magnitude less than those given in Section 6.6 and Section 6.8.

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