Removal Efficiency of Free Water Surface Constructed Wetlands Treating Landfill Leachate

Constituent pH

TSS (mg/L) TDS (mg/L) COD (mg/L) TOC (mg/L) Copper (mg/L) Lead (mg/L) Mercury (mg/L) Nickel (mg/L) Zinc (mg/L)

Influent

6.32 1008 1078 456 129 0.05 0.078 0.0019 0.082 0.08

Effluent

6.86 30 396 45 17

0.024

0.004

0.0019

0.01

0.03

Percent Removal (%)

97 63 90

87 52 94

88 62

Note: TSS, total suspended solids; TDS, total dissolved solids; COD, chemical oxygen demand; TOC, total organic carbon.

Source: Johnson, K.D. et al., in Constructed Wetlands for the Treatment of Landfill Leachates, Mulamoottil, G. et al., Eds., Lewis Publishers, Boca Raton, FL. 1998. With permission.

If sufficient alkalinity is not present to provide a buffering capacity, the hydrolysis of the ferric iron (Fe3+) will further decrease the pH in the wetland effluent:

Several wetland systems described by Brodie et al. (1993) are effective in the removal of iron and manganese but the pH decreases from 6 to about 3 because of the reaction defined above. Previous attempts utilizing exposed limestone filter beds and the addition of buffering agents have been either ineffective or too expensive. Oxides of iron and aluminum would precipitate on the exposed limestone surfaces under aerobic conditions and that surface coating would prevent further calcium dissolution and eliminate any further buffering capacity. To correct this problem, the Tennessee Valley Authority (TVA) has developed an anoxic limestone drain (ALD). Crushed high-calcium-content limestone aggregate (20-to 40-mm size) is placed in a trench 10 to 16 ft (3 to 5 m) wide and to a depth ranging from 2 to 5 ft (0.6 to 1.5 m). The bed cross-section must be large enough

TABLE 6.16

Nutrients and Microorganisms Required for Biological Oxidation

TABLE 6.16

Nutrients and Microorganisms Required for Biological Oxidation

Minimum Required

Quantity

Parameter

(kg/kg BOD)

Nitrogen

0.043

Phosphorus

0.006

Manganese

0.0001

Copper

0.00146

Zinc

0.00016

Molybdenum

0.00043

Selenium

14 x 10-10

Magnesium

0.0030

Cobalt

0.00013

Calcium

0.0062

Sodium

0.00005

Potassium

0.0045

Iron

0.012

Carbonate

0.0027

to pass the maximum expected flow as defined by Darcy's law (see Section 6.5 in this chapter). The exposed portion of the trench is backfilled with compacted clay to seal the bed and ensure anoxic conditions in the limestone. The interface between the clay and the limestone is usually protected with a plastic geotextile. The upstream end of the trench or bed is located to intercept the source of the acid mine drainage.

Brodie et al. (1993) suggested specific guidelines for utilization of the ALD component:

• Existing alkalinity >80 mg/L, Fe <20 mg/L — Only the wetlands system is required.

• Existing alkalinity >80 mg/L, Fe >20 mg/L — A wetlands system without an ALD is probably adequate, although the ALD would be beneficial.

• Existing alkalinity <80 mg/L, Fe >20 mg/L — An ALD is recommended.

• Existing alkalinity <80 mg/L, Fe <20 mg/L — The ALD is not essential but is still recommended.

• Existing alkalinity = 0 mg/L, Fe <20 mg/L — The ALD will be necessary as the Fe concentration approaches 20 mg/L.

• Dissolved oxygen in liquid >2 mg/L or pH >6 and eH >100 mV — These conditions will result in oxide coatings and negate the benefits of an ALD.

A sedimentation pond is recommended as a treatment component prior to a wetland whether or not an ALD component is used in the system. This allows precipitation of a large fraction of the dissolved iron in a basin that can be dredged more easily than the wetland component.

The current practice for design of the wetland component is based on empirical evaluation of the performance of successfully operating systems. The TVA recommends a hydraulic loading from 0.37 to 1.0 gal/ft-d for iron removal depending on the pH, alkalinity, and iron concentration in the inflow. Others recommend a hydraulic loading rate of up to 3.5 gal/ft-d (0.14 m/d) for the same purpose. The treatment cells are designed for the base flow and then sufficient freeboard is provided to accommodate the design storm event. Multiple cells with a water depth in treatment zones of less than 1.5 ft (0.5 m) are recommended. Deep-water zones can also be provided if supplemental habitat values are a project goal. Recommended flow velocities in the wetland cells range from 0.1 to 1.0 ft/s (0.03 to 0.3 m/s). A separate wetland cell should be constructed for each 50 mg/L of iron content in the inflow because of the need for reaeration after oxidation of this amount of iron. If topography permits, a cascade spillway is recommended between these wetland cells.

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