Reed bed systems are similar in some ways to the vertical flow constructed wetlands described in Chapter 7. In this case, the bed is composed of selected media supporting emergent vegetation, and the flow path for liquid is vertical rather than horizontal. These systems have been used for wastewater treatment, landfill leachate treatment, and sludge dewatering. This section describes the sludge dewatering use, where the bed is typically underdrained and the percolate is returned to the basic process for further treatment. These beds are similar in concept and function to conventional sand drying beds.
In conventional sand beds, each layer of sludge must be removed when it reaches the desired moisture content, prior to application of the next sludge layer. In the reed bed concept, the sludge layers remain on the bed and accumulate over a period of many years before removal is necessary. The significant cost savings from this infrequent cleaning are the major advantage of reed beds. Frequent sludge removal is necessary on conventional sand beds, as the sludge layer develops a crust and becomes relatively impermeable, with the result that subsequent layers do not drain properly and the new crust prevents complete evaporation. When reeds are used on the bed, the penetration of the stems through the previous layers of sludge maintains adequate drainage pathways and the plant contributes directly to dewatering through evapotranspiration.
This sludge dewatering method is in use in Europe, and approximately 50 operational systems are located in the United States. All of the operational beds have been planted with the common reed Phragmites. Experience has shown that it is necessary to apply well-stabilized wastewater sludges to these beds. Aero-bically or anaerobically digested sludges are acceptable, but untreated raw sludges with a high organic content will overwhelm the oxygen-transfer capability of the plants and may kill the vegetation. The concept will also work successfully with inorganic water treatment plant sludges and high-pH lime sludges.
The structural facility for a reed bed is similar in construction to an open, underdrained sand drying bed. Typically, either concrete or a heavy membrane liner is used to prevent groundwater contamination. The bottom medium layer is usually 10 in. (25 cm) of washed gravel (20 mm) and contains the underdrain piping for percolate collection. An intermediate layer of pea gravel about 3 in. (8 cm) thick prevents intrusion of sand into the lower gravel. The top layer is 4 in. (10 cm) of filter sand (0.3 to 0.6 mm). The Phragmites rhizomes are planted at the interface between the sand and gravel layers. At least 3 ft (1 m) of freeboard is provided for long-term sludge accumulation. The Phragmites are planted on about 12-in. (30-cm) centers, and the vegetation is allowed to become well established before the first sludge application (Banks and Davis, 1983b).
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