Reed Beds

Reed beds constructed in wetlands provide long-term storage and volume reduction of sludges (sludge stabilization and dewatering) to mitigate environmental concerns. Widely used throughout Europe, Asia, and Aus-

Table 7.3. Design considerations for aerobic sludge composting (adapted from Metcalf and Eddy, Inc., 1991).

Item

Design Considerations

Sludge sources

Untreated sludge or digested sludge

Amendments and bulking agents

Wood chips, sawdust, recycled compost, and

straw

Carbon:nitrogen ratio

Ranging from 25:1 to 35:1

Volatile solids

>50%

Air requirement

>50% remaining in all parts of compost

Moisture content

<60% for static pile type; <65% for

windrow

pH

6-9

Temperature

Initially 50-55°C and later 55-66°C

Mixing and turning

Periodically, depending on type of compost

operations

Heavy metals and trance organics

Do not exceed the limits for compost

disposal

Site constraints

Considerations: available area (minimum

area for accommodating 30 days of

composting production), access, proxim-

ity to treatment plant, climatic conditions,

and availability of buffer zone

tralia, and in more than 50 locations in the United States, reed-bed technology features low construction costs and minimal day-to-day operation and maintenance costs due to infrequent cleaning of the beds (usually every several years). The reed-bed system reduces water content, minimizes sludges, and provides sufficient storage time to stabilize sludges prior to disposal.

Reed beds use common reed plants (Phragmites communis) to dewater solids in a confined area (Fig. 7.3). The phragmite was first used years ago in Europe for handling iron oxide sludges. The beds can be any shape to accommodate existing land conditions and areas. Specially designed ponds with under-drains covered by a sand and gravel mixture are constructed and filled with reed plants. Modified sludge drying beds also work well and are an ideal retrofit. They already have sidewalls, layers of sand and gravel, an under-drain system that collects and carries away filtrate, and an impervious membrane liner. Solids are pumped into the reed

Figure 7.3. A photo of a reed bed sludge treatment before planting vegetation.

beds. Dewatering occurs through evaporation, plant transpiration, and decantation. Decanted water seeps through the bottom of the bed and through the layers of sand and gravel into the under-drains, traveling back to the wastewater treatment plant for secondary treatment. During dewa-tering the solids change from liquid to "cake." Six inches of solids and water will compress to a half-inch of solid cake. The cake is left in the bed and the process is repeated (Fig. 7.4).

The reeds are planted one foot on center throughout the bed. Aero-bically stabilized sludge is typically applied uniformly through a grid-perforated tile. Sludges must be well stabilized, 60% or less volatilized to be used successfully with reed beds. Optimum application rates range between 2% and 4% solids. While plants are young they should be watered with plant effluent. After they are established, they can be fed heavier sludge mixtures. Loading rates (gallons per square foot per year) depend on temperature for well-established beds.

The phragmite is one of the most widespread flowering plants in the world. It is a tough adaptable plant, which can grow in polluted waters and find sustenance in sludge. This reed has a voracious appetite for water. The plant is tolerant to low oxygen levels and to waterlogged conditions. The reeds hold themselves in the soil through roots and rhizomes, an in-

Figure 7.4. A photo of a reed bed sludge treatment after planting vegetation.

tricate network of underground stems. New plants in turn will sprout from these stems. These rapidly growing roots provide air passages through the sludge that in turn provide a host area for many biological communities to develop and continue to mineralize the sludge.

Reed beds perform three basic functions: (1) dewater the sludge, (2) transform it into mineral and humus like components, and (3) store sludge for a number of years. Dewatering is accomplished through evaporation (as in a normal sludge drying bed operation), transpiration through the plants root stem and leaf structure, and filtration through the bed's sand and gravel layers and the plant's root system. The leachate is channeled back to the treatment plant through the under-drain.

The plants should be harvested annually to prevent drainage backup. The vegetation can be composted or burned.

Researchers recommend the installation of multiple beds to handle emergencies and downtime due to cleaning. Beds may be out of service for up to a year while rootstalks grow new tops after cleaning. The top level of sand and material removed during clean-out is similar in pathogen content to composted sludge and can be used in the same way. Many beds have gone 8 to 10 years without having to be cleaned out.

Sludge reed beds are a significant improvement over existing drying beds. Sludge can be dewatered and converted into biomass and low-grade compost without chemical addition or energy. They have lengthy turnover time and are capable of reducing sludge volumes by up to 95% over time.

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