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Using Reed Beds for Sludge Treatment

Using Reed Beds for Sludge Treatment

The use of reed beds in both central and decentralized wastewater treatment systems can offer a low cost and energy efficient opportunity to process sludge. Originally developed in Germany, the practice was brought to the United States in the 1980s. Under this technology, a variety of marsh grass, also known as Phragmites, is planted in reed beds built with concrete walls and lined with an impermeable layer to protect groundwater. TPO magazine suggests using a concrete bottom because PVC liner can be easily damaged during maintenance. The beds themselves contain a porous, finely aggregated media such as sand or recycled glass (pg. 12). This media allows the reeds to grow and excess liquid to pass through an underdrain system connected to the head of the plant for recycling. Risers can help distribute and load the sludge.

After the reeds have been established during a period of roughly three months, sludge can be loaded into the beds every three weeks. As the plants’ extensive root structure absorbs sludge moisture, water will be released through leaves and into the atmosphere via evapotranspiration. The microbes found in the root rhizome will help the sludge continue to break down. During the winter months when the reeds are dormant, the freeze-thaw cycle will allow liquid to easily separate from sludge to continue dewatering. When spring arrives, the reeds will return to their active growing cycle.

According to TPO Magazine, reed beds can adequately manage facilities that treat up to two million gallons per day provided that the required land is available. The reeds themselves can handle climates that experience several weeks of freezing temperatures during the winter. Before temperatures drop too low, operators will typically burn off the reeds in the fall. Alternatively, the reeds can be composted or disposed in a landfill. After approximately 8 years, the solids must be removed. At this time, the beds will be taken out of service in the summer and given an additional 90 days to dry out. Once the sludge is removed, the reeds will need to be re-established. A presentation by the Constructed Wetland Group provides a detailed overview of how to perform maintenance on reed beds.

While this technology is low maintenance and energy efficient, there are still pros and cons. As an advantage, reed beds can help to remove heavy metals from sludge. This should be considered during reed harvesting. As a drawback, constructing new beds requires significant capital costs, however utilities may be able to convert existing sand pits or drying beds to reduce costs. TPO Magazine notes that unpleasant odors can emerge during the spring when winter ice melts. Many scientists also worry that wastewater facilities using non-native grasses can encourage the establishment of invasive species. Phragmites spread predominantly through their underground rhizomes, laterally growing stems with roots. Furthermore, when non-native grasses escape into a new area, they can easily take over since their native competitors aren’t present. Facilities should practice careful harvesting and monitor the integrity of their bed structures to ensure containment. Despite these drawbacks, reed bed systems can be a successful and efficient form of sludge treatment even in comparison to conventional treatment methods.

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