Reed bed technology for biosolids dewatering and storage

Interested in Dewatering/Biosolids?

Get Dewatering/Biosolids articles, news and videos right in your inbox! Sign up now.

Dewatering/Biosolids + Get Alerts

As the cost of treating and storing biosolids continues to rise, some wastewater treatment plant operators are using an alternative to conventional drying beds. This technology, known as constructed reed bed technology, has been gaining acceptance in the United States for its simple and low-cost operation, minimal maintenance and significant sludge volume reduction. 

Developed in Germany, reed bed technology has been widely used for biosolids dewatering and stabilization in Europe, Asia and Australia. It was introduced to the U.S. in the early 1980s, where there are now more than 50 installations. 

Reed bed systems dewater digested sludge, minimize the solids and provide sufficient storage time to stabilize the biosolids before disposal. A type of marsh grass, Phragmites communis, is planted inside “beds” with concrete sides, an impermeable liner, risers to distribute the sludge, media to hold the reeds, and a drain to recycle the liquid. 

The technology is suitable for wastewater plants that treat up to 2 million gallons per day and that experience at least several weeks of freezing weather. “The freeze-thaw process changes the texture of the solids so they can be easily separated from the liquid,” says Diane Garvey, president of Garvey Resources in Lansdale, Penn. Reed beds also work well in climates warm enough for the reeds to grow year-round. 

How they work 

Phragmites is planted on 1-foot centers in the beds, where they quickly grow and colonize. The reeds are held in the soil through roots and rhizomes, an intricate network of underground stems. 

Dewatering is accomplished through evapotranspiration and filtration through the plants’ root system and the bed’s sand and gravel layers. 

Once the plants are established, typically within three months, wastewater solids are pumped a few inches deep into the bed every seven to 21 days. The leachate is channeled back to the treatment plant through the underdrain for secondary treatment. 

The solids accumulate over six to eight years and are then removed. They can be landfilled or further processed into compost. “The solids have also been used successfully as a fertilizer and soil conditioner to re-vegetate disturbed land such as surface mines, coal, sand and gravel,” says Garvey. 

Design considerations 

Reed beds should have the following:

  • Impermeable liner or base to protect groundwater
  • Outer walls 6 to 8 feet high, made of concrete with footers
  • Loading of liquid biosolids from manifold and risers along the long sides, typically three per side
  • Beds of up to 100 feet long and up to 50 feet wide
  • Adequate access for bed clean-out 

The beds should be sized based on allowable loading rate and biosolids production during the winter and early spring dormant phase. 

Design the beds with enough redundancy so that, after six years, one bed at a time can be removed from service to dry out, remove solids and re-establish the plants. 

David King, plant manager/operator at RAE Water & Sewer in Bozeman, Mont., stresses the importance of winterizing the riser pipes. RAE has three risers for each reed bed, with heat tape and insulation jackets to prevent freezing. He also suggests that the underdrain be designed with clean-outs. 

Minimal maintenance 

Reed beds require little maintenance. Some operators burn off the reeds in the fall. King uses a propane tank. “This requires all-metal piping, and the timing is tricky, since you have to do it after the reeds turn brown and before the snow flattens them,” he says. 

The reeds can also be harvested and either landfilled or composted. According to King, doing nothing and allowing the reeds to build up can cause drainage problems. 

Although King says his reed beds have required very little maintenance, the plant has made some additions, including adding a fence to keep animals out. 

Few disadvantages 

While capital costs of reed beds vary, the beds can significantly reduce staff hours and costs required to dewater with conventional sand drying beds or with mechanical systems such as a belt press or centrifuge. 

“For dewatering equipment, you need a separate building,” says King, “and you also have to pay people to haul the sludge away and landfill.” 

Adds Garvey: “It costs a lot to run even a small belt press, so if you have the land available, reed beds may make sense.” 

Says King, “About the only disadvantage is there can be some odor in the spring when the ice melts, but that usually only lasts a couple of weeks.” He suggests a concrete bottom instead of a PVC liner. “Operators have to be careful that they don’t go through the liner when they clean out the beds,” he says. “It’s not a question of if the liner will become damaged, but when.” 

Garvey stresses the importance of protecting the perforated pipes and the drains from damage. During clean-out or maintenance, the weight of even a small front-end loader can crush the underdrains. 

One misconception is that the reeds can spread and become invasive. “It was thought that the roots would start to re-grow in the fields, but that’s not a problem if the soils are well drained,” says Garvey. 

Positive performance 

Reed beds initially dewater biosolids to around 20 percent solids, which are typically 70 percent volatile organics and 30 percent non-volatile organics, by weight. As the dewatered material ages, there is a 60 to 80 percent reduction in volatile solids. 

When the bed is ready for clean-out, it is taken out of service for at least 90 days in the summer, which allows the reeds to further dewater the material. Over time, the biosolids will be significantly reduced, for a final product that is 50 to 75 percent total solids. 

Garvey recommends that plant operators check the quality of their biosolids to make sure it meets percent solids and ammonia limits. “Also, allow enough time to obtain your permits for beneficial use of the biosolids before cleaning out the beds,” she says. 

 

References 

An Evaluation of Reed Bed Technology to Dewater Army Wastewater Treatment Plant Sludge, U.S. Army Corps of Engineers, USACERL Technical Report EP-93/09, September 1993. 

“Reed Beds: An Alternative Way to Handle Wastewater Solids,” David King, RAE Water & Sewer, PowerPoint Presentation, August 2011. 

“Reedbeds – An Alternative Method for Sludge Treatment.” Lagoon Systems in Maine: an Informational Resource for Operators of Lagoon Systems. State of Maine Department of Environmental Protection. Online Newsletter, 2003. 



Discussion

Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.