The proper utilization and disposal of sludge is one of the most critical issues facing wastewater treatment plants today. Nearly all wastewater treatment plant operators face the problem of storing and disposing biosolids. Landfill costs are skyrocketing; incineration permits are expensive and difficult to obtain; and land application is limited by availability of permitted land. However, constructed wetland technology such as reed beds provide long-term storage and volume reduction of biosolids to mitigate these concerns. Widely used throughout Europe, Asia, and Australia, and in more than 50 locations in the United States, reedbed technology features low construction costs and minimal day-to-day operation and maintenance costs. Much interest is also being shown in Canada for their use. The system reduces water content, minimizes solids, and provides sufficient storage time to stabilize biosolids prior to disposal.
The solution is slowly gaining acceptance in Maine is sludge reed beds. Phragmites are only one of natures age old processes which have been adapted by man in the battle against pollution. They were first used years ago in Europe in an attempt to deal with iron oxide sludges.
How Do Reed Beds Work?
Reed beds use common reed plants (phragnmites communis, a second cousin of the common marsh plant) to dewater solids in a confined area. The beds can be any shape to accommodate existing land conditions and areas. Specially designed ponds with underdrains 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 side walls, layers of sand and gravel, an underdrain system which collects and carries away filtrate, and an impervious membrane liner. Solids are pumped into the reed 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 underdrains, traveling back to the wastewater treatment plant for secondary treatment. During dewatering 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.
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 intricate network of underground stems. New plants in turn will sprout from these stems. These rapidly growing roots provide air passages through the sludge which 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. Leachate is channeled back to the treatment plant through the underdrain.
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 root stalks 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 eight to ten 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 a 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.
Reed bed photos in this
article are from the