Although initial designs assumed that the winter months dictated the most restrictive loading rates, operational experience shows that the most problems are experienced in the spring. There are several possible reasons for this:
- The reeds which were harvested in the fall have not developed a lot of vegetative growth to accomplish evapotranspiration.
- More rainfall occurs in the spring.
- Most aerobic digesters achieve the lowest volatile solids reduction in the spring when the wastewater temperatures are lowest.
- Winter operations benefit from the freeze/thaw process.
There are several steps which can accommodate the reed bed spring limitations.
T Optimize digester operation to perform during winter & spring conditions - topic of another article. T Arrange for hauling a portion of the digested biosolids in liquid form and load the beds at a lower rate. T Provide aerated liquid storage to allow reduced bed loadings during the spring and higher loadings in the summer months when reeds operate at the highest rates. T Isolate and discontinue loading areas where the reeds appear stressed. They will usually grow back and the area can again be used for biosolids loading. Preliminary Treatment Requirements -Both aerobically and anaerobically digested biosolids work well in this process (Sassaman and Fair, 1998). Ideally, volatile solids concentration should be 70 percent or less. Higher concentrations may result in odors, slime layers, septic conditions, and reed mortality. If the final product is to be land applied, removal of grit and screenings should take place to avoid any visible waste materials, plastics, or sanitary products.
Pathogen Reduction During Drying - Reed beds have been demonstrated to achieve vector attraction reduction measured by volatile solids reduction across the digesters and the beds. Several facilities analyzed for fecal coliform, Helminth Ova and Enteric Virus and found that the end product met Class A pathogen reduction. At this time, there is not enough operating data or analyses to predict the likelihood of this outcome. The mechanisms by which pathogens are reduced in reed beds are not clearly understood. Facilities evaluating reed beds should not rely on Class A pathogen reduction when predicting life cycle costs, but operating facilities should perform pathogen enumeration to see the level of pathogen reduction achieved. Applicability- The size of typical facilities using reed beds range up to 2 MGD, but the largest system in the United States is planned for upgrade to service an 8 MGD facility. A general rule of thumb for sizing is 40 gallons/square foot/year in climates that undergo a freeze - thaw cycle. Digested biosolids at 3-4% Total Solids with volatile solids concentration of 70 percent or less are ideal. Advantages and Disadvantages - The following compares features of reed beds to conventional sand drying beds or mechanical methods: Advantages
? The use of reed beds can significantly reduce the staff hours and costs required to dewater biosolids.
? The reed beds are described by operators as a simple and manageable method of removing biosolids from the WWTP.
? The reed beds give the operators more control over their biosolids removal at an annual cost that is affordable (Sassaman and Fair, 1998).
? Reed bed cleaning can be scheduled to meet the local demand for soil amendment rather than trying to land apply year round when fields may not be open or building expensive storage facilities.
? The contents of the reed bed biosolids product has less moisture than the product from most other dewatering methods.
? Decomposition of the organic matter is enhanced.
? The quality of the percolated water is better than conventional beds. Percolated water has specifically higher dissolved oxygen, lower BOD, COD, TOC, ammonia, and total nitrogen.
? Denitrification occurs before the liquid drains back to the head of the plant.
? If the excavated product is screened of roots, or composted, the reed beds may produce a high quality biosolids, which are suitable for use as a soil amendment.
Disadvantages - ? There is concern about reeds spreading to nearby areas by means of seeds, or the rhizomes of the plant that have many buds and large carbohydrate reserves for growth, becoming a dominant species in nearby wetlands environments. (Note:Studies in Wisconsin found no evidence to support this assumption.)
? There is a potential for onsite odor generation if the solids are not stabilized prior to loading the reed beds.
? Accumulating the solids for years, although greatly reducing volume, merely delays the end use or disposal costs.
? Excavation of reed beds can cost up to $57/ton, plus hauling and disposal or land application.
? Metals concentration in the reed bed may increase as VS are destroyed.
? Currently, data are not available to determine if the delayed end use of large quantities of product is more economical than daily end use or disposal.
Ongoing Research- The University of Delaware, Dr Jack Gallagher, Graduate College of Marine Studies, Halophyte Biotechnology Center, is doing research on reed beds. In a project funded by a NOAA Sea Grant, the University released a report titled, Optimizing Reed (Phragmites Australis) Function In Sludge Drying Beds: Producing A More Effective Genotype. The goal of this project was to develop a variety or varieties of Phragmites especially designed to enhance the drying and decomposition processes in the biosolids-drying bed. The design criteria for the improved Phragmites variety also includes plant sterility. At this time, there is one variety that meets this criteria, a distinctive variegated line. Additional research is planned in 2002 to determine if reeds will grow on biosolids amended corn fields, and if so, will herbicides prevent reed growth. Utilizing the Biosolids from Reed Beds - In Pennsylvania, there is reluctance to use the final product in agriculture because regulators do not want to introduce either the rhizomes or the Phragmites seeds to agricultural land. Proponents point out that phragmites will not proliferate in deep and well drained soils that are managed for crop production. In Aliquippa, Pennsylvania, the biosolids in reed beds were shown to be Class A with respect to pathogens and high quality with respect to metals. Aliquippa requested permission to distribute the product as a soil amendment but, at this stage, Pennsylvania Department of Environmental Protection approval is pending. In Mahonoy City, Pennsylvania the final product has been approved as an exceptional quality product and was used in reclamation of mined lands through a contract with SynaGrow. In Myerstown Pennsylvania, the product was used to supplement daily cover at the landfill. In the Southwest part of the US, the product is composted prior to land application. In Toledo Ohio, the product was screened and used as mulch in landscape plantings with no reed emergence. References provided upon request. | 
