Sludge reed beds (Figure 1) have been used for dewater-ing (draining and evapotranspiration) and mineralization of sludge in Denmark and other countries in Europe since 1986-88 when the first sludge-processing systems were introduced. Sludge from wastewater treatment plants (2500-125 000 pe) is treated in sludge reed bed systems with 1-18 reed bed basins with loading rates of 25-2200 tonnes dry solids (TDS) per annum for 10 years. In 2004, ~ 110 systems were in operation in Denmark.
Long-term sludge reduction takes place in reedplanted basins, partly due to dewatering (draining, evapotranspiration) and partly due to mineralization of the organic solids in the sludge. Sludge from wastewater treatment plants is pumped onto the basin surface. The dewatering phase results in the dry solids content of the sludge remaining on the basin surface as sludge residue, whereas the majority of its water content continues to flow vertically through the sludge residue. The water content is further reduced through evapotranspiration.
In addition to dewatering, the organic solids in the sludge are mineralized, thereby minimizing the sludge volume. Oxygen diffusion via filter aeration and through the cracked sludge surface and oxygen diffusion from the roots into the sludge residue enable aerobic microorganisms to exist close
to the roots and in the sludge residue (Figure 1). The overall reduction of the sludge volume occurs without the use of chemicals. The process involves only a very low level of energy consumption for pumping the sludge and reject water (Figure 2). Experience from reference plants is that this type of system is capable of treating many types of sludge having a dry solids content in the range of c. 0.5-5%.
The sizing and design of reed bed systems depends on the sludge production (TDS per annum), sludge type, quality, and regional climate. A treatment period of 8-12 years means that the depth of the basin above the filter needs to be at least 1.8 m. The operation of the system may be divided into a number of phases related to different periods in the lifetime of a system. A system generally runs for a total of at least 30 years: this period being divided into two to three phases each lasting around 8-12 years. Each phase consists of commissioning, full operation, emptying, and reestablishment of the system.
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