Sludge Dewatering and Mineralization in Sludge Treatment Reed Beds
Hans Brix
2017
Water
Sludge Treatment Reed Beds (STRBs) are widely used in Northern Europe to dewater and mineralize surplus sludge from activated sludge systems used to treat urban domestic sewage. STRBs are low-technology, energy-efficient, and do not require addition of chemicals. They dewater and stabilize the sludge and produce a final product that can be safely used as a fertilizer for agricultural crops. Long-term sludge reduction takes place in the reed beds due to dewatering and mineralization of the
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... c matter in the sludge. Although, in theory, a simple technique relying largely on natural processes, experience has shown that it is very important to understand and respect the basic design and operation requirements of STRBs. This paper describes the basic design and operation requirements of STRBs, with special focus on pivotal requirements to respect in order to secure proper functioning. Also, the paper summarizes performance experience concerning final dry matter content, degree of mineralization, emission of greenhouse gases, and degradation of micro-pollutants in STRBs. There are still a number of outstanding issues that are not fully understood, particularly in relation to the importance of the sludge quality for the dewatering in an STRB. Therefore, extreme care should be taken when attempting to extrapolate the use of STRBs to applications and regions outside of their 'normal' and documented area of application. Compared to conventional sludge handling techniques, Sludge Treatment Reed Beds (STRBs) are low-technology, energy-efficient, and do not require addition of chemicals. STRBs dewater and stabilize the sludge and produce a final product that can be safely disposed of or used for agricultural purposes [4, 5] . Using conventional technologies such as centrifuges and filter presses, the sludge typically reaches a dry matter content of around 20%, whereas the sludge in correctly designed and operated STRBs can reach a dry matter content of 20%-30% [2,6] and in optimal conditions up to 40% [7, 8] . An additional advantage of STRBs as compared to most other technologies is that the water released from the sludge during dewatering is treated as it percolates through the bed. The typically high concentrations of Chemical Oxygen Demand (COD) in the reject water are reduced by more than 60%, and ammonium-N is usually nitrified. In addition, long-term sludge reduction takes place in the reed beds due to dewatering and mineralization of the organic matter in the sludge [9,10]. STRBs have been used in Europe for sludge dewatering and stabilization since the late 1980s. The largest experience comes from Denmark, where there are +100 full-scale systems in operation [11, 12] . Experiences from these show that STRBs are capable of treating many types of sludge with a dry solid content between 0.5% and 5%, including sludge from water works [13, 14] . Dimensioning and design of the STRBs depend on the sludge production (tons of dry solids/year), sludge type, sludge quality, and regional climate [7, 11] . The operation of a system is divided into a number of phases related to different periods in the lifetime of the system. These phases comprise the periods of commissioning, full operation, emptying, and re-establishment of the system. Loading cycles are related to the sludge type and the maturity of the STRB. The sludge residue will, after approximately 10 years of operation, reach an approximate dry solids content of 25%-30% [14, 15] . Experience has shown that the quality of the final product with respect to heavy metals, hazardous organic compounds, and pathogens after 10 years of treatment make it possible to recycle the sludge residue to agriculture [5, 9, 14, [16] [17] [18] .
doi:10.3390/w9030160
fatcat:gsmew6o3j5ax7nrtkcgvnnvymy