Abstract
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 organic 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.
Highlights
Wastewater treatment processes such as activated sludge treatment systems produce surplus sludge which has to be disposed of
Sludge treatment processes generally have two main purposes: (i) thickening and dewatering whereby the sludge volume and the costs of subsequent handling, transportation, and disposal are reduced [2]; and (ii) stabilization of labile organic matter remaining in the sludge through microbial decomposition and desinfection [3]
A sludge treatment reed bed system generally consists of a number of reed beds that are loaded in sequence by liquid surplus sludge from the wastewater treatment plant
Summary
Wastewater treatment processes such as activated sludge treatment systems produce surplus sludge which has to be disposed of. 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. 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]. 10 years of treatment make it possible to recycle the sludge residue to agriculture [5,9,14,16,17,18]
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