Dewatering of primary settled urban sludge in a vertical flow wetland

Sludge drying reed beds (SDRBs) are a combination of traditional sludge drying beds and constructed wetlands. Their main advantages include low investment, infrequent biosolids removal (of the order of years), dewatering and mineralization of biosolids, minimization of biosolid volume, simplicity and economy, and production of a beneficial, well-composted side product. A constructed wetland facility is used to treat the sewage from the municipality of Nea Madytos, Thessaloniki Province, North Greece. The primarily produced biosolids in the Imhoff tank of this facility are treated with a vertical flow SDRB. The performance and dewatering efficiency of this system was monitored for 1 year. Over the 12 years of operation of the treatment plant, 7884 m3 of biosolids from the Imhoff tank were transferred to the SDRBs. Until today, no biosolids were removed, and the level of the dewatered sludge on the bed is built to about 20 cm, with a volume of residue sludge of 28 m3. This corresponds to a biosolid reduction of 99.64%. At the end of the examination period the TS (VS) content varied from 55% (40%) in the top layer to 65% (35%) in the bottom layer. The heavy metal concentration meets the European Union standards for heavy metals in case of agricultural disposal of the treated sludge.