Innovative sludge handling through pelletization/thickening

Abstract

For the past decades, it has been observed that the organic contents of sewage sludge produced in Japan has been increasing. As a result, difficulties have been experienced with thickening and dewatering operations. Furthermore in Japan, where space is limited, space saving has been an urgent necessity for the sludge thickening–dewatering process. A new ultra-compact sludge treatment system has been developed to solve these problems. In this system, primary and excess activated sludge is directed to a sludge conditioning tank, where it is dosed with an inorganic coagulant and mixed to neutralize the surface charge of the sludge particles. This conditioned sludge is then sent to a pelletizing/thickening tank, where an amphoteric polymer is added to pelletize the sludge. This results in pellets having a diameter of 1–2 cm. Using the same tank, water can then be removed with a special built-in device, resulting in a sludge with an increased concentration of solids. The sludge pellets produced have a high mechanical strength with a very low moisture content. The retention time of the sludge in the system (conditioning plus pelletizing/thickening) is only 10 to 20 min, as compared to about 12 h when conventional thickeners are used. Pilot plant scale studies in the laboratory and full scale investigations at treatment plants have demonstrated that this new system can eliminate conventional thickeners, enabling “direct” sludge dewatering. Compared with conventional thickening, the new system produces a sludge cake with 3–5% less moisture content and at a rate greater than 200 kg-ds/m/h using a belt press filter. Further, the soluble phosphorus in the sludge can be converted to an insoluble form through the addition of an inorganic coagulant. Also, because of the short retention time of the sludge in the system, there is less opportunity for the sludge to become septic and to release phosphorus into the supernatant which normally is returned to the wastewater flow.