Enhancing the dewaterability of anaerobically digested sludge using fibrous materials recovered from primary sludge: demonstration from a field study

Sludge dewatering is an important process for determining the operation cost of sludge disposal. Hence, improving the dewaterability of anaerobically digested sludge containing high water content is of paramount significance. For this challenge, we developed a novel process in which fibrous materials initially collected from a primary sedimentation tank, namely recovered fibers, were used as an auxiliary agent to improve the dewaterability of anaerobically digested sludge. The objective of this study was to investigate the feasibility of using the developed process to improve sludge dewaterability. Three fibrous materials individually recovered from the respective primary sedimentation tanks in different wastewater treatment plants (WWTPs) were used to investigate sludge dewaterability, organic C compositions, and calorific values. The recovered fibers showed comparable compositions irrespective of the WWTP. Six different WWTP sludge samples after anaerobic digestion were continuously supplied to three different dewatering devices, i.e., screw press, centrifuge, and belt press machines, with or without the supply of recovered fibers. Irrespective of the type of a dewatering device, the supply of recovered fibers mixed with the tested sludge samples at a feed ratio of 0.18–0.20 g-fibers/g-sludge (dry weight) reduced the amount of polymer flocculant by 13–50%, thereby displaying the superiority of the addition of recovered fibers. Furthermore, at fiber feed ratios of 0.20 g-fibers/g-sludge and 0.40 g-fibers/g-sludge when the rates of sludge supply and polymer flocculant were kept constant, the corresponding water contents in the dewatered sludge cakes decreased to about 74–76% and 69–72%, respectively. The consistent results regardless of the dewatering device or sludge sample showed the significance of the addition of recovered fibers for improvement in sludge dewaterability.