Abstract
The objective of this study was to investigate the effect of waste activated sludge (WAS) predation by the aquatic worm Tubifex tubifex (T. tubifex) on the overall biodegradability of WAS. The initial WAS biodegradability potential was determined in 80 days sequential batch-fed anaerobic and aerobic treatment combinations. These treatment combinations were used as a reference for comparison with the effect of 5-day predation and 40-day anaerobic treatment combinations. Predation and the subsequent anaerobic digestion of the predated solids shows superior solids removal and superior overall conversion rates compared to solely conventional anaerobic digestion. Strikingly, the predation and anaerobic treatment combinations reached the same chemical oxygen demand (COD) and volatile solids (VS) reduction as the reference processes, i.e. 58% and 49% for COD and VS, respectively. Our results show that predation and anaerobic treatment combinations increase solids removal rates, but do not alter the overall biodegradability potential of WAS.
Highlights
Waste activated sludge is a by-product from conventional sewage treatment
To put solids removal potential of a certain treatment method in the proper perspective, it is necessary to determine to what extent the solids potentially could be biodegraded in a given time frame
The results clearly show that the first aerobic or anaerobic digestion stage showed the largest contribution to the total volatile solids (VS) and chemical oxygen demand (COD) removal, which is in agreement with other research (Novak et al, 2003; Park et al, 2006) and can be explained by the sequenced degradation of readily biodegradable sludge parts followed by the more complex parts
Summary
Waste activated sludge is a by-product from conventional sewage treatment. Due to stringent legislation (Commission Of European Com munities, 1998), preventing the agricultural use of stabilised WAS in countries like the Netherlands, WAS treatment and final disposal largely contributes to the total sewage treatment costs (Metcalf et al, 2004). WAS biodegradability can be improved by applying pre- or in-linesludge treatment methods prior to the anaerobic digestion process These additional treatments improve the solids reduction by an additional 5 to 35% (Gonzalez et al, 2018). A positive effect on the biodegradability of WAS is observed in aerobic worm predated treatment Aquatic worms, such as T. tubifex have been found to naturally inhabit the aerobic zones of WWTPs. Sudden worm growth or worm blooms, have been associated with improved sludge settling characteristics and a lower WAS pro duction. Tamis et al (2011) suggested that worm predation as pre-treatment prior to anaerobic digestion enhances the overall WAS biodegrad ability compared to conventional AD in terms of solids removal and treatment time. In our previous work (de Valk et al, 2017b), we suggested that the aforementioned overall increased biodegradability was possibly due to presence of a sludge fraction that is only degradable under aerobic conditions, such as the worm predation process and not under anaerobic conditions (Park et al, 2006)
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