Cascade Anaerobic Digestion to enhance Waste Activated Sludge Degradation

Abstract

Large amounts of residual waste activated sludge are produced as by-products during biological wastewater treatment processes. Anaerobic digestion is a widely accepted stabilisation method for waste activated sludge (WAS) treatment. However, the application of anaerobic digestion is limited by the long retention time and low degradation efficiency of compounds. Structural extra cellular polymeric substances (EPS) are metabolic products released by microorganisms that play an important role in the disintegration of sludge structure. The limitations in anaerobic digestion mentioned above pertain to the hydrolysis step in anaerobic digestion. The cascade reactor (cascade AD) system i.e. continuous stirred tank reactor (CSTR) in series, is a robust reactor system that is expected to enhance the hydrolysis step and to show a superior performance at low solid retention time compared to a conventional CSTR.
This research is aimed at observing the difference in performance of cascade AD and a conventional CSTR at shortened retention time. Various indicators were used to understand the performance enhancement of a cascade AD in comparison to a conventional CSTR. Moreover, the degradation of structural EPS by selected enzyme groups such as protease, cellulase and polygalacturonase were also studied.
The cascade AD showed better performance than a conventional CSTR at a retention time of 22 and 15 days. This improved performance was enabled by the smaller reactors in cascade AD that provided higher hydrolysis rate. Higher removal efficiency of protein, carbohydrate and structural EPS was observed in cascade AD. Improved ammonium and phosphate release were also indications of better performance of cascade AD. Although the mass balance in nitrogen was maintained in the reactor system, phosphorous mass balance indicated possibilities of precipitation.
The batch tests performed with the enzyme protease, cellulase and polygalacturonase were aimed at understanding the degradation of SEPS. It was inferred from the test that volatile suspended solids proved to be a better indicator for solubilisation compared to COD and ammonium concentration. Protease showed higher solubilisation compared to cellulase and polygalacturonase. Particle size distribution did not indicate a significant difference upon the addition of all three enzyme groups; indicating that a significant change in structure was not caused by the enzymes. Hydrolysis kinetics and SEPS degradation could not be derived from the test because of the variations in results based on the substrates used. Nevertheless, the tests proved to be useful in improving methodology for deriving hydrolysis kinetics of WAS.
In conclusion, the novel cascade AD showed better performance at shortened retention time. The system also showed stable performance despite the shortened retention time compared to a conventional CSTR. Thus, the stable performance suggests the opportunities to further lower the retention time in cascade AD.