A Comparative Study of the Effects of External Selection on Settleability and Formation of Aerobic Granular Sludge

Aerobic granular sludge (AGS) has shown much promise in the advancement of the wastewater treatment industry. AGS has been studied intensely since the early 1990’s due to highly desirable characteristics of nutrient removal efficiency, low footprint, and abnormally faster settling rates than conventional activated sludge. With the exception of a few on-going research projects, all AGS systems have been studied and implemented through the use of sequencing batch reactors (SBR). Recently, a novel approach to developing AGS and improving settling characteristics of activated sludge within conventional activated sludge (CAS) processes has being attempted through the use of external selection. Activated sludge was fed to a hydrocyclone for approximately 374 days to select for denser material that would improve settling and potentially cultivate AGS. The effect of the hydrocyclone on the activated sludge was directly compared to the settling characteristics of an identical parallel treatment train that did not utilize the external selector. Data suggests that with the correct nozzle and operating pressure, much of the faster settling sludge and ballast material for the cultivation of AGS can be selected for, retained, and returned to the activated sludge process. During the warmer months, while the wastewater temperature was consistently \u3e19ᵒC, the hydrocyclone improved settling rates in the activated sludge by up to 15-20x the settling rates of the activated sludge in the parallel train. While a shift to granulation did not occur, further research and optimization could potentially lead to the cultivation of AGS and the further improvement of settling characteristics of activated sludge. The settling performance imparted on the activated sludge by the hydrocyclone shows that it is feasible to improve the overall settleability of activated sludge in a suspended growth process through external selection