Most of the activated sludge models developed for the substrate removal kinetics have commonly assumed that the organic matter is dissolved, and do not include an identification of the fraction of the total COD (TCOD) that is truly dissolved (DCOD), nor the fraction that is particulate COD (PCOD). Recognizing that not all the COD in wastewater is dissolved, recent research efforts have concentrated on modeling the hydrolysis of particulate organic matter once it is trapped in the biological floc particles. However, little effort has been made in identifying how the particulate matter is actually removed form the liquid phase. Research performed at the University of New Orleans experimental station at Marrero, Louisiana, USA, has revealed that most of the TCOD in wastewater is PCOD, and that only 20% is DCOD. Therefore, biological flocculation must play an important role on PCOD removal. To ascertain this role, carefully controlled experiments were run using a 6 m~3/d continuous flow aeration chamber with variable hydraulic retention times. More than 90 % PCOD removal was accomplished with HRTs as low as 20 minutes. Deflocculation of the bacterial suspension by adding an ion exchange resin resulted in minimal TCOD removal, thus demonstrating that biological flocculation plays a major role on both PCOD and TCOD removal in suspended growth reactors.
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