Experimental investigation of the external nitrification biological nutrient removal activated sludge (ENBNRAS) system

Abstract

A systematic lab-scale experimental investigation is reported for the external nitrification (EN) biological nutrient removal (BNR) activated sludge (ENBNRAS) system, which is a combined fixed and suspended medium system. The ENBNRAS system was proposed to intensify the treatment capacity of BNR-activated sludge (BNRAS) systems by addressing two difficulties often encountered in practice: (a) the long sludge age for nitrification requirement; and (b) sludge bulking. In the ENBNRAS system, nitrification is transferred from the aerobic reactor in the suspended medium activated sludge system to a fixed medium nitrification system. Thus, the sludge age of the suspended medium activated sludge system can be reduced from 20 to 25 days to 8 to 10 days, resulting in a decrease in reactor volume per ML wastewater treated of about 30%. Furthermore, the aerobic mass fraction can also be reduced from 50% to 60% to <30% and concommitantly the anoxic mass fraction can be increased from 25% to 35% to >55% (if the anaerobic mass fraction is 15%), and thus complete denitrification in the anoxic reactors becomes possible. Research indicates that both the short sludge age and complete denitrification could ameliorate anoxic aerobic (AA) or low food/microorganism (F/M) ratio filamentous bulking, and hence reduce the surface area of secondary settling tanks or increase the treatment capacity of existing systems. The lab-scale experimental investigations indicate that the ENBNRAS system can obtain: (i) very good chemical oxygen demand (COD) removal, even with an aerobic mass fraction as low as 20%; (ii) high nitrogen removal, even for a wastewater with a high total kjeldahl nitrogen (TKN)/COD ratio, up to 0.14; (iii) adequate settling sludge (diluted sludge volume index [DSVI] <100 mL/g); and (iv) a significant reduction in oxygen demand.

Comparative study of biological nutrient removal (BNR) processes with sedimentation and membrane‐based separation

“…Similar recoveries have been documented in the literature for BNR processes (Barker and Dold, 1995;Hu et al, 2003) with the ''missing'' COD being attributed to fermentation processes occurring in the anaerobic zone. Although direct evidence to support this hypothesis was not generated in the present study, the COD recovery improved during the higher rate operating conditions.…”

Comparison between UCT type and DPAO biomass phosphorus removal efficiency under aerobic and anoxic conditions

“…The concept of biological phosphorus removal under anoxic conditions in municipal wastewater treatment, coupled by a fixed film nitrification which provided the nitrate for the anoxic zone, was applied by Bortone et al (1996), introducing for the first time the so-called "DEPHANOX" process. Other researchers based their investigation on this pattern (Hu et al 2003;Wang et al 2004a;Torrico et al 2008). Sludge recirculation also occurs for the nitrifying sludge (sludge II) from ST2 back to the nitrification tank.…”

Mathematical Modelling and Computer Simulation of Activated Sludge Systems

“…Also Hu et al (2002b) indicated that the main factor affecting the occurrence of DPAOs and associated anoxic phosphate uptake was the nitrate load to the main anoxic reactor. The other potentially stimulating factors include the small aerobic mass fraction, sequence of reactors (nitrification and then denitrification) and reduced frequency of sludge alternation between the aerobic and anoxic states (Hu et al 2002b;Hu et al 2002c;Hu et al 2003a). The other potentially stimulating factors include the small aerobic mass fraction, sequence of reactors (nitrification and then denitrification) and reduced frequency of sludge alternation between the aerobic and anoxic states (Hu et al 2002b;Hu et al 2002c;Hu et al 2003a).…”