The impact of high mixed liquor concentration (3-13 gVSS/ℓ) on the kinetic rates of the N and P removal bioprocesses in membrane biological nutrient removal activated sludge systems

Abstract

The impact of including membranes for solid liquid separation and high volatile suspended solids (VSS) concentration (3-12 gVSS/ℓ) on the kinetics of biological nitrogen and phosphorus removal (BNR) was investigated. To achieve this, a membrane bioreactor (MBR) biological nutrient removal (BNR) activated sludge (AS) system was operated for 450 days in parallel with a conventional BNR system with a settling tank (CAS). The influence of high VSS concentration (up to 12 gVSS/ℓ) in the MBR system on the system performance and the nitrification, denitrification and phosphorus release and uptake kinetic rates were measured with aerobic, anoxic and anaerobic batch tests on mixed liquor (ML) harvested from the MBR system, diluted to different VSS concentrations, and from the CAS system. Also, the limitation of ammonia, oxygen, nitrate and acetate on the kinetic rates was investigated with batch tests. The results show that the BNRAS steady state and kinetic models developed for low VSS concentration BNRAS systems with secondary settling tanks can be applied with reasonable confidence to predict the performance of high VSS concentration BNRAS systems with membranes, except for the maximum specific growth rate of the nitrifiers, which was observed to be significantly lower in the MBR system.