Removal of carbon and nutrients from wastewater in a moving bed membrane biofilm reactor: the influence of the sludge retention time

A University of Cape Town (UCT) pilot plant combining both membrane bioreactor (MBR) and moving bed biofilm reactor (MBBR) technology was monitored. Three experimental Phases were carried out by varying the mixed liquor sludge retention time (SRT) (indefinite, 30 and 15 days, respectively). The system performance has been investigated during experiments in terms of: organic carbon, nitrogen and phosphorus removal, biokinetic/stoichiometric constants, membrane fouling tendency and sludge dewaterability. The observed results showed that by decreasing the SRT the UCT pilot plant was able to maintain very high total COD removal efficiencies, whilst the biological COD removal efficiency showed a slight decrease. Nitrification was only slightly affected by the decrease of the mixed liquor SRT, showing high performance (as average). This result could be related to the presence of the biofilm able to sustain nitrification throughout experiments. Conversely, the average P removal efficiency was quite moderate, likely due to the increase of the ammonium loading rate that could promote an increased NO3-N recycled from the anoxic to the anaerobic tank, interfering with phosphorus accumulating organisms (PAOs) activity inside the anaerobic tank. Membrane fouling increased at 30 days SRT likely due to a reduction of protective cake pre-filter effect. Moreover, it was noticed the increase of the resistance due to pore blocking and a general worsening of the membrane filtration properties.