Regular articleMoving bed membrane bioreactors for carbon and nutrient removal: The effect of C/N variation

- A UCT-IFAS-MBR pilot plant subjected to C/N variations was investigated.
- Biological performances were significantly affected by the lowest C/N ratio.
- Competition between PAOs and denitrifying bacteria decreased P removal dramatically.
- Respirometry showed a dramatic decrease of biomass activity with the lowest C/N.
- Membrane filtration worsened due to the abrupt decrease of the C/N ratio.

In this paper, an experimental campaign was carried out on a University of Cape Town Integrated Fixed Film Activated Sludge Membrane Bioreactor (UCT-IFAS-MBR) pilot plant. The aim of the study was to evaluate the effect of the influent C/N ratio on the system performance in terms of organic carbon, nitrogen and phosphorus removal, biomass viability (through respirometry), activated sludge features and membrane filtration properties. The experiments were organized into three phases, characterized by a variation of the C/N ratio (namely, Phase I: C/N = 5, Phase II: C/N = 10; Phase III: C/N = 2). The results highlighted that the system performance was significantly affected by C/N ratio. The removal efficiencies were satisfactory for C/N ratio equal to 10 and 5, with average removal in Phases I and II of 98-98%, 53-69% and 67-87% for COD, nitrogen and phosphorus, respectively. Conversely, with a C/N ratio of 2, a significant worsening of the pilot plant performance was observed, with average COD, nitrogen and phosphorus removal equal to 70%, 44% and 26%, respectively, much lower compared to the previous phases. Respirometry highlighted a significant decrease of bacterial activity when the C/N was reduced to 2, even if the biofilm seemed to be more resilient in terms of activity compared to the activated sludge.

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