Development of an algorithm for air-scour optimization in membrane bioreactors G. Ferrero*, H. Monclus**, G. Buttiglieri*, S. Gabarron**, J. Comas**, I. Rodriguez-Roda* , ** * ICRA (Catalan Institute for Water Research). Scientific and Technological Park of the University of Girona. H2O Building. c/ Emili Grahit 101. E17003. Girona. Spain. Tel: (+34) 972 18 33 80 (email:[gferrero, gbuttiglieri, irodriguezroda]@icra.cat) ** Laboratory of Chemical and Environmental Engineering (LEQUiA). Environmental Institute. University of Girona. Girona. E17071. Spain. (email:[hector, sara.gabarron, quim, ignasi]@lequia.udg.cat) Abstract: Membrane Bioreactors are used in an increasing number of wastewater treatment facilities because of their compactness and efficiency in solid-liquid separation. In this paper the development of an air-scour control algorithm based upon short term and long term membranes permeability evolution is presented. An open loop calibration and partial validation was carried out in a semi-industrial scale pilot plant where manual changes in air-scour flow had been previously carried out. The control system was successfully tested in closed loop in an industrial scale pilot plant, defining a maximum daily air-scour decrease or increase of 6% of the air-scour recommended by membranes suppliers. A maximum air-scour saving of 20%, calculated in terms of air flow saved, was achieved without interfering with the biological nutrient removal and without any apparent long term effect. Keywords: Aeration, Control, Energy saving, Membrane bioreactor. 1. INTRODUCTION Membrane Bioreactors (MBR) improve conventional activated sludge process for wastewater treatment, using an ultra or micro filtration for solid-liquid separation instead of secondary settlers, obtaining higher effluent quality (Monclús et al., 2010, Judd, 2006). The MBR process presents many advantages (higher effluent quality, reduced excess sludge production, drastically enhanced elimination of pathogens and viruses, potential degradation of specific refractory pollutants, higher stability and persistence to shock loads, etc.), but as in most of membrane filtration processes the permeate flux declines (or transmembrane pressure increases) during filtration due to membranes fouling (Judd, 2005). MBR use can be always justified in case of discharge of treated wastewater in very sensitive areas, water reuse, limited space available for plants retrofitting and high loaded or complex industrial wastewater treatment and with a significant seasonal component (Ferrero et al., 2010). Nonetheless, the technology is still limited by the high operating costs and by membrane fouling (Meng et al., 2009). Through the implementation of automatic control systems, optimal results are achievable regarding both energy optimization and fouling mitigation. The most important achievements can be found in the patent literature; however there is still a lack of robust control systems capable to reduce aeration requirements maintaining optimum filtration performances. Some suppliers propose to decrease air scour flow rate during filtration and to increase it during relaxation, such as to reduce overall air scour flow. The air scour flow rate varies in approximate proportion to increases and decreases in the flow rate of permeate through the membrane bioreactors. The filtration-relaxation cycles are frequent and shorter than suppliers’ specifications (Livingston, 2007). Ginzburg et al. (2007) developed an on-line process control system that considers resistance values and adjust operational parameters such as membrane aeration frequency and membrane aeration flow in order to reduce operational costs related to fouling removal. An on-line fouling measurement for controlling membrane cleaning actions by measuring the reversible and irreversible fouling was developed (Brauns et al., 2002, 2005) but it has not been applied to an advanced control system yet. This paper presents the development of an innovative control algorithm for air-scour optimization that dynamically regulates air flow based on long term and short term permeability trends. Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011 Copyright by the International Federation of Automatic Control (IFAC) 3795