Energy demand is a key cost factor when considering MBR technology. The main contributors to energy costs are sludge transfer, permeate production and, most significantly, aeration.
Membrane aeration is normally achieved via coarse bubble aerators positioned beneath the membrane units.
Aeration demanded for membrane scouring can be normalised to produce the specific aeration demand (SAD). Normalisation can be either against the membrane area (SADm, taking units of Nm3/(m2.h)) or against the permeate volume (SADp, Nm3/m3).
SADp is then directly proportional to the specific energy demanded for membrane air scouring, or SEDAm, since all other components of air pumping (namely the blower efficiency, aerator depth, inlet air pressure, aerator type, and ratio of air and water enthalpies) are constant for a specific installation. SADp is the ratio of SADm to the flux in consistent units, i.e. in m/h; energy efficiency is improved both by increasing the sustainable flux and decreasing the membrane air scour rate required to sustain this flux.
Aeration for biological treatment, often referred to as the ‘process aeration’, is dependent primarily on the inlet and outlet biodegradable organic carbon and Kjeldahl nitrogen levels, Kjeldahl nitrogen (KN) being biologically oxidisable nitrogen. The absolute difference between inlet and outlet defines the load, and thus the oxygen demanded by this load to convert organic carbon to carbon dioxide and KN to nitrate (nitrification).
In many cases, further biological treatment is required for nutrient removal (nitrate and phosphate, usually abbreviated to N and P respectively). Such removal is essentially brought about by pumping the sludgeto different tanks or regions of tanks where different oxidation conditions prevail, allowing conversion of nitrate to nitrogen gas (denitrification). This modification, known as the ‘Modified Ludzack-Ettinger’ process (MLE), then affects the OPEX in two ways.
Firstly, additional energy is required for transferring sludge between the two regions. Secondly, the oxygen demand is reduced since some of the oxygen required for ammoniacal nitrogen oxidation to nitrate (nitrification) is supplied by the nitrate.
For biological phosphorous (P) removal, further pumping or sludge transfers to anaerobic tanks and/or chemicals addition may be employed. The sludge pumping and chemicals further add to OPEX, and the introduction of further tankage increases the capital costs to some extent.
The actual aeration rate demanded to deliver a specific dissolved oxygen (DO) concentration is dependent on key design parameters, such as the aerator type and the depth to which it is submerged in the tank, and the residual DO and sludge solids concentration. The efficiency of transfer of oxygen from air into the sludge is defined by the alpha factor (α), which is simply the ratio of the mass transfer coefficient of water to that of the sludge. α decreases with solids concentration according to trends which appear to vary widely between different studies but which generally follow a pseudo-exponential relationship. Thus, according some relationships, the aeration rate required to maintain a specified DO residual may double between a process sludge solids concentration of 7 and 12 g/L.
Henkel, J., Cornel, P., and Wagner, M. (2011). Oxygen transfer in activated sludge – new insights and potentials for cost saving. Water Science & Technology, 63(2) 2011.
Wilson, T.E., and McGettigan, J. (2006). A critical new look at nutrient removal processes, Proceedings of WEFTEC, 21-25 Oct, Dallas.