Modular operation of membrane bioreactors for higher hydraulic capacity utilisation

Abstract

Using data from 6 full-scale municipal membrane bioreactors (MBR) in Germany the hydraulic capacity utilisation and specific energy consumption were studied and their connexion shown. The average hydraulic capacity utilisation lies between 14% and 45%. These low values are justified by the necessity to deal with intense rain events and cater for future flow increases. However, this low hydraulic capacity utilisation leads to high specific energy consumption. The optimisation of MBR operation requires a better utilisation of MBR hydraulic capacity, particularly under consideration of the energy-intensive membrane aeration. A first approach to respond to large influent flow fluctuations consists in adjusting the number of operating modules. This is practised by most MBR operators but so far mostly with variable flux and constant membrane aeration. A second approach is the real-time adjustment of membrane aeration in line with flux variations. This adjustment is not permitted under current manufacturers' warranty conditions. A further opportunity is a discontinuous operation, in which filtration takes place over short periods at high flux and energy for membrane aeration is saved during filtration pauses. The integration of a buffer volume is thereby indispensable. Overall a modular design with small units, which can be activated/ inactivated according to the influent flow and always operate under optimum conditions, enables a better utilisation of MBR hydraulic capacity and forms a solid base to reduce MBR energy demand.

Membrane bioreactors – A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects

“…Many authors [27,67,70,73,74] stressed the importance of hydraulic capacity utilization of the membranes and operation at optimal flow conditions, i.e., hydraulic load close to the design flow rate, for energy efficient operation of MBRs. Since operation below optimal flow conditions is associated with energy penalty, implementation of flow equalization or adjusting operational settings to the incoming flow was proposed.…”

Municipal Sewage Treatment Technology: A2/O-VMBR Integrated Technology for Municipal Treatment and Improved Pollutant Removal

“…Unfortunately, the nitrifying bacteria were unable to absorb or transform ammonia nitrogen at a water temperature of 16 • C after 20 days of operation. Nitrifying bacteria have stringent growth needs and a long growth cycle; hence, a decrease in their numbers may impede the rate of ammonia nitrogen removal [41][42][43][44].…”

Sustaining membrane permeability during unsteady-state operation of anaerobic membrane bioreactors for municipal wastewater treatment following peak-flow

“…Furthermore, by specifying membrane surface area based on peak flow, severe membrane under-utilisation has been reported [8]. To illustrate, in several surveys of full-scale municipal aerobic MBRs [13,14], the average flow was typically less than 50 % of the peak flow used for design. This also incurred an increased operational cost of around 54 %, due to the excess specific aeration demand per unit membrane area (SAD m ) required [8].…”

“…However, Metcalf [9] observed a significant membrane permeability decline in a pilot scale aerobic MBR treating settled municipal wastewater, when the flux returned to the average flux of 20 L m -2 h -1 , from a peak flux of 25 L m -2 h -1 that was sustained for 24 h. The authors attributed the increased fouling to the operating flux exceeding the critical flux during peak flow. In recognition of such behaviour, several studies sought to identify fouling control strategies that could be deployed during peak flow, such as increasing SAD m , shortening filtration cycle time, or increasing backwash flux [4,14]. Following evaluation of a laboratory scale aerobic MBR treating synthetic settled municipal wastewater, Howell et al [18] concluded that membrane fouling introduced by a temporary increase in flux could be controlled by an increase in SAD m , with the residual foulant removed following flux restoration to a sub-critical level.…”