Membrane Filtration

Question 1

Membrane Bioreactor Systems

Answer 1

• conventionally in the activated sludge system there is one or more biodegradation tank, followed by a sedimentation tank
• in membraned bioreactor systems the settling is replaced by membrane filtration

Question 2

Membrane Processes

Answer 2

• Different types of membrane filtration based on pore size in membrane (microfiltration > ultrafiltration > nanofiltration > reserve osmosis)
• the smaller the pore size, the smaller the retained particles but a higher transmembrane pressure is required to push particles through
• microfiltration membrane retains suspended particles, ultrafiltration macromolecules such as big proteins, nanofiltration multivalent ions such as calcium ions and reserve osmosis retains all material so pure water comes out (high pressure required)
• micro and ultra are most common

Question 3

Types of membrane configurations

Answer 3

• submerged configuration
• cross-flow configuration

Question 4

Submerged configuration

Answer 4

• hollow fibers with pores in their walls or plate membranes
• submerged in wastewater and water is pushed or sucked through the membrane from outside to inside where clean water is collected and led to the outlet of the system

Question 5

Cross-flow configuration

Answer 5

• an alternative configuration are tubular membranes which are often placed outside the bioreactor in cross-flow configuration
• flow goes parallel to the membrane direction and works inside out, dirty water flows in the inner channels and clean water is sucked out

Question 6

Advantages of membrane filtration

Answer 6

• all biomass is retained such that high activated sludge concs are possible
• high biomass concs means a very compact system can be obtained such that less space is need and a more intensified process is made meaning a smaller footprint
• very high and constant effluent quality is guaranteed

Question 7

Disadvantages of membrane filtration

Answer 7

• membrane fouling where sludge piles up on the membrane surface which can clog the pores and can hamper good flow through it (extra operational costs)
• screening/pre-treatment of the wastestream is necessary to avoid extra clogging of the membrane
• can’t get too high in biomass concs as then there will be oxygen transfer issues
• more pumps and the movement of sludge will induce shear on the sludge which can increase the fouling propensity of the sludge