MEMBRANE FILTRATION

Description

Membrane filtration is a separation technology suitable for a range of industrial processes. Nowadays, membranes are extensively used in water and waste water treatment processes. Membranes are engineered barriers that exclude or separate colloids, molecules or salt. The key property exploited in membrane technology is the ability of the membrane to control the permeation of a chemical through the system.

This technology is different from a filter, which is limited to separate particles larger than 10µm and only by size exclusion. Membranes separate particles, molecules and chemical species because of their size, shape, chemical and physical properties.

There are four main classifications of filtration:

Microfiltration (MF) is the more open type of membrane. MF is a low pressure process. The flow is predominantly though the pore of the membrane. It is used to retain suspended materials such in a clarification process.

Ultrafiltration (UF) is a less open type of membrane. UF is usually operated at moderated pressure. UF process retains very fine particles and large molecules such as proteins. The permeate flow results partially from the pore and partially by diffusion through the material.

Nanofiltration (NF) is a nearly closed type of membrane operating at moderate to high pressure. Most of the permeate flow results from the diffusion through the material and partially through very selective pores that allow small molecule such as sugars to pass in the permeate.

Reverse Osmosis (RO) is a closed type of membrane operated at high pressure. Nearly all the flow is realised by permeation of the solvent through the material.

Application

There are many different applications of membrane in the industry. Here we will restrict ourselves to the applications in water and waste water treatment and liquid product conditioning.

Water treatment
Water purification: production of demineralised and de-ionised {Fig 1}
Desalination {Fig 6}
Removal of iron and Organic matter from ground water

Effluent treatment
Effluent volume reduction{Fig 8}
Effluent treatment and/or water recycling
Recovery and re-use of caustic cleaning solutions {Fig 2}
Concentration of cutting and cooling oils for disposal {Fig 8}
Colour removal from effluent at textile factories and dye works {Fig 10}
Energy recovery from hot effluent {Fig 5}

Product conditioning
Filtration and separation of process liquids in the food industry for example
Concentration and purification of enzymes and vitamins without heat damage {Fig 3}
Decontamination and clarification of product and water {Fig 4}
Concentration of fruit juice, whey {Fig 3}
Clarification of product beer wine juice… {Fig 7}
Whey processing recovers protein, lactose and water {Fig 9}

The recoveries that can be performed by using membrane separation can be combined under one system.

A typical example where this process could be used is a laundry using hot water containing soap and colloids. This can be treated by Ultrafiltration which permeates more then 80% of the waste and will enable recovery of 70% of the soap (free surfactants) that hasn't been used in the cleaning. The remaining hot water can be reused for further cleaning needing only 20% of the energy to get a hot wash.

Benefits

The separation process is rapid and very selective. Membrane systems are easy to integrate in an exiting process. Membranes enable product recovery, increased product yield and allow product quality improvement. The technology allows the removal of unwanted product in an effluent or product stream such as TSS and COD for discharge to sewage. They allow water recovery and energy recovery water purification, disinfection reduction in effluent volume and produce a concentrate easier to transport and dispose of in the case of a waste, or store in the case of a product.

Membrane technology can be a stand alone solution or can be integrated in and existing system. They have a low foot print to volume treated ratio.

Five main benefits:

Water can be recovered for reuse
Product can be recovered or concentrated
Energy can be recovered
Decontamination
Generation of pure water or standardised water