Membrane filtration

Purpose

• Separation of organic substances

• Concentration of salts, organic compounds, microorganisms, solids

• Purification, disinfection

Principle

Membrane filtration is a separation technique in which wastewater is filtered through a semipermeable membrane by applying pressure. The pore size of the membrane determines what type of pollutants are withheld. Generally the distinction is made between:

• Microfiltration: 5 µm to 0,1 µm (bacteria and suspended solids)

• Ultrafiltration: 0,1 µm to 20 nm (proteins, macro-molecules, viruses)

• Nanofiltration: 20 nm to 1 nm (multivalent salts, small dissolved organics)

The membrane characteristics (pore size, porosity, …) define the pressure needed to obtain the desired water flux through the membrane. During the filtration process the membrane is increasingly fouled, resulting in an intensifying transmembrane pressure. In order to control membrane fouling different continuous and discontinuous cleaning techniques are in use (i.e. back flushing, aeration, cross flow filtration, chemical cleaning).

Plenty of membrane configurations are possible, varying from spiral wound membranes to plate membranes, hollow fiber membranes and tubular membranes. To protect the membranes a suitable pretreatment is of crucial importance.

Membrane filtration is a separation technique that generates a purified water stream (permeate) and a concentrated stream that contains all the separated pollutants. This concentrate needs further treatment.

Scheme

Applications

There are lots of applications for membrane filtration. A few examples with respect to wastewater treatment are:

• MF/UF: membrane bioreactor (MBR), pretreatment for reverse osmosis (RO) installations, production of process water from surface water or treated effluent

• NF: removal of residual organic pollutants during drinking water production, recovery of dyes

Example of realisation

Operational costs

Compared with other filtration techniques like sand filtration, membrane filtration is relatively energy intensive. The energy consumption is mostly linked to measures to control membrane fouling like aeration and pumping energy. There is also a cost for chemicals (chemical cleaning). Because the membrane life is limited, these membranes have to be exchanged every 5 to 10 years.

Approach Trevi

The type and nature of the waste water determines the filterability by means of membranes. Trevi always performs pilot tests during the dimensioning of a new membrane filtration plant. This way a realistic flux is selected, with a maximum service life for the membranes and a minimal cleaning frequency.