| Contamination of membranes causes a higher energy use, a higher cleaning frequency and a shorter life span of the membrane. Membrane contamination is usually called fouling. |
The International Union of Pure and Applied Chemistry defines fouling as follows:
The process that results in a decrease in performance of a membrane, caused by the deposition of suspended or dissolved solids on the external membrane surface, on the membrane pores, or within the membrane pores. (Koros 1996)
When clean water is filtered, the membrane material is the only resistance caused (Rm). The flux is than called the clean water-flux. As a result of the accumulation of particles on the membrane through the filtration of water with a certain level of suspended solids, a cake will form on the membrane (Rc; particles). When particles block the membrane pores, this is called pore plugging (Rpb; scaling). Resistance as a consequence of adsorption in or on the membrane is called biofouling (Ra).
Rm = membrane resistance
Ra = adsorption, biofouling
Rpb = pore plugging
Rc = cake layer
Particles, biofouling and scaling are three main groups of pollutants that can be distinguished from membrane fouling. These will cause the need of a higher workload, to keep the filtration capacity at a certain level. At a certain point the pressure has increased so much that it is no longer economical.
For the control of membrane fouling there are many different techniques.
One way to predict fouling is by using the Silt Density Index (SDI) of the feed water. The SDI, which is based on experience, can be defined as the time that is needed to filtrate an amount of water with a noted concentration of salts through a standard 0.45 mm micro filtration membrane. When the SDI is high, one can conclude that the feed water contains a high amount of membrane plugging matter.