membrane fouling
A review of the types of fouling found in reverse osmosis systems
Fouling of reverse osmosis systems
Membrane fouling is the most common operational problem reported in membrane systems of all types and is a major consideration for the designer. Systems processing wastewater would generally be expected to foul more rapidly than systems treating mains water although the picture is rarely straightforward.
Generally fouling results in a decline of performance – most commonly manifested as a decrease in product flow and/or an increase in the operating pressure of the system. Also differential pressure (feed – concentrate) or ‘delta P’ is commonly affected. In extreme circumstances elevated differential pressure can lead to mechanical failure of the membrane element, or of the end plate of the pressure vessel.
Generally the most commonly reported fouling falls into four different categories:
- inorganic scaling
- fouling by iron and / or manganese
- biofouling
- organic fouling
- composite fouling
Inorganic scaling
Inorganic scaling – this occurs when mineral constituents in the feed water become supersaturated during passage through the RO system, resulting in precipitation of scale on the membrane surface. This type of fouling can occur in any RO application where the feed water contains dissolved minerals in significant quantity.
Common scales include:
- calcium carbonate (potable water, brackish water, wastewater)
- calcium sulphate (brackish water, sea water)
- calcium phosphate (wastewater)
- magnesium ammonium phosphate aka struvite (wastewater
- silica
Fouling by iron or manganese
Fouling by iron is an acute problem most commonly experienced on systems fed by borehole or spring water – in particular those fed from the lower greensand aquifer which underlies many chalk formations. Dissolved ferrous iron (Fe2+) is oxidised to the ferric (Fe3+) form when it is pumped from the well and is aerated, and forms amorphous ferric hydroxide precipitates which stain the affected membranes a rusty red colour.
Fouling by manganese is an additional problem most commonly encountered when water is taken from a deep reservoir where anaerobic conditions may prevail. When dissolved manganese becomes oxidised it can form black deposits which foul the membrane surface. Over time (and even with very low levels of manganese) a significant build up of manganese can accumulate.
Control measures for iron are not always straightforward since they mostly involve dosing chlorine ahead of the membrane system, and any chlorine residual reaching the RO membranes can quickly damage them. In such circumstances, the system designer may opt to use chlorine resistant RO membranes.