The special group "Water and Wastewater Technology" of the German Process Plant and Equipment Association (VDMA; Frankfurt, Germany; www.vdma.org) has released a new VDMA Specification on prevention and reduction of scaling and fouling in membrane plants.
The objective of this VDMA Specification is to help to ensure that measures are taken to prevent and reduce scaling and fouling when planning, designing and operating membrane plants. In many areas of industry and for many municipalities, membrane technologies have proved to be a successful method of water and wastewater treatment and today are an integral part of combination or treatment processes. Due to different water compositions and reaction mechanisms, fouling and scaling can affect the operation of a plant and even cause complete plant failure by blockage of the membrane.
Fouling is a compressible surface layer that is the result of the deposit and/or adsorption of organic and/or colloidal substances on the membrane or in the membrane pores. Scaling is the term for the precipitation of inorganic substances present in the water on and within the membrane.
Microorganisms can cause irreparable damage to membrane plants. The type of microorganism, their growth rate and concentration on the membrane mainly depends on critical factors such as temperature, pH, the concentration of dissolved oxygen and the presence of organic and inorganic nutrients.
In addition to the salinity and the operating temperature, the fouling potential of the raw water has the biggest influence on the life of the modules and on plant availability.
Fouling within a membrane system reduces the performance of a membrane due to deposits on the interface and can, for example, also become a problem if biofilms accumulate on the inner walls of the pipework, pipe bends and flanges in pipework systems. The table below lists examples of colloidal fouling and biofouling.
Table– Typical scaling and fouling phenomena
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Microfiltration |
Ultrafiltration |
Nanofiltration |
Reverse osmosis |
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Dead end filtration |
Negligible scaling; exponential increase in fouling with increasing pressure. |
See microfiltration, usually with a low solids content. |
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Cross flow filtration |
Negligible scaling, higher flow velocity at the surfaces of the membranes reduces fouling; here, too, pressure increase shall be taken into consideration. |
see microfiltration |
Usually, filtration shall be performed with a low solids content, more susceptible to scaling, fouling caused by humic matter. |
Biofouling/EPS hardly preventable, latent scaling danger in the case of wrong pre-treatment. |
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Immersed system (membrane bioreactor procedure) |
PH value displacements caused by membrane ventilation (CO2 stripping) can result in scaling on the membrane; fouling problems associated with upstream biology. |
see microfiltration |
not relevant |
not relevant |
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Immersed system for filtration of effluent from secondary settlement |
Fouling problems in the case of insufficient biological pre-treatment, e.g. too low a sludge age. |
see microfiltration |
not relevant |
not relevant |
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Remarks |
Due to the relatively open pore structure, microfiltration membranes are especially susceptible to fouling, in particular fouling occurring inside the membrane pores (so‑called pore-fouling). |
Basically, the conditions are similar to those applying to microfiltration. Due to their asymmetrical pore structure and lower pore size, however, UF membranes are less susceptible to fouling. |
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Biofilm deposits can develop into a continuous layer whose components dissolve and can lead to an increase in microorganisms when accumulating in the membranes, which has an adverse effect on plant throughput. If the same amount of water to be treated is to be filtered, the operating pressure has to be increased, which in turn affects the economic efficiency and may damage the membranes. Therefore, the fouling potential should undergo critical evaluation as early as during the planning stage.
This VDMA Specification has been prepared by the member companies of the special group "Water and Wastewater Technology" of the VDMA Process Plant and Equipment Association in cooperation with other companies. It contains preventive and cleaning strategies for different membrane technologies in order to achieve a safe and economic plant operation.
As a guideline for action its objective is to help to ensure that the essential process, plant and operational factors for minimizing and preventing the formation of surface layers are taken into account when planning and designing membrane plants.
For more information, please contact VDMA technical manager, Hans Birle ([email protected]) or visit www.vdma.org/verfahrenstechnik