Cleaning

2.1 Why is cleaning of membranes essential?

Membrane surfaces - for potable water production - have to be cleaned like any other surface that are in contact with food or pharmaceuticals to guarantee a high quality product as well as hygienic standards. In addition a good cleaning procedure is also essential for the functionality and capacity of the membrane plant, even if bacteriological requirements are as low as in most wastewater applications.

2.2 What are the requirements for a sufficient cleaning?

A sufficient cleaning can only be guaranteed, if the applied cleaner is optimised for removing the soil and does not adversely influence the membrane characteristics. As a simple example, the use of a caustic soda applied in highest concentration will not be able to remove a mineral scaling.

Additionally the physical aspects of membrane cleaning like temperature, mechanical forces, time and last but not least the chemical activity are points of major concern in membrane cleaning processes. Even the most important "solvent" in membrane cleaning - the water - should be of the best quality; otherwise the cleaning itself could become a cause for fouling.

2.3 Physical aspects of membrane cleaning

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The "Sinnersche Kreis", Figure 1 and 2, is one of the most important and well known descriptions for cleaning in general that can be used for membrane cleaning. To get the optimum cleaning result, time, temperature, mechanical force and the right chemical activity is needed. If one of the parameters is decreased at least one of the others has to be increased in many cases two or all three have to be increased.

2.3.1 Temperature It is well known that an increase of the temperature increases the reactivity of processes. This is the same for cleaning processes. As a major value it can be stated that an increase of 10°C doubles the cleaning activity. Nevertheless depending on the different foulants a minimum temperature might be necessary. Many polymer membranes unfortunately allow only a certain temperature. Any exceeding of these limits can cause sever damage of the membrane. For the user it is complicated to understand the reasons for different limitations even if the membrane material applied is the same (Figure3).

Stafcäty of dfferent modules with membranes made of pdysiifone temperature 100

Stafcäty of dfferent modules with membranes made of pdysiifone temperature 100

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Even if the membrane materials are the same, the spacer material, the support material, and the glue may influence the module stability. For that reason it is important to have all data available so as to make good cleaning recommendation

Especially in case of water and wastewater treatment plants there is no option to heat up the cleaning solution. That is one of the major differences compared to food application where on very large plants with a cleaning volume of 5000 litres a possibility for heating is state of the art. If there is a wastewater treatment plant producing a latex emulsion at 40°C the pre rinse should already be done at the same temperature. If the pre rinse can be done only with cold water the emulsion may break down and will lead to much more difficult cleaning conditions.

In some cases cleaning might be impossible. Comparable may happen if a wastewater with high amounts of proteins is treated. High concentrated proteins can be liquid at high temperature but can become solid if they are cooled down.

2.3.2 Mechanics The relationship between flow and cleaning results are well known from different models and practical experiences. Higher flow rates lead to higher turbulence and to better emulsifying and dispersing properties. The increase of flow rates is limited by pump capacity, mechanical stability and of course in the economics. During membrane cleaning it is important to watch the pressures both parallel to and vertical to the membrane surface. Therefore it is necessary to illustrate the different types of fouling depending on the different membranes. On ultra- and nanofiltration membranes as well as on reverse osmosis membranes mostly the fouling or scaling layer is directly on the surfaces (figure 4). Using microfiltration membranes as well as surface fouling an inner porous fouling appear (figureS).

During cleaning of UF, NF, and RO plants the filtering capacity should be as low as possible, otherwise it is possible that a secondary membrane builds up during cleaning. For cleaning MF plants it is the best to clean first with a low pressure to take away the examples regardng dfferent merrtrane manufacturers" reœmmendations

loose cake on the surface then discharge this first dirty solution and finally change to a higher filtration capacity to clean the pores of the membranes.

An often discussed cleaning procedure is backflushing (many membranes and modules cannot be back flushed). In practice we have learned that back flushing is very effective in improving membrane capacity when a hard cake layer is present, although a totally clean situation will never be achieved. By using backflushing, as a method of removing crude surface fouling, care must also be taken that the monomolecular and oligomer fouling is removed. The removal of monomolecular and oligomer fouling is independent of the direction of flow; chemical aspects are most responsible for the clean. Another method is a cleaning from the permeate to the concentrate side. This may be an alternative process on membranes which allow this type of cleaning but it will only function well if the dirt is discharged and not recirculated. If the cleaning solution is recirculated then emulsified fouling will come from the feed side (dirty side) to the permeate side (clean side) and may foul the membrane from the permeate side. However even if the permeate side is not fouled, an additional risk is the possibility of contamination with bacteria.

2.3.3 Time Generally it is stated that the effect of an increased time increases the cleaning efficiency. Tliat is valid for many cases. Especially in membrane cleaning the practical experiences have shown also different effects. During cleaning our membrane system is still able to filter out some only emulsified or dispersed soils.

The exact time for a cleaning can only be given by the practical experience or by direct control of the cleaning process.

FouJmg of VF - membranas

Fouing of UF - membranes

FouJmg of VF - membranas

Fouing of UF - membranes

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