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Figure 6. Effect of cleaning time

2.4 Chemical aspects of membrane cleaning

Generally the chemicals applied for membrane cleaning are as important as the water quality. Many users are not aware of the problems that can appear if the wrong chemicals or water quality is applied.

2.4.1 Water quality Water itself is a very good solvent and also a good cleaner. Water used in membrane cleaning should be of good quality. Any impurities can be filtered out during cleaning and may block the membrane rather than cleaning them. Prefilters should remove any suspended solids. The bacteriological contamination should be at a very low level to prevent biofouling. Several metals form salts, which precipitate especially iron and manganese, these; together with silicates can form insoluble salts. The normal deposition of non-silicate salts of iron and manganese may be removed but silicates can only be removed by hydrofluoric acid. This is not a viable proposition as, using this, most membranes and nearly every plant would be destroyed, not including any human risks. Water hardness is not a problem. With formulated detergents, both acidic or alkaline hardness can be complexed or removed. Very often demineralised water is used, being produced by ion exchange, RO plants or evaporators. Water from evaporators can sometimes cause problems on membranes, as antifoamers are often used in defoaming evaporated solution. These antifoamers can escape into the vapours. Contamination with even very small amounts can decrease the membrane capacity with time.

Measuring the silt density index (SDI) can easily test the water quality. There are published values for water quality from which it can be determined whether or not they can be treated by RO plants. Here the SDI should not be higher than 5. For cleaning of membrane plants the SDI should be less than 3, otherwise problems may occur.

2.4.2 Influence of the soil The soil itself has a direct influence on the chemicals, which should be used for cleaning the membranes. For membrane cleaning it is necessary to determine the type of soil present much more than for usual stainless steel surfaces.

2.4.2.1 Fat, oil and other hydrophobic fouling The removal of hydrophobic fouling like fat from hydrophobic surfaces like organic polymers is more difficult than from steal or glass. This is depending on the hydrophobic characteristic of fat that adsorbs more strongly on the membrane surface. The removal can be done with surfactant based products above the melting point of the fat or grease in aqueous solution. Below the melting point it is nearly impossible to remove fat and grease residues without using a real solvent like ethanol. The surfactants must be chosen precisely, because they have to be compatible with the membrane, the spacer and the support. Surfactants and defoamers used in normal CIP detergents or washing powders are often not compatible with membranes even if temperature and pH limitations are suitable. Some membrane manufacturers have approved normal washing powders in the past and could reach quite good cleaning results. However a normal washing powder is usually changed at least once a year and of course nobody will take care that the old version was approved on a membrane system.

2.4.2.2 Proteins Alkaline detergents remove proteins best, this is well known from practical experience. The higher the pH the faster the protein hydrolysis and the better the solubility. Protein solubility is poor in the neutral pH range. At a pH of 4-5 milk proteins for example are denatured and precipitated. The initially used pH-sensitive membranes like cellulose acetate led to the development of enzymatic detergents. Many of today's commonly used membranes are pH resistant. Initially, only inorganic ceramic membranes were pH stable. Latterly also organic polymers like polysulforte, polypropylene and polyvinyldifluoride with high pH stability were developed. The exact specifications vary from manufacturer to manufacturer. The best cleaning process is not achieved by attaining the correct pH alone. A pH of 11 -11.5 is very often the limit; even a small amount of caustic may be enough to reach this value but not sufficient to clean well. Additionally to the alkalinity there is a need for dispersants, emulsifiers, soil carrying agents, stabilisers for hardness salts, buffering systems and available chlorine or oxygen as cleaning boosters. Enzymatic cleaners are usually applied if the pH limitation is at or below 10 or if a high level of dirt is present.

2.4.2.3 Minerals and salts Most of the mineral scale can be easily removed by an acid cleaning with organic or inorganic acids. Alkaline products containing complexing agents can remove some others. In cases of silicate and sulphide residues it is best to prevent than to clean.

2.4.2.4 High molecular polysaccharide, EPS extra cellular polysaccharide Polysaccharides, in particular, cause severe problems if they appear on non-oxidising stable membranes. Here two typical phenomena appear. The one is that EPS is directly fixed on the membrane surface and lead to a strong decrease of the capacity. The other especially in spiral wound membrane systems appearing is that the EPS settle down in the spacer material and rise the difference pressure. Both can lead to a loss in the performance of the membrane system. The cleaning process in such cases has to be adapted individually. One successful process to decrease the difference pressure is the enzymatic cleaning followed by an acidic sanitation and another enzymatic cleaning step (Figure 7).

Development of difference pressure with two different cleaning processes

Development of difference pressure with two different cleaning processes

(Figure 7)

Whereas with the old cleaning procedure a cleaning became necessary every 7 to 10 days, the new enzymatic based process could lengthen the cleaning cycles to around 30 days. Trials that were done during optimisation of the plant performance with additional disinfection steps could kill most of the biofilm producing bacteria but did not remove them from the membrane and spacer material. So the dead bacteria became ideal food for new bacteria and speeded up finally the developments of new colonies.

2.4.2.5 Cleaning agents from industrial companies Many wastewaters contain cleaning agents from the factories that may influence the behaviour of the membrane system. If only part of the total wastewater of a factory is treated then it is easier to control what detergents are used. Within screening test it can be controlled what influence the detergent has on a membrane system. If there is a decrease in capacity another detergent for the factory cleaning has to be applied. The producer of the cleaning products should be able to offer a product, which fulfil as well the cleaning in the factory as the treatment by a membrane unit.

In some cases the pH during recycling is of major interest. There might be a cleaning agent that can be recycled at a pH of 8 without any problems but if decrease the pH to value of 5 a blocking of the membrane may appear. It is well known that the temperature has a strong influence on the capacity of membrane systems. Usually by increasing the temperature the capacity will increase as well. If a wastewater with some cloud point depending antifoamer is treated the reaction may be the opposite. Meanwhile the product is clear and good soluble at 25°C it becomes insoluble at 45°C and decrease the capacity of the membrane system or even block it.

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