Method for preventing the formation of biological films

Description

This invention relates to the use of formulations which, after addition to aqueous media passing through filtration units, improve the performance of the filtration units with time in relation to the performance with time of filtration units for the filtration of aqueous media to which these formulations have not been added.

It is basically known that aqueous media can be cleaned by filtration through filtration units. For example, the filtration units in question can be membrane units for the conditioning of water. The function of filtration units such as these is substantially to remove mineral, organic or microbiological impurities from the aqueous media to be filtered according to requirements. The filtration units are normally in operation for several days, weeks or even months before they are cleaned or overhauled. Depending on the type of filtration unit and the nature of the aqueous medium to be filtered, the performance of the filtration units deteriorates with time. This can be reflected, for example, in the fact that the differential pressure between the inflow and outflow side of the filtration unit is increased with the effect that the materials used in the filtration unit are subjected to greater mechanical stressing and hence to accelerated wear. As operation of the filtration unit continues, blockages can occur to the point where the filtration unit is brought to a standstill. The reduction in the performance of the filtration units is generally attributable to the fact that a layer of extracellular material is gradually formed over the filter materials used. For example, in the water conditioning field, experts often speak of the formation of biofilms in membrane units—so-called biofouling. The effect of the buildup of extracellular layers is that frequent stoppages for cleaning and disinfection are necessary. The result of this is that the filtration unit is not available for conditioning water during such stoppages which involves costs for interrupting the filtration process and for cleaning. If the necessary cleaning and disinfection intervals are not observed, the risk of failure of the water supply through blockage and the risk of microbiological impurities in the permeate increase because the extracellular layers can form a matrix for the growth of microorganisms under protected conditions. On the other hand, over-frequent cleaning and disinfection lead to increased wear, particularly where oxidative formulations are used, and hence to a reduced life of the filtration materials used.

The problem addressed by the present invention was to make it possible, by the addition of selected formulations to the aqueous media to be filtered, to increase the performance of the filtration units with time in relation to the performance with time of filtration units for the filtration of aqueous media to which these formulations have not been added.

The present invention relates to the use of formulations which

• • a) contain at least one compound selected from the groups of nitrogen and phosphorus compounds, at least one ammonium compound preferably being present as the nitrogen compound, more particularly in quantities of 0.1 to 8 mg per liter of aqueous medium, and—in another preferred embodiment—the phosphorus compound being at least one phosphate compound which is preferably added in a quantity of 0.3 to 20 mg per liter of aqueous medium
  • and
  • b) are added to aqueous media passed through filtration units, the filtration units preferably containing at least one membrane filter element with pore sizes of 0.0001 to 1 μm and, more particularly, a membrane filter element with pore sizes of 0.0001 to 0.005 μm and—in a particularly preferred embodiment—achieving retention levels of 90 to 99.9% for sodium chloride
    in order to increase the performance of the filtration units with time in relation to the performance with time of filtration units for the filtration of aqueous media to which the formulations have not been added. The expert generally characterizes the various membrane filter elements as microfiltration, ultrafiltration, nanofiltration and reverse osmosis membranes. Characterizing the membrane filter elements on the basis of pore size over the entire range from microfiltration to reverse osmosis is only meant to be taken as a basic characterization. For example, where the formulations according to the invention are used to increase the performance of the particularly preferred reverse osmosis membranes with time, the corresponding membrane filter will not be characterized by its pore size but instead by its sodium chloride retention levels.

Where membrane filter elements are used in the filtration units, they preferably contain at least one material component selected from the groups of ceramic materials and/or organic polymers. In a particularly preferred embodiment, the membrane filter element contains at least one polyamide and/or polypiperazine amide.

In a preferred embodiment of the present invention, the performance of the filtration unit as a function of time is determined by measuring the differential pressure and/or the permeate flow. Where the formulations to be used in accordance with the invention are added to the aqueous media to be filtered, the differential pressure does not increase as much with time and the permeate flow does not decrease as much as is the case in the filtration of aqueous media to which the formulations to be used in accordance with the invention have not been added.

In another preferred embodiment of the use according to the invention, the formulations additionally contain at least one enzyme which is preferably selected from the group of hydrolases and more particularly from the group of proteases.

In the use according to the invention, the buildup of extracellular layers on the filtration units can take place more slowly or extracellular layers already present can even be removed. Since microorganisms can hide in such layers and are able to proliferate undisturbed, another particularly preferred embodiment of the formulation used in accordance with the invention is characterized by the additional presence of at least one biocide component selected from the groups of alcohols, aldehydes, antimicrobial acids, carboxylic acid esters, acid amides, phenols, phenol derivatives, diphenyls, diphenyl alkanals, urea derivatives, oxygen and nitrogen acetals and formals, benzamidines, isothazolines, phthalimide derivatives, pyridine derivatives, antimicrobial surface-active compounds, guanidines, antimicrobial amphoteric compounds, quinolines, 1,2-dibromo-2,4-dicyanobutane, iodo-2-propynyl butyl carbamate, iodine, iodophores, peroxides. In a most particularly preferred embodiment, the formulation contains as antimicrobial components one or more compounds selected from ethanol, n-propanol, i-propanol, butane-1,3-diol, phenoxyethanol, 1,2-propylene glycol, glycerol, undecylenic acid, citric acid, 2-benzyl-4-chlorophenol, 2,2′-methylene-bis-(6-bromo-4-chlorophenol), 2,4,4′-trichloro-2′-hydroxydiphenylether, N-(4-chlorophenyl)-N-(3,4-dichlorophenyl)-urea, N,N′-(1,10-decanediyldi-1-pyridinyl-4-ylidene)-bis-(1-octanoamine)-dihydro-chloride, N,N′-bis-(4-chlorophenyl)-3,12-diimino-2,4,11,13-tetra-azatetra-decane diimidoamide, quaternary ammonium compounds or alkylamines, guanidines, amphoteric compounds.

In a preferred use, the formulations are added proportionally to the water stream of the aqueous medium. In another preferred use, the quantity of formulation added to the aqueous medium is measured via conductivity. In a particularly preferred embodiment, the quantities added to the aqueous medium are measured and controlled via conductivity. In the use according to the invention, a mixing unit is preferably disposed between the point of addition and the filtration unit.

In another preferred embodiment of the use according to the invention, the formulations are made up in a form selected from aqueous solutions, gels, emulsions, pastes, dispersions, powders, granules, flakes, beads, tablets, blocks, extrudates. In a particularly preferred variant, the formulations used consist of a nonaqueous component and a water-soluble capsule preferably based on polyvinyl alcohol. In a most particularly preferred variant, the water-soluble capsule contains part of the formulation ingredients.

As already mentioned, formulations containing ammonium compounds may be employed in the use according to the invention. Where ammonium compounds are used, they are preferably selected from ammonium acetate, ammonium alum, ammonium carbonate, ammonium chloride, ammonium dihydrogen phosphate, ammonium magnesium phosphate, ammonium nitrate, ammonium phosphate, ammonium sulfate and ammonium sulfate saltpetre. In a preferred embodiment, the formulations used in accordance with the invention contain phosphate compounds preferably selected from at least one of the groups of primary orthophosphates with the general formula M′H₂PO₄ or M″(H₂PO₄)₂, where M′ for example represents an alkali metal and M″ represents an alkaline earth metal, and secondary orthophosphates with the general formula M′₂HPO₄ or M″HPO₄ or tertiary orthophosphates with the general formula M′₃PO₄ or M″₃(PO₄)₂ and condensed phosphates derived therefrom—so-called metaphosphates and polyphosphates. In a most particularly preferred embodiment, the formulations used in accordance with the invention additionally contain at least one component selected from organic carbon compounds, preferably carbohydrates, lipids, proteins or hydrocarbons, and from the groups of fatty acids, amino acids, alcohols and other carboxylic acids. In very special cases, the formulations used in accordance with the invention additionally contain at least one trace element selected from the group of sulfur compounds and minerals, for example iron, calcium, potassium, magnesium, manganese, zinc compounds, and the trace elements, for example nickel, cobalt, tin, copper, aluminium, iodide, bromide, borate, molybdate compounds.

EXAMPLES

Procedure:

In an industrial trial, water of defined quality was delivered to two parallel 2.5 inch reverse osmosis membranes at a constant pressure of 9 bar. The analysis of the water used is shown in Table 1.

Water of the quality detailed in Table 1 was continuously fed into the first membrane with no additives over a period of 30 days under a constant pressure of 9 bar. A formulation was added to the water just before the second membrane, being designed to release 3 mg/L ammonium chloride and 6 mg/L potassium hydrogen phosphate hourly into the water. During continuous operation for 30 days, the differential pressure was determined as the difference between the entry pressure and the exit pressure for both membranes. At the same time, the quantity of permeate obtained was recorded in liters per hour. The results of this long-term investigation are set out in Table 2 and show that, where the formulations used in accordance with the invention are added, the performance of the reverse osmosis membranes is improved. This is all the clearer, the longer the filtration units are operated without interruption, i.e. without cleaning and disinfection. In the Comparison Example, i.e. without the addition of a formulation to be used in accordance with the invention, operation had to be stopped after 30 days through blockage of the membranes. By this time, the pressure difference was 2 bar and the permeate flow was only 35 liters per hour. In the case of the Example according to the invention, the pressure difference at the same stage was 1.20 bar and the permeate flow 48 liters per hour.