System for controlling microbial fouling

Details System for controlling microbial fouling System for controlling microbial fouling

2001-09-13

2003-03-18

Hruskoci, Peter A. (Department: 1724)

Compositions

Water-softening or purifying or scale-inhibiting agents

Packages or heterogeneous arrangements

C210S698000, C210S755000, C210S756000, C210S764000, C252S180000, C252S181000, C422S015000, C422S037000, C424S661000, C424S723000

Reexamination Certificate

active

06533958

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of controlling microbial fouling and a control system therefor, more specifically a method of controlling microbial fouling using bromination method wherein hypobromous acid, HOBr, formed by the reaction between hypochlorous acid and bromide ion, is used for controlling the microbial fouling in aqueous system, and a control system therefor.
BACKGROUND OF THE INVENTION
In aqueous system such as cooling water towers, industrial water system, paper processing water and oil field waters, microorganisms including bacteria, fungi and algae may cause biological problems including microbiologically influenced corrosion. In addition, the microorganisms form slime. These microbiologically influenced corrosion and slime formation may deteriorate structures, reduce cooling efficiency, change colors as well as increase environmental and health problems.
Generally, methods using oxidizing biocides and non-oxidizing biocides have been used to control microbial fouling in aqueous systems.
Oxidizing biocides sterilize microorganisms by oxidizing cell proteins while non-oxidizing biocides sterilize microorganisms by inhibiting metabolism.
Examples of such non-oxidizing biocides include isothiazolone, methylenebisisocyanate, glutaraldehyde, quartanary ammonium, and the like.
Generally the biocidal power of non-oxidizing biocides is not comparable with that of oxidizing biocides, but the biocidal activity of non-oxidizing biocides lasts longer than that of oxidizing biocides.
The method using oxidizing biocides includes chlorination method and bromination method, and each reaction mechanism is as follows;
Chlorination
Cl
2
+H
2
O.HCl+HOCl
HOCl.H
+
+OCl
−
Bromination
HOCl+Br
−
HOBr+Cl
−
HOBr.H
+
+OBr
−
The produced HOCl and OCl
−
in chlorination, and HOBr and OBr
−
function as biocides. In chlorination, the concentration of HOCl and OCl
−
vary depending on pH. As pH increases, the concentration of HOCl decrease more rapidly than that of OCl
−
as HOCl is more effective as a biocide than OCl
−
, the chlorination becomes less effective in the system having the pH higher than 7.
Further in chlorination method, the ammonia or amine compound contained in an aqueous system such as cooling water may react with HOCl or OCl
−
to form chlorinated amines which is less effective as biocide, resulting in a reduction of biocidal power.
On the other hand, in bromination method, the concentration of HOBr can still be maintained even in high pH system, and thus, the biocidal power is not reduced.
Further the brominated amine formed by the reaction with ammonia in aqueous system exerts almost same biocidal power as HOBr.
Thus the bromination method is effective compared to the chlorination method.
As a HOCl or OCl
−
source in both method, alkali or alkaline earth metal hypochlorites, for example sodium hypochlorite (NaOCl) are widely used to control microbial foule in various kinds of aqueous system including cooling water towers, bleaching process, swimming pools, petroleum industry.
However, the alkali or alkaline earth metal hypochlorite is not stable under typical storage conditions and several methods have been suggested to stabilize NaOCl. One of them was suggested in U.S. Pat. No. 3,328,294 to Self.
The '294 patent teaches to stabilize the unstable NaOCl by reacting with an equal molar ratio of sulfamic acid.
Hypobromite having various advantages over hypochlorite such as better biocidal performance in high pH or amine environments and lower volatility also have the unstable problems under typical storage conditions.
U.S. Pat. No. 5,795,487 (corresponding to Korea Patent Applications number 97-708350)(hereinafter referring to as simply '487 patent) assigned to NALCO Chemical Company disclosed a process to manufacture stabilized alkali or alkaline earth metal hypobromite.
The method comprises the steps of: mixing an aqueous solution of alkali or alkaline earth metal hypochlorite with a water soluble bromide ion source; allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form unstable alkali or alkaline earth metal hypobromite; and adding to the unstable solution of alkali or alkaline earth metal hypobromite an alkali metal sulfmate as stabilizer.
In the '487 patent, the inventors emphasized that the surprisingly increased stability of the stabilized sodium hypobromite achieved by patent method is basically due to the specific order of reagent addition in the process of manufacture, that is, in the order of an aqueous solution of alkali or alkaline earth metal hypochlorite, a water soluble bromide ion source and an aqueous solution of an alkali metal sulfamate. In addition, '487 patent teaches that addition of the stabilizer prior to bromide oxidation would not permit the formations of NaOBr because NaOBr is synthesized by the reaction formula NaOCl+NaBr→NaOBr+NaCl.
However since in the '487 patent the aqueous solution of hypochlorite reacts with an equal molar ratio of the bromide ion source which is expensive on the basis of the above reaction formula, the method according to the '487 patent is costly and requires continuous supply of bromide ion source to maintain the biocidal power in the system.
An object of the present invention is to provide an economic method of controlling microfouling in aqueous system using a stabilized hydrobromite.
Another object of the present invention is to provide an efficient method of controlling microfouling in aqueous system having low volatility, a high free halogen residual long-lasting biocidal power.
It is an object of the present invention to provide a method for preparing a stabilized aqueous alkali or alkaline earth metal hydrobromite solution to be used for the above method.
Further another object of the present invention is to provide an improved anti-microfouling system.
SUMMARY OF THE INVENTION
We have been studying an efficient method of controlling microbiofouling in aqueous system using an aqueous hypochlorite solution, water soluble bromide ion source, metal sulfamate as stabilizer, and unexpectedly found that addition of the stabilizer prior to bromide oxidation could permit the formation of NaOBr, which is contrary to the teaching of the '487 patent.
Specifically, soon after the stabilizer has been added to an aqueous solution of alkali or alkaline earth metal hypochlorite prior to the formation of hypobromite (NaOBr), the stabilized hypochlorite did not react with the water soluble bromide ion source and thus did not allow the NaOBr formation, which met with the teaching of the '487 patent.
However it has been surprisingly found that several to several tens hours after the stabilized hypochlorite and bromide ion source have been added to into aqueous system, the concentration of free halogen residual begun to dramatically increase and the biocidal power lasted long.
Based on the above discovery the inventors continued to study and completed the present invention.
The main feature of the present invention is to allow hypochlorous acid, HOCl, to react with a stabilizer before hypobromites have been formed by the reaction between the hypochlorous acid and a water soluble bromide source in controling microbial fouling in aqueous system using hypobromite produced by the reaction of hypochlorite and bromide ion source.
In one embodiment of the invention, when alkali or alkaline earth metal hypochlorite is used as a hypochlorous acid source, it is preferred to stabilize the hypochlorite by reacting with a stabilizer and then to allow the stabilized hypochlorite to contact a water soluble bromide ion source to form hypobromites.
However since the hypochlorites preferentially react with the stabilizer than with the bromide ion source, the hypochlorite, the stabilizer and the bromide ion source may simultaneously be added to an aqueous system to be treated.
Also it may be acceptable to mix the

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