AU621309B2 - Cooling tower treatment - Google Patents

Description

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FILING DATE: WOBELEA PTY. LIMITED PI 4237/87 9/ 9/87 Form COMMONWEALTH OF AUSTRALIA The Patents Act 1952 COMPLETE SPECIFICATION FOR AN INVENTION ENTITLED: COOLING TOWER TREATMENT The following statement is a full description of this invention, including the best method of performing it known to me/us: -1- This invention relates to a cooling tower treatment and, in particular, to the treatment of cooling towers to substantially destroy Legionnaire's disease bacteria resident therein.

It is accepted that most cooling towers have at least some Legionnaire's disease bacteria and, because the water in towers is recirculated, there e. tends to be, over a period of time, a build-up of such bacteria.

o 0 °ioo* There have been cases where the build-up has been of such an order that a dangerous concentration of the bacteria occurs and these bacteria can 1 a i enter the airconditioning system and can cause substantial illness, and even

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i death, to persons breathing the polluted air.

One such case occurred in 1986 in Newcastle, Australia, where there was a substantial death toll as well as a high degree of illness from Legionnaire's disease from a polluted cooling tower.

I Substantial difficulties can occur when oxidisers, for example, halogens, peroxides, persulphates and ozone are used as biocides in cooling towers.

Each of these has a very short life, so there needs to be continuous dosing for them to be effective and, in most cases, particularly with the halogens, A 1 Ishould there be any excess, then the metal components of the cooling tower itself can be substantially damaged.

Non-oxidising biocides, which are organic based materials, such as phenolic quaternary amines, etc., have a disadvantage in that it is necessary to effect an analysis to ascertain the concentration of the material and such an analysis is continually needed before the required quantity can be added.

There is no way available which can automatically dose water with biocides -2- Wobelea/66/Sep88 3 of this type to maintain the required concentration.

The present invention provides a treatment for a cooling tower system which comprises initially treating the system so that the water has the properties, pH 7.8-8.2, total alkalinity 100- 200 mg/1, calcium hardness 100-200 mg/1 and initial chlorination/bromination 5-10 mg/l, and then adding a bromo or bromo chloro hydantoin compound in an amount to maintain a S, selected redox value.

In certain application the bromo or bromo chloro hydantoin compound may be replaced by a chloroisocyanuric acid or salt thereof.

Total alkalinity is expressed as concentration of CO 2 expressed S. as calcium carbonate. A redox value is selected to provide an active biocide concentration sufficient to maintain control of target microorganisms.

Redox values are measured by reference to the concentration of hypobromous acid and of bromamines and chloramines.

It is preferred that overall monitoring of the redox value is done in such a way that, when the redox value reaches a high set point, an operator can be notified so that the system can be retreated so that the water is returned to the initial parameters.

51/SPEC/WOBELEA.SPC/Dec'91 I6II v I~kF~I 4 It is also preferred that the addition of the bromo or chlorine compound is controlled automatically on a predetermined redox value being met.

The preferred hydantoin compound is bromo-3-chloro-5,5-dimethylhydantoin (BCDMH).

The hydantoin compounds referred to above are particularly suitable for this application as they provide an effective biocide and they are also highly active so that there is little increase in the total dissolved solids in the system.

In order that the invention may be more readily understood, we shall describe one particular method of treating a cooling tower water system to minimise, or effectively eliminate, the gLegionnaire's disease bacteria therefrom.

a It is necessary to adjust the parameters of the cooling tower within specified limits:pH 7.8-8.2 Total Alkalinity 100-200 mg/l Calcium Hardness 100-200 mg/l Initial Chlorination/Bromination 5-10 mg/l.

In order to make this initial adjustment we may use a mixture as follows:- U WP51/SPEC/WOBELEA.SPC/Dec'91 4l i CT t

ISC

4 Ct Sodium Bicarbonate 10.0 50.00% Gypsum 7.0 35.00% Sodium Dichloroisocyanurate 0.85 4.25% Sodium Hexametaphosphate 2.15 10.75% 20.0 100.00% The mixture, used at the rate of 2 gram for 5 litres added to soft water will raise total alkalinity by 100 mg/l and calcium hardness by 100 mg/l. The pH will be 8.0 and the initial chlorine level 10 mg/l.

It will be see that this brings the system water into the parameter range set out above.

If required, the sodium dichloroisocyanurate (SDIC) could be replaced by BCDMH or other bromo or bromo chloro hydantoin compound.

Once the initial dosing has been effected, the preferred treatment chemical is BCDMH although similar bromo or bromo chloro hydantoin compounds can also be used. Other compounds which may be useful in specific applications are TICA (trichloroisocyanuric acid) and SDIC.

As discussed below, the BCDMH is added to maintain a required redox value.

/SPEC/WOBELEA.SPC/Dec'91 For example, using chlorine at pH 8.0 only about 20% exists as the active HOC1 whereas, at a similar pH BCDMH has approximately available HOBr.

BCDMH also has a further advantage in closed systems as its active content is 90% so continual dosing of the system does not substantially add to the total dissolved solids in the system.

Where the water to be used has a high alkalinity, it may be preferred to use TICA rather than BCDMH as the active biocide.

rtE S* a e 6( TICA is very acidic and thus its addition tends to decrease the alkalinity of the system.

In common with BCDMH, TICA is 90% active so, again, it does not substantially increase the total dissolved solids in the system.

Dosing with either of these products can be done in one of several ways.

In the first methods, the compounds are used as tablets which can be added to an erosion feeder or a soaker feeder in which the concentration of the product can increase at a known rate and, when the water in the system SPEC/WOBELEA.SPC/Dec'91 i2, v.' Ut /7r q 0 1_11___ reaches a pre-set redox value, as will be described later herein, a solenoid valve or dosing pump is caused to operate permitting the water from the feeder to be added to the water in the system.

Alternatively, the product concerned can be dissolved in water to provide a solution having a required concentration and, again, on the required redox value being met, can be delivered to the water by way of a solenoid valve 0 a oBo or dosing pump.

0 0 0 It will be appreciated that, because the water is recirculating in a cooling tower, the water can contact various atmospheric acid formingr."ss", such as carbon dioxide, sulphur compounds and oxides of nitrogen, amongst others and thus the pH can vary depending upon the concentrations of these.

As redox is a function of pH, any decrease in pH will cause a rise in redox.

o°0 °B By setting up a buffered system, of the type described, with a pH of 8.2, the required redox value is 700 my.

As the pH increases, because of usage of the BCDMH or TICA, the redox value will drop and, when it reaches a predetermined point, the solenoid valve or dosing pump will be caused to operate to add a predetermined iquantity of the compound to the water, which will bring the redox reading back to the required reading.

However, over time, there will be an increase in acidification in the water, as previously described, the pH value overall will drop and the redox I reading will increase.

This will not generally adversely effect the dosing operation and, elea/66/Se-6- 'ohel e a/66/Sep88 effectively, the saturation index of the HOC, or HOBr- will be maintained at a satisfactory level.

However, the redox probe can be arranged to have a set point, say, at 750 my, at which it is deemed that the overall pH is too -h4g.land, when the probe value reaches this level, as a result of overall acidification of the water and the addition of the compound, then an alarm will sound and an t tf operator will need to re-treat the water as a whole with the mixture previously described and used to set the initial parameters.

i f, S *Thus, it will be seen that, during normal operation, the water will be automatically dosed with biocide but, when the overall parameters of the I J water become unsatisfactory an alarm condition will be initiated to enable the required parameters to be established.

It would be possible to provide a second automatic operation at this stage, which would add a predetermined quantity of the mixture and this could operate quite satisfactorily although, at intervals, it would be necessary to check the various parameters so that these could be corrected individually should this be required.

It is also possible to provide a further alarm which operates at low redox values, say below 700 my, which could indicate an empty feeder or a V blocked valve or pump.

Also, the redox probe could be used to provide an indication of low water level in the system and could automatically operate to introduce make-up water into the system when this is necessary.

It will be seen that the use of the system of the invention provides a -7- Wobelea/66/Sep88 i 1 0 06 od 0 1 0004b o 00 o *4 o I 0 o ol I 0 to r method of maintaining a required biocide level in the system without the introduction of any excessive quantity of free HOC1/HOBr and, as a corollary to this, with a minimal likelihood of damage to the metal components of the system.

At the same time, the use of the selected compounds ensure that the bacteria which causes Legionnaire's disease and other bacteria can be fully controlled and the number of these bacteria in the system be maintained at a level at which there will be no substantial likelihood of adversely effecting humans.

It will be appreciated that whilst in this description we have described the operation of particular compounds, the ones described could be replaced by other4bdae4 a organo bromo or organo chlorine compounds.

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Claims (9)

1. A treatment for a cooling tower system which comprises initially treating the system so that the water has the properties, pH 7.8-8.2, total alkalinity 100-200 mg/l, calcium hardness 100-200 mg/l and initial chlorination/bromination 5-10 mg/l, and then adding a bromo or bromo chloro hydantoin compound in an amount to maintain a selected redox value.

2. A treatment as claimed in Claim 1 wherein the bromo or bromo chloro hydantoin compound is replaced by a :I chloroisocyanuric acid or salt thereof. K

3. A treatment as claimed in Claim 1 wherein the hydantoin compound is BCDMH. i

4. A treatment system as claimed in any one of the preceding claims wherein the initial conditions are set by using a mixture of sodium bicarbonate, gypsum, sodium I dichloroisocyanurate and sodium hexametaphosphate in water K and adding this until the required parameters are mete. I

5. A treatment system as claimed in any one of the preceding I claims wherein on a predetermined redox value being reached the hydantoin compound is dosed into the system.

6. A treatment system as claimed in claim 5 wherein the redox value is of the order of 700 mv.

7. A treatment system as claimed in claim 5 or claim 6 wherein on a predetermined set point redox being reached an indication is provided to enable treatment of the system to the initial properties.

8. A treatment system as claimed in claim 7 wherein the redox value at which this occurs may be of the order of 750 mv. P51/SPEC/WOBELEA.SPC/Dec'91 AI A

9. A treatment system as claimed in any preceding claim wherein there is a further alarm operable on a low redox value which indicates a fault in the supply of the additive chemical and/or water. A treatment for a cooling tower system es substantially as hereinbefore described. Dated this 9th day of September, 1988 4* 4 rt LI 44 4' 9* Q 04a 0, 4 44 a4 4 4'f 4' 44 44 0 44 44 4' I A #4 4 WOBELEA PTY. LIMITED By its Patent Attorneys A. TATLOCK ASSOCIATES Fellows Institute of Patent Attorneys of Australia. Wobelea/66/Sep88