HORROCKS TEST IN THE CHLORINATION OF WATER …Horrocks Test~For. [4J Automatic compensa'tion for...

9

FIXED DOSE VERSUS HORROCKS TEST IN THE CHLORINATION OF WATER SUPPLIES IN THE FIELD

By MAJOR R ST. JOHN LYBURN, M.D., M.RC.P.(I.),

AND

SERJEANT ]. E. RIDLEY,

Royal Army Medical Corps.

INTRODlJCTION.

THERE are four methods in general use for the chemical sterilization of water supplies in the field.

Type of Dose of chlorine Contact wate!' iV1ethod parts per million period A ntichlor

1. Filtered .. Chlorination 1·0 in excess of chlor- 30 mins. None . ine deviated

2. Filtered .. Super-chlorination 2·0 in excess of chlor- 15 mins. Sodium thiosulphate ine deviated.

3. Filtered or Fixed dose chlor- 4·0 30 mins. Sodium thiosulphate unfiltered. ination.

4. Filtei-ed .. Chloramination .. 2·0 60 mins. None

In methods 1 and 2, the Horrocks Test is used to estimate the" chlorine absorption figure" of the water so that sufficient water sterilizing powder can be added to maintain a concentration of either :-(1) one part per millioQ of free chlorine over a period of thirty minutes or (2) two parts per million of free chl~rine over a period of fifteen minutes as outlined in Army Training Manuals.

Method 3 is used whenever a Horrocks Box is not available and Method 4 for bulk supplies at Bases where Water Points are established with conditions similar to civilian supplies.

THE HORROCKS TEST.

iVlany ·sources of error can occur in this test due to manufacturer's inaccuracies, e.g. (a) 6 white cups selected at random from 72 cups and filled to within onequarter of. an inch of the,brim. Volumes ranged from 167 to 198 rnl.(b) 6 black cups selected at random from 24 cups and filled to white line on inside. Volumes ranged from 230 to 258 ml. (c)' IOscoops selected from tins of W.S.P. and a level scoopful of powder weighed. Variation from 2·2 to 2·7 grammes. (RA.s.C. Spec. 2 grammes) [1].

In the teaching of the Horrocks Test, the selection of the correct Indicator Cup is of importance for accurate interpretation. A white cup of distilled water to which 1 part per million of chlorine and 3 drops of standard cadmium iodide and starch solution have been added will show an Oxford blue colour with the bottom of the cup only just visible. This sh~mld be the correct colour to select when petforming the Horrocks Test if 1 part per million of chlm;ine' is' required as a residuum after thIrty minutes contact \vith the water under test. It has beeri found, however, that a water containing 0·5 part per million of free chlorine will

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10 Fixed Dose versus Horrocks Test in the Chlorination of Water

give different blue colours with increasing amounts of indicator solution. If 3 drops of indicator are added, a Cambridge blue colour will result, and by adding further. drops the colour deepens to an Oxford blue and then to an inky blue black colour. This suggest'$. that the i~dicator solution should contain more iodide as there is not a complete replacement of chlorine by iodine when 3 drops are added, and instead of free iodine there may be iodine chloride formed which does not give a blue colour with starch.

In many cases the Water Duty man adds more than 3 drops of indicator to one or two of the cups with a result that cups 3 and 4 may be paler in colour than cups 1 and 2. It is a mistake, therefore, to attempt to estimate the free chlorine content of a water by depth of blue colour after addition of starch iodide solutions.

When teaching the Horrocks Test, it should be pointed out that the addition of drops of standard W.S.P. solution to white cups of the sample simply compares with the addition of scoopfuls of the particular powder to 100 gallons of the water. The introduction of " parts per milliori" tends to confuse the Water Duty man with no knowledge of chemistry.

By calculation, a water sterilizing powder containing 25 per cent available chlorine when used to perform the Horrocks Test should impart an additional 1 part per million of chlorine to each successive cup but, when titrated by standard laboratory methods, the addition is in the region of 0·7 to 0·8 part per million. No explanation of this discrepancy is offered, and similar results have been noted in the unpublished work of a civili~n waterworks chemist who found that the estimated chlorine figure from bleach powders was higher than the actual figure obtained in distilled water.

The Horrocks Test is too often regarded as a chemical test for the purity of a source of water whereas a dangerous water with large numbers of pathogeriic bacteria may deviate only a little chlorine and a relatively safe water with a fair amouIit of organic or inorganic matter will deviate considerably more. Thus the operator can interpret a " 2 scoop water" as being purer than a " 4 scoop water" as a source of supply.

In the field, notice boards are erected' to indicate the number of scoopfuls of W.S.P. required to sterilize a source of water. Although this figure is obtained by daily tests, it is only true for the particl1lar powder in use at the time and allowance must-.be made for weaker powders and possible changes in the chlorine absorption figure due to land washings, etc., after heavy rainfall.

Finally the Horrocks Test should be taught as a prac,tical one, which ensures a residuum of chlorine at the end of the prescribed contact period, whereas the introduction of "parts per million" terids to make the test like look an exact chemical estimation.

FIXED DOSE CHLORINATION.

Instructors are invariably questioned about the addition of a gross amount of chlorine capable of sterilizing water under the worst conditions to be expected ' followed by dechlorination with thiosulphate. Mackenzie [2J suggested that if the Horrocks Box were discarded and a Fixed Dose substituted, clarified water',_




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R. St. John Lyburn and ]. E. Ridley 11

would be dosed with 3 parts per million of. chlorine and unclarified water 4 parts bver a contact period of fifteen minutes.

The Halazone tablet in the Individual Outfit for use with water bottles is one e;'ample of Fixed Dose chlorination, 4 parts per million of chlorine being imparted to the water (if tablets up to strength) and dechlorination effected with a tablet of thiosulphate after thirty minutes contact with filtered or unfiltered water. Whilst this article was being written, such an outfit was reported in the British Medical Journal (July 18) for civilian use. An ampoule of sodium hypochlorite is provided to give an extravagant dose of chlorine to 2 gallons of water and dechlorination is effected after five minutes contact. No specific type of water, filtered or unfiltered, is mentioned in the instructions. . The advantage of the civilian outfit is the stability of the chlorine compound, as :Halazone tablets must 'be tested from time to time for deterioration.

In the field, a method of demonstrating the presence'of chlorine at the end of the contact period is. desir~ble to establish confidence in the. stetllization before adding the antichlor. Starch iodide solutions. can be used, but if orthotolidine is substituted and used with a FixedGI~ss .chloroscope, compensating for natural' colour in the water, the operator ca~be cert.rtn that at least 2 parts per million of chlorine have been in contact with the water (see fig. I). .

\

$AMPLt: ONLY -----_+_ ii--il----S.4.MPL[; PLUS A

~=~~~~:::~ n:w OQOPS or ,. ORTHOTOLIOIN~

rix[o COLOUREO' <'CA~S,---I-+-,---+i TO MATCH 2. PA~T5 P(Q MILLION or CHLORINE..

FIG. 1.

INOI~TOR '

RELATIVE MERITS OF THE Two METHODS •

. The arguments for and agai~st Fixed Dose and Horrocks Test methods are briefly summarized from Mackenzie [2].

Fixed Dose-For. [I] Simplification and elimination of the Horrocks Test.

Fixed Dose-Against. , [2] No method of estimating strength of W.S.P .

. [3J May be uneconomical on most occasions. Horrocks Test~For.

[4J Automatic compensa'tion for chlorine deficiency of weak powders and for rflpid deviation of chlorine by reducing substances in natural waters.. .

[5] Apparatus available for testing strength of W.S.P. with reasonable accuracy.

[6J Treatment of small quantities of water possible, e.g. water bottles, etc.

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1 Z Fixed Dose versus Horrucks Test trl the Chlorination uf Water

Horrocks Test-Against.

[7J Does not compensate for deficiency of chlorine in weak powders when superchlorinating. Hilton Sergeant [5J has added the follo.wing advantages to the Horrocks Box :- -

Horrocks Test-For.

(S) Deviation of large'amount of chlorine as shown by Horrocks Test suggests presence of mustard gas. '

. (9) The colour test, performed after treatment of water, demOli~trates presence of free chlorine and, therefore, absence of mustard gas.

Regarding the above arguments the following observations are ~ade. Fixed Dose-For.

(1) It is agreed that simplification is desirable, particularly when large numbers of men are receiving h:aining in the minimum possible time. Fixed Dose-Against..

(2) If deterioration of W.S.P. can be eliminated, no estimation of strength ~ould be necessary. , This could possibly be effected':by packing a Fixed Dose for 100 gallons of water ina sealed waxed envelope or ampoule.

(3) Relative costs will- be discussed later. Horrocks Test~For.

, (4) This is the strong {i>olnt of the test and is agreed to be an important factor but, under active service'conditions, the quick turnover ensures a strong powder which rarely falls below 20, per cent before the tin is exhausted.

(5) To ascertain the practical value and possible error of this test, observations were made under field conditions. Trained and untrained observers were given a W.S.P. for test after laboratory titration. '.

Results :-No. of Tl'ained or Average of Maximum Minimum Laboratory

'observers untrained : readings reading reading titration Set A '19 All trained 19·1 % 25% 13% 19·8%.

B .17 All untrained 19'.06% 26% 12% 19·8% C 1 * Trained 27·4% 3.0% 24% 24·8% D 1 * Trained , 27·4% 31% 21% 24·8%

* In set t the observer used different boxes and in set D the same box for each of IQ observations. .

It will be seen from St;!ts A and B that there is little difference in accuracy between skilled and unskilled observers and both sets show wide -individual variations. In Sets C and D, the single trained observer is more consistently accurate than mixed observers but shows a definite bias towards high reading's, wh.'ether using the same box or different boxes. No space is available-for complete tabulation of individual results.

(6) The us.e of the Horrocks Box for treatment of small qual1tities of water can be eliminated if a scoop to contain 3 mL of liquid is incorporated in a water bottle (see fig. 2).

Horrocks Test-Against. '

(7) This defect has always been apparent and the teaching should stress a full strength powder wherever possible. With a weak powder, the extra scoopful




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required for superchlorination may only impart an additional 0·5 part per million instead of 1 part.

Regarding the arguments put forward by Hilton Sergeant.

Horrocks Test-For.

" (8) -The use of the Horrocks Test as a presumptive mustard gas test is superfluous when a test for poisons has been carried out by the Medical Officer. When a Poisons Test Case is not available, it should be remembered that other war gases will also deviate comparatively large amounts of chlorine.

(9) This point is important but a Fixed Dose method incorporating a qualit~tive test for free chlorine would be as efficient.

After consideration of the above arguments the following method is suggested to eliminate the H'orrocks Test and its consequent difficulties.

SCOOP TO CONTAI'N 3"L6 or LIQUID ~ nXtD TO v'ATl:R r.oTTL& CORK

I

FIG. 2.

FIXED DOSE LIMITESTER METHOD.

Apparatus and reagents required :-(1) Lovibond Fixed Glass chloroscope (similar to phosphatase Limitester used

in milk analysis) to read 2 parts p~r million of chlorine with orthotolidine reagent. (2) Sealed waxed envelopes or ampoules containing sufficient W.S.P. to impart

5 parts per million of chlorine to 100 gallons of water (about 12 grammes would be required).

(3) Sealed waxed envelopes or ampoules containing sufficient anhydrous sod,ium thiosulphate to remove 4 parts per million of free chlorine from 100

. gallons of water (about 4 or 5 grammes would be required). Method.-(l) Add contents of 1 envelope of W.S.P. to a small tin or vessel, .

make into a thin paste with a little water, dilute ,and add to 100 gallons of water and mix thoroughly.

(2) Allow fifteen minutes contact and draw off a sample into tlie Limitester tubes. Add a few drops of orthotolidine reagent to tube marked B and allow to stand for five minutes. If the colour which has developed in tube B is q.eeper than that in tube A, the water is sterilized and can be dechlorinated if required for use by adding the contents of 1 envelope of thiosulphate and stirring well. If the colour in tube B is lighter than that in A, add a further envelope of W.S.P.

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as above, allow ten minutes contact and re-test with Limitester before dechlorina-tion. f

(3) For small quantities of water ·use the " Water Bottle Method."

INVESTIGATION OF THE METHOD.

Mackenzie has demonstrated the efficiency of chlorine in providing a· safe water' with doses of 0·5 to 2·0 parts per million in excess of the amount deviated over. short contact periods, using Bact. coli as a control. Thresh [3J expresses the opillion that sewage effluents conforming to Royal Commission standards can be effec- . tively chlorinated with doses ranging from 1 to 5 parts per million, Bact. coli 'being destroyed so thaf the majority of samples are negative in 10 nil., but does not specify any contact period . .-

The suggested Fixed Dose Limitester method would ensure an immediate concent~ation of 5 parts per million of chlorine and by testing quantitatively at the end of the contact period would indicate that at least 2 parts per million had been in contact with the:water. As a confirmation of the efficiency of chlorine in sterilizing grossly contaminated waters and to ascertain the value of the suggested method, several experim:ents.were conducted. ,

Before starting the experiments, it was decided to use a lake water which had been clarified by filtni.tion through metal filters using filter powder (Kieselguhr) as filter aid. Measured 'amounts of a crude sewage containing particiIJate matter were then added to this clarified water for testing the method. Salinl suspensions

J " . '. of Bact. coli added to 'distilled or dechlorinated tap water are more easily killed by chlorine than ·Bact. coli introduced by crude sewage with particulate matter and ammonia to retard sterilization.

All chlorinated water samples' were taken in bottles into which a crystal of sodium thiosulphate had been' introduced before laboratory sterilization so that dechlorination was immediate. Bacteriological examinations were based on the Ministry of Health presumptive coliform test [4J.

Experiment 1.-A 200 gallon water tank truck was filled with clarified lake water and crude sewage containing particulate matter added to obtain a concentration of 1 part of sewage to 5,000 parts of water. Water Sterilizing Powder was added to give an immediate concentration of 5 parts per million of ch,lorine.

Sample A. (Clarified lake

water plus . sewage)

50

+

Sample B. (Sample A after Neg.

15 mins. con-tact)·

10

5+1.

1·0

5+1.

Bacteriological results. mls. of sample Residual chlorine

0·1 0·01 0·001 at 15 mins.

Neg.

4·6 parts per million.

... Sample taken at 5 mins. was negative in 50 mls.

Experiment 2.-Before increasing the sewage content, the possibility of partic~late matter retarding sterilization was cOl}sidered and a decision made to

. perform two tests.




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R. St. John Lyburn and J. E. Ridley 15

WPollute the clarified lake water to obtain a concentration of 1 part of sewage to 1,000 parts of water and add 5 parts per million of chlorine.

(2) To filter this sewage polluted lake water through metal candles using Kicselguhr followed by 5 parts per million of chlorine.

Test 1. Sample A.

(Clarified lake water plus sewage)

Sample B; (Sample A after

15 m'ins. con'tact)

,Test' 2.' Sample C.

50

+

+

(Sample A after + filtration)

Sample D. (Sample C after Neg.

15 mins. con-tact)*

10

"+1.

1+ I.

Bacteriological results mls. of sample

1·0 0·1 0·01 0·001

5+1. '+1. '+!. 2+;'

Neg.

Neg.

* Sample taken at 5 mins. was 'negative in 50, mls.

This experiment stresses two im~ortant points :-

Residual chlorine at 15 mins.

3·9 parts per mil-lion.

4·2 parts per million.

(a) efficiency of metal ~lters when using filter powder Kieselguhr as filter aid in reducing numbers of bacteria and, more important, the removal of small particles containing " clumps" of bacteria ....

(b) with unfiltered water, a dose of 5 parts per million of chlorine left a residuum of 3·9 parts after fifteen minutes contact and, although insufficient to. kill all coliform organisms embedded in particulate matter, reduced the number to 3 per 100 m!. It is doubtful whether the Hon:ocks Test with its maximum residual chlorine figure of 2 parts per million would have reduced the number to this extent.

Experiment 3.-To demonstrate the importance of filtration more clearly, the concentration of sewage was increased to 1 in 10 and Experiment 2' repeated.

Bacteriological results mls. of sample Residual chlorzne

Test 1. SO 10 1·0 0·1 . 0·01 0·001 at 15 mins. Sample A.

(Clarified lake + 5+ I. 5+1. 5+1. 5+1. 5+1. water plus sewage)

Sample B. (Sample A after +.' 1+1, Ng. 1·2 parts per mil-

lS mins. con- lion. tact)

Test 2. Sample C.

(Sample A after filtration) "

+ '+1. 5+1. "+/. .+ is ,+ /.' Sample D.

(Sample Cafter Neg .. ' - 2·5 parts per mil-lS mins. con- lion. tact) *

* Sample taken at 5 mins. was negative in SO mls.




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A striking example of the efficiency of filtration in removing " clumps," the chlorine being effective in five minutes with filtered water. With unfiltered water, 5 parts per million of chlorine reduced the number of coliform organisms to 3 per 100 ml. in fifteen minutes. ' No experiments were conducted to ascertain the amount of chlorine required to provide a coliform free water in the presence of particulate matter.

THE IMPORTANCE OF FILTRATION, . It will be seen from the above experiments that chlorination of filtered waters

with a Fixed Dose of 5 parts per million is extremely effective but that all waters containing particulate matter will require larger doses or a longer contact period.

Filtration is too often regarded as only a necessary, rather than an essential, stage in the purification of water supplies and sterilization regarded as the" be all and end all" rather than a finishing barrier, In tropical and sub-tropical regions, efficient filtration of water is of the greatest importance in removing

~~~=---It-~w~-~~ """"'-"

.:. 10 n. SUCTION HOSt iD $CURet.

FIG, 3.

pathogenic protozoa such as the cysts of Entamrxba histolytica and Giardia lamblia. Disease producing helminths such as the larva! of Ancylostomes, ova of Tcenia solium (Somatic Ta!niasis), the c;yclops of Dracontiasis, the larva! of Strongyloides stercoralis, and the great majority of pathogenic schistosomes are likewise removed by efficient filtration. If reliance were placed upon chlorine to\ kill cysts ~f E. histolytica, the quantity required would be in the-region of 500 parts per million. By using thiosulphate, this amount of chlorine could be removed but the end products would render the water quite undrinkable.

For bulk supplies and unit purification, adequate filtration is provided by using· Mobile Water Purifiers (3,000 gallons per hour) and Water Tank Trucks or Trailers fitted with high pressure metal filters using filter powder (Kieselguhr). To units such as companies, the Portable Filter~ng Apparatus is issued but, as this apparatus weighs from 137 to lS0pounds, depending upon the method of packing in panniers, it would be better named" Transportable."

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R. si. John Lyburn and /. E. Ridley 17

For small parties of men on patrol, no filtering apparatus is available and reliance is placed upon the Halazone tablet for sterilization of water bottles. As there is no filtration barrier, it is not considered that 4 parts per million of chlorine is sufficient under these Circumstances for grossly polluted waters containing particulate matter which may be the only available source and a larger dose of chlorine should be given. .

The alternative is a truly portable filter to be carried easily by one man and able to fill the water bottles of 50 men in fifteen minutes followed by sterilization with the existing Halazone tablet.

Several designs have been suggested to reduce the weight of the existing Portable Filter, a successful one being a semi-rotary pump (with air bottle removed) fitted to'a single pack domestic type Metafilter, the whole apparatus with chemicals being packed in a light pannier for transport (see fig. 3). The total

FIG. ,I.

STAND4A.O MtTAr'LTtR w.c:K

weight of this apparatus (exclUding pannier) is about 20 pounds and it will effectively filter 12 to 15 gallons of a water containing 5 parts per 100,000 suspended solids in fifteen minutes, i.e. sufficient for 50 water bottles.

Although efficient and capable of being carried by one man, the apparatus was considered to be too heavy and a suggestion was made that the civilian type stirrup pump could be substituted for the semi-rotary pump with a consequent reduction in weight (see fig. 4). Total weight excluding canvas bag and chemicals -:-15 pounds. The apparatus was as efficient as the semi-rotary pump model and filled 50 water bottles in fifteen minutes without undue effort. .

On both types, backfiushing arrangements were purposely omitted to reduce weight, the method of clean~ng being the removal of the filter head with filter attached and washing in the source. On return from patrol, the rings of the filter pa~k are loosened and b~shed in clean water, this process beingo equiValent to

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-1"8 Fixed Dose versus Horrocks Test in the Chlorination of Water

\ backftushing. follows :-

The amount of chemicals required for both filte1s is.: as

1 day patrol of SO men using 1 gallon per head per day.

Filter Powder Kieselguhr 4 to 6 charges of ~ oz. each Halazone outfits (50 tablets in each)'

.. 2 or 3 oz.

.. 40r5

The weights of the components could be considerably reduced by cutting the diameter of the filter chamber to a minimum and using a lightweight hose apd strainer. The cost of manufacture should not be great and it iscorisidered that small parties of men on short patrols would' be afforded greater protection f~om grossly contaminated water supplies, particularly 'in the tropics. The filtercou'ld also be used with advantage for emergency civilian supplies as stirrup pumps can be easily adapted to fit the filter. The output per hour of this stirr~ppump type is about SO to 60 gallons and could be used for smallis6lated groups where hO large scale emergency clarification is available.

DISCUSSION.

The object of the tests was to show that simplification of teaching and operation could be accomplished by elimination of the Horrocks Test and substitution of a Fixed Dose Method of chlorination of water supplies in the field.

The Horrocks Test has been in operation for many years but has always been difficult to teach and the introduction of " parts per million " has tended to increase these difficulties. Confusion has arisen over Simple chlorination, Superchlorination, the Modified Horrocks Test and the selection of the correct colour for the indicator cup ll:nd it is considered the introduction of a standard method for filtered waters would be of great value. All unfiltered waters containing particula:te matter should receive special treatment, probably 10 to 20 parts per million of chlorine, and heavily contaminated unfiltered waters would probably need 50 parts if the \contact period is not to be increased.

The existing method of filtration and chloramination of bulk supplies at Bases is considered adequate with installations such as the Mobile Water Purifiers (3,000g.p.h.) Mk. 1 and Mk. 2. , The~ccessory uses of the Horrocks Box can be ellminated'o~m~difiedi e.g .

. - .' . '-' , . ", ,.: ... ' -'I'

deterioration and testing ofW.S.P. prevented, by packing in sealed waxed·Gon-tainers. The designing of such an envelope sliouidpresen't no gre,at manufacturing . "(,'.. .: r. " . .) . . difficulties. . . . . , (,

The Fixed .Dose Method includes a colour' test with o~thotolldiu'e as the indi-.' '. .

cator solution, this solution being cheaper an(i m,ore stable than starch iodide solutions. ' . , ,

As regards relative ,costs of the methods, the introduction q! superchJorination as the standard method of sterilization increased the cost of operationdueto ,extra W.S.P. and the provision of taste remover tablets. The Fixed Dose metI}od suggested would increase the. general cost sltghtIy but, reduce th~ ini~ial Gost, q£ appara tus, .e.g. Limitester 135. ,9d., Horrocks Box £2 {tllese .~recivili(j.n 'estim5l:t~s




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R. St. John Lyburn and J. E. Ridley 19

and under Anny contract the prices are proportionately reduced}. No space is available for a detailed discussion of costs.

It may be argued that the introduction of a Limitester into a Fixed, pose method complicates operation and teaching unnecessarily and merely substitutes a cheaper apparatus whilst increasing" running costs" but the sugge.sted method can be taught to untrained men with no previous knowledge of water sterili~atiQn methods in one hour, whereas six hours are normally devoted to the teaching of the Horrocks Test and its accessory tests. - Of the failures on Water Duty Courses over 90 per cent have little or no idea of theinterpret~ti~n of the Horrocks Test but can perform the practical work of the test quite well.

The Limitester was introduced to ensure a direct quantitaNve as well as qualitative test at the end of the contact period which is of considerable importance in establishingc()nfidencein the steri!ization where thereisnobacteriolog~cal contro~.

It will be noticed that 5 parts per million of chlorine ar,e added to the water and, only 4 'parts per million removed by thiosulphate. This is based on the assumption:tliat ill natural waters, even after fift~ation, 'will de~iate a littie chlorine and the maximum residual chlorine figure to be expected after dechlorina

,tion is about 0·5 part per million. This approximates to the residual chlorine tigure if si~ple chlorination is used with the Horrocks Box. The average person will describe this amount as " faintly chlorinous taste" and only a few will describe it as a definite taste of chlorine.

CONCLUSIONS.

The experiments have shown that a Fixed Dose of 5 parts per million of chlorine is extremely efficient ih dealing with grossly contaminated waters. The importance of filtration has been clearly demonstrated, a grossly contaminated water being effectively sterilized in five minutes after filtration through metal filters using Kieselguhr as filter aid.

An attempt has been made to design a very light portable filter for small parties of men on patrol and it has been shown that a filter weighing considerably less than 20 pounds can effectivelyfin--the water bottles of 50 men in fifteen minutes. The inclusion of such a' filter is considered to be an essential in the equipment of small parties of men -on patrol in tropical 'and sub-tropical areas for protection against tropical worm diseases and the removal of " clumps" of bacteria in grossly polluted waters.

SUMMARY.

The relative merits of the HOITocks Test and Fixed Dose methods of sterilization of water supplies in the field have been discussed.

An attempt,has been made to simplify teaching and operation by substituting a Fixed Dose Method incorporating a chloroscope with a fixed glass to match 2 parts per million of chlorine using orthotolidine indicator solution.

Experiments were conducted with the proposed method using grossly contaminated waters and the importance of filtration demonstrated.




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The case for a very light portable filter to be used by smafl detachments of men on patrol has been put forward and a new design suggested.

It is proposed to perform comparative Field Tests with contaminated waters using (a) Horrocks Test Method and (b) Fixed Dose-Limitester Method and to compare the bacteriological efficiency of both methods with filtered' and un filtered water in a second paper.

We are much indebted to Colonel E. B. ~llnutt, M.C., for his helpful encouragement and for his peI111ission to submit this article for publication

REFERENCES.

[1[ Handbook of Specification for Supplies, 1939. [2] Mackenzie, E. F. W. (1938). JOURNAL OF THE ROYAL ARMY MEDICAL CORPS, 71, 289. [3] THRESH, J. C., BEALE, J. F., and SUCKLING, E. V. (1933). "The Examination of Waters -".

and Water Supplies." [4] "The Bacteriological Examination of Water Supplies," 1939. Ministry of Health.

- [5] HILTON SERGEANT, F. C., JOURNAL OF THE ROYAL ARMY MEDICAL CORPS, 1941.




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HORROCKS TEST IN THE CHLORINATION OF WATER …Horrocks Test~For. [4J Automatic compensa'tion for...

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