Residual Chlorine Methods and Disinfection of Sewage Robert V. Day, Donn H. Horchler, and Henry C. Marks RESEARCH DEPARTMENT, WALLACE & TIERNAN CO., INC., NEWARK 1, N. J.
A n important problem in sewage chlorination is the selection of a residual chlorine method that parallels disinfection. Residual chlorine includes a great variety of chloramines, both simple and complex, which differ in their bactericidal properties, Their exact chemical structure being unknown, it is necessary to determine empirically the method giving the best correlation with bacterial kill. The o-tolidine and two amperometric titration procedures were investigated on three different types of domestic sewage. By studying a large number of samples and using statistical evaluation, they were compared with respect to correlation with bacterial kill. The amperometric titration was significantly better for sewages with a high organic load. The result of the study permits the selection of the proper chlorine method for a given purpose. The degree of correlation is of great importance in dictating the amount of chlorine required to meet a given bacterial standard for a sewage effluent.
A
N IMPORTANT problem in sewage chlorination is the selection of a residual chlorine method that parallels disinfection. Available chlorine in sewage includes different chlorine compounds in varying amounts, depending on the characteristics of the sample. These compounds are chloramines, both ammonia and organic. Free residual chlorine does not exist in sewage with t h e present practice of chlorination. Therefore, the residual chlorine method should give a reproducible measure of this complex group of chloramines t o predict successfully the degree of disinfection. The question immediately arises whether all the chloramines are important or whether only a small fraction contributes t o the disinfectant properties of the residual. As their exact chemical structure is unknown, it is not possible to determine this directly. It is necessary t o employ the empirical approach of determining residual chlorine by methods which indicate different percentages of the chloramines and by comparing correlation between disinfection and residual chlorine. This paper presents the results of such a study using three different types of domestic sewage. I n choosing methods responding to different fractions of the chloramine mixture, the o-tolidine method was selected as indicating a relatively small portion of the total. The acid starchiodide and amperometric titration procedures (6) apparently respond t o all chloramines present and give high results. The more precise amperometric titration was chosen over the starchiodide procedure. I n a third method, the available chlorine capable of reacting with an organic sulfide was determined by difference and called the “effective residual.” This eliminated one part of the chloramines, indicating a fraction intermediate between the other two methods. It has been suggested t h a t chlorine kills bacteria by reacting with sulfhydryl groups ( 5 ) .
by the amperometric, o-tolidine, and “effective” methods. Bacterial samples were also taken at each contact period and treated with sodium sulfite t o inactivate the chlorine. The most probable number (MPN) of coliform organisms was determined on the basis of the partially confirmed test using brilliant green lactose bile as the confirmatory medium, Three or four dilutions were used with five tubes for each dilution.
Residual Chlorine Determination The o-tolidine method followed the American Public Health Association method (1). A 15-ml. sample was added t o 1.5 ml. of o-tolidine reagent and t h e maximum color was compared in a Wallace & Tiernan comparator with permanent glass standards. The amperometric method as published by Marks, Joiner, and Strandskov ( 8 ) was modified. The sample was buffered t o a p H of 3.5 to 4.0 with 4 ml. of a n acetate buffer for each 200 ml. of sample. The acetate buffer consisted of 82 grams of sodium acetate and 540 grams of glacial acetic acid per liter. The acidification t o a p H of 2.0 (6) causes interference when iron or nitrites are present. The critical p H for the elimination of the interference is 3.5 t o 4.0. For the titration of effective residual, a 500-ml. aliquot of the chlorinated sample was treated with a computed dosage of the organic sulfide, ethyl thioglycollic acid. After a 30-second reaction period the residual chlorine was determined by the amperometric method; this was called the inert fraction of the residual chlorine. The effective residual was the difference between the total residual chlorine as determined by the amperometric method and the inert fraction. The dosage of ethyl thioglycollic acid was computed on a molar basis and was 1.2 times the residual chlorine determined 5 minutes prior by amperometric titration. The ethyl thioglycollic acid solution was prepared by dissolving approximately 8.5 grams in 1 liter of distilled water. The solution was standardized (8)and then diluted so that 1ml. was equivalent to 0.5 mg. of chlorine.
Procedure Samples of settled sewage, trickling filter effluent, and chemical treatment effluent were collected a t two sewage treatment plants in North Jersey, both primary and secondary effluents being sampled at one plant. The plants received only domestic sewage and prechlorination was not practiced during the period of sampling. Each sewage plant was visited about 20 days during a 2-month period; two or three fresh samples were collected each day. Aliquots of each sample were treated with different chlorine dosages t o provide a complete range of bacterial kill. After 15- and 30-minute contact periods the residual chlorine was determined
Experimental Results Table I, which gives the relation between chlorine dosage and residual chlorine by amperometric titration and o-tolidine, shows the consistently greater magnitude of the amperometric result. The difference can be as much as 3 p.p.m. and is greater the higher the organic load. Trickling filter effluent shows a smaller difference than the primary effluents. There are a large number of cases where the amperometric titration shows a significant residual and the o-tolidine none. I n comparing successive ranges of residual chlorine and dosage, o-tolidine shows less consistent results from sample to sample. The values given by the effective
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Table I.
Chlorine Dosage
(Residual chlorine, p.p.m., 30-minute chlorine contact period) Chlorine D ~ P.P.M.
jyo, ~ of ~ Expts.
AmperoInetrio ~ ~ , Range Median
o-Tolidine Range Median
Settled Sewage 0- 1 . 0 1.1- 2 . 0 2.1- 3 . 0 3.1- 4 . 0 4.1- 5 . 0 5.1- 6 . 0 6.1- 7 . 0 7.1- 8 . 0 8.1- 9 . 0 9.1-10.0 10.1-11.0 11.1-12.0 2.1- 3 . 0 3 1- 4 . 0 4.1- 5 . 0 5.1- 6.0 6.1- 7 . 0 7.1- 8 . 0 8.1- 9 . 0 9.1-10.0
2 5 9 8 5 7 9 10 7 4 6
0 0.05-0.30 0.40-0.75 0.90-1.45 1.65-1.90 1.35-2.00 1.60-2.80 0-3.10 3.25-4.10 0.90-4.80 3.35-4.00 3.70
1
0:25 0.50 1.35 1.85 1.75 2.15 2.70 3.75 3.75 3.72
..
Chemical Treatment Effluent 0.05-0.65 0.15
6 9 5 4 8
0.25-1.10 0.60-1.35 1.45-1.90 1.10-2.50 1.45-3.10 1.70-3.20 2.65-3.75
11
9 3
'
0.65 1.25 1.80 1.70 1.90 2.55 3.40
..
0 0 0 0-0.05 0.12-0.25 0-0.05 0.05-0.60 0.05-0.45 0.65-1.4 0-2.0 0.40-1.1 0.60
0 0 0 0.15 0 0.13 0.25 1.0 0.8 0.75
0 0 0-0.13 0-0.35 0-0.50 0-0.70 0-1.00 0.35-0.80
0 0 0 0.08 0 0.10 0.18 0.60
0-0.05 0-0.70 0.15-1.1 0.30-1.6 0.90-2.0 1 . 6 -2.0
0 0.14 0.55 0.95 1.2 1.8
..
Trickling Filter Effluent 0- 1.0 1.1- 2 . 0 2.1- 3 . 0 3.1- 4 . 0 4.1- 5 . 0 5.1- 6 . 0
0-0.55 0.30-1.15 0.95-2.30 1.60-2.80 2.05-3.86 2.85-3.96
8 8 13 11 8 6
0.05 0.65 1.40 2.15 2.50 3.30
method folloiy the same pattern as the amperometric, but amount to 50 to 75% of the latter. To show variation of most probable number of coliform organisms with residual chlorine, all results are summarized in Tables 11, 111, and IV. The greater the organic load the more erratic the results, settled sewage being the worst. There is generally a wider variation in most probable number for o-tolidine values than amperometric. The large range of most probable number values a t zero by o-tolidine account for much of this. The drop in bacterial numbers from 15 to 30 minutes shows t h a t there is active residual present' in most of these cases. Referring back t o Table I, it is evident t h a t the reduction in most probable number obtained with a given residual chlorine is the same, regardless of the analytical method, but the o-tolidine responds t o a smaller fraction of the chlorine. A given median most probable number, such as 10 per ml., can be obtained with 0.5 to 1 p.p.m. less residual chlorine (amperometrically) by increasing contact time from 15 to 30 minutes. The total effect of contact time is discussed more' fully herewith.
Residual Chlorine, P.P.M.
No. of expts.
0 0.01 0.49 0.60 0.99 1.00 1.49 1.50-1.99 2.00-2.49 3.50-2.99 8.00-3.49 3.60-3.99 4.00-4.49
3 5 7 7 9 10 7 2 3 1
0
7
0.01-0.19 0.20-0.29 0.40-0.59 0.60-0.79 0.80-0.99 1.00-1.19 1.20-1.39
5 2 5 4 6 7
1.40-1.59 1.60-1.79
5
1.80-2,oo
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8
1
4
RESIDUAL CHLORINE-ppm
Figure 1.
zffectof
Residual Chlorine on Most probable Number
Settled sewage, IO-minute contacl period, amperometric titration
Correlation between Most Probable Number and Residual Chlorine
I n using a statistical method to summarize and interpret the results, it is logical t o assume a logarithmic relation between residual chlorine and most probable number from the very nature of the latter ( 3 ) . Both a reciprocal and a Table 11. Trickling Filter Effluent direct logarithmic relationship were tried 15-Minute Contact 30-Minute Contact on representative data, the direct giving Median No. of Median the best agreement. For each case the M P N per ml. MPN expts. M P N per ml. MPN correlation coefficient, T , between log Amperometric Method most probable number and residual 3300 - 35,000 7000 4 1700 -24,000 7000 chlorine was computed. In summariz3300 5,400 3500 4 450 - 1,700 850 460 ->16,000 1700 9 24 - 5,400 170 ing these calculations in Table V, it is 64 - >1,600 330 7 17 - >160 54 17 280 130 9 1.733 4.0 more useful to give the value of r2. This 28 8 0.29.5 0.6 9.3>160 is a direct mea.sure of the per cent of 0.220 3.9 7 160 280 3 24 - >160 33 dist,ribution ( d ) , and of the critical differ64 350 130 5 2.792 14 ence for each set of data, such that if the 79 170 150 7 0.417 2.1 20 120 38 3 0 .22 .. 3 1 .. 7 difference between any two 2 values ex3.9-33 13 6 0.29 5 0 6 17 . . 1 9.5 .. ceeds the critical value there is a 95% 0.26.1 1.0 5 0.22.2 0.8 probability that the two correlation 0.89.2 4.4 4 0.20.8 0.2 coefficients are significantly different.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 5
Water Treatment Table HI. Residual Chlorine, P.P.M. 0 -0 49 0 .50-0 99
1 00-1.49 1 50-1.99 2 00-2.49 2 50-2.99 3 00-3.49 3 50-3.99 4 00-4 49 4 50-4 99 5 00-5 49 0 0.01-0.19 0.20-0.39 0.40-0.59 0.60-0.79 0.80-0.99 1.00-1.19 1.20-1.39 1.40-1.59 1.60-1.79 1.80-2.00
15-Minute Contact No. of expts.
8 9 4
' 10
11 6 7 4 13
..1
25 18 5 6 4 3 6
2 2 1 1
0 -0.49 0.50-0.99 1.00-1.49 1.50-1.99 2.00-2.49 2.50-2.99 3.00-3.49 3.50-3.99 0
0.01-0.19 0.20-0.39 0.40-0.59 0.60-0.79 0.80-0.99 1.00-1.19
I
-
30-Minute Contact Median MPN
M P N per ml.
No. of expts.
Amperometric Method 10 54,000 -160,000 92,000 9 35,000 7,000 - 54,000 8 3,500 - 13,000 6,000 1,600 13 330 - 5,400 540 7 16:000 5.6540 1.7540 2.135 2.335 2.035 0.754 1.27.9 3.3
35,000 5,400 90 7.0 3.2 4.9 11 7.2 2.8
a-Tolidine Method 330 -160,000 35,000 27 920 18 1,600 130 540 350 6 13 54 35 4 1.735 8.4 7 0.67.0 0.8 3 0.64.9 3.3 4 .. 1.47.9 1.513 2 0.9 0.4 'i
3 . 3 - 160,000 1.7>160 2.635 2.011
-
n 7-
..
.. .. ..
Table IV. Residual Chlorine, P.P.M.
Settled Sewage
4
8
5 12 13 4 6 3 23 16 4 5 4 1 2
35 7.9
1.2-
3.3
5,400 6.3 14 3.6 6.4 7.9 2.8
3.3
Chemical Treatment Effluent
15-Minute Contact No. of expts.
A4 ..
2.42.3-
..
M P N per ml.
30-Minute Contact Median MPN
No. of expts.
Amperometric Method >16,000 8 7 6,400 790 10 90 14 22 5 8.2 4 5 5 6 0 8 1 26 ->16,000 3.9540 3.3160 1.735 0.217 0.8 0.8
o-Tolidine Method 2,400 27 21 13 2.3
.. ..
M P N per ml.
Median MPN
280 ->16,000 70 540 240 0.718 0.70.20.9 0 20 8 0216,000 0.227 0.74.5 0.20.9 0.20.4 0.2 0.2
170 1.2 2.4 0.5
31 12 4 4
2 1 1
-
The trickling filter effluent shows a high degree of correlation between the two variables, with no difference between the two analytical methods. The correlation is equally good for chemical treatment effluent when amperometric titration is used. It is significantly poorer for the o-tolidine method, as shown by the difference between the Z values. By this criterion the amperometric method also gives a better correlation for settled sewage. Here the 2 values show the results for 30-minute contact to be significantly poorer than 15 minutes for both analytical methods. There is no well-established reason for this, but it may be connected with the continued formation of less active chloramines from the organic matter present with increased time. Because the effective residual method gave no better correlation than the simple amperometric titration in any case, there is no advantage in investigating i t further.
Chlorine Required to Meet Stream Standards
..
..
.. ..
fication on a statistical basis. I n these terms it means that not only a regression curve but confidence limits are needed. This may be illustrated for the settled sewage data as in Figure 1. I n addition to the best straight line for log of most probable number as a function of residual chlorine, the upper limits for 80 and 95% confidence are shown. Applying this to the Ohio River objectives, 2.2 p.p.m. of residual chlorine is all that is required for an average most probable number of 5000 per 100 ml. B u t 3 p.p.m. is necessary to have SO% of the samples not greater than 5000 and 3.15 p.p.m. to ensure that 95% of the samples shall not exceed 20,000 per 100 ml. The 95% limit, therefore, governs the chlorine required. This results in an average most probable number of 600 per 100 ml., far less than the requirement. The importance of a control method which affords good correlation is evident. Table VI shows the results of applying the Ohio River objectives in this way to the three types of sewage and both chlorine methods. I n trickling filter effluent where both analytical methods have been shown to give the same degree of correlation, the same amount of chlorine is required, regardless of the method of control. The different residual chlorine values result from the different sensitivities of the two methods. I n this case, the SO% requirement dictates the level of residual chlorine to give an average most probable number of 1600
I n the chemical treatment effluent the poor correlation given by the o-tolidine method imposes such a high chlorine dosage to meet the 95% requirement that the average most probable number would be 260 per 100 ml. The better degree of correlation with the amperometric methods permits less chlorine, as the average can be 1400 per 100 ml., the 80% requirement being the controlling factor. The importance of correlation is equally evident for settled sewage. The great variation exhibited by the o-tolidine method calls for a chlorine dosage a t the 15-minute contact sufficient to Table V. Contact Period, Min. 15
Correlation Indexes
Chlorine Metbod a-Tolidine Amperometric Effective +Tolidine Amperometric Effective Critical difference
Settled Sewage 71
2
0.73 0.93 0.91 0.32 0.65 0.58
1.28 2.04 1.89 0.64 1.12 1.01 0.34
Trickling Filter Effluent rs 2 0.85 1 . 6 1 0.81 1.46 0.77 1.37 0.79 1.42 0.83 1.53 0 . 7 1 1.23 0.40
Chemical Treatment Effluent rp
Z
0.58 0.84
1.00 1.58
..
*.
30 0.37 0.71 Most stream standards specify not only average bacterial count 0.80 1.42 but also upper limits to be exceeded in less than a stipulated per.. . . centage of samples examined. Bacterial 15 and 30 0.40 quality objectives for the Ohio River ( 7 ) Table VI. Chlorine Required to Meet Stream Standards require a monthly average most probTrickling Filter Chemical Treatment able number of not more' than 5000 per Contact Settled Sewage Effluent Effluent Period, Residual, Dosage, Residual, Dosage, Residual, Dosage, 100 ml., but, in addition, the count must Min. Chlorine Method p.p.m. lb./mg. p.p.m. Ib./mg. p.p.m. lb./mg. not exceed 5000 per 100 ml. in 20% of the 15 a-Tolidine 0.98 31.9 0.47 78.5 76.1 1.20 samples examined in any one month, nor 15 Amperometric 3.54 69.0 2.50 32.5 2.38 62.5 30 o-Tolidine 1.15 85.0 0.75 23.2 0.26 68.4 20,000 per 100 ml. in more than 5%. 30 Amperometrio 3.15 71.4 1.60 1.57 53.4 23.3 This correctly puts the quality speci-
May 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
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give a n average most probable number of 970 per 100 ml. t o comply with the 95% requirement, compared with an average of 2240 per 100 ml. to meet the 80% requirement for the amperometric method. The limitations dictated by the statistical method are even better illustrated by comparing 15- and 30-minute contact periods for settled sewage. The unusually poor correlation for the longer contact period, as previously discussed, dictates the use of more chlorine than for the 15-minute period when o-tolidine is the control method. I t is necessary to provide higher residual chlorine for the longer contact period because the average must be reduced to 170 per 100 mi. while 970 per 100 ml. suffices for the 15-minute period, the 90% requirement being the controlling factor in both cases. Use of a chlorine method giving unnecessarily poor correlation means application of excessive quantities of chlorine and waste of money.
Summary and Conclusions The o-tolidine and two amperometric titration methods have been compared with respect to correlation between bacterial kill and residual chlorine for three types of domestic sewage. blodification of the usual amperometric titration failed to improve the correlation. In all samples the amperometric method indicated higher values than o-tolidine. I n trickling filter effluent correlation was equally good for both
methods, in spibe of the difference in absolute values of residual chlorine. I n settled sewage and chemical treatment effluent the amperometric method gave significantly better correlation. It is apparently less affected by organic matter. As most stream standards specify not only average bacterial count but also a statistical upper limit, poor correlation means an unnecessarily low average count and excessively high chlorine to comply with this limit.
References (1) American Public Health Association, New York, “Standard Methods for the Examination of W a t e r a n d Sewage,” 9th ed., 1946. ( 2 ) Eliassen, R., Heller, A. S . ,a n d Krieger, H. L., Sewage Works J . , 20, 1008 (1948). (3) Eliassen, R., a n d Krieger, H. L . , SewaQe and Ind. Wastes, 22, 47 (1950). (4) Hoel, P. G., “Introduction t o Mathematical Statistics,” Xew York, J o h n %‘iley & Sons, 1946. (5) Knox, IT. E., Stumpf, P. K., Dreen, D. E., a n d Auerbach, V. H., J . Bact., 5 5 , 4 5 1 (1948). (6) Marks, H. C., Joiner, R . R., a n d Stiandskov, F. B., Water and Sewage Works, 95, 176 (1948). ( 7 ) Ohio River Valley R7ater Sanitation Commission, Cincinnati, Ohio, “Bacterial Quality Objectives for t h e Ohio River,” 1961. (8) Siggia, S., and Edsberg, R. L., Anal. Chem., 20, 938 (1948).
RECEIYED for review November 21, 1952.
ACCEPTED December 3, 1952.
Disinf ectio Wastes by Chlorin H. Heukelekian, Robert Day1, and Raymond Manganelli NEW JERSEY AGRICULTURE EXPERIMENT STATION, RUTGERS UNIVERSITY, RUTGERS, N. J.
The purpose of this study was to obtain the best chemical yardstick of residual chlorine indicating the effectiveness of chlorination for disinfection of mixtures of wastes and sewage in terms of the most probable number (MPN) of coliform organisms remaining. The three methods of residual chlorine determinations used in this study were o-tolidine, modified starch iodide, and amperometric methods. The residuals obtained by these methods with different dosages of chlorine applied to sewage; sewage plus spent yeast broth; sewage plus titanium pigment waste; waste from a chemical manufacturing plant; and municipal sewage containing large volumes of various industrial wastes were compared and correlated with the MPN of coliforms with the use of statistical methods. The results obtained indicated approximately similar magnitudes of residuals by the modified starch iodide and amperometric methods with lower values for the o-tolidine method for nearly equivalent MPN of coliforms remaining. Statistical analyses of the results showed only a fair correlation between the residual chlorine determined by the three methods employed and the coliform organisms remaining. This is primarily due to the inaccuracies of m.p.n. determinations which do not allow a more precise evaluation of the differences in the method of residual chlorine determinations.
I
N THE evaluation of the action of chlorine as a disinfecting agent the ultimate criterion should be the number of coliform organisms remaining in the effluent. The percentage reduction of coliform organism as a result of chlorination, although valuable as an index of efficiency, is not significant with respect to acceptability of the effluent from the regulatory standpoint whatever the standards may be. The coliform organisms remaining in the effluent after chlorination are determined by the dilution technique and are expressed in terms of most probable numbers. The technique of enumeration and mathematical expression on the basis of the most probable numbers are subject t o great inherent inconsistencies even with a large number of replications which are not 1
Present address, Wallace & Tiernan Co., Inc.
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commonly employed under routine conditions. A sa basis of practical control of the chlorination process, the enumeration of coliform organisms has little value as the results are obtained some 4 days later. For these reasons a chemical short cut such as residual chlorine determination is employed under practical conditions t o indicate the quality and the acceptability of the effluent. The residual chlorine determination is a test involving the contact of the chlorine with the substrate for a definite length of time. The portion of chlorine which has not reacted with the various ingredients in the substrate is then determined by one of several methods. There are a number of reactions which take place with the various materials in the substrate. The nature and concentration of these materials vary with the different substrates and even with the same substrate from time to time.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 5
