Chlorination of Water Part 5

The results show the effect of organic matter on the reduction of the chlorine concentration available for germicidal action and also the importance of avoiding a local excess of chlorine (_vide_ p. 41).

An effort has been made by some observers to find a quant.i.tative relation between the organic matter, expressed as oxygen absorbed in parts per million, and the chlorine required for oxidation, but without definite result. Some of the results obtained are given in Table VII.

TABLE VII.--OXYGEN TO CHLORINE RATIO

-------------------------+------------------------- | Oxygen Absorbed Observer. | Ratio -----------------.

| Chlorine Absorbed -------------------------+------------------------- Rouquette | 1 Bonjean | 0.5 Orticoni | Less than 1 Valeski and Elmanovitsch | 0.4 Race | 0.4 Theoretical | 0.22 -------------------------+-------------------------

The value of 0.4 (0.39) obtained by the author is the average of over one hundred determinations covering a period of two years. The experiments of Zaleski and Elmanovitsch were made with the water of the Neva River.

The divergence in the ratios affords additional evidence in favor of reaction (2) mentioned on page 28 and also shows that the chlorinated compounds are less readily oxidized than those from which they are produced. Heise[8] has found that the amount of chlorine consumed is usually proportional to the concentration in which it is added though not necessarily a function of the concentration of the organic matter.

=Temperature.= The evidence regarding the effect of temperature upon the dosage required is somewhat conflicting. Ellms (_vide supra_) found that the velocity of the germicidal action varied directly with the temperature and this has also been the author's experience with laboratory experiments. Typical examples of these are given in Tables VIII and IX.

TABLE VIII.[A]--EFFECT OF TEMPERATURE

AVAILABLE CHLORINE 0.4 PART PER MILLION

---------------+--------------------------------- |Temperature, degrees, Fahrenheit.

Contact Period.+-----------+----------+---------- | 36 | 70 | 98 ---------------+-----------+----------+---------- Nil | 424 | 424 | 424 5 minutes | 320 | 280 | 240 1.5 hours | 148 | 76 | 12 4.5 hours | 38 | 14 | 3 24 hours | 2 | 0 | 0 48 hours | 2 | 0 | 0 ---------------+-----------+----------+----------

[A] Results are _B. coli_ per 10 c.cms.

TABLE IX.[B]--EFFECT OF TEMPERATURE

AVAILABLE CHLORINE 0.2 PARTS PER MILLION

---------------+--------------------------------- |Temperature, degrees, Fahrenheit.

Contact Period.+-----------+----------+---------- | 36 | 70 | 98 ---------------+-----------+----------+---------- Nil | 240 | 240 | 240 5 minutes | 240 | 250 | 235 1 hour | 245 | 235 | 195 4 hours | 215 | 190 | 170 24 hours | 143 | 130 | 115 48 hours | 130 | 59 | 19 72 hours | ... | 28 | ...

96 hours | ... | 16 | ...

120 hours | ... | 6 | ...

[B] Results are _B. coli_ per 10 c.cms.

The reaction velocity of a germicide is proportional to the temperature[9] and the influence of temperature may be mathematically expressed by the formula _K__{1}/_K__{2} = _[theta]_(_T__{2} - _T__{1}), in which _K__{1} and _K__{2} are the constants of the reaction at temperatures _T__{2} and _T__{1}, respectively, and _[theta]_ is the temperature coefficient. From the value of _[theta]_, the velocity constant of a germicide for any temperature may be calculated from the equation _K__{_T_} = _K__{20} _[theta]_^{(_T_ - _T__{20})}.

_K__{1} and _K__{2} are obtained from the formula _K__{_T_} = log(_N__{1}/_N__{2})/(_t__{2} - _t__{1}) in which _N__{1} - _N__{2} is the number of bacteria destroyed in the interval _t__{2} - _t__{1}.

A reduction of temperature also lowers the oxidizing activity of the chlorine so that a greater concentration is available for germicidal action. This is shown by the results plotted in Diagram II.

[Ill.u.s.tration: DIAGRAM II

EFFECT OF TEMPERATURE ON ABSORPTION OF CHLORINE BY WATER

+----------------------------+----------------------------------+ | | Value of K calculated from | | | absorption at 63 F. | | Absorption of Chlorine by | | | water containing 40 p.p.m. | Log(N_{1}/N_{2}) | | of colour | K = ----------------- | | | _t__{2} - _t__{1} | +-------+--------------------+ | |Time of|Temperature of Water+-------+--------+--------+--------+ |Contact+--------------------+_t__{2}|_t__{1} |_t__{1} |_t__{1} | |Minutes|32 F.|46 F.|63 F.|minutes| = 0 | = 5 | = 10 | +-------+------+------+------+-------+--------+--------+--------+ | Nil |10.00 |10.00 |10.00 | | | | | | 5 | 8.00 | 7.45 | 6.50 | 5 | 0.0374 | ---- | ---- | | 10 | 7.23 | 7.09 | 5.91 | 10 | 0.0228 | 0.0082 | ---- | | 20 | 7.00 | 6.60 | 5.18 | 20 | 0.0190 | 0.0066 | 0.0057 | | 40 | 6.42 | 6.05 | 4.47 | 40 | 0.0087 | 0.0043 | 0.0040 | | 60 | 6.22 | 5.60 | 3.90 | 60 | 0.0068 | 0.0040 | 0.0036 | | 80 | 6.13 | 5.40 | 3.65 | 80 | 0.0056 | 0.0033 | 0.0029 | +-------+------+------+------+-------+--------+--------+--------+]

Tables VIII and IX, however, show that the temperature coefficient of the germicidal action has a greater effect than the reduction in the amount of chlorine absorbed and removed from the reaction.

The results obtained on the works scale with these waters are very different to the laboratory ones and show that more chlorine is required during the summer season than in winter. The results with bleach and liquid chlorine are in the same direction (_vide_ Diagrams III and IV).

The bleach was regulated so as to maintain a constant purity, whilst in the other case the dosage was constant with a varying _B. coli_ content.

In Diagram IV the _B. coli_ is plotted; this does not represent all the factors involved as the _B. coli_ content of the treated water is also a function of that of the raw water, but in the example given this factor is of no moment because it was comparatively constant during the period plotted (extreme variation 80 per cent).

The discrepancies between the laboratory and works results cannot be easily explained. The only difference in the conditions is the nature of the containing vessel. Gla.s.s is practically inert at all temperatures but the iron pipes, through which the water pa.s.sed before the samples were taken, may exert an absorptive influence on the chlorine at the higher temperatures experienced during the summer months.

Waters containing organic matter that differs much in quant.i.ty from the examples above may yield very different results and no generalisation can be made that will cover all cases. An increase of temperature increases the germicidal velocity and also the rate of absorption of chlorine by the organic matter; other factors determine which of these compet.i.tive actions predominates.

=Method of Application (admixture).= A thorough admixture of the water and chlorine is a _sine qua non_ for successful operation. This should, if possible, be attained by natural means, but if there is any doubt as to the efficiency of the mixing process, mechanical appliances should be utilised. Pumps, especially centrifugal pumps, const.i.tute a very convenient and efficacious method of mixing the germicide and the water, and the solutions should never be injected into the discharge pipes when it is possible to make connections with the suctions.

[Ill.u.s.tration: DIAGRAM III

EFFECT OF TEMPERATURE]

[Ill.u.s.tration: DIAGRAM IV

EFFECT OF TEMPERATURE]

Inefficient admixture leads to local concentration of the chlorine, a condition which (_vide_ p. 35), results in a wastage of the disinfectant. Two practical examples of this effect may be cited. In one case the water was free from colour and contained very little organic matter. This water was chlorinated at one plant by allowing the bleach solution to drop into one vertical limb of a syphon approximately 6,000 feet long, the other vertical limb being used as a suction well for the pumps which discharged into the distribution mains. At the other plant the bleach solution was injected into the discharge pipe of a reciprocating pump through a pipe perforated with a number of small holes. The results for two typical months are given in Table X.

TABLE X.--EFFECT OF EFFICIENT MIXING

------+-----------+-----------------------+--------------- | Available | BACTERIA PER C.CM. | | Chlorine +-----------------------+ | Parts Per | | | B. Coli Index Month.| Million. | Raw |Treated Water. | Per 100 c.cms.

+-----+-----+ Water.+-------+-------+-------+------- | A. | B. | | A. | B. | A. | B.

------+-----+-----+-------+-------+-------+-------+------- July | 0.20| 0.25| 864 | 27 | 93 | <0.2 |="" 8.5="" august|="" 0.20|="" 0.27|="" 1.108="" |="" 12="" |="" 120="" |=""><0.2 |="" 10.2="" ------+-----+-----+-------+-------+-------+-------+-------="" a="efficient" mixing.="" b="inefficient">

The results with the "B" plant were very irregular. The hypochlorite and water did not mix thoroughly and, as several suctions pipes were situated in the suction shaft, there was no subsequent admixture in the pumps; this also caused complaints regarding taste and odour but the complaints were localised, and not general as would result from an overdose of solution due to irregularities at the plant.

The second example deals with a water containing 40-45 p.p.m. of colour.

This supply was taken from the river by low-lift pumps and discharged into a header which was connected with the high-lift pumps by two intake pipes about 5,000 feet in length. During 1914 a baffled storage basin of two hours capacity was constructed and in June the hypochlorite was added at the inlet to this basin by means of a perforated pipe. The object was to increase the contact period prior to the delivery of the water into the header. The results for this month were as follows:

AVAILABLE CHLORINE 1.88 PARTS PER MILLION

------------------------+-------------------------+---------- | BACTERIA PER C.CM. AGAR.| +------------+------------+ B. Coli.

| 3 Days at | 1 day at | Index | 20 C. | 37 C. | Per c.cm.

------------------------+------------+------------+---------- Raw water | 410 | 104 | 0.280 Treated water | 49 | 26 | 0.036 Percentage purification | 88.2 | 75.0 | 87.5 ------------------------+------------+------------+----------

During August the point of application of the hypochlorite was changed from the inlet of the basin to the suctions of the pumps and the solution proportioned to the amount of water pumped by the starch and iodide test. The average of the daily tests for this month were:

AVAILABLE CHLORINE 1.55 PARTS PER MILLION

------------------------+-------------------------+------------ | BACTERIA PER C.CM. AGAR.| +------------+------------+ B. Coli.

| 3 Days at | 1 day at | Index | 20 C. | 37 C. | Per c.cm.

------------------------+------------+------------+---------- Raw water | 448 | 100 | 0.600 Treated water | 26 | 12 | 0.005 Percentage purification | 91.9 | 88.0 | 99.2 ------------------------+------------+------------+----------