SCG GE, 
HE SELF-PURIFICATION OF RIVERS. 
bs JHARLES V. CHAPIN, M.D., SUPERINTENDENT OF 
. HEALTH, PROVIDENCE, R. I. 
A GREAT deal has been said and written about the self- 
purification of rivers, and both the advocates and oppo- 
nents of the theory that rivers readily purify themselves 
have taken a very firm stand in the matter. The ques- 
tion, however, is not one which is easily settled. <A 
great many factors must be taken into consideration be- 
fore we can reach a just conclusion. The question of 
dilution is an important one. A polluted stream may ap- 
parently become purified when in reality there is simply a 
dilution owing to the addition of pure water either from 
tributaries or from springs. On the other hand a river 
may apparently continue without showing any signs of 
improvement, the improvement being masked by the 
addition of pollution at various places. It is very seldom 
that in these discussions this factor has been taken into 
*‘account and more rarely still that actual measurements 
have been made, for it is no easy matterto do this. In 
the first place the amount of water entering the stream 
between the two points where the tests are made must be 
approximately known, whether it comes from springs or 
tributaries. Then the character of this addition must be 
known, for in most cases the water of tributaries contains 
very appreciable amounts of pollution. 
Another difficulty is the collection of samples. It is 
evident that if samples are taken from different stations 
on the same day and hour the sample from the lower 
station will not by any means necessarily represent the 
water at the upper station. And even if allowance be 
made for the flow of the river, it would be extremely 
dangerous to assume that the water passing the lower 
station at the calculated time was really the same as that 
passing the upper station. Yet this method of taking 
samples at different times should in all cases be followed, 
though it is by no means the invariable custom. 
Another point to be borne in mind “is in regard to the 
chemical analysis of the water. ‘Tests should be made 
upon the water both before and after filtration through 
filter paper. In estimating the free ammonia we must 
take into consideration also the oxidation of albuminoid 
into the free and the escape of the latter into the air. It 
is very seldom indeed that all these factors have received 
due consideration. Perhaps the most careful study of the 
chemical self-purification of rivers has been made upon 
the Blackstone by the Massachusetts State Board of 
Health. A large number of samples were taken with much 
care during a period of several years, and allowance was 
made for the addition of natural impurities by tributaries. 
No allowance was made for the addition of sewage 
impurities by tributaries, but from a study of the 
chlorine contents it can fairly be assumed that 
the amount is small. The first sample was taken 
just below Worcester, which is the point of pollu- 
tion, and the last below Millville, twenty-three miles 
below. The average of all the tests showed a diminution 
of free ammonia from 1728 to 1299 parts, of albuminoid 
ammonia from 826 parts to 382, an increase of nitrogen 
as nitric acid from 218 to 457 and a diminution of total 
nitrogen from 3000 to 2156. ‘There has evidently then 
been a very considerable improvement as regards the 
organic contents of the water, and this notwithstanding a 
slight addition by tributaries not allowed for. How much 
of this improvement is due to sedimentation it is impos- 
sible to say, but the increase of nitrates shows that it is 
largely at least due to a true oxidation. © Other observers 
on other rivers have obtained different results,as Percy 
Frankland from the Thames and Ure, but he does not 
appear to have taken sufficient account of the addition of 
Vol. XXIII. No. 581 
pollution by tributaries. On the whole the work of the 
Massachusetts Board is the most thorough published, and 
from it we can fairly assume that in a river moderately 
polluted there is in the course of a few miles a very con- 
siderable destruction of dissolved organic matter by 
oxidation, as well as loss of solid particles by subsidence. 
The aeration of the water of rivers in falling over 
dams and natural obstructions has been supposed to exert 
an important influence in purification, but this supposition 
is not correct; for while dissolved oxygen is necessary 
for the working of certain purifying changes, an excess 
of oxygen, introduced by however so efficient aeration, 
has been shown not to extend such process. Careful ex- 
perimenting in the artificial aeration of water by the 
Massachusetts State Board of Health has demonstrated 
that it has little or no effect in decreasing the organic in- 
gredients, and Professor Leed’s experiments upon the 
water both above and below Niagara Falls, where a na- 
tural aeration is carried on On a most stupendous scale, 
showed that there was no chemical purification. 
But by far the most important consideration when 
river-water is to be used for drinking is the presence of 
pathogenic micro-organisms. We know that certain dis- 
eases may be transmitted by means of the contamination 
of water with their specific poison, and it is known that 
the poison consists of bacteria. Cholera: and typhoid 
fever are diseases of this type. If the intestinal dis- 
charges from persons suffering from these diseases find 
their way into a river and thus reach a public water supply 
the most disastrous consequences may occur to the users 
of the water. The vital question is, do these bacteria 
disappear as the result of natural agencies at work in the 
river, and if so, how longan interval is required to accom- 
plish this result. It is well known that most pathogenic 
bacteria do not propagate in ordinary river water, but on 
the other hand tend to die out. But they die slowly and 
may live for days and perhaps weeks. Investigation 
alone can show whether in running rivers there are 
factors which operate more unfavorably than in confined 
waters. Observations on the bacterial contents of a 
river are not readily made, some of the difficulties of 
chemical analysis holding for biological as well, and ob- 
servers are not all agreed upon results. 
Frankland found in the Thames and Ure that there was 
nO apparent diminution in the number of organisms as 
the river flowed along. Elaborate tests were made on 
the River Spree at Berlin by Frank. Specimens were 
taken at fifteen different stations above, below and in the 
city and subjected to bacteriological examination. It 
was shown that although sometimes hundreds of thou- 
sands of bacteria were added to each centimetre of water 
flowing through the city the number was in the course of 
a few miles reduced to 3000 to 8000, about the same as 
above the city. It is probable that this result was due 
largely to sedimentation. 
In the Isar, thirty-three kilometres below Munich, the 
point of pollution, the bacteria were reduced from 15,231 
to 2378. 
The following is the average result of ten tests on the 
Limmat, which flows out from Lake Zurich, past the 
sity he same name. 
city of the same NUMBER OF BAC- 
STATIONS. DISTANCE. TERIA PER C. C. 
Outflow from lake, o 225 
Station 1, 1.86 noi 
Sewer outlet, 2.175 296,670 
Station 4, 2.485 12,870 
Ph ae 2.7096 10,892 
meen Oe RANG) 5,902 
sor 5-903 4,208 
sbameiaO 3 6.214 2,346 
ees 8.078 2,110 
