therefore probably be shown if each quantitative determination for the effluent, diminished by 30% of the amount of the same substance found in normal ground-water, be multiplied by about 1.45.

It appears from analyses 5 and 6 that the free ammonia in sewage increased during its flow through the sewers at the expense of the albuminoid. Also that the number of bacteria was practically doubled.

It is believed that this increase in

this and in all sewage is of non-pathogenic bacteria only, since sewage appears to be an unfavorable breeding-place for the pathogenic varieties.

The result of a bacterial analysis is generally stated as a certain number of bacteria per cubic centimeter. This number frequently runs up into the millions, of which it is evident. that no direct count could have been made. To obtain practi cable conditions a small amount of sewage is diluted with 1000 to 100,000 times its volume of sterile water, and the number of bacteria found in this mixture per cubic centimeter is mul tiplied by the proportion of dilution. How many of the bacteria are pathogenic it is impossible to say with our present knowledge of bacteriology and methods of analyzing; for the finding of the bacterium of typhoid fever or cholera in sewage is an unusual occurrence, so few are they in comparison with. the total number present. If there was one such bacterium in each cubic centimeter of a given sewage, and this was diluted 10,000 times for analysis, the chance of this bacterium. being present in the analyzed sample would be but one in ten thousand; but if this sewage be discharged into 50 times its volume of water, each glassful of this would be likely to contain five or six typhoid bacilli. It is therefore apparent that the absence of pathogenic bacteria from an analyzed sample by no means indicates that they are not present in the sewage in great numbers. This is of little importance in an analysis of sewage, since it should be assumed that the excreta of a

typhoid patient containing millions of these may at any time enter the sewer. It is desirable, however, to learn to what extent such bacteria are removed by purification or otherwise, and on this point there is still great uncertainty. But it is in general assumed that any reduction in the total number of bacteria is at least no greater than that in the number of pathogenic ones in proportion to the number originally present. It is now thought that liquefying organisms have a germicidal effect upon typhoid bacilli; and also that the latter increase in number in sewage but slowly, if at all; consequently that they disappear even more rapidly than a general analysis would indicate.

It is desirable to distinguish between aerobic, anaerobic, and facultative bacteria (see page 398), and between the liquefying and non-liquefying; largely because of the effect of these in the decomposition and purification of sewage. Also to ascertain the presence or absence of B. coli communis, especially in the case of a stream in which the presence of sewage is suspected, since these are considered almost positive proof of such pollution.

The incubator test of purified effluent and of diluted sewage has come into prominence in the past two or three years, notably through the Manchester (England) investigation of sewage purification. This test consists of determining the oxygen absorbed by a sample; then completely filling a bottle with the same and placing it in an incubator at 80° F. for five days; after which it is again tested for oxygen absorbed. Increase in this is an indication of putrefaction during incubation; but if the sample has remained sweet there will be a somewhat less amount of oxygen absorbed after incubation. An effluent or diluted sewage which remains sweet after this test is in no danger of further putrefaction-that is, will not create a nuisance—unless further polluted. This test is not applicable to effluents discharged into streams to be afterwards used for water-supplies.

ART. 88. AIMS OF TREATMENT.

The aim of any treatment of sewage may be either to prevent the creation of a nuisance or to produce an effluent which, if discharged into a river, will not render it unsuitable for city supplies. The former case may exist where the sewage is discharged into a river, a lake, or a salt-water bay; the latter where into potable fresh water only. The purification must be considered from both the chemical and bacteri

ological sides. For either of the above cases a standard of purity is most difficult to decide upon, although many standards have been advanced.

Where it is desired only to prevent a nuisance the bacteriological condition need hardly be considered, unless oysters or other shell-fish are reached by the effluent. In such a case also the purification need be carried to such a point only that all matters in suspension are removed and danger of future putrefaction averted.

The maintaining of a river-water potable, however, calls for a much higher standard. To be perfectly safe it would seem, from our present knowledge, that all bacteria should be removed, since we are not certain that some of those escaping are not pathogenic. (See also Art. 12.) The removal of 99.98% of the bacteria, however, probably reduces the chance of infection by at least that amount: and if the effluent be then diluted with ten times its volume of pure water, the chance of infection by drinking such dilution would be but one fifty-thousandth that by drinking the sewage. The only standard for the permissible number of bacteria in sewage effluents is the least possible. It may be possible to sterilize sewage, but since bacteria are necessary for the liquefying and oxidation of organic matter in the sewage, this would mean only a temporary delay in decomposing such matter, and

This

would leave it and the poisonous by-products of putrefaction to create a nuisance and to produce enteric diseases. might be avoided by completely sterilizing (if such a thing is practicable) the effluent from a process by which a sufficient percentage of the organic impurities have been removed to permit of complete oxidation by dilution. But sterilizing to avoid decomposition should not be attempted, since decomposition must precede any purification, and in most cases the sooner it occurs the better. It need not involve the giving off of noxious vapors or odors, if proceeding in the presence of sufficient oxygen..

Several chemical standards have been suggested. The Rivers Pollution Commission (England) in 1868 recommended as a limit for effluents discharged into streams:

Mr. F. P. Stearns has given .0080 parts of albuminoid and .0399 of free ammonia as quantities below which nuisances have not been known to result, and .0233 parts of albuminoid and .1116 of free ammonia as limits above which nuisances will be created, if the pollution be from sewage. If the impurities are of a more stable character, it is possible that these limits may be exceeded. Most authorities consider, however, that no general standard can be set for all effluents.

The standards given are for prevention of nuisance only. Any standard, however, which does not take into account the condition of the stream into which the effluent is to discharge,

is incomplete. A more reliable rule would be to ascertain the amount of free oxygen in the diluting stream passing the effluent outlet per second, and to permit no more unoxidized organic matter to reach such stream per second than can be fully oxidized by one-half to three-fourths of this amount of oxygen (since the intermingling of sewage and stream probably will not be complete). So far as all organic matter except bacteria is concerned, the above standard would also insure a safe potable water if time and opportunity for complete intermingling and oxidation be afforded.

It should be remembered that the presence of chlorine in excess of the local normal is generally an indication of past sewage pollution; nitrates indicate the amount of organic matter rendered innocuous; and albuminoid ammonia is taken as an index of the polluting organic matters still present. The character of an effluent should not be judged alone by its appearance, by its chemical or its bacteriological analyses, but by all three combined; since it may be clear, but contain many pathogenic bacteria, or dissolved matter which may be precipitated or putrefy and create a nuisance; also a turbid effluent may contain only mineral matters or such organic ones as will undergo no change but oxidation.