The Pollution of Ice Supplies.
In the annual report of the Massachusetts State Board of Health are given the results of a special investigation made by direction of the legislature into the matter of the pollution of bodies of water in the State used as sources of ice supply with reference to the effect of such pollution upon the wholesomeness of the ice.
“The collection and sale of ice in Massachusetts,” says the report, “is one of the well-established industries of the State The entire ice product of the State as stated in the State census of 1885. was a little less than one million tons (980,170), valued at $1,672,932. Since ice is largely employed, not only for the preservation of food, with which it often comes into direct contact, but also for the cooling of water and other beverages, in which it is frequently melted or dissolved, it is a matter of great importance that it should be obtained from sources that are free from pollution.
“The lakes, ponds and streams from which ice is cut are in many instances in the immediate neighborhood of populous towns and villages; and while these sources may have been comparatively pure in earlier years, in the beginning of ice collection, the increasing density of population has in many ways contributed to the pollution of such sources in such manner as to greatly imperil them for use as sources of ice supply. The purity of water supplies has for many years been considered an essential condition to the health of communities making use of them, but it has not been until a more recent period that the same condition was also considered as essential to a good ice supply,
“It was atone time a popular belief that all impurities were excluded from ice in the process of freezing. Recent observations, however, have proven that while a considerable portion of the impurities of water are excluded or disappear in the process of freezing, it is equally true that some of the more dangerous impurities may survive the ordinary process of freezing and reappear in the melted ice.” Samples of water and ice from many sources were collected and analyzed. As the different parts of a cake of ice may differ much in quality a division was made with snow-ice, “bubbly-ice” and clear ice.
From the examinations that have been made, it appears probable that, when ice first forms on the surface of a pond or river, a considerable part of the impurity in the water near the surface is entangled in the first inch or less in depth, and that the ice that forms below this first inch contains but a very small percentage of the impurities of the water. If snow falls upon the thin ice, causing it to sink, so that water from below saturates the snow, it will freeze without purification; or, if rain falls upon the snow and freezes, the ice thus formed contains the impurities of the snow and of the rain water and whatever else may have settled out of the air. The method often pursued of flooding the ice of a pond or river by cutting holes through it, gives a layer of ice as impure as the water of which it is formed.
The purifying effect of freezing is greater upon substances held in solution than upon those in suspension. For example, upon freezing the upper part of a body of sewage to a depth of an inch it was found that an average of eleven per cent of the impurities in solution remained in the ice, while of the much smaller parts in suspension an average of forty-nine per cent remained. The unfrozen sewage under the ice contained the impurities which the ice had expelled.
It appears that the parts that are in suspension, particularly particles that have some buoyancy in water, are not so easily expelled as the parts that are in solution. This is confirmed by the fact that a large part of the organic matter, one-half or three-quarters and sometimes more, that is found in good ice, is of particles in suspension, and is readily removed by filter paper.
The inch in depth of frozen sewage contained ten per cent of the organic impurity of the sewage, as indicated by the sum of ammonias; and from other experiments we have reason to conclude that, if another inch in depth had formed under the first, it would have contained a still smaller percentage of organic impurity; but if the first inch had been pressed down, and the sewage had risen above it and then frozen, this last layer would have been as impure as the sewage. This is an extreme case of impurity of the source.
Taking an average of all of the water and ice used for ice supplies which we have examined wc find that the organic impurities of the snow ice, as shown by the sum of ammonias, amount to sixty-nine per cent of those of the waters; that the organic impurities ot all the ice, except the snow ice, amount to twelve per cent, and those of what we have called clear ice amount to six per cent, of the impurities of the waters. The color of the waters was entirely removed, and the salt that they contained was nearly all removed by the process of freezing.
There were eighty-one per cent as many bacteria in the snow ice as in the waters; ten j>er cent as many in all other ice, and two per cent as many in the clear ice as in the waters.
While the board, as before stated, was unable in these warm wdntcrs to make the experiments desired to settle many points of the inquiry, the results obtained lead to the conclusions that, while clear ice from polluted sources may contain so small a percentage of the impurities of the source that it may not be regarded as injurious to health, the snow ice and any ice, how. ever clear, that may have been formed by flooding, is likely to contain so large a percentage of the impurities of the source, and with these impurities some of the disease germs that may be in the source, that the board feels bound to warn th« public against using ice for domestic purposes that is obtained from a source polluted by sewage beyond that which would be allowable in a drinking water stream or pond; and that in general it is much safer to use, for drinking water and for placing in contact with food, that portion of the ice that is clear.
As to the small number of bacteria remaining in the ice, it is not safe to depend upon numbers alone.
A large number of bacteria of one kind may be harmless, and a small number of another kind may communicate a most serious disease. It is known, from these experiments as well as from others, that many kinds of bacteria survive a long season in ice; and it has been shown by Dr. Prudden that the bacillus of typhoid fever will live in decreasing numbers in ice for three months at least. It is, then, the quality of the bacteria rather than the quantity that we are to consider, and the best judgment in regard to this includes the source from which they came. If the source is one which is liable to be polluted by disease-producing bacteria, as is likely to be the case whereever sewage enters, this fact should have much more weight than the small number of bacteria found.