173. The impurities which occur in ordinary waters are of two kinds; namely, mechanical, or those held in suspension by the water; and physical, or those held in solution. The mechanical impurities are mud, leaves, vegetation, fish, frog spawn, insects, insect eggs, etc. The physical impurities are solutions of minerals, putrescent animal matter, albuminous slimes, etc. The teachings from privy vaults and drains are the most harmful poisons that usually get into the water supply.

The mechanical impurities are far less dangerous. They are easily seen and may be removed by passing the water through a bed of sand. This will strain out everything which is visible. The danger, however, lies mainly in the pollutions which are invisible. Mineral poisons can be neutralized by the use of chemicals, and sometimes by heating and settling. The organic poisons from sewage, etc. can be removed only by careful filtration through sand to remove all mechanical impurities, etc., and then through bone charcoal. This material exerts a chemical action upon the organic matter in the water, and renders it inert or harmless. The charcoal, however, gradually becomes saturated and clogged with the refuse, and loses its chemical powers. Therefore, it must be renewed at intervals.

The animal charcoal is made from bones, and is hard and dense. When its pores become clogged with refuse, it can be restored to usefulness only by reburning. There is no practicable way by which this can be done upon a small scale. Unless the air is carefully excluded during the whole process, the material will be consumed, like other charcoal, and will be destroyed.

Charcoal which is made from wood has little or no value for the purpose of filtration.

174. Water which has grown stale by standing may be greatly improved, and be made suitable for drinking purposes, by the process called aeration, provided it has not been otherwise polluted.

Aeration may be accomplished in several ways. The water may be squirted into the air in fine streams; air may be forced through the water in fine bubbles; or air and water may be shaken up or otherwise agitated together. The object to be attained in every case is to expose the water to the action of the air to the greatest practicable extent.

In the process of aeration the water absorbs a considerable quantity of air, and is thereby greatly improved in appearance and taste. The air has a mild oxidizing effect, which is sufficient to destroy a small amount of vegetable matter and render it harmless. But this purifying influence is very limited in extent, and is of no use whatever for removing or destroying the germs of putrefaction, fermentation, and disease which are imparted to the water by sewage or house drainage. These germs can be killed only by boiling, and some certain disease germs cannot be certainly killed even in that way.

The process of aeration is thus adapted only to the purpose of freshening water and rendering it more palatable, and is not serviceable for actual purification.

In all apparatus designed to aerate water, care must be taken to thoroughly exclude all dust from the air, because dust is very apt to carry with it many kinds of germs which give rise to putrefaction and disease. Dust must be kept out of food and drinking water.

175. Rain water which is taken from the roofs of buildings is always more or less contaminated with leaves, dust, excreta of birds, dead insects, etc. If it is desired to use any of this water for drinking or cooking purposes, the pipes which lead it to the cistern, or the cistern itself, should be supplied with a device called a rain-water cut-off. This is an apparatus which turns all of the water from the rain leaders to waste until the roofs are washed clean, and then turns the water into the cistern. The water which falls during the first few minutes of a rain storm is loaded with dust and insects, but after twenty minutes or so it is usually very pure.