This shows a mean decrease of almost 93% in the number of bacteria in 24 miles; probably largely due to sedimentation, which is encouraged by the sluggish flow. The table also illustrates the slow intermingling of sewage and stream under such conditions. With a rapid, current the intermingling would be more thorough, but the stream might flow many times this distance before losing the same number of bacteria. What percentage of the bacteria remaining are pathogenic is not known; but there seem to be good reasons for supposing that the percentage of these removed is greater than of the non-pathogenic. (See also page 27.)

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ART. 94. BROAD IRRIGATION.

Broad irrigation means the distribution of sewage over a large surface of ordinary agricultural ground, having in view a maximum growth of vegetation (consistently with due purification) for the amount of sewage supplied. Filtration means the concentration of sewage at short intervals, on an area of specially chosen porous ground, as small as will absorb and clean it, not excluding vegetation, but making the produce of secondary importance.' (Royal Commissioners on Metropolitan Sewage Discharge.) No more definite line could be drawn between irrigation and filtration than is indicated by these definitions. In many plants the same land is used alternately for both methods. The nitrates which would pass. off with the effluent in filtration are to a certain extent (10% to 20% probably) absorbed by vegetation.

In broad irrigation much of the sewage must at times be diverted from the crops-as in rainy weather or after the fruit has matured. If this is not done, the crops cannot be raised to advantage. In some locations it will not be seriously objectionable to turn the sewage at these times into the streams, particularly in rainy weather when these will be in flood; but where this is not permissible provision must be made to treat the sewage otherwise, as on filtration-beds. If this plan is adopted sewage should be turned upon the filtration-beds two or three times a week to keep alive in them the nitrifying bacteria.

Irrigation-fields are ordinarily odorless, but on close, humid days in summer the moist deposit on the surface gives off an appreciable dish-water smell, which, however, is seldom noticeable more than 100 yards from the field. The intensity of the odor seems to increase not directly with but as the square or some higher power of the area irrigated. It is not advisable to place such grounds in the midst of a settled community, but a quarter of a mile should be sufficient intervening space.

Sewage is used in irrigation much as water is, except that it should not come into direct contact with berries, celery, cabbage, or the edible portions of any plant. In some cases, generally where grass of some kind is grown, the sewage flows slowly all over the land in a thin layer. Where corn or vegetables are grown they are usually planted on the narrow ridges between ploughed furrows into which the sewage flows, and where it stands, soaking downward and sideways into the soil. The roots of vegetation and the vegetable mould which forms on the surface of the ground prevent the rapid absorption of the sewage, and unless the subsurface soil be clayey or quite non-porous, sub-drains are not often necessary, but ditches are carried through the farm at intervals to receive the drainage. If the sewage is not clarified before being

applied to the soil, an impervious skin shortly forms, composed of filaments of paper, rags, and similar matters, together with grease and the more stable organic matter; and this must be frequently removed if the ground is to be re-aerated and kept absorptive. This matter, which has little odor, can be piled in a dry spot and burned occasionally.

If the ground is not level, the furrows should follow contours, that the sewage may stand in them. If, on a sloping land, furrows are not desired, the catchment system may be employed. In this a series of ridges following the contours are placed at intervals of 15 to 100 feet down the slope; the sewage is held behind each ridge until it overflows it, when the surplus runs over the surface until intercepted by the next ridge. The object of the ridges is to prevent the sewage from gathering into channels and attaining erosive velocity. Hence the steeper the land the closer should be the ridges to each other. This method was adopted at Wayne, Pa., on a steep rocky hill 100 feet high with a soil of micaceous loam.

The ridge-and-furrow system is particularly applicable to evel land. In this system the ground is divided into beds. sloping from a central ridge to gutters or furrows on each side, each furrow being common to two adjacent beds. Another furrow for distributing the sewage runs along each ridge, from each side of which the sewage overflows in a thin sheet. The beds are generally 15 to 20 feet from each ridge to either furrow, and of any convenient length. The slope of the beds. is a matter of judgment, being steeper the more porous the soil in order that the sewage may be evenly distributed.

Sewage is in some cases distributed through main carriers of iron or of vitrified pipes, under pressure produced either by gravity or by pump, to hydrants, as at Pulman, Ill.; through vitrified pipes by gravity, as at Summit, N. J.; or through open channels, lined with concrete or with split pipe;

and in many recent works the channels are used without any lining whatever. From the main carriers the sewage is diverted by means of simple gates to secondary carriers, which are often but ploughed furrows, the location of which is changed when they become clogged with sewage. These furrows should be closer together the more pervious the soil, to effect uniform distribution. If the subsoil is clayey, or the water-table is near the surface, it may be necessary to lay sub-drains. These are generally placed under the ridges if the ridge-and-furrow method is used. From 3 to 6 feet is the customary depth, depending upon the porosity of the soil and the crops grown. Sub-drains cannot be used near osiers,

since these root deep and stop up the drains.

Open, porous soils are best adapted to irrigation; although they should not absorb the sewage faster than 25,000 to 30,000 gallons per acre per day to obtain good results from crops. But if the crops are only an incident (" intermittent filtration"), the more porous the soil the better. Clay land may be improved for irrigation by ploughing-under ashes or sand, but can never be made as desirable as naturally porous soil. The sewage from 50 to 150 or 200 persons can be used for irrigating one acre, depending upon the quality of the soil. At the Paris sewage farm at Achères 11,766 gallons per acre daily is fixed as the limit, but this is largely streetwater. At Berlin the population contributing to each acre of the irrigation-fields is 156.

ART. 95. CROPS.

Crops of all kinds have been grown on sewage farms. Italian rye-grass seems particularly well adapted to this purpose, absorbing sewage indefinitely and growing so closely as to choke out weeds, but is not very hardy in this country north of Washington, D. C. It is grown in flat beds.

It

makes excellent fodder and is a good crop for dairy farms,* but when cut can be kept only by ensilage. It is sown at the rate of 45 to 50 pounds per acre.

In the northern United States corn has given excellent satisfaction. At South Framingham, Mass., 100 bushels of shelled corn per acre has been grown; at Brocton, Mass., 70 bushels is obtained. The corn is grown in hills 3 feet apart, the ridges being about 4 feet apart, and is irrigated through the furrows.

Wheat has been grown at the Salt Lake City farm, 36 bushels per acre, and barley 28 bushels per acre; but cereals are apt to develop stalk rather than grain on sewage farms. Walnuts give good results in Pasadena, Cal. Cabbages,

parsnips, carrots, potatoes, rhubarb, turnips, cauliflower, celery, onions, squashes, beans, peas, asparagus, as well as other garden truck, and tobacco, have all been grown on sewage farms, as have timothy, alfalfa, and other grasses. Only actual trial in a given section of country will determine the crop which there grows best and finds the best market.

Meadow-land at Paris (Gennevilliers) is uninjured by a flow of 50,000 gallons per acre per day. Lucerne grass takes 36,000 gallons; artichokes 12,000 gallons; flowers and parsley 11,000; leeks, cabbage, and celery 7000; beets, carrots, and beans 4000; potatoes, asparagus, and peas 3000 gallons per acre per day.

* Dairy products are considered by many English cities the most profitable yet tried; Birmingham selling $20,000 to $30,000 worth of milk annually from its sewage farm.