The principle involved in this method of sewage purification is that of any general method of sewage reduction in whatever form carried on, namely, its oxidation or nitrification. This oxidation, or breaking down of the organic matter in the sewage, is accomplished in this case, as in the case of intermittent sand filters, contact beds, and sprinkling filters, through the agency of bacterial action.

Householders have long been familiar with the fact that although solids contained in sewage may have been discharged for long periods of time into a cesspool, the latter, if located in dry, porous soil, did not seem to become filled with the solid residue. This is due to the liquefaction of the solid matter in the sewage after its discharge into the cesspool, and to the seepage and bacterial reduction of the liquid matter in the surrounding soil. To replace the cesspool and eliminate the insanitary conditions which, in most instances, result from its use, other methods have been devised which utilize the agencies of nature to the best advantage. Thus the sedimentation and, in some cases, the liquefaction of the solid matters in sewage are carried on in specially designed settling tanks which are easily cleaned and which provide for greater efficiency in settling out suspended matters than the cesspool. Similarly, the filtration of liquids from cesspools through the soil is replaced by the scientific method of sub-surface irrigation, which is much more efficient in three distinct ways: (1) the limited seepage area represented by the walls of the cesspool is increased many times by distributing the effluent from the settling tank over a large area of soil in a system of sub-surface tiling; (2) the bacterial reduction is more effective, since it has been found that the bacterial action necessary to purify sewage takes place in the upper layers of the soil and is almost absent at depths of five feet or more; (3) the soil is given an opportunity to rest and to dry out by alternately using different portions of the sub-surface irrigation system. In the cesspool the seepage of the effluent and whatever bacterial action takes place in the surrounding soil must go on continuously, which often results in clogging of the soil and overflowing of the cesspool.

The purification and final disposition of sewage by means of sub-surface irrigation is the method best adapted to the single residence, and oftentimes to the hotel or institution, if soil conditions are favorable and proper area is available. This system requires less oversight in its operation than the various forms of artificial filters. Furthermore, the sewage is entirely hidden from sight after it leaves the settling tank, and this is usually desirable near private residences and on the grounds of country homes, country clubs, and summer hotels. Also, where the sewage must be treated in close proximity to a residence or hotel or at a point on the windward side of a residence, this method, more effectually than any other, precludes the possibility of a nuisance resulting from the operation carried on, since the settling-tank effluent is at no point exposed directly to the air. Furthermore, its cost is less than that of other works for final treatment of sewage, and, finally, the system is more easily installed.

The method is in reality modified broad irrigation, but the sub-surface irrigation field can be utilized much more effectively and with considerably less attention than a broad irrigation area, and, as noted above, is less liable to be the cause of a nuisance or to be the means of spreading infectious disease through the agencies of flies and other insects.

If an area of sandy soil is available on which to locate the sub-surface irrigation field, if the settling tank and siphon chamber have been correctly built, and if the sub-surface tiling system has been properly laid, the success of the system is well assured. On the other hand, failure is certain if either broad irrigation or sub-surface irrigation methods of sewage disposal are attempted on stiff, impervious clay soils. Between the ranges of porosity of soil represented by these limits there are many soils in which sewage may be successfully disposed of by sub-surface irrigation. A sandy or gravelly loam will, without question, successfully care for sewage effluent when such effluent is properly distributed by sub-surface tiling, and even in a rather heavy soil the effluent from a settling tank may often be disposed of satisfactorily by providing for a greater length of sub-surface tiling per person served by the settling tank than that which would suffice in the more porous soils. However, if the soil is very heavy so that surface water does not readily seep away, or if the ground-water level is within two or three feet of the surface, this method is not suitable and some form of filter, described in the succeeding chapter, should be used for final treatment of sewage.

When soil conditions and the area available are favorable to this method and such a system is to be installed, the irrigation area selected should be at the point where the ground-water level is lowest, and this will generally be on a plateau or bench at the head of a slope of ground. The relative elevation of the ground surface should, of course, be low enough to insure operating head or fall to operate the siphon in the chamber adjoining the settling tank and to distribute the effluent by gravity to the sub-surface tiling. If the soil is composed of loose gravel, or lies over a limestone or shale formation, the location of the irrigation area should be selected with a view to preventing the contamination of any wells or springs which may exist on the premises,—that is, the area should be on lower ground, and as far removed from wells as is convenient.

As will be explained later, the length of sub-surface tiling necessary to receive a given quantity of sewage effluent should vary, within certain limits, with the character and porosity of the soil, thus requiring larger quantities of effluent to be delivered from the siphon or dosing chamber in the case of the more compact soils. Also the size of this chamber should be determined with reference to the number of sections into which the sub-surface tiling system is divided.

TABLE II
For Use in the Construction of Sub-surface Irrigation Systems in Sandy or Very Porous Soils
Dimensions given are for inside measurement.
Number of Persons Served by Sewer.Total Length of 3–inch Sub-surface Tiling.No. of Sections in Sub-surface System.Mean Width of Siphon Chamber (Feet).Mean Length of Siphon Chamber (Feet).Depth of Siphon Chamber from Roof of Tank (Feet).Diameter of Siphon (Inches).Discharging Depth or Maximum Depth of Effluent above Lower Edge of Siphon Bell (Inches).Distance from Roof to Top of Wall between Settling Tank and Siphon Chamber (Inches).
414022´2´2´ 4˝3˝13˝12˝
828022´ 6˝2´ 6˝2´ 4˝3˝13˝12˝
1242022´ 6˝4´2´ 4˝3˝13˝12˝
1552523´ 6˝4´2´ 4˝3˝13˝12˝
2587523´4´3´ 2˝5˝23˝12˝
35122523´ 8˝4´ 6˝3´ 2˝5˝23˝12˝
50175024´6´3´ 2˝5˝23˝12˝
75262523´6´3´ 2˝5˝23˝16˝
100350024´7´3´ 2˝5˝23˝16˝
125437535´ 6˝8´3´ 2˝5˝23˝16˝
150525037´8´3´ 2˝5˝23˝16˝
175612537´ 6˝8´3´ 9˝6˝30˝16˝
200700038´8´3´ 9˝6˝30˝18˝
TABLE III
For Use in the Construction of Sub-surface Irrigation Systems in the Heavier Loams (not Clay or Hardpan)
Dimensions given are for inside measurements.
Number of Persons Served by Sewer.Total Length of 3–inch Sub-surface Tiling.No. of Sections in Sub-surface System.Mean Width of Siphon Chamber (Feet).Mean Length of Siphon Chamber (Feet).Depth of Siphon Chamber from Roof of Tank (Feet).Diameter of Siphon (Inches).Discharging Depth or Maximum Depth of Effluent above Lower Edge of Siphon Bell (Inches).Distance from Roof to Top of Wall between Settling Tank and Siphon Chamber (Inches).
43002 2´ 5˝ 3´2´ 4˝3˝13˝12˝
86002 3´ 5´2´ 4˝3˝13˝12˝
129002 4´ 5´2´ 4˝3˝13˝12˝
1511252 4´ 6´ 6˝2´ 4˝3˝13˝12˝
2518752 4´ 6´3´ 2˝5˝23˝12˝
3526252 4´ 4´ 6˝3´ 2˝5˝23˝16˝
5037502 4´ 6˝ 6´3´ 2˝5˝23˝16˝
7556252 6´ 7´3´ 2˝5˝23˝16˝
10075002 7´ 8´3´ 2˝5˝23˝16˝
12593753 8´10´3´ 9˝6˝30˝20˝
150112503 9´12´3´ 9˝6˝30˝20˝
17513125310´12´4´ 2˝8˝35˝20˝
20015000311´12´4´ 2˝8˝35˝20˝

The dimensions of siphon chambers to effectively deliver the effluent in proper volumes to the sub-surface irrigation system are given in the following tables, which indicate widths of siphon chambers to agree in general with the widths of the settling tanks to serve a given number of persons, as shown in Chapter II. These tables of dimensions for siphon chambers provide for two different capacities where the same number of persons are served by the sewer, depending on the total lengths of sub-surface tiling required, which in turn depend on the character of the soil in which the sub-surface system is laid. The tables provide for a division of the sub-surface tiling system into two parts up to a system for 100 persons, and into three parts for a greater number of persons. These tables also show the total length of lateral distributing tiling in the sub-surface irrigation system necessary to distribute over a sufficient area at the irrigation field, in both sandy soils and in the heavier loams, the various quantities of sewage to be treated in the different-sized tanks and discharged from the siphon chambers. The tables also indicate the diameter of the siphon and the discharging depth of each siphon.