Night-soil is generally disposed of by being buried in trenches, but these are frequently too deep: purification largely depends on the action of the Ærobic bacilli, that is, the group of microbes which live close to the surface of the soil and require air for their existence; these swarm in the top layers of the earth, but are not found at a greater depth than 1 foot and diminish enormously in number as this is approached. In order to obtain the best results, both from a purificative as well as a manurial point of view, the burial should be merely superficial and not exceeding in depth the limit of ordinary cultivation, in order to ensure the ground being thoroughly aËrated.

The sites for trenching grounds should be carefully selected; light sandy soil unshaded by trees and well open to the south and west will give the best results. They should be well above flood level, at least 5 feet above the highest known flood, situated on the down stream side of the bazaar, connected by good pucca roads with it to allow of quick carriage, as time is the most important factor in the disposal of night-soil, and should not be in the direction of the prevailing winds with the town. A fringe or belt of bamboos between the town and trenching grounds will be found of the greatest use in keeping off flies which might be blown into inhabited neighbourhoods, and which are a most dangerous factor in disseminating disease.

At Burdwan in Bengal there is a successful example of what can be done with trenching grounds from a financial point of view. There the night-soil of the Northern Section of the Municipality, where the latrine system is in force, and which has a population of about 23,000, is mainly disposed of in a permanent trenching ground of 18 bighas in area. This is collected from private latrines in covered buckets and deposited in the conservancy carts at the public latrines, whence it is removed to the trenching grounds, where trenchers are told off for each latrine group, who excavate the trenches and are in charge of them; these trenches are 3 feet wide, 1½ feet wide, 12 to 15 feet in length and 1 foot apart. The carts empty the night-soil in from one end until a height of 9 inches of liquid is attained, the excavated earth being then replaced. During the rainy season Indian corn is grown, and in the cold weather cabbage, cauliflower, Bengal pumpkin, and other kitchen vegetables, for which there is a ready sale in the local bazaar.

The trenching ground during the current year 1913–14 has been leased out for Rs. 600.

In the damp climate of Bengal jute has been found to grow very luxuriantly on newly-trenched ground and yields an abundant crop; it exhausts the soil so much that after the crop has been cut the field can be re-trenched, an important point when the subsoil water is practically 12 or 18 inches from the surface while the jute is growing. Another advantage in growing jute is that in certain localities there is a difficulty in finding a market for vegetables grown on a trenching ground.

In Muzaffarpur Jail the trenches were made 1 foot deep, 1 foot wide, and 1 foot apart, and varying in length according to requirements; 3 inches of night-soil are filled in and covered over with the excavated earth; 20 trenches are always kept ready, being excavated the day before. Corn is found to grow luxuriantly after the trench has been filled in for a month, and other kinds of vegetables, excepting potatoes, after two months. Urine and sullage water are buried in different fields and at a considerably greater depth.

At the Government Farm at Allahabad (United Provinces) the system known as the Allahabad Shallow Trench was in successful operation, and provided for the whole of the cantonment and half the municipal population; there is an unlimited area of land available, the soil being of various kinds, black cotton, sandy loam and stiff clay. The only crops grown are grass and sorghum, and it is found that the manurial value of the night-soil is not exhausted for three or four years. Trenching can, however, be done every third year on the same land without making it “sewage sick;” land which was worth Rs. 2 will, after trenching, fetch about Rs. 10 per bigha for seven or eight years.

At Meerut for the year 1911–12 the sale-proceeds of night-soil and city sweepings amounted to Rs. 12,871, and at Furrakhabad cum Fatehgarh to Rs. 18,317.

The area required for the contents of a 60–gallon conservancy cart is 80 sq. ft., the most suitable dimensions being 16 feet long by 5 feet wide; 3 inches of the top surface of this space is removed and placed on the embankments of the plot near which the first line of trenches is dug; the subsoil thus exposed is then well cultivated and pulverized to a depth of 9 inches, when the contents of the cart are tipped into the centre of the trench; the liquid matter rapidly sinking into the loosened soil, while the solid excreta remains on the top in a layer less than ? inch in thickness; 3 inches of earth are then similarly removed from the next trench which is parallel to the first, no intervening space being left, and thrown over the night-soil in the latter. The practical working is very simple, as all that has to be done, is to see that a sufficient number of trenches for the daily supply are dug the day before and the earth from them placed over the filled ones. It has been found from experience that night-soil, thus treated, decomposes in less than a week and, if dug then, no trace is observable; the effluvium disappears after three days, and crops are successfully grown immediately after trenching. The Shallow Trench System is by far the most scientific one, but requires a large area of land, working out to 545 sixty-gallon carts per acre. At Muzaffarpur during 1912–13, part of the night-soil was sold for Rs. 1,200, and part of it trenched in the municipal trenching ground, which was leased out for Rs. 1,045. The practical objection to this system is the fly pest. It is therefore only applicable where the trenching grounds are remote, and to the leeward side of the town. Sprinkling chloride of lime or quicklime on the top of the trenches prevents the breeding of flies. The researches of Majors Firth and Horrocks, R.A.M.C., published in the British Medical Journal, however, show that the enteric bacilli is capable of surviving in soil for much longer periods than has been believed possible. These can exist apparently in ordinary soil for 65 days, in sewage-polluted soil for at least 53 days, while in soil sufficiently dry to be blown about in dust for 25 days, and for about a similar period when exposed to a hot summer’s sun. The authors of this note have also proved experimentally the translation of infective material from sun-dried and dusty soil by means of wind, as also by flies which have walked over or fed on polluted earth, indicating the advisability of treating night-soil, especially from Military Cantonments, in septic tanks and filter beds before applying it to soil—vide Chapter IX, page 50.

In order to arrive at the area necessary for a deep trenching ground for a bazaar of 10,000 inhabitants, and assuming that not more than ?th of a gallon of night-soil per head per diem is removed, provision will have to be made for 1,250 gallons or, say, 200 cub. ft. If the trenches are made 1 foot wide, 1 foot deep, and 1 foot apart, and are filled with 6 inches of night-soil, 400 cub. ft. will be required daily, or allowing for roads and divisions between the plots, say 12 acres, for a year’s work, or, in other words, the area required is, one acre for 833 persons. The trenches should be divided up into plots or sections for each latrine circle, excavated, when weather permits, at least a week before they are used so as to aËrate the earth; the bottom of the trench should also be dug up to a depth of 9 inches for the same reason.

The following extract describes somewhat forcibly what may, and undoubtedly does frequently occur in many instances:—

“Trenching again is a success in dry soils, but a good deal of ground is required, and sufficient trenching ground is not always available within practically workable distances of the night-soil producing areas (public and private latrines), and often enough, whereas it may be quite successful in a given ground in the hot weather, it will, in the same ground, however fresh, prove an absolute failure in the rains, when, owing to the high water-level in the subsoil, everything trenched is brought to the surface by the gases of putrefaction, and the entire area trenched becomes a pestilential bog, crawling with maggots, bubbling with the foulest odours and swarming with blue-bottle flies, whose chief delight is to frequent the houses in the neighbourhood and infect both food and drink.”