Construction of Tanks.—The excavation for the tanks was in soft earth for a depth of 11⁄2 m.; the lower depths were in a firm foundation of sillar and calcareous clay. The total excavation in the tanks, channels, etc., was 8,335 cu. m., and the actual cost was 453⁄4 cents per cu. m. To facilitate the construction, about six-tenths of the concrete beams were cast as single monoliths and placed in position by sliding them across the tanks on temporary timbers. The remainder of the beams, the roof, and the slab were placed in position in the ordinary way with timber forms. The total quantity of concrete placed was 1,360 cu. m. A 1:21⁄2:41⁄2 concrete was used for the walls, channels, etc., and a 1:2:3 mixture for the roof slab and beams.
Table 14 gives the average cost per cubic meter for all the concrete work.
TABLE 14.—Average Cost per Cubic Meter for Concrete in Tanks.
| Pesos per cubic meter. | Pesos per cubic meter. | |
| Labor: | ||
| Mixing and placing | 5.20 | |
| Carpenter work in forms, framing, etc. | 4.20 | |
| ——— | ||
| Total labor cost | 9.40 | |
| Materials: | ||
| Screened gravel | 4.04 | |
| Sand (from neighboring arroyo) | 4.98 | |
| Cement (including hauling) | 15.19 | |
| Lumber, nails, and other supplies | 1.90 | |
| ——— | ||
| Total materials cost | 26.11 | |
| Total cost of concrete per cubic meter | 35.51 |
Sludge-pit.—The sludge-pit, used when cleaning out the tanks, is carried 639 m. northward, far enough to get the available fall to drain the bottom of the detritus chambers and liquefying tanks. The drainage
pipe was formed of 56-cm. (22-in.) concrete tubes. The sludge-pit is merely an excavation in the earth 20 m. square and 2 m. deep, the sides having a slope of 11⁄2 to 1. An overflow drains the pit to an irrigation ditch, the solid matter being allowed to settle and the liquid to drain off. From time to time it is proposed to dig out the solids and plow them into the land.
General.—To the east of the tanks a 3-roomed house has been built for the inspector.
In order to provide a good supply of water for cleaning operations, a well 22 m. deep has been sunk and is fitted with pumps operated by an Eclipse windmill, 4 m. in diameter, on a tower 22 m. high, which delivers the pump water to a circular wooden tank of 20,000 liters capacity.
The work in connection with the purification tanks was carried out by the Company's own staff; it was begun on September 10th, 1908, and practically completed by the first week in January, 1909.
At the time of writing, the tanks have to deal with the sewage from a population of only 10,000 persons, as only from 15 to 20% of the connections have been made. The sewage, therefore, has been diluted with several times its volume of surplus water, and the necessary scum on the top of the sewage in the tanks has not yet assumed the usual thick matty condition observed in most systems. As there are no available means in Monterrey of having proper determinations made of the degree of purification which takes place in the passage of the sewage through the liquefying tanks, a few simple tests have been made. These tests were limited to the determination of the amount of oxygen absorbed in 4 hours, and show a purification of 50% in passing from the detritus chambers to the outlet. The sewage, although very black and full of suspended matter as it enters the tanks, leaves them in a very clarified condition.