For the purpose of disposing of the sewage and using it profitably, the
Company purchased 909 hectares (2,246 acres) of land from the Community of
San Nicolas de los Garzas, the outfall sewer being carried to the
southwestern boundary of the land acquired. This area has a general fall
in all directions to the northeastern boundary, with a gradual fall of
about 25 m. across the diagonal of the land. The area purchased was
practically virgin land, only small portions having been cultivated. The
greater part was covered with a growth of mezquite trees and small shrubs.
The quality of the land is excellent, if properly irrigated, and capable
of yielding abundant crops of every description. The limits of this land
are shown on Plate II.
_Sewage Purification Tanks._--For the purpose of obtaining a satisfactory
effluent to discharge on the land without causing nuisance, the Company
built a system of detritus chambers and liquefying tanks at the end of the
outfall sewer. One difficulty to be faced, in designing these works, was
the fact that there were no data regarding the probable quantity of
dry-weather sewage, nor any particulars as to its general character;
there was also the probability that the outfall sewer would have to carry
large quantities of surplus water. Therefore, the system was designed so
as to be capable of extension if necessary, and the sizes of the various
tanks were limited at present, because of the septic processes which would
be set up in the long length of outfall sewer. The tanks were designed to
deal with 10,000,000 liters of sewage proper per day, and the channels,
etc., were proportioned to take the full flow of the sewer if necessary.
Provision was also made for discharging large volumes of surplus water
directly on the land, independent of the tanks. To do this a by-pass was
taken from the sewer a short distance before reaching the site of the
tanks. By properly timing the flow, arrangements could be made to
discharge these waters in the early hours of the morning, by allowing the
scour-pipes in the distribution system to be opened at night when the
domestic sewage flow was at its minimum. As the area of land available is
very great, the degree of purification in the tanks was relatively
unimportant; the object to be obtained consisted chiefly in distributing
on the land an effluent which would be innocuous and clear.
The general design of the works is shown on Plate XXIV, and they consist
essentially of a screen chamber, duplicate detritus tanks, and three
liquefying tanks. There is also a sludge-pit 629 m. from the tanks.
[Illustration: PLATE XXIV.--SEWAGE DISPOSAL WORKS AT SAN NICOLAS DE LOS
GARZAS; GENERAL PLAN OF DETRITUS AND LIQUEFYING TANKS, WITH DETAILS OF THE
LATTER.]
_Screen Chamber and Detritus Tanks._--Enlarged details of the screen
chamber are shown on Plate XXV. The invert, where the sewer enters the
screen chamber, is 489.45 m. above datum. This chamber has duplicate
screens which are fully detailed on Plate XXX. For cleaning purposes the
screens are raised by a steel-framed head-gear, which is arranged so that
they may be lowered to a small traveling bogie, out of the way of the
screen chamber.
[Illustration: PLATE XXV.--SEWAGE DISPOSAL WORKS AT SAN NICOLAS DE LOS
GARZAS. DETAILS OF DETRITUS CHAMBERS AND INLET CHANNELS.]
[Illustration: PLATE XXVI.--SEWAGE DISPOSAL WORKS AT SAN NICOLAS DE LOS
GARZAS DETRITUS AND LIQUEFYING TANKS; DETAILS OF DISTRIBUTING CHANNELS.]
[Illustration: PLATE XXX.--SEWAGE DISPOSAL WORKS AT SAN NICOLAS DE LOS
GARZAS. DETAILS OF SCREENING APPARATUS.]
From the screen chamber there are two main channels, 1.22 m. wide,
branching to the two concrete detritus chambers. Each channel has a square
penstock, so that the sewage can be diverted into either chamber when
necessary.
The detritus chambers are octagonal in plan, 4 m. in diameter, and each is
provided with an outlet weir 1.50 m. wide. At the weir level the chambers
have a depth of 1.75 m., with drainage channels below that level. The
coping is 1 m. above the outlet weir of the detritus tanks. To drain off
these chambers, each has a scour-out pipe, 30 cm. in diameter, controlled
from valves with spindles carried above the coping level. Each of these
pipes is connected to a central chamber, and leads to a 56-cm. (22-in.)
sludge-pipe. The chambers as designed are of smaller capacity than those
usually provided, but, as all surface water is strictly excluded from the
sewerage system, the quantity of detritus reaching the chambers may be
small. The velocity through them when both are in use will be
approximately 0.082 m. (0.27 ft.) per sec.
From these chambers the sewage is carried to the three liquefying tanks by
a main channel, 11.5 m. long and 1.50 m. wide.
[Illustration: PLATE XXVII, FIG. 1.--CAST CONCRETE BEAMS BEING PLACED IN
POSITION, LIQUEFYING TANKS.]
[Illustration: PLATE XXVII, FIG. 2.--INLET WEIRS TO LIQUEFYING TANKS,
DURING CONSTRUCTION.]
[Illustration: PLATE XXVIII, FIG. 1.--VIEW OF LIQUEFYING TANKS, FROM INLET
END.]
The tanks are of concrete and have reinforced concrete roofs. Each is 66
m. long and 6 m. wide; the minimum depth for the sewage is 1.50 m. at the
outlet end, and 2.25 m. at the inlet, increasing to a maximum depth of
2.75 m. at the lowest depth at the scour-out channel. Their combined
capacity is 2,500,000 liters, which is equivalent to 6 hours' flow of the
quantity of sewage for which they were designed. The sewage passes from
the main channel, through penstock-valves which control the flow, into one
or the other of the tanks. From these valve openings it flows over
concrete weirs, 5 m. long, and is deflected to the bottom of the tank by a
reinforced concrete scum-plate, extending across each tank, with a
clearance of 15 cm. at each end. This scum-plate is 1.5 m. deep and 10 cm.
thick, and is placed 40 cm. from the end walls.
[Illustration: PLATE XXIX.--SEWAGE DISPOSAL WORKS AT SAN NICOLAS DE LOS
GARZAS; DETAILS OF OUTLET CHANNELS AND WEIR BOX.]
The details of the concrete division and outside walls are shown on Plate
XXIX. The floor was constructed in two layers, and its surface is divided
into 6 channels formed by small walls, 20 cm. wide and 15 cm. deep, the
object of these channels being to facilitate the cleaning of the floor by
scouring it out to a specially arranged channel at the deepest point of
the tank, near the inlet end. Each scour-out channel has a 30-cm. (12-in.)
gate-valve, controlled from the roof of the tank, the three scour-pipes
meeting in a concrete chamber outside of the tanks, from which a 56-cm.
(22-in.) concrete pipe discharges the contents of the tanks to the
sludge-pit during cleaning operations. The velocity through the tanks,
when they are used in combination, is 0.0253 m. (0.083 ft.) per sec., the
tanks being made as long as economically possible, in order to obtain this
low velocity and thus permit the proper sedimentation of the suspended
matters. The roof of each tank is 1 m. above the weir level. Each tank has
four ventilating columns, 3.7 m. high and 30 cm. in diameter, vitrified
clay pipes, with an exterior casing of contrete, being used for the
shafts. The roof is enclosed within parapet walls, and is covered with a
layer of earth 25 cm. thick.
The outlet channel from the tanks leads to a measuring chamber, 3 m.
square, as shown on Plate XXIX. This chamber is fitted with penstocks,
1.83 m. wide, and measuring weirs. From this chamber the sewage is
delivered to two main irrigation ditches, which distribute the sewage in
two directions, one northward and the other to the western extremity of
the lands.
_Construction of Tanks._--The excavation for the tanks was in soft earth
for a depth of 1-1/2 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 45-3/4 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 1:2-1/2:4-1/2
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.