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.