_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.]