In recent years the provision of proper means of measuring and recording the flow of sewage at disposal works is becoming more general, but there are still a very large number of works at which it is impossible to obtain any trustworthy information as to the volume of sewage treated. As long as all works smoothly, and there is no trouble with the effluent, it is considered superfluous to trouble about the quantity of the liquid which passes through. When, however, difficulties arise, and it becomes necessary to investigate the cause of the trouble, it is of the utmost importance to be in a position to ascertain the daily volume of the sewage arriving at the works and the variations in the rate of flow, as well as the quantities dealt with by each separate tank and filter. It is also of great assistance, in making investigations at such times, to have a definite record of the volumes treated day by day during the preceding six months; indeed the possession of a complete record of the daily flow of sewage over the whole period during which a sewage works has been in operation, is a valuable asset not only to those responsible for the works themselves but also to the authorities who control the streams and watercourses, and to investigators in search of information to be used for the public benefit. The initial cost of suitable measuring and recording appliances is not excessive, but, when once the works are completed and in good working order, the local surveyor or manager has great difficulty, and usually finds it impossible, to persuade his committee that the outlay is justified. It is, therefore, desirable that engineers should in all cases make provision for such apparatus in the contract for the construction of the works, and thus make sure that it will be available when required.

Various types of measuring appliances are in use. Among these the simplest is the gauge weir penstock Fig. 148, manufactured by Messrs. Adams Hydraulics, Ltd., from which the depth of water flowing over the weir can be observed at any time. This, however, does not provide a record of the volume flowing at all times. For this purpose the same firm supply several types of recorders, among which the simplest is the type shown in Fig. 149. This is supplied with 24-hour or 7-day movement, and charts which register the volume in gallons per minute, per hour, or per 24 hours.

Another measuring apparatus is that supplied by Messrs. George Jennings, Ltd., Fig. 150. This consists briefly of a hollow copper float attached to a brass rod, which is carried up through a hollow column into the indicator box provided with a glass hinged door. The brass rod is provided with pointers, which move up and down with the rods and indicate the water level.

Messrs. George Kent, Ltd., supply a large number of various types of recorders and measuring instruments, including the well-known “Venturi” meter and others actuated by floats to indicate the discharge over a weir. The Venturi meter type of measuring apparatus adapted for sewage, is shown in Fig. 151. In this case the Venturi tube may form part of the ordinary supply pipe, or it may be fitted in a chamber built in the supply channel, the tube connected through the chamber with its ends terminating in the channel at either side. This measuring apparatus may be fitted with the following types of recorders. A diagram only, counter only, or combined counter and diagram in one instrument. The recorder may be placed in any convenient position within a thousand feet of the meter tube.

Several types of measuring apparatus are manufactured by Messrs. Glenfield and Kennedy. One of these is illustrated in Fig. 152. This consists of an apparatus for recording the volume of water or sewage flowing over a weir, a chart being revolved by clockwork, and the volume indicated by a pen actuated by a float and cord working over pulleys. This instrument can be fitted with cam and pen carriage to show the rate of discharge on the chart (24 hours or 7 days), in gallons of cubic feet per minute over a V-notch, rectangular weir, or in an open channel of known dimensions. It is claimed that by simply taking the area of the diagram, the total discharge for any period can be ascertained much more quickly and more correctly than from a diagram simply giving the height flowing over the weir.

A somewhat novel form of apparatus, recently introduced by Messrs. Adams Hydraulics, Ltd., for measuring the flow of sewage in a channel, consists of a water-wheel by means of which the velocity of the flow is registered. In order to maintain the paddles of the wheel at a uniform depth below the surface of the liquid, the wheel is carried upon a shaft supported by two floats provided with vertical guide rods working in brackets attached to the sides of the channel. By this means the whole of the apparatus rises and falls with the liquid in the channel. The wheel-shaft is provided with a bevel-toothed wheel, which engages with another similar wheel attached to a flexible shaft, and this drives a set of geared wheels similar to a flush-tank counter, which thus record the number of revolutions made by the paddle-wheel.