Sewage Disposal Works: Their Design and Construction

by William Charles Easdale

Fig. 154.—Electrolytic Hypochlorite of Magnesium Plant.

It consists of four cells, each containing ten elements, consisting of one positive and two negative plates. The positive plates consist of thin platinum wire, wound upon slate slabs, and the negative electrodes are of zinc. The four cells are placed one above the other, so that the liquid passes through from one to the other by gravitation. The feed-tank at the top contains a solution of sodium chloride and magnesium chloride, and is fitted with a glass gauge to indicate the amount of solution in the tank. From this tank the solution passes through a small ball-valve cistern, so as to maintain a constant rate of flow. The feed-tank is also provided with a plate, operated by a chain carried over to the outside of the tank, by means of which the liquid to be electrolysed can be stirred from time to time, so as to secure a uniform strength of solution. The solution passes through the four electrolysers in series, being subjected to the action of a regulated current of 15 to 17 amperes at 230 to 250 volts, being 5·7 to 6·2 volts per cell. After the electrolysed solution leaves the last cell it runs into a small tank, where a fixed amount of hydroxide of magnesium is mixed with it, and it is claimed that by this means the solution is rendered stable, a quality which should be of much value where the solution has to be stored for any length of time.

The question as to what is the most suitable sterilising agent to use under certain conditions, and in what proportion it should be added to the sewage effluent, is a matter for the chemist and biologist. The method of application is, however, the duty of the engineer. As in the case of other chemicals, there are two ways in which it may be applied. The solution may be added to the sewage effluent in equal doses of varying strength, or in varying doses of equal strength. There is a third method, involving the variation of the dose and the strength of the solution, but while this is not impossible it is probably not practical. The chief difficulty to be overcome is the variation of both the rate of flow and the strength of the sewage, and the most practical solution is to prepare the sterilising agent of a uniform strength, and vary the doses in direct proportion to the flow of the effluent, the minimum dose being sufficient for the maximum strength of the sewage. This method was adopted by the author in the case of a small scheme of sewage disposal, which he prepared for a place where the only outlet for the final effluent was a discharge over an area of chalk subsoil, from which the water supply of a large town is drawn. In this case, he designed a simple apparatus which does not involve any special appliances, and which would be quite satisfactory for ordinary practical purposes. A new apparatus for the purpose in question has, however, recently been introduced by Messrs. Nixon and Mannock. As will be seen from the illustration, [Fig. 155], it is based upon the application of the Venturi principle, and involves the use of a Venturi tube, as previously described under the heading of “Measuring Apparatus.” In fact, the same Venturi tube can be utilised to serve both for measuring the flow of the effluent, and for applying the sterilising agent in direct proportion to the flow of the effluent.

Fig. 155.—Nixon and Mannock’s Apparatus
for Injecting Chemical Solution.

The apparatus consists of a cylinder C, the top of which is connected by means of a pipe fitted with a three-way cock to the “Upstream” end of the tube A. A similar connection is made from the bottom of the cylinder to the “throat” B. A piston of the type used in the Kent Standard Water Meters, and provided with a counterbalance weight, works in the cylinder by means of the difference of the pressure on the two sides of the Venturi tube. The chemical solution (e.g. a 5 per cent. solution of chlorine) is supplied to the underside of the piston, and the pressure on the upper side of the piston being greater than the pressure on the underside, the chemical is forced down by the piston and injected through the injection tube and regulating valve into the effluent at the “throat” of the Venturi tube. As the flow of the effluent through the Venturi tube produces a difference of pressure which varies as the square root of the velocity, the rate of injection will also vary in the same proportion. The injection is thus in exact proportion to the flow, and any variation of the flow will automatically cause a corresponding variation in the rate of injection.

When the chemical re-agent is exhausted, the piston will be at the bottom of the cylinder, and the pointer at zero. In order to recharge the cylinder with the chemical, the three-way cocks must be reversed by means of the hand lever, thereby cutting off pipes A and B, and simultaneously connecting the top of the cylinder to the waste pipe, and the bottom to the supply from the chemical storage tank, which is fixed at such a height that the head will rapidly force the piston up and re-fill the cylinder with the chemical. The three-way valves are then reversed, and the apparatus is again in full working order. The apparatus shown is applicable to the treatment of 1000 gallons per hour, and will only need recharging once per day of 24 hours.

A feature of this apparatus is that it is self-starting, and should the flow cease, the injection will also automatically stop, the static head on both sides of the piston being equal. There is absolute immunity from danger or over-injection of the chemical by this system, and this is a valuable factor in the treatment of potable water. Where absolutely necessary, the same firm can supply a de-chlorinating apparatus. By means of the indicator, the works manager is constantly informed of the exact amount of chemical injected, and the scale readings can be compared with those of a Venturi Meter operated by the same Venturi tube. This apparatus can be supplied of a larger size, and provided with automatic recharging gear for larger installations.

Whatever method may be adopted for applying the sterilising agent, it is essential in all cases to have a storage tank to receive the mixture in order to provide time for the chemical to have full effect. So far as can be ascertained at present, a storage capacity equal to one hour’s flow of the liquid to be sterilised will be sufficient under ordinary circumstances, but provision should be made for thoroughly mixing the chemical with the effluent and for drawing off any deposit which may occur in the tank without interfering with the normal working of the plant.