In cases where it is necessary to raise the sludge to the disposal area, a hand-operated chain-pump may be used for small schemes, or for large volumes, and where power is available, sludge elevators of the bucket type, as shown on pages 40 to 42, and manufactured by Messrs. S. S. Stott and Co., Messrs. Ham, Baker and Co., Ltd., and Messrs. Adams Hydraulics, Ltd., will be found convenient. These appliances are usually erected in special sludge wells, to which the sludge is delivered by gravity. In the case of long tanks, in which the floors are comparatively flat, and especially where the sludge is allowed to accumulate until it has become consolidated to a great extent, difficulties are experienced in causing the sludge to flow to the outlet by gravity. This usually involves the employment of men to descend into the tank and force the sludge towards the outlet by means of squeegees, a slow and laborious process.
Fig. 50.—Chemical Mixer.
Chemical Mixers.—The methods adopted for adding the necessary chemicals to sewage for chemical precipitation are various. Where alumina-ferric is used, the simplest method is to place blocks of the precipitant in wire cages placed in the inlet channel so that the flow of the sewage itself dissolves the block as required. It has been found that this method is not economical in some cases, and the precipitant is dissolved beforehand in a suitable mixer in order that it may be added to the sewage in the form of a solution. This applies specially to the lime process, and several forms of these mixing machines are shown in [Figs. 50], [51] and [52], made by Messrs. Goddard, Massey and Warner, Messrs. Manlove, Alliott and Co., Ltd., and Messrs. S. H. Johnson and Co., Ltd. These may be driven by power or by the flow of the sewage itself, but the most important point which requires attention is that the strength of the solution shall vary with the strength of the sewage, either by varying the rate of flow of a solution of uniform strength, or by varying the strength of a solution flowing at a uniform rate.
Fig. 51.—Chemical Mixer.