Studies on the Filter-thickener

Abstract

There are several types of filter-thickener now available in commercial scale, but in this experiment, we studied the fundamental characteristics of the simplest type of filter-thickener as shown in Fig. 1. This filter-thickener consists of a cylindrical settling tank (50cm in diameter and 90cm in depth) with perforated floor covered with a filter medium. (Poly-vinyl chloride filter cloth).A part of the liquid contained in the sludge sedimented on the bottom of the settling tank is withdrawn as filtrate through the filter cloth by liquid head and suction of the gear pump. The cake sludges on the filter cloth are scraped and gathered to the centre of the tank bottom and withdrawn by the reciprocating diaphragm pump as thickened sludges. Overflow, underflow and filtrate are all sent back to the reservoir, where they are mixed by the stirrer, and recirculated by the pump.After a long and continuous running under constant feed-, underflow- and filtration-rate conditions. the whole system comes to its equilibrium. Then the concentration distribution of slurry along the height of the settling tank and the underflow concentration are measured. The results obtained are shown in Table 1 and Fig. 5.According to our study of the capacity of the ordinary continuous thickener, the concentration Cu of underflow sludge is determined mainly by the underflow rate per unit settling area (Qu/A) and the settling characteristics of that slurry. Solid flux G which may settle down across a horizontal unit area of the slurry layer with a concentration C, is represented by the equation, G=C(R+Qu/A), where R is a settling velocity of slurry having a concentration C and Qu/A is a downward bulk flow of liquid induced by the withdrawal of underflow. As shown in Fig. 6 (dotted curve), solid flux G at a constant rate Qu/A takes a minimum value Gm at a certain value of C(Cmu), and the slurry layer of that concentration controlls the capacity of the thickener.From the material balance, the underflow concentration to be obtained under this condition may be calculated by means of the following equation.Gm=Cmu(Rcmu+Qu/A)=CuQu/AIn the filter-thickener, besides the underflow Qu, the filtrate Qff is withdrawn from the bottom of the tank. Therefore the downward bulk flow of liquid in the lower part of the settling tank becomes (Qu+Qff)/A.If the sum of Qu and Qff is designated by Qu', solid handling capacity in the settling part of the filter-thickener is the same as that of ordinary thickener with an underflow rate Qu'. Then if all the sludge (Qu') but no filtrate is withdrawn as underflow, the concentration of the underflow sludge becomes Cu' in Fig. 6.If Qu (one part of Qu') is withdrawn as underflow and Qff (=Qu'-Qu) as filtrate, the concentration of the thickened sludge becomes Cu2 in Fig. 6. In other words, the volume of sludge decreases from Qu' to Qu, and solids contained remain the same, so the sludge concentration increases from Cu' to Cu2 which equals Cu'×Qu'/Qu. Accordingly, when corrected values of observed Cu2 at various operating conditions are plotted as Cu2Qu2/Qu' against