This invention relates to liquid purification and separation, for example to the dewatering of a slurry or the like from sewage treatment plants, where a flocculation compound is mixed with a slurry. The solids of the mixture are filtered from the liquids of the mixture, and a relatively dry sludge cake is discharged from the system.
It is desirable to dewater slurries in various manufacturing processes so as to recapture and use the solids and to dispose of the water in an uncontaminated condition. For example, in the disposal of sewage, a sludge slurry is removed from settling tanks of sewage treatment plants which contains both solids and liquid, with a substantial amount of solids suspended in the liquid. One common procedure for treating sludge is to add a flocculation compound as a filter aid to the sludge to free the trapped liquid and filter the solids from the liquid. The flocculants are added to the slurry for increasing the filterability of the slurry and for coagulating the solids, therefore reducing the tendency of the small solids from passing with the liquid through the filter medium. Even after flocculation of the sludge mixture some of the solids are so small and the quantity of liquid so great that the smaller solids tend to move from the mixture with the liquid through the filter medium if the liquid is urged too vigorously from the mixture. This condition requires that the sludge be carefully handled and gently dewatered.
In the past, several sludge dewatering systems have been developed which comprise pairs of moving conveyor belts usually fabricated from woven textile material that function as filters. The prior art systems deposit coagulated sludge on one moving belt, and as the sludge moves with the first moving belt the second moving belt is moved down into contact with the sludge so as to squeeze and therefore dewater the sludge. Examples of such prior art systems are disclosed in U.S. Pat. Nos. 3,800,952, 3,601,039, 3,699,881, and 3,896,030.
In a sewage treating process it is desirable to process as much sludge as possible in a short period of time, to extract as much liquid as possible from the sludge during the process to produce a dry sludge cake, and to capture a high percentage of the solids from the original slurry. These three objectives are in conflict with each other since more water can be extracted from sludge if the sludge is given more time to drain. Although the dewatering capacity of a dewatering system can be increased by merely speeding up the system, the increase in speed also usually causes a lower percent of solids to be filtered from the liquid and the sludge is not squeezed for a time long enough to remove the desired amount of liquid. When the draining of the sludge is assisted by squeezing the sludge between sandwiched endless conveyor belts, the smaller sludge particles tend to pass through the conveyor belts. Also, if the sandwiched conveyor belts are run through a sinuous path so as to create shear forces in the sludge filter cake, the shifting of the filter cake tends to break the filter cake and allow some of the finer solids to pass with the liquid through the filter belts. On the other hand, after a high percentage of the liquid has been removed from the sludge cake, the sludge cake can be squeezed and higher compression forces can be applied to the sludge cake to remove more liquid substantially without hazard of allowing the small solid particles to pass through the filter belt, since the reduced quantity of water present in the sludge cake is insufficient to wash out the small particles.
Once the sludge has been dewatered, it is desirable to remove the sludge from the pores of the filter belts, so that a porous belt is available on the next cycle of the belt for receiving the sludge slurry. The cleaning of the filter belts is usually accomplished by directing a continuous flow of water against the belt, and requires the excessive use of water.
Dewatered sludge can be used for various purposes, such as for a landfill as a fertilizer, or as a fuel. If the dewatered sludge has a low liquid content, it weighs less for a given volume, which means that it can be transported at a lower cost, and the dry product can be better used in subsequent processes since the energy required to extract the residual water would be less and the dryer the sludge, the better the sludge sustains combustion. Also, a dryer sludge makes a better landfill.