This invention relates to a method of electroosmotically dehydrating sludge produced in processing service water, sewage or various types of industrial sewage. More particularly, the invention relates to such a method using a filter press.
The particles of sludge contained in service water sludge and sewage sludge, for example, are electrically negatively charged, while the water contained in the sludge is positively charged. In the dehydration method known as electroosmotic dehydration, the water contained in sludge moves electroosmotically toward a cathode, and the salt contained in the sludge is electrolyzed, so that the filtrate is acid on the anode side and alkaline on the cathode side.
Generally, metal ions precipitate as insoluble metal hydroxide on the alkaline side, and the precipitate sticks to the electrode surfaces and filter cloths on the cathode side and disadvantageously affects the electroosmotic dehydration. This can be prevented by a method of reversing the polarity of the electrodes for a few minutes at the last step, or for every several batches, of electroosmotic dehydration, in order for the acid filtrate to dissolve and remove the precipitate.
U.S. Pat. No. 5,092,974 dated Mar. 3, 1992 to Kondo et al. describes a process for compressive (pressurized) and electroosmotic dehydration using carbonaceous electrodes which contain carbon fibers, and the polarity of which is reversed during the dehydration process.
However, depending on the type of sludge, the reversal of polarity may not recover the blinded or clogged filter cloths. In particular, sludge containing a large amount of calcium blinds the filter cloths in a short time, so that the dehydration and the sludge cake release from the cloths may deteriorate.
When the liquid pH is 13 or higher near the cathode, Ca.sup.++ ions having moved together with the liquid to the cathode side, and OH.sup.- ions produced on the cathode surfaces, are bound together to precipitate as insoluble calcium hydroxide [Ca(OH).sub.2 ] in the filter cloth fibers. This disadvantageously affects the filtering speed and the cake release.