PURPOSE: To dehydrate uniformly cake by shortening the time of compression by the procedure in which the liquid level of filtrate tank for dehydrator is measured, the varied amount of liquid is calculated, the value is compared with the liquid level changing rate set values for sludge pressing and compression processes, and when they coincide with each other, sludge is advanced to the next process.

CONSTITUTION: Concentrated sludge in the mixing tank 1 is dehydrated by the dehydrator 3, and the filtrate is introduced into the filtrate tank 4. When reaching a hydrating rate, little change occurs in the relation between the times T1 and T2 of the pressing and compression processes of the dehydrator 3 and the liquid level 1, and the amount of filtrate generated becomes small or Δl. The liquid level of the filtrate tank 4 is measured by the liquid level meter 6, the varied amount of liquid level is calculated by an arithmetic processer, and when the value coincides with the liquid level changing rate Δl/ΔT, set up previously, a command is sent out to delivery sludge to the next process. Thus, each process can be operated with good efficiency, hydrating time can be shortened, and at the same time the cake can be dehydrated uniformly.