Filtration device

A filtration device comprising:

• a first membrane filtration unit including a first storage tank configured to store therein a liquid to be treated, a first separation membrane part configured to separate the liquid to be treated into a first permeated liquid and a first non-permeated liquid, a first liquid delivery pipe configured to deliver the liquid to be treated from the first storage tank to the first separation membrane part, a first non-permeated liquid pipe configured to return the first non-permeated liquid to either one of the first storage tank or the first liquid delivery pipe, a first permeated liquid pipe configured to cause the first permeated liquid to flow from the first separation membrane part, a first extraction pipe through which a portion of the first non-permeated liquid is extracted from either one of the first liquid delivery pipe or the first non-permeated liquid pipe, a first adjustment unit configured to adjust a flow rate of the first permeated liquid to be substantially constant, and a first liquid scale configured to detect an amount of a stored liquid in the first storage tank;
• a second membrane filtration unit including a second storage tank configured to store therein the first permeated liquid from the first permeated liquid pipe of the first membrane filtration unit, a second separation membrane part configured to separate a stored liquid in the second storage tank into a second permeated liquid and a second non-permeated liquid, a second liquid delivery pipe configured to deliver the stored liquid in the second storage tank from the second storage tank to the second separation membrane part, a second non-permeated liquid pipe configured to return the second non-permeated liquid to either one of the second storage tank or the second liquid delivery pipe, a second permeated liquid pipe configured to cause the second permeated liquid to flow from the second separation membrane part, a second extraction pipe through which a portion of the second non-permeated liquid is extracted from the second non-permeated liquid pipe, a second adjustment unit configured to adjust a flow rate of the second permeated liquid to be substantially constant, and a second liquid scale configured to detect an amount of the stored liquid in the second storage tank;
• a first controller unit configured to control an amount of a stored liquid stored in the first storage tank based on measurement values from the first and second liquid scales of the first and second membrane filtration units;
• and
• a second controller unit configured to control an amount of a stored liquid stored in the second storage tank based on a measurement value from the second liquid scale, wherein based on a preceding-stage measurement value from the first liquid scale of the first membrane filtration unit and a subsequent-stage measurement value from the second liquid scale in the second membrane filtration unit, the first controller unit stops a flow of the first permeated liquid of the first membrane filtration unit.