Membrane filtration system

Abstract

According to one embodiment, a membrane filtration system includes a raw water tank, a pretreatment membrane module, a raw water feed line, a high-pressure RO membrane module, a high-pressure line, a preceding power recovery unit which pressurize the pretreated water by transmitting the pressure of the concentrate to the pretreated water, a succeeding power recovery unit which pressurize the raw water by transmitting a remaining pressure of the concentrate to the raw water, a concentrate discharge line, a first pressure transmission line communicating to the power recovery unit by being branched from the high-pressure line, a second pressure transmission line communicating to the power recovery unit by being branched from the raw water feed line, and a valve provided in the drain line to regulate a discharge of the concentrate from the power recovery unit in accordance with a pressure loss in the pretreatment membrane module.

What is claimed is: 
     
         1 . A membrane filtration system, comprising:
 (A) a raw water tank configured to accommodate a raw water containing solutes and insoluble components;   (B) a pretreatment membrane module that separates and removes the insoluble components from the raw water fed from the raw water tank;   (C) a raw water feed line having a raw water feed pump to feed the raw water from the raw water tank to the pretreatment membrane module;   (D) a high-pressure reverse osmosis membrane module provided in a subsequent stage of the pretreatment membrane module to separate and remove the solutes from pretreated water providing treated water as permeate and concentrate as retentate;   (E) a low-pressure reverse osmosis membrane module provided in the subsequent stage of the high-pressure reverse osmosis membrane module and to which a pressure lower than that applied to the high-pressure reverse osmosis membrane module is applied to separate and remove remaining solutes from the treated water;   (F) a high-pressure line having a high-pressure pump to feed the pretreated water to the high-pressure reverse osmosis membrane module at predetermined high pressure;   (G) a preceding power recovery unit having a positive-displacement pump to which a portion of each of the brine and the pretreated water are fed and which pressurize the pretreated water by transmitting the pressure of the concentrate to the pretreated water;   (H) a succeeding power recovery unit having a positive-displacement pump to which a portion of each of the concentrate from the preceding power recovery unit and the raw water are fed and which pressurize the raw water by transmitting a remaining pressure of the concentrate to the raw water;   (I) a concentrate discharge line through which the concentrate discharged from the high-pressure reverse osmosis membrane module flows to transmit the pressure of the discharged concentrate to the positive-displacement pump of the preceding power recovery unit;   (J) a communicating line communicating the preceding power recovery unit and the succeeding power recovery unit and through which the concentrate from the preceding power recovery unit flows;   (K) a first pressure transmission line branched from the high-pressure line and communicating to the preceding power recovery unit and through which a portion of the pretreated water to be fed to the high-pressure reverse osmosis membrane module flows;   (L) a second pressure transmission line branched from the raw water feed line and communicating to the succeeding power recovery unit and through which a portion of the raw water to be fed to the pretreatment membrane module flows;   (M) a drain line to discharge the concentrate from the succeeding power recovery unit; and   (N) a pressure regulating valve provided in the drain line to regulate a discharge of the concentrate from the succeeding power recovery unit in accordance with a pressure loss in the pretreatment membrane module.   
     
     
         2 . The system according to  claim 1 , further comprising:
 a first manometer that measures a pressure in upstream of the pretreatment membrane module;   a second manometer that measures a pressure in downstream of the pretreatment membrane module; and   a controller that determines a membrane differential pressure ΔP of the pretreatment membrane module from measured pressures of the first and second manometers, regulates the pressure of the concentrate discharged from the high-pressure reverse osmosis membrane module by opening the pressure regulating valve if the determined membrane differential pressure ΔP is smaller than or equal to a set pressure value Pc, and maintains the pressure regulating valve closed if the determined membrane differential pressure ΔP is larger than the set pressure value Pc.   
     
     
         3 . The system according to  claim 1 , further comprising:
 a treated water tank arranged in a subsequent stage of the pretreatment membrane module to accommodate the pretreated water having passed through the pretreatment membrane module;   a water conveyance line having a conveying pump to feed the pretreated water from the treated water tank to the high-pressure pump; and   a back washing line branched from the water conveyance line and communicating to the pretreatment membrane module, to guide the pretreated water to the pretreatment membrane module by driving of the conveying pump.   
     
     
         4 . The system according to  claim 1 , further comprising:
 a direct transfer line connected to the high-pressure line to directly guide the pretreated water having passed through the pretreatment membrane module to the high-pressure reverse osmosis membrane module;   a treated water tank that accommodates the treated water having passed through the high-pressure reverse osmosis membrane module;   a water conveyance line having a conveying pump to feed the treated water from the treated water tank to the low-pressure reverse osmosis membrane module; and   a back washing line branched from the water conveyance line and communicating to the pretreatment membrane module, to guide the treated water to the pretreatment membrane module by driving of the conveying pump.   
     
     
         5 . The system according to  claim 1 , wherein there are a plurality of succeeding reverse osmosis membrane modules, and the system further comprises:
 a treated water tank that accommodates product water having passed through the last stage, succeeding reverse osmosis membrane module;   a water conveyance line having a conveying pump to send out the product water from the treated water tank; and   a back washing line communicating to the pretreatment membrane module by being branched from the water conveyance line to guide the treated water to the pretreatment membrane module by driving of the conveying pump.   
     
     
         6 . The system according to  claim 1 , further comprising:
 a concentrate tank that accommodates the concentrate discharged from the reverse osmosis membrane module; and   a back washing line having a washing pump to feed the concentrate from the concentrate tank to the pretreatment membrane module.   
     
     
         7 . The system according to  claim 1 , further comprising:
 a hot water tank that accommodates hot water;   a washing line provided between at least one of the pretreatment membrane module, the high-pressure reverse osmosis membrane module and the low-pressure reverse osmosis membrane module, and the hot water tank and having a conveying pump to feed the hot water to at least one of the high-pressure reverse osmosis membrane module and the low-pressure reverse osmosis membrane module; and   a controller that controls driving of the conveying pump at preset intervals or in preset timing to cause the conveying pump to feed the hot water to at least one of the high-pressure reverse osmosis membrane module and the low-pressure reverse osmosis membrane module.