System for cleaning membrane, and method for cleaning membrane using same
A system for cleaning a membrane of the present invention includes: a first membrane filtration tank which includes a membrane and an aerator, filters inflow raw water by membrane filtration, and discharges treated water and water to be treated, respectively; a pressure gauge for measuring a transmembrane pressure and a flow meter for measuring a permeation flux which are provided at a treated water discharge line of the first membrane filtration tank; and a controller which calculates a water permeability recovery rate by calculating the water permeability from the differential pressure measured at the pressure gauge and the permeation flux measured at the flow meter, predicts a membrane contamination degree from the calculated water permeability recovery rate, and controls a back-pulsing interval and an intermittent aeration interval.
1. A membrane cleaning system comprising:
a first membrane filtration tank provided with a membrane and an aerator and filtering inflow raw water by membrane filtration and discharging treated water and water to be treated respectively;
a pressure gauge configured to measure a transmembrane pressure and a flow meter configured to measure a permeation flux, the pressure gauge and the flow meter being on a treated water discharge line of the first membrane filtration tank;
a pump configured to deliver treated water for back-pulsing to the first membrane filtration tank; and
a controller calculating a water permeability based on the transmembrane pressure measured by the pressure gauge and the permeation flux measured by the flow meter to calculate a water permeability recovery rate and predicting a degree of membrane contamination based on the calculated water permeability recovery rate through simulation and controlling a back-pulsing interval and an intermittent aeration interval based on the predicted degree of membrane contamination,
wherein the degree of membrane contamination is predicted by the controller using a cumulative prediction index given by Equation 1:
k
n
=
1
∑
i
=
1
n
R
i
+
K
c
K
0
-
n
where kn denotes a cumulative prediction index immediately after performing back-pulsing, or back-pulsing plus intermittent aeration, n times from a current time point; K0 denotes an initial water permeability of the membrane before use of the membrane; Kc denotes a water permeability of the membrane immediately after a latest back-pulsing; and Ri denotes a water permeability recovery rate obtained by an i th cleaning after the latest back-pulsing; and
a predictive value of Ri for calculating the cumulative prediction index kn is determined by a linear regression of j data (R1′ to Rj′) accumulated in the past, as given by Equation 2:
R
i
=
t
j
R
j
′
_
-
t
j
_
·
R
j
′
_
t
j
2
_
-
t
j
_
2
(
t
i
-
t
j
_
)
+
R
j
′
_
where ti denotes a time point requiring prediction and is selected from time points immediately after the i th cleaning expected in the future; ‘-’ (bar) denotes arithmetic mean; and Rj′ denotes a water permeability recovery rate at the past time point of tj and is defined as in Equation 3 based on the initial water permeability K0:
R
j
′
=
1
-
K
j
-
1
-
K
j
K
0
[
Equations
3
]
where j>1, Kj−1 denotes a water permeability of the membrane immediately after a j−1 th cleaning, and Kj denotes a water permeability of the membrane immediately after the j th cleaning.
2. The membrane cleaning system according to claim 1 , wherein the first membrane filtration tank is a submerged membrane filtration tank.
3. The membrane cleaning system according to claim 1 , wherein if the degree of membrane contamination is less than a first set value, the controller maintains a current operating condition, whereas if the degree of membrane contamination is greater than or equal to the first set value, the controller compares the degree of membrane contamination with a second set value; and
if the degree of membrane contamination is less than the second set value, the controller adjusts the back-pulsing interval to optimize cleaning conditions, whereas if the degree of membrane contamination is greater than or equal to the second set value, the controller adjusts the aeration interval.
4. The membrane cleaning system according to claim 1 ,
wherein the pump configured to deliver treated water for back-pulsing is in the treated water discharge line of the first membrane filtration tank.
5. The membrane cleaning system according to claim 1 , wherein an aeration amount by the aerator is adjusted using a blower based on information received by the controller.
6. The membrane cleaning system according to claim 1 , wherein water to be treated, discharged from the first membrane filtration tank, is introduced into a second membrane filtration tank, and water to be treated in the second membrane filtration tank is recovered by being introduced into the first membrane filtration tank as raw water.
7. A method for cleaning a membrane of a membrane cleaning system including a membrane filtration tank provided with the membrane and an aerator, a pressure gauge and a flow meter on a treated water discharge line of the membrane filtration tank, a pump configured to deliver treated water for back-pulsing to the membrane filtration tank, and a controller, the method comprising:
measuring a transmembrane pressure using the pressure gauge and a permeation flux of treated water from the membrane filtration tank using the flow meter to calculate a water permeability;
calculating a water permeability recovery rate using the controller based on information of the water permeability;
predicting a degree of membrane contamination using the controller based on the water permeability recovery rate through simulation;
if the degree of membrane contamination is less than a first set value, maintaining a current operating condition, whereas if the degree of membrane contamination is greater than or equal to the first set value, comparing the degree of membrane contamination with a second set value; and
if the degree of membrane contamination is less than the second set value, adjusting a back-pulsing interval to optimize cleaning conditions, whereas if the degree of membrane contamination is greater than or equal to the second set value, adjusting an aeration interval,
wherein the degree of membrane contamination is predicted using a cumulative prediction index given by Equation 1:
k
n
=
1
∑
i
=
1
n
R
i
+
K
c
K
0
-
n
where kn denotes a cumulative prediction index immediately after performing back-pulsing, or back-pulsing plus intermittent aeration, n times from a current time point; K0 denotes an initial water permeability of the membrane before use of the membrane; Kc denotes a water permeability of the membrane immediately after a latest back-pulsing; and Ri denotes a water permeability recovery rate obtained by an i th cleaning after the latest back-pulsing; and
a predictive value of Ri for calculating the cumulative prediction index kn is determined by a linear regression of j data (R1′ to Rj′) accumulated in the past, as given by Equation 2:
R
i
=
t
j
R
j
′
_
-
t
j
_
·
R
j
′
_
t
j
2
_
-
t
j
_
2
(
t
i
-
t
j
_
)
+
R
j
′
_
where ti denotes a time point requiring prediction and is selected from time points immediately after the i th cleaning expected in the future; ‘-’ (bar) denotes arithmetic mean; and Rj′ denotes a water permeability recovery rate at the past tj and is defined as in Equation 3 based on the initial water permeability K0:
R
j
′
=
1
-
K
j
-
1
-
K
j
K
0
[
Equations
3
]
where j>1, Kj−1 denotes a water permeability of the membrane immediately after the j−1 th cleaning, and Kj denotes a water permeability of the membrane immediately after the j th cleaning.