US 2022/0027706 A1 · Membrane Fouling Warning Method Based on Knowledge-Fuzzy Learning Algorithm

Abstract

An intelligent warning method based on knowledge-fuzzy learning algorithm is designed for membrane fouling with high accuracy. A multi-step prediction strategy, using the least-squares linear regression model, is developed to predict the characteristic variables of membrane fouling Meanwhile, the knowledge of membrane fouling category, which is extracted from the real wastewater treatment process, can be expressed as the form of fuzzy rules. Moreover, a knowledge-based fuzzy neural network is designed to establish the membrane fouling warning model, thus deal with the problem of difficult warning of membrane fouling. The results reveal that the intelligent warning method can improve the ability to solve the membrane fouling, mitigate the deleterious effect on the process performance and ensure the safety operation of the wastewater treatment process.

A membrane fouling warning method based on knowledge-fuzzy learning algorithm comprising:

(1) determine characteristic variables of membrane fouling:
for a sewage treatment process of membrane bioreactor, sewage treatment process variables are analyzed and the characteristic variables of membrane fouling are selected, including water permeability (P), permeability decay (PD), water flow (WF), gas washing size (GWS), sludge concentration (SC), transmembrane pressure (TMP), water turbidity (WT) and permeability recovery rate (PR);
(2) predict the characteristic variables of membrane fouling:
the characteristic variables are obtained by a multi-step prediction strategy based on a least-squares linear regression model;
values of a characteristic variable with first four moments are used to predict the characteristic variable at a fifth moment;
the estimation of a characteristic variable using the least-squares linear regression model can be expressed as:
xd(n+4)=ad0+ad1xd(n)+ad2xd(n+1)+ad3xd(n+2)+ad4xd(n+3), (1)
where xd(n+h) is dth characteristic variable value, n=1, 2, . . . , N−4, Nis the number of samples, d=1, 2, . . . , 8, h=0, 1, . . . , 4, x1(n+h) is a P value, x2(n+h) is a PD value, x3(n+h) is a WF value, x4(n+h) is a GWS value, x5(n+h) is a SC value, x6(n+h) is a TMP value, x7(n+h) is a WT value, x8(n+h) is a PR value, ad0, ad1, ad2, ad3, ad4 are different regression coefficients of the dth characteristic variable, and the dth characteristic variable satisfies:
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where Qd is the sum of squared errors of the dth characteristic variable with N samples, ad0, ad1, ad2, ad3, ad4 are calculated by Eq. (3);
(3) construct a knowledge base of membrane fouling:
a knowledge of membrane fouling, which is extracted from expert experience, can be expressed as the form of fuzzy rules:
Rule I:
if the WF value is more than 460 m3/h, then the WF value has reached its peak;
Rule II:
if the P value is less than 30 LMH/bar and the TMP value is less than −5 kPa, then membrane is in a state of extreme contamination;
Rule III:
if the GWS value is less than 2400 m3/h and the TMP value is less than −20 kPa, then an aeration rate is too low and the membrane is facing serious pollution threat;
Rule IV:
if the WF value is less than 300 m3/h and the GWS value is more than 8000 m3/h, then the aeration rate is too high and membrane silk will be damaged;
Rule V:
if the WT value is more than 5 NTU, then effluent quality has exceeded a standard;
Rule VI:
if the P value is more than 90 LMH/bar and the TMP value is less than −40 kPa, then the TMP value is too high and the membrane is facing serious pollution threat;
Rule VII:
if the SC value is more than 13000 mg/L, then the SC value is too high and the membrane is facing serious pollution threat;
Rule VIII:
if the SC value is less than 6000 mg/L, then the effluent quality has exceeded the standard;
Rule IX:
if the P value is less than 80 LMH/bar, the WF value is more than 300 m3/h and the GWS value is less than 4200 m3/h, then the aeration rate is too low and the membrane is facing serious pollution threat;
Rule X:
if the WF value is more than 200 m3/h and the WT value is more than 4 NTU, then the effluent quality has exceeded the standard;
Rule XI:
if the P value is less than 60 LMH/bar and the TMP value is less than −5 kPa, then the membrane has been seriously polluted;
Rule XII:
if the PD value is more than 30 LMH/bar·h, then there is rapid abnormal membrane fouling;
Rule XIII:
if the P value is less than 60 LMH/bar, the WF value is more than 200 m3/h and the GWS value is less than 5000 m3/h, then there is a tendency of sludge hardening inside the membrane silk;
Rule XIV:
if the WF value is less than 300 m3/h, the GWS value is less than 5000 m3/h and the SC value is more than 12000 mg/L, then operating parameters of membrane tank are unreasonable;
Rule XV:
if the WF value is less than 200 m3/h and the PR value is less than 90%, then online chemical cleaning effect fails to meet requirements;
Rule XVI:
if the P value is more than 80 LMH/bar, the PD value is less than 30 LMH/bar·h, the GWS value is more than 5000 m3/h, the SC value is less than 13000 mg/L, the WT value is less than 4 NTU and the PR value is more than 90%, then there is no warning;
Rule XVII:
if the P value is more than 80 LMH/bar, the PD value is less than 30 LMH/bar·h, the GWS value is less than 8000 m3/h, the SC value is less than 13000 mg/L, the WT value is less than 4 NTU and the PR value is more than 90%, then there is no warning;
Rule XVIII:
if the P value is more than 80 LMH/bar, the PD value is less than 30 LMH/bar·h, the GWS value is more than 5000 m3/h, the SC value is more than 6000 mg/L, the WT value is less than 4 NTU and the PR value is more than 90%, then there is no warning;
Rule XIX:
if the P value is more than 80 LMH/bar, the PD value is less than 30 LMH/bar·h, the GWS value is less than 8000 m3/h, the SC value is more than 6000 mg/L, the WT value is less than 4 NTU and the PR value is more than 90%, then there is no warning;
the above fuzzy rules of membrane fouling categories can be concluded as
if x1(n)∈L1k(n) and . . . xd(n)∈Ldk(n) and . . . x8(n)∈L8k(n), then yj(n)=rj(n), (4)
where k is kth fuzzy rule, k=1, 2, . . . , 19, Ldk(n) is a linguistic term of the dth characteristic variable in nth fuzzy rule, yj(n) is jth membrane fouling category, rj(n) is a linguistic term of the jth membrane fouling category and is formulated as:
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where wkj(n) is a weight between the kth fuzzy rule in a rule layer and the jth membrane fouling category in an output layer, vk(n) is an output of the rule layer:
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where cdk(n) and σdk(n) are a center and a width of dth membership function in the kth fuzzy rule:
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where Adk(n) and Bdk(n) are upper and lower values of the dth membership function in the kth fuzzy rule;
(4) establish an intelligent warning model for membrane fouling:
the characteristic variables are input variables of the intelligent warning model;
output variables are selected from the knowledge base of membrane fouling;
then an intelligent warning model based on fuzzy neural network (FNN) is established;
a structure of FNN comprises four layers:
an input layer, a membership function layer, a rule layer and an output layer;
a network is 8-19-19-16, including 8 neurons in the input layer, 19 neurons in the membership function layer, 19 neurons in the rule layer and 16 neurons in the output layer;
connection weights between the input layer and the membership function layer are assigned to 1;
an input of FNN is X(t)=[x1(t), x2(t), . . . , xN(t)], xn(t)=[x1n(t), x2n(t), . . . , x8n(t)], t=1, 2, . . . , T, T is the number of iterations;
an expectation output of neural network is expressed as Ye(t) and an actual output is expressed as Y(t);
four layers of FNN are specifically as follows:
the input layer:
there are 8 neurons which represent input variables in this layer, an output value of each neuron is
ui(t)=xi(t),i=1,2, . . . ,8, (10)
where ui(t) is ith output vector at tth iteration and an input vector is xi(t)=[x1(t), x2(t), . . . , xN(t)];
the membership function layer:
there are 19 neurons of the membership function layer;
outputs of the membership function neurons are:
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where cdk(t) denotes a center vector of kth membership function neuron at the tth iteration, cdk(t)=[cdk1(t), cdk2(t), . . . , cdkN(t)]T, and σdk(t) is a width vector of the kth membership function neuron at the tth iteration, σdk(t)=[σdk1(t), σdk2(t), . . . , σdkN(t)]T,
the rule layer:
there are 19 neurons of the rule layer;
outputs of the rule layer are:
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where vk(t) is an output vector of kth rule neuron at the tth iteration, vk(t)=[vk1(t), vk2(t), . . . , vkN(t)]T;
the output layer:
this layer contains 16 neurons;
output values of the output layer are:
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where Yj(t)=[y1(t), y2(t), . . . , yN(t)], which is the membrane fouling category, represents the output of FNN at the tth iteration, wkj(t)=[wkj1(t), wkj2(t), . . . , wkjN(t)]T is a connection weight between the kth rule neuron and the jth output neuron at the tth iteration;
based on a softmax function, probabilities of the output neurons are:
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where pj(t) is a probability vector of the jth output neuron at the tth iteration, pj(t)=[pj1(t), pj2(t), . . . , pjN(t)]T, the jth membrane fouling category with a corresponding maximum value of pj(t) is selected to warn future events of membrane fouling;
(5) train the FNN model:
{circle around (1)} given the FNN model, inputs of FNN are x(1), x(2), . . . , x(t), . . . , x(T), correspondingly, expectation outputs are Yd(1) Yd(2), . . . , Yd(t), . . . , Yd(T);
{circle around (2)} set learning step s=1;
{circle around (3)} t=s;
according to Eqs. (1)-(14), calculate probabilities of the output neurons, exploiting gradient descent algorithm:
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(
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where wkj(t+1) is a connection weight between the kth rule neuron and the jth output neuron at t+1th iteration, ηw∈(0, 0.01] is a learning rate, I(t)=[1{Y1(t)}, 1{Y2(t)}, . . . , 1{Y16(t)}], 1{⋅} represents an indicator function;
{circle around (4)} if t≤T, go to step {circle around (3)};
if t>T, stop the training process;
(6) realize membrane fouling warning:
the least-squares linear regression model utilizes testing samples of P, PD, WF, GWS, SC, TMP, WT and PR to predict multiple values of the characteristic variables;
then, the testing samples of P, PD, WF, GWS, SC, TMP, WT and PR are used as the inputs of FNN, the outputs of FNN are the membrane fouling categories.

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Membrane Fouling Warning Method Based on Knowledge-Fuzzy Learning Algorithm

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Membrane Fouling Warning Method Based on Knowledge-Fuzzy Learning Algorithm

Life of this patent

Abstract

Claims1