Application of fusion control strategy in purification treatment for workshop wastewater*

Zheng-qiao LEI

Dean’s Office，Chongqing Industry Polytechnic College，Chongqing 401120，China

1.Introduction

Organic matter content is very rich in the production wastewater and garbage percolate fluid of production workshop，it is difficult to use conventional techniques to deal with it，and it is more difficult to make control for biochemical oxygen demand(BOD)and chemical oxygen demand(COD)specially．The former describes the oxidation decomposition process resulted from microbial and biochemical effects of organic substances in wastewater，BOD is the total dissolved oxygen consumption when the sewage wastewater treatment combined with air，and BOD shows the size of the pollution degree of sewage．If the value is high then it shows the sewage with organic pollutants more，and the degree of pollution is more serious．In China，the legal regulations shows that the BOD maximum allowable concentration must be less than 100 mg/L in industrial wastewater，and surface water BOD must be less than 4 mg/L．The latter is same as the former，it is an important indicator of the extent of water pollution，its measure unit is ppm or mg/L，it shows the pollution degree of water quality and the smaller the better in value of water．In China，the legal regulations shows that COD maximum allowable concentration must be less than 100 mg/L in the industrial waste water．The oxygen dissolved in water for the air is known as DO(Dissolved Oxygen)，its significance is that the oxygen is dissolved into the water．It is a measure of its purification ability of water index，and its measure unit is mg/L．Under the action of aerobic bacteria，Some organic chemical compounds will elicit a biological response，and the dissolved oxygen in water would be consumed．If the organic matter is substituted by car-bon，then each 12 g carbon would consume 32 g oxygen according to the chemical formula C+O2=CO2．When the water dissolved oxygen decreased to 5 mg/L The breath of some fish will become very difficult．Dissolved oxygen in water could get continuous supplement because of the oxygen in the air going into water and aquatic plant photosynthesis，but when the waters suffer from the organic chemical compounds pollution the oxygen is seriously consumed，the dissolved oxygen can not be added in a timely manner，then anaerobic bacteria will be immediately and quickly perfunctory，and the corrosion of organic substances of plant corruption will make the water black and stink．Therefore，the importance of dissolved oxygen is that it is a basic research of water self purification，the time from the dissolved oxygen is consumed to return to the initial state represents that if the time required is short then itself ability to purify water is strong，or that the water pollution is not serious．Conversely the water pollution is serious，and even the water loses its ability to clean it own．

The above shows that the wastewater treatment process is very complex，the key technology is how to control the dissolved oxygen，and that is to say how to control the DO parameters．

2.Cybernetics characteristic of wastewater treatment

The above has been pointed out that the dissolved oxygen DO is the most important parameter for wastewater purification．Dissolved oxygen of high concentration is propitious to degrade contaminants，and accordingly，it makes the water be quickly purified．In order to improve the process efficiency and reduce energy consumption，many scholars have done a lot of research work［1-4］．In the wastewater treatment process，the most important parameter is the outflow and inflow water quality parameters，and the biochemical treatment system itself has no choice but to maintain its balance．If the DO parameter is too low or too high，so it is necessary to adjust the fan speed that increases or reduces air tolerance in order to keep the dynamic balance．Otherwise too much tolerance will lead to great waste of energy，and if the air volume limit is not enough then it will cause that the fan is difficult to provide air tolerance．Because the wastewater biochemical treatment process is a typical complex processes，it is difficult to describe the biological and chemical processes with a rigorous mathematical method such as nonlinearity，time variation，randomness，fuzziness and instability and so on．Generally speaking，the process can be simplified as follows．

① The unknown，time-varying，random and distribution of the process parameters．

②Process parameters having the time lag with unknown and time-varying．

③The process has serious nonlinearity．

④The correlation between process parameters．

⑤Wastewater treatment owns the randomness，unknown and diversity in interference environment．

In the face of these characteristics，it is difficult to effectively control the wastewater treatment process by the traditional PID control strategy．The traditional control method is based on the exact mathematical model，but the sewage treatment process is difficult to establish accurate mathematical model because of being changeable and random in water quality and quantity of wastewater．According to technological requirement，only when the provided oxygen in wastewater treatment pool is equal to the consumption oxygen in wastewater treatment pool required over the same period，then it can ensure the quality of water．The DO parameter control is very complex in the wastewater treatment process，and therefore it is necessary to study the control strategy corresponding to wastewater treatment process characteristics．

3.Fusion control strategy

It is difficult to obtain good control effect for wastewater treatment process control with the help of conventional PID control strategy and modern control theory method，and the problem can be expressed as 5 aspects as follows．

① There is uncertainty problem in the wastewater treatment process．

②It is a highly nonlinear process．

③It makes the presence of a half structured and unstructured problems．

④There is the system complexity．

⑤There is the reliability problem．

The above mentioned problems cause that the conventional control strategy can not effectively control the wastewater treatment process，and therefore it is necessary to further study the wastewater treatment process control strategies．There are many control strategies that can provide choice for reference，but there are still many problems needed to be solved．For instance，the neural network control requires a certain number of samples，but it is difficult to obtain samples from the known experimental and pre-experiment because of the influence of uncertainty．In view of the limitations of the method，usually the neural network control is difficult to achieve the effective control for wastewater treatment process．The expert control system is a knowledge based control．Because it is difficult to capture the feature information，make expression of feature information as well as construct a mature and complete knowledge base，therefore，the expert control system is also difficult to achieve control of the wastewater treatment process．Human control experience can be summarized and described by means of human language，it can be described by fuzzy language using the fuzzy set，and it also can be implemented by"IF condition THEN action"statements．But because there is too much uncertainty，the fuzzy control of wastewater treatment process is not necessarily a good choice．The basic characteristics of human simulated intelligent control(HSIC)simulate control behavior of control expert，and therefore its control algorithm is multimodal．The specific method is performed using alternating multi mode control，such a feature can improve the coordination of many contradictions in control system，and so it is a more sensible choice．In this paper，starting from strategy optimization，it proposes a fusion control strategy based on expert control system and HSIC．Fusion control strategy summarized up the control skills and experience in field experts，and it can be summarized as engineering control algorithm．

4.Control model and algorithm

4.1.Control model

This paper combines the human simulated intelligent control and expert control system technology，when the expert system technology is introduced into the design of control system，the expert system structure can be simplified．All of the knowledge base，reasoning machine and a set of rules can be integrated into the human simulated intelligent controller．It can make that the controller has excellent performance and the system structure is simple，and therefore，it can be very good control of the water quality and the input air oxygen．Expert system is an intelligent program system，and it can solve the problem that only experts can solve according to the knowledge base and inference engine．The DO parameter control system based on Fusion control strategy can neatly use the expert system technology to realize multi modal control because of control strategy being based on the basic characteristic of opening-closed loop control combination，and so it improves the performance of the system of judgment and reasoning．The structure model of the control system is shown in Figure 1．

Figure 1．Structure model of DO parameter control system

In fact，the establishment of knowledge set is how to express that it has acquired knowledge，usually control system uses production rules to establish the knowledge set，and its basic structure can be expressed as“IF(condition)THEN(action)”．The distinct advantage of system constituted by production rule rests with the module being good，and the module is added with independence．The addition，deletion and modification of each rule do not affect each other，and so the naturality is good，especially suitable for industrial process control．

Figure 2 shows the structure of general control system，and in which，r(t)is the process input，y(t)is the process output，e(t)is the process deviation，and e(t)=r(t)-y(t)．The controller of generalized control model is based on characteristic recognition，and if it knows the feature then it can select corresponding control pattern．Figure 1 can fuse the technology of HSIC and ES to make control system structure be simplified．It can easily fuse the control and inference rule as well as knowledge base into an intelligent controller，and it is better in modularity and naturality，no direct relation among the rules，and it owns strong self-adaptability．

Figure 2．Structure of generalized control process

4.2.Control pattern

The prototype algorithm of HSIC can be ex-pressed as the following．

Where u，Kp，k，e，˙e and em，irespectively represents the control output，proportion coefficient，constraint coefficient，system deviation，change rate of system deviation and the value of the ithdeviation peak．

4.3.Control algorithm design

The control rule set of DO parameter，based on the prototype algorithm of HSIC，further summarizes the human control experience，and a set of suitable for different status control rules are established，Combined HSIC and ES，the control algorithm can be designed by structured English description as below．

5.Simulation experiment

For not losing the generality，it assumes the model as G(S)=K0e-τS/(T1s+1)(T2s+1)．In which，K0=4．134，T1=1．0 s，T2=2．0 s，τ=10 s．It adapts respectively the PID control algorithm and intelligence fusion control algorithm(simplified as HSIC)to actualize the control．At time t=15 s，it joins a pulse signal to be as the outer interference，the pulse amplitude is 1．0，and the pulse width is 0．2 s．The response curve of two sorts of control algorithm is respectively shown in Figure 3．Compared with PID control algorithm，it is short to transient process time for the HSIC algorithm，and reveals very strong robustness．The analysis of curve shows that the control effect is satisfactory，and it makes the robustness and accuracy of vulcanizing process control all reach the expected result．From the comparison of simulation control result respectively by HSIC control and PID control，it can be seen that the intelligent fusion control can make system achieve stable quickly．

Figure 3．Response curve under pulse interference

6.Conclusion

The oxygen requirement is changeable in real time in the wastewater treatment aeration process．In order to reduce energy consumption，reduce the fluctuation of system，and make the treated wastewater to meet the discharge standard，by means of intelligent fusion control strategy it can make the DO concentration in aeration tank control in the admissible range．The simulation result demonstrates that the intelligent fusion control strategy can make control system structure simple，the response of the system is real-time，and the control quality is more ideal．

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