Method of controlling waste water purification plants using multiple control functions

A method of automatically controlling a wastewater purification plant comprises the steps of measuring two or more of a number of parameters, determining a control parameter on the basis of the measurement results obtained and at least two selected control functions, selecting a control action on the basis of the determined control parameter and subsequently implementing the selected control action.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of automatically controlling a 
waste water purification plant wherein one or more of a number of system 
parameters are measured, a control parameter on the basis of the 
measurement results obtained and a selected control function are 
determined, a control action on the basis of the determined control is 
selected parameter, and the selected control action is implemented. 
2. The Prior Art 
In a prior art method of the type mentioned above, control is effected of 
biological waste water plants, wherein it is desired to carry out a 
microbial removal of the nitrogen and phosphorous containing compounds as 
well as organic matter. 
A variety of embodiments of such biological purification plants are known, 
but they generally have the common feature that they comprise a 
nitrification tank or zone operated in aerobic conditions, a 
denitrification tank or zone operated in anoxic conditions and a 
clarification tank in which a sedimentation of active sludge is carried 
out and from which tank part of the active sludge is generally recycled to 
the nitrification and/or denitrification tank. 
The above-mentioned group of purification plant types comprises two main 
types, viz. plants wherein recycling of completely or partially treated 
waste water is effected and plants comprising two treatment tanks which 
are alternately operated in anoxic and aerobic conditions, and in which 
plants no recycling of completely or partially treated waste water is 
effected. 
In the prior art method an initial measurement is carried out of a given 
parameter, such as the oxygen concentration and the ammonium 
concentration, in the aerobic tank. On the basis of the measurement result 
obtained the state of the aerobic tank is identified, and subsequently the 
identified plant state and a preselected control criterium, e.g., 
maintenance of the oxygen concentration in the aerobic tank at a desired 
level, form the basis of a selection of a control action, e.g., in the 
form of a change in the oxygen supply rate. 
Identification of the plant state is effected by means of a mathematical 
model for the relevant purification process quantitatively describing the 
correlation between the various measurement parameters. 
The mathematical model allows i.a. control carried out using a given 
control criterium related to a specific measurement parameter, to be 
carried out on the basis of a measurement of another parameter, e.g. the 
oxygen set point may be controlled on the basis of a measurement of the 
ammonium concentration. 
In another prior art method of the type described above, control is 
effected on the basis of measurement results for one of two or more 
measurement parameters. Priorities are given to the individual measurement 
parameters on the basis of their information value, suitability and 
credibility, and in normal conditions control is effected on the basis of 
measurement results for the measurement parameter of first priority. 
If, for a period of time, it is impossible to obtain measurement results 
for the measurement parameter of first priority, or in case the 
measurement results obtained are considered erroneous and therefore have 
to be rejected, control is instead effected on the basis of the 
measurement parameter of second priority etc. This is generally known as a 
priority sequence of control criteria. 
In a third prior art control method, control is carried out using 
measurements for two or more parameters simultaneously. In this control 
method, the measurement result of one parameter, e.g., the ammonium 
concentration, is used to determine the desired value (the set point) of a 
second parameter, e.g., the oxygen concentration. The fixed set point of 
the second parameter is then compared to a measurement of said parameter 
carried out simultaneously with the measurement of the first parameter, 
and on the basis of the said comparison a control action is then selected 
for the change of the second parameter from the measured value to the set 
point value. Such control method is generally known as a cascade control. 
"Computer Control of an Alternating Activated Sludge Process", Kummel M. 
and Nielsen M. K., published at The International Symposium on Process 
Systems Engineering, Kyoto, August 23-27, 1982, discloses a method of 
controlling a biological purification plant comprising two treatment tanks 
which are alternately operated in anoxic and aerobic conditions, and 
wherein the flow pattern is changed accordingly and so that the untreated 
waste water is supplied to the anoxic tank, from which it is carried to 
the aerobic tank and therefrom further on in the plant to a clarification 
tank in which a sedimentation of active sludge is carried out, which 
sludge is subsequently recycled in the plant for introduction into the 
anoxic tank, and from which clarification tank the effluent is discharged. 
The control is effected by means of a computer collecting the measurement 
results, analyzing the results on the basis of a mathematical model and 
implementing new control strategies. 
In the prior art method, measurements of oxygen, ammonium and nitrate are 
carried out using suitable sensors, the control parameters used being the 
oxygen supply rate and the nitrification and denitrification period ratio. 
In the prior art method, the ammonium and nitrate concentration methods are 
used continuously to determine the corresponding optimum oxygen 
concentration (the set point) during the nitrification and denitrification 
processes, respectively. 
Furthermore the nitrification and denitrification period ratio is 
controlled relative to the ammonium content of the untreated waste water, 
i.e., such that the nitrification period is prolonged when the ammonium 
load is high and shortened when the ammonium load is low, and vice versa 
for the denitrification period. 
EP-A-0,446,036 discloses an apparatus for controlling a system, e.g. a 
waste water purification plant, the apparatus:comprising 1) a number of 
measuring units, 2) means for analysing measurement data in order to 
select a characteristic data set, 3) means for analysing the 
characteristic data set in order to identify a possible operation problem, 
4) means for analysing the operation problem in order to find a strategy 
for resolution of the problem, and 5) means for controlling the system on 
the basis of the strategy. 
SUMMARY OF THE INVENTION 
Use of the means for analysing the operation problem includes a strategy 
determination mechanism, wherein measurement results for a number of 
parameters are used as input data to a neural network, and the information 
contained in the output data from the neural network is used as a basis 
for determining a set of setpoints for the control parameters. 
It is the object of the present invention to provided method of the type 
described which provides a more efficient, and accurate control than the 
prior art methods. 
The method according to the invention is characterized in that the control 
parameter is determined on the basis of the measurement results for at 
least two parameters and the control functions associated with said 
parameters. 
The invention is based on the discovery that some of the parameters 
measured during the control of a waste water purification plant provide 
information about the same physical conditions, and that consequently such 
parameters may be used to obtain a more accurate, quick and reliable 
identification of the state of the plant and determination of the control 
parameter, thereby resulting in a more efficient control of the plant. 
Furthermore, the use of the method according to the invention allows an 
improved utilization of the capacity of the purification plant. 
In addition it is possible to identify variations in the amount and 
concentration of polluting substances in the waste water supplied to the 
plant more quickly than with the prior art methods, and consequently a 
more efficient control is obtained. 
As used herein the term "control function" means the function according to 
which a given control parameter according to a mathematical model of the 
deterministic/stocastic type is determined relative to a given measurement 
parameter or a parameter derived therefrom, i.e. the control function 
defines the correlation between the control parameter and the measurement 
parameter or the parameter derived therefrom. 
The control functions used according to the invention are preferable 
determined on the basis of past data and experience from earlier 
operations. 
The control functions used are typically discontinuous functions which are 
a combination of various continuous functions. 
An example of a control parameter in a control function is the desired 
value (the set point) of the oxygen concentration in a nitrification tank. 
Examples of measurement parameters associated with this control parameter 
comprise the nitrate concentration, the redox potential and the phosphate 
concentration. 
The fixed set point of the oxygen concentration in the nitrification tank 
forms, e.g. in combination with a measurement value for the same oxygen 
concentration, the basis of the selection of a suitable control action in 
the form of an increase or a reduction in the oxygen supply to the 
nitrification tank. 
Another example of a control parameter of a control function is the actual 
ammonium concentration in a denitrification tank. Examples of measurement 
parameters associated with this control parameter comprise the ammonium 
concentration, the oxygen concentration and the oxygen supply. 
The fixed value for the ammonium concentration may e.g. form the basis of 
determining whether the operation conditions should be shifted between the 
nitrification and denitrification tanks during the control of a 
purification plant of the type described above in connection with the 
disclosure of the article "Computer Control of an Alternating Activated 
Sludge Process". 
A preferred embodiment of the invention is characterized in that the 
control parameter is determined on the basis of a weighted combination of 
control functions, the control functions being weighted relative to their 
suitability in connection with the control action in question. 
The use of this embodiment of the invention allows the control functions 
used in the determination of the control parameter to be weighted 
differently depending on the magnitude of the parameter. 
The control parameter may, e.g., be determined by use of one of the 
following two formulas: 
##EQU1## 
wherein CP is the determined control parameter, w are weights, c.sub.pi 
are the control parameter determined with the individual control functions 
and m is a positive integer above 1, 
##EQU2## 
wherein CP, w.sub.i, c.sub.pi and m have the meaning defined above, and 
wherein n.sub.i are real numbers. 
Due to the use of the n.sub.i- values, formula (2) allows different 
weighting of the individual control functions depending on the magnitude 
of the control parameter. 
The determination of the control parameter using weights for the individual 
control functions may further be carried out by using a combination of 
different continuous functions, such as exponential, logarithmic and 
potency functions, i.e., using conventional statistic and stocastic 
models. 
The determination of the control parameter and the subsequent selection of 
the control action are preferably carried out using a mathematical model 
for the purification plant which defines the correlation between 
measurement parameters, derived measurement parameters and control 
parameters and which can describe the state of the purification plant at 
the relevant point of time. Alternatively, the control action may be 
determined on the basis of a predetermined set of rules. 
Another preferred embodiment of the invention is characterized in that the 
quality of the measurement results is evaluated and that the control 
parameter is determined on the basis of the evaluated measurement results. 
The evaluation of the quality of the measurement results is preferably 
carried out using the method described in DK patent application No. 
1677/91 having the title "Method of controlling waste water purification 
plants using quality evaluation of measurement data", said application 
being filed on the same day as the present application. 
Reference is made to the application for a more detailed description of the 
way in which the evaluation of the quality of the measurement results is 
carried out in the above-mentioned preferred embodiment of the invention. 
The quality evaluation of the measurement results is preferably carried out 
on the basis of a comparison of the measurement value for at least one 
parameter with an expected dynamic value interval calculated continuously 
on the basis of the mathematical model and a simultaneous and/or previous 
measurement of one or several other parameters and/or a previous 
measurement of the same parameter. 
The expected dynamic value interval is preferably determined by calculation 
of an expected dynamic value and maximum variations therefrom. 
More preferably, the quality evaluation of the measurement results is 
carried out by evaluating the credibility of the measurement value on the 
basis of the comparison of the measurement value with the expected dynamic 
value interval by the allocation of a credibility factor which, in 
combination with the measurement value, is used in the subsequent 
determination of the control parameter. 
Prior to the determination of the control parameter, the measurement 
results may possibly be corrected with a value corresponding to the 
magnitude of the identifiable measurement interference, if any. 
As used herein the term "identifiable measurement interference" means 
measurement interference caused by influences imposed on the purification 
plant in connection with the control of same. 
The quantification of the identifiable measurement interference is 
preferably carried out on the basis of the mathematical model and past 
data of response courses for control modifications of the same type 
carried out previously. 
In the above preferred embodiment of the invention, the control parameter 
is determined at any time on the basis of the most credible measurement 
results of those available and by weighting of same according to their 
credibility, thereby obtaining an optimum control compared to the 
collected material of measurement results, and which control is far more 
efficient than the control obtained with the prior art methods. 
In this preferred embodiment of the invention, the control parameter is 
determined on the basis of the collected measurement values for two or 
more parameters and the control functions and credibility factors 
associated therewith. 
In this case the control parameter may e.g. be determined by using one of 
the following two formulas: 
##EQU3## 
wherein CP, w.sub.i, cp.sub.i and m have the meaning defined above, and 
wherein cf.sub.i is a credibility factor, 
##EQU4## 
wherein CP, w.sub.i, cp.sub.i, cf.sub.i, n.sub.i and m have the meaning 
defined above. 
The method according to the invention is preferably carried out using an 
integral control and computer system (control apparatus) collecting and 
storing measurement results and control signals, processing the collected 
data using a mathematical model and implementing new control actions. 
The waste water purification plant controlled according to the method of 
the invention may be a biological waste water purification plant wherein 
the purification is carried out by means of microorganisms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The action steps shown in FIG. 1 will now be explained in further details. 
By using various measurement apparatuses, measurements of a number of 
parameters are carried out sequentially at different places in the waste 
water purification plant, and the measurement data thus obtained are 
collected (step 1) in the data base of a control apparatus. Examples of 
such measurement parameters comprise the concentration of ammonium, 
nitrate, oxygen, phosphate, cell dry matter and biomass in the untreated 
waste water, at various places in the purification plant and in the 
effluent, the amount of supplied untreated waste water and the amount of 
the oxygen supplied to the plant. 
Furthermore sequential data are collected in the data base of the control 
apparatus for a variety of different control parameters (step 2), 
sequential data for a number of parameters (step 3) describing the state 
of the purification plant, such as time of the day and flow pattern, and 
data for the response course of the purification plant (step 4) on control 
actions previously made. 
On the basis of the collected measurement data, derived measurement data 
are computed in the control apparatus (step 5), such as the rate of change 
of the oxygen concentration, the oxygen consumption rate and the 
nitrification and denitrification rate. 
On the basis of the data collected during steps 1-5, a quality evaluation 
and correction of the measurement data collected in step 1 is carried out 
in step 6. 
The set of quality evaluated and corrected measurement data obtained in 
step 6 forms the basis of the determination of the control parameter and a 
selection of the final control action (step 7). 
This step may be carried out using a mathematical model defining the 
correlation between the measurement parameters, the derived measurement 
parameters and the control parameters and describing the state of the 
purification plant at the relevant point of time. Alternatively the 
control action may be determined on the basis of a predetermined set of 
rules. 
After the final control action has been selected, it is implemented (step 
8). The control apparatus effects the implementation by modifications of 
the setting of the control apparatus associated with the individual 
control parameters. 
With reference to FIG. 2 it will now be explained in further details how 
the above-mentioned quality evaluation and correction (step 6) is carried 
out. 
A given measurement value is initially subjected to a primary evaluation 
(step 10) comprising investigating whether the measurement value is 
comprised within a value interval having fixed and relatively wide limits 
corresponding to the maximum and minimum, respectively, values of the 
relevant measurement parameter appearing in ordinary operation conditions. 
Furthermore, the primary evaluation comprises investigating whether the 
change of the measurement value as compared to the latest measurement 
carried out is comprised within a value change interval set so as also to 
have fixed and relatively wide limit values corresponding to the maximum 
values of the relevant measurement parameter appearing in ordinary 
operation conditions. 
If the measurement value is not comprised within the above value interval, 
or if the change of the measurement value is not comprised within the 
above value change interval, the measurement value is rejected as 
erroneous. 
Thus, the primary evaluation serves to discard the evidently erroneous 
measurements. 
The state of the waste water purification plant at the time of the 
measurement is then identified (step 11), cf. step 3 described above. 
In steps 12 and 13 the measurement value is verified, i.e. it is evaluated 
whether the value is correct or not. 
The verification is effected by determining (step 13) whether the 
measurement value is comprised within a value interval determined on the 
basis of an expected value and maximum deviations therefrom, which may be 
computed (step 12) on the basis of the data collected in steps 1-5 and the 
mathematical model quantitatively describing the correlation between 
different parameters. 
An example of such calculation of the expected value and maximum deviations 
is that the ammonium concentration in a given treatment tank is calculated 
on the basis of measurements of the amount of waste water supplied to the 
plant and the time of the day, providing indirect information about the 
ammonium concentration of the supplied waste water, and/or the past course 
for the ammonium concentration in the treatment tank, and/or the past 
course for the nitrate concentration in the treatment tank, and/or the 
oxygen concentration in the treatment tank, and the amount of oxygen 
supplied thereto, together providing information about the oxygen 
consumption rate. 
When using several methods to calculate the expected value and maximum 
deviations therefrom, the methods are weighted according to their 
credibility. 
If the measurement value is not comprised within the calculated value 
interval, the deviation of the measurement value from the expected value 
is calculated and stored (step 14). 
Subsequently, it is investigated whether the measurement value includes 
identifiable measurement interference (step 15). Such identifiable 
measurement interference results from modifications made in the state of 
the purification plant in order to control said plant, such as 
modifications in the flow pattern of the purification plant by control of 
the pump operation and change in the oxygen supply rate to a treatment 
tank by control of the supply pump. 
Such control modifications give rise to a relatively brief change of the 
measured parameter, which change of measurement parameter is not 
symptomatic of the general state dynamics of the plant. 
Consequently, such brief change of the measurement parameter is neglected 
by correcting the measurement value with a value corresponding to the 
interference (step 16). The quantification of the interference is carried 
out on the basis of the mathematical model and past data of the response 
courses for modifications of the same type previously made, which data are 
collected and stored in the memory of the control apparatus. 
After the measurement value has been corrected, it is investigated again 
whether the corrected measurement value is comprised within the value 
interval computed in step 12. 
If it is found in step 16 that the measurement value does not include any 
identifiable interference, it is investiagted whether the value interval 
calculation made in step 12 is incorrect (step 17), which e.g. may be the 
case if sudden changes in the load of the purification plant occur, i.e. 
changes in the amount and/or concentration of the waste water supplied to 
the plant. Thus, step 17 includes measurement values for further 
measurement parameters compared to the measurement parameters forming the 
basis of the value interval calculation made in step 12. 
If it is found in step 17 that the state of the purification plant has 
changed so that the value interval calculation made in step 12 is 
incorrect, a revised value interval (step 18) is computed on the basis of 
the measurement parameters used in steps 12 and 17, which revised value 
interval is used for comparison with the measurement value approved in 
step 10 and possibly corrected in step 15. 
As explained above, initially only measurement results for a limited set of 
measurement parameters are preferably used in the value interval 
calculation made in step 12, as measurement results for a further set of 
measurement results are only included, if it is found that the measurement 
value is beyond the value interval initially computed. Such division of 
the verification procedure is preferred in order to limit the calculation 
work associated therewith and hence the necessary computer capacity. 
Alternatively, all the measurement parameters used in steps 12 and 17 may 
be included in the value interval calculation initially made, 
corresponding to the cancellation of steps 17 and 18 from the flow diagram 
shown in FIG. 2. 
After the verification and a correction, if any, the measurement value is 
evaluated as to credibility (step 19), irrespective of whether said value 
is comprised within the value interval calculated in steps 12 or 18, or 
not. 
Of course measurement values beyond the above mentioned value interval have 
a low credibility and are generally not used in the subsequent selection 
of the final control action, except in particular situations where the 
measurement results obtained are few or of a poor quality. 
The credibility evaluation is effected by comparing said measurement value 
with the value interval computed in step 12 or the revised value interval 
calculated in step 18, and on the basis of the result of this comparison 
by subsequently allotting the measurement value a credibility factor which 
is stored in the data base of the computer system (step 20), and using 
said factor in combination with the possibly corrected measurement value 
for the subsequent selection of the final control action. 
The invention will now be explained in further detail with reference to the 
following example. 
EXAMPLE 
It is desired to control a biological waste water purification plant 
comprising two treatment tanks which are alternately operated in anoxic 
and aerobic conditions, and wherein the flow pattern is changed 
accordingly and so that the untreated waste water is supplied to the 
anoxic tank (denitrification tank), from which it is carried to the 
aerobic tank (nitrification tank) and therefrom further on in the plant to 
a clarification tank, in which a sedimentation of active sludge is carried 
out, the sludge subsequently being recycled in the plant for introduction 
into the anoxic tank and from which clarification tank the effluent is 
dicharged. 
The general control strategy comprises two control criteria, viz. 1) 
shifting the operation state between the two treatment tanks, i.e., a 
change in the set point of the oxygen concentration in the two tanks and a 
change in the flow pattern of the plant, if both the nitrate concentration 
in the denitrification tank and the ammonium concentration in the 
nitrification tank are less than or equal to predetermined respective 
minimum limit values, and 2) controlling the oxygen concentration during 
the course of the nitrification and denitrification phase in the two 
respective tanks in relation to the desired oxygen concentration (set 
point) determined on the basis of measurements of other parameters. 
Control according to control criterium 1) is effected by use of, e.g., the 
ammonium concentration in the nitrification tank as control parameter, and 
the measurement parameters associated therewith are the oxygen 
concentration, the oxygen supply and the ammonium concentration in the 
same tank. 
On the basis of the measurement values for the oxygen concentration of and 
the oxygen supply to the aerobic tank, it is possible to compute the 
oxygen consumption rate in the tank. The change in the oxygen consumption 
rate is coupled to the ammonium concentration, and the correlation between 
the two noted parameters, i.e., the control function used is determined on 
the basis of past data and experience from earlier operations and 
calculations using a mathematical model. The control function is shown in 
FIG. 3. 
Control according to control criterium 2) is effected by use of e.g. the 
set point of the oxygen concentration in the denitrification tank as 
control parameter, and the measurement parameters associated therewith 
comprise the nitrate concentration, the phosphate concentration and the 
redox potential in the same tank. 
The control functions used for the three measurement parameters are 
determined on the basis of past data and experience from earlier 
operations, and the functions will appear from FIGS. 4-6 showing the 
desired oxygen concentration measured in mg O.sub.2 per liter as a 
function of the nitrate concentration, the rate of change of the phosphate 
concentration (calculated on the basis of the phosphate measurements) and 
the redox potential, respectively. 
Control according to control criterium 1) results in a measurement of an 
ammonium concentration in the nitrification tank of 1.5 mg NH.sub.4 --N 
per liter, and the measured values for the oxygen concentration and the 
oxygen supply in the same tank are calculated to correspond to a change in 
the oxygen consumption rate of -0.5 mg O.sub.2 per liter per hour. On the 
basis of the control function shown in FIG. 3 it is found that the 
measured oxygen consumption rate corresponds to an ammonium concentration 
of 0.9 mg NH.sub.4 --N per liter. 
The control functions for the measurement parameter of ammonium 
concentration and the derived measurement parameter of oxygen consumption 
rate are allotted the weights 0.8 and 0.2, respectively. 
The control parameter, i.e. the ammonium concentration (AC), is then 
determined using the above formula (1): 
##EQU5## 
As the determined value for the control parameter is greater than the 
minimum limit value causing a shift in the operation conditions between 
the two treatment tanks, no such shift is effected. 
Control according to control criterium 2) results in a measurement of a 
nitrate concentration in the denitrification tank of 0.5 mg NO.sub.3 --N. 
per liter and a redox potential of 90 mV, and on the basis of measurements 
of the phosphate concentration in the same tank the rate of change of the 
phosphate concentration is calculated to amount to 2 g PO.sub.4 --P per 
m.sup.3 per hour. On the basis of the control functions shown in FIGS. 
4-6, three different values for the set point of the oxygen concentration 
are found, viz. 0.2 mg O.sub.2 per liter, 0 mg O.sub.2 per liter and 0.7 
mg O.sub.2 per liter, respectively. 
The control functions for the measurement parameter of nitrate 
concentration, the derived measurement parameter of rate of change of the 
phosphate concentration and the measurement parameter of redox potential 
are allotted the weights 7, 5 and 3, respectively. 
The control parameter, i.e. the set point of the oxygen concentration 
(SOC), is then determined using the above formula (1): 
##EQU6## 
On the basis of the determined control parameter, a control action can now 
be selected, causing the oxygen concentration in the denitrification tank 
to be raised from the previous 0 mg O.sub.2 per liter to 0.23 mg O.sub.2 
per liter. This increase in the oxygen concentration expresses the fact 
that the information contained in the measurement values used for the 
three measurement parameters indicates that the nitrate concentration is 
less than the minimum limit value causing a shift in the operation state 
to be effected, and that the set point of the oxygen concentration 
therefore may be slightly raised to allow reaction of ammonium during the 
period up to the point of time where a shift in the operation state 
between the tanks is carried out. Consequently, an improved utilization of 
the volume capacity of the plant and a more efficient purification of the 
waste water are obtained. 
Furthermore, the above control procedure results in a very quick and 
reliable identification of the state of the denitrification tank and hence 
a more efficient control of the same.