- Email: [email protected]
Simple control strategies of methanol dosing for post-denitrification
e>
Pergamon
po: S0273-1223(98)00482-X
War.Sci. T~ch. Vol. 38. No.3. pp, 291-297. 1998. IAWQ C 1998 Published by Elsevier Science Ltd. Printed in Great Britain. All rights reserved 0273-1223198 $19'00 + 0'00
SIMPLE CONTROL STRATEGIES OF METHANOL DOSING FOR POST-DENITRIFICATION N. Puznava, S. Zeghal and E. Reddet Anjou Recherche. Research Center ofCompagnie Gin/rale des Eaux - 01V Chemin de la Digue, BP 76. 78603 Maisons-Laffitte Cedex; France
ABSTRACT The objective of this work is to propose a simple but efficient way of controlling the carbon addition for the post-denitrification process in order to comply with regulatory constraints and optimize operating cost. A Biostyr~ pilot column filled with polystyrene beads was used for the experiments. In order to simulate a secondary treated water from a nitrifying stage with a carbon source addition. the feed water was composed of river water dosed with nitrates. phosphates and methanol. Methanol was added initially with no control (at different constant rates) and in a second stage with different control slrategies based on the on-line measurement of inlet and/or outlet nitrate concentrations. This simple dosing mode proved to be very efficient in set-point tracking to ensure the effluent quality and in minimizing the methanol addition (up to 20% less methanol consumption). thus optimizing operation costs. ce 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Biostyr; methanol; on-linesensor; post-denitrification; process control; upflow floating biofilter. INTRODUCTION Upflow floating biofilters are well known and can be used in several configurations such as secondary nitrification-denitrification in one cell (Rogalla et al., 1992) or tertiary denitrification (Zeghal et al., 1995). For some areas whereeffluent nitrogen requirements are very low and where the carbon/nitrogen ratios are not very favourable for denitrification, the post-denitrification configuration can be a valuable option. High denitrification ratesusingthe upflow floating biofilter are obtained. This requires however the addition of an external carbon source. Methanol is commonly carbon source (La Cour Jansen et al., 1994) but some industrial effluents can be alsoused. Implementation of process control at wastewater treatment plants (WWTP) has increased considerably in recent years thanks to the development of on-line sensorsfor monitoring effluentdischarge. The target was to optimize operation of.~s by. meas~ring with on-li?e sensors the processvariables such as dissolved oxygen or redox potential 10 the mixed liquor (Charpentier et al.; 1989) and to control it. Moreover, for advanced treatment objectives, chemical addition control for phosphorus removal (Zeghal et al., 1996) based on turbidity measurements and expertsystems to manage sewage treatment plantsare implemented (Beutler et al., 1989).
291
292
N. PUZNAVA et al .
The aim of the present study was to investigate simple control strategies such as the feed-back (FB) and/or feed-forward (FF) control algorithms using one or two on-line sensors. This paper shows a comparison between the results obtained and also the costs for each example. MATERIALS AND METHODS Experimental set-up Figure I presents the Biostyr® pilot used for the post-denitrification . A 300 mm diameter column filled with polystyrene beads of 3.5 mm diameter was used. The height of the floating bed filter was 2.0 m. Below the filter bed, a height of 1.4 m was allowed for the bed expansion during backwash. A treated water storage tank was used for the backwash of the filter. In order to simulate a secondary treated water from a nitrifying stage with a carbon source addition, the feed water was composed of Seine river water dosed with nitrates, phosphates and methanol.
nu rate , ph Olphlit
f······
I n / C'C ll o n
;
ro", wa l
~
nnr ate ana ly lc r
" lIf_Ie anal yzC'r
r---'
Figure I. Diostyr
Nitrate measurements were made on-line using a DATALINK NTIOO analyser for water coming out of the filter (location 5 and 6). The measurement was based on the UV absorption spectrum of nitrates . Two pumps (location 2 and 3) with external command were used: one for nitrates injection and one for methanol dosing . The methanol pump was controlled by a control station (location 4), an MCC Rhapsodie unit used as datalogger and multiple input-output controller . COD was measured by using HACH methods. Transfer function identification Different methods can be used for the identification of the transfer function, the most practical known methods being based on the identification of the transfer function in open loop or closed loop systems . Identification in an open loop system was chosen , because of its rapidity. Step inputs were injected with the methanol pump and nitrate concentrations in the treated water were monitored. As expected, the response of the system (bbtFigure 2) was a first order function with delay: where Gs is the system gain, J is the first order constant, t is the pure time delay and s is the Laplace operator . For an inlet flow equal to 400 1111 and an inlet nitratre concentration of 20 mg/l the pure time delay in the filter was 17.6 minutes. The first order constant was 16.5 minutes for the same operating conditions.
Methanol dosing for post-denitrification
18 .0 0
~
T
I
N03 ·N
15 .0 0
t
. .7........................-
o0 .6 S
E
'" 0 Z z
293
12 .0 0
:I:
0
MeOH
0"
f
9 .00
r:
6 .00 12 :4 5
13 :4 5
02
0 14 :4 5
I
13 : 15
" ::E
14 : 15
Figure 2. Response to a step input of MeOH.
Controller desi~n As the transfer function was a first order function with delay, the methods of the linear control theory could be easily applied. In this case, a rather practical method has been used for the PI-controller tuning (Pollinger, 1994). It consisted of compensating the first order constant from the transfer function with the integral term I and then calculating the gain P so that the system did not become unstable in the sense of the Nyquist theory of stability. The block diagram of the control loop is shown in Figure 3.
;' " ·FF.i.;.;p· ············ 'N03 :N'i~i;, "
"
.. ... . . . . . .. . . . . . . .. . .... . . . ...
-
N03 ·N
+
Sct Point
L . . - - - - - - - l N03·N mei er I+-
•
"
-
-
-
FB·loop
-
-
-
---J
. ~
Figure 3. Block diagram of the control loop.
RESULTS AND DISCUSSION Methanol was added initially without control and subsequently using different control strategies. The following strategies were tested: • FB control based on the on-line measurement of nitrate outlet concentrations • FF control based on the on-line measurement of nitrate inlet concentrations • FB and FF control based on the on-line measurement of outlet and inlet nitrate concentrations (Fig.3). Depending of the wastewater inlet quality, one of these three control modes could be chosen. They all proved to be very efficient in set-point tracking to ensure the effluent quality and in minimizing the methanol addition. thus optimizing the operation cost. During all the experiments the pilot was operated at different upflow velocities (from 5.5 mIh to 10 m1h) with nitrate applied loads varying between 0.5 and 3.6 kglm3.d and a temperature between 15°C to 20°C. The COD inlet concentration ranged from 80-130 mgll and the suspended solids average was about 25 mg SS/\.
294
N. PUZNAVA et al.
1. Constant Dowrate and nitrate inlet concentration Figure 4 represents the modification of the nitrate set-point from 5 mgll to 2 mg/1. This result demonstrates that a simple FB control with a PI-controller gives satisfactory results when nitrate inlet variations are not present and the flowrate is constant. N 0 3 · N outlet
-
0 .95 0 .9 0 .8 5 § J:
.,
0 .8 0 0 .75 :::!i 0 .7 0 .65
O +--
-
,..--
17 :00
18 :00
--t-
-
-+-
19 :00
-
20 :00
-+-21 :00
-
-t--
-
22 :00
...--
--l- 0 .6 0 :00
23 :00
Figure 4. Changing of N03-N set - point value from 5 mgll to 2 mgll.
2. Constant Dowrate and variable nitrate inlet concentration Firstly, a sensor default was simulated. An increase in the inlet nitrate concentration was applied, the sensor was supposed to be out of service and the control was made by a stewise methanol addition as it would be the case in a fall back position.
! :: f········.. ~
15
8Z
10
Z c
Z M
~
.
.
.
r:
. ..
<
.
1 .2 .S J:
5
+I . .
t- -
0 10 :00
.
a
G>
:::!i
~~ _ ..:._---~. -
12 :00 I-
I 0 .8 T0 .4
N 0 3 -N outlet
~
14 :00 -N03 ·N in le t
. 16 :00
-
Me O H
0 .0 18 :00
I
Figure 5. Control response with stepwise chang ing for methanol.
The response was satisfactory since the N03-N outlet concentration was kept below the set-point value of 2 mgll. Compared to the following example this control mode is less interesting due to a methanol consumption which is 18% higher. Figure 6 shows the response of the nitrate using an FF and FB control algorithm. In this case, capital costs are higher but the use of two on-line sensors gives all the necessary information for control. One benefit is the methanol consumption which is 18% less than in the case above where there was no on-line sensor in use. The advantage of this control mode is that the nitrate inlet perturbation has been taken into account by the FF loop which increased the methanol addition. The PI-controller in the FB loop has just a security role to ensure that the outlet nitrate concentration satisfies the set-point. This control mode is very robust against all kind of perturbations but the use of two on-line sensors implies higher capital and maintenance costs.
295
Methanol dosing for post-den itrification
. -
30
I::
11th "
2:
I
~
I' .~eo~ , . , '
~ =,.......... ,' .. ,.. ,:.. , , " .. , .. , t
E ' ::: :. ; 7. "
S 15 ~ 10
N 0 3· N o ulle t
-.- N 0 3 N i n le t
•
.~.
.
. " " . " . ,"
.. ' .
.
.: . . .
.
. ,""
.
. . .
~ :~. ~.
2 2 :00
0 :00
. . . ','
.
___ . " fIII"W; t'I'$ , , , :' , , , , , , , , , "
o 2 0 :00
'
4 :0 0
2 :0 0
6 :00
3
S
:3 ; 2
-\
~
1
I 0 6 :0 0
Figure 6, Control response with FF and FB control loop,
Therefore , it is highly desirable to control the process with just one on-line sensor such as in an FF control loop, The use of an FB control loop is normally more interesting than using an FF loop, but in the process studied the time delay was too important and made the use of a PI-controller in FB loop impossible. To be sure that the on-line sensor worked well, we took one sample per day of the inlet nitrate concentration and measured it with another analyser. In this way we compared this value with the on-line sensor value. This kind of measurement was necessary to check out the reliability of the on-line sensor.
3. Variable Dowrate and nitrate inlet concentration The FF controller consists of a gain proportional to the N03-N inlet concentration and flowrate. In this way the quantity of methanol needed to remove a fixed load is calculated (acceptable if equal to the biofilter treatment capacity). Since the control mode is a open loop system, an external set-point value for the residual N03-N concentration cannot be given. The residual N03-N can be fixed with the proportional gain of the FF controller. 1
r
10
9 8 7
0 0 .",
i
0
6
u:
.
5
"0
4 + 3
S
2
0 ::E
1
c:
,S :I:
'"
o +------+------+-------+---~= 0
12:30
10:30 I-
N0 3.N oull et -
14:30 N03- N inlel -
16:30 MeOH
Flowrate l
Figure 7, Control response /I I with FF control loop .
Figure 7 shows the response of the biofilter operated under variable flow and nitrate inlet concentration. The N03-N outlet concentration was kept all the time close to a fixed value of 3 mg/I. It must be noticed that the nitrate applied load change from 0.5 to 2.0 kg/m3.d which is much more important than in the previous examples. In the following example. the variations in flowrate and N03-N inlet concentration were delayed. Two hours separated the peak of flowrate and the peak of inlet N03-N concentration . The response of residual N03-N
296
N. PUZNAVA
~I
al.
is similar to that of the previous example. The fixed value of 3 mgll was well maintained apart from a dip towards during the end of the perturbation. Both examples demonstrate the satisfactory results which can be obtained with this simple control mode which required only one on-line sensor.
1-
N0 3·N outlet
-0-
N0 3-N inlel -
MeOH •. Flowrele
I
Figure 8. Control response #2 with FF control loop .
Regarding the costs for methanol consumption, these are 20% lower than for the stepwise methanol addition (for the case with constant flowrate and variable nitrate inlet concentration). This is still 2% less than the control system with FF and FB. CONCLUSIONS FB and/or FF control of methanol dosing for post-denitrification in upflow floating biofilters are attractive solutions due to their simplicity . This study presented implementation of these control algorithms to control the methanol addition for post-denitrification process in an upflow floating biofilter. After several experiments on a pilot filter operated in different conditions the following conclusions were reached : the FF control algorithm gives the best results throughout the examples . It was more economical than the other two different control algorithms used in this study; the FF and FB control seems to be very complete as a control algorithm, but in the case studied was not economically the best one. The use of .wo on-line sensors gave much interesting information but the maintenance is also high; a good control system improves the biological process as well. In this case, a good control means that nitrate .lI.llil carbon source are eliminated during the denitrification process ; there are very important improvement margins over the stepwise methanol addition (20% less consumption with FF control algorithm and 18% less consumption with FF and FB control algorithm) . ACKNOWLEDGEMENT This work is part of the Eureka program SCIMBIS EU12l2. Financial support from the French Ministry of Education and Research is gratefully acknowledged. REFERENCES Beutler. E. and Legrand. Ph. (1989). Expert systems in sewage treatment plants, L'Eau, l'lndustrie , les Nuisances, 132(11),57-59 (in French)
Methanol dosing for post-den itrification
297
Charpentier, J., Godart, H. and Mongo, Y. (1989). Oxidation- Reduction Potential (ORP) regulation as a way to optimise nutrient removal. War. Sci. Tech., 21,1209-1223 Fiillinger, O. (1994) . Regelungstechnik; EinfUhrung in die Methoden und ihre Anwendungen, Hilthig Buch Verlag Heidelberg, pp 240 La Cour Jansen, 1.. Jepsen, S.-E. and Dahlgren Laursen, K. (1994). Carbon utilization in denitrifying biolilters. Wat. Sci. Tech.• 29(5-6), 101-109 Rogalla, F., Badard, M., Hansen, F. and Dansholm (1992). Up-scaling a compact nitrogen removal process. War. Sci. Tech.• 26(5 6),1067-1076 Zeghal, 5.• Nagem Nogueira, F.• Salzer. C. and Rogalla, F. (1995). Tertiary denitrification using an upflow floating biofilter, WEFTEC95. 68th Annual Conference and Exposition, Miami Beach . Florida. October 21-25 Zeghal, 5.• Subra, J. Ph., Sauvegrain, P. and Vignoles, C. (1996) . Chemicals addition control for phosphorus removal in primary sedimentation tanks. Proc. 7th Gotheburg Symp., Edinburgh. September 23-25
Pergamon
po: S0273-1223(98)00482-X
War.Sci. T~ch. Vol. 38. No.3. pp, 291-297. 1998. IAWQ C 1998 Published by Elsevier Science Ltd. Printed in Great Britain. All rights reserved 0273-1223198 $19'00 + 0'00
SIMPLE CONTROL STRATEGIES OF METHANOL DOSING FOR POST-DENITRIFICATION N. Puznava, S. Zeghal and E. Reddet Anjou Recherche. Research Center ofCompagnie Gin/rale des Eaux - 01V Chemin de la Digue, BP 76. 78603 Maisons-Laffitte Cedex; France
ABSTRACT The objective of this work is to propose a simple but efficient way of controlling the carbon addition for the post-denitrification process in order to comply with regulatory constraints and optimize operating cost. A Biostyr~ pilot column filled with polystyrene beads was used for the experiments. In order to simulate a secondary treated water from a nitrifying stage with a carbon source addition. the feed water was composed of river water dosed with nitrates. phosphates and methanol. Methanol was added initially with no control (at different constant rates) and in a second stage with different control slrategies based on the on-line measurement of inlet and/or outlet nitrate concentrations. This simple dosing mode proved to be very efficient in set-point tracking to ensure the effluent quality and in minimizing the methanol addition (up to 20% less methanol consumption). thus optimizing operation costs. ce 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Biostyr; methanol; on-linesensor; post-denitrification; process control; upflow floating biofilter. INTRODUCTION Upflow floating biofilters are well known and can be used in several configurations such as secondary nitrification-denitrification in one cell (Rogalla et al., 1992) or tertiary denitrification (Zeghal et al., 1995). For some areas whereeffluent nitrogen requirements are very low and where the carbon/nitrogen ratios are not very favourable for denitrification, the post-denitrification configuration can be a valuable option. High denitrification ratesusingthe upflow floating biofilter are obtained. This requires however the addition of an external carbon source. Methanol is commonly carbon source (La Cour Jansen et al., 1994) but some industrial effluents can be alsoused. Implementation of process control at wastewater treatment plants (WWTP) has increased considerably in recent years thanks to the development of on-line sensorsfor monitoring effluentdischarge. The target was to optimize operation of.~s by. meas~ring with on-li?e sensors the processvariables such as dissolved oxygen or redox potential 10 the mixed liquor (Charpentier et al.; 1989) and to control it. Moreover, for advanced treatment objectives, chemical addition control for phosphorus removal (Zeghal et al., 1996) based on turbidity measurements and expertsystems to manage sewage treatment plantsare implemented (Beutler et al., 1989).
291
292
N. PUZNAVA et al .
The aim of the present study was to investigate simple control strategies such as the feed-back (FB) and/or feed-forward (FF) control algorithms using one or two on-line sensors. This paper shows a comparison between the results obtained and also the costs for each example. MATERIALS AND METHODS Experimental set-up Figure I presents the Biostyr® pilot used for the post-denitrification . A 300 mm diameter column filled with polystyrene beads of 3.5 mm diameter was used. The height of the floating bed filter was 2.0 m. Below the filter bed, a height of 1.4 m was allowed for the bed expansion during backwash. A treated water storage tank was used for the backwash of the filter. In order to simulate a secondary treated water from a nitrifying stage with a carbon source addition, the feed water was composed of Seine river water dosed with nitrates, phosphates and methanol.
nu rate , ph Olphlit
f······
I n / C'C ll o n
;
ro", wa l
~
nnr ate ana ly lc r
" lIf_Ie anal yzC'r
r---'
Figure I. Diostyr
Nitrate measurements were made on-line using a DATALINK NTIOO analyser for water coming out of the filter (location 5 and 6). The measurement was based on the UV absorption spectrum of nitrates . Two pumps (location 2 and 3) with external command were used: one for nitrates injection and one for methanol dosing . The methanol pump was controlled by a control station (location 4), an MCC Rhapsodie unit used as datalogger and multiple input-output controller . COD was measured by using HACH methods. Transfer function identification Different methods can be used for the identification of the transfer function, the most practical known methods being based on the identification of the transfer function in open loop or closed loop systems . Identification in an open loop system was chosen , because of its rapidity. Step inputs were injected with the methanol pump and nitrate concentrations in the treated water were monitored. As expected, the response of the system (bbtFigure 2) was a first order function with delay: where Gs is the system gain, J is the first order constant, t is the pure time delay and s is the Laplace operator . For an inlet flow equal to 400 1111 and an inlet nitratre concentration of 20 mg/l the pure time delay in the filter was 17.6 minutes. The first order constant was 16.5 minutes for the same operating conditions.
Methanol dosing for post-denitrification
18 .0 0
~
T
I
N03 ·N
15 .0 0
t
. .7........................-
o0 .6 S
E
'" 0 Z z
293
12 .0 0
:I:
0
MeOH
0"
f
9 .00
r:
6 .00 12 :4 5
13 :4 5
02
0 14 :4 5
I
13 : 15
" ::E
14 : 15
Figure 2. Response to a step input of MeOH.
Controller desi~n As the transfer function was a first order function with delay, the methods of the linear control theory could be easily applied. In this case, a rather practical method has been used for the PI-controller tuning (Pollinger, 1994). It consisted of compensating the first order constant from the transfer function with the integral term I and then calculating the gain P so that the system did not become unstable in the sense of the Nyquist theory of stability. The block diagram of the control loop is shown in Figure 3.
;' " ·FF.i.;.;p· ············ 'N03 :N'i~i;, "
"
.. ... . . . . . .. . . . . . . .. . .... . . . ...
-
N03 ·N
+
Sct Point
L . . - - - - - - - l N03·N mei er I+-
•
"
-
-
-
FB·loop
-
-
-
---J
. ~
Figure 3. Block diagram of the control loop.
RESULTS AND DISCUSSION Methanol was added initially without control and subsequently using different control strategies. The following strategies were tested: • FB control based on the on-line measurement of nitrate outlet concentrations • FF control based on the on-line measurement of nitrate inlet concentrations • FB and FF control based on the on-line measurement of outlet and inlet nitrate concentrations (Fig.3). Depending of the wastewater inlet quality, one of these three control modes could be chosen. They all proved to be very efficient in set-point tracking to ensure the effluent quality and in minimizing the methanol addition. thus optimizing the operation cost. During all the experiments the pilot was operated at different upflow velocities (from 5.5 mIh to 10 m1h) with nitrate applied loads varying between 0.5 and 3.6 kglm3.d and a temperature between 15°C to 20°C. The COD inlet concentration ranged from 80-130 mgll and the suspended solids average was about 25 mg SS/\.
294
N. PUZNAVA et al.
1. Constant Dowrate and nitrate inlet concentration Figure 4 represents the modification of the nitrate set-point from 5 mgll to 2 mg/1. This result demonstrates that a simple FB control with a PI-controller gives satisfactory results when nitrate inlet variations are not present and the flowrate is constant. N 0 3 · N outlet
-
0 .95 0 .9 0 .8 5 § J:
.,
0 .8 0 0 .75 :::!i 0 .7 0 .65
O +--
-
,..--
17 :00
18 :00
--t-
-
-+-
19 :00
-
20 :00
-+-21 :00
-
-t--
-
22 :00
...--
--l- 0 .6 0 :00
23 :00
Figure 4. Changing of N03-N set - point value from 5 mgll to 2 mgll.
2. Constant Dowrate and variable nitrate inlet concentration Firstly, a sensor default was simulated. An increase in the inlet nitrate concentration was applied, the sensor was supposed to be out of service and the control was made by a stewise methanol addition as it would be the case in a fall back position.
! :: f········.. ~
15
8Z
10
Z c
Z M
~
.
.
.
r:
. ..
<
.
1 .2 .S J:
5
+I . .
t- -
0 10 :00
.
a
G>
:::!i
~~ _ ..:._---~. -
12 :00 I-
I 0 .8 T0 .4
N 0 3 -N outlet
~
14 :00 -N03 ·N in le t
. 16 :00
-
Me O H
0 .0 18 :00
I
Figure 5. Control response with stepwise chang ing for methanol.
The response was satisfactory since the N03-N outlet concentration was kept below the set-point value of 2 mgll. Compared to the following example this control mode is less interesting due to a methanol consumption which is 18% higher. Figure 6 shows the response of the nitrate using an FF and FB control algorithm. In this case, capital costs are higher but the use of two on-line sensors gives all the necessary information for control. One benefit is the methanol consumption which is 18% less than in the case above where there was no on-line sensor in use. The advantage of this control mode is that the nitrate inlet perturbation has been taken into account by the FF loop which increased the methanol addition. The PI-controller in the FB loop has just a security role to ensure that the outlet nitrate concentration satisfies the set-point. This control mode is very robust against all kind of perturbations but the use of two on-line sensors implies higher capital and maintenance costs.
295
Methanol dosing for post-den itrification
. -
30
I::
11th "
2:
I
~
I' .~eo~ , . , '
~ =,.......... ,' .. ,.. ,:.. , , " .. , .. , t
E ' ::: :. ; 7. "
S 15 ~ 10
N 0 3· N o ulle t
-.- N 0 3 N i n le t
•
.~.
.
. " " . " . ,"
.. ' .
.
.: . . .
.
. ,""
.
. . .
~ :~. ~.
2 2 :00
0 :00
. . . ','
.
___ . " fIII"W; t'I'$ , , , :' , , , , , , , , , "
o 2 0 :00
'
4 :0 0
2 :0 0
6 :00
3
S
:3 ; 2
-\
~
1
I 0 6 :0 0
Figure 6, Control response with FF and FB control loop,
Therefore , it is highly desirable to control the process with just one on-line sensor such as in an FF control loop, The use of an FB control loop is normally more interesting than using an FF loop, but in the process studied the time delay was too important and made the use of a PI-controller in FB loop impossible. To be sure that the on-line sensor worked well, we took one sample per day of the inlet nitrate concentration and measured it with another analyser. In this way we compared this value with the on-line sensor value. This kind of measurement was necessary to check out the reliability of the on-line sensor.
3. Variable Dowrate and nitrate inlet concentration The FF controller consists of a gain proportional to the N03-N inlet concentration and flowrate. In this way the quantity of methanol needed to remove a fixed load is calculated (acceptable if equal to the biofilter treatment capacity). Since the control mode is a open loop system, an external set-point value for the residual N03-N concentration cannot be given. The residual N03-N can be fixed with the proportional gain of the FF controller. 1
r
10
9 8 7
0 0 .",
i
0
6
u:
.
5
"0
4 + 3
S
2
0 ::E
1
c:
,S :I:
'"
o +------+------+-------+---~= 0
12:30
10:30 I-
N0 3.N oull et -
14:30 N03- N inlel -
16:30 MeOH
Flowrate l
Figure 7, Control response /I I with FF control loop .
Figure 7 shows the response of the biofilter operated under variable flow and nitrate inlet concentration. The N03-N outlet concentration was kept all the time close to a fixed value of 3 mg/I. It must be noticed that the nitrate applied load change from 0.5 to 2.0 kg/m3.d which is much more important than in the previous examples. In the following example. the variations in flowrate and N03-N inlet concentration were delayed. Two hours separated the peak of flowrate and the peak of inlet N03-N concentration . The response of residual N03-N
296
N. PUZNAVA
~I
al.
is similar to that of the previous example. The fixed value of 3 mgll was well maintained apart from a dip towards during the end of the perturbation. Both examples demonstrate the satisfactory results which can be obtained with this simple control mode which required only one on-line sensor.
1-
N0 3·N outlet
-0-
N0 3-N inlel -
MeOH •. Flowrele
I
Figure 8. Control response #2 with FF control loop .
Regarding the costs for methanol consumption, these are 20% lower than for the stepwise methanol addition (for the case with constant flowrate and variable nitrate inlet concentration). This is still 2% less than the control system with FF and FB. CONCLUSIONS FB and/or FF control of methanol dosing for post-denitrification in upflow floating biofilters are attractive solutions due to their simplicity . This study presented implementation of these control algorithms to control the methanol addition for post-denitrification process in an upflow floating biofilter. After several experiments on a pilot filter operated in different conditions the following conclusions were reached : the FF control algorithm gives the best results throughout the examples . It was more economical than the other two different control algorithms used in this study; the FF and FB control seems to be very complete as a control algorithm, but in the case studied was not economically the best one. The use of .wo on-line sensors gave much interesting information but the maintenance is also high; a good control system improves the biological process as well. In this case, a good control means that nitrate .lI.llil carbon source are eliminated during the denitrification process ; there are very important improvement margins over the stepwise methanol addition (20% less consumption with FF control algorithm and 18% less consumption with FF and FB control algorithm) . ACKNOWLEDGEMENT This work is part of the Eureka program SCIMBIS EU12l2. Financial support from the French Ministry of Education and Research is gratefully acknowledged. REFERENCES Beutler. E. and Legrand. Ph. (1989). Expert systems in sewage treatment plants, L'Eau, l'lndustrie , les Nuisances, 132(11),57-59 (in French)
Methanol dosing for post-den itrification
297
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