membrane air diffusers (Supratec, Simmern, Germany). The pilot plant is, further, provided with a Programmable Logic. Controller (PLC) and Supervisory Control ...
Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011
Development of an algorithm for air-scour optimization in membrane bioreactors G. Ferrero*, H. Monclus**, G. Buttiglieri*, S. Gabarron**, J. Comas**, I. Rodriguez-Roda*,**
* ICRA (Catalan Institute for Water Research). Scientific and Technological Park of the University of Girona. H2O Building. c/ Emili Grahit 101. E17003. Girona. Spain. Tel: (+34) 972 18 33 80 (email:[gferrero, gbuttiglieri, irodriguezroda]@icra.cat) ** Laboratory of Chemical and Environmental Engineering (LEQUiA). Environmental Institute. University of Girona. Girona. E17071. Spain. (email:[hector, sara.gabarron, quim, ignasi]@lequia.udg.cat)
Abstract: Membrane Bioreactors are used in an increasing number of wastewater treatment facilities because of their compactness and efficiency in solid-liquid separation. In this paper the development of an air-scour control algorithm based upon short term and long term membranes permeability evolution is presented. An open loop calibration and partial validation was carried out in a semi-industrial scale pilot plant where manual changes in air-scour flow had been previously carried out. The control system was successfully tested in closed loop in an industrial scale pilot plant, defining a maximum daily air-scour decrease or increase of 6% of the air-scour recommended by membranes suppliers. A maximum air-scour saving of 20%, calculated in terms of air flow saved, was achieved without interfering with the biological nutrient removal and without any apparent long term effect. Keywords: Aeration, Control, Energy saving, Membrane bioreactor.
1. INTRODUCTION Membrane Bioreactors (MBR) improve conventional activated sludge process for wastewater treatment, using an ultra or micro filtration for solid-liquid separation instead of secondary settlers, obtaining higher effluent quality (Monclús et al., 2010, Judd, 2006). The MBR process presents many advantages (higher effluent quality, reduced excess sludge production, drastically enhanced elimination of pathogens and viruses, potential degradation of specific refractory pollutants, higher stability and persistence to shock loads, etc.), but as in most of membrane filtration processes the permeate flux declines (or transmembrane pressure increases) during filtration due to membranes fouling (Judd, 2005). MBR use can be always justified in case of discharge of treated wastewater in very sensitive areas, water reuse, limited space available for plants retrofitting and high loaded or complex industrial wastewater treatment and with a significant seasonal component (Ferrero et al., 2010). Nonetheless, the technology is still limited by the high operating costs and by membrane fouling (Meng et al., 2009). Through the implementation of automatic control systems, optimal results are achievable regarding both energy optimization and fouling mitigation. The most important Copyright by the International Federation of Automatic Control (IFAC)
achievements can be found in the patent literature; however there is still a lack of robust control systems capable to reduce aeration requirements maintaining optimum filtration performances. Some suppliers propose to decrease air scour flow rate during filtration and to increase it during relaxation, such as to reduce overall air scour flow. The air scour flow rate varies in approximate proportion to increases and decreases in the flow rate of permeate through the membrane bioreactors. The filtration-relaxation cycles are frequent and shorter than suppliers’ specifications (Livingston, 2007). Ginzburg et al. (2007) developed an on-line process control system that considers resistance values and adjust operational parameters such as membrane aeration frequency and membrane aeration flow in order to reduce operational costs related to fouling removal. An on-line fouling measurement for controlling membrane cleaning actions by measuring the reversible and irreversible fouling was developed (Brauns et al., 2002, 2005) but it has not been applied to an advanced control system yet. This paper presents the development of an innovative control algorithm for air-scour optimization that dynamically regulates air flow based on long term and short term permeability trends.
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Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011
2. MATERIAL AND METHODS 2.1 Castell d’Aro semi-industrial scale pilot plant The semi industrial scale pilot plant is equipped with a prescreening system to prevent the entrance of large particles. The bioreactor with a total volume of 2,26 m3 was designed according to the University of Cape Town (UCT) configuration, i.e. the MBR consists of an anaerobic (An) (14% of the total volume), an anoxic (Ax) (14%) and an aerobic (Ox) compartment (23%), that are ultimately followed by a compartment (49%) with submerged hollow fibre membranes (Mem). The used microfiltration membranes with a total membrane area of 12,5 m2 (MicrozaTM, Asahi Kasei Chemicals Corporation, Tokyo, Japan) are characterised by a nominal pore size of 0,1 µm. Total suspended solids (TSS) sensors (Solitax; Hach Lange, Düsseldorf, Germany) are installed in the anaerobic and membrane compartments. The anoxic reactor is equipped with an oxidation reduction potential (ORP) sensor (Alldos, Reinach, Switzerland). The anaerobic and anoxic compartments are supplemented with a mixer. In the aerobic reactor a pH sensor (ProMinent, Heidelberg, Germany) is installed. Furthermore, in the aerobic and membrane compartment combined dissolved oxygen (DO)-temperature sensors (Crison, Barcelona, Spain) are installed. The membrane compartment contains an ammonium sensor (Hach Lange, Düsseldorf, Germany). In the aerobic reactor a PID controller maintains the DO at 1,5 mg·L-1 using two membrane air diffusers (Supratec, Simmern, Germany). The pilot plant is, further, provided with a Programmable Logic Controller (PLC) and Supervisory Control and Data Acquisition (SCADA) system that acquires digital and analogical data and controls all the automatic control loops of the plant, i.e. aeration, permeate and backwash fluxes, hydraulic retention time (HRT), sludge retention time (SRT), mixed liquor suspended solids (MLSS) concentration and recycle flows. The wastewater is obtained from the sewer that enters the full-scale wastewater treatment plant at Castell d´Aro (CA) where the pilot MBR is located. The wastewater, after passing a first coarse screen (5cm), is pumped to the pilot plant with the use of a centrifuge pump (Grundfos, Bjerringbro, Denmark), crossing a 1 mm nominal pore size filter and stored in a 500 L buffer tank that is continuously mixed. From this buffer tank the wastewater is pumped to the anaerobic reactor with a positive advance pump (Seepex, Bottrop, Germany) passing a second filter with a nominal pore size of 0,6 mm to prevent large particles from entering the bioreactor and damaging the membranes.
pumps (Grundfos AP35.40.06.3V) alternatively fed the pilot plant and a 1 mm pore size filter (FMS Filterron, RFA 2450) was used in order to prevent large solids from entering the bioreactor and damaging the membranes. Permeate is obtained by applying a vacuum pressure drop over the membranes using an auto aspiring centrifuge pump; the TMP is monitored by means of a pressure transducer (Edress + Hauser, model Cerabar-MPMC 41). Ultimately, the treated effluent collected in the permeate tank is discharged from the pilot plant to the WWTP sewer. The anaerobic and anoxic reactors are equipped with ORP sensors (Crison) and mixers. In the membrane compartment a DO-Temperature sensor (Crison) and a level transmitter (Waterpilot FMX 167) are installed. The pilot plant is provided of a PLC and a SCADA. Data of both pilot plants are accessible on-line at www.colmatar.es. Pilot plant characteristics are presented in Table 1. Table 1. Slope ratio and control actions Location Volume An/Ax/Ox/Mem Type
m3 %
Supplier Total Area Porus diameter Maximum flux Cycles
m2 µm LMH
CA 2,26 14/14/23/49 Hollow Fiber
GR 12,5 20/20/49/11 Hollow Fiber
Microza/Puron
Zenon
12,5 0,1 24 9’ filtration/ 1’ backpulse
46,45 0,04 36 10’ filtration/ 40’’ relaxation
3. RESULTS 3.1 Theoretical development of the aeration control system The aeration algorithm is the one of the modules of a more complex multi-level control system (Ferrero et al. 2010). It consists of a control for energy reduction built on permeability trends, where
d
Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011
Ě< Ěƚ ^d ^Z = Ě< Ěƚ >d
Ě< < 0, SR is compared to a set of reference Ěƚ >d
110
values specified during the calibration phase and depending on the different membrane characteristics and users’ necessities (i.e. users can define maximum and minimum aeration values and reduction). The aeration is modified according to the current propensity of the membranes to get fouled. Hence, different control actions (in terms of aeration increase or reduction) are associated to each range of values.
Ě< When > 0, which means that on long term basis Ěƚ >d
scour reduction can be applied;
•
Ě< Ě< Ě< > 0 and > , a smaller air Ěƚ ^d Ěƚ >d Ěƚ ^d scour reduction can be applied (i.e. 50% of the maximum);
•
Ě< < 0, a moderate aeration increase is applied. Ěƚ ^d
Minimum air flow reccomended by suppliers
90
6,0
80 70
5,5
60 50 5,0
40 30 20 10
4,5 air flow increase
Permeability Air flow
air flow decrease
0
4,0 110
membranes permeability is improving, minor changes on aeration will depend only on short term permeability slope:
Ě< Ě< Ě< • > 0 and ≤ , maximum air Ěƚ ^d Ěƚ >d Ěƚ ^d
6,5
100
Permeability [LMH/bar]
When
(2)
permeability decrease. The control system was then calibrated focusing on a period when manual changes on membranes aeration were taken in order to observe the effect of aeration reduction on permeability evolution first and in order to prevent a significant loss of permeability then (Fig. 1). The air flow was initially 6 m3·h-1; at day 120 it was decreased to 5 m3·h-1, then at day 134 increased at 5,5 m3·h-1, and at day 140 initial conditions were re-established.
Membranes airflow [m3/h]
A slope ratio (SR) is defined as the ratio of short term and long term permeability slopes:
115
120
125
130
135
140
Time [Days]
Fig. 1. Manual control actions taken at CA pilot plant. Fouling evolution was monitored using the permeability trends as control parameter. The short term permeability slope was calculated as the mobile slope of the daily permeability values of the last 4 days
Ě< and the long Ěƚ 4
term permeability slope was calculated as the mobile slope of the daily permeability values of the last 14 days
Ě< , on Ěƚ 14
the basis of the best fit of the data. Generally, the time frames can be adapted to experimental conditions.
The control was initially studied for membrane bioreactors operating with constant flux. The air-scour control was only activated when the loss of permeability evolves according to standard rates (considered to be inferior to a daily permeability loss of 30%, but the value can be calibrated depending on a specific case), and no other critical problems, such as malfunctioning, alarms and/or equipment failure affect the process.
Both short term and long term slopes and slope ratios were iteratively calculated every day (Table 2).
3.1 Open loop validation of the control system in CA pilot plant An exhaustive monitoring of the permeability values was done during a specific period of time, where all the operational parameters were set as recommended by membranes suppliers (i.e. air flux was fixed at 6 m3·h-1 (SADm of 0,48 m3·m-2·h-1), permeate flux of 16 LMH. Historical data from the pilot plant operation were analyzed, and as expected, it was possible to appreciate a gradual
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Table 2. Slope ratio and control air actions (CA) Day
K
Ě< Ěƚ 4
Ě< Ěƚ 14
(SR) 4/14
∆ Air (m3·h-1)
106
90,9
1,6
0,7
-
-1
107
99,9
3,2
1,3
-
-1
108
90,9
1,3
1,3
-
-1
109
84,4
-2,8
1,3
-
0,5
110
81,7
-6,1
1,2
-
0,5
111
77,5
-4,3
0,8
-
0,5
112
74,2
-3,5
0,3
-
0,5
113
70,4
-3,7
-0,3
11,6
1
114
66,1
-3,8
-1,0
3,8
1
115
59,1
-4,9
-1,7
2,9
1
116
60,3
-3,7
-2,3
1,6
0,5
117
57,3
-2,5
-2,9
0,9
-0,25
118
56,4
-1,1
-3,4
0,3
-0,5
Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011
As general consideration, we can state that an aeration decrease of 1 m3·h-1, which represent a decrease of nearly 17% of the aeration recommended by membranes suppliers, is not recommended in one step only and in this case another automatic option might be added in order to split this change in more days.
119
65,0
1,3
-3,2
-0,4
-1
120
72,9
5,5
-2,6
-2,1
-1
121
76,9
7,0
-1,6
-4,3
-1
122
77,8
4,2
-0,7
-5,7
-1
123
75,2
0,8
-0,1
-6,3
-1
124
70,9
-2,1
0,3
-
0,5
125
67,4
-3,5
0,6
-
0,5
126
68,0
-2,5
0,8
-
0,5
127
70,1
-0,2
1,0
-
0,5
128
70,3
1,1
1,0
-
-1
129
69,9
0,6
0,8
-
-0,5
130
71,8
0,5
0,6
-
-0,5
131
68,7
-0,3
0,2
-
0,5
132
66,5
-1,3
-0,3
3,9
1
133
65,8
-2,0
-0,7
3,0
1
134
64,7
-1,3
-0,8
1,6
0,5
135
61,2
-1,7
-0,8
2,0
1
136
62,3
-1,4
-0,7
1,9
0,75
137
61,0
-1,0
-0,7
1,4
0,5
138
59,2
-0,7
-0,8
0,9
0
139
58,9
-1,2
-1,0
1,2
0,25
140
56,2
-1,5
-1,2
1,3
0,25
2
3
* in grey the days that correspond to the manual control actions explained in the text
In order to achieve the best fit with the real data, the control system was calibrated as presented in Table 3. Table 3. Slope ratio and control actions (CA pilot plant)
-0,75
If SR < 0,
-0,5
0,6 – 0,9
-0,25
0,9 – 1,1 1,1 – 1,4 1,4 – 1,7
0 0,25 0,5
1,7 – 2,0
0,75
>2
1
2000
Ě< > 0 and a maximum decrease of aeration ( Ěƚ 4
1 m3·h-1) is associated to a positive trend of permeability in the last four days. When 0 < SR 1.1 the short term evolution is steeper than the longer term evolution and the aeration is increased accordingly.
22 20
1500 18 1000
16 14
500 12
Membranes air flow Permeability
0 10
15
20
10 25
30
35
Membranes airflow [m3/h]
0 – 0,3 0,3 – 0,6
During the calibration of the membranes aeration control system, manual changes were carried out in order to observe the effect of aeration decrease on the membranes module used during the experimentation (Fig.2).
Permeability [LMH/bar]
