Optimization Of Membrane Cleaning Process in Membrane Bioreactors (MBR) Treating Refinery Wastewater M. M. O. CARV ALHO*, T. F. A. L. EUGÊNIO*, A. R. ALKMIM*, P. COSTA*, M. C. S. AMARAL*, L. S. FRANÇA NETA**, A.C. CERQUEI RA***, A.P TORRES*** AND V.M.J SANTI AGO***. * Department of Sanitary and Environmental Engineering - Federal University of Minas Gerais - Brazil Phone: + 55 31 3409-3669 - E-mail:
[email protected] ** Chemistry Department - Federal Center of Technological Education of Minas Gerais - Brazil *** Research Center of PETROBRAS - Brazil
ABSTRACT This paper approaches the study to optimize the conditions of membrane cleaning in a MBR used in oil refinery wastewater reclamation. Therefore, an early study was done concerning the type of chemicals used in washings, its concentration and its sequence. To quantify efficiency of cleaning, permeability of six membranes used in tests were measured before and after every step of washing sequence. From these results, it can be concluded that use of sodium hypochlorite is better than use of products based in hydrogen peroxide for organic fouling removal. Moreover, when acid washing happened at the first step of washing, membrane efficiency of recovery increased significantly. It was also noted that recovery with use of oxalic acid was even better than with use of citric acid. Permeability results were confirmed by observation of SEM and EDS analyses. Keywords: Membrane cleaning; membrane bioreactor; refinery wastewater
INTRODUCTION Membrane fouling is still the major challenge for the consolidation of the MBR technology in wastewater reclamation. To mitigate this problem it is necessary to perform periodic cleaning of the membranes. There are two main types of cleaning: physical and chemical cleaning. After some time of operation, it forms an irreversible fouling, which cannot be removed by physical processes (WEI, et al., 2011). In general, oxidizing agents such as sodium hypochlorite (NaClO) or hydrogen peroxide (H2O2) are used to remove organic compounds and acids such as citric and oxalic to remove the inorganic (JUDD, 2006). Determining the concentration, cleaning time and the frequency of them is the key to make the process less costly (KIM, et al., 2011). However, there is no protocol for membrane cleaning in MBRs, since the efficiency of this procedure depends directly on the nature of the effluent being treated and the conditions of operation. This study is led in partnership with Petrobras to improve process of treating wastewater from oil refineries. The wastewater generated from the petrochemical industry can be classified into uncontaminated and Organic according to their physicochemical characteristics. Typically, the system receives not contaminated effluent streams with characteristics inorganic, such as: system purges of cooling water and steam generation. Organic load is result of contaminated wastewater with organic compounds, found in the drainage of process water and tanks. (MUSTAFA, 1998) MBRs are used to promove water treatment. Cleaning process of membrane modules is carried out according to the measures suggested by the manufacturer. Therefore, this study aims to optimize the conditions of membrane cleaning in a MBR used in oil refinery wastewater reclamation, concerned to the type of chemicals, its concentration and sequence.
METHODOLOGY Tests were carried out using tThe chlorinated polyethylene membrane sheet, wiith a nominal (maximum) pore size of 0.4μm used in the treatment of petroleum refining wastewater. The flats were cut into rectangular shape of 15.2 cm long and 7.1 cm high, to fit an acrylic module and have the permeability (Lp) determined. Previous studies have shown a great variation in permeability over this flat membrane. Thus, permeability of each piece was taken before and after each cleaning to obtain nominal new membrane permeability was done using the parameter η: η = ((Lpused – Lpwashe d) / (Lp used– Lpn ew)) × 100 Cleaning with acid (citric and oxalic) combined with sodium hypochlorite or cleaning with a commercial surfactant based in H2O2 were investigated. In order to determine the curve of flux against pressure, permeate flux measurements were taken under different pressure, ranging between 0.9 and 2.0 bar. A total of 6 used membranes were employed. For the evaluation of the best sequence of the reagents three membranes were used, designated 1, 2 and 3. Membrane (1) was washed as recommended by the manufacturer, i.e. with NaClO 5000ppm for 2h and then with citric acid pH 2.5 for another 2h. Membrane (2) was washed using the same procedure but with the reverse sequence. Membrane (3) was washed with oxalic acid in pH 2.5 for 2h and then with 5000ppm NaClO also for 2h. Membranes (4-6) were washed with commercial surfactant using concentrations of 500, 1000 and 2000 ppm. Table 1 shows the sequence of washings for all membranes tested. Table 1 – Membranes washing sequence. 1st Washing
2nd Washing
Membrane Agent
Concentration
Time
Agent
Concentration
Time
1
NaClO
5000 ppm
2h
Citric Acid
pH 2.5
2h
2
Citric Acid
pH 2.5
2h
NaClO
5000 ppm
2h
3
Oxalic Acid
pH 2.5
2h
NaClO
5000 ppm
2h
4
Commercial Surfactant
500 ppm
2h
-
-
-
5
Commercial Surfactant
1000 ppm
2h
-
-
-
6
Commercial Surfactant
2000 ppm
2h
-
-
-
For each step of cleaning, a piece of membrane was collected in order to provide visualization of the washing effects on membrane’s surface with a Scanning Electron Microscope (SEM) apparatus. Samples were subjected to pre-treatment coating with gold (≈ 30ηM) in a metallizer BALTEC, model MED020. In addition, Energy Disperse Spectroscopy (EDS) analyses were made to verify composition of membrane surface fouling. Thus, the efficiency of washing sequence to remove inorganic or organic fouling can be discussed. For this test, samples were pre-treated with a thin carbon film.
Samples were analysed by a JEOL, model JSM-6360LV, in UFMG Centre of Microscopy, with current in the range 90-100 µA, under high vacuum.
RESULTS Values of permeability (Lp) for membranes (1-6) in each stage of washing sequence are displayed on Table 2.
Table 2 – Permeability (Lp) for membranes (1-6) measured in each stage of washing sequence.
Permeability (LMH/bar)
Membrane Lp used
Lp 1st wash
Lp 2nd wash
1
112
443
479
2
83
87
539
3
69
72
629
4
137
124
-
5
84
79
-
6
168
196
-
Firstly, it can be noticed that commercial surfactant didn’t presented a great efficiency when compared to sodium hypochlorite. Permeability after 1st washing (with NaClO) of Membrane (1) was recovered in 74,7%, while after washing with the highest concentration of surfactant, Membrane (6) only reached 16,7% of recovery. And this is the reason why the washing sequence was not taken ahead for Membranes (4-6). Figure 1 shows Permeability Recovery for Membranes (1-3) in each step of washing. From these results it also can be seen that the recovery of permeability is higher when the membrane is washed first with acid. This is probably because there is an inorganic layer on the surface, formed due to the concentration of the solution inside the reactor. By removing this layer, the organic part is completely exposed to NaClO, increasing the effectiveness of cleaning. Once determined the best cleaning sequence, citric acid was replaced by oxalic acid to compare the performance of these two substances. Membrane (3) was washed with oxalic acid in pH 2.5 for 2h and then with 5000ppm NaClO also for 2h. Oxalic acid led to better permeability recovery, as can be seen in Figure 1.
Figure 1 - Permeability recovery (%) of the membranes 1, 2 and 3 after each cleaning procedure and total recovery related to the used membrane and related to the new membrane.
Better efficiency for acid-oxidant sequence is confirmed with SEM images.
Figure 2 shows the surface of membranes in each step of washing and it can be observed that final conditions for Membrane (2) and Membrane (3) presented less amount of fouling. The prediction that there is a layer of inorganic material covering surface of fouling was confirmed by EDS test. Figure 3 provides the results of EDS analyses for Membranes (1-3) and shows that inorganic material is present in membrane used. It can also be observed that the acidic washings are followed by a reduction of ions such as iron, calcium, phosphorus. But as EDS is a spot analysis, other interferences, even due to membrane material and washing reagents, are shown.
M EM BR AN E (1 )
Used
After 1st Washing
After 2nd Washing
M EM BR AN E (2 )
Used
After 1st Washing
After 2nd Washing
M EM BR AN E (3 )
Used
After 1st Washing
After 2nd Washing
Figure 2 – Images made by a SEM apparatus from the surface of Membranes (1-3) for each step of st nd washing sequence (membrane used, 1 Washing and 2 Washing) – Zoom of 1500 x.
M EM BR AN E (1 ) 500
500
500
S
Ca C
Cl
400
400
400
300
300
300
Ba
Cl
Si
200
200
Al S
200
P 100
Fe O Fe Ca
0
P Cl S P Si Cl
100
Ca
Si Al S Na S Cl Fe P Fe
Fe Ca
Fe
0
5
keV 10
0
15
Ba
Fe Ca
Ca O C
O Ca C
100
Fe
Ca
0
keV
5
10
Sr Al Cl Sr S
15
0
Sr 5
st
Used
Ca Ca
Ca
0
10
keV 15
After 2nd Washing
After 1 Washing
M EM BR AN E (2 ) 500
500
500
Ca C
Ca C 400
400
300
300
Ca C 400
300
Cl 200
200
Si
S 100
100
O
0
Cl
200
P Cl Si P Cl Si S
O
Ca Ca
0
keV 5
10
S Al P P
Cl
Si S
0
15
Na
100
Ca
O
Ca Ca
keV
0
5
10
0
15
Cl
Ca 0
keV 5
After 1st Washing
Used
Ca 10
15
After 2nd Washing
M EM BR AN E (3 ) 1000
500
Ca C
1000
C
900
Si Al
900
400
800
800 700
700
300
600
600 500
500
200
400
400
Si
Cl 300
300
S
200
Fe Si Fe Al P O S P Cl Ca Si
100 0 0
Ca C
Cl
S 100
Ti O Fe Fe Ti Ca
200
O Al P Cl S Fe P Si Cl
Ca Fe
Ca
Fe
5
keV 10
Used
15
0 0
100
Fe keV 5
10
After 1st Washing
15
0 0
P P Si Cl
Ca Ti Ca Ti 5
Fe
Fe
keV 10
After 2nd Washing
Figure 3 - Results of EDS analyses for Membranes (1-3) for each step of washing st nd sequence (membrane used, 1 Washing and 2 Washing).
15
CONCLUSION From the present study it is concluded that for membranes used in MBRs for the treatment of refinery wastewater, better wash sequence is first acid and then oxidizing agent. Commercial surfactant based in H2O2, even in high concentration, wasn’t efficient enough to remove organic compounds of fouling. It was also observed that the oxalic acid has better performance than citric acid. However, further studies on the impact of oxalic acid in the lifecycle of the membrane are needed. Next, the efficiency of cleaning with different concentrations of NaClO (5000, 2000, 1000, 500ppm) are going to be tested. The best membrane exposure time related to the oxidizing agent will also be evaluated (2-24 hours). The best combination of reagents and time will be used for evaluating the combined use of ultrasound, aeration and heating (30°C and 40°C) separately. Studies on the optimal concentration, time and use of temperature, agitation and ultrasonic cleaning of the membranes was not completed and, only after attainment of these results, it will be possible to construct an optimized procedure for this purpose.
References JUDD, S. (2006). The MBR Book: Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment . Elsevier, Great Britain. KIM, M. J., SANKAR ARAO, B. e YOO, C. K. (2011). Determination of MBR fouling and chemical cleaning interval using statistical methods applied on dynamic index data. Journal of Membrane Science.375, 345-353. MUSTAFA, G. d. (1998). Reutilização de Efluentes Líquidos em Indústria Petroquímica. UFBA. WEI, C.-h., et al. (2011). Critical flux and chemical cleaning-in-place during the long-term operation of a pilot-scale submerged membrane bioreactor for municipal wastewater treatment. Water Research.45, 863-871.
