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E. Katsoyannos, A. Hatzikioseyian, E. Remoundaki and M. Tsezos National Technical University of Athens (NTUA) - School of Mining and Metallurgical Engineering
Laboratory of Environmental Science and Engineering, Heroon Polytechniou 9, Zografou 15780, Athens, Greece Tel: +30 210 772 2172, +30 210 772 2271, Fax: +30 210 772 2173,
e-mails: [email protected], [email protected],[email protected] [email protected]
Reference1. Katsoyannos E. and Tsezos M. Biotechnology application in the OMW disposal. Pyrforos, NTUA,1999.
2. Kyritsakis A. The olive oil. Athens,1989.
3. Balis K., Chatzipavlidis I. and Flouri F. Integrated system for OMW disposal. Athens ,1991.
4. Bauer R., Waldner G., Fallmann H., Hager S., Klare M.,Krutzler T. Malato S.,Meletzky P. The Photo-Fenton reaction.
5. TiO2/UV process for waste treatment – novel developments.Catalysis Today, 53 (1999) 131-144.
Results - Conclusions•1.With both photocatalytic systems used phenols reduction is possible to values that are below under the phytotoxicity limit (500ppm) using physical sun light and within reasonable time.
•2. Total COD reduction is satisfactory ( 74 – 87%), while the de-colorization and the deodorisation of OMW are complete.
•3. A pre-treatment of raw OMW is necessary prior to the application of the photocatalysis.
•4. ΤiO2 was more efficient in terms of phenols-degradation, COD reduction rate and H2O2 consumption under the experimental conductions.
5. ΤiO2 use is simpler as it requires no pH control while, its release in the environment is not recommended. Tube reactors with fixed ΤiO2 are required for any full scale application .
6. Photo – Fenton spent solutions can be released into the environment, FFR is suitable for Photo – Fenton application.
IntroductionIn Greece, 2000 olive mill units produce annually 300-450.000 tons olive oil, corresponding to more than 2.500.000 tons olive mill wastewaters (OMW) with extremely high BOD (50- 80.000 mgO2/L) and COD (120- 200.000 mgO2/L) values (1).
The rainfall during the olive mill units operation as well as the uncontrolled OMW disposal cause considerable and esthetic problems in agricultural and touristicregions, extending over the summer months with significant pollution of surface and underground waters.
The common disposal practice of OMW in Greece can be summarized as follows (2);
60% of OMW disposed in an uncontrolled way into dry river beds
20% in neighboring non cultivated agricultural areas
8% in sewers
2% in septic tanks
2% in crevasses
The table that follows summarizes the OMW composition of two phase olive mills
Table 1. Composition of three phase olive mill wastewater (1)
Objectives(a) To study of the possibility of the application of the photocatalytic oxidation with the aid of natural sunlight for the treatment and de-colorization of the OMW.
(b) To compare the efficiency of two photocatalytic systems (Photo-Fenton and TiO2) in the reduction of Polyphenols, COD and BOD values of OMW.
(c) To derive useful technical-economical data for larger scale applications of the tested photocatalytic systems.
Photocatalytic treatment of olive mill wastewaters (OMW) in pilot scale
AbstractOlive mill wastewaters (OMW) are considered one of the most difficult to treat agro-industrial pollutants, due to their excessive COD and BOD values which cause serious environmental problems in touristic and agricultural Mediterranean regions. A good number of treatment and disposal methods have been proposed, though none offers an applicable techno-economical feasible solution. The use of the sunlight energy with the aid of inorganic
catalysts (photocatalytic oxidation) is a promising alternative. In the present study the results of the treatment of OMW with two different photocatalytic systems (Photo - Fenton and TiO2 ) under natural sunlight in a pilot Falling Film Reactor (FFR) are presented and compared. Practically total degradation of the phytotoxic phenolics and a remarkable COD reduction as well as a complete OMW de-colorization were achieved by both systems tested.
For both photocatalytic systems , data of H2O2 consumption and energy requirements are also presented. On the basis of the results concerning the COD reduction, the Total Phenols degradation and the energy requirements, the comparison of the photocatalytic systems tested, leads to positive technical and economical conclusions.
Materials and Methods1. OMW pre-treatmentConsiderable (60-45% )COD and BOD reduction by efficient removal of suspended solids (final values 5 -7 g/L) after CaO addition, sedimentation and filtration.
Photocatalysis conditions
pH 2,2 - 2,5 (to avoid Fe 2+ sedimentation)
H2O2 concentration 2-4 g/L
Natural sun light
Mirror area slope 45°
Waste volume 10 L
pump; OMW recycling flow = 0,25 L / sec
Photocatalysis Parameter
pH
Η2Ο2
COD and photometric after digestion at 150oC
Sunlight irradiation Q (kJ / L) and Sun energy E(W/m2)
Fe 2+ concentration
Total phenols (expressed as ppm of equivalent caffeic acid) are determined photometrically after extraction with methanol with theFolin-Ciocalteu reagent.
Daily, sun radiation energy values (W/m2) were recorded and the sun energy ( kJ/ L) irradiating the reactor surface was related to the waste volume treated (figure 1).
The efficiency of the photocatalytic system was expressed in terms of COD-, BOD- and Total Phenols – reduction per kJ of irradiating Sun energy.
2. Experimental conditions
2.1. OMW Pre - treatment
CaO addition (pH = 10) in order to remove (by sedimentation and filtration) most of solids (60-45%) for achieving satisfactory sunlight penetration.
Final solid concentration of the OMW before photocatalytic treatment = 5 - 7 g/ L.
Observed COD-reduction due to solids separation was; from initial values of ca.120.000 ppm to final values of ca.50.000 ppm.
Final concentration of Total Phenols ca. 2,3 g / L .
2.2. Photocatalytic systems implemented
a ) Photo-Fenton Fe2+ = 1-5-10 mM
pH = 2,2 - 2,5 (pH control with 1n H2SO4)
b ) TiO2 4 g/L, H2O2 3-7,5-12 g/L
Reactor irradiating surface : 0,254 m2
PhotocatalysisThe oxidation of organic substances catalyzed by metal oxides (Fe, Zn, Ti) in acidic solutions with the aid of sunlight is an relative new, interesting alternative for a considerable reduction of toxic pollutants.
Under the effect of the (in the visible and better in the UV region) H2O2 oxidation leads to Hydroxyl - radical formation. . OH is an extremely active agent, degrading most of the toxic organic pollutants following the simplified pathway listed bellow:
Fenton reagent ; Fe2+ + H2O2 ----------- Fe3+ + OH- + . OH
TiO2 when radiated in the region under 387 nm leads also to . OH formation ;
The Photo-Fenton reagent has no adverse consequences on the environment when it is released in the environment after use. Use of TiO2, is interesting because its application does not require pH control. (4,5)
Practical photocatalysis application is implemented with the aid of three types of reactors ;
• Batch stirred tank reactor with liquid waste recycling.
• Continuous falling film reactor (FFR) with recycling. The sunlight is reflected on the sloping mirror glass or white polymeric surface penetrating thus twice the thin liquid waste falling film. Double , rotating mirror reactors convert more efficiently the sunlight.
• Artificial UV light source central inside a glass tube in the shape of a double glass tube reactor, while the liquid waste is recycled inside the outer glass tube.TiO2 is often fixed using special thermal techniques onto the outer surface of a central tube.
Disposal methodsSeveral disposal methods have been proposed, though none offers a successful solution with the most common being :
- Direct irrigation of agricultural land or simple CaO addition prior to disposal in agricultural areas.
- Disposal in lagoons for physical concentration or thermal concentration prior to use as compost.
- Aerobic (for animal feed and compost production) or anaerobic (for biogas production) fermentation.
- Oxidation with O2 under high pressure or O3 or by electrolysis.
- Reverse osmosis and irrigation.
Solids removal with CaO and poly-electrolytes addition followed by solids-composting and multi-step aerobic fermentation of the liquids in tower-fermentors before reverse osmosis.
- Combination of evaporation, hydrolysis, oxidation with hot air after sludge removal and fat separation by ceramic filters.
Water content 88 -92 % b.w. pH 4,5 – 5,8 Fat 0,05 – 1,5 b.w. Total phenols 3,15 – 6,05 ppm Tannins and lignins 0,30 – 0,71 % Cellulose 13,8 – 33,0 % T.S. Soluble sugars 1,45 – 1,77 % b.w. Organic C 42,9 – 45,0 b.w Nitrogen (ΤΚΝ) 0,15 – 0,23 % b.w. Woody material 34,2 – 37,8 % b.w. Ash Total Solids
9,5 – 13,6 % T.S. 8,4 – 13,2 % b.w.
Conductivity 1750 – 2300 µS/cm
Figure 2 Schematic presentation of the FFR reactor
ResultsThe results below summarise the observed reactor performance:
OMW photocatalytic oxidation with Photo – Fenton in pilot Falling Film Reactor( FFR) with natural sun light(5mM Fe + 7,5 g/L H2O2 )
Total phenols reduction; 1,94 g/L (98%) within 12,5 hours under energy consumption per OMW volume 642 kJ / L
Reduction of soluble COD; 82% within 12,5 hours (642 kJ /L)
Reduction of total COD ; 74 % within 12,5 hours (642 kJ /L)
H2O2 Addition ; 19,5 g/L
H2O2 Consumption ; 16,5 g/L
OMW photocatalytic oxidation with TiO2 in pilot Falling Film Reactor( FFR) with natural sun light(3 g/LTiO2 + 7,5 g/L H2O2 )
Total phenols reduction; 2,11 g/L ( 98% ) within 10 hours under energy consumption per OMW volume 670 kJ / L
Reduction of soluble COD; 91% within 7,5 hours (584 kJ/L)
Reduction of total COD ; 87 % within 11,5 hours (670 kJ/L)
H2O2 Addition ; 13,3 g/L
H2O2 Consumption ; 12,3 g/L
Comparison of efficiency and consumption of the photocatalytic systems used
ΤiO2 Photo – Fenton
Total phenols reduction 0,21g / L / hour 0,15 g/L/h
( 318 kJ /g ) ( 331 kJ /g )
COD Reduction 11,6 % COD / h 5,9 g/L/h
(7,7 kJ /% COD) ( 8,8 kJ /% COD)
H2O2 Consumption 13,3 g / L 19,5 g/L
Figure 3. FFR/OMW/Photo-Fenton/Sol.Phenols-Time
0
0.5
1
1.5
2
2.5
0 5 10 15
Time (hours)
To
tal S
olu
ble
Ph
en
ols
(g
Figure 7. FFR/OMW/TiO2/Sol.Phenols - Q
0
0.5
1
1.5
2
2.5
0 100 200 300 400 500 600 700 800
Q ( kJ / L)
To
tal S
olu
ble
Ph
eno
ls(
g/L
)
Figure 8. FFR/OMW/TiO2/Sol.+Tot. COD - Time
0
2000
4000
6000
8000
10000
12000
14000
16000
0 5 10 15
Time (g/L)
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tal a
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OD
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Tot. COD
Figure 9. FFR/OMW/TiO2/Sol.Phenols - Time
0
0.5
1
1.5
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0 2 4 6 8 10 12 14
Time (hours)
To
tal S
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Ph
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g/L
)
Figure 10. FFR/OMW/TiO2/Tot.+Sol. COD - Q
0
20004000
60008000
10000
1200014000
16000
0 200 400 600 800
Q (kJ / L)
To
tal a
nd
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lub
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OD
Sol. COD
Tot. COD
OMW photocatalytic oxidation with Photo – Fenton in pilot Falling Film Reactor( FFR) with natural sun light (5mM Fe + 7,5 g/L H2O2 )
Figure 4. FFR/OMW/Photo-Fenton/Sol.Phenols - Q
0
0.5
1
1.5
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2.5
0 100 200 300 400 500 600 700
Q ( kJ / L )
To
tal S
olu
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Ph
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(
Figure 5. FFR/OMW/Photo-Fenton/Sol.+tot. COD - Time
0
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3000
3500
4000
0 5 10 15
Time ( hours )
To
tal an
d S
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Tot. COD
Figure 6. FFR/OMW/Photo-Fenton/Sol.+Tot. COD - Q
0
500
1000
1500
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3500
4000
0 200 400 600 800
Q ( kJ/L )
So
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Sol. COD
Tot. COD
OMW photocatalytic oxidation with TiO2 in pilot Falling Film Reactor( FFR) with natural sun light (3 g/LTiO2 + 7,5 g/L H2O2 )
