the wastewater, followed by flocculation and flotation or settling. Dissolved air flotation process proved to separate oil-water wastes (Luthy et al., 1976; Franzen,.
Environrnentlnternational, Vol. 7, pp. 267-270, 1982 Printed in the USA. All rights reserved.
0160-4120/82/040267-04503.00/0 Copyright ©1982 Pergamon Press Ltd.
TREATMENT OF OILY REFINERY WASTES USING A DISSOLVED AIR FLOTATION PROCESS Ahmed S. Moursy and Sohair E. Abo EI-Ela Water PollutionLaboratory, EnvironmentalScience Division,NationalResearchCentre, Dokki, Cairo, Egypt
Refinery wastewater from Moustorod oil refinery, Cairo, Egypt, was treated by chemical coagulation. This step was followed by rapid solid-liquid separation using flotation under pressure of 4 atm. A flotation tank was designed for this purpose. Gas liquid chromatography was used for determination and identification of hydrocarbon compounds in refinery wastewater. The treatment techniqueproved to be very promising for reducing the hydrocarbon compounds, as well as other pollutants, to a very low concentration. The results obtained showed that some of the hydrocarbon compounds were completely removed.
the wastewater, followed by flocculation and flotation or settling. Dissolved air flotation process proved to separate oil-water wastes (Luthy et al., 1976; Franzen, 1971; Boyd, 1971; Sokolov, 1974). Flotation is a relatively new technique in the field of industrial water treatment. For the purification of refinery effluents, several flotation methods have been developed; they offer a promising and relatively low-cost means of providing high-efficiency oil removal. Flocculation and flotation achieve separation o f oil from water by gravity settling. The flocculation process aims at the formation o f a floc, while the flotation leads to the formation of an oil-foam which is skimmed o f f the surface of the water. The objective of this study was to evaluate the use of chemical coagulation followed by dissolved air flotation as a treatment process for the primary effluent of refinery wastewater.
Introduction The major sources of hydrocarbon compounds in water are attributed to oily leakage during the transportation and the discharge of industrial wastewater into rivers or streams. Refining of crude oil results in an oil wastewater. Industries using oils in processing or as raw materials are also faced with handling oily wastewater. The conventional processes applied in refineries, industries, and water treatment plants often cannot remove these compounds down to a desired level. Refinery wastewater characteristics vary with refinery size and process sophistication. An oil refinery is comprised of many interrelated processes which generally separate, alter, and rebuild the molecular configuration into the desired products (Ford, 1976). Egyptian Suez Oil Company has a large refinery at Moustorod, Cairo, Egypt, which produces a diverse array o f oil products. Because of its complexity and size, a significant amount of process water that has been in contact with oil and chemicals is generated and discharged to Ismaillia Canal. The major portion o f free and emulsified oil in refinery effluent is removed by using American Petroleum Institute (API) separators. Recognizing the need for further improvement of effluent water quality, this step must be followed by further treatment. Chemical methods are the widest used for demulsifing oily wastewater (Patterson, 1975; Ford, 1976). The process may consist o f rapidly mixing chemicals with
Material and Methods Following primary treatment in an API separator, refinery wastewater was subjected to physicochemical treatment consisting of flocculation with alum, then rapid solid-liquid separation using flotation under pressure of 4 atm. Composite wastewater samples were collected from the Moustorod oil refinery, Cairo, at different times from the point of discharge to the canal. Physicochemical 267
268
A h m e d S. Moursy and Sohair E. A b o EI-EIa
analyses of wastewater, including the determination of total oil and grease, were performed according to the American Standard Method (1975). The concentrations of hydrocarbons were determined using gas liquid chromatography (GLC) according to McNair (1969).
Hydrocarbon recovery One liter refinery wastewater sample was acidified to pH 3 with HCl and extracted twice with 30 ml portions of CHC13. The combined extracts were saponified to separate the saponifiable materials. The nonsaponifiable materials were dehydrated, reduced to l0 ml, and analyzed using gas chromatography (Varian Aerograph 2400, Detector: flame ionization; Column: stainless steel, 6 ft (1.83 m) long, 1/8 in. (3.2 mm) o.d.; Packing: 1.5°70 OV-101 on chromosorb G. 100/120 mesh; Temperatures: injector 250 °C, detector 250 °C, column temperature program from 70 to 230 °C at 5 °C/min; Attenuation: 16 x 10-11. Twenty hydrocarbon compounds were taken as reference for the identification and determination of the hydrocarbon compounds in refinery wastewater. Chemical coagulation Chemical coagulation was carried out using jar test procedures to gain information on the optimum pH value and the coagulant dosage required for the best removal of organic matter. Alum was used as a metallic coagulant. This phase was followed by dissolved air flotation. Dissolved air flotation unit A laboratory-scale flotation under pressure system was designed similar to that used by E1-Gohary (1980) (Fig. 1). Alum was introduced just prior to entry to the flotation cell and before applying the pressurized airwater mixture. Factors affecting the dissolved air flota-
Pressure ~
Water I Inlet
tion process, such as detention period and air/solids ratio, have been investigated.
Determination of optimal detention time To determine the optimum time of flotation, the pressurized air was fixed. The clarified effluent was drawn off at several time intervals and analyzed for turbidity and COD contents. Determination of the optimum A / S ratio Air/solids (A/S) ratio is the parameter most commonly used in the development of design criteria of dissolved air flotation system. This was determined by fixing the solids content of the waste while changing the volume of pressurized air applied to the flotation cell. The effluent was drawn off for analysis at the predetermined optimum time. After these runs, the A/S ratio was computed from the following formula: A/S -
1.3s~R(P- 1),
(1)
QSo where So P R Q So
= = = = =
air saturation in cm 3 L-L; absolute pressure in atm; pressurized volume in litre; waste flow in litre; influent suspended solids in mg L-'.
R e s u l t s and D i s c u s s i o n
Physicochemical characters of wastewater discharged from the refinery are shown in Table 1. From this table it can be seen that the concentration of phenols was 140 ~g L-'. This high value is potentially toxic to marine life, creates an oxygen demand in receiving water, and imparts a taste to drinking water with even minute concentrations of the chlorinated derivatives (Ford, 1976). The low value of BOD is attributed to the high concentration of phenol which has a biotoxic or biostatic effect on the seed microorganisms. The high level of oil in
Safety Valve
Table 1. Characteristics of refinery wastewater.
- 11:1 ::o wrL
ijol
"u''..... '
J
Dim. in : m m
Scale
Fig. 1. Retention tank.
1 : 50
Parameters
Results
pH Turbidity, N T U COD, m g 02 L "1 BOD, m g 02 L -1 Total SS. at 105 °C, m g L-' Fixed SS. at 550 °C, m g L-' Volatile SS. at 550 °C, nag L-' Total residue at 105 °C, m g L -I Fixed residue at 550 °C, m g L-' Volatile residue at 550 °C, m g L-' Total dissolved residue at 105 °C, m g L l Tot',d oil and grease, m g L -1 Total oils, m g L -I Phenols, #g L "
8.1 18.0 150.0 6.0 56.0 31.0 25.0 506.0 386.0 120.0 450.0 86.84 71.21 140.0
Treatment of oily refinery wastes
269
refinery wastewater indicated that the emulsified and soluble oil fractions are not removed by gravity oil separation equipment ( A P I Separator), and are discharged in the effluent into Ismaillia Canal. In order to determine the o p t i m u m p H value for alum, a fixed dose equivalent to 4.3 mg AP ÷ L-' was used. The p H value was changed to cover the range f r o m 4-7. The results obtained showed that the best turbidity removal values were achieved at p H 5.9-6.2. T o study the effect o f coagulant different alum doses were employed, covering the range 1.44-10.8 mg AP ÷ L -1. The results obtained showed that the optimum alum dose was 4.3 AP ÷ L -1, which gave the best removal values for both C O D and turbidity. To determine the o p t i m u m detention time required for flotation, the A / S ratio was fixed at a value equivalent to 0.001. Wastewater samples were withdrawn for turbidity and C O D analysis at different time intervals. Available data (Fig. 2) tends to show that the 7 min was adequate for best solid-liquid separation. To determine the o p t i m u m A / S ratio, the volume of pressurized air was changed f r o m 100 to 300 ml at a fixed dose of total oil and grease equivalent to 86.0 mg L-'. The detention time was kept constant at the predetermined value, namely 7 rain. Accordingly, the A / S ratio was completed from Eq. (1). The results obtained (Fig. 3) showed that the o p t i m u m A / S ratio was 0.001. The overall efficiency o f the treatment is illustrated in Table 2. Considerable reductions in turbidity, COD, total oil and grease, phenols, and total oils were obtained They were 94.4070, 95o70, 95o-/0, 85o70, and 96.5o70, respectively. The identification and determination of chromatographed hydrocarbon compounds using gas c h r o m a t o g r a p h y after the separation o f the saponifiable materials are presented in Table 3. Available data showed that the refinery wastewater contained aromatic, unidentified aromatic and n-pariffin compounds. For
-= " 100
=- Turbidity ~' COD
A/S = 0.001
100
80-
E
o (]:
60-
40-
20-
0
I
2
I
I
4
6
I
8
I
~
10
12
_4
10
AIS ratio
Fig. 3. Percentage removal of turbidity as a function of A/S ratio. the unidentified aromatic and n-paraffin compounds, the percentage removal was measured by calculation of peak area of the c h r o m a t o g r a m of the raw sample, then it was compared with the peak area obtained f r o m treatment process. Data showed that when the wastewater is chemically pretreated to break the oil emulsion, dissolved air flotation units are capable of removing most of the emulsified oil in addition to the original free oil content. Removal rate o f hydrocarbon compounds, as indicated by the chromatograms, was generally found to increase as the solubility o f the compounds decreased. The results obtained after treatment o f refinery wastewater showed that some o f the hydrocarbon compounds were completely removed. On the other hand, the percentage removal of the other hydrocarbon compounds ranged between 79°7o and 98°70. These may be attributed to the difference in solubility and molecular weight o f the hydrocarbon compounds. Finally, it m a y be concluded that the treatment o f refinery wastewater via flocculation/dissolved air flotation is a promising technique for removing the hydrocarbon compounds to very low concentrations as well as other pollutants.
90 o
](
E
.,,
----._,g.
Table 2. Average results of treatment of refinery wastewater.
80
Parameters 70 L -
601 -
/
I
I
I
I
I
5
I
10
~
i
,
~
I
15
Tirnelmin.
Fig. 2. Percentage removal of turbidity and COD as a function of time.
pH Turbidity, NTU COD, mg 02 L-1 Total SS. at 105°C, mg L-' Fixed SS. at 550°C, mg L-' Volatile at 550°C, mg L-I Total oil and grease, mg L~ Total oils, mg L-1 Phenols, #g L-'
After Treatment
% Removal
6.7 1.0 8.0 12 4 10 4.62 2.56 22.0
94.4 95.0 79.0 87.0 60.0 95.0 96.5 85.0
A h m e d S. Moursy and Sohair E. A b o EI-EIa
270
Table 3. Percentage removal of chromatographed hydrocarbon c o m p o u n d s from refinery wastewater. Before Treatment Peak Hydrocarbon N u m b e r of C o m p o u n d s
After Treatment
Peak Area ( m m ~)
Concentration (mg L "~)
Peak Area (nun a)
Concentration (mg L " )
% Removal
Aromatics 1 Ethylbenzene 2 m-, p-Xylene 2 o-Xylene 4 Isopropylbenzene 5 Unidentified (X~) 6 Unidentified (X~) 7 Ter-butylbenzene 8 Unidentified (X3) 9 Unidentified (X,) 10 1,2,3-trimethylbenzene 11 Unidentified (Xs) 12 n-butylbenzene
39 24 22 15 14 40 5 4 22 104 28 99
0.503 0.310 0.281 0.192 ~ 0.064 1.315 1.277
2.0 0.0 1.5 0.0 0.0 2.0 0.0 0.0 2.0 4.0
0.025 0.0 0.018 0.0 0.0 0.0 0.054 0.042
95 100 93 100 100 95 100 100 91 96 95 97
n-paraffins 13 n-C,~ 14 n-Cl~ 15 n-C,, 16 n-C16 17 n-CI7 18 n-C,8 19 n-C,9 20 n-C2o 21 n-C21 22 n-C22 23 n-C~3 24 n-C~4 25 n-C25
440 221 460 550 50 260 36 112 62 99 15 18 42
-
-
98 79 93 92 100 100 100 100 100 82 100 100 95
Acknowledgements-This study was supported in part by the funds provided by the Multidisciplinary Environmental Studies research project sponsored by the Egyptian National Research Centre, Dokki, Cairo, and the U.S. Environmental Protection Agency.
References American Public Health Association, American Water Works Association (1975) Standard Methods for the Examination of Water and Wastewater, 14th ed. A P H A , A W W A , Washington, DC. Boyd, J. L., Shell, G. L., and Dahlstrom, D. A. (1971) Treatment of oily wastewater to meet regulatory standards. American Institute of Chemical Engineering Conference, Toledo, OH. EI-Gohary, F. A. and A b o EI-Ela, S. E. (1980) The optimization of wastewater treatment via combined techniques. Part II: Combined biological dissolved air flotation, Environ. Int. 3, 219~223.
1.5
3,0
10 48.0 30.0 44.0 0.0 0.0 0.0 0.0 0.0 18.0 0.0 0.0 2.0
-
Ford, D. L. (1976) Water Pollution Control in the Petroleum Industry, in Industrial Wastewater Management Handbook, H. S. Azad, ed. McGraw Hill, New York. Franzen, A. E., Skogan, V. G., and Grutsch, J. F. (1971) Tertiary treatment of refinery process water effluent by chemical coagulation and air flotation. 64th A n n u a l Meeting and Water Quality Engineering S y m p o s i u m , A m e r i c a n Institute o f Chemical Engineers, 28 November-2 December, San Francisco, CA. Luthy, R. G., Selleck, R. E., and Galloway, T. R. (1978) Removal of emulsified oil with organic coagulants and dissolved air flotation, J. Water Pollut. Control Fed. 50, 331-46. McNair, H. M. and Bonelli, E. J. (1969) Basic Gas Chromatography. Patterson, J. W. (1975) Wastewater Treatment Technology, A n n Arbor Science Publishers, A n n Arbor, MI. Sokolov, V. P., and Pustoseleva, Z. I. (1974) Purification o f petroleum refinery wastewater studied by the reagent pressure flotation method, Nefteperab Neftekhim 2, 11.
