Dissolved air flotation for abattoir wastewater

Wat. Res. Vol. 20, No. 4, pp. 421-426, 1986 Printed in Great Britain. All rights reserved

0043-1354/86 $3.00+0.00 Copyright © 1986 Pergamon Press Ltd

DISSOLVED AIR FLOTATION FOR ABATTOIR WASTEWATER D. A. LOVETT and S. M. TRAVERS CSIRO Division of Food Research, Meat Research Laboratory, P.O. Box 12, Cannon Hill, Queensland 4170, Australia

(Received September 1984) Abstract--The performance of laboratory-scale dissolved air flotation plants treating abattoir wastewater has been measured for saturator pressures from I00 to 500 kPa, wastewater suspended solids concentrations from 170 to 2100 mg l -I and air-to-solids ratios from 0 to 0.17. Maximum removal efficiencies for chemical oxygen demand, suspended solids and fat were 70, 50 and 95% respectively, at high wastewater suspended solids contents and high air-to-solids ratios. Pressure did not greatly influence the size of the bubbles generated in the process, or the removal efficiencies. High air-to-solids ratios were necessary to obtain maximum removal efficiencies and to prevent appreciable settling of the suspended solids.

Key words-- abattoir, dissolved air flotation, removal efficiency, air-to-solids ratio, fat, chemical oxygen demand (COD), suspended solids, dissolved solids, laboratory-scale, bubble size, saturator pressure

INTRODUCTION

liquid turbulence. Its value, in joules, is given by:

Dissolved air flotation (DAF) is an efficient and rapid method of separating particulate matter from wastewater. Up to 98% of suspended solids can be removed from industrial wastewaters using D A F and coagulants (Hopwood, 1980; Bratby, 1982). The process is more rapid than settling and produces a drier sludge. It is used in many industries, but a shortage of design data for particular wastewaters such as those from abattoirs, often leads to poor performance (Roberts et aL, 1978). This paper provides design data relating to four aspects of the D A F treatment of abattoir wastewaters: the relationship between bubble size and saturator pressure; the applicability of Henry's law to dissolution of air in wastewaters; removal efficiencies for suspended solids, chemical oxygen demand (COD) and fat at different air-to-solids ratios and suspended solids concentrations; and the effect of air-to-solids ratio on the relative quantities of floated and settled solids.

AF = ~ ltt73/(po _ pQ)2

(1)

where cr is the liquid-air interfacial tension (N m - l ) , Pa is atmospheric pressure (Pascal) and P0 is the applied pressure (Pascal). Equation (1) may be used to deduce the effect of pressure and surface tension on bubble size. It shows that the energy necessary for bubble formation can be decreased by decreasing the surface tension or by increasing the pressure. For the same concentration of dissolved air it is reasonable to assume that a larger number of smaller bubbles will be generated as the energy required for bubble formation is reduced (Takahashi et al., 1979). By this reasoning, bubble size should be directly related to surface tension and inversely related to pressure. The effect of pressure on bubble size however is uncertain, because pressure not only affects the energy required for bubble formation, but also the equilibrium concentration of dissolved air (Henry's law). The effect of pressure on bubble size has been investigated in this work.

Theory o f bubble formation in D A F The size of bubbles has been shown to greatly affect the performance of flotation processes (Cassell et al., 1975). Although the bubbles produced in D A F are small, of the order of 100#m (Vrablik, 1959), even smaller bubbles are generally considered advantageous (Kitchener and Gochin, 1981; Wood and Dick, 1973). Studies of the nucleation and growth of microbubbles by Ward et al. (1970) and Takahashi et al. (1979) have shown that energy must be imparted to a liquid supersaturated with air, before bubbles will form. This energy, AF, is usually provided by 421

MATERIALS AND M E T H O D S

The size distributions of bubbles formed at different dissolved pressures were measured with apparatus described elsewhere (Lovett et al., 1984). Air saturated water from a pressure vessel was fed into a Perspex bubble tank through a diaphragm valve. A sequence of photomicrographs of the bubble cloud was taken at 5 s intervals for each pressure, to observe the growth of the bubbles. Maximum size was reached after 10-15s. Bubble diameters were measured from photographic prints, 50 x magnification, taken at this time. Henry's law [equation (2)] predicts a linear relationship between the mass concentration of dissolved air at satur-

422

D . A . LOVETT and S. M. TRAVERS

ation C (mg

1 I ) and

the saturator pressure P (kPa): (2)

C = kP

The constant of proportionality, k, is called Henry's Law constant and has a value of 0.226 mg I ~ kPa-~ for air in water at 20'C. Bratby and Marais (1975) have demonstrated that the presence of suspended solids in the water does not alter k or affect the relationship, but Vrablik (1959) has reported that the value o f k varies with the concentration of dissolved solids. To test the accuracy of equation (2) for abattoir wastewater and to determine a value for k, the mass of air dissolved in wastewater at different pressures was measured by a method due to Maddock (1979) and Conway et al. (1981). Air was bubbled through wastewater in a saturator (a 20 1., 50 cm deep pressure vessel) for periods of 6 or 16min at pressures of 100, 200, 300, 400 and 500 kPa. Air was introduced at the bottom of the saturator through a sparger which consisted of a loop of stainless steel tubing with fifty 0.95 m m holes. The free air flow rate through the pressure vessel was 151 min -~ at 100 kPa and was increased with the pressure to maintain a constant volume sparging rate in the saturator. To measure the concentration of dissolved air at each pressure, a volume of the aerated wastewater was passed through a needle valve and into a chamber at atmospheric pressure where the dissolved air was separated and its volume measured. The volume of the wastewater sample was also measured, and the concentration of dissolved air calculated. Measurements of the removal efficiency of suspended solids, C O D and fat were made in batch tests. The experimental apparatus is shown in Fig. 1. Four 1800 ml flotation cylinders made of Perspex were used in the tests. The pressure vessel described earlier was used as the saturator. In a typical run, one of the flotation cylinders was filled with a mixture of air-saturated tap water (at 26~C) fed from the saturator, and wastewater (at 34°C, pH 6.8-7.2) pumped from the 21. graduated measuring cylinder. The wastewater was collected from a local abattoir, and had widely different concentrations of suspended solids, C O D and fat on different days. The two flows were mixed just before entry into the flotation cylinder. The other flotation cylinders were filled in like manner with wastewater/air-saturated tapwater mixtures, giving different air-to-solids ratios in each. The air-to-solids ratio was varied by changing the flowrates of

the air-saturated tap water and wastewater. Needle valves were used to control the flow of the air-saturated tapwater, and the speed of the peristaltic p u m p (maximum flow 51rain -~) was adjusted to control the flow of wastewater. The solenoid valves were electrically coupled to the peristaltic p u m p so that both flows could be started or stopped simultaneously. The volume of wastewater used to fill each cylinder was taken as the decrease in volume in the graduated cylinder. The volume of air-saturated tap water was obtained by subtracting the known volume of wastewater from the volume of the flotation cylinders. The volume of precipitated air was calculated from Henry's law for water at the saturator temperature, Full saturation was achieved at retention times of about 15 min. After filling, each cylinder was allowed to stand lbr 20 min and the s u b n a t a n t sampled for suspended solids, C O D and fat through a port 12 cm above the base of the cylinder. Chemical analyses were done by the methods described in S t a n d a r d M e t h o d ~ (APHA, 1971), except that a Freon 112/alcohol solvent was used for fat extraction. The suspended solids, C O D and fat concentrations in the wastewaters tested ranged from 170 to 2100, 930 to 6500 and 45 to 600 mg I ~ respectively. Saturator pressures from 50 to 500kPa were used, and air-to-solids ratios were varied between 0 and 0.17. During the early tests, it was noted that at low air-tosolids ratios some solids settled to the bottom of the flotation vessels. As this settled mass contributed to the measured removal efficiency, the relative quantities of settied and floated solids were measured in a number of subsequent flotation tests by carefully decanting off most of the liquid and float from the flotation cylinders after the subnatant sample had been taken. The total settled solids was then measured and corrected for solids contained in the excess water taken with the sample. The float solids were calculated as the difference between the influent suspended solids and the subnatant and settled solids.

RESULTS AND DISCUSSION B u b b l e size d i s t r i b u t i o n s at 50, 100, 200 a n d 500 k P a are s h o w n in Fig. 2. T h e m o d e d i a m e t e r s o f

Solenoid valve

Compressed air supply Pl--~ressure _ ---

® Flotation cylinder --,ig-

Sludge

e

Air flowmeter L_'~_.~ soturat°r

Needle valve

e

oe

•

t •

Wastewater 2 supply Peristaltic pump

Fig. 1. Dissolved air flotation apparatus.

°¢..

Sampling port

Dissolved air flotation for abattoir wastewater

21I 1o15 Pressure S0kPo .-e g

t~ :>0

l-]

ameters and lengths. Vrablik does not describe his apparatus. Figure 3 shows the concentration of dissolved air in wastewater at different pressures after retention times of 6 and 16 min. The mass predicted by Henry's law for water is also plotted as a straight line. The wastewater samples contained 1000 mg 1-1 total dissolved solids. The meaured values were close to those predicted by Henry's law when sufficiently long retention times were used. The values above the line are assumed to be due to entrainment of undissolved bubbles in the wastewater samples. We conclude that for wastewater with dissolved solids up to 1000 mg 1-~ and for pressures up to 500 kPa, the mass of dissolved air is given by Henry's law with the value of k being equal to that for air in pure water at the same temperature. Measured removal efficiences for suspended solids, COD and fat are shown plotted against air-to-solids ratio in Figs 4, 5 and 6. In Fig. 7 the suspended solids removal efficiency has been plotted against the suspended solids concentration in the flotation cylinder. The curves in Figs 4-7 are curves of best fit. Several results from full-scale abattoir D A F plants have also been plotted. The labels A and B on these points indicate diffeent plants. The large scatter observed in some of the plots may be due to differences in the surface chemistry of the particles and colloids for different samples (Roberts et al., 1978). The trends of the results for suspended solids, COD and fat removal were similar. Fat removal efficiencies were higher than those for suspended solids due to the natural tendency of the fat to float, and because the hydrophobic nature of the fat particles increased the probability that a collison with a bubble would result in adhesion (Vrablik, 1959; Kitchener and Gochin, 1981). The COD removal efficiency results on the other hand were lower than those for suspended solids because the soluble fraction of the COD could not be removed by the flotation process. In general, removal efficiencies for the three components increased with air-to-solids ratio and with suspended solids concentration reach-

Pressure= 100 kPe

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__,_ O

50

100 150 200 Bubble diameter (p.m)

423

250

Fig. 2. Bubble size distributions at four saturator pressures.

the distributions were 60/~m at 50kPa, 90/lm at 100kPa, 8 0 # m at 200kPa and l l 0 / t m at 500 kPa. The change in bubble diameter was thus about 20% in the pressure range normally used in D A F plants (200-500kPa). This change is similar to that observed by Vrablik (1959) and Takahashi et al. (1979), and confirms the conclusion drawn from their data, that pressure does not appreciably influence flotation performance. Our results and those of Vrablik indicate that bubble size increases with pressure, while those of Takahashi et al. show a decrease with pressure. These differences may be related to the different methods used to generate the bubbles. In our work a 6 mm diaphragm valve was used for pressure release. This was followed by 40mm of 4 mm dia tube with a right-angle bend. Takahashi et al. used a needle valve, or nozzles of various di140 tm 120 E

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Derived from Henry's law for water 6 rain retention time 16 rain retention time

I 300

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Pressure (kPo)

Fig. 3. Concentration of air dissolved in wastewater at different saturator pressures.

D . A . LOVETT a n d S. M

424

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F i g . 6. E f f e c t o f a i r - t o - s o l i d s r a t i o o n r e m o v a l different suspended solids contents.

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of

Dissolved air flotation for abattoir wastewater

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Fig. 8. Effect of air-to-solids ratio on removal of suspended solids by settling and flotation (wastewater suspended solids content 1250mg 1-~). ing a maximum of about 70% for suspended solids, 50% for COD and 95% for fat. At zero air-to-solids ratio removal efficiencies were still relatively high due to natural flotation of fat and settling of other solids. As the suspended solids concentration approached zero the removal efficiency for suspended solids also approached zero (Fig. 7). This was an interesting result and indicated that particle sizes decreased at low suspended solids concentrations. The effect of pressure on the removal efficiencies was small. The data in Figs 4-7 were obtained at pressures from 50 to 500 kPa, but no relationship with pressure could be observed. This result agrees with our earlier conclusion that pressure does not appreciably affect bubble size. It is contrary to the results of Wood and Dick (1973) who reported differences in float rise rate at different pressures, presumably due to differences in bubble size. Bratby (1983) was critical however of their experimental method. He had earlier concluded that "the often reported dependence of flotation performance on saturator pressure is not supported" (Bratby, 1982). The fraction of solids removed by settling and by flotation for different air-to-solids ratios has been plotted in Fig. 8. At low air-to-solids ratios, solids removal was by both settling and flotation, with much of the flotation being due to the low density of fat particles and fat/protein agglomerates rather than to bubble attachment. At higher air-to-solids ratios, settling decreased in importance. Air-to-solids ratios from 0.004 to 0.07 are commonly recommended in the literature for full-scale plants. Figure 8 shows that at the low end of this range the settled fraction can be relatively high. This fraction will increase with suspended solids concentration, particle size and degree of flocculation (Travers and Lovett, 1985). The removal efficiencies measured in this work should represent the maximum values obtainable for each set of operating conditions. How closely these values are approached in full-scale plants will depend on the individual plant design. The results from plants A and B in Fig. 7 show that removal

efficiencies for full-scale, continuously operated plants can be similar to those measured in this work.

CONCLUSIONS In this work the maximum efficiencies for removal of suspended solids, COD and fat from noncoagulated abattoir wastewater were 70, 50 and 95% respectively, and occurred at high suspended solids concentrations and high air-to-solids ratios. Pressure had little effect on bubble size or removal efficiency. The air-to-solids ratio did not greatly affect removal efficiencies, but higher values than those commonly recommended in the literature were necessary to prevent excessive settling. Dissolved solids at concentrations up to 1000 mg 1-~ did not affect the mass of air which could be dissolved in the wastewater. Acknowledgements--The authors are grateful to Mr L. S.

Herbert for valuable discussions and advice, and to Mr B. Rumley who constructed the apparatus.

REFERENCES

APHA (1971) Standard Methods for the Examination o f Water and Wastewater, 13th edition. American Public Health Association, New York. Bratby J. R. (1982) Treatment of raw wastewater overflows by disolved air flotation. J. Wat. Pollut. Control Fed. 54, 1558-1565. Bratby J. R. (1983) Batch flotation tests: how useful are they? J. Wat. Pollut. Control Fed. 55, 110-113. Bratby J. R. and Marais G. V. R. (1975) Saturator performance in dissolved-air (pressure) flotation. Water Res. 9, 929-936. Cassell E. A., Kaufman K. M. and Matijevic E. (1975) The effects of bubble size on microflotation. Water Res. 9, 1017-1024. Conway R. A., Nelson R. F. and Young B. A. (1981) High solubility gas flotation. J. Wat. Pollut. Control Fed. 53, 1198-1205.

Hopwood A. P. (1980) Recovery of protein and fat from food industry waste waters. Wat. Pollut. Control 79, 225-232.

426

D . A . LOVETT and S. M. TRAVERS

Kitchener J. A. and Gochin R. J. (1981) The mechanism of dissolved air flotation for potable water: basic analysis and a proposal. Water Res. 15, 585 690. Lovett D. A., Travers S. M. and Maas R. L. (1984) Treatment of abattoir wastewater by dissolved air flotation. Part I. Wastewater not pretreated. Meat Research Report No. 9, CSIRO Meat Research Laboratory. Maddock J. (1979) Attainment of consistent effluent quality by the use of dissolved-air flotation. Technical Report TR 106, Water Research Centre. Roberts K. L., Wetter D. W. and Ball R. O. (1978) Dissolved air flotation performance. Proc. 33rd Indust. Waste ConiC, Purdue University.

Takahashi T., Miyahara T. and Mochizuki H. (1979) Fundamental study of dissolved air pressure flotation. J. chem. Engng Japan 12, 275-280. Travers S. M. and Lovett D. A. (1985) Pressure flotation of abattoir wastewaters using carbon dioxide. Water Res. 19, 1479-1482. Ward C. A., Balakrishnan A. and Hooper F. C. (1970) On the thermodynamics of nucleation in weak gas liquid solutions. J. Basic Engng 695-701. Wood R. F. and Dick R. L (1973) Factors influencing batch flotation tests. J. War. Pollut. Control Fed. 45, 304-315. Vrablik E. R. (1959) Fundamental principles of dissolved air flotation of industrial wastes. Proc. 14th lndust. Waste Conf, Purdue University.