Asbestos Fiber Removal During Effluent Wastewater Treatment. Pilot Plant Evaluation

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ASBESTOS FIBER REMOVAL DURING EFFLUENT WASTEWATER TREATMENT. PILOT PLANT EVALUATION MASSIMO OTTAVIANI, ACHILLE MARCONI* and PAOLA MAGNATTI

Laboratorio di Igiene del Territorio - Istituto Superiore di Sanita. Viale Regina EIena 299-130161 Rome, Italy

* Laboratorio di Igiene degli Ambienti Conjinati - Istituto Superiore di Sanita, Viale Regina EIena 299, 001 61 Rome, Italy

ABSTRACT A pilot plant study bs been undertaken to optimize coagulation and filtration pretreatment for asbestotioform fiber removal from industrial waste water. Experiments were conducted to compare the effectiveness of simple sand fiitration and magnesium oxide filtration. The first tests showed that magnesium oxide is an excellent medium to filter asbestos and operates longer than sand, flow rate being equal. Based on laboratory jar testes, the optimum pH range for flocculation-destabilization was defined. Asbestiform fiber reduction during effluent treatment, after flocculation-sedimentation and filtration, was also evaluated. The arificial asbestos suspensions were prepared using a well-defined water and following the same details already published elsewhere. For determinations of asbestos fibers in the liquid samples methods that involve filtering the liquids through a polycarbonate or cellulosic filter of 0.2 pm pore size were used. Accurate fiber counts were carried out using both scanning electron microscopy (SEM), and high magnification phase light microscopy.

1. INTRODUCTION

In Italy, regulations which protect waters from pollution, do not provide specifically for an asbestos contribution (as natural mineral fibre) to water pollution but only for settling and total suspended solids increasing (points 6 and 7 in Tables A and C, law No. 319 [I]. We are convinced that these limits do not hold in due consideration the potential ecological damage and the possible human health hazards associated with the presence of asbestos in effluents. In addition it appears that the use of parameters related to the

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concentration of suspended particles, as a surrogate measure of the asbestos fibre level, do not represent a guarantee for the fibre concentration [2]. Therefore, studies have been undertaken t o achieve, through water treatments, as low a level of asbestos as is possible in industrial effluents, too. Pollution nature and levels [2-41 vary very much according t o the different branches of asbestos industries. Before any other consideration yet, all asbestos industrial processes should apply the principle of full recycling of the processing water t o reduce the pollution risk t o a minimum. In the case of this not being possible out of technological reasons, technologies should be utilized to keep the pollution level below the level obtained through the only Sedimentation pre-treatment (fiber concentration: lo9 f/l) [5]. (Even if concentrations less than l o 5 f/l are recommended for drinking water in quality criteria published by EPA, to maintain the risk of gastrointestinal tumor below lo-' [6-81. The purpose of this work was to optimize mixing, various pretreatment (coagulation and sedimentation) and filtration processes, utilizing artificial asbestos suspensions containing asbestos levels of the order of 5.10" to 10" f/l. At first, tests were carried out through a batch process to obtain the optimum conditions which were then applied to the pilot plant. Also, the pilot plant effectiveness was evaluated for asbestos fiber removal.

2. MATERIALS AND METHODS 2.1. ASBESTOS FIBRE ANALYSIS

Artificial asbestos fibre suspensions were prepared by tap (drinking) water, previously analyzed by scanning electron microscopy (SEM) and showing a fibre content, after prefiltration on 0.2 pm pore size cellulosic filter, of about 0.05 million fibres per litre (MFL) [9]. The chemical characteristics of this water are reported inTable 1. Table 1. Raw Water Quality Characteristics. Alcalinity Hardness Permanent Hardness Temporary Hardness PH Temperature

310 mg/l CaCO, 33°F 8"F 25°F 7.30 15-18°C

The asbestos fibre reference material used in this study was chrysotile (long range) supplied by the NIEHS [ 101. 50 mg of this powder was suspended in 5 liters of the prefiltered water and treated by ultrasound probe for 30 minutes at a power of 0.015 in order to obtain a more uniform dispersion and t o separate as much as possible the fibres bundles. This suspension was the starting solution used in all experiments. The analyses

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have been carried out by using scanning electron microscopy (SEM) and phase contrast light microscopy (PCLM) at magnification of 1250X by an oil immersion objective. Both the analytical techniques have been used in order t o evaluate the suitability of a less expensive and time consuming method like PCLM when compared to the more expensive and time consuming electron microscopic method. For PCLM an opportunely diluted sample of the starting suspension (few milliliters, depending on the fibres density on the filter) was filtered onto a cellulosic filter of 0.2 pm pore size, 25 mm diameter, by means of a millipore filter funnel. The filter obtained was the mounted on a microscope slide by the acetone vapourtriacetine technique [ 11J and then analyzed by a Leitz Ortholux phase contrast microscope equipped with an oil immersion objective 100 X (N.A. 1.25) at 1250 X magnification. For SEM examination the same amount of the starting suspension was filtered on a polycarbonate filter, 0.2 pm pore size and 25 nim diameter, placed on a backing cellulosic filter of 1.2 pm pore size, in order to facilitate a more uniform distribution of particles on the filter. A small portion of the particles-laden filter was then mounted on SEM stubs by double sided sticky tape and then coated by gold of about 300 nm thick in a sputter coater [ 111. The same preparation procedure was followed for all liquid samples collected at the different stages of the flocculation and filtration tests. The counting criteria adopted during both the PCLM and SEM analyses were: all particles having an aspect ratio (length: diameter) more than 3 :1 were counted and sized by means of an eyepiece graticule [ 121 for light microscopy and directly, by means of marks on the screen, for SEM. In order to obtain comparable areas examined in both the instruments, 200 fields have been counted in the case of PCLM at 1250 X, corresponding to an area of about 0.62 mm’, and 250 fields for SEM at 5000 X, corresponding to an area of about 0.12 mm2. The resolving power of both the instruments have been tested for the light microscope by especially prepared phase contrast test slides and for SEM directly on the screen: it has been found t o be about 0.1 p m for both the instruments, therefore all fibres showing a diameter 0.1 pm or more have been counted, considering as half those fibres crossing the side of the SEM screen or the edges of the graticule and laying with one end inside them. During the analyses background counts were carried out on a certain number of blank filters of each batch used and, when they exceeded 10 percent of the sample counts, the number of blank fibre count was subtracted from the total number of fibre counts found for the samples. The minimum detectable concentration of asbestos fibres varies with the volume of the liquid passed through the filter, which depends on the amount of the background particles present in the liquid. For the experimental conditions of this study the detection limit (SEM counts) typically ranged from 0.03 MFL t o 0.08 MFL.

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3 . JAR TESTS AND PILOT PLANT

The optimum pH range for destabilization of asbestos fibers (mainly chrysotile) was defined during coagulation tests. A number of experiments in the literature confirmed that the optimum range for aluminum sulfate concentrations up t o 100 mg/l, is 6.3-6.8 pH units [13]. The pH was obtained adding calcium hydroxide, its concentration being a function o f t h e a h m amount and of the starting water alkalinity (Table 1). Afterwards, maintaining t h s pH range for all the samples, asbestos fibre reduction was evaluated as a function of the alum amount, through jar tests (sedimentation time, after mixing-flocculation: 120 min). The data obtained are reported in figure 1. Tests were then carried out to evaluate the filter effectiveness. The clarified water, after coagulation-sedimentation pre-treatment, fed the filters. Flow was adjusted to 6.8 mh-' (14.4 l/h). The mean results obtained are reported in Figures 2 and 3 , where details of the filters are given, t o (2able 2 ) .

f/l

1

6 -

6 4-

2-

1

I

10

I

20

I

30

I

40

Iig. 1 . Residual fibre concentration as a function of alum dosage.

I

50

6'0 "91,

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2 x 1 0 6 -Suspension

A sbestgs

Pos ts,ed ime n t a t i o n

, F l o w 6,Exh'-' XIO! 6

4 2

1

30

I 60

1

120

1 90

min,

Fi. 2. Sand Filter. Residual fibre concentration.

Flow

I

6,E

mx h-'

1 I

30

I

60

9'0

Fig. 3. MgO Filter. Residual fibre concentration.

I20

mi n

340 Tab. 2. Operating Filter Features. Sand Filter column, cm 5.2

9 .o

MgO Filter column, cm 5.2

9 .o

@J

r#~

depth, cm

effective size, mm 0.500 (UNI No. 20)

depth, cm

effective size, mm 0.500 size 0.710

(UNI No. 20)

(UNI No. 1 7 ) ~

~~

The parameter's values obtained were applied t o the pilot plant schematized in figure 4. The artificial asbestos suspension (51 at a time) was steadily decanted into a well agitated 10 1 tank. The suspension flowed then by gravity t o a 8 1 flocculation tank at a rate of 270 ml min-' . Here the additives were added so as t o maintain an alum dosage of 20 mg I-' and 6.5 pH units. In this tank the suspension was stirred at 25 rpm for approximatively 30 min. From the flocculation tank, the water flowed by gravity to a 35 1 sedimentation tank providing a retention time of 120 min. The clarified water was collected in a tank. From this tank water was pumped through two filters with a centrifugal pump. This operation kept constant either the liquid'seal or the flow in the filters. Flow was adjusted t o 240 ml min-' . The pilot plant was set up and monitored.

Alum Feed

Sygtem

Sedimentatf6n

Fig. 4. Pilot water treatment plant.

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When it had .reached a steady state condition, sampling was started. The sampling locations were as follows: 1 - prior to the water entering the flocculation tank to be fed; 2 - at the end of the launders in the sedimentation tank; 3 - at the outflow from the filters. The mean results of the tests are reported in tables.

4. RESULTS AND DISCUSSION

The analyses carried out by SEM and PCLM, side by side on the same liquid samples from jar tests experiments, showed a very good agreement between the two instruments. In figure 1 are plotted the asbestos fibre concentrations obtained by SEM and PCLM for a series of jar tests. Through the positive results obtained, all the subsequent analyses were performed using SEM method and PCLM was used only in a smaller number of samples in order to obtain further terms of comparison. In all cases the two techniques gave results in good agreement, but it seems to us that, for a definitive assessment, further studies on a larger number or samples are needed. The results reported in Table 3 are intentionally generalized. Tab. 3. Asbestos Removal Pilot Plant Data* (fibres per liter). feed water 3-4 109

postsediment. 1.9-2,0 lo6

* Values from several determination.

sand filter 0.5-0.6 lo6

MgO filter 0.2-0.3 lo6 ~~

In fact they represent only those data that gave a significant asbestos fiber difference between raw feed water and post-sedimentation water. In some data this difference was not significant. It could be attributed to two concomitant factors: 1 - the continuous change of asbestos fiber concentration in feed water and so there was no comparison term; 2 - the large variability of the analytical methods used for counting fibres. The change of asbestos fibre concentration in feed water appears to be caused by the heterogeneity of suspended particles, this heterogeneity being due to chemical-physical phenomena and agglomerations attributable to the bacterial charge (Figs.4,5 and 6 ) . However, a general examination of the data showing a significant reduction after sedimentation indicates that this effluent treatment is effective in the case of waters containing approximatively 10" to 10" f/l. To obtain these concentrations it is necessary to carry out a sedimentation pretreatment [4].

Fig. 5 . Pilot Plant evaluation. I.

Fig. 6. Pilot plant evaluation. 11.

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Filtration significantly reduced the amount of asbestos in water. Sand filter produced an effluent containing asbestos levels of the order of 0.6 lo6 f/l magnesium oxide filter produced an effluent containing asbestos levels of the order of 0.3 lo6 f/l. Magnesium oxide filter resulted more effective in reducing asbestos fibre concentration. However, further optimization studies should t o be undertaken (for example filter bed granulometry, duration of the filter, liquid flow, mechanical and electrical combination in removing asbestos fibres) t o justify also economically, the utilization of magnesium oxide filter instead of sand filter. It is interesting t o point out that for the tests carried out during this study, the asbestos fibre reduction factor was the same for all pilot plant concentrations of feed water. That is to say, the change of fibre concentration in feed water produced a corresponding change of fiber concentration in post-sedimentation water. This, however, does not appear to be enough t o assume any direct relationship between the initial and final asbestos fiber concentrations across the process.

REFERENCES 1. Gazzetta Ufficiale della Repubblica Italiana, 141, 29-5-1976. 2. M. J. McGuire, A. E. Bowers and D. A. Bowers, Journ. AWWA, (1983) 364-370. 3. M. Sitting, Pollution control in the asbestos, cement, glass and allied mineral industries, NDC, London, 1975. E. C. Report EUR, Report G. 953, November 1980. 4. R. W. Lanting and J . den Boeft, Environmental Pollution by the Asbestos Industries, Problems and Proposals for Solutions, 5. J. Lawrence and H. W. Zimmerman, Journ. WPCF, (1977) 156-160. 6. Federal Register, 44 (191) 56628, 1979. 7. P. C. Elmes and M. Simpson, Br. J. Ind. Med., 34 (1977) 174-180. 8. I. J. Selikoff, E. C. Hammond and H. Seidman, Ann. N.Y. Acad. Sci., 330 (1979) 91-116. 9. A. Marconi, G . Cecchetti and M. Barbieri, Proc. 5th International Colloqium on Dust Measuring Technique and Strategy, Johannesburgh, October 29-31, 1984, Asbestos International Association (in press). 10. W. J. Campbell, W. C. Huggins and A. G. Wylie, Bureau of Mines Report of Investigation/1980, RI 8452,1980. 1 1 . AIA, Health and Safety Pubblication, Recommended Technical Method No. 1 (RTMI), January 1982. 12. W. H. Walton and S. T. Beckett, Ann. Occup. Hyg., 20 (1977) 19-23. 13. M. Falleni and M. Ottaviani, Inquinamento, 24 (10) (1982) 123-128.