NOTES
Chrysotile Asbestos Fiber Removal during Potable Water Treatment. Pilot Plant Studies Ronald 6. Hunsinger” and Kenneth J. Roberts Ontario Ministry of the Environment, 135 St. Clair Avenue West, Toronto, Ontario, Canada M4V 1P5
John Lawrence Water Chemistry Section, National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario, Canada L7R 4A6
w Pilot plant studies incorporating conventional potable water pretreatment (coagulation, flocculation, sedimentation) and filtration to assess chrysotile asbestiform fiber reduction are reported. Some problems were encountered in consistent fortification of the pilot plant feed water with sufficient asbestos t o make removal measurement possible. A subjective analysis of the pilot plant runs indicated t h a t optimized conventional water treatment is effective in reducing asbestiform fiber concentrations. Difficulties experienced in analyt ical precision led to the recommendation that further pilot work be curtailed until analytical methods can measure significant reductions in asbestos across treatment processes a t ambient levels. No direct relationship could be found between asbestos concentration and turbidity.
Ministry of the Environment, Ontario (MOE), and the Canada Centre for Inland Waters, Department of Fisheries and Environment, Canada (CCIW), between 1976 and 1977. T h e object was to evaluate various pretreatment (coagulation, flocculation, and sedimentation) and filtration processes for chrysotile asbestos fiber removal from surface water a t a pilot scale treatment facility equipped with full process control. Also, it was planned to reexamine for any possible turbidity-asbestos concentration correlation.
Canada has traditionally been blessed with an abundance of high quality water from which to draw its municipal supplies. Howecer, in 1971 Cunningham and Pontefract ( I ) identified chrysotile asbestos fibers in t h e drinking water of several major cities in Eastern Canada and these occurrences have since been confirmed a t other locations ( 2 , 3 ) .In fact, the presence of chrysotile asbestos in Canadian waters now appears ubiquitous with values ranging from about lo5-10’ fibers L-* [higher values of u p t o lo9 fibers L-’ have been reported in the vicinity of asbestos mining communities ( 4 ) ] .Chrysotile is t h e most comrnon asbestos species found in Canadian natural waters and in most cases is the only identified fiber present in numbers above the detection limit. T h e health significance of continuous, low level ingestion of chrysotile asbestos has not been determined, b u t in the absence of established risk information it would seem reasonable that water treatment engineers aim to achieve as low a level as is possible in potable water. Lawrence et al. (4,5) have reported results of bench scale studies for the removal of chrysotile fibers from water. These authors showed that asbestos fibers reacted t o conventional potable water pretreatment processes (coagulation, flocculation, and Sedimentation) and filtration in a manner similar t o other particulate matter. However, their experiments also pointed out that the usual parameter of effectiveness of t h e coagulation and filtration processes, namely turbidity, did not relate with any reliability to t h e asbestos particle content of the finished water a t concentrations below 5 X 1O1O fibers L-I. Consequently, detailed asbestos analyses have to be carried out on all samples to determine the fiber concentrations. Pilot studies have also been reported by the US.Environmental Protection Agency (6) for the removal of amphibole fibers from Lake Superior (amphibole fibers have a negative surface charge, whereas chrysotile fibers are positively charged). This project was undertaken as a joint venture between the 0013-936X/80/0914-0333$01 .OO/O @ 1980 American Chemical Society
Asbestos Analysis At the time that this study was undertaken, there were three common analytical methods available for determination of asbestos in water ( 7 ) . All used the transmission electron microscope ( T E M ) for the actual counting of the fibers, but the sample preparation techniques differed significantly. T h e three methods are designated below as A, B, and C. Method A involves ultracentrifuging t h e sample, ultrasonically resuspending t h e residue in 1 mL of water, and placing a l - p L drop of this suspension on a 3-mm carbon-coated electron microscope grid. In method B, t h e sample is vacuum filtered through a membrane filter of O.l-pm nominal pore size, t h e membrane is ashed in an oxygen plasma a t 100 “C, and the ash is dispersed ultrasonically as in method A. Method C involved filtering the sample through a membrane filter, carbon coating t h e filter, placing a small portion of t h e filter directly on a T E M grid, and dissolving away the filter medium using a Jaffewick assembly. T h e carbon-coated particles were then supported directly on the T E M grid. Method A was used throughout this study because that was the method being used by the electron microscope laboratory at CCIW a t that time. T h e dried grids were examined under a “Siemens 101” transmission electron microscope (X20 000 magnification). When t h e concentration of fibers was low ( l o 5 - l O 6 L-l), it was necessary to count all of the approximately 100 grid openings covered by the l - p L drop. At higher concentrations, it was sufficient to count only 20-50 random squares a n d assume t h a t these squares were representative of the entire drop. Background counts were carried out on each grid before use, and this number was subtracted from the total number of fibers counted for the sample. T h e limit of detection was 5 X lo4 fibers L-l. With the complex sample preparation procedures and t h e visual difficulty of counting t h e fibers under t h e electron microscope, it was felt t h a t all the analytical results obtained during the current project contained an overall uncertainty of a t least a factor of 2 (i.e., a sample reported as 1 X lo6 fibers L-’ could have values ranging from 5 X lo5 t o 2 X lo6 fibers L-I). T h e precision of T E M analysis of chrysotile fibers has recently been discussed in detail by Hallenbeck e t al. (8). Volume 14, Number 3, March 1980
333
7
dSQEST0S rEEC SYSTEM
..,,...... ..... ,.... ....... .........
9 ,
3UCL MEDIA FILTER VO
h
>LCRIhE CEED SYSTEM
I
U
d
4LUM FEED SYSTEM
DOlELECTROLITE FEED SYSTEM
1-4
I
....... ....... .......
....... ....... .......
CUAL ME)#,. NO 2
FILTER
S I N 0 F LTER
Figure 1. Pilot scale water treatment plant
Since this work was completed, a Ministerial Committee on Asbestos Analysis has recommended that method C be adopted as the “interim standard” method (9).
Pilot Plant Facility T h e pilot plant was set u p in the Ministry of the Environment Research Test Facility in Toronto. This facility uses the Humber River as its raw water source. Unfortunately, initial analysis of the Humber River water indicated asbestos levels only of the order of 5 X lo5 fibers L-l. Hence, it was necessary t o spike the river water to approach those higher asbestos levels indicated in the earlier surveys ( 2 , 3 ) . Figure 1 shows the schematic of the treatment processes employed. Raw water was pumped a t a rate of approximately 0.8 L s x l (10 imperial gallons per minute (Igpm)) from the Humber River into a 980-L (200 Ig) supply tank with the excess flowing to waste. The water was then pumped a t a rate of 0.4 L s-l ( 5 Igpm) through a rotameter to a 3814-L (840 Ig) asbestos contact chamber, giving a retention time of 2.8 h. A long shaft mixer kept the contents of the chamber well agitated a t all times. A metering pump added the asbestos suspension to the raw water in this chamber. From the asbestos contact chamber, the raw water containing asbestos flowed by gravity to a flocculation chamber. A second chemical metering pump added sodium hypochlorite solution for prechlorination of the raw water-asbestos mixture as it left the contact chamber. A primary coagulant (alum) was added to the chlorinated raw water-asbestos suspension as it entered the rapid mix compartment of the flocculation tank. Two mixers gave a n initial rapid mix, Leaving the rapid mix chamber, t h e water flowed under a baffle, a t which point a multiported tube was installed and a coagulant aid (activated silica) was added with a metering pump. The water flowed over a weir into the second slow mix chamber and then under another baffle to the outer chamber. Both slow mix chambers were equipped with large propeller mixers operating a t approximately 50 rpm, giving in each chamber. a G value of 20 From the flocculation tank, the water flowed by gravity to a 4086-L (900 Ig) sedimentation tank providing a retention 334
Environmental Science & Technology
time of 180 min. T h e water entered t h e sedimentation tank a t mid-depth and was taken off via launders near the surface a t the other end of the tank. On leaving the sedimentation tank, the water entered another header system which fed three filter columns located below the flocculation-sedimentation equipment. T h e filters consisted of 14-cm i.d. acrylic columns, 2.24 m in height. Details of the media are given in Table I. Water was pumped through the filters with centrifugal pumps, and the flow rate was controlled manually with throttling valves and monitored on rotameters. Flow was adjusted to 8.9 m h-’ ( 3 Igpm/ft2) for each filter. The dosage and selection of primary coagulant and coagulant aid were determined by using a standard jar test procedure prior t o each run. Coagulant aids were found to be necessary for good floc formation, especially during the winter period when low water temperatures were encountered. Generally, the dosage required for the Humber River raw water averaged 30 mg L-I alum plus 5 mg L-’activated silica. The initial dosage of chlorine was determined with a chlorine demand test, and the hypochlorite dosage was adjusted, as necessary, to maintain a free chlorine residual of 0.3 mg L-’ throughout the system. Asbestos spiking was achieved by metering in a 50 mg L-I suspension of chrysotile asbestos, containing approximately 4 X 1 O I 2 fibers L-’. Details of the preparation of this suspension and the size distribution have already been published elsewhere ( 4 ) . T h e size distribution of t h e fibers from t h e suspension after dilution with the feed water was very similar to t h a t of natural samples obtained from various locations in Canada. T h e asbestos concentrate reservoir was continuously agitated ultrasonically, and t h e feed lines were kept very short (15 cm) to minimize settling of the suspension. After the first series of runs, it became evident that the fortified water frequently did not contain the expected concentration of fibers. Consequently, it was necessary to investigate the problem of diluting concentrated suspensions in some detail, and the results have been documented in a separate report ( I O ) . Other researchers have reported similar difficulties when working with asbestos suspensions ( 7 ) . T h e sampling locations were as follows: (1)Humber River water, taken from 908-L 1200 Ig) supply tank prior to the water entering the asbestos contact chamber; (2) feed water, taken a t the inflow to the flocculation chamber prior to coagulant addition and after asbestos and sodium hypochlorite have been added (at this point, the asbestos had been contacted with the Humber River water for -3 h); (3) postsedimentation water, taken from the collector box a t the end of the launders in the sedimentation tank, just before the water enters the header for the filters; (4) dual media effluent 1 (DM-l), the effluent of the dual media filter no. 1 containing 0.45-mm effective size (es.) sand and 0.9-mm e s . anthracite; (5) dual media effluent 2 (DM-2), the effluent of the dual media filter no. 2 containing 0.45-mm e s . sand and 1.2-mm e.s. anthracite; (6) sand filter effluent, the effluent of the single media column containing 0.70-mm e.s. sand. T h e pilot plant was set up and monitored hourly over a t least one retention period of the complete process. A t the end of this period, intensive half-hourly monitoring was begun and when the turbidity of the effluent had reached a steady state, sampling was started. New, 250-mL “Nalgene” bottles were rinsed 4 times in the water to be sampled and 1/4 portions taken a t 0.5 h intervals until t h e bottles were filled. T h e Humber River water and feed water were also sampled on a %-bottle portion method, taking into account the respective retention times of the system. In this way, after reaching a steady-state condition, approximately the same “block” of water was sampled as it passed through the treatment system. If any problems were encountered during the
.. - -~.
Table I. Filter Media in the Experimental Asbestos Removal Pilot Plant sand filter no.
dual media
depth, cm (in.)
anthracite
effective size, mm
uniformity coeff
depth, cm (in.)
effective size, mm
20.3 (8)
0.45
1.5
40.6 (16)
0.9
uniformity coeff
1.7
20.3 (8)
0.45
1.5
40.6 (16)
1.2
1.4
45.7 (18)
0.70
1.5
no. 1
dual media no. 2
sand
Table II. Asbestos'Removal Pilot Plant Data run no.
alum a
act. Si a
2
30
5
3
24
3
4
36
6
12
30
13
30
5
14
30
5
15
30
5
*
5
a Alum and activated silica dosage in mg L-'. units, Ftu).
feed water
4.02 (16) 0.81 (25) 7.8 (20) 17 (22) 2.3 (8) 3.4 (20) 6.9 (28)
postsed.
1.5 (4.1) (5) 1.4 (5.1) 17.0 (4) 5.4 (3.0) 0.8 (4.8) 1.4 (5.3)
Asbestos results in millions of fibers per liter.
run (e.g., with t h e asbestos feeding system or turbidity breakthrough in one of the filters), the sampling was abandoned, the unit process corrected, and, one retention time later, the sampling resumed.
Ke.sult,s a n d Discussion T h e results of t h e asbestos counts and turbidity measurements for the pilot plant runs are presented in Table 11. Numerous other runs were carried through to completion only to find t h a t the asbestos lecels in the feed water were not sufficient to justify counting of postsedimentation and postfiltration samples. Examination of Table I1 leads to certain general observations. However, these observations must be approached with caution for many reasons; these include: the present state-of-the-art in asbestos counting technology, the experimental pyoblems encountered in the feeding and retrieval of the asbestos, and also the relatively small amount of data obtained. In three of the six postsedimentation samples reported in Table 11, there was a significant reduction in the asbestos fiber counts (with corresponding reduction in turbidity levels) from those levels encountered in the feed water. This is in keeping with the earlier bench scale studies in which conventional potable water pretreatment processes did, in fact, produce substantial reductions in chrysotile concentrations ( 4 , 5 ) Similar observal ions have been made regarding the response of other asbestos species to conventional potable water treatment proctlsses in work carried out by t h e US.Environmental Protection Agency (6).However, the fact that three of t h e six postsedimentation samples reported exhibited asbestos counts t h a t were not significantly different (keeping in mind the unmrtainty of a factor of 2 in the analyses) from those encountered in the feed water makes any generalization difficult. T h e in herent inaccuracies in the analytical methodology also made it impossible to determine the effect of changes in coagulant and coagulant aid on the concentration of fibers in the postsedimentation samples. Filtration, in the majority of cases, significantly reduced the amount of asbestos in the water from t h a t which was encountered in the postsedimentation samples. In the results
filters DM-2
DM-1
0.31 (0.7) 0.23 (0.3) 0.07 (0.15) 0.14 (0.14) 1.6 (0.1 1)
