Water Res. Vol. 19, No. 9, pp. !.19%t203, 1985 0043-1354 85 53.00+0.00 Printed in Great Britain. All rights reserved Copyright C 1985 Pergamon Press Ltd

TECHNICAL NOTE

DISSOLVED ORGANIC CARBON OF COAL SLURRY TRANSPORT WATER

M. C. REID I, J. W. DAVIS t, G. S. SAYLER I'* and R. A. MINEAR 2 ~Department of Microbiology and The Graduate Program in Ecology and

:Department of Civil Engineering, Environmental Engineering Program, The University of Tennessee, Knoxville, TN 37996, U.S.A.

( Receiced Jamzar y 1985)

Abstract--The qualitative and quantitative occurrence and fate of dissolved organic carbon (DOC) leaching into coal slurry transport water was examined in laboratory-generated coal slurries and wastewaters from the Black Mesa coal slurry pipeline. Laboratory slurries were formulated for both western coals (Wyodak, Montana Rosebud and Black Mesa) and eastern coals (Illinois No. 6 and Pittsburgh No. 8). Sephadex G-25 elution profiles and ultrafiltration studies indicate that the majority of the organic compounds in western co,'tl slurry wastewaters were lower (less than 1000) molecular weight species (62.°~, for Wyodak and 74"~o for Montana Rosebud). Biochemical Oxygen Demand (BOD) for these particular wastewaters ranged from 50 to 150mgl -~ as determined through the use of an electrolytic respirometer. Also, there was a concomitant 51-74 °/reduction in the DOC levels in the wastewaters. This removal was primarily due to the removal of the lower ( < 1000) molecular weight compounds by the seed inoculum. There was no evidence for the presence of mutagenic organics in the raw wastewater.

Key words--wastewater, organics, biotreatability, mutagenicity

INTRODUCTION

Coal slurry pipelines have recently received in- creasing attention as a potentially efficient and cost-effective means of transporting coal over large distances (Jackson, 1978; Anonymous, 1976). At present, the only operative pipeline is the Black Mesa pipeline which transports coal from northeastern Arizona to southern Nevada. The Black Mesa pipe- line is 440 km in length and has been transporting approx. 5 million metric tons of coal per year since 1970 (Carleton and Cheng, 1977). Furthermore, a minimum of eight additional coal slurry pipelines spanning approx. 7500 km have been proposed in the United States or are in the pre-planning stages (Slurry Transport Association Newsletter, February, 1983).

The potential environmental risks associated with coal slurry transport have received only modest at- tention (Peavy, 1981). A primary environmental con- cern yet to be fully addressed is the quality and fate of wastewater resulting from slurry transport. Using anticipated data of slurry transport of 175 million tons of coal per annum via pipelines; approx. 120 million gallons of water per day will be required .. (Slurry Transport Association Newsletter, February, 1983). Previous studies indicate that during trans- port, slurry water may undergo significant deterio- ration in quality and reuse potential (Godwin and

*To whom correspondence should be addressed.

Manahan, 1979; Moore, 1981). These studies focused

primarily on metal ion contaminants and sulfates in the transport wastewater. Little attention has been given to the organic contaminants remaining in the wastewater. Godwin and Manahan (1979) noted that chemical oxygen demand (COD) levels reached con- centrations as high as 2000mgl -~ in lignite-based slurry wastewaters, although no information was available concerning the types of organic con- taminants present in the wastewater. However, it is expected that ferrous ion may contribute to this COD. Concern must be given to the disposal of coal slurry wastewaters containing high levels of organic contaminants which may represent an unacceptable environmental burden and a potential threat to human health.

The objective of this study was a preliminary qualitative and quantitative assessment of the classes of organic compounds present and their potential for removal by biological mechanisms. Environmental health risks associated with the potential release of chemical mutagens in the wastewater were also examined.

SLURRY FORMULATION AND WASTEWATER GENERATION

Illinois No. 6 (II1. No. 6), Pittsburgh No. 8 (Pitt. No. 8), Montana Rosebud (MRB) and Wyodak coal samples were obtained from Oak Ridge National Laboratory, Oak Ridge, TN (courtesy of Dr Ed

1199

1200 Technical Note

Davis). All four coal samples were mined in 1978 and Table I. Variability of dissohed organic carbon values present in obtained the same year by ORNL. Both MRB and ~,estern coal slurry waste'.,.aters Wyodak are samples of surface mined low-sulfur D O C l m g l - ' )

No. or" - sub-bituminous coals. Pitt. No. 8 is an anthracite coal Coal type samples Mean = SD Range

while I11. No. 6 is bituminous in rank. Total sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . W,,odak 30 153.2 -__ 40.8 93-298

concentrations of Pitt. No. 8 and Ill. No. 6 were determined to be 4.4 and 2.8°,~ respectively (Dr Ed Davis, personal communication). All samples were obtained as lump coal and were stored at ORNL in sealed 55 gallon drums and were 2 years old at the time of these studies. Additional Montana Rosebud samples were obtained from Peabody Coal Co., St Louis, MO (courtesy of Dr John Arnold). Black Mesa coal used in the Black Mesa Coal Slurry Pipeline was obtained from Black Mesa Pipeline Co., Flagstaff, AZ (courtesy of Mr Jim Shelley). Black Mesa coal slurry samples were obtained from the Southern California Edison, Mohave Generating Station, Laughlin, NV (courtesy of Mr Ken Smith). MRB samples obtained from Peabody Coal Company were freshly mined (2 weeks old) and delivered as lump coal which was pulverized and ground to specification in the laboratory. Black Mesa coal samples were obtained as pulverized samples from the coal storage stockpile taken immediately before entering into the rod mill/slurry mixer of the slurry processing unit at the head of the Black Mesa pipe- line.

Simulated coal slurries were prepared as a 50:50 mixture (w/w) of surface dry coal (40 mesh) with distilled water. Slurries were prepared in various volumes up to 121. in glass vessels equipped with Teflon-sealed screwcaps. When appreciable head- space existed the atmosphere was replaced with nitro- gen gas. The prepared slurries were mixed for various time intervals up to 14 days to simulate transport. Coal water separation was achieved by centrifugation at 10,000 rpm. The centrate (supernatant transport water following centrifugation) was subjected to filtration through Gelman-Spectra Type A-E glass fiber filters (Gelman Sciences Inc., Ann Arbor, MI) and 0.4.urn (Nucleopore Corp., Pleasanton, CA) filtration membranes to remove coal fines (micro- scopic coal particles). The residual transport water was then stored at 5:C when necessary prior to further analysis.

WASTEWATER DISSOLVED ORGANIC CARBON

The dissolved organic carbon (DOC) of slurry wastewater was determined by direct combustion, Infrared CO,. analysis using either an Oceanography International 064B carbon analysis system (Ocean- ography International, College Station, TX)..or a Beckman Model 915 Total Organic Carbon analyzer (Beckman Int., Fullerton, CA).

In general, wastewater DOC reaches maximum concentrations in 1-3 days simulated transport and decreases slightly on prolonged transport (Sayler et al., 1983). Mean DOC values for the five coal types

Montana Rosebud 15 175.8 - 110.9 25-324 Black Mesa 16 99._" ~ 66.5 15-220 Illinois No. 6 3 38 Z 5 29-4.1 Pittsburgh No. 8 3 30 z 6 15-38

All values reported as mg I -~ of dissolved organic carbon. *Tv, o separate shipments of Montana Rosebud and Black Mesa coal

were used in the generation of these values.

examined are given in Table 1. Wastewaters from western coals exhibited mean DOC concentration 3-6-fold higher than the eastern coal examined. Max- imum DOC concentration approaching or exceeding 3 0 0 m g C l -~ were obtained for Wyodak and MRB slurry wastewaters.

Preliminary data (Sayler et al., 1983) indicate that less than 5'~ of the DOC was non polar organic matter extractable by organic solvents. Studies were then undertaken to examine the molecular weight ratios of the DOC. Molecular weight characterization of wastewater DOC was performed by gel perme- ation chromatography of raw or concentrated (10: 1, roto-evaporation) wastewater using Sephadex G-25 gel (Pharmacia, Uppsala, Sweden) or by ultrafiltra- tion using AMICON (Amicon Corp., Lexington, MA) YM5 (mol. wt cutoff, 5000 MN) or YMZ (1000 mol. wt cutoff) ultrafiltration membranes. Sample ultraviolet absorbance (254nm) measure- ments were performed on a Perkin-Elmer Model 200 spectrophotometer (Perkin-Elmer Corp., Norwalk, CT). Fluorescence was measured with a Turner Model III fluorometer (Sequoia-Turner Corp., Mountain View, CA). A 7-54 filter with a maximum of 34 nm was used. Maximum transmission at max was 0.79 T and the band width was 140 nm.

Typical Sephadex gel elution profiles are presented in Fig. I. The elution profiles of both Wyodak and Montana Rosebud slurries demonstrate that the ma- jority of organic constituents fractionated are lower molecular sized compounds. The elution pattern of Black Mesa pipeline DOC indicates a broader distri- bution of organic constituents present. The peak eluting beyond the inclusion volume (Vt) in the Black Mesa profile is indicative of gel matrix-solute inter- actions.

Ultrafiltration results are presented in Fig. 2. A mass balance of the organic materials present in the three size classes was performed by total organic carbon analysis. These combined data demonstrate that for both Wyodak and MRB laboratory prepared coal slurries, the majority of the organic con- taminants in the resulting wastewater are comprised of compounds with molecular weights of 1000 or less. For Wyodak samples, 62,°/o of the DOC present in the slurry wastewater was comprised of compounds of molecular weights of 1000 or less. Montana Rosebud

Technical Note 1201

,.o ,., .--. oo~ j I \",I ] "" g / : " " " " ,/ \1 / -

00" 0 " - - " I I 1'5 ~ Montana RONb*~d a Z 20

N I 0 30O ~ . 8 ..e. > uJ

O0 "0 ~ Ln - -~ .

2C BlOCk MeSa II 600 ~"

0.0 - - . 60 V 0 100 120 Vl IBO ELUTION VOLUME (ml}

Fig. I. G-25 sephadex fractionation of the dissolved organic carbon compounds present in coal slurry wastewaters.

a--Lab generated slurry; b---pipeline generated slurry.

samples were characterized as having 74~o of the total DOC in the < 1000 mol. wt category. Both Wyodak and MRB demonstrated approximately equivalent amounts of organic material which fractionate in the >1000 but <5000 cutoff range. In this molecular weight category, approx. 28% of the DOC present in Wyodak samples could be classified as intermediate in molecular weight, whereas for the MRB samples the value was approx. 26~. Organic constituents with molecular weights >5000 were not found in the MRB wastewater whereas this same size class ac- counted for 9% (on a carbon mass balance basis) of the total organic load in the Wyodak wastewater. Organic compounds present in the Black Mesa sam- ple exhibited a more even distribution across the three size classes.

BOD A N D D O C R E M O V A L

Biochemical Oxygen Demand (BOD) for the slurry wastewater was determined using the E/BOD Respirometer system (Oceanography International, College Station, TX) (Young and Baumann, 1976a,b). The coal slurry wastewater (CSW) was supplemented with phosphate buffer (1.0 ml 1 -~) and then adjusted to pH 7.0-7.2 using I N NaOH. After the adjusted slurry wastewater was equilibrated to incubation temperature (25°C), duplicate samples., were inoculated with a seed culture consisting of microorganisms originating from activated sludge collected from a wastewater treatment plant.

Theoretical BOD curves were constructed from the estimated velocity constant (k) and ultimate BOD (L) contained in the equation

Y= L(I - 10-k'). (1)

The Thomas graphical method (Thomas, 1950) was employed to estimate both k and L where k was the

0 velocity or rate constant for the reaction and L the ultimate BOD. DOC removal from the wastewater was monitored during the respirometer studies.

The BOD of the Wyodak wastewater samples designated 3, 4 and 5 are described in Fig. 3. The experimental curves as well as the theoretical curves predicted by equation (1) are depicted in this figure. The plateau achieved by each curve varies from approx. 50 to 150 mg 1 -~. Two reasons for this dis- crepancy are the differences in total amount of or- ganic carbon removed and total amount of carbon dioxide (CO_,) respired. The oxygen consumed by the seed inoculum is directly related to the total amount of organic carbon oxidized (Larson and Perry, 1981). For sample 3 approx. 58 mg of organic carbon were removed from the slurry wastewater as compared to 44 mg of organic carbon removed for sample 5. In addition, the majority or dissolved oragnic carbon removed was either accumulated into biomass or mineralized completely to CO,. The ratio of CO, to biomass varied from experiment to experiment and this affected the oxygen uptake of the seed inoculum; thus affecting the ultimate height of the BOD curve (Fig. 3). BOD rate constants for oxygen demand (K~) ranged from 0.09 to 0.18day -~ for five Wyodak samples examined.

An analysis was conducted, using samples desig- nated 1 and 5, to relate the molecular weight fraction- ation to biological removal of wastewater DOC.

I 00 f Wy°doka

'°t Z~ Montana Rosebud o I00 IJJ U n." UJ 60 n

IJJ

-> zo I ) - <~ J LtJ E I00 Block "'mesa h

6O

2O

> 5 , 0 0 0 >1 ,000 < 5 , 0 0 0

/ I OOC 160 mg t - I I

< 1 , 0 0 0

MOLECULAR WEIGHT

Fig. 2. Comparative wastewater DOC molecular weight distributions among western coal slurries (a--lab simulated;

b--pipeline generated; c--concentrated 10: I).

1202 Technical Note

225

2O0

175

150

7 - - 1 2 5 3E

o 1 0 0 0 n n

7 5

5 0

25

0

B . . . . - . . . . . . . . . . . . . . . . . . . . .

¢ . . . . . . : . : # . - . . . . . . . "_. ,o . . : : : ; o _ o - O

fro ,..e / . .j.oO

_ - - - . - - . . . . '~ clp,~ .~~'~-e-°

d P"

TIMF IN DAYS

Fig. 3. Biochemical oxygen demand of Wyodak coal slurry wastewater. [] Observed BOD curves; • the theoretical BOD curves predicted by equation. A--Sample 3 DOC 197 mg l-t: B--Sample 4; C--Sample 5, DOC, 142 mg I-L

Initial levels of DOC present in the reactor vessels varied from 57 to 149 mg of carbon but the residual levels after biological treatment averaged 33 + 6 mg of carbon for the five Wyodak samples (although between 1-6 mg carbon were removed during sam- pling). These residual carbon levels remained in the wastewater regardless of the length of time of the study.

As indicated in Table 2 there was preferential utilization of DOC from the < 1000 mol. wt class during the course of BOD exertion. Recalcitrant organic carbon was comprised primarily of the mo- lecular weight classes > 1000 (Table 2). In the begin- ning of the respirometer experiments the two molec- ular weight fractions > 1000 comprised 33---40~ of the DOC of the slurry wastewater. At the termination of the run these same fractions comprised 69-71~ of the residual DOC. As is indicated by Table 2, a negligible reduction in the DOC of the higher molec- ular weight fraction was achieved, while there was an average 77~o reduction in the < 1000 fraction.

The net removal of DOC from the wastewaters by the activated sludge were primarily from the low molecular weight fractions (< I000) and there was little change in the concentration of DOC in the > 1000 fractions (1000-5000 and >5000). These re- sults are in agreement with other investigators who found that higher molecular weight organic com- pounds comprised the major fraction of activated sludge treated effluent. For example, Manka and Rebhun (1982) reported that 61~ of the DOC in the effluent from a sewage treatment plant was of higher mol. wt (> 500) and that only 17~/o was less than 500. Furthermore, Grady and co-workers (Grady et al., 1984) examined the effluent of synthetic waste, bench scale activated sludge reactors and found that the majority of DOC removal was accomplished by removal of the < 1000 mol. wt fractions.

The higher molecular weight DOC in the slurry wastewaters appears to be unaffected by the activated sludge since there was little net change in the DOC of these fractions. However, these higher molecular weight fractions may not contain the same organic compounds which were present in the original waste- waters. The original higher molecular weight DOC compounds may have been removed by the activated sludge and the residual DOC in these fractions may have originated through the metabolic activity of the microorganisms present in the activated sludge acting on the lower molecular weight DOC (Grady et al., 1984; Daigger and Grady, 1977; DeWalle and Chian, 1974). The methods employed in this study to deter- mine DOC were unable to differentiate between these two possibilities but the end result remains un- changed: the higher mol. wt (> 1000) DOC comprises the major component of the slurry wastewater after treatment with activated sludge.

MUTAGENICITY ASSESSMENT

In order to determine if the wastewater was a potential health hazard, concentrated (10:1) waste- waters were examined for the presence of mutagenic agents using the Salmonella typhimurium histidine auxotroph reversion assay (Ames et al., 1975). The wastewaters were prepared as previously described with a coal-water contact time of 3 days. Results are presented in Table 3. Prior to the testing of samples,

Table 2. Molecular weight characterization of Wyodak coal slurry wastewater DOC before and after BOD* analysis

Dissolved organic carbon (mg I -~) • . Sample It Sample 5

DOC tool. wt class Initial Final+ Initial Final > 5000 tool. wt 13 + 5(I1~)~ 14(24%) 18 + 2 (13'~) 14+1(235o) < 5000 and > 1000 tool. wt 25 _.+ 7 (22%) 26 (45%) 39 _ 8 (27%) 30 + I (48°~) < 1000 mol. wt 76 __. 2 (677£) 18(31%) 85_+8(60%) 18 + 1 (29'~,~) Total DOC 114 (100%) 58 (100%) 142 (100°/~) 62 (100%)

*BOD.,o. tBOD.m: Sample I, 78mgl -I and Sample 5, 52mgl -t. **After BOD2o. §% Of total DOC.

Technical Note

Table 3. Mutagenicity evaluation of coal slurry concentrates using the Ames assay

1203

Salmonella typhimurium Dose

Coal type range II) S-9 TA 97 TA 98 TA I00

His- revertants

S-9 TA 97 TA 98 TA 100

Wyodak 0 (Control) + 140 38 91 50 + 170 28 107

100 + 172 26 98 200 + 169 20 103 250 + 173 21 88

Montana Rosebud 0 (Control) + 207 --* - - 50 + 203 -- 99

I00 + 228 - - 94 200 + 192 - - 98 250 + 229 -- - -

Black Mesa 0 (Control) + - - 20 125 50 + -- 3l I21

100 + -- 18 122 200 + - - - - 116 250 + - - 19 117

145 24 108 156 32 165 II9 36 128 147 29 1~2 153 25 I57

96 20 100 98 18 I00

151 2t 99 145 31 115 150 20 96

- - 27 120 - - 1 7 1 1 2

- - 1 6 1 2 5

- - - - 1 2 0

- - 1 3 1 1 6

*Indicates microsomal activation. Aroclor 1254 induced rat fiver. ?Not determined.

Salmonella strains TA 97, TA 98, TA 100 were checked for histidine auxotrophy, uvr B and LPS/rfa mutat ions and the presence of the R factor plasmid. Positive controls using 2-aminofluorene (+S-9 ) for strains TA 97 and TA 98 and M M N G ( - S - 9 ) for TA 100, were performed to insure that the tester strains were functionally responsive to known mutagenic agents. As indicated by the data in Table 3, no linear

dose- response was obtained with the slurry waste- waters tested. The number of revertants per plate

generated from the dose regime used did not significantly differ from those scored in the control

plates. Thus, the results indicate that under the condit ions employed, no mutagenic activity could be detected in the coal slurry wastewaters.

These results, however, must be regarded as pre- liminary in nature. The present study used samples which had been concentra ted 10:1 prior to analysis. DOC values for a typical Wyodak concentrate were

on the order of 1400mgl -~, which represents a maximum dose per plate o f 0.35 mg organic carbon.

Given the fact that the DOC of slurry wastewaters is comprised of large numbers of compounds , the con- centration used may represent an insufficient amount of sample. Thus, the available data indicate that the wastewater does not appear to represent a particular mutagenic threat but concentra t ion factors of 100 or greater may be required to adequately assess the mutagenic potential o f these wastewaters.

Acknowledgement--This investigation was supported by U.S. Department of Interior grant, 14-34-0001-1455; from the Office of Water Research and Technology, Bureau of Reclamation.

R E F E R E N C E S

Ames B. N., Lee F. D. and Darstow W. E. (1975) An improved bacterial test system for the detection and

classification of mutagens and carcinogens. Proc. natn. Acad. Sci. 70, 782-786.

Anonymous (1976) How the slurry pipeline in Arizona is working. Enrir. Sci. Technol. 10, 1080.

Carleton A. J. and Cheng D. C. H. (1977) Pipeline design for industrial slurries. Chem. Engng 84, 111-132.

Daigger G. T. and Grady C. P. L. Jr (1977) A model for the bio-oxidation process based on product formation con- cepts. Water Res. 11, 1049-1057.

DeWa[le F. B. and Chian E. S. K. (1974) Kinetics of formation of humic substances in activated sludge systems and their effects on flocculation. Biotechnol. Bioengng 16, 739-755.

Godwin J. and Manahan S. E. (I 979) Interchange of metals and organic matter and sub-bituminous coal or lignite under simulated coal slurry pipeline conditions. Envir. Sci. Technol. 13, 1100-1104.

Grady C. P. L. Jr, Kirsch E. J., Kocwara M. K., Trgovcich B. and Watt R. D. (1984) Molecular weight distribution in activated sludge effluents. Water Res. 18, 239-246.

Jackson O. (1978) Outlook shines for coal slurry lines. Coal Age June, 88-89.

Larson R. J. and Perry R. L. (1981) Use of the electrolytic respirometer to measure biodegradation in natural waters. Water Res. 15, 697-702.

Manka J. and Rebhun M. (1982) Organic groups and molecular weight distribution in tertiary effluents and renovated waters. Water Res. 16, 399-403.

Moore J. W. (1981) Water quality considerations in the slurry pipeline of coal. Arkansas Water Resources Research Center, University of Arkansas, Publication No. 84.

Peavy H. S. (1981) Water pollution potential of coal slurry pipelines. EPA 600/7-81-082.

Sayler G. S., Minear R. A., Reid M. C. and Davis J. W. (1983) Enhanced reuse potential of coal slurry, transport water: toxic organics assessment and removal. Office of Water Research and Technology Report No. RU-84/4.

Thomas H. A. Jr (1950) Graphical determination of BOD curve constants. Wat. Sewage Wks 97, 123-130.

.. Young J. C. and Baumann E. R. (1976a) The electrolytic respirometer--i. Factors affecting oxygen uptake mea- sures. Water Res. 10, 1031-1040.

Young J. C. and Baumann E. R. (1976b) The electrolytic respirometer--ll. Use in water pollution control plant laboratories. Water Res. 10, 1141-1149.