- Email: [email protected]
Fate of natural organic matter (NOM) during groundwater recharge using reclaimed water
~
Pergamon
Wal. Sc. Tech. Vol. 40. No.9. pp. 241-248. 1999 e 1999
Publishedby Elsevier ScienceLtdon behalfof the IAWQ Printedin GreatBntain. Allnghts reserved 0273-1223199 S20.00 + 0.00
PU: S0273-1223(99)00662-9
FATE OF NATURAL ORGANIC MATTER (NOM) DURING GROUNDWATER RECHARGE USING RECLAIMED WATER Jorg E. Drewes and Peter Fox Department ofCivil and Environmental Engineering. Arizona State University. Tempe AZ 85287-5306. USA
ABSTRACT The scope of this study was to investigate how natural organic matter (NOM) from drinking water and soluble microbial products (SMP) generated In the wastewater treatment process influence the character of DOC in reclaimed water used for indirect potable reuse. Biodegradanon studies in conjunction with XADfractionat ion and IlC-NMR spectroscopy were applied to both characterize organic matter and to study removal mechanisms during subsequent soil-aquifer treatment (SAT). Based on hydraulically corresponding samples of drinking water and reclaimed water from reuse sites in Arizona and California. residual DOC in reclaimed water after SAT was dominated in concentration and character by NOM. Changes in DOC character were observed with increasmg retention times during SATin the direction to more ahphatic and less aromatic compounds indicative of hurnification with biodegradat ion as the dominant transformation process for bulk organics. Q 1999 Published by Elsevier Science Ltd on behalf of the IAWQ . All rights reserved
KEYWORDS UC-NMR-spectroscopy; groundwater recharge; indirect potable reuse; natural organic matter (NOM); soil-aquifer treatment (SAT); soluble microbial products (SMP). INTRODUCTION An increasing number of municipalities are applying indirect reuse of treated wastewater as a feasible option to augment potable water supplies. Regarding potable and non-potable reuse, residual dissolved organic carbon (DOC) is of concern in soil-aquifer treatment (SAT) systems using treated domestic effluents, because of the uncertainty associated with a broad spectrum of potential health concerns (Crook, 1998). There are three groups of residual organic chemicals that require attention concerning groundwater quality in water reuse systems: a. natural organic matter (NOM) already present in drinking water, b. synthetic organic compounds (SOC) added by consumers and disinfection byproducts (DBP's) generated during chlorination of water and wastewater, and c. soluble microbial products (SMP) formed during the wastewater treatment process due to decomposition of organic compounds (Hejzlar and Chudoba 1986; Link et al., 1989; Murthy and Novak, 1998). The unidentified bulk of residual TOC in reclaimed water, often designated as effluent organic matter (EfOM), has been often characterized as humic and fulvic acids (Amy et al. , 1987; Manka and Rebhun, 1982). However, it is still difficult to distinguish between the amount of organic compounds derived from the treatment process itself (SMP's) and the portion contributed by consumers and drinking water sources (NOM). It is currently believed that these residual compounds might significantly influence the Toe of groundwater at the point of recovery. In addition, refractory TOC is of 241
242
J. E. DREWES and P. FOX
concern, because it might mobilize organic contaminants and trace metals in the subsurface (Chiou et al., 1986), and is also a precursor for disinfection by-product (DBP) formation. Because of that, TOC or DOC is considered to be a suitable parameter for determing organic removal efficiencies in practice, and therefore it is subject to regulations for water reuse (National Research Council, 1998). The existing uncertainties in establishing local water reuse systems for indirect potable reuse requires a greater understanding of the constituents in reclaimed water and investigations on the removal mechanisms during SAT (National Research Council, 1998). To focus on these issues, the research project entitled "Soilaquifer Treatment for Sustainable Water Reuse" was established in the Southwestern U.S., and investigated different water reuse field sites in Arizona and California. As part of this project, the central objective of this study was to investigate how natural organic matter from drinking water and soluble microbial products generated during wastewater treatment influence the organic chemical composition of reclaimed water. Additional investigations focused on removal processes for the bulk of organics during subsequent SAT. Investigations reported here focused on a water reuse field site in Arizona. Examination considered the entire watershed including drinking water sources, water treatment, water usage, wastewater treatment and subsequent soil-aquifer treatment. To characterize the fate of organic matter, techniques such as biodegradation studies using adapted soil-columns, hydrophilic/hydrophobic fractionation and isolation, and solid-state 13C-NM R spectroscopy were applied. METHODS The Northwest Water Reclamation Plant in the City of Mesa (Arizona) was investigated over a period of two years. The facility employed activated sludge treatment including nitrification/denitrification with tertiary filtration and disinfection. The plant is using 50% of its total capacity of 30,500 m 3/d for groundwater recharge using four infiltration basins. A cluster of piezometer wells (OW) was located 280 m downgradient of the infiltration basins representing 6 m-screened intervals at depths of 20, 30, 40, and 60 m below ground surface. In addition, monitoring wells NW -4 and NW -2 with a screening interval of 23-56 m below ground surface, and Superfund site wells (37U, 6U) screened from 14 to 25 m below ground surface were located approximately 600 m and 1500 m, respectively downgradient of the basins. Figure 1 presents a simplified cross-section of the site.
1'1'/'1' 1{ 11 11\'IAI
~1II)1l1
rn
I· AI I I '\" I A'
,,.1
Figure 1. Cross section of the Northwest Water Reclamation Facility, Mesa (Arizona), (distance of wells downgradient of infiltration basins not on scale).
To determine the portion of non-biodegradable DOC in different reclaimed waters, an adapted soil-column system which simulated aquifer conditions in a series of four I m-columns was used, and operated under saturated, anoxic redox conditions (Drewes and Fox, 1999). This system characterized DOC removal between 0 and 30 days, representing short-term SAT.
Fate of NOM during groundwater recharge using reclaimed water
243
The DOC fractionation method used in this study is based on methods following Aiken et al. (1992) and Malcolm (1990), and is described in detail in Drewes et al. (in press). Accordingly, DOC was separated into three fractions which are operationally defined as: hydrophobic acids fraction reversibly adsorbed onto XAD-8 at pH 2 (representing mainly aquatic fulvic and humic acids), hydrophilic acids fraction that reversibly adsorbed onto XAD-4 at pH 2, and ultra-hydrophilic fraction that passes through both resins. A volume of 36 litres of each sample (filtered through 0.1 11m and acidified to pH 2) was applied to 580 mL columns of Amberlite XAD-8 resin and XAD-4 in series. The hydrophobic acid fraction and the hydrophilic fraction were back-eluted parallel from the XAD columns with 0.1 N NaOH at pH 13 and passed directly on two parallel 150 mL cation exchange columns. Each eluted fraction was collected in a volume of 500 mL and freeze-dried. The overall rate of recovery of organic carbon for the applied XAD-approach varied between 66% and 77%. All samples were filtered using 0.45 11m cellulose nitrate membrane filter and stored at 4°C prior to analysis. The dissolved organic carbon was analyzed using a Shimadzu 5050A TOC-analyzer with autosampler. UVabsorbance (UVA) was measured at a wavelength of 254 nm on a Hewlett Packard 8452A spectrophotometer (path length 1 ern), Specific UVA (SUVA) was calculated from the ratio of UVA to DOC. The solid-state 13C-NMR experiments were performed on a Varian UnityPlus 400 spectrometer using a multinuclear Varian 5 mm CP/MAS probe operating at 100.58 MHz. The acquisation parameters included a 50 kHz sweep width, 3 s pulse delay, 3 ms l l-step contact time and an applied line broadening of 150. The percentage of integrated shift ranges representing different functional groups can be compared to describe potential changes of chemical properties (Malcolm, 1989; Aiken et al., 1992). Suwannee River Fulvic Acid (SRF) was used as a reference for NOM, and was chosen to validate the 13C-NMR analysis. RESULTS AND DISCUSSION DOC removal during soil-aquifer treatment (SAT) To study DOC removal during subsequent SAT, six different Mesa tertiary effluents were percolated through the soil-columns for a period of two months each (Drewes and Fox, in press). The behavior of DOC removal during short-term SAT (retention time - 30 days) for all effluents followed first-order kinetics in accordance with other SAT-studies (Drewes and Jekel, 1998; Quanrud et al., 1998). After several months of operation and in contrast to the observed DOC-removal, UVA elimination continued at increasing retention times indicating an ongoing loss of aromaticity. This behavior was probably due to acclimation of microorganisms over time leading to a loss in UV absorbance without mineralisation of organic compounds. A similar improvement in UVA removal was reported in soil-column studies after several month of operation using tertiary effluent in Germany (Drewes and Jekel, 1998).
2
.
4
10
12
,~
10 20 I
upper anuv rat urat
30
s:
g. 40
"0
rndd le alluv'il l unit
50 -• • UV254 (1Iml
60
DOC [mgll)
086
70
-
SUVA
Figure 2. Mesa monitoring well OW-I, depth profile for organic parameter based on monthly samples 2.99-6.99.
J. E. DREWESand P. FOX
244
The considerable decrease in UV-absorbance relative to DOC-removal (observed in the soil-column study) was confirmed by field measurements at different depths of the upper alluvial unit. Figure 2 demonstrates how DOC and UVA varies with depth at monitoring well OW-l less than 300 m downgradient of the infiltration basin. The DOC concentration at the top and the bottom of the upper alluvial unit were 1.62 mg/L and 1.42 mg/L, respectively. The vertical variations in concentration may be explained by the water in the top of the upper alluvial unit having a shorter travel time as compared to the water at the bottom of the upper alluvial unit. The top of the middle alluvial unit represents a mixture of native groundwater and reclaimed water while native groundwater dominates at greater depths. While changes in the upper alluvial unit did occur as a function of depth, changes in DOC concentrations as a function of downgradient distance or travel times were also observed. Immediately below the recharge basins, DOC concentrations of 3 mg/L were consistently observed at depths of 1.5 m to 4.5 m. As horizontal flow occurs downgradient to monitoring wells NW-4 and NW-210cated 300 to 500 m south of the basins, DOC concentrations decreased to 1.6 and 1.2 mg/L and at distances greater than 1,500 m DOC concentrations in the reclaimed water plume varied from 0.8 to 1.1 mg/l, The SUVA represents the relative aromaticity of the bulk of organics. Previous research has indicated that as organics are initially removed during SAT, the SUVA increases as preferential removal of non-aromatic compounds occurs (Drewes and Jekel, 1998). The effect of increasing SUVA during short-term SAT occurs as the effluent SUVA of 1.7 Umg m increases to greater than 2.5 Umg m directly below the recharge basins, and is also observed in the multi-depth sampling well OW-l (Figure 2). As downgradient distance and associated travel time increases, the SUVA tends to decrease to values less than 1.5 Umg m. This trend represents long-term transformations of bulk organic carbon where aromatic compounds are either preferentially removed or converted to non-aromatic products.
7.0 6.0
-~ C100aI
=1.49 mgll *8(04 011') + 2.8 mgll * 8(04 005') + 1.02 mg/L
-
ooII-coIumn sludy
5.0
\ 2.0
~
OW1-1
1.0
Su~ ."" wells
NW-2
0.0
o
1000
1500
2000
2500
3000
3500
travel time (d)
Figure 3. DOC removal during short-termand long-termSAT.
Figure 3 presents a model to estimate DOC concentration as a function of travel time within the reclaimed water plume assuming an easily biodegradable fraction that is removed during short-term SAT « 30 days retention time) and a less biodegradable fraction that is removed over long-term SAT (> 30 days retention time). Based on these results, significant decreases in DOC concentration will not occur at greater than 500 days. For the Mesa reclaimed water, a DOC concentration of 1 mg/L seems to be refractory. Ouantification of soluble microbial products (SMP's) derived in the treatment process Water sources used for the City of Mesa's (Arizona) drinking water supply vary between surface water and groundwater during the year. During surface water supply and groundwater supply, hydraulically corresponding samples of drinking water and reclaimed water were taken. Using the soil-column study, the
Fate of NOM during groundwater recharge using reclaimed water
245
remaining DOC concentrations of reclaimed water after short-term SAT (- 30 days) were found to be 2.16 to 3.84 mgIL. Reclaimed water DOC after SAT was correlated with the drinking water DOC for specific sampling days. Based on this correlation, an equation (Creclaimed water DOC = 1.0 Cdrinking water DOC + 1.15 mg/L) presented previously was derived describing the relationship between drinking water DOC and Mesa reclaimed water DOC after SAT (Drewes and Fox, submitted). According to these results, a lower reclaimed water DOC can be reached by a drinking water supply based on lower source water DOC. According to the y-intercept of this equation, the portion of DOC not related to drinking water was approximately 1.15 mglL for the City of Mesa. This remaining DOC consists of anthropogenic compounds added by consumers, and soluble microbial products (SMP's) generated in the wastewater treatment process. Several characterization studies on effluent organic matter showed that humic substances represent a considerable part of the soluble organic substances derived in the treatment process (Manka and Rebhun, 1982; Hejzlar and Chudoba, 1986; Link et al., 1989; Murthy and Novak, 1998). Similarity of NOM and SMP's Potential structural changes in the bulk of organics were examined with XAD fractionation and isolation, and solid-state 13C-NMR spectroscopy. In addition, Drewes and Fox (submitted) showed that 50% of the surface water DOC used in the drinking water supply of the City of Mesa and 50% of reclaimed water DOC had molecular weight fractions less than 1,000 Dalton. Fractions> 10 kDalton were small or negligible in Mesa reclaimed water. Fractionation of Mesa's main surface water source classified 51% of total DOC as ultra-hydrophilic, 21% as hydrophilic acids, and 28% as hydrophobic acids, which was expected for surface water DOC (Aiken et al. 1992). For the corresponding reclaimed water the fractionation characterized 50% of DOC also as ultra-hydrophilic, 18% as hydrophilic acids, and 32% as hydrophobic acids, confirming results for effluents reported in similar studies (Fujita et al., 1996; Debroux, 1998). Since a significant portion of DOC in reclaimed water may be attributed to SMP's, a specific change in the character of effluent organic matter as compared to natural organic matter was not evident based on ultrafiltration and XADfractionation results. DOC in mgJI
00
1.0
I
I
~ _
i' e
20
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I
-'-= -
30
4.0
50
60
~
[ I Meoa'ert"'Y.-"'oo, tGS
!··~" I--·-'
,
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I~r 1
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."~J__. 1111 1~e
t
l'ahydrophilic acids ~d~eu~~ 'p u'tra -hY~ ~hiiiCl
Figure 4. XAD-fractionation of Mesa reclaimed water prior to and after short-term and long-term SAT compared to background groundwater.
Fate of NOM during soil-aquifer treatment During subsequent SAT, a preferred removal of ultra-hydrophilic compounds occured (Figure 4). According to Namour and Mueller (1998) and Huber and Frimmel (1996), the ultra-hydrophilic DOC fraction is
246
J. E. DREWES and P. FOX
preferentiaIly derived by amino acids, proteins, and polysaccharides, which are potentiaIly biodegradable. The hydrophobiclhydrophilic acid DOC fractions seemed to persist during short-term SAT, however SUVA changed significantly in both fractions (Table I), indicating structural changes of aromatic compounds due to biodegradation of organic matter. Removal during long-term SAT (> 30 days) was similar for hydrophobic and hydrophilic acids with a 63% decrease in theDOC of these fractions, whereas the ultra-hydrophilic fraction DOC removal was 42%. Since hydrophobic acid compounds tend to be better adsorbable compared to hydrophilic acids due to larger molecular weight and hydrophobic character, a dominant sorption process during long-term SAT seemed to be unlikely. Mesa groundwater not affected by infiltrated reclaimed water, represents water quality after long retention times in the subsurface. This water showed a less hydrophobic/hydrophilic character with only a minor difference in the ultra-hydrophilic fraction as compared to long-term SAT water. Table 1. DOC and UVA for Mesa reclaimed water during SAT Hydrophobic acids DOCmg/L UVA 11m 1.73 4.3 1.7 2.87 0.68 1.14
Sample Prior to SAT Short-term SAT « 30 d) Long-term SAT (> 30 d)
Hydrophilic acids DOCmg/L UVA 11m 0.96 1.82 0.96 1.55 0.56 0.35
Ultra-hydrophilic DOCmg/L UVA 11m 2.67 2.15 1.08 1.87 0.63 1.54
Applying 13C-NMR spectroscopy on XAD-8 and XAD-4 isolates provided insight into stuctural differences. Figure 5 presents NMR-spectra of corresponding XAD-8 drinking water and reclaimed water isolates prior to and after SAT. AIl spectra were dominated by four broad unresolved humps representing complex mixtures which are typical for hydrophobic acids isolated from streams and rivers (Malcolm, 1985).
Drinking water
Mesa reclaimed water 10/01/98
Mesa reclaimed water after short-term SAT (soil-column study)
, 250
, 200
,
,
150
100
I
50
o
, ppm
Figure S. 13C-NMRspectra for XAD-8 isolates of corresponding samples (Mesa, Arizona).
The spectra of Mesa drinking water and reclaimed water after removal of biodegradable compounds by simulated short-term SAT were similar to one another. Table 2 summarizes the chemical shift areas representing the dominant functional groups. The reclaimed water after short-term SAT has significantly more aromatic character and less carboxylic functional groups as compared to drinking water NOM. However, during long-term SAT reclaimed water DOC is losing its aromatic and carboxylic character resulting in a relative increase in aliphatic compounds. AdditionaIly, Figure 6 shows that the 13C_NMR spectra of monitoring weIl 6U XAD-8 isolate resembles the organic matter of background groundwater not impacted by reclaimed water. The similarity of drinking water and reclaimed water after SAT in structure and composition was also evident for XAD-4 isolates (Drewes et al., in press). Based on these results,
Fate of NOM during groundwater recharge using reclaimed water
247
SMP's derived from the wastewater treatment process do not cause differences in structure as compared to NOM.
Reclaimed water after long-term SAT (6U)
Mesa background groundwater
Figure 6. 13C_NMR spectra for XAD-S isolates of monitoring we1l6U and background groundwater.
Table 2. Percentage of chemical shift areas for 13C-NMR spectra of XAD-8 isolates
Chemical shift Aliphatic (D-60ppm) Carbohydrates (60-90 ppm) Aromatic (110-160 ppm) Carboxylic (190-230 ppm)
Drinking water CitvofMesa 61.5
Hydrophobic acids Reclaimed Reclaimed water Reclaimed water short-term SAT after SAT (NW-2) water 53.7 52.1 50.8
Reclaimed water after SAT (6U) 63.6
11.3
16.0
14.1
14.4
12.6
5.3
13.8
11.1
10.8
7.2
15.9
13.2
14.1
13.5
12.9
CONCLUSIONS This study showed that not only natural organic matter (NOM) from drinking water but also soluble microbial products (SMP's) generated in the wastewater treatment process engrave their character on DOC in water reuse systems. Reclaimed water organic matter after SAT was low in molecular weight and was only slightly more aromatic and less aliphatic as compared to drinking water from surface water sources. Although, residual organic carbon in reclaimed water is increased by SMP 's derived in the wastewater treatment process, the character of organic matter did not change according to XAD-8 and XAD-4 fractionation results. The similarity of drinking water and reclaimed water I3C NMR spectra proved that structure and composition of the isolates are representative of natural organic matter. Soil-column studies simulating short-term SAT and field samples representing long-term SAT seemed to confirm a rapid removal of ultra-hydrophilic compounds low in molecular weight, and a sustained biodegradation of poorly degradable organic compounds during SAT. During subsequent SAT, biodegradation seemed to be the dominant removal process for DOC resulting in continuous structural changes of organic matter. ACKNOWLEDGEMENT We gratefully acknowledge the technical and financial support received from the City of Mesa (Arizona). Principal funding was provided by the American Water Works Association Research Foundation (AWWARF) and the U.S. Environmental Protection Agency. Additional financial assistance was received through a post-doc scholarship administered by the Deutsche Forschungsgemeinschaft (DFG). The funding agencies assume no responsibility for the content of the research reported in this publication or for the opinions or statements of fact expressed in the report.
J. E. DREWES and P. FOX
248
REFERENCES Aiken, G., McKnight, D. M., Thorn, K. A. and Thurman, E. M. (1992). Isolation of hydrophilic organic acids from water using nonionic macroporous resin. Org. Geochem., 18(4),567-573. Amy, G., Bryant, C. and Belyani, M. (1987). Molecular weight distribution of soluble organic matter in various secondary and tertiary effluents. Wat Sci Tech., 19(3-4), 529-538. Crook, J. (1998). Water reclamation and reuse criteria. In: Wastewater Reclamation and Reuse. T. Asano (ed), Vol. 10, Water Quality Management Library, Technomic Publishing, Lancaster PA, pp. 627-704. Debroux, J. F. (1998). The physical-chemical and oxidant-reactive properties of effluent organic matter (EjDM) intended for potable reuse. Ph.D. thesis, University of Colorado. Drewes, J. E. and Jekel, M. (1998). Behavior of DOC and AOX using advanced treated wastewater for groundwater recharge. Wat. Res. 32(10), 3125-3133. Drewes, J. E. and Fox, P. (submitted). Impact of drinking water sources on reclaimed water quality. Water Environment Research. Drewes, J. E. and Fox., P. (1999). Behavior and characterization of residual organic compounds in wastewater used for indirect potable reuse. Wat Sci Tech, 40(4-5),391-398. Drewes, J. E., Sprinzl, M., Wendrock, A., Williams, M. D., Fox, P. and Westerhoff, P. (in press). Tracking residual dissolved organic carbon using XAD-fracttonallon and 13C-NMR sprectroscopy in indirect potable reuse systems. Vom Wasser. Fujita, Y., Ding, W. H. and Reinhard, M. (1996). Idennfication of wastewater dissolved organic carbon characteristics in reclaimed wastewater and recharged groundwater. Water Environment Research, 68(5), 868-876. Hedges, J. I. (1988). Polymerization of Humic Substances in Natural Environments. Humic Substances and Their Role in the Environment. Dahlem Konferenzen. F. H. Frimmel, R. F. Christman, J. Wiley and Sons Ltd., 45-58. Hejzlar, J. and Chudoba, J. (1986). Microbial polymers in the aquatic environment - I-III. Wat. Res. 20(10), 1209-1227. Huber, S. and Frimmel, F. H. (1996). Size-Exclusion Chromatography with Organic Carbon Detection (LC-OCD): A Fast and Reliable Method for the Characterization of Hydrophilic Organic Matter in Natural Waters. Vom Wasser. 86,277-290 (in German). Link, J., Gilbert, E. and Eberle, S. H. (1989). Investigation of properties and quantities of residual compounds in secondary effluents. Vom Wasser, 72, 349-370 (in German). Malcolm, R. L. (1985). Geochemistry of Stream Fulvic and Humic Substances. Aiken et al., Humic substances in soil, sediments and water. John Wiley and Sons. 181-209. Malcolm, R. L. (1989). Apphcations of Solid-state 13C_NMR Spectroscopy to Geochemical Studies of Humic Substances. Hayes et al., Humic Substances II. John WIley and Sons. 339-372. Malcolm, R. L. (1990). Factors to be considered in the isolation and characterization of aquatic humic substances. Meeting on Humic Substances In the Environment, Proceedings of Linkoping. Linkoeing University, Sweden, 390-417. Manka, J. and Rebhun, M. (1982). Organic Groups and Molecular Weight Distribution in Tertiary Effluents and Renovated Waters. Wat. Res., 16, 399pp. . Murthy, S. N. and Novak, J. T. (1998). Influence of Cations on Activated Sludge Effluent Quality. Proceedings WEFTEC '98. 71" Annual Conference Water Environment Federation, Orlando FL. Volume I. 309-324. National Research Council (1998). Issues in Potable Reuse. National Academy Press, Washington, D.C. Quanrud, D., Arnold, R. G., Clark, A., Massaro, M. and Wilson, L. G. (1998). Efficiency and sustainability of soil-aquifer treatment leading to wastewater reclamation and reuse. Proceedings AWWA Water Reuse 1998, 579-591.
Pergamon
Wal. Sc. Tech. Vol. 40. No.9. pp. 241-248. 1999 e 1999
Publishedby Elsevier ScienceLtdon behalfof the IAWQ Printedin GreatBntain. Allnghts reserved 0273-1223199 S20.00 + 0.00
PU: S0273-1223(99)00662-9
FATE OF NATURAL ORGANIC MATTER (NOM) DURING GROUNDWATER RECHARGE USING RECLAIMED WATER Jorg E. Drewes and Peter Fox Department ofCivil and Environmental Engineering. Arizona State University. Tempe AZ 85287-5306. USA
ABSTRACT The scope of this study was to investigate how natural organic matter (NOM) from drinking water and soluble microbial products (SMP) generated In the wastewater treatment process influence the character of DOC in reclaimed water used for indirect potable reuse. Biodegradanon studies in conjunction with XADfractionat ion and IlC-NMR spectroscopy were applied to both characterize organic matter and to study removal mechanisms during subsequent soil-aquifer treatment (SAT). Based on hydraulically corresponding samples of drinking water and reclaimed water from reuse sites in Arizona and California. residual DOC in reclaimed water after SAT was dominated in concentration and character by NOM. Changes in DOC character were observed with increasmg retention times during SATin the direction to more ahphatic and less aromatic compounds indicative of hurnification with biodegradat ion as the dominant transformation process for bulk organics. Q 1999 Published by Elsevier Science Ltd on behalf of the IAWQ . All rights reserved
KEYWORDS UC-NMR-spectroscopy; groundwater recharge; indirect potable reuse; natural organic matter (NOM); soil-aquifer treatment (SAT); soluble microbial products (SMP). INTRODUCTION An increasing number of municipalities are applying indirect reuse of treated wastewater as a feasible option to augment potable water supplies. Regarding potable and non-potable reuse, residual dissolved organic carbon (DOC) is of concern in soil-aquifer treatment (SAT) systems using treated domestic effluents, because of the uncertainty associated with a broad spectrum of potential health concerns (Crook, 1998). There are three groups of residual organic chemicals that require attention concerning groundwater quality in water reuse systems: a. natural organic matter (NOM) already present in drinking water, b. synthetic organic compounds (SOC) added by consumers and disinfection byproducts (DBP's) generated during chlorination of water and wastewater, and c. soluble microbial products (SMP) formed during the wastewater treatment process due to decomposition of organic compounds (Hejzlar and Chudoba 1986; Link et al., 1989; Murthy and Novak, 1998). The unidentified bulk of residual TOC in reclaimed water, often designated as effluent organic matter (EfOM), has been often characterized as humic and fulvic acids (Amy et al. , 1987; Manka and Rebhun, 1982). However, it is still difficult to distinguish between the amount of organic compounds derived from the treatment process itself (SMP's) and the portion contributed by consumers and drinking water sources (NOM). It is currently believed that these residual compounds might significantly influence the Toe of groundwater at the point of recovery. In addition, refractory TOC is of 241
242
J. E. DREWES and P. FOX
concern, because it might mobilize organic contaminants and trace metals in the subsurface (Chiou et al., 1986), and is also a precursor for disinfection by-product (DBP) formation. Because of that, TOC or DOC is considered to be a suitable parameter for determing organic removal efficiencies in practice, and therefore it is subject to regulations for water reuse (National Research Council, 1998). The existing uncertainties in establishing local water reuse systems for indirect potable reuse requires a greater understanding of the constituents in reclaimed water and investigations on the removal mechanisms during SAT (National Research Council, 1998). To focus on these issues, the research project entitled "Soilaquifer Treatment for Sustainable Water Reuse" was established in the Southwestern U.S., and investigated different water reuse field sites in Arizona and California. As part of this project, the central objective of this study was to investigate how natural organic matter from drinking water and soluble microbial products generated during wastewater treatment influence the organic chemical composition of reclaimed water. Additional investigations focused on removal processes for the bulk of organics during subsequent SAT. Investigations reported here focused on a water reuse field site in Arizona. Examination considered the entire watershed including drinking water sources, water treatment, water usage, wastewater treatment and subsequent soil-aquifer treatment. To characterize the fate of organic matter, techniques such as biodegradation studies using adapted soil-columns, hydrophilic/hydrophobic fractionation and isolation, and solid-state 13C-NM R spectroscopy were applied. METHODS The Northwest Water Reclamation Plant in the City of Mesa (Arizona) was investigated over a period of two years. The facility employed activated sludge treatment including nitrification/denitrification with tertiary filtration and disinfection. The plant is using 50% of its total capacity of 30,500 m 3/d for groundwater recharge using four infiltration basins. A cluster of piezometer wells (OW) was located 280 m downgradient of the infiltration basins representing 6 m-screened intervals at depths of 20, 30, 40, and 60 m below ground surface. In addition, monitoring wells NW -4 and NW -2 with a screening interval of 23-56 m below ground surface, and Superfund site wells (37U, 6U) screened from 14 to 25 m below ground surface were located approximately 600 m and 1500 m, respectively downgradient of the basins. Figure 1 presents a simplified cross-section of the site.
1'1'/'1' 1{ 11 11\'IAI
~1II)1l1
rn
I· AI I I '\" I A'
,,.1
Figure 1. Cross section of the Northwest Water Reclamation Facility, Mesa (Arizona), (distance of wells downgradient of infiltration basins not on scale).
To determine the portion of non-biodegradable DOC in different reclaimed waters, an adapted soil-column system which simulated aquifer conditions in a series of four I m-columns was used, and operated under saturated, anoxic redox conditions (Drewes and Fox, 1999). This system characterized DOC removal between 0 and 30 days, representing short-term SAT.
Fate of NOM during groundwater recharge using reclaimed water
243
The DOC fractionation method used in this study is based on methods following Aiken et al. (1992) and Malcolm (1990), and is described in detail in Drewes et al. (in press). Accordingly, DOC was separated into three fractions which are operationally defined as: hydrophobic acids fraction reversibly adsorbed onto XAD-8 at pH 2 (representing mainly aquatic fulvic and humic acids), hydrophilic acids fraction that reversibly adsorbed onto XAD-4 at pH 2, and ultra-hydrophilic fraction that passes through both resins. A volume of 36 litres of each sample (filtered through 0.1 11m and acidified to pH 2) was applied to 580 mL columns of Amberlite XAD-8 resin and XAD-4 in series. The hydrophobic acid fraction and the hydrophilic fraction were back-eluted parallel from the XAD columns with 0.1 N NaOH at pH 13 and passed directly on two parallel 150 mL cation exchange columns. Each eluted fraction was collected in a volume of 500 mL and freeze-dried. The overall rate of recovery of organic carbon for the applied XAD-approach varied between 66% and 77%. All samples were filtered using 0.45 11m cellulose nitrate membrane filter and stored at 4°C prior to analysis. The dissolved organic carbon was analyzed using a Shimadzu 5050A TOC-analyzer with autosampler. UVabsorbance (UVA) was measured at a wavelength of 254 nm on a Hewlett Packard 8452A spectrophotometer (path length 1 ern), Specific UVA (SUVA) was calculated from the ratio of UVA to DOC. The solid-state 13C-NMR experiments were performed on a Varian UnityPlus 400 spectrometer using a multinuclear Varian 5 mm CP/MAS probe operating at 100.58 MHz. The acquisation parameters included a 50 kHz sweep width, 3 s pulse delay, 3 ms l l-step contact time and an applied line broadening of 150. The percentage of integrated shift ranges representing different functional groups can be compared to describe potential changes of chemical properties (Malcolm, 1989; Aiken et al., 1992). Suwannee River Fulvic Acid (SRF) was used as a reference for NOM, and was chosen to validate the 13C-NMR analysis. RESULTS AND DISCUSSION DOC removal during soil-aquifer treatment (SAT) To study DOC removal during subsequent SAT, six different Mesa tertiary effluents were percolated through the soil-columns for a period of two months each (Drewes and Fox, in press). The behavior of DOC removal during short-term SAT (retention time - 30 days) for all effluents followed first-order kinetics in accordance with other SAT-studies (Drewes and Jekel, 1998; Quanrud et al., 1998). After several months of operation and in contrast to the observed DOC-removal, UVA elimination continued at increasing retention times indicating an ongoing loss of aromaticity. This behavior was probably due to acclimation of microorganisms over time leading to a loss in UV absorbance without mineralisation of organic compounds. A similar improvement in UVA removal was reported in soil-column studies after several month of operation using tertiary effluent in Germany (Drewes and Jekel, 1998).
2
.
4
10
12
,~
10 20 I
upper anuv rat urat
30
s:
g. 40
"0
rndd le alluv'il l unit
50 -• • UV254 (1Iml
60
DOC [mgll)
086
70
-
SUVA
Figure 2. Mesa monitoring well OW-I, depth profile for organic parameter based on monthly samples 2.99-6.99.
J. E. DREWESand P. FOX
244
The considerable decrease in UV-absorbance relative to DOC-removal (observed in the soil-column study) was confirmed by field measurements at different depths of the upper alluvial unit. Figure 2 demonstrates how DOC and UVA varies with depth at monitoring well OW-l less than 300 m downgradient of the infiltration basin. The DOC concentration at the top and the bottom of the upper alluvial unit were 1.62 mg/L and 1.42 mg/L, respectively. The vertical variations in concentration may be explained by the water in the top of the upper alluvial unit having a shorter travel time as compared to the water at the bottom of the upper alluvial unit. The top of the middle alluvial unit represents a mixture of native groundwater and reclaimed water while native groundwater dominates at greater depths. While changes in the upper alluvial unit did occur as a function of depth, changes in DOC concentrations as a function of downgradient distance or travel times were also observed. Immediately below the recharge basins, DOC concentrations of 3 mg/L were consistently observed at depths of 1.5 m to 4.5 m. As horizontal flow occurs downgradient to monitoring wells NW-4 and NW-210cated 300 to 500 m south of the basins, DOC concentrations decreased to 1.6 and 1.2 mg/L and at distances greater than 1,500 m DOC concentrations in the reclaimed water plume varied from 0.8 to 1.1 mg/l, The SUVA represents the relative aromaticity of the bulk of organics. Previous research has indicated that as organics are initially removed during SAT, the SUVA increases as preferential removal of non-aromatic compounds occurs (Drewes and Jekel, 1998). The effect of increasing SUVA during short-term SAT occurs as the effluent SUVA of 1.7 Umg m increases to greater than 2.5 Umg m directly below the recharge basins, and is also observed in the multi-depth sampling well OW-l (Figure 2). As downgradient distance and associated travel time increases, the SUVA tends to decrease to values less than 1.5 Umg m. This trend represents long-term transformations of bulk organic carbon where aromatic compounds are either preferentially removed or converted to non-aromatic products.
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~
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NW-2
0.0
o
1000
1500
2000
2500
3000
3500
travel time (d)
Figure 3. DOC removal during short-termand long-termSAT.
Figure 3 presents a model to estimate DOC concentration as a function of travel time within the reclaimed water plume assuming an easily biodegradable fraction that is removed during short-term SAT « 30 days retention time) and a less biodegradable fraction that is removed over long-term SAT (> 30 days retention time). Based on these results, significant decreases in DOC concentration will not occur at greater than 500 days. For the Mesa reclaimed water, a DOC concentration of 1 mg/L seems to be refractory. Ouantification of soluble microbial products (SMP's) derived in the treatment process Water sources used for the City of Mesa's (Arizona) drinking water supply vary between surface water and groundwater during the year. During surface water supply and groundwater supply, hydraulically corresponding samples of drinking water and reclaimed water were taken. Using the soil-column study, the
Fate of NOM during groundwater recharge using reclaimed water
245
remaining DOC concentrations of reclaimed water after short-term SAT (- 30 days) were found to be 2.16 to 3.84 mgIL. Reclaimed water DOC after SAT was correlated with the drinking water DOC for specific sampling days. Based on this correlation, an equation (Creclaimed water DOC = 1.0 Cdrinking water DOC + 1.15 mg/L) presented previously was derived describing the relationship between drinking water DOC and Mesa reclaimed water DOC after SAT (Drewes and Fox, submitted). According to these results, a lower reclaimed water DOC can be reached by a drinking water supply based on lower source water DOC. According to the y-intercept of this equation, the portion of DOC not related to drinking water was approximately 1.15 mglL for the City of Mesa. This remaining DOC consists of anthropogenic compounds added by consumers, and soluble microbial products (SMP's) generated in the wastewater treatment process. Several characterization studies on effluent organic matter showed that humic substances represent a considerable part of the soluble organic substances derived in the treatment process (Manka and Rebhun, 1982; Hejzlar and Chudoba, 1986; Link et al., 1989; Murthy and Novak, 1998). Similarity of NOM and SMP's Potential structural changes in the bulk of organics were examined with XAD fractionation and isolation, and solid-state 13C-NMR spectroscopy. In addition, Drewes and Fox (submitted) showed that 50% of the surface water DOC used in the drinking water supply of the City of Mesa and 50% of reclaimed water DOC had molecular weight fractions less than 1,000 Dalton. Fractions> 10 kDalton were small or negligible in Mesa reclaimed water. Fractionation of Mesa's main surface water source classified 51% of total DOC as ultra-hydrophilic, 21% as hydrophilic acids, and 28% as hydrophobic acids, which was expected for surface water DOC (Aiken et al. 1992). For the corresponding reclaimed water the fractionation characterized 50% of DOC also as ultra-hydrophilic, 18% as hydrophilic acids, and 32% as hydrophobic acids, confirming results for effluents reported in similar studies (Fujita et al., 1996; Debroux, 1998). Since a significant portion of DOC in reclaimed water may be attributed to SMP's, a specific change in the character of effluent organic matter as compared to natural organic matter was not evident based on ultrafiltration and XADfractionation results. DOC in mgJI
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Figure 4. XAD-fractionation of Mesa reclaimed water prior to and after short-term and long-term SAT compared to background groundwater.
Fate of NOM during soil-aquifer treatment During subsequent SAT, a preferred removal of ultra-hydrophilic compounds occured (Figure 4). According to Namour and Mueller (1998) and Huber and Frimmel (1996), the ultra-hydrophilic DOC fraction is
246
J. E. DREWES and P. FOX
preferentiaIly derived by amino acids, proteins, and polysaccharides, which are potentiaIly biodegradable. The hydrophobiclhydrophilic acid DOC fractions seemed to persist during short-term SAT, however SUVA changed significantly in both fractions (Table I), indicating structural changes of aromatic compounds due to biodegradation of organic matter. Removal during long-term SAT (> 30 days) was similar for hydrophobic and hydrophilic acids with a 63% decrease in theDOC of these fractions, whereas the ultra-hydrophilic fraction DOC removal was 42%. Since hydrophobic acid compounds tend to be better adsorbable compared to hydrophilic acids due to larger molecular weight and hydrophobic character, a dominant sorption process during long-term SAT seemed to be unlikely. Mesa groundwater not affected by infiltrated reclaimed water, represents water quality after long retention times in the subsurface. This water showed a less hydrophobic/hydrophilic character with only a minor difference in the ultra-hydrophilic fraction as compared to long-term SAT water. Table 1. DOC and UVA for Mesa reclaimed water during SAT Hydrophobic acids DOCmg/L UVA 11m 1.73 4.3 1.7 2.87 0.68 1.14
Sample Prior to SAT Short-term SAT « 30 d) Long-term SAT (> 30 d)
Hydrophilic acids DOCmg/L UVA 11m 0.96 1.82 0.96 1.55 0.56 0.35
Ultra-hydrophilic DOCmg/L UVA 11m 2.67 2.15 1.08 1.87 0.63 1.54
Applying 13C-NMR spectroscopy on XAD-8 and XAD-4 isolates provided insight into stuctural differences. Figure 5 presents NMR-spectra of corresponding XAD-8 drinking water and reclaimed water isolates prior to and after SAT. AIl spectra were dominated by four broad unresolved humps representing complex mixtures which are typical for hydrophobic acids isolated from streams and rivers (Malcolm, 1985).
Drinking water
Mesa reclaimed water 10/01/98
Mesa reclaimed water after short-term SAT (soil-column study)
, 250
, 200
,
,
150
100
I
50
o
, ppm
Figure S. 13C-NMRspectra for XAD-8 isolates of corresponding samples (Mesa, Arizona).
The spectra of Mesa drinking water and reclaimed water after removal of biodegradable compounds by simulated short-term SAT were similar to one another. Table 2 summarizes the chemical shift areas representing the dominant functional groups. The reclaimed water after short-term SAT has significantly more aromatic character and less carboxylic functional groups as compared to drinking water NOM. However, during long-term SAT reclaimed water DOC is losing its aromatic and carboxylic character resulting in a relative increase in aliphatic compounds. AdditionaIly, Figure 6 shows that the 13C_NMR spectra of monitoring weIl 6U XAD-8 isolate resembles the organic matter of background groundwater not impacted by reclaimed water. The similarity of drinking water and reclaimed water after SAT in structure and composition was also evident for XAD-4 isolates (Drewes et al., in press). Based on these results,
Fate of NOM during groundwater recharge using reclaimed water
247
SMP's derived from the wastewater treatment process do not cause differences in structure as compared to NOM.
Reclaimed water after long-term SAT (6U)
Mesa background groundwater
Figure 6. 13C_NMR spectra for XAD-S isolates of monitoring we1l6U and background groundwater.
Table 2. Percentage of chemical shift areas for 13C-NMR spectra of XAD-8 isolates
Chemical shift Aliphatic (D-60ppm) Carbohydrates (60-90 ppm) Aromatic (110-160 ppm) Carboxylic (190-230 ppm)
Drinking water CitvofMesa 61.5
Hydrophobic acids Reclaimed Reclaimed water Reclaimed water short-term SAT after SAT (NW-2) water 53.7 52.1 50.8
Reclaimed water after SAT (6U) 63.6
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12.6
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13.2
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13.5
12.9
CONCLUSIONS This study showed that not only natural organic matter (NOM) from drinking water but also soluble microbial products (SMP's) generated in the wastewater treatment process engrave their character on DOC in water reuse systems. Reclaimed water organic matter after SAT was low in molecular weight and was only slightly more aromatic and less aliphatic as compared to drinking water from surface water sources. Although, residual organic carbon in reclaimed water is increased by SMP 's derived in the wastewater treatment process, the character of organic matter did not change according to XAD-8 and XAD-4 fractionation results. The similarity of drinking water and reclaimed water I3C NMR spectra proved that structure and composition of the isolates are representative of natural organic matter. Soil-column studies simulating short-term SAT and field samples representing long-term SAT seemed to confirm a rapid removal of ultra-hydrophilic compounds low in molecular weight, and a sustained biodegradation of poorly degradable organic compounds during SAT. During subsequent SAT, biodegradation seemed to be the dominant removal process for DOC resulting in continuous structural changes of organic matter. ACKNOWLEDGEMENT We gratefully acknowledge the technical and financial support received from the City of Mesa (Arizona). Principal funding was provided by the American Water Works Association Research Foundation (AWWARF) and the U.S. Environmental Protection Agency. Additional financial assistance was received through a post-doc scholarship administered by the Deutsche Forschungsgemeinschaft (DFG). The funding agencies assume no responsibility for the content of the research reported in this publication or for the opinions or statements of fact expressed in the report.
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REFERENCES Aiken, G., McKnight, D. M., Thorn, K. A. and Thurman, E. M. (1992). Isolation of hydrophilic organic acids from water using nonionic macroporous resin. Org. Geochem., 18(4),567-573. Amy, G., Bryant, C. and Belyani, M. (1987). Molecular weight distribution of soluble organic matter in various secondary and tertiary effluents. Wat Sci Tech., 19(3-4), 529-538. Crook, J. (1998). Water reclamation and reuse criteria. In: Wastewater Reclamation and Reuse. T. Asano (ed), Vol. 10, Water Quality Management Library, Technomic Publishing, Lancaster PA, pp. 627-704. Debroux, J. F. (1998). The physical-chemical and oxidant-reactive properties of effluent organic matter (EjDM) intended for potable reuse. Ph.D. thesis, University of Colorado. Drewes, J. E. and Jekel, M. (1998). Behavior of DOC and AOX using advanced treated wastewater for groundwater recharge. Wat. Res. 32(10), 3125-3133. Drewes, J. E. and Fox, P. (submitted). Impact of drinking water sources on reclaimed water quality. Water Environment Research. Drewes, J. E. and Fox., P. (1999). Behavior and characterization of residual organic compounds in wastewater used for indirect potable reuse. Wat Sci Tech, 40(4-5),391-398. Drewes, J. E., Sprinzl, M., Wendrock, A., Williams, M. D., Fox, P. and Westerhoff, P. (in press). Tracking residual dissolved organic carbon using XAD-fracttonallon and 13C-NMR sprectroscopy in indirect potable reuse systems. Vom Wasser. Fujita, Y., Ding, W. H. and Reinhard, M. (1996). Idennfication of wastewater dissolved organic carbon characteristics in reclaimed wastewater and recharged groundwater. Water Environment Research, 68(5), 868-876. Hedges, J. I. (1988). Polymerization of Humic Substances in Natural Environments. Humic Substances and Their Role in the Environment. Dahlem Konferenzen. F. H. Frimmel, R. F. Christman, J. Wiley and Sons Ltd., 45-58. Hejzlar, J. and Chudoba, J. (1986). Microbial polymers in the aquatic environment - I-III. Wat. Res. 20(10), 1209-1227. Huber, S. and Frimmel, F. H. (1996). Size-Exclusion Chromatography with Organic Carbon Detection (LC-OCD): A Fast and Reliable Method for the Characterization of Hydrophilic Organic Matter in Natural Waters. Vom Wasser. 86,277-290 (in German). Link, J., Gilbert, E. and Eberle, S. H. (1989). Investigation of properties and quantities of residual compounds in secondary effluents. Vom Wasser, 72, 349-370 (in German). Malcolm, R. L. (1985). Geochemistry of Stream Fulvic and Humic Substances. Aiken et al., Humic substances in soil, sediments and water. John Wiley and Sons. 181-209. Malcolm, R. L. (1989). Apphcations of Solid-state 13C_NMR Spectroscopy to Geochemical Studies of Humic Substances. Hayes et al., Humic Substances II. John WIley and Sons. 339-372. Malcolm, R. L. (1990). Factors to be considered in the isolation and characterization of aquatic humic substances. Meeting on Humic Substances In the Environment, Proceedings of Linkoping. Linkoeing University, Sweden, 390-417. Manka, J. and Rebhun, M. (1982). Organic Groups and Molecular Weight Distribution in Tertiary Effluents and Renovated Waters. Wat. Res., 16, 399pp. . Murthy, S. N. and Novak, J. T. (1998). Influence of Cations on Activated Sludge Effluent Quality. Proceedings WEFTEC '98. 71" Annual Conference Water Environment Federation, Orlando FL. Volume I. 309-324. National Research Council (1998). Issues in Potable Reuse. National Academy Press, Washington, D.C. Quanrud, D., Arnold, R. G., Clark, A., Massaro, M. and Wilson, L. G. (1998). Efficiency and sustainability of soil-aquifer treatment leading to wastewater reclamation and reuse. Proceedings AWWA Water Reuse 1998, 579-591.