Monitoring Estrogen Compounds in Wastewater ... - Springer Link

Water Air Soil Pollut (2008) 188:31–40 DOI 10.1007/s11270-007-9498-6

Monitoring Estrogen Compounds in Wastewater Recycling Systems Deborah M. Kvanli & Sreelatha Marisetty & Todd A. Anderson & W. Andrew Jackson & Audra N. Morse

Received: 15 May 2007 / Accepted: 13 August 2007 / Published online: 21 November 2007 # Springer Science + Business Media B.V. 2007

Abstract The presence of pharmaceuticals and personal care products (PPCPs) and endocrine disrupting chemicals (EDCs) in treated wastewater is gaining attention due to their potential environmental impact. An analytical method was developed to quantify estrogen compounds in samples from a concentrated wastewater matrix typical of water recycling systems used in space. The method employed conventional HPLC with UV detection. Solid phase extraction (SPE) was used to isolate the compounds of interest from wastewater. Spikerecovery tests in clean and wastewater matrices were used to test the extraction process. The results of these experiments suggest that deconjugation is the most predominant reaction occurring in the systems, as effluent concentrations of free estrogens typically exceeded influent concentrations. Despite the long retention times of the system or the near infinite solids retention time, free estrogens were not removed from graywater representative of space D. M. Kvanli : T. A. Anderson Department of Environmental Toxicology, Texas Tech University, Lubbock, TX 79409, USA S. Marisetty : W. A. Jackson : A. N. Morse (*) Department of Civil and Environmental Engineering, Texas Tech University, Box 41023, Lubbock, TX 79409-1023, USA e-mail: [email protected]

waste streams. For a closed-loop wastewater treatment system, these compounds may accumulate to levels requiring other removal mechanisms (i.e., reverse osmosis). Keywords Estradiol . Estrone . 17β-estradiol . 17α-ethynylestradiol . Graywater . Water reuse

1 Introduction Micropollutants are compounds present in μg/l (ppb) quantities in environmental or waste samples. The term is currently applied to pharmaceuticals (antibiotics, endocrine-disrupting compounds (EDCs), etc.) and other personal care products (PCPs) that may have environmental consequences. Although some PPCPs have been identified in the environment for several decades, the fate and persistence of these compounds are not well understood. PPCPs are of interest due to their wide occurrence in the aquatic environment and potential hazard to aquatic organisms. The release of these chemicals into the environment is primarily attributed to their incomplete removal by waste treatment systems (Gomes et al. 2005). For example, EDCs such as natural and synthetic estrogens are of potential concern because of their endocrine activity at low concentrations. A majority of these estrogens are excreted in urine as conjugates, which are biologically inactive (Belfroid et al. 1999;

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Gomes et al. 2005). In raw wastewater and in the wastewater treatment process, the inactive conjugates of natural and synthetic estrogens can potentially be cleaved, resulting in the release of the active (parent) compound (Ternes et al. 1999). Excretion rates of estrogen compounds are highly variable depending on age, phase of menstrual cycle, and race, resulting in varying EDC concentrations in wastewater. The two most important EDC removal processes in biological wastewater treatment such as an activated sludge treatment plant are sorption to sludge particulate matter and biodegradation (Andersen et al. 2004). Bacterial populations may vary in their ability to degrade estrogens (Layton et al. 2000). A long sludge retention time (SRT) and hydraulic retention time (HRT) have a positive influence on the ability of an activated sludge system to remove estrogens in wastewater treatment plants (WWTPs; Andersen et al. 2004; Johnson et al. 2004; Chang et al. 2006). Estrogens may be partially incorporated into sludge by sorption during wastewater treatment (Andersen et al. 2004); therefore, increasing the concentration of sludge solids may reduce sludge estrogenic activity more readily and increase removal by sorption (Hermanowicz and Wozei 2002). Several factors, such as type and age of the plant, geographic location, composition of feed stream, estrogen substrate specificity, and temperature, affect estrogen removal in WWTPs (Layton et al. 2000). Estrogenic activity has been identified in wastewater solids and field studies have confirmed that estrogens can partition to the solid phase in WWTPs (Gomes et al. 2005). However, it is reported that less than 3% of the estrogenic activity was found in the sludge (Körner et al. 2000fs) and only about 5% of the estrogens were adsorbed onto digested wastewater sludge (Andersen et al. 2003). Thus, the occurrence and behavior of these compounds in wastewater is important for evaluating the removal efficiency of treatment and the environmental risk associated with the discharge of treated wastewater (Gomes et al. 2005). Even more importantly, the risk of reusing such water should be considered. Due to drought and population growth in the U.S. and the rest of the world, countries and municipalities are implementing various levels of wastewater recycling. Currently, most municipal wastewater is recycled for non-potable uses such as irrigation of highway-rights of way, golf courses, and yards (Angelakis et al. 2003). However, drought may force municipalities to pursue

Water Air Soil Pollut (2008) 188:31–40

wastewater as a potable water source and the fate and persistence of EDCs should be considered. In wastewater recycling, nanofiltration and reverse osmosis (RO) has been considered fail safe mechanisms to remove EDCs from wastewater; however, a study has shown that EDCs may be released from the membrane during backwashing or at high pH variations (Nghiem et al. 2004). As such, RO and other membrane processes may be best used as a polishing step rather than a treatment process to reduce human exposure to EDCs. Therefore, the fate and removal of EDCs during wastewater treatment and recycling is critical so that the loading to downstream processes (i.e., RO) is known. In order to support long-term space flights, such as a lunar base or a mission to Mars, NASA must recycle wastewater for potable water consumption. However, their wastewater is different than terrestrial wastewater as fecal matter is separated at the source; the wastewater would be more appropriately described as graywater. Space graywater composition varies depending on mission duration and facilities available on the mission, including crop production, laundry facilities, dishwashing facilities, and food production. The highest strength graywater waste stream is the transit mission graywater (Table 1; Morse et al. 2007). The graywater produced during the transit mission comes from urine, collected humidity condensate from human and machine respiration, and urinal flush water. The most commonly studied space related graywater is the early planetary base waste stream, which is composed of waste streams from urine, urinal flushing, hand washing,

Table 1 Transit mission and early planetary base graywater composition (Jackson and Morse 2005; Morse et al. 2007)

Urine Soap Disodium cocoamphodiacetate Sodium laureth sulfate Humidity Condensate A Humidity Condensate B Humidity Condensate C Water All units are g/kg

Early Planetary Base

Transit Mission

131

396 – –

0.08 0.05 0.205 0.205 0.205 863

– 2.2 2.2 2.2 598


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laundry, showers, humidity condensate, and food processing waste streams (Table 1; Jackson and Morse 2005). Disodium cocoamphodiacetate and sodium laureth sulfate are the surfactants in the space graywater, resulting from astronaut hand washing and showering activities. The early planetary base graywater composition is unique due to the high concentration of urine compared to general terrestrial graywater streams. As such, the early planetary base graywater contains high concentrations of dissolved organic carbon (DOC; approximately 470 mg/l), NH4þ N (approximately 480 mg/l), and total nitrogen (620 mg/l; Jackson and Morse 2005) when compared to typical terrestrial wastewater containing 17 mg/l of NH4þ N and TKN-N of 29 mg/l (Metcalf and Eddy, Inc. 2003). The transit mission graywater contains DOC (approximately 1,100 mg/l), NH4þ N (approximately 1100 mg/l), and total nitrogen (1,900 mg/l; Morse et al. 2007). The fraction of urine in space waste streams is 40% for transit mission graywater and 13% for early planetary base wastewater; therefore, constituents excreted in urine, such as PPCPs and EDCs, are expected to enter space graywater processing systems at higher concentrations than terrestrial systems due to the low dilution of urine by other waste streams. Consequently, the presence and fate of these compounds may be different than in terrestrial systems. The purpose of this work was to evaluate the fate of EDCs (natural and synthetic estrogens) present in high strength graywater in biological treatment systems. The experiments focused on wastewater recycling systems (WRS) such as those used (and proposed for use) by NASA in space and the fate of estrogen compounds in these systems. An analytical method for the analysis of samples from a concentrated wastewater matrix was developed and applied to monitor EDCs in influent and effluent reactor samples. These experiments were

performed using the Texas Tech University-Water Recovery System (TTU-WRS), which treats the early planetary base waste stream and the Urine-Humidity Condensate System (UHC), which treats the transit mission wastewater (Table 2). Although the work was performed for NASA to answer their concerns of the fate of EDCs in space systems, the results are applicable to terrestrial systems. As freshwater supplies for potable water consumption become scarce, municipalities will be forced to recycle wastewater and/or implement water conservation measures. In the case of the latter, the volume of dilution streams will decrease and the percent urine contribution to the wastewater will increase. As such, the loading of EDCs will increase. Although the urine in terrestrial graywater may never be as concentrated as the transit mission graywater, the early planetary base graywater is an applicable graywater to study as more and more water conservation controls are implemented terrestrially.

2 Materials and Methods 2.1 Estrogen Analysis Estrogen compounds (estrone, E1; 17β-estradiol, E2; 17α-ethynylestradiol, EE2; estriol, E3, estradiol acetate, EA) were obtained as solids from Sigma (St. Louis, MO). Purities for the various estrogen compounds ranged from 97 to 100%. All stock standards were made in acetonitrile (HPLC grade) and diluted with milli-Q water (>18 MΩ) to the final concentration of working standards (50 μg/l). Initially, efforts focused on the analysis of the estrogen compounds using HPLC. Estradiol acetate was used as an internal standard. An HPLC-UV method was developed based in part on published

Table 2 TTU-WRS and UHC system configuration (Jackson and Morse 2005; Morse et al. 2007) TTU-WRS

Length (m) Internal diameter (m) Total volume (l) Working volume (l) Packing surface area (m2)

UHC System

Packed Bed Reactor

Tubular Reactor

Packed Bed Reactor

Membrane-aerated Reactor

0.45 0.008 3.5 2.2 0.12

15.5 3.18×10−3 0.12 0.10 0.31

0.457 0.008 1.6 1.1 0.12

0.457 0.01 3.7 3.6 0.825

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methods in the literature (Ferguson et al. 2001; Lopez de Alda and Barcelo 2001). The method provided excellent resolution of the compounds of interest in both clean (milli-Q) water and wastewater matrices (Fig. 1). In addition, the method provided an adequate calculated limit of detection (determined using guidelines in U.S. EPA 2000), assuming that 1 l of water could be extracted (Table 3). With an adequate analytical method in place, work focused on developing extraction methods for the estrogen compounds in water. This method development exercise was conducted in two phases. Initially, various solid phase extraction (SPE) cartridges were tested for the ability to remove estrogen compounds from clean (milli-Q) water. Various SPE sorbent types as well as sorbent masses were investigated. Subsequently, method development experiments on “dirty” water (influent/effluent water from the TTU-WRS) were initiated based on the results of the previous tests. 2.2 Graywater Treatment Systems The fate of free estrogen compounds in two graywater treatment systems was evaluated. Both systems were denitrification-nitrification systems; however, the reactors within the systems differed. The TTU-WRS is a scaled-down version of a water recovery system designed at Johnson Space Center (Campbell et al. 1000

Detector Response

800 E1 600

EE2 E2 EA

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Fig. 1 HPLC-UV chromatogram of estrogen compounds (estriol, E3; estradiol, E2; ethynylestradiol, EE2; estrone, E1; estradiol acetate, EA) in a wastewater matrix. The mobile phase was a water and acetonitrile gradient at 0.8 ml/min; 30:70 v/v initially then to 70:30 over 35 min). A RP-C18 column (250 mm×4 mm i.e., 5 μm) was used for separation. Detection of the analytes of interest was by UV at 220 nm

Table 3 Calculated detection limits for estrogen compounds obtained from HPLC-UV analysis of spiked water Chemical

SDa

MDL in water (μg/l)

Estriol (E3) 17β-estradiol (E2) 17α-ethynyl estradiol (EE2) Estrone (E1) Estradiol acetate (EA)

0.011 0.007 0.012 0.012 0.007

0.06 0.04 0.07 0.07 0.04

MDL was determined using guidelines from EPA, 2000 where MDL=SD×t (99%; n-1 df) and assuming 1 l of water was extracted z

Standard deviation of seven replicate measurements.

2003a, b). The TTU-WRS utilized two biological attached growth reactors: an anaerobic packed bed reactor for simultaneous denitrification and DOC removal, and a tubular aerobic reactor for nitrification with a recycle loop (Muirhead et al. 2003; Jackson et al. 2004). The major function of the recycle loop was to supply oxidized nitrogen to the packed bed reactor in order to reduce DOC and to release the nitrogen gas produced to the atmosphere. The graywater entered the system in the raw feed tank and was collected in an effluent tank. The liquid raw feed flow rate for the system was 0.7 ml/min, while the recycle flow rate was 7 ml/min. The system hydraulic residence time (HRT) was 2.5 days. The solids residence time (SRT) for the packed bed reactor was considered infinite; however, the SRT for the tubular reactor was 7 days. Due to the reactor configuration, the tubular reactor shed an average of 0.33 g of dried biomass per week (Diaz et al. 2006). In general, the system achieved 80% nitrification, 90% DOC removal, and 60% denitrification. The estrogen study period in the TTU-WRS was approximately 60 days. Urine donations in this study were approximately 50/50 male/female urine. Influent and effluent EDC samples from the TTUWRS were collected daily for 9 weeks and composited to generate an average weekly influent and effluent. Prior to the analysis, water samples were filtered through 0.2 μm filter. Other water quality samples were also collected and analyzed for pH, DOC, TN, and NH4þ N to assess the performance of the TTU system (Jackson and Morse 2005). For treatment of the transit mission graywater, the UHC system consisted of a packed bed reactor and a membrane-aerated reactor (Morse et al. 2007; McLamore et al. 2007). Nitrification occurred in the


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membrane-aerated reactor, while the packed bed reactor was used for denitrification and DOC removal. The influent was fed to the reactors at a flow rate of 0.6 ml/min. The recycle flow rate was 10 ml/min. The HRT for the system was 3.24 days. Unlike the TTU-WRS, the SRT for both the membrane-aerated reactor and packed bed reactor were infinite (Morse et al. 2007). Influent and effluent EDC samples from the UHC system were collected daily for approximately 1 month in a procedure similar to that for the TTUWRS. Additionally, the system was monitored by comparing the free estrogen influent and effluent water quality. The study period for the UHC experiment was 20 days. Urine donations in this study were exclusively female for an evaluation of the worst-case estrogen loading to the treatment system.

study. A 0.5-g Strata-X® SPE cartridge actually provided better recovery of the test compounds than a 1-g Strata-X® SPE cartridge (Table 4). Once confident that adequate and reproducible recoveries of estrogen compounds could be obtained from clean water, method development experiments on dirty water (influent/effluent water from the reactor) were initiated. Results of these experiments (Table 5) were consistent with recoveries from clean water, although the overall recoveries were slightly lower (66–102%), especially for the internal standard, estradiol acetate (EA). HPLC-UV chromatograms from these tests had slightly more background than extracts of clean water (data not shown); however, detection and resolution of estrogen compounds was not compromised. 3.2 WRS Samples

3 Results and Discussion 3.1 Extraction Method Development Results of the evaluation of SPE sorbents for estrogen compounds spiked into clean water are presented in Table 4. HPLC-UV chromatograms from the analysis of this water were consistent with previous results for estrogen compound standards, indicating no analytical complications with this type of sample (data not shown). In addition, recoveries of estrogen compounds, including the internal standard, were good (83–118% for a 2-g SPE cartridge). Larger masses of SPE sorbents typically provide increased extraction capacity, translating to better recoveries of compounds from water. Larger capacity SPE cartridges were also useful for samples with a less than clean matrix. However, this was only partially true in this

Table 4 Percent recoveries (±SD) obtained from HPLC-UV analysis of mili-Q water (>18 MΩ) spiked (50 μg/l) with each estrogen compound and extracted using SPE (n=3) Chemical

Strata-X® (1 g)

Strata-X® (0.5 g)

C18 (2 g)

Estriol 17β-estradiol 17α-ethynyl estradiol Estrone Estradiol acetate

44±20 26±18 27±17 55±14 16±16

97±25 94±29 93±28 94±12 75±24

118±1 100±10 98±9 97±7 83±8

The results obtained from the analysis of influent and effluent EDC samples from the TTU-WRS are presented in Fig. 2. The analytical method developed was successful in determining estrogen compounds in these concentrated graywater samples. Estradiol and estrone were more frequently detected in both sample types than estriol and ethynylestradiol. Estradiol concentrations in the influent and effluent of the TTU-WRS system were ND–40.3 μg/l and 10.8– 25.2 μg/l, respectively. Estrone concentrations in the system influent were all less than 36.1 μg/l; however, the effluent concentrations ranged from ND to 30 μg/l. Estriol concentrations exceeded the detection limit on three occasions, but the concentrations were less than 6 μg/l. Fluctuations in the influent concentrations of estrogen compounds occurred; this is likely due to the high variation in excretion rates attributed to the menstrual cycles of individuals contributing urine. The age range of the individuals donating urine during the project was 18 to 24. Therefore, the variation of estrogen concentrations with age is expected to be less significant (Gomes et al. 2005). Additionally, none of the donors were pregnant during the study; however, donors may have been using contraception. Further analysis of estradiol in the influent and effluent EDC samples indicated that initially, estradiol concentrations in the effluent were lower than concentrations in corresponding influent samples.

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Table 5 Percent recoveries (±SD) obtained from HPLC-UV analysis of different types of water spiked (50 μg/l) with each estrogen compound and extracted using C-18 SPE (n=3) Chemical

Milli-Q water

Influent/effluent water

Estriol 17β-estradiol 17α-ethynyl estradiol Estrone Estradiol acetate

118±1 100±10 98±9 97±7 83±8

79±22 102±3 98±2 102±3 66±7

gation, resulting in higher effluent concentrations. The results of this study are supported by a study performed by D’Ascenzo et al. (2003), wherein deconjugation of estrogens was observed in a wastewater holding tank and sewer system. Deconjugation was confirmed via laboratory biodegradation tests. 3.3 UHC Samples

This suggests that estradiol was microbially transformed in the reactor. Subsequently, effluent concentrations of estradiol increased to levels higher than those in the corresponding influent samples. This suggests that a conjugated form of estradiol may have entered the reactor and was deconjugated by microbial processes, but not further degraded. The researchers had hypothesized effluent concentrations of estrogen compounds would be higher than influent concentrations based on conjugated forms of the compounds entering the reactor followed by deconju-

The results obtained from the analysis of influent and effluent samples from the UHC are presented in Fig. 3. As expected, the estrogen concentrations were higher than those observed in the TTU-WRS, which is due to the higher concentration of urine in the feed solution of the UHC system and the use of all female urine during the study period. One noticeable difference between the estrogen concentrations in the UHC graywater stream as compared to the graywater treated in the TTU-WRS is the presence of ethynylestradiol and the reduced estrone concentrations. Although one sample had an estrone concentration exceeding 40 μg/l, estrone, when detected was

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Fig. 2 Influent and effluent concentrations of estradiol, estrone, estriol, and ethynylestradiol in the TTU-WRS

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Fig. 3 Influent and effluent concentrations of estradiol, estrone, estriol, and ethynylestradiol in the UHC

typically less than 10 μg/l. Ethynylestradiol concentrations peaked at the beginning of the study (concentrations exceeding 80 μg/l); however, ethynylestradiol concentrations were less than 20 μg/l during the remainder of the study. Estriol, when detected (four influent samples, four effluent samples), peaked up to 60 μg/l but were typically less than 20 μg/l. Overall, the observed concentrations of estrogen concentrations fluctuated in influent samples from the UHC system. Again, this may be attributed menstrual cycle phase of the persons contributing the urine. Only estradiol concentrations were consistently present in the UHC waste stream, likely due to insufficient time for production and metabolism of natural estrogens in the treatment process. Even though other free estrogen compounds did not occur consistently, on certain days their concentrations peaked as high as 68.9 μg/l for estradiol, 140 μg/l for ethynylestradiol, 59 μg/l for estriol, and 129 μg/l for estrone.

3.4 Discussion As observed herein and confirmed in the literature (Gomes et al. 2005), estrogen concentrations in graywater influent are highly variable. Another compounding factor was the different stages of the menstrual cycle, which was not monitored during the study. However, the increase in estradiol concentrations in the UHC system from October 5–October 8 suggests that donors may have been entering the pre-ovulation phase of their menstrual cycle whereas the increase in estradiol between October 17 and October 19, suggests the donors may have been entering the luteal phase of their menstrual cycle. The estrone and estriol data follow similar patterns as estriol, further supporting the menstrual cycle phase changes. Using batch culture, Ternes et al. (1999) demonstrated that ethynylestradiol persists for a relatively long time as the half-life is 10 times longer than natural estrogens. The data herein supports those

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observations. Additionally, ethynylestradiol was more frequently detected in the system effluent (n=9) than the influent (n=7). Overall, the free estrogen concentrations, when detected, exceeded values reported by others in the literature. Gomes et al. (2005) reported estrone concentrations of 0.02 μg/l, estriol concentrations of 0.04 μg/l, and estradiol concentrations of 0.06 μg/l in raw wastewater. However, ethynylestradiol was not detected in the raw wastewater of that study. The estrogen concentrations measured in this study were approximately 1,000 times greater than estrogen concentrations in the Gomes et al. (2005) study and the Komori et al. (2004) study investigating free and conjugate estrogen wastewater concentrations in Japan, highlighting the uniqueness of small dilution streams and their impact on estrogen concentrations in municipal wastewater. As municipalities continue to push water conservation techniques such as low flush toilets, low flow shower heads, or in the case of water rationing during drought conditions, estrogen concentrations are likely to increase, but still be less than the values reported herein this study. Complicating a comparison between potential estrogen compounds during drought water conservation methods is fecal matter. Fecal matter will be included in the wastewater but was not included in the space graywater and subsequently may increase the load of estrogens in the wastewater. HRT is an important factor in free estrogen removal in wastewater treatment systems. Previous research (Chang et al. 2006) demonstrated that estrone may be completely removed in an activated sludge batch culture in 72 h. Although the HRT for the TTU-WRS was less than 72 h (60 h) the HRT of the UHC systems was approximately 78 h. Therefore, influent estrone should have been removed; however, the detention time does not account for flux of estrone produced due to estradiol breakdown, which has been observed to occur in approximately 10 h (Chang et al. 2006). In the same study, 50% of the estradiol was removed in an activated batch culture in 10 and 25.2 min, which was a rate significantly faster than observed in this study. In fact, estradiol was typically detected at the highest concentrations of all the free estrogens in both systems and was typically detected in the effluent of the system. Estradiol and estrone removal was monitored in membrane bioreactors and both compounds were removed in approximately 50 h


(Chang et al. 2006). As such the retention times in this study should have been sufficient for free estrogen removal but were not. Deconjugation is another factor important to the results of this study. Estrogens are typically excreted in the conjugate, inactive form (Gomes et al. 2005); however, the compounds detected herein are the free form. The presence of the free compound suggests that the urine holding time (i.e., time from donation to feed solution preparation) and the retention time of the treatment systems was sufficient to allow for the formation of active estriol and estrone in both the treatment systems. Typically, the highest concentrations of estrogens excreted in urine are estriol > estrone > estradiol. However, in this study, the predominant form of free estrogens were estradiol > estrone > estriol. The free estrogen concentrations in this study suggest the space graywater would have estrogenic activity and additional treatment steps such as RO or nanofiltration (Nghiem et al. 2004) would be needed to protect the health of users of the graywater, which in this case would be astronauts. As indicated previously the sorption of estrogens to solids is a primary removal mechanism. However, the data in this study show that free estrogen concentrations are highly variable and no clear conclusions could be drawn regarding sorption to biosolids. Additionally, an infinite SRT did not appear to aid in estrogen removal. In general, free estrogen concentrations were typically greater in the effluent of the systems than the influent suggesting that deconjugation was occurring rather than removal mechanisms such as sorption to solids or biodegradation (D’Ascenzo et al. 2003).

4 Conclusion The purpose of this study was to evaluate the fate of free estrogens (estradiol, estrone, estriol) and a synthetic estrogen (ethynylestradiol) in high strength graywater biologically treated. The estrogen concentrations measured in this study were highly variable; however, estradiol and estrone concentrations were greater than estriol, which was typically not detected in the system. Ethynylestradiol was detected at high levels (up to 140 μg/l) in the UHC system wherein the only urine source for the study was female urine.


The results of these experiments suggest that deconjugation is the most predominant reaction occurring in the systems, as effluent concentrations of free estrogens typically exceeded influent concentrations. Free estrogens were not typically removed. In the case of reusing this graywater in space applications, additional RO or other membrane filtration techniques to remove the free estrogens from the graywater would be required (Chang et al. 2006; Nghiem et al. 2004). Despite the long retention times of the system or the infinite solids retention time, free estrogens were not removed from graywater representative of space waste streams.

Acknowledgements The authors would like to thank the Center for Space Sciences at Texas Tech University and NASA Johnson Space Center for funding the work.

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