Indirect ammonium oxidation by the product of bromide ozonation, hypobromous acid (HOBr), is expected at Br−/N ratios above 0.4 (Eusebi and Battistoni, 2016; Tanaka and Matsumura, 2003). Thus, Eusebi & Battistoni (2016) removed >40 mg ammonium nitrogen applying ozone dosage similar to that applied in the present study (7.3 mg O3/mg N) at high Cl− (14 g/L) concentration and high COD (11 g/L). Despite the Br/NH4-N ratio was equal to 4.4 in this study, no removal of NH4-N was observed.
Application of GAC for flowback water treatment
COD and DOC removal with three studied GAC types did not exceed 25% (Fig. 2; Table S4). Biopolymers and building blocks were removed to the highest extent (67% and 58% respectively), whereas removal of LMW acids did not exceed 30% and removal of LMW neutrals was negative. A possible explanation is that most of the LMW organic compounds are weak adsorbates at near-neutral pH of DAF-treated flowback water, e.g. adsorption of acetate rapidly decreases at pH > 6.0 (Kipling, 1948). Moreover, displacement of LMW organic compounds with larger molecules might occur (Velten et al., 2011).
Chemviron F400 showed the best performance (23% COD removal and 20% DOC removal), followed by Norit C Gran and Norit 830w. Meanwhile, HOC removal was higher with Norit 830w (45% removal) than with two other carbon types (27% removal). This can be attributed to the differences in the carbon surface chemistry, hence Chemviron F400 and Norit C Gran have acidic surface whereas Norit 830w has alkaline surface (Table S1). Hydrophobic compounds with a negative charge would not be removed by GAC with acidic surface due to the repulsion forces observed at near–neutral pH of DAF-treated flowback water.
Application of aerobic degradation for flowback water treatment
The efficacies of the COD and DOC removal reached 73% and 83% respectively after 48 h of incubation. (Fig. 2; Table S4). The COD was also measured in the liquid samples that were taken during incubation at 2, 5, 10 and 24 h showing that 71% removal was achieved already after 10 h of incubation. Further removal was not observed during next 38 h of incubation, indicating that ≈30% of organic matter was non-biodegradable. Similar COD removal efficacies were observed by other researchers, who used activated sludge adapted to high chloride concentrations for flowback water treatment (>50% COD removal after 6 h of batch incubation) and conventional oil produced water treatment (81% COD removal after 21 h of aeration in SBR) (Lester et al., 2015; Pendashteh et al., 2010).
SOUR during first hour of the incubation were between 25 and 40 mg/g VSS*h (Fig. 3), indicating high biological activity of sludge (He et al., 2017). Further decrease of SOUR can be attributed rather to the substrate depletion in the batch experiment, than to the inhibition of bioactivity. Neither oxygen consumption nor COD removal were registered in the abiotic control batches (results not shown), indicating that observed removal is attributed solely to biodegradation.
Fig. 3. COD removal and specific oxygen uptake rate (SOUR) (A), NH4-N and NO2-N removal (B), acetic acid and ethanol removal (C) during aerobic degradation experiments.
Partial NH4-N removal (47%) was observed during first 10 h of incubation (Fig. 3), Theoretical amount of nitrogen, which could be assimilated in the batch experiment by biomass assuming 50% carbon assimilation and empirical bacterial composition CH1.666N0.20O0.27 is equal to 18.1 mg/L. This value is close to the observed NH4-N removal of 17.6 mg/L after 10 h of incubation.
Additionally, formation of NO2, which is the main product of ammonium oxidation at high salinities, and NO3 was not observed (Cui et al., 2016; Vendramel et al., 2011). Therefore, nitrogen removal during aerobic treatment of flowback water occurs via assimilation by the biomass, and not via the nitrification.
Analysis of DOC in the flowback water and sludge liquor used for biodegradation experiments have shown that flowback water was the main source of organic carbon. Minimal possible removal of flowback water DOC fractions was calculated assuming that all DOC of sludge liquor is removed in biological experiments. The calculations showed high removal of DOC (79%) as well as its separate fractions (HOC, biopolymers, LMW neutrals and LMW acids) (Fig. 2; Table S4).
GC-FID analysis of VFA and alcohols in the liquid samples taken during aerobic incubation showed complete removal of acetic acid within 1 h, whereas ethanol removal required 5 h of incubation. Catabolic free energy of ethanol oxidation (-ΔG0 = 1325.5 kJ/mol) is higher than that of acetate oxidation (-ΔG0 = 893.7 kJ/mol), which in turn leads to higher cell yields coupled to growth on ethanol and, in theory, preferable utilization of ethanol as a substrate (Roden and Jin, 2011). However, first step of ethanol metabolism (oxidation to acetaldehyde) is endergonic, which limits bacterial catabolism of this substrate. Faster metabolism of acetic acid can be also explained by competitive inhibition of the ethanol catabolism by acetate.
Removal of individual organic compounds
Total IS-eq concentrations of the individual organic compounds detected in flowback water by LC-LTQ/HRMS in positive and negative ionization modes was 5.2 mg/L. Organic carbon makes up roughly 50% of the organic compounds, therefore, respective TOC is 2.6 mg/L (Mackinnon, 1981). This value is considerably lower than the organic carbon content of the building blocks fraction (46 mg/l) determined by LC-OCD analysis.
Molecular weight of the compounds, which make up fraction of building blocks, is 300–500 Da, thus the fraction is completely covered by the LC-LTQ/HRMS molecular weight resolution of 115–1300 Da. Therefore, <10% of potentially present structures were identified by LC-LTQ/HRMS, either because of the analytical limitations of the chromatographic separation, or due to the high detection limits (0.05 μg/L IS-eq).
DAF and ozonation did not improve removal of detected individual organic compounds, as shown by the IS-eq concentrations assigned to unique exact masses (Table S5) and total IS-eq concentrations (Fig. 4). 1,6-dioxacyclododecane, 7,12-dione was the only compound removed by DAF to <0.01 μg/L IS-eq among 27 unique exact masses determined in flowback water (Table 1). Removal of this volatile cyclic polyester dimer was most probably achieved by stripping after pressurized air release. At the same time IS-eq concentrations of PEG oligomers in flowback water increased, possibly due to the breakdown of higher molecular weight PEG polymers (Table 1).
Fig. 4. The fraction of the organic compounds detected in positive (A) and negative (B) ionization mode (Ct/C0), which was left after DAF, ozonation, bioegradation and adsorption to GAC. The horizontal line represents the average total concentration (n = 3) and the vertical line – minimal and maximal concentrations.
Application of GAC with acidic surface chemistry (Chemviron F400 and Norit C Gran) led to >99% removal of organic compounds (Fig. 4). Activated carbon with alkaline surface chemistry (Norit 830w) showed lower performance with 98% and 78% removal of organic compounds analysed in positive and negative mode respectively. These results are complementary to the observed removal of COD and DOC, which is higher for GAC with acidic surfaces (Fig. 2).
Removal of individual organic compounds detected in positive ionization mode did not change significantly in biodegradation experiments, whereas removal of the compounds measured in negative ionization mode decreased (Fig. 4). Organic compounds with higher molecular weight (Mw > 350 Da) were removed, whereas lower molecular weight compounds (Mw 200–350 Da) were formed after aerobic incubation (Table S5).
These compounds can be identified as intermediates of aerobic degradation, because their presence was proved neither in the flowback water, nor in the sludge liquor. One of these intermediates was identified as 1,3-dicyclohexylurea. However it is unclear, whether the intermediates are resulting from biodegradation of compounds present in flowback water or sludge liquor.
The only compound, identified by the suspect screening, namely 2-(2-butoxyethoxy)ethanol, was not removed during DAF. The compound was removed by all subsequent treatments tested: ozonation, sorption to activated carbon and biodegradation. 2-(2-butoxyethoxy) ethanol is a transformation product of 2-butoxyethanol, which is frequently used in the fracturing fluids and is not rejected by membranes, therefore removal of this compound and its transformation products prior to membrane filtration is highly desired.
Implications for flowback water management
Aerobic biological treatment is inexpensive relatively to physic-chemical processes for organic contaminants removal. However, it is rarely used for oil and gas produced water due to potential problems related to high salinity and presence of inhibitory compounds (Lefebvre and Moletta, 2006). Yet the present study together with the work of Lester et al. (2015) has shown that aerobic degradation targets easily degradable LMW organics that comprise the majority of organics in shale gas FPW. Yet, aerobic degradation at higher chloride concentrations than applied in this study (32 g/L) will be a challenging task (Akyon et al., 2015; Jiménez et al., 2018). Combination with other industrial wastewaters with lower salinities or adaptation of sludge to higher concentrations of Cl-may be a solution for flowback waters with high salinities.
Whereas LMW organic acids and alcohols are removed by aerobic degradation, fractions with higher molecular weight are only partially removed, as shown by DOC fractionation and analysis of individual organic compounds. In contrast, activated carbon can effectively remove compounds with Mw 115–1300 Da not affecting LMW organic acids and alcohols. Sorption to activated carbon is also efficient towards removal of other classes of organic pollutants, which commonly occur in shale gas FPW but were not detected in this study, namely PAHs, chlorinated organics and total petroleum hydrocarbons (Alzahrani and Mohammad, 2014; Pavoni et al., 2006; Rosenblum et al., 2016).
Hence combination of aerobic degradation with GAC filtration can target different fractions and classes of organic compounds. The processes can be applied as an intermediate treatment step between primary treatment and TDS removal in order to decrease membrane fouling, or prior to the direct flowback water reuse, if equipment fouling due to elevated concentrations of organic compounds is observed at the production sites (Alzahrani and Mohammad, 2014). Additional studies on the changes of flowback water toxicity during aerobic degradation and GAC filtration is needed, since only a small number of organic compounds can be identified using existing non-target screening methods.
Composition of shale gas flowback water differs significantly between production sites and even between wells within a single play (Shih et al., 2015). Hence, each full-scale application will require an individual approach, including characterization of flowback water and evaluation of its biological treatability. Flowback water may be combined with other types of industrial wastewater with lower salinity to offer optimal solutions for equalization of peaks in flow and composition, including salinity. Thus, environmentally safe discharge or re-use of such water streams may be facilitated, which is especially relevant for agricultural/industrial regions where water scarcity is paralleled by shale gas production.
The studied flowback water was characterised by the high concentrations of organic compounds, as indicated by COD (1800 mg/L) and DOC (649 mg/L) concentrations. Organic compounds in the studied flowback water were dominated by LMW acids and neutrals, with acetic acid and ethanol being the most abundant LMW compounds. Only a small number of individual organic compounds, including PEG-oligomers and presumable fracturing fluid additives 1,6-dioxacyclododecane, 7,12-dione and 2-(2-butoxyethoxy)ethanol were identified using non-target LC-LTQ/HRMS screening.
DAF applied for flowback water pre-treatment did not change COD and DOC, as well as concentrations of organic fractions and individual organic compounds significantly. This indicates necessity for specific organic removal, especially when the present organics cause fouling on the membranes frequently applied for TDS removal or on the drilling and fracturing equipment, when the water is going to be reused for the next fracturing operations without desalination step (Shaffer et al., 2013).
Ozonation was shown to be inefficient towards removal of organic compounds. Aerobic degradation removed >70% of DOC, mainly targeting LMW organics. In addition, GAC filtration removed fractions of DOC with higher molecular weight, including LC-LTQ/HRMS-detected individual organic compounds with molecular weight between 115 and 1300 Da. Combination of both technologies is proposed for efficient organic removal prior to desalination or flowback water reuse.
This work was funded by the Netherlands Organisation for Scientific Research (NWO) Earth and Life Sciences (ALW), project number 859.14.001, and the water utilities Brabant Water, Oasen and WML. The express their gratitude to prof. Grzegorz Pieńkowski, dr. Monika Konieczyńska (Polish Geological Institute) and Andrzej Panuszewski (Conspan sp. Z.o.o.) for their support in delivery of the shale gas flowback water samples for this study. The authors would like to thank Perry van der Marel (WLN) for providing activated sludge adapted to high salinities from industrial WWTP at Delfzijl (the Netherlands). The authors appreciate the contribution of Wolter Siegers and Margo van der Kooi (KWR) to the experimental part of the study.
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