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Evaluation of a Pilot Anaerobic Secondary Effluent for Potable Reuse: Impact of Different Disinfection Schemes on Organic Fouling of RO Membranes and DBP Formation Aleksandra Szczuka, Juliana Berglund-Brown, Hannah Chen, Amanda Quay, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05473 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019
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Evaluation of a Pilot Anaerobic Secondary Effluent for Potable Reuse: Impact of
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Different Disinfection Schemes on Organic Fouling of RO Membranes and DBP
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Formation
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Aleksandra Szczuka1,2, Juliana P. Berglund-Brown1,2, Hannah K. Chen1, Amanda N.
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Quay1, and William A. Mitch1,2,*
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1 Department
of Civil and Environmental Engineering, Stanford University, 473 Via
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Ortega, Stanford, California 94305, United States
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2National
Science Foundation Engineering Research Center for Re-Inventing the
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Nation's Urban Water Infrastructure (ReNUWIt), 473 Via Ortega, Stanford, California
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94305, United States
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*Contact Information: email:
[email protected], Phone: 650-725-9298, Fax: 650-
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723-7058
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ABSTRACT
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Anaerobic biological secondary treatment has the potential to substantially
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reduce the energy cost and footprint of wastewater treatment. However, for utilities
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seeking to meet future water demand through potable reuse, the compatibility of
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anaerobically-treated secondary effluent with potable reuse trains has not been
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evaluated. This study characterized the effects of different combinations of chloramines,
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ozone and biological activated carbon (BAC), applied as pretreatments to mitigate
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organic chemical fouling of reverse osmosis (RO) membranes and the production of 43
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disinfection byproducts (DBPs). The study employed effluent from a pilot-scale
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anaerobic reactor and soluble microbial products (SMPs) generated from a synthetic
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wastewater. Ozonation alone minimized RO flux decline by rendering the dissolved
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organic carbon (DOC) more hydrophilic. When combined with chloramination, ozone
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addition after chloramines maintained a higher RO flux. BAC treatment was ineffective
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for reducing the pressure and energy requirements for a set permeate flux. Regardless
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of pre-treatment method prior to RO, the total DBP concentrations were 1 mg/L as Cl2 total chlorine residual. The
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total chlorine residual was quenched with 33 mg/L ascorbic acid, and extracted within 4 h for
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DBP analysis.
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Analyses and toxicity calculation: Analytical methods for basic water quality parameters
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applied to filtered water samples are provided in Text S1. Forty-three compounds were
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measured in samples collected from the sample points indicated in Figure 1 using gas
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chromatography and mass spectrometry-based modified US EPA Methods described
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previously47 and summarized in Text S2. Thirty-five halogenated DBPs, including 4
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regulated THMs, 6 iodinated THMs (I-THMs), 10 HAAs (including iodoacetic acid), 4
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haloacetonitriles, 4 haloacetamides, 4 haloacetaldehydes, 2 haloketones and
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chloropicrin were measured in triplicate with ~0.2 g/L method reporting limits (MRLs).
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Eight N-nitrosamines were measured in duplicate with ~2 ng/L MRLs.
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The contribution of a DBP to the toxicity of a disinfected water is a function of both its concentration and its toxic potency. Measured DBP concentrations were divided
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by metrics of toxic potency to evaluate their contribution to the DBP-associated toxicity
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of the disinfected waters. The metrics of toxic potency focused on DBP concentrations
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associated with a 50% reduction in the growth of Chinese hamster ovary (CHO) cells
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compared to untreated controls (i.e., LC50 cytotoxicity values) for halogenated DBPs.
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Concentrations associated with a 50% lifetime excess cancer risk (i.e., LECR50 values)
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were used for N-nitrosamines, because these values are lower than LC50 cytotoxicity
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values (see Text S3 for further discussion). The purpose was not to estimate an
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absolute risk, but to estimate the relative importance of the individual DBPs for the DBP-
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associated toxicity. The toxicity-weighting procedure, and the same toxic potency
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metrics, have been employed previously 22,33,47-50.
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RESULTS AND DISCUSSION
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Water quality. Table S1 provides the basic water quality parameters for the
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untreated effluents of the SAF-MBR and the lab-scale AFBR reactors. The bromide
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concentration was higher in the SAF-MBR effluent (227 g/L) than the lab-scale AFBR 19 ACS Paragon Plus Environment
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effluents (~50 g/L). DOC, the other major difference between the effluents, was 3.5
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mg/L for the SAF-MBR effluent. The batches of effluent collected from the AFBRs fell
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into two groups, with effluents used for ozonation, chloramination, and
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ozonation/chloramination experiments (Group I) featuring 2.2 mg/L DOC and samples
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used for ozonation/BAC and ozonation/BAC/chloramination experiments (Group II)
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containing 1.6 mg/L DOC. Because the DOC concentration should impact RO fouling,
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each fouling experiment with AFBR effluent compared pre-treated effluent to a
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concurrent untreated control. The SAF-MBR treated filtered sewage with a higher
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strength (600 mg-COD/L) than the acetate and propionate-based synthetic sewage (250
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mg-COD/L) supplied to the AFBRs. The higher DOC concentration in the SAF-MBR
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effluent may reflect the higher influent COD and the occurrence of non-biodegradable
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organics in the sewage. Regardless, all of the reactors achieved ~20 mg-COD/L in the
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effluent (≥90% removal).
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At 44-62 mg-N/L, ammonia was the main inorganic nitrogen species, as
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expected for the anaerobic effluent. The effluent pH ranged from 6.9-7.1 upon sample
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collection, but increased within 5 h to pH 8.0, likely due to off-gassing of carbon dioxide. 20 ACS Paragon Plus Environment
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While pH has been shown to affect RO membrane fouling35, 36, back-titrating the pH to
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7.0 with HCl showed that the difference in pH did not affect membrane fouling (Figure
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S1). The carbohydrate and protein concentrations measured in a filtered pilot-scale
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AFBR sample were 13.8 ( 1.1) mg-glucose/L and 12.6 ( 1.3) mg-BSA/L, respectively.
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The carbohydrate concentration was comparable to the 4.9-12.3 mg-glucose/L levels
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measured in aerobic MBR effluents, while the protein concentration was higher than the
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1.0-4.3 mg-BSA/L levels measured in aerobic MBR effluents51,52. In the AFBR influent,
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6.0 ( 0.1) mg-S/L sulfate and
