Sulfide-Induced Dissimilatory Nitrate Reduction to Ammonium Supports Anaerobic Ammonium Oxidation (Anammox) in an Open-Water Unit Process Wetland.

Open-water unit process wetlands host a benthic diatomaceous and bacterial assemblage capable of nitrate removal from treated municipal wastewater with unexpected contributions from anammox processes. In exploring mechanistic drivers of anammox, 16S rRNA gene sequencing profiles of the biomat revealed significant microbial community shifts along the flow path and with depth. Notably, there was an increasing abundance of sulfate reducers (Desulfococcus and other Deltaproteobacteria) and anammox microorganisms (Brocadiaceae) with depth. Pore water profiles demonstrated that nitrate and sulfate concentrations exhibited a commensurate decrease with biomat depth accompanied by the accumulation of ammonium. Quantitative PCR targeting the anammox hydrazine synthase gene, hzsA, revealed a 3-fold increase in abundance with biomat depth as well as a 2-fold increase in the sulfate reductase gene, dsrA These microbial and geochemical trends were most pronounced in proximity to the influent region of the wetland where the biomat was thickest and influent nitrate concentrations were highest. While direct genetic queries for dissimilatory nitrate reduction to ammonium (DNRA) microorganisms proved unsuccessful, an increasing depth-dependent dominance of Gammaproteobacteria and diatoms that have previously been functionally linked to DNRA was observed. To further explore this potential, a series of microcosms containing field-derived biomat material confirmed the ability of the community to produce sulfide and reduce nitrate; however, significant ammonium production was observed only in the presence of hydrogen sulfide. Collectively, these results suggest that biogenic sulfide induces DNRA, which in turn can explain the requisite coproduction of ammonium and nitrite from nitrified effluent necessary to sustain the anammox community.IMPORTANCE This study aims to increase understanding of why and how anammox is occurring in an engineered wetland with limited exogenous contributions of ammonium and nitrite. In doing so, the study has implications for how geochemical parameters could potentially be leveraged to impact nutrient cycling and attenuation during the operation of treatment wetlands. The work also contributes to ongoing discussions about biogeochemical signatures surrounding anammox processes and enhances our understanding of the contributions of anammox processes in freshwater environments.

Toxic Byproduct Formation during Electrochemical Treatment of Latrine Wastewater.

Electrochemical systems are an attractive option for onsite latrine wastewater treatment due to their high efficiency and small footprint. While concerns remain over formation of toxic byproducts during treatment, rigorous studies examining byproduct formation are lacking. Experiments treating authentic latrine wastewater over variable treatment times, current densities, chloride concentrations, and anode materials were conducted to characterize byproducts and identify conditions that minimize their formation. Production of inorganic byproducts (chlorate and perchlorate) and indicator organic byproducts (haloacetic acids and trihalomethanes) during electrolysis dramatically exceeded recommendations for drinking water after one treatment cycle (∼10-30 000 times), raising concerns for contamination of downstream water supplies. Stopping the reaction after ammonium was removed (i.e., the chlorination breakpoint) was a promising method to minimize byproduct formation without compromising disinfection and nutrient removal. Though treatment was accelerated at increased chloride concentrations and current densities, byproduct concentrations remained similar near the breakpoint. On TiO2/IrO2 anodes, haloacetic acids (up to ∼50 µM) and chlorate (up to ∼2 µM) were of most concern. Although boron-doped diamond anodes mineralized haloacetic acids after formation, high production rates of chlorate and perchlorate (up to ∼4 and 25 µM) made them inferior to TiO2/IrO2 anodes in terms of toxic byproduct formation. Organic byproduct formation was similar during chemical chlorination and electrolysis of wastewater, suggesting that organic byproducts are formed by similar pathways in both cases (i.e., reactions with chloramines and free chlorine).

Multilayer Heterojunction Anodes for Saline Wastewater Treatment: Design Strategies and Reactive Species Generation Mechanisms.

Multilayer heterojunction SbSn/CoTi/Ir anodes, which consist of Ir0.7Ta0.3O2 bottom layers coated onto a titanium base, Co-TiO2 interlayers, and overcoated discrete Sb-SnO2 islands, were prepared by spray pyrolysis. The Ir0.7Ta0.3O2 bottom layer serves as an Ohmic contact to facilitate electron transfer from semiconductor layers to the Ti base. The Co-TiO2 interlayer and overcoated Sb-SnO2 islands enhance the evolution of reactive chlorine. The surficial Sb-SnO2 islands also serve as the reactive sites for free radical generation. Experiments coupled with computational kinetic simulations show that while ·OH and Cl· are initially produced on the SbSn/CoTi/Ir anode surface, the dominant radical formed in solution is the dichlorine radical anion, Cl2·(-). The steady-state concentration of reactive radicals is 10 orders of magnitude lower than that of reactive chlorine. The SbSn/CoTi/Ir anode was applied to electrochemically treat human wastewater. These test results show that COD and NH4(+) can be removed after 2 h of electrolysis with minimal energy consumption (370 kWh/kg COD and 383 kWh/kg NH4(+)). Although free radical species contribute to COD removal, anodes designed to enhance reactive chlorine production are more effective than those designed to enhance free radical production.

Electrochemical Transformation of Trace Organic Contaminants in Latrine Wastewater.

Solar-powered electrochemical systems have shown promise for onsite wastewater treatment in regions where basic infrastructure for conventional wastewater treatment is not available. To assess the applicability of these systems for trace organic contaminant treatment, test compound electrolysis rate constants were measured in authentic latrine wastewater using mixed-metal oxide anodes coupled with stainless steel cathodes. Complete removal of ranitidine and cimetidine was achieved within 30 min of electrolysis at an applied potential of 3.5 V (0.7 A L(-1)). Removal of acetaminophen, ciprofloxacin, trimethoprim, propranolol, and carbamazepine (>80%) was achieved within 3 h of electrolysis. Oxidation of ranitidine, cimetidine, and ciprofloxacin was primarily attributed to reaction with NH2Cl. Transformation of trimethoprim, propranolol, and carbamazepine was attributed to direct electron transfer and to reactions with surface-bound reactive chlorine species. Relative contributions of aqueous phase ·OH, ·Cl, ·Cl2(-), HOCl/OCl(-), and Cl2 were determined to be negligible based on measured second-order reaction rate constants, probe compound reaction rates, and experiments in buffered Cl(-) solutions. Electrical energy per order of removal (EEO) increased with increasing applied potentials and current densities. Test compound removal was most efficient at elevated Cl(-) concentrations present when treated wastewater is recycled for use as flushing water (i.e., ∼ 75 mM Cl(-); EEO = 0.2-6.9 kWh log(-1) m(-3)). Identified halogenated and oxygenated electrolysis products typically underwent further transformations to unidentifiable products within the 3 h treatment cycle. Identifiable halogenated byproduct formation and accumulation was minimized during electrolysis of wastewater containing 75 mM Cl(-).

Modular advanced oxidation process enabled by cathodic hydrogen peroxide production.

Hydrogen peroxide (H2O2) is frequently used in combination with ultraviolet (UV) light to treat trace organic contaminants in advanced oxidation processes (AOPs). In small-scale applications, such as wellhead and point-of-entry water treatment systems, the need to maintain a stock solution of concentrated H2O2 increases the operational cost and complicates the operation of AOPs. To avoid the need for replenishing a stock solution of H2O2, a gas diffusion electrode was used to generate low concentrations of H2O2 directly in the water prior to its exposure to UV light. Following the AOP, the solution was passed through an anodic chamber to lower the solution pH and remove the residual H2O2. The effectiveness of the technology was evaluated using a suite of trace contaminants that spanned a range of reactivity with UV light and hydroxyl radical (HO(•)) in three different types of source waters (i.e., simulated groundwater, simulated surface water, and municipal wastewater effluent) as well as a sodium chloride solution. Irrespective of the source water, the system produced enough H2O2 to treat up to 120 L water d(-1). The extent of transformation of trace organic contaminants was affected by the current density and the concentrations of HO(•) scavengers in the source water. The electrical energy per order (EEO) ranged from 1 to 3 kWh m(-3), with the UV lamp accounting for most of the energy consumption. The gas diffusion electrode exhibited high efficiency for H2O2 production over extended periods and did not show a diminution in performance in any of the matrices.

Co-occurrence of Photochemical and Microbiological Transformation Processes in Open-Water Unit Process Wetlands.

The fate of anthropogenic trace organic contaminants in surface waters can be complex due to the occurrence of multiple parallel and consecutive transformation processes. In this study, the removal of five antiviral drugs (abacavir, acyclovir, emtricitabine, lamivudine and zidovudine) via both bio- and phototransformation processes, was investigated in laboratory microcosm experiments simulating an open-water unit process wetland receiving municipal wastewater effluent. Phototransformation was the main removal mechanism for abacavir, zidovudine, and emtricitabine, with half-lives (t1/2,photo) in wetland water of 1.6, 7.6, and 25 h, respectively. In contrast, removal of acyclovir and lamivudine was mainly attributable to slower microbial processes (t1/2,bio = 74 and 120 h, respectively). Identification of transformation products revealed that bio- and phototransformation reactions took place at different moieties. For abacavir and zidovudine, rapid transformation was attributable to high reactivity of the cyclopropylamine and azido moieties, respectively. Despite substantial differences in kinetics of different antiviral drugs, biotransformation reactions mainly involved oxidation of hydroxyl groups to the corresponding carboxylic acids. Phototransformation rates of parent antiviral drugs and their biotransformation products were similar, indicating that prior exposure to microorganisms (e.g., in a wastewater treatment plant or a vegetated wetland) would not affect the rate of transformation of the part of the molecule susceptible to phototransformation. However, phototransformation strongly affected the rates of biotransformation of the hydroxyl groups, which in some cases resulted in greater persistence of phototransformation products.

Nitrate removal in shallow, open-water treatment wetlands.

The diffuse biomat formed on the bottom of shallow, open-water unit process wetland cells contains suboxic zones that provide conditions conducive to NO3(-) removal via microbial denitrification, as well as anaerobic ammonium oxidation (anammox). To assess these processes, nitrogen cycling was evaluated over a 3-year period in a pilot-scale wetland cell receiving nitrified municipal wastewater effluent. NO3(-) removal varied seasonally, with approximately two-thirds of the NO3(-) entering the cell removed on an annual basis. Microcosm studies indicated that NO3(-) removal was mainly attributable to denitrification within the diffuse biomat (i.e., 80 ± 20%), with accretion of assimilated nitrogen accounting for less than 3% of the NO3(-) removed. The importance of denitrification to NO3(-) removal was supported by the presence of denitrifying genes (nirS and nirK) within the biomat. While modest when compared to the presence of denitrifying genes, a higher abundance of the anammox-specific gene hydrazine synthase (hzs) at the biomat bottom than at the biomat surface, the simultaneous presence of NH4(+) and NO3(-) within the biomat, and NH4(+) removal coupled to NO2(-) and NO3(-) removal in microcosm studies, suggested that anammox may have been responsible for some NO3(-) removal, following reduction of NO3(-) to NO2(-) within the biomat. The annual temperature-corrected areal first-order NO3(-) removal rate (k20 = 59.4 ± 6.2 m yr(-1)) was higher than values reported for more than 75% of vegetated wetlands that treated water in which NO3(-) was the primary nitrogen species (e.g., nitrified secondary wastewater effluent and agricultural runoff). The inclusion of open-water cells, originally designed for the removal of trace organic contaminants and pathogens, in unit-process wetlands may enhance NO3(-) removal as compared to existing vegetated wetland systems.

Biotransformation of trace organic contaminants in open-water unit process treatment wetlands.

The bottoms of shallow, open-water wetland cells are rapidly colonized by a biomat consisting of an assemblage of photosynthetic and heterotrophic microorganisms. To assess the contribution of biotransformation in this biomat to the overall attenuation of trace organic contaminants, transformation rates of test compounds measured in microcosms were compared with attenuation rates measured in a pilot-scale system. The biomat in the pilot-scale system was composed of diatoms (Staurosira construens) and a bacterial community dominated by ß- and γ-Proteobacteria. Biotransformation was the dominant removal mechanism in the pilot-scale system for atenolol, metoprolol, and trimethoprim, while sulfamethoxazole and propranolol were attenuated mainly via photolysis. In microcosm experiments, biotransformation rates increased for metoprolol and propranolol when algal photosynthesis was supported by irradiation with visible light. Biotransformation rates increased for trimethoprim and sulfamethoxazole in the dark, when microbial respiration depleted dissolved oxygen concentrations within the biomat. During summer, atenolol, metoprolol, and propranolol were rapidly attenuated in the pilot-scale system (t1/2 < 0.5 d), trimethoprim and sulfamethoxazole were transformed more slowly (t1/2 ≈ 1.5-2 d), and carbamazepine was recalcitrant. The combination of biotransformation and photolysis resulted in overall transformation rates that were 10 to 100 times faster than those previously measured in vegetated wetlands, allowing for over 90% attenuation of all compounds studied except carbamazepine within an area similar to that typical of existing full-scale vegetated treatment wetlands.

Phototransformation of wastewater-derived trace organic contaminants in open-water unit process treatment wetlands.

Open-water cells in unit process treatment wetlands can be used to exploit sunlight photolysis to remove trace organic contaminants from municipal wastewater effluent. To assess the performance of these novel systems, a photochemical model was calibrated using measured photolysis rates for atenolol, carbamazepine, propranolol, and sulfamethoxazole in wetland water under representative conditions. Contaminant transformation by hydroxyl radical ((•)OH) and carbonate radical ((•)CO3(-)) were predicted from steady-state radical concentrations measured at pH values between 8 and 10. Direct photolysis rates and the effects of light screening by dissolved organic matter on photolysis rates were estimated using solar irradiance data, contaminant quantum yields, and light screening factors. The model was applied to predict the land area required for 90% removal of a suite of wastewater-derived organic contaminants by sunlight-induced reactions under a variety of conditions. Results suggest that during summer, open-water cells that receive a million gallons of water per day (i.e., about 4.4 × 10(-2) m(3) s(-1)) of nitrified wastewater effluent can achieve 90% removal of most compounds in an area of about 15 ha. Transformation rates were strongly affected by pH, with some compounds exhibiting faster transformation rates under the high pH conditions associated with photosynthetic algae at the sediment-water interface and other contaminants exhibiting faster transformation rates at the circumneutral pH values characteristic of algae-free cells. Lower dissolved organic carbon concentrations typically resulted in increased transformation rates.

Unit Process Wetlands for Removal of Trace Organic Contaminants and Pathogens from Municipal Wastewater Effluents.

Treatment wetlands have become an attractive option for the removal of nutrients from municipal wastewater effluents due to their low energy requirements and operational costs, as well as the ancillary benefits they provide, including creating aesthetically appealing spaces and wildlife habitats. Treatment wetlands also hold promise as a means of removing other wastewater-derived contaminants, such as trace organic contaminants and pathogens. However, concerns about variations in treatment efficacy of these pollutants, coupled with an incomplete mechanistic understanding of their removal in wetlands, hinder the widespread adoption of constructed wetlands for these two classes of contaminants. A better understanding is needed so that wetlands as a unit process can be designed for their removal, with individual wetland cells optimized for the removal of specific contaminants, and connected in series or integrated with other engineered or natural treatment processes. In this article, removal mechanisms of trace organic contaminants and pathogens are reviewed, including sorption and sedimentation, biotransformation and predation, photolysis and photoinactivation, and remaining knowledge gaps are identified. In addition, suggestions are provided for how these treatment mechanisms can be enhanced in commonly employed unit process wetland cells or how they might be harnessed in novel unit process cells. It is hoped that application of the unit process concept to a wider range of contaminants will lead to more widespread application of wetland treatment trains as components of urban water infrastructure in the United States and around the globe.

Enhancing the activity of oxygen-evolution and chlorine-evolution electrocatalysts by atomic layer deposition of TiO2.

We report that TiO2 coatings formed via atomic layer deposition (ALD) may tune the activity of IrO2, RuO2, and FTO for the oxygen-evolution and chlorine-evolution reactions (OER and CER). Electrocatalysts exposed to ~3-30 ALD cycles of TiO2 exhibited overpotentials at 10 mA cm-2 of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities. For example, the deposition of TiO2 onto IrO2 yielded a 9-fold increase in the OER-specific activity in 1.0 M H2SO4 (0.1 to 0.9 mA cmECSA -2 at 350 mV overpotential). The oxidation state of titanium and the potential of zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalysts.

Phosphate Recovery from Human Waste via the Formation of Hydroxyapatite during Electrochemical Wastewater Treatment.

Electrolysis of toilet wastewater with TiO2-coated semiconductor anodes and stainless steel cathodes is a potentially viable onsite sanitation solution in parts of the world without infrastructure for centralized wastewater treatment. In addition to treating toilet wastewater, pilot-scale and bench-scale experiments demonstrated that electrolysis can remove phosphate by cathodic precipitation as hydroxyapatite at no additional energy cost. Phosphate removal could be predicted based on initial phosphate and calcium concentrations, and up to 80% total phosphate removal was achieved. While calcium was critical for phosphate removal, magnesium and bicarbonate had only minor impacts on phosphate removal rates at concentrations typical of toilet wastewater. Optimal conditions for phosphate removal were 3 to 4 h treatment at about 5 mA cm-2 (∼3.4 V), with greater than 20 m2 m-3 electrode surface area to reactor volume ratios. Pilot-scale systems are currently operated under similar conditions, suggesting that phosphate removal can be viewed as an ancillary benefit of electrochemical wastewater treatment, adding utility to the process without requiring additional energy inputs. Further value may be provided by designing reactors to recover precipitated hydroxyapatite for use as a low solubility phosphorus-rich fertilizer.

Identification of transformation products from ß-blocking agents formed in wetland microcosms using LC-Q-ToF.

Identification of degradation products from trace organic compounds, which may retain the biological activity of the parent compound, is an important step in understanding the long-term effects of these compounds on the environment. Constructed wetlands have been successfully utilized to remove contaminants from wastewater effluent, including pharmacologically active compounds. However, relatively little is known about the transformation products formed during wetland treatment. In this study, three different wetland microcosm treatments were used to determine the biotransformation products of the ß-adrenoreceptor antagonists atenolol, metoprolol and propranolol. LC/ESI-Q-ToF run in the MS(E) and MS/MS modes was used to identify and characterize the degradation products through the accurate masses of precursor and product ions. The results were compared with those of a reference standard when available. Several compounds not previously described as biotransformation products produced in wetlands were identified, including propranolol-O-sulfate, 1-naphthol and the human metabolite N-deaminated metoprolol. Transformation pathways were significantly affected by microcosm conditions and differed between compounds, despite the compounds' structural similarities. Altogether, a diverse range of transformation products in wetland microcosms were identified and elucidated using high resolving MS. This work shows that transformation products are not always easily predicted, nor formed via the same pathways even for structurally similar compounds.

Sunlight inactivation of fecal indicator bacteria in open-water unit process treatment wetlands: Modeling endogenous and exogenous inactivation rates.

A pilot-scale open-water unit process wetland was monitored for one year and found to be effective in enhancing sunlight inactivation of fecal indicator bacteria (FIB). The removal of Escherichia coli and enterococci in the open-water wetland receiving non-disinfected secondary municipal wastewater reached 3 logs and 2 logs in summer time, respectively. Pigmented enterococci were shown to be significantly more resistant to sunlight inactivation than non-pigmented enterococci. A model was developed to predict the inactivation of E. coli, and pigmented and non-pigmented enterococci that accounts for endogenous and exogenous sunlight inactivation mechanisms and dark processes. Endogenous inactivation rates were modeled using the sum of UVA and UVB irradiance. Exogenous inactivation was only significant for enterococci, and was modeled as a function of steady-state singlet oxygen concentration. The rate constants were determined from lab experiments and an empirical correction factor was used to account for differences between lab and field conditions. The model was used to predict removal rate constants for FIB in the pilot-scale wetland; considering the variability of the monitoring data, there was general agreement between the modeled values and those determined from measurements. Using the model, we estimate that open-water wetlands at 40° latitude with practical sizes can achieve 3-log (99.9%) removal of E. coli and non-pigmented enterococci throughout the year [5.5 ha and 7.0 ha per million gallons of wastewater effluent per day (MGD), respectively]. Differences in sunlight inactivation rates observed between pigmented and non-pigmented enterococci, as well as between lab-cultured and indigenous wastewater bacteria highlight the challenges of using FIB as model organisms for actual pathogens in natural sunlit environments.

Rapid chiral separation of atenolol, metoprolol, propranolol and the zwitterionic metoprolol acid using supercritical fluid chromatography-tandem mass spectrometry - Application to wetland microcosms.

A method for enantiomeric separation of the three ß-blocking agents atenolol, metoprolol, propranolol and the zwitterionic metoprolol acid, a major metabolite of both metoprolol and in environmental matrices also atenolol, has been developed. By use of supercritical fluid chromatography and the polysaccharide-based Chiralpak(®) IB-3, all four compounds were simultaneously enantiomerically separated (Rs>1.5) within 8min. Detection was performed using tandem mass spectrometry, and to avoid isobaric interference between the co-eluting metoprolol and metoprolol acid, the achiral column Acquity(®) UPC(2) BEH 2-EP was attached ahead of to the chiral column. Carbon dioxide with 18% methanol containing 0.5% (v/v) of the additives trifluoroacetic acid and ammonia in a 2:1 molar ratio were used as mobile phase. A post column make-up flow (0.3mL/min) of methanol containing 0.1% (v/v) formic acid was used to enhance the positive electrospray ionization. Detection was carried out using a triple quadrupole mass spectrometer operating in the selected reaction monitoring mode, using one transition per analyte and internal standard. The method was successfully applied for monitoring the enantiomeric fraction change over time in a laboratory scale wetland degradation study. It showed good precision, recovery, sensitivity and low effect of the sample matrix.