Systematic evaluation of nitrate and perchlorate bioreduction kinetics in groundwater using a hydrogen-based membrane biofilm reactor

To evaluate the simultaneous reduction kinetics of the oxidized compounds, we treated nitrate-contaminated groundwater (∼9.4 mg-N/L) containing low concentrations of perchlorate (∼12.5 μg/L) and saturated with dissolved oxygen (∼8 mg/L) in a hydrogen-based membrane biofilm reactor (MBfR). We systematically increased the hydrogen availability and simultaneously varied the surface loading of the oxidized compounds on the biofilm in order to provide a comprehensive, quantitative data set with which to evaluate the relationship between electron donor (H₂) availability, surface loading of the electron acceptors (oxidized compounds), and simultaneous bioreduction of the electron acceptors. Increasing the H₂ pressure delivered more H₂ gas, and the total H₂ flux increased linearly from ∼0.04 mg/cm²-d for 0.5 psig (0.034 atm) to 0.13 mg/cm²-d for 9.5 psig (0.65 atm). This increased rate of H₂ delivery allowed for continued reduction of the acceptors as their surface loading increased. The electron acceptors had a clear hydrogen-utilization order when the availability of hydrogen was limited: oxygen, nitrate, nitrite, and then perchlorate. Spiking the influent with perchlorate or nitrate allowed us to identify the maximum surface loadings that still achieved more than 99.5% reduction of both oxidized contaminants: 0.21 mg NO₃-N/cm²-d and 3.4 μg ClO₄/cm²-d. Both maximum values appear to be controlled by factors other than hydrogen availability.     

Fluidized bed reactor for the biological treatment of ion-exchange brine containing perchlorate and nitrate

The removal of perchlorate and nitrate from contaminated drinking water using regenerable ion-exchange processes produces a high salt brine (3-10% NaCl) laden with high concentrations of perchlorate and nitrate. This bench-scale research describes the operation of acetate-fed granular activated carbon (GAC) based fluidized bed reactors (FBR) for perchlorate-only, and combined nitrate and perchlorate removal from synthetic brine (6% NaCl). The GAC was inoculated with a salt-tolerant culture developed by the authors and used previously in batch systems. An FBR was an effective design for perchlorate reduction and exhibited first-order degradation kinetics with respect to perchlorate concentrations. Nitrate was also removed by the organisms in the column and had no negative effects on the removal of perchlorate using the FBR design. However, at higher concentrations of nitrate the FBR was more difficult to operate due to loss of carbon and biomass from the formation of nitrogen bubbles and the high recycle flow rates needed.     

Multidimensional modeling of biofilm development and fluid dynamics in a hydrogen-based,membrane biofilm reactor (MBfR)

A two-dimensional, particle-based biofilm model coupled with mass transport and computational fluid dynamics was developed to simulate autotrophic denitrification in a spiral-wound membrane biofilm reactor (MBfR), where hydrogen is supplied via hollow-fiber membrane fabric. The spiral-wound configuration consists of alternating layers of plastic spacer net and membrane fabric that create rows of flow channels, with the top and bottom walls comprised of membranes. The transversal filaments of the spacer partially obstruct the channel flow, producing complex mixing and shear patterns that require multidimensional representation. This study investigated the effect of hydrogen and nitrate concentrations, as well as spacer configuration, on biofilm development and denitrification fluxes. The model results indicate that the cavity spacer filaments, which rest on the bottom membranes, cause uneven biofilm growth. Most biofilm resided on the bottom membranes, only in the wake of the filaments where low shear zones formed. In this way, filament configuration may help achieve a desired biofilm thickness. For the conditions tested in this study, the highest nitrate fluxes were attained by minimizing the filament diameter and maximizing the filament spacing. This lowered the shear stress at the top membranes, allowing for more biofilm growth. For the scenarios studied, biomass limitation at the top membranes hindered performance more significantly than diffusion limitation in the thick biofilms at the bottom membranes. The results also highlighted the importance of two-dimensional modeling to capture uneven biofilm growth on a substratum with geometrical complexity.     

Rapid and complete destruction of perchlorate in water and ion-exchange brine using stabilized zero-valent iron nanoparticles

Perchlorate has emerged as a widespread contaminant in groundwater and surface water. Because of the unique chemistry of perchlorate, it has been challenging to destroy perchlorate. This study tested the feasibility of using a new class of stabilized zero-valent iron (ZVI) nanoparticles for complete transformation of perchlorate in water or ion-exchange brine. Batch kinetic tests showed that at an iron dosage of 1.8 g L(-1) and at moderately elevated temperatures (90-95 degrees C), approximately 90% of perchlorate in both fresh water and a simulated ion-exchange brine (NaCl=6% (w/w)) was destroyed within 7h. An activation energy (Ea) of 52.59+/-8.41 kJ mol(-1) was determined for the reaction. Kinetic tests suggested that Cl(VII) in perchlorate was rapidly reduced to chloride without accumulation of any intermediate products. Based on the surface-area-normalized rate constant k(SA), starch- and CMC-stabilized ZVI nanoparticles degraded perchlorate 1.8 and 3.3 times, respectively, faster than non-stabilized ZVI particles. Addition of a metal catalyst (Al, Cu, Co, Ni, Pd, or Re) did not show any reaction improvement. This technology provides an effective method for complete destruction of perchlorate in both contaminated water and brine.     

Kinetics of nitrate and perchlorate removal and biofilm stratification in an ion exchange membrane bioreactor

The biological degradation of nitrate and perchlorate was investigated in an ion exchange membrane bioreactor (IEMB) using a mixed anoxic microbial culture and ethanol as the carbon source. In this process, a membrane-supported biofilm reduces nitrate and perchlorate delivered through an anion exchange membrane from a polluted water stream, containing 60 mg/L of NO₃⁻ and 100 μg/L of ClO₄⁻. Under ammonia limiting conditions, the perchlorate reduction rate decreased by 10%, whereas the nitrate reduction rate was unaffected. Though nitrate and perchlorate accumulated in the bioreactor, their concentrations in the treated water (2.8 ± 0.5 mg/L of NO₃⁻ and 7.0 ± 0.8 μg/L of ClO₄⁻, respectively) were always below the drinking water regulatory levels, due to Donnan dialysis control of the ionic transport in the system.Kinetic parameters determined for the mixed microbial culture in suspension showed that the nitrate reduction rate was 35 times higher than the maximum perchlorate reduction rate. It was found that perchlorate reduction was inhibited by nitrate, since after nitrate depletion perchlorate reduction rate increased by 77%. The biofilm developed in the IEMB was cryosectioned and the microbial population was analyzed by fluorescence in situ hybridization (FISH). The results obtained seem to indicate that the kinetic advantage of nitrate reduction favored accumulation of denitrifiers near the membrane, whereas per(chlorate) reducing bacteria were mainly positioned at the biofilm outer surface, contacting the biomedium. As a consequence of the biofilm stratification, the reduction of perchlorate and nitrate occur sequentially in space allowing for the removal of both ions in the IEMB.     

Simultaneous removal of perchlorate and nitrate from drinking water using the ion exchange membrane bioreactor concept

This work evaluates the feasibility of the ion exchange membrane bioreactor (IEMB) concept for the simultaneous removal of perchlorate and nitrate from drinking water, when nitrate is present in the ppm range and perchlorate in the ppb range. The IEMB concept combines Donnan dialysis and simultaneous biological degradation of both pollutants. Membrane transport studies showed that Donnan dialysis is suitable for obtaining water with concentrations of perchlorate and nitrate below the recommended levels. However, the pollutants were accumulated in a receiving stream, thus requiring additional treatment before disposal. On the other hand, the IEMB process operated with hydraulic retention times ranging from 1.4 to 8.3h in the water compartment, proved to remove effectively perchlorate and nitrate while preserving the water composition with respect to other ions, thus avoiding secondary contamination of the treated water. For a polluted water stream containing 100 ppb of ClO(4)(-) and 60 ppm of NO(3)(-), the concentrations of both ions in the treated stream were kept below the recommended levels of 4 ppb for ClO(4)(-) and 25 ppm for NO(3)(-). The IEMB system was operated under ethanol limitation, but even under these conditions, an increase of the perchlorate and nitrate concentrations in the treated water was not observed for up to 6 days.     

Fatty acids production from hydrogen and carbon dioxide by mixed culture in the membrane biofilm reactor

Gasification of waste to syngas (H₂/CO₂) is seen as a promising route to a circular economy. Biological conversion of the gaseous compounds into a liquid fuel or chemical, preferably medium chain fatty acids (caproate and caprylate) is an attractive concept. This study for the first time demonstrated in-situ production of medium chain fatty acids from H₂ and CO₂ in a hollow-fiber membrane biofilm reactor by mixed microbial culture. The hydrogen was for 100% utilized within the biofilms attached on the outer surface of the hollow-fiber membrane. The obtained concentrations of acetate, butyrate, caproate and caprylate were 7.4, 1.8, 0.98 and 0.42 g/L, respectively. The biomass specific production rate of caproate (31.4 mmol-C/(L day g-biomass)) was similar to literature reports for suspended cell cultures while for caprylate the rate (19.1 mmol-C/(L day g-biomass)) was more than 6 times higher. Microbial community analysis showed the biofilms were dominated by Clostridium spp., such as Clostridium ljungdahlii and Clostridium kluyveri. This study demonstrates a potential technology for syngas fermentation in the hollow-fiber membrane biofilm reactors.     

Removing low ppb level perchlorate, RDX, and HMX from groundwater with cetyltrimethylammonium chloride (CTAC) pre-loaded activated carbon

Perchlorate contaminates vast amounts of groundwater throughout the United States which could potentially be used as potable water. Activated carbon pre-loaded with cetyltrimethylammonium chloride has been shown in this research to be an effective adsorbent for removing perchlorate from three low conductivity (50-66 microS/cm) groundwaters containing perchlorate (ClO(4)(-)) concentrations of 0.85, 1.0, and 5.6 parts per billion (ppb), respectively. In rapid small-scale column tests (RSSCTs), the virgin granular activated carbon (GAC) (used as a control) treated between 20,000 and 40,000 bed volumes (BV) of water. In contrast, the activated carbon that was pre-loaded with CTAC processed 170,000-270,000 BV before perchlorate was detected above 0.25 ppb in the effluent. Though this pre-loading significantly increased the capacity for perchlorate, it also diminished the GAC's capacity to remove organics. The groundwater containing 1 ppb ClO(4)(-) also contained the nitro-organics HMX (0.6 ppb) and RDX (5.5-6.6 ppb). RDX was detected in the effluent from the CTAC-pre-loaded bed after only 8000 BV had been processed whereas 308,000 BV could be processed through the virgin bed before RDX was detected. Likewise, HMX breakthrough was observed after 116,000 BV in the CTAC-pre-loaded bed while the virgin RSSCT exhibited no breakthrough of HMX during a test that was operated for 309,000 BV. However, by combining a CTAC-pre-loaded bed followed by a virgin GAC bed in series, both perchlorate and RDX could be removed for the same length of time.     

Kinetics and mechanisms of DMSO (dimethylsulfoxide) degradation by UV/H(2)O(2) process

The objective of this study was to elucidate the degradation pathways of dimethylsulfoxide (DMSO) during its mineralization caused by UV/H(2)O(2) treatment. In order to accomplish this, we measured the concentration time-profiles of DMSO and its degradation intermediates during the UV/H(2)O(2) treatment. In addition, we proposed a kinetic model that could account for the degradation pathways of DMSO during its UV/H(2)O(2) treatment. The results show that the degradation of DMSO by the UV/H(2)O(2) treatment can be classified into two major pathways, and this is supported by both the analysis of the intermediates and total organic carbon (TOC) measurements. Firstly, DMSO was degraded into sulfate (SO(4)(2-)) through the formation of methansulfinate (CH(3)SO(2)(-)) and methansulfonate (CH(3)SO(3)(-)) as sulfur-containing intermediates. One of the two carbon constituents of DMSO was highly resistant to mineralization, due to the formation of methansulfonate, which reacted very slowly with (.-)OH k = 0.8 x 10(7) M(-1)s(-1)). Secondly, the other carbon constituent of DMSO was relatively easily mineralized through the formation of formaldehyde (HCHO) and formate (HCO(2)(-)) as non-sulfur-containing intermediates. The kinetic model proposed in this study for the degradation of DMSO by (.-)OH in the UV/H(2)O(2) process was able to successfully predict the patterns of concentration time-profiles of all components during the UV/H(2)O(2) treatment of DMSO.     

Cultivation of Phanerochaete chrysosporium and production of lignin peroxidase in novel biofilm reactor systems: Hollow fiber reactor and silicone membrane reactor

The white rot fungus Phanerochaete chrysosporium and its extracellular enzyme lignin peroxidase are both known to catalyze the transformation and, in many cases, the degradation of several hazardous compounds and are, therefore, promising candidates for application in hazardous waste treatment. The application of P. chrysosporium in large-scale waste treatment and commercial production of lignin peroxidase has been impeded by the lack of bioreactor systems yielding consistent production of high levels of lignin peroxidase under long-term steady state conditions and controlled growth of the fungus. The use of innovative biofilm systems, which minimize intensive shear and provide for fungal growth as a biofilm, was investigated. The viability of the use of a hollow fiber reactor and a stirred tank reactor modified into a unique silicone membrane reactor for the cultivation of P. chrysosporium and production of high levels of lignin peroxidase was demonstrated. The membrane reactor utilizes silicone tubing as a growth support and for oxygenation. The silicone membrane reactor was operated using a repeated batch technique, consisting of alternating growth and production phases, to yield production of lignin peroxidase over a period of 5 weeks and appears promising for application as a hazardous waste treatment process.     

Influence of the agitation rate on the treatment of partially soluble wastewater in anaerobic sequencing batch biofilm reactor

This work reports on the influence of the agitation rate on the organic matter degradation in an anaerobic sequencing batch reactor, containing biomass immobilized on 3 cm cubic polyurethane matrices, stirred mechanically and fed with partially soluble soymilk substrate with mean chemical oxygen demand (COD) of 974+/-70 mg l(-1). Hydrodynamic studies informed on the homogenization time under agitagion rates from 500 to 1100 rpm provided by three propeller impellers. It occurred very quickly compared to the total cycle time. The results showed that agitation provided good mixing and improved the overall organic matter consumption rates. A modified first-order kinetic model represented adequately the data in the entire range of agitation rate. The apparent first-order kinetic constant for suspended COD rose approximately 360% when the agitation rate was changed from 500 to 900 rpm, whereas the apparent first-order kinetic constant for soluble COD did not vary significantly.     

Surface modification of polypropylene macroporous membrane to improve its antifouling characteristics in a submerged membrane-bioreactor: H(2)O plasma treatment

To improve the antifouling characteristics of polypropylene hollow fiber macroporous membranes in a submerged membrane-bioreactor for wastewater treatment, the membranes were surface modified by H₂O plasma treatment. Structural and morphological changes on the membrane surface were characterized by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE-SEM). The change of surface wettability was monitored by contact angle measurement. The static water contact angle of the modified membrane reduced obviously with the increase of plasma treatment time. The total surface free energy and its dispersive component decreased, while the polar component increased with the increase of treatment time. The relative pure water flux for the modified membranes increased gradually with the increase of plasma treatment time. The tensile strength and the tensile elongation at break for the membranes decreased after plasma treatment. After continuous operation in a submerged membrane-bioreactor for about 68 h, flux recovery after water and caustic cleaning, flux ratio after fouling were improved by 2.0, 3.6 and 22.0%, while reduction of flux was reduced by 1.1% for the 1 min H₂O plasma treated membrane, compared to those of the unmodified membrane.     

An investigation of the formation of chlorate and perchlorate during electrolysis using Pt/Ti electrodes: The effects of pH and reactive oxygen species and the results of kinetic studies

The characteristics of chlorate (ClO₃⁻) and perchlorate (ClO₄⁻) formation were studied during the electrolysis of water containing chloride ions (Cl⁻). The experiments were performed using an undivided Pt/Ti plate electrode under different pH conditions (pH 3.6, 5.5, 7.2, 8.0 and 9.0). ClO₃⁻ and ClO₄⁻ were formed during electrolysis in proportion to the Cl⁻ concentration. The generation rates of ClO₃⁻ and ClO₄⁻ under acidic conditions (pH 3.6 and 5.5) were lower than in basic pH conditions (pH 7.2, 8.0 and 9.0). However, the pH of the solution did not influence the conversion of ClO₃⁻ to ClO₄⁻. The effects of intermediately formed oxidants on the production of ClO₃⁻ and ClO₄⁻ were observed using sodium thiosulfate (Na₂S₂O₃) as the active chlorine scavenger and tertiary butyl alcohol (t-BuOH) as the hydroxyl radical (OH) scavenger. The results revealed that electrolysis reactions that involved active chlorine contributed dominantly to ClO₃⁻ production. The direct oxidation reaction rate of Cl⁻ to ClO₃⁻ was 13%. The OH species that were intermediately formed during electrolysis were also found to significantly affect ClO₃⁻ and ClO₄⁻ production. The key formation pathways of ClO₃⁻ and ClO₄⁻ were studied using kinetic model development.     

Comparative study of the uses of poly(4-vinylpyridine) and poly(diallyldimethylammonium) chloride for the removal of perchlorate from aqueous solution by polyelectrolyte-enhanced ultrafiltration

An application of polyelectrolyte-enhanced ultrafiltration utilizes cationic polyelectrolytes to electrostatically bind anionic species. The colloid and target anion are then concentrated using an ultrafilter, producing a filtrate with a lower concentration of the target. This study compared the performances of poly(4-vinylpyridine) (P4VP) and poly(diallyldimethylammonium) chloride (PDADMAC) for the removal of perchlorate. Potentiometric titration data revealed that the ionization properties of P4VP in aqueous solution vary as functions of titrant utilized, degree of protonation, and counterion concentration. The greater affinity of perchlorate over chloride for the protonated pyridine residues of P4VP provided up to 95.8% retention of perchlorate under the solution conditions investigated. Through ultrafiltration experiments, the effects solution pH, counterion concentration, and polymer concentration were examined for both P4VP and PDADMAC. In addition, the effectiveness of P4VP recovery and reuse was also assessed.     

Impact of UV/H(2)O(2) advanced oxidation treatment on molecular weight distribution of NOM and biostability of water

The presence of natural organic matter (NOM) poses several challenges to the commercial practice of UV/H₂O₂ process for micropollutant removal. During the commercial application of UV/H₂O₂ advanced oxidation treatment, NOM is broken down into smaller species potentially affecting biostability by increasing Assimilable Organic Carbon (AOC) and Biodegradable Organic Carbon (BDOC) of water. This work investigated the potential impact of UV/H₂O₂ treatment on the molecular weight distribution of NOM and biostability of different water sources. A recently developed flow cytometric method for enumeration of bacteria was utilized to assess biological stability of the treated water at various stages through measurement of AOC. BDOC was also assessed for comparison and to better study the biostability of water. Both AOC and BDOC increased by about 3-4 times over the course of treatment, indicating the reduction of biological stability. Initial TOC and the source of NOM were found to be influencing the biostability profile of the treated water. Using high performance size exclusion chromatography, a wide range of organic molecule weights were found responsible for AOC increase; however, low molecular weight organics seemed to contribute more. Positive and meaningful correlations were observed between BDOC and AOC of different waters that underwent different treatments.     

Distributions and activities of ammonia oxidizing bacteria and polyphosphate accumulating organisms in a pumped-flow biofilm reactor

The spatial distributions and activities of ammonia oxidizing bacteria (AOB) and polyphosphate accumulating organisms (PAOs) were investigated for a novel laboratory-scale sequencing batch pumped-flow biofilm reactor (PFBR) system that was operated for carbon, nitrogen and phosphorus removal. The PFBR comprised of two 16.5 l tanks (Reactors 1 and 2), each with a biofilm module of 2 m² surface area. To facilitate the growth of AOB and PAOs in the reactor biofilms, the influent wastewater was held in Reactor 1 under stagnant un-aerated conditions for 6 h after feeding, and was then pumped over and back between Reactors 1 and 2 for 12 h, creating aerobic conditions in the two reactors during this period; as a consequence, the biofilm in Reactor 2 was in an aerobic environment for almost all the 18.2 h operating cycle. A combination of micro-sensor measurements, molecular techniques, batch experiments and reactor studies were carried out to analyse the performance of the PFBR system. After 100 days operation at a filtered chemical oxygen demand (COD_f) loading rate of 3.46 g/m² per day, the removal efficiencies were 95% COD_f, 87% TN_f and 74% TP_f. While the PFBR microbial community structure and function were found to be highly diversified with substantial AOB and PAO populations, about 70% of the phosphorus release potential and almost 100% of the nitrification potential were located in Reactors 1 and 2, respectively. Co-enrichment of AOB and PAOs was realized in the Reactor 2 biofilm, where molecular analyses revealed unexpected microbial distributions at micro-scale, with population peaks of AOB in a 100–250 μm deep sub-surface zone and of PAOs in the 0–150 μm surface zone. The micro-distribution of AOB coincided with the position of the nitrification peak identified during micro-sensor analyses. The study demonstrates that enrichment of PAOs can be realized in a constant or near constant aerobic biofilm environment. Furthermore, the findings suggest that when successful co-enrichment of AOB and PAOs occur in biofilm environments, such as in the PFBR system, they do so at different zone depths in the biofilm.     

Soil organic matter-hydrogen peroxide dynamics in the treatment of contaminated soils and groundwater using catalyzed H2O2 propagations (modified Fenton's reagent)

The interactions between catalyzed H(2)O(2) propagations (CHP-i.e. modified Fenton's reagent) and soil organic matter (SOM) during the treatment of contaminated soils and groundwater was studied in a well-characterized surface soil. The fate of two fractions of SOM, particulate organic matter (POM) and nonparticulate organic matter (NPOM), during CHP reactions was evaluated using concentrations of hydrogen peroxide from 0.5 to 3M catalyzed by soluble iron (III), an iron (III)-ethylenediamine tetraacetic acid (EDTA) chelate, or naturally-occurring soil minerals. The destruction of total SOM in CHP systems was directly proportional to the hydrogen peroxide dosage, and was significantly greater at pH 3 than at neutral pH; furthermore, SOM destruction occurred predominantly in the NPOM fraction. At pH 3, SOM did not affect hydrogen peroxide decomposition rates or hydroxyl radical activity in CHP reactions. However, at neutral pH, increasing the mass of SOM decreased the hydrogen peroxide decomposition rate and increased the rate of hydroxyl radical generation in CHP systems. These results show that, while CHP reactions destroy some of the organic carbon pools, SOM does not have a significant effect on the CHP treatment of soils and groundwater.     

Long-term formation of microbial products in a sequencing batch reactor

Activated sludge in a sequencing batch reactor (SBR) is subjected to alternating feast-and-famine conditions, which may result in the enhanced production of microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and internal storage products (X_STO). In this work, the long-term formation of these three microbial products by activated sludge in an SBR is investigated using an expanded unified model with a parallel experimental study. We also use the model to compare the impacts in an SBR to those in a continuous-flow activated sludge system. The model captures all experimental trends for all components with solids retention time (SRT) for global steady state and within a cycle. At an SRT of 20 days, the active microorganisms constitute about 28% of the mixed liquor volatile suspended solids (MLVSS); the remaining biomass is comprised of residual inert biomass (X_I) of 40%, EPS of 31%, and X_STO of ∼1%. The active biomass becomes a smaller fraction with the increasing SRT, while the inert biomass becomes increasingly dominant. For soluble components, effluent chemical oxygen demand (COD) is dominated by SMP, which varies to some degree in a cycle, peaking as external substrate becomes depleted. Within the SBR cycle, external substrate (S) declines strongly in the first part of the cycle, and SMP shows a small peak at the time of S depletion. X_STO is the only biomass component that varies significantly during the cycle. It peaks at the time that the input substrate (S) is depleted. Simulation for a continuous-flow activated sludge system and comparison with an SBR reveals that the constant “famine” conditions of the continuous system lead to lower EPS and X_STO, but higher MLVSS and X_I.