Accumulation of nitrite in denitrifying barriers when phosphate is limiting

Denitrification kinetics during aquifer storage and recovery of drainage water from agricultural land

An aquifer storage transfer and recovery (ASTR) system was studied in which tile drainage water (TDW) was injected with relatively high NO₃ (about 14 mg/L) concentrations originating from fertilizers. Here we present the evolution of denitrification kinetics at 6 different depths in the aquifer before, and during ASTR operation. First-order denitrification rate constants increased over time before and during the first days of ASTR operation, likely due to microbial adaptation of the native bacterial community and/or bioaugmentation of the aquifer by denitrifying bacteria present in injected TDW. Push-pull tests were performed in the native aquifer before ASTR operation. Obtained first-order denitrification rate constants were negligible (0.00-0.03 d^-1) at the start, but increased to 0.17-0.83 d^-1 after a lag-phase of about 6 days. During the first days of ASTR operation in autumn 2019, the arrival of injected TDW was studied at 2.5 m distance from the injection well. First-order denitrification rate constants increased again over time (maximum >1 d^-1). Three storage periods without injection were monitored in winter 2019, fall 2020, and spring 2021 during ASTR operation. First-order rate constants ranged between 0.12 and 0.61 d^-1. Denitrification coupled to pyrite oxidation occurred at all depths, but other oxidation processes were indicated as well. NO₃ concentration trends resembled Monod kinetics but were fitted also to a first-order decay rate model to facilitate comparison. Rate constants during the storage periods were substantially lower than during injection, probably due to a reduction in the exchange rate between aquifer solid phases and injected water during the stagnant conditions. Denitrification rate constants deviated maximally a factor 5 over time and depth for all in-situ measurement approaches after the lag-phase. The combination of these in-situ approaches enabled to obtain more detailed insights in the evolution of denitrification kinetics during AS(T)R.

Influence of Temperature on Denitrification and Microbial Community Structure and Diversity: A Laboratory Study on Nitrate Removal from Groundwater

Temperature is an extremely important environmental condition in the application of microbial denitrification for nitrate removal from groundwater. Understanding the nitrate removal efficiency of groundwater and the diversity, composition, and structure of microbial communities under different temperature conditions is of great significance for effective mitigation of groundwater nitrate pollution. This study investigated the effects of temperature on denitrification at 15 °C, 25 °C, 40 °C, and 45 °C. Moreover, the characteristics of microbial community structure and diversity were analyzed by combining high-throughput sequencing and polymerase chain reaction methods in order to fully clarify the denitrification efficiency under different temperature conditions. According to laboratory batch experiments and the findings of previous research, glucose was set as the carbon source and changes in “three nitrogen” indicators of the four temperature systems were mainly tested to clarify the effectiveness of nitrate removal. The maximum removal rates of nitrate nitrogen at 15 °C, 25 °C, 40 °C, and 45 °C were 44.05%, 87.03%, 99.26%, and 92.79%, respectively. Therefore, the most efficient nitrate removal can be achieved at 40℃. The Chao abundance indexes in the denitrification systems at 15 °C, 25 °C, 40 °C, and 45 °C were 1873, 352, 466, and 640, respectively. Therefore, the highest species richness was observed at 15 °C, but there were only a few dominant bacteria species. The composition of the bacterial community and the most dominant phylum varied at different temperatures. Among them, Gammaproteobacteria in Proteobacteria phylum plays an important role in the degradation of nitrate nitrogen. The relative abundance of Gammaproteobacteria at 15 °C, 25 °C, 40 °C, and 45 °C were 25.32%, 66.56%, 72.83%, and 3.47%. Tolumonas belongs to Gammaproteobacteria. The relative abundance of Tolumonas at 15 °C, 25 °C, 40 °C, and 45 °C were 9.41%, 65.47%, 62.49%, and 0.03%, respectively. The results of this study show that different temperature conditions affect the diversity, composition, and structure of the microbial community, thereby affecting the efficiency of denitrification for nitrate removal from groundwater.

Degradation of High-Concentration Nitrate Nitrogen in Groundwater: A Laboratory Study

To investigate effective and reasonable methods for the remediation of nitrate nitrogen pollution in groundwater, two groups of laboratory denitrification experiments were conducted: one on the effect of native denitrifying microbes in groundwater and another on the effect of artificially added denitrifying microbes. The water used in the experiment was typical groundwater with a high concentration of nitrate nitrogen. The temperature was controlled at 15°C. Both groups of experiments established four types of culture environments: anaerobic, anaerobic with an added carbon source (glucose), aerobic, and aerobic with an added carbon source (glucose). The results indicated that native denitrifying microbes in the groundwater have almost no ability to remove high concentrations of nitrate nitrogen. However, artificially added denitrifying microbes can effectively promote denitrification. Artificially added denitrifying microbes had the highest activity in an anaerobic environment in which a carbon source had been added, and the rate removal of a high concentration of nitrate nitrogen in groundwater was the highest and reached as high as 89.52%.

Denitrification using permeable reactive barriers with organic substrate or zero-valent iron fillers: controlling mechanisms, challenges, and future perspectives

Nitrate as a diffusive agricultural contaminant has been causing substantial groundwater quality deterioration worldwide. In situ groundwater remediation techniques using permeable reactive barriers (PRBs) have attracted increasing interest. Particularly, PRBs based on biological denitrification, using the organic substrate as a biostimulator, and chemical nitrate reduction, using zero-valent iron (ZVI) as a reductant, are two major PRB approaches for groundwater denitrification. This review paper analyzed the published studies over the past 10 years (2010-2020) using laboratory, modeling, and field-scale approaches to explore the performance and mechanisms of these two types of PRBs. Important factors affecting the denitrification efficiencies as well as the influential mechanisms were discussed. Several research gaps have been identified and further research needs are discussed in the end.

Denitrification performance and bacterial flora analysis of immobilized denitrification filler in industrial wastewater

The study aimed at evaluating the nitrogen removal performance of the immobilized denitrification filler, and the influence of shock loading on the high-rate denitrification process. A pilot scale reactor was operated for treatment of aniline production wastewater. The nitrogen removal activity significantly increased in the continuous feed experiments, reaching 5.23 kg N m^-3 day^-1 on day 31 (30 °C) at Hydraulic Retention Time (HRT) = 10 h. In the impact experiment, the denitrification filler was inhibited by Free Nitrite Acid (FNA) when the shock load flowed 1.5 times into the bioreactor and recovered after the load was restored for 20 h. The high-throughput results demonstrated that the dominant position of the denitrifying bacteria further enhanced in a micro toxic and high-salinity environment, providing a basis for the dominance of the composite denitrifying bacteria and the efficacy of the immobilization technology.

Performance and mechanism of synchronous nitrate and phosphorus removal in constructed pyrite-based mixotrophic denitrification system from secondary effluent

The performance and process of the constructed pyrite-based mixotrophic denitrification (POMD) system using pyrite and residual organic matters as the co-electron donors were investigated for simultaneous removal of N and P from secondary effluent. After the batch experiments, 61.80 ± 3.26% of phosphate and 99.99 ± 0.01% of nitrate were removed, and the obtained nitrate removal rate constant can reach 2.09 days^-1 in POMD system, which was significantly superior to that reported (0.95 day^-1) in pyrite-based autotrophic denitrification (PAD) system. PO₄^3--P removal was mainly achieved via chemical precipitation as FePO₄ with iron, and it was irrelevant with the initial nitrate and ammonium concentrations. High-throughput 16S rRNA gene sequencing analysis showed the coexistence of heterotrophic and autotrophic denitrifiers in the mixotrophic environment. The denitrification process could be divided into two stages according to the carbon balance and calculation of sulfate accumulation: (a) nitrate was mainly reduced heterotrophically during 12-36 h and (b) nitrate was reduced autotrophically after 36 h. The calculated proportion of heterotrophic denitrification was 58.17 ± 3.78%, which was promoted by a higher ammonium concentration. These findings are likely to be useful in understanding the mixotrophic denitrification process and developing a cost-effective technology to simultaneously remove N and P from secondary effluent. Graphical abstract.

Performance enhancement of H2S-based autotrophic denitrification with bio-gaseous CO2 as sole carbon source through new pH adjustment materials

H₂S-based denitrification could achieve synchronous removal of nitrate and H₂S and had been regarded as an efficient way for biogas desulfurization and wastewater denitrification. Using CO₂ in biogas as carbon source had a potential of saving cost further, but the performance deteriorated due to the drop in pH. Two kinds of nature ore, medical stone and phosphate ore, were added as new pH adjustment materials in this study, and feasibility of using CO₂ as sole carbon source for H₂S-based denitrification was investigated. As a result, both materials could increase the pH from 4.5 to above 6.0. Compared with medical stone, higher level of pH (up to 6.39) and nitrate removal efficiency (99.1%) were obtained with phosphate ore. In addition, ATP increased more rapidly than the control, reflecting improvement on microbial activities. Therefore, phosphate ore as the pH adjustment material could improve H₂S-based denitrification performance obviously.

Comparison of heterotrophic and autotrophic denitrification processes for nitrate removal from phosphorus-limited surface water

Phosphorus (P) limitation has been demonstrated for micro-polluted surface water denitrification treatment in previous study. In this paper, a lab-scale comparative study of autotrophic denitrification (ADN) and heterotrophic denitrification (HDN) in phosphorus-limited surface water was investigated, aiming to find out the optimal nitrogen/phosphorus (N/P) ratio and the mechanism of the effect of P limitation on ADN and HDN. Furthermore, the optimal denitrification process was applied to the West Lake denitrification project, aiming to improve the water quality of the West Lake from worse than grade V to grade IV (GB3838-2006). The lab-scale study showed that the lack of P indeed inhibited HDN more greatly than ADN. The optimal N/P ratio for ADN and HDN was 25 and a 0.15 mg PO₄^3--P L^-1 of microbial available phosphorus (MAP) was observed. P additions could greatly enhance the resistance of ADN and HDN to hydraulic loading shock. Besides, The P addition could effectively stimulate the HDN performance via enriching the heterotrophic denitrifiers and the denitrifying phosphate-accumulating organisms (DNPAOs). Additionally, HDN was more effective and cost-effective than ADN for treating P-limited surface water. The study of the full-scale HDBF (heterotrophic denitrification biofilter) indicated that the denitrification performance was periodically impacted by P limitation, particularly at low water temperatures.

Groundwater nitrate remediation using plant-chip bioreactors under phosphorus-limited environment

Groundwater denitrification bioreactors under limited phosphorus conditions were studied in column experiments using four types of plant-chips. When the phosphate-P concentration in the influent increased from 0.04mg/L to 0.4mg/L, the nitrate removal ratio increased from 61.6% to 86.1% in reed, from 7.2% to 12.6% in Japanese cedar, from 37.0% to 73.6% in Moso bamboo, and from 19.2% to 50.5% in Lithocarpus edulis. The carbon source of the denitrifiers' growth was indicated by the content of acid detergent soluble organic matter in the chips. Furthermore, according to the modified Michaelis-Menten-type equation proposed in the study, the denitrification rate was largely limited by the phosphate-P concentration in reed and L. eduilis, and by the dissolved organic carbon (DOC) in Japanese cedar. Denitrification in Moso bamboo was affected by both phosphate-P and DOC. Besides the DOC, phosphorus emerged as an important limiting element of denitrification in some bioreactor plant-chips.

Nitrate and phosphate removal from agricultural subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters

Woodchip bioreactors have been increasingly used as an edge-of-field treatment technology to reduce the nitrate loadings to surface waters from agricultural subsurface drainage. Recent studies have shown that subsurface drainage can also contribute substantially to the loss of phosphate from agricultural soils. The objective of this study was to investigate nitrate and phosphate removal in subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters. The woodchip bioreactor demonstrated average nitrate removal efficiencies of 53.5-100% and removal rates of 10.1-21.6 g N/m(3)/d for an influent concentration of 20 mg N/L and hydraulic retention times (HRTs) of 6-24 h. When the influent nitrate concentration increased to 50 mg N/L, the bioreactor nitrate removal efficiency and rate averaged 75% and 18.9 g N/m(3)/d at an HRT of 24 h. Nitrate removal by the woodchips followed zero-order kinetics with rate constants of 1.42-1.80 mg N/L/h when nitrate was non-limiting. The steel byproduct filter effectively removed phosphate in the bioreactor effluent and the total phosphate adsorption capacity was 3.70 mg P/g under continuous flow conditions. Nitrite accumulation occurred in the woodchip bioreactor and the effluent nitrite concentrations increased with decreasing HRTs and increasing influent nitrate concentrations. The steel byproduct filter efficiently reduced the level of nitrite in the bioreactor effluent. Overall, the results of this study suggest that woodchip denitrification followed by steel byproduct filtration is an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Evaluation of sustainable electron donors for nitrate removal in different water media

An external electron donor is usually included in wastewater and groundwater treatment systems to enhance nitrate removal through denitrification. The choice of electron donor is critical for both satisfactory denitrification rates and sustainable long-term performance. Electron donors that are waste products are preferred to pure organic chemicals. Different electron donors have been used to treat different water types and little is known as to whether there are any electron donors that are suitable for multiple applications. Seven different carbon rich waste products, including liquid and solid electron donors, were studied in comparison to pure acetate. Batch-scale tests were used to measure their ability to reduce nitrate concentrations in a pure nutrient solution, light greywater, secondary-treated wastewater and tertiary-treated wastewater. The tested electron donors removed oxidised nitrogen (NOx) at varying rates, ranging from 48 mg N/L/d (acetate) to 0.3 mg N/L/d (hardwood). The concentrations of transient nitrite accumulation also varied across the electron donors. The different water types had an influence on NOx removal rates, the extent of which was dependent on the type of electron donor. Overall, the highest rates were recorded in light greywater, followed by the pure nutrient solution and the two partially treated wastewaters. Cotton wool and rice hulls were found to be promising electron donors with good NOx removal rates, lower leachable nutrients and had the least variation in performance across water types.

Remediation of nitrate-contaminated water by solid-phase denitrification process-a review

The paper presents a compilation of various autotrophic and heterotrophic ways of solid-phase denitrification. It covers a complete understanding of various pathways followed during denitrification process. The paper gives a brief review on various governing factors on which the process depends. It focuses mainly on the solid-phase denitrification process, its applicability, efficiency, and disadvantages associated. It presents a critical review on various methodologies associated with denitrification process reported in past years. A comparative study has also been carried out to have a better understanding of advantages and disadvantages of a particular method. We summarize the various organic and inorganic substances and various techniques that have been used for enhancing denitrification process and suggest possible gaps in the research areas whi'ch are worthy of future research.

In situ biostimulation of petroleum hydrocarbon degradation by nitrate and phosphate injection using a dipole well configuration

The main aim of this study was to explore the feasibility of source zone bioremediation by nitrate and nutrient injection in a crude-oil contaminated aquifer using a recirculating well dipole. Groundwater pumped from a downgradient well at a rate of 2.5m(3)h(-1) was enriched with bromide (tracer), nitrate and ammonium phosphate and injected in a well 40 m upgradient. The test was run for 49 days with solute injection, followed by 65 days of dipole operation without solute addition. The resulting bromide breakthrough curve allowed quantifying a first-order leakage coefficient of 0.017 day(-1) from the dipole, whereas from the nitrate data a first-order nitrate consumption rate of 0.075 day(-1) was determined. Dissolved hydrocarbon concentrations including benzene decreased to non-detect in 84days but experienced important rebounds after ending circulation. Nitrite accumulated temporarily but was consumed entirely when solute injection stopped. The mass balance calculations revealed that about 83% of the nitrate was used for hydrocarbon degradation, with the remaining being used for oxidation of reduced sulfur. A reactive transport model was used for the delineation of the treated zone. This model suggested that denitrification influenced flow and transport in the dipole. It is concluded that successful promotion of denitrifying hydrocarbon degradation is easily obtained in this aquifer and enables to abate dissolved concentrations, and that dipole configuration is a good option.

Optimization of C/N and current density in a heterotrophic/biofilm-electrode autotrophic denitrification reactor (HAD-BER)

In this study, central composite design (CCD) and response surface methodology (RSM) were applied to optimize the C/N and current density in a heterotrophic/biofilm-electrode autotrophic denitrification reactor (HAD-BER). Results showed that nitrate could be effectively reduced over a wide range of C/Ns (0.84-1.3535) and current densities (96.8-370.0 mA/m(2)); however, an optimum C/N of 1.13 and optimum current density of 239.6 mA/m(2) were obtained by RSM. Moreover, the HAD-BER performance under the optimum conditions resulted in almost 100% nitrate-N removal efficiency and low nitrite-N and ammonia-N accumulation. Furthermore, under the optimum conditions, H2 generated from water electrolysis matched the CO2 produced by heterotrophic denitrification by stoichiometric calculation. Therefore, CCD and RSM could be used to acquire optimum operational conditions and improve the nitrate removal efficiency and energy consumption in the HAD-BER.

Achieving partial denitrification with sludge fermentation liquid as carbon source: the effect of seeding sludge

The partial denitrification (nitrate to nitrite) has been a promising way for nitrate wastewater treatment combined with ANAMMOX system subsequently. This work investigated the effect of seeding sludge on partial denitrification by using sludge fermentation liquid as carbon source, with the sludge taken from: anoxic/oxic reactor (SA), anaerobic-anoxic-oxic reactor (SA-A-O) and alternately anaerobic sludge fermentation coupling anoxic denitrification reactor (SA-A). The results showed that transient accumulation of nitrite was observed in SA and SA-A-O. However, at the initial nitrate concentration of 30 mg/L, a high nitrite of 20.91 ± 0.52 mg/L was accumulated under complete nitrate reduction in the SA-A system, which indicated that partial denitrification could be realized. Furthermore, as much as 80% nitrate-to-nitrite transformation ratio (NTR) was achieved in a 108-day operation with inoculating SA-A, which illustrated the stability of partial denitrification under long-term operation.

Denitrification in presence of acetate and glucose for bioremediation of nitrate-contaminated groundwater

With the current increasing interest in aquifer denitrification, recent attention has been given to cost-effective in-situ treatments such as Enhanced In-Situ Biological Denitrification (EISBD), which intends to stimulate the indigenous bacterial activity by injecting an external organic substrate and/or nutrients to the aquifer matrix. Within this context, laboratory batch assays have been conducted to develop a strategy for in-situ denitrification of a nitrate-contaminated aquifer in Argentona, Catalonia (Spain). The assays were run under aerobic and anaerobic conditions at a temperature of 17 degrees C to better simulate the conditions of the aquifer. Acetate and glucose were added to assess their potential to promote heterotrophic denitrifying bacteria activity. Overall, the results revealed that indigenous micro-organisms had the potential of reducing nitrate under appropriate conditions. Nitrate removal was complete and faster under anaerobic conditions, though high nitrate removals were also attained under initial aerobic conditions when a readily organic compound was amended at a sufficient dosage. The results also revealed that a significant amount of the available organic carbon was consumed by processes other than denitrification, namely aerobic oxidation and other microbial oxidation processes. To sum up, the results of this study demonstrated that addition of organic compounds into the groundwater is a promising method for in-situ bioremediation of nitrate in the Argentona aquifer. This approach could potentially be applied to a number of situations in which nitrate concentration is elevated and where indigenous micro-organisms with potential to reduce nitrate are present within the aquifer material.

Effect of operating parameters on denitrification in an anoxic rotating biological contactor

The presence of nitrate in water and wastewater is a serious environmental problem. Anoxic rotating biological contactors (RBC) are a promising novel technology for nitrate removal. In this study the effect of two carbon/nitrogen (C/N) molar ratios (1.5 and 3.0) on denitrification, using acetate as a carbon source, were investigated in an anoxic bench-scale RBC, treating synthetic wastewater. The effect of different hydraulic retention times (HRTs) and different nitrogen and carbon influent concentrations on the reactor performance, at constant C/N, were also analysed. The average removal efficiency in terms of nitrogen-nitrate was about 90.4% at C/N = 1.5, lowering to 73.7% at C/ N = 3.0. Considering carbon-acetate removal, overall efficiencies of 82.0% and 63.6% were attained at C/N ratios of 1.5 and 3.0, respectively. The increase in nitrogen-nitrate (from 50 to 100 mg N-NO3- L(-1)) and carbon-acetate influent concentrations and the decrease in HRT, keeping C/N constant, had a slight negative effect in terms of substrate removal. It was found that, for the tested conditions, the use of C/N = 1.5 is advantageous to denitrification. The anoxic RBC was significantly effective at reducing nitrate concentrations within a relatively short HRT. These reactors may be a feasible option for the treatment of nitrate-rich wastewaters.

The characteristics and application of sludge-fly ash ceramic particles (SFCP) as novel filter media

Novel filter media-sludge-fly ash ceramic particles (SFCP) were prepared using dewatered sludge, fly ash and clay with a mass ratio of 1:1:1. Compared with commercial ceramic particles (CCP), SFCP had higher total porosity, larger total surface area and lower bulk and apparent density. Tests of heavy metal elements in lixivium proved that SFCP were safe for wastewater treatment. A lab-scale upflow anaerobic bioreactor was employed to ascertain the application of SFCP in denitrification process using acetate as carbon source. The results showed that SFCP reactor brought a relative superiority to CCP reactor in terms of total nitrogen (TN) removal at the optimum C/N ratio of 4.03 when volumetric loading rates (VLR) ranged from 0.33 to 3.69 kg TN (m(3)d)(-1). Therefore, SFCP application, as a novel process of treating wastes with wastes, provided a promising way in sludge and fly ash utilization.

Selection of organic substrates as potential reactive materials for use in a denitrification permeable reactive barrier (PRB)

The aim of the present study was to select a suitable natural organic substrate as a potential carbon source for use in a denitrification permeable reactive barrier (PRB). A number of seven organic substrates were first tested in batch tests. The materials attained varying degrees of success at promoting denitrification. Some of the organic substrates performed very well, achieving complete nitrate removal (>98%), while others were considered unsuitable for a variety of reasons, including: insufficient nitrate or nitrogen removal, excessive release of leachable nitrogen from the substrate or excessive reduction of nitrate to ammonium rather than removing it as gaseous N2. The top performing substrate in terms of denitrification extent (>98%) and rate (0.067 mgNO3(-)-N dm(-3)d(-1)g(sub)(-1)) was then selected for two bench-scale column experiments in an attempt to simulate the PRB. The inlet concentration was 50 mg dm(-3) NO3(-)-N and the columns operated at two different flow rates: 0.3 cm3 min(-1) (Column 1) and 1.1cm3 min(-1) (Column 2). The two columns showed different general patterns, making it clear that the flow rate was a key factor at the nitrate removal. Nitrate was completely removed (>96%) by the passage through Column 1, while only partially removed in Column 2 (66%). The results indicated that the selected organic substrate (Softwood) was applicable for further use as a filling material for a PRB.

A long-term performance test on an autotrophic denitrification column for application as a permeable reactive barrier

The long-term performance of a sulfur-based reactive barrier system was evaluated using autotrophic denitrification in a large-scale column. A bacterial consortium, containing autotrophic denitrifiers attached on sulfur particles, serving as an electron donor, was able to transform 60 mg N L(-1) of nitrate into dinitrogen. In the absence of phosphate, the consortium was unable to remove nitrate, but after the addition of phosphate, nitrate removal was readily evident. Once the column operation had stabilized, seepage velocities of 1.0 x 10(-3) and 0.5 x 10(-3)cms(-1), corresponding to hydraulic residence times of 24 and 48 h, respectively, did not affect the nitrate removal efficiency, as determined by the nitrate concentration in the effluent. However, data on the nitrate, nitrite and sulfate distribution along the column indicated differential transformation patterns with column depths. Based on the dinitrogen concentration in the total gas collected, the denitrification efficiency of the tested column was estimated to be more than 95%. After 500 d operation, the hydrodynamic characteristics of the column slightly changed, but these changes did not inhibit the nitrate removal efficiency. Data from a bacterial community analysis obtained from four parts of the column demonstrated the selective a spatial distribution of predominant species depending on available electron acceptors or donors.

Nitrate attenuation in groundwater: a review of biogeochemical controlling processes

Biogeochemical processes controlling nitrate attenuation in aquifers are critically reviewed. An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution, and to safeguard groundwater supplies and groundwater-dependent surface waters. Denitrification is focused upon as the dominant nitrate attenuation process in groundwater. As denitrifying bacteria are essentially ubiquitous in the subsurface, the critical limiting factors are oxygen and electron donor concentration and availability. Variability in other environmental conditions such as nitrate concentration, nutrient availability, pH, temperature, presence of toxins and microbial acclimation appears to be less important, exerting only secondary influences on denitrification rates. Other nitrate depletion mechanisms such as dissimilatory nitrate reduction to ammonium and assimilation of nitrate into microbial biomass are unlikely to be important in most subsurface settings relative to denitrification. Further research is recommended to improve current understanding on the influence of organic carbon, sulphur and iron electron donors, physical restrictions on microbial activity in dual porosity aquifers, influences of environmental condition (e.g. pH in poorly buffered environments and salinity in coastal or salinized soil settings), co-contaminant influences (particularly the contrasting inhibitory and electron donor influences of pesticides) and improved quantification of denitrification rates in the laboratory and field.

Removing selenate from groundwater with a vegetable oil-based biobarrier

Vegetable oil-based permeable reactive biobarriers (PRBs) were evaluated as a method for remediating groundwater containing unacceptable amounts of selenate. PRBs formed by packing laboratory columns with sand coated with soybean oil were used. In an initial 24-week study a simulated groundwater containing 10 mg L(-1) selenate-Se was supplied to three soil columns and the selenate and selenite content of the effluent waters monitored. Two of the soil columns were effective at removing selenate and, during the final 21 weeks of the study, effluents from these columns contained almost no selenate or selenite. Almost all (95%) of the selenate removed was recovered as immobilized selenium sequestered in the solid matrix of the column. For unknown reasons, the third column failed to reduce selenate. A second study looked at the ability of PRBs to remove selenate when nitrate was present. As was done in the first study, three columns were evaluated but this time the water supplied to the columns contained 20 mg L(-1 )nitrate-N and 10 mg L(-1) selenate-Se. Nitrate quickly disappeared from the effluents of these columns and during the final 23 weeks of the study, the nitrate content of the effluent water averaged less than 0.03 microg ml(-1) nitrate-N. Selenate was also removed by these columns but at a slower rate than observed with nitrate. In the final 6 weeks of the study, about 95% of the selenate applied to the columns was removed. In situ PRBs containing soybean oil might be used to remediate groundwater contaminated with both selenate and nitrate.

Injection of innocuous oils to create reactive barriers for bioremediation: laboratory studies

In situ groundwater remediation may be achieved using stationary permeable barriers created by the injection of a substrate, such as innocuous vegetable oil, into the contaminated aquifer. The oil provides the electron donor stimulating microorganisms to degrade or sequester many contaminants. At present, little is known about the best procedures to use when injecting oil into an aquifer. In this investigation, laboratory column and sand tank studies were used as model systems to explore the effect of different injection parameters on the distribution of oil emulsions into water-saturated sand. The parameters investigated included injection pressures of 70, 1400 and 18,000 KPa; injection times of 15, 30, 60 or 120 s; and the influence of an emulsifier, polyoxyethylenesorbitan monooleate (Tween 80), upon the distribution of the injected oil. The longest injection patterns were achieved at 18,000 KPa. A pattern that was 46+/-1.8 cm long was produced with an 18,000 KPa injection for 60 s. Increasing the injection time to 120 s increased the length of the pattern by only 6.5%. Tween 80 at concentrations of 0.05% increased the width of the injection patterns but did not increase the length of the pattern. A multi-ported injection probe might be used to create in situ permeable barriers approximately 1 m wide.

Removing selenite from groundwater with an in situ biobarrier: laboratory studies

Laboratory biobarriers were evaluated for their ability to remove selenite from flowing groundwater. Microbial activity in aquifers is usually limited by substrate availability, and biobarriers stimulate microbial activity by providing a substrate; for these studies soybean oil was used. Water containing 10 mg L(-1) selenite-Se was pumped through the biobarriers for 74 days and the amount present in the effluent monitored. The amounts remained high for the first 2 weeks of the study but then declined. From day 28 until the end of the study the amount of selenite-Se in the column effluents averaged 0.20 +/- 0.04 mg L(-1), a decrease of approximately 98%. At the end of the study about half of the selenite-Se applied to the columns was recovered as immobilized selenium trapped by the biobarrier. This study suggests that biobarriers containing vegetable oil might be used as a process for removing selenite from contaminated groundwater.