Tracing the role of endogenous carbon in denitrification using wine industry by-product as an external electron donor: Coupling isotopic tools with mathematical modeling

Modified microbiology through enhanced denitrification by addition of various organic substances-temperature effect

Worldwide, the environmental nitrate (NO₃^-) problem is increasingly coming into focus. These increases in NO₃^- concentration result mainly from agricultural inputs and are further exacerbated by decreasing and finite geogenic NO₃^- degradation capacity in aquifers. Thus, treatment methods are becoming more and more important. In this study, the effects of enhanced denitrification with addition of organic carbon (C) on thereby autochthonous occurring microbiology and compared at room temperature as well as 10 °C were investigated. Incubation of bacteria and fungi was carried out using natural sediments without degradation capacity and groundwater with high NO₃^- concentrations. Addition of the four applied substrates (acetate, glucose, ascorbic acid, and ethanol) results in major differences in microbial community. Cooling to 10 °C changes the microbiology again. Relative abundances of bacteria are strongly influenced by temperature, which is probably the explanation for different denitrification rates. Fungi are much more sensitive to the milieu change with organic C. Different fungi taxa preferentially occur at one of the two temperature approaches. Major modifications of the microbial community are mainly observed whose denitrification rates strongly depend on the temperature effect. Therefore, we assume a temperature optimum of enhanced denitrification specific to each substrate, which is influenced by the microbiology.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

Impact of compost reactive layer on hydraulic transport and C & N cycles: Biogeochemical modeling of infiltration column experiments

Managed Aquifer Recharge (MAR) is a key strategy to increase freshwater resources in many regions facing water scarcity. MAR issues are related to both quantity and quality of the infiltrating water. In most countries, very high quality of the infiltrating water is required, to limit the impact on the aquifer geochemistry. In this paper, the possibility of injecting water of lower quality in the aquifer and letting the biogeochemical reactions take place in order to enhance its quality is explored. Here, we present the fate of nutrients (C, N) in the biogeochemical system of a reactive barrier formed by mixture of different proportions of sand and compost, supplied with treated wastewater to mimic MAR. An integrated conceptual model involving the nutrient cycles and biomass dynamics (auto- and heterotrophic) was developed, and then tested with a number of solute transport experiments in columns with different compost fraction in the column filling. The model incorporated both saturation and inhibition processes (regarding the nutrients and their byproducts) to provide a comprehensive picture of the nutrient dynamics within the column. The model developed (three if considering the 3 column setups) allowed to discriminate the processes that govern the fate of nutrients in relation with the compost enhancing long-term nutrient degradation, yet hindering hydraulic parameters that affect infiltration rates.

Bioavailable electron donors from ultrasound-treated biomass for stimulating denitrification

Finding low-cost electron donors to drive denitrification is an important target for many municipal wastewater treatment plants (MWTPs). Excess sludge (biomass) potentially is a low-cost electron donor generated internally to the MWTP, but it has to be made more bioavailable. Aerobic and anoxic biomasses were treated with ultrasound, and their supernatants were used as electron donors for stimulating denitrification. The supernatant from ultrasound-treated anoxic biomass achieved 54% faster nitrate-N removal than did supernatant from the treated aerobic biomass, and the supernatant of untreated biomass was ineffective as an electron donor. UV illumination of the supernatants further enhanced the rates, with increments of 19% and 14%, respectively for the aerobic and anoxic supernatants. Sodium acetate at a range of initial concentrations was compared as a readily bioavailable electron donor to gauge the acceleration impact of the supernatants as equivalent bioavailable chemical oxygen demand (COD). The total chemical oxygen demand (TCOD) of the supernatant harvested from anoxic biomass without UV illumination was 76% bioavailable, while its bioavailable TCOD was 78% after UV illumination. For the supernatant from the aerobic biomass, the bioavailable fractions were, respectively, 56% and 58% without and with UV illumination. The greatest impact for converting excess biomass into a source of bioavailable electron donor to drive denitrification came from ultrasound treatment of the biomass, which disrupted the biomass to form particulate chemical oxygen demand (PCOD) that was bioavailable. PCOD was at least 51% bioavailable, and it contributed no less than 82% of the bioavailable COD.

Use of nitrogen and oxygen isotopes of dissolved nitrate to trace field-scale induced denitrification efficiency throughout an in-situ groundwater remediation strategy

In the framework of the Life+ InSiTrate project, a pilot-plant was established to demonstrate the viability of inducing in-situ heterotrophic denitrification to remediate nitrate (NO₃^-)-polluted groundwater. Two injection wells supplied acetic acid by pulses to an alluvial aquifer for 22months. The monitoring was performed by regular sampling at three piezometers and two wells located downstream. In the present work, the pilot-plant monitoring samples were used to test the usefulness of the isotopic tools to evaluate the efficiency of the treatment. The laboratory microcosm experiments determined an isotopic fractionation (ε) for N-NO₃^- of -12.6‰ and for O-NO₃^- of -13.3‰. These ε¹⁵N_NO3/N2 and ε¹⁸O_NO3/N2 values were modelled by using a Rayleigh distillation equation to estimate the percentage of the induced denitrification at the pilot-plant while avoiding a possible interference from dilution due to non-polluted water inputs. In some of the field samples, the induced NO₃^- reduction was higher than 50% with respect to the background concentration. The field samples showed a reduced slope between δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- (0.7) compared to the laboratory experiments (1.1). This finding was attributed to the reoxidation of NO₂^- to NO₃^- during the treatment. The NO₃^- isotopic characterization also permitted the recognition of a mixture between the denitrified and partially or non-denitrified groundwater in one of the sampling points. Therefore, the isotopic tools demonstrated usefulness in assessing the implementation of the field-scale induced denitrification strategy.

Evaluating the potential use of a dairy industry residue to induce denitrification in polluted water bodies: A flow-through experiment

Improving the effectiveness and economics of strategies to remediate groundwater nitrate pollution is a matter of concern. In this context, the addition of whey into aquifers could provide a feasible solution to attenuate nitrate contamination by inducing heterotrophic denitrification, while recycling an industry residue. Before its application, the efficacy of the treatment must be studied at laboratory-scale to optimize the application strategy in order to avoid the generation of harmful intermediate compounds. To do this, a flow-through denitrification experiment using whey as organic C source was performed, and different C/N ratios and injection periodicities were tested. The collected samples were analyzed to determine the chemical and isotopic composition of N and C compounds. The results proved that whey could promote denitrification. Nitrate was completely removed when using either a 3.0 or 2.0 C/N ratio. However, daily injection with C/N ratios from 1.25 to 1.5 seemed advantageous, since this strategy decreased nitrate concentration to values below the threshold for water consumption while avoiding nitrite accumulation and whey release with the outflow. The isotopic results confirmed that nitrate attenuation was due to denitrification and that the production of DIC was related to bacterial whey oxidation. Furthermore, the isotopic data suggested that when denitrification was not complete, the outflow could present a mix of denitrified and nondenitrified water. The calculated isotopic fractionation values (ε¹⁵N_NO3/N2 and ε¹⁸O_NO3/N2) might be applied in the future to quantify the efficiency of the bioremediation treatments by whey application at field-scale.