Strategies to Overcome Intermediate Accumulation During in situ Nitrate Remediation in Groundwater by Hydrogenotrophic Denitrification

Heterotrophic nitrate reduction potential of an aquifer microbial community from psychrophilic to thermophilic conditions

High temperature-aquifer thermal energy storage (HT-ATES) aims at the seasonal storage and extraction of large quantities of heat in the subsurface. However, the impacts of temperature fluctuations caused by HT-ATES toward biodiversity and ecosystem services in the subsurface environment with respect to the nitrogen cycle remain unclear. Hence, understanding possible temperature adaptation mechanisms of aquifer microbial communities is crucial to assess potential environmental risks associated with HT-ATES. In this study, we investigated the effects of temperatures between 12 °C and 80 °C on a pristine aquifer microbial community and its capacity to reduce nitrate, a common global groundwater contaminant. 13C-labeled acetate was used as easily consumable carbon and energy source for nitrate reduction, allowing precise activity measurement by analysis of released 13CO2. We observed nitrate reduction coupled to acetate mineralization at 12 °C, 25 °C, 38 °C, 45 °C and 60 °C but not at 80 °C. The rates of acetate mineralization at 12 °C to 38 °C were significantly higher than rates at 45 °C and 60 °C. Temperature significantly affected the composition of the acetate-mineralizing, nitrate-reducing microbial communities. Members of the families Pseudomonadaceae and Comamonadaceae mainly developed in enrichments incubated at 12 °C and 25 °C, whereas phylotypes affiliated to Rhizobiaceae dominated at 38 °C. At 45 °C and 60 °C, phylotypes belonging to Symbiobacteriaceae, Paenibacillaceae and Planococcaceae mainly developed. These findings indicate that the indigenous aquifer microbiome can maintain the ability to reduce nitrate over a wide temperature range, providing support that HT-ATES may allow thermal energy storage while simultaneously attenuating nitrate pollution.

Groundwater denitrification using a continuous flow mode hybrid system combining a hydrogenotrophic biofilter and an electrooxidation cell

An attached-growth continuous flow hydrogenotrophic denitrification system was investigated for groundwater treatment. Two bench-scale packed-bed reactors were used in series, without external pH adjustment or carbon source addition, while inorganic carbonate salts already contained in the groundwater were the sole carbon source used by the denitrifying bacteria. The hydrogen was produced by water electrolysis using renewable energy sources thus minimizing resource-draining factors of the treatment process. The biofilter was subjected to a combination of three groundwater retention times (13.5, 27 and 54 min, corresponding to 20, 10 and 5 mL min^-1 inlet water flow rates) and two hydrogen flow values (10 and 20 mL min^-1) to evaluate its efficiency under different operating parameters. In all cases, significant nitrate percentage removals were achieved, ranged between 64.1% and 100%. The treatment process appears to slow down with lower retention times and H₂ flow rate values, although residual nitrate concentrations were always in the range of 0-5.1 mg L^-1, values below the maximum permitted limit of 11.3 mg L^-1. In cases where nitrite accumulation was detected, a continuous flow electrochemical oxidation process with three different current density values (5.0, 7.5 and 10.0 mA cm^-2) was examined as a post-treatment step aiming to completely remove the toxic nitrite anions. Finally, an advanced mathematical model of the attached growth hydrogenotrophic denitrification process was developed to predict concentrations of all the substrates examined in the bio-filter (nitrate, nitrite, inorganic carbon and hydrogen).

Simultaneous removal of nitrate, nitrobenzene and aniline from groundwater in a vertical baffled biofilm reactor

The challenge of simultaneous removal of nitrobenzene (NB), aniline (AN) and nitrate from groundwater in a single bioreactor is mainly attributed to the persistence of AN to degradation with anoxic denitrification conditions. In this work, simultaneous removal of NB (100 μM), AN (100 μM) and nitrate (1 mM) was achieved within 8 h with a COD/N ratio of 8 in a vertical baffled biofilm reactor (VBBR). By setting DO concentration at 0.4-0.5 mg L^-1 to create a micro-aerobic condition, NB removal rate was accelerated without accumulation of AN, and AN could serve as electron donors for denitrification after ring cleavage. High-throughput sequencing showed that biofilm was predominated by denitrifiers (Luteimonas, Planctomyces, Thiobacillus, Thauera and so on) and NB-degrading bacteria (Pseudomonas), and biodiversity varied vertically along the height of the reactor. A dominantly anaerobic pathway for reducing NB to AN was identified by PICRUSt analysis, as the predicted genes involved in aerobic transformation of NB were several magnitudes lower than those in the anaerobic pathway. This study provided a new insight to the role of oxygen in robust bioremediation groundwater contaminated with NB, AN and nitrate.

Development of Bio-Electrochemical Reactor for Groundwater Denitrification: Effect of Electric Current and Water Hardness

Nitrate-nitrogen (NO3-N) contaminating groundwater is an environmental issue in many areas, and is difficult to treat by simple processes. A bio-electrochemical reactor (BER) using copper wire and graphite plate was developed to purify the NO3-N-contaminated groundwater. The low (of 10 mA) and high (of 20 mA) electric currents were applied to the BERs, and various influent hardness levels from 20 to 80 mg/L as CaCO3 due to groundwater characteristics were supplied to clarify the total nitrogen removal efficiency and NO3-N removal mechanisms. In the BER-10, the bio-electrochemical reactions caused 85% of total nitrogen to be removed through heterotrophic and autohydrogenotrophic denitrification in the suspended sludge and biofilm. However, the chemical deposit occurring at the cathode from water hardness affected the decreasing denitrification performance; 12.6% of Mg and 8.8% of Ca elements were observed in the biofilm. The enhancement of electrochemical reactions in the BER-20 caused integrating electrochemical and bio-electrochemical reactions; the NO3-N was electrochemically reduced to NO2-N, and it was further biologically reduced to N2. A better total nitrogen removal of 95% was found; although, a larger deposit of Mg (22.8%) and Ca (10.8%) was observed. The relatively low dissolved H2 in the BER-20 confirmed that the deposit affected the decreasing gaseous H2 transfer and inhibition of autohydrogenotrophic denitrification in the suspended sludge. According to the microbial analysis, both heterotrophic and autohydrogenotrophic denitrification were obtained in the suspended sludge of both BERs; Nocadia (26.8%) was the most abundant genus in the BER-10, whereas Flavobacterium (27.1%) and Nocadia (25.0%) were the dominant genera in the BER-20.

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.