Combination of aquifer thermal energy storage and enhanced bioremediation: Biological and chemical clogging

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart-Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C-70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C-70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h-1 to 2.9 ± 2.2 h-1 (GM) and 0.1 ± 0.1 h-1 to 1.8 ± 0.0 h-1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h-1 to 5.1 ± 3.9 h-1 at GM. Activation energies for PCE degradation yielded 32 kJ mol-1 (GM) and 56 kJ mol-1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Experimental investigation on chemical clogging mechanism of loose porous media in recharge process of groundwater heat pump

Groundwater heat pumps (GWHP) are an efficient utilisation of shallow geothermal energy technology and of great significance in terms of promoting energy conservation and reducing emissions. However, recharge clogging has been a key problem restricting the continuous operation of GWHP. In this study, a simulation test device for sand column was designed with the aim of addressing chemical clogging induced by heat pump reinjection in a porous saline aquifer in the Huaibei Plain, China. The trend in the variation of the permeability coefficient was studied based on the detection of the sand sample composition, recharge water quality, and sand layer temperature, and the cause of formation was analysed using the saturation index (SI) and ion ratio method. The results indicated that the permeability coefficient in the sand column decreased exponentially, with a maximum and minimum decrease of 8.14% and 71.65% of the original coefficient, respectively, found in sections P2-P3 and P8-P9. Therefore, the clogging effect of the aquifer at approximately 200-400 mm from the recharge well was significant. Water-rock interactions predominantly involved the dissolution of halite, albite, chlorite, anhydrite, and dolomite and the precipitation of calcite, as well as the exchange adsorption of Ca²⁺ and Mg²⁺ to Na⁺, which were the key sources of ions during the water chemical evolution process. Finally, quartz was formed by the weathering and dissolution of aluminosilicate minerals such as albite, and particle migration and precipitation during the hydrodynamic disturbance were the primary causes of the front-end blockage of the column.

Temperature Effects on PCE Degradation on ZVI in Column Experiments with Deionized Water

The effects of rising groundwater temperatures on zerovalent iron (ZVI)-based remediation techniques will be critical in accelerating chlorinated hydrocarbon (CHC) degradation and side reactions. Therefore, tetrachloroethylene (PCE) degradation with three ZVIs widely used in permeable reactive barriers (Gotthart-Maier cast iron [GM], Peerless cast iron [PL], and ISPAT sponge iron [IS]) was evaluated at 10-70 °C in deionized water. From 10 to 70 °C, PCE degradation half-lives decreased from 25 ± 2 to 0.9 ± 0.1 h (PL), 24 ± 3 to 0.7 ± 0.1 h (GM), and 2.5 ± 0.01 to 0.3 ± 0.005 h (IS). Trichloroethylene (TCE) degradation half-lives at PL and GM decreased from 14.3 ± 3 to 0.2 ± 0.1 h (PL) and 7.6 ± 2 to 0.4 ± 0.1 h (GM). This acceleration of CHC degradation and the stronger shift toward reductive β-elimination reduced the concentration of potentially harmful metabolites with increasing temperatures. PCE and TCE degradation yields an activation energy of 28 (IS), 58 and 40 kJ mol^-1 (GM), and 62 and 53 kJ mol^-1 (PL). Hydrogen gas production by ZVI corrosion increased by 3 orders of magnitude from 10 to 70 °C, and an increased chance of gas clogging was observed at high temperatures.

Thermally enhanced bioremediation: A review of the fundamentals and applications in soil and groundwater remediation

Thermally enhanced bioremediation (TEB), a new concept proposed in recent years, explores the combination of thermal treatment and bioremediation to address the challenges of the low efficiency and long duration of bioremediation. This study presented a comprehensive review regarding the fundamentals of TEB and its applications in soil and groundwater remediation. The temperature effects on the bioremediation of contaminants were systematically reviewed. The thermal effects on the physical, chemical and biological characteristics of soil, and the corresponding changes of contaminants bioavailability and microbial metabolic activities were summarized. Specifically, the increase in temperature within a suitable range can proliferate enzymes enrichment, extracellular polysaccharides and biosurfactants production, and further enhancing bioremediation. Furthermore, a systematic evaluation of TEB applications by utilizing traditional in situ heating technologies, as well as renewable energy (e.g., stored aquifer thermal energy and solar energy), was provided. Additionally, TEB has been applied as a biological polishing technology post thermal treatment, which can be a cost-effective method to address the contaminants rebounds in groundwater remediation. However, there are still various challenges to be addressed in TEB, and future research perspectives to further improve the basic understanding and applications of TEB for the remediation of contaminated soil and groundwater are presented.

Geothermal heating: Is it a boon or a bane for bioremediation?

There has been a worldwide interest in renewable energy technologies, as a means of decreasing reliance on fossil fuels, minimizing climate change effects, and reducing greenhouse emissions. One such technology is geothermal heating where the constant subsurface temperature is used to cool or heat building interiors via heat pumps. In Canada, the use of geothermal heating has become a popular option for heating and cooling buildings, and it is anticipated that, in the near term, most large buildings will include geothermal heating as part of their climate control strategy. However, little is known about the environmental impacts of geothermal heating on the subsurface environment. The present review will examine the effect of geothermal heating on groundwater flow and remediation efforts, whereby the heat generated by geothermal systems may help with urban pollution. "Geothermal Remediation" could leverage the subsurface heating resulting from geothermal systems to accelerate biodegradation of certain petroleum-based pollutants at brown-field sites, while providing building(s) with sustainable heating and cooling. This idea coincides with the rising momentum towards sustainable and green remediation in Europe and the United States. To ensure that Geothermal Remediation is achievable, the effect of heat on bioremediation needs to be examined. This review provides an insight into what we know about heat effects on bioremediation activities and subsurface transport.

Henry's law constants of volatile organic compounds between 0 and 95 °C - Data compilation and complementation in context of urban temperature increases of the subsurface

In urban areas with frequently occurring contamination by volatile organic hydrocarbons (VOC) possible uncontrolled contaminant mobilization e.g. by volatilization is feared in case of subsurface temperature increases induced by high temperature underground thermal energy storage (HT-UTES) or due to urban heat islands (UHI). However, volatilization of VOC is the main process utilized by thermal remediation approaches, wherefore a combination of contaminant remediation with UTES is increasingly discussed. To predict VOC volatilization under changing temperature conditions, which is substantially characterized by Henry's law constants (H), temperature dependency of H must be known substance-specifically in the total concerned temperature range. To provide an overview of available H and to evaluate their quality, published data of 41 pollutants were compiled, revealing the need for further measurements above 40 °C for at least 24 compounds (as e.g. TCE/PCE, BTEX). Additionally, the temperature dependence of H was calculated based on the compiled data using an established temperature regression function. Furthermore, H was complementarily measured for 10 relevant VOCs between 10 and 90 °C using the EPICS-method (Equilibrium Partitioning In Closed Systems). The measurements quantified the expected strong increase in H with rising temperature while providing high data quality (R² = 0.84-0.99, mostly low standard deviations), thus improving the general data availability of H for VOCs and extending the scope of the regression function. The newly measured data and enhanced evaluation of compiled data contribute to a more reliable assessment of the pollutant behaviour in terms of volatilization at elevated temperatures caused e.g. by UTES or UHI.

Evaluating a new injection method of liquid/gas mixture spray injection via performing long-term in situ bioremediation tests

During in situ bioremediation, continuous injection of growth substrates such as carbon sources, electron donors, or electron acceptors inevitably results in microbial growth, resulting in biological clogging in an aquifer. Therefore, for successful bioremediation, development of a new injection method is needed to reduce or alleviate this clogging problem. In this study, we carried out field tracer tests using single-well push-pull tests (SWPPTs), single-well natural gradient drift tests (SWNGDTs), and long-term in situ well-to-well tests to develop and evaluate a new method of liquid/gas mixture spray injection. The effectiveness of the new method was evaluated by estimating the factors as follow: longitudinal dispersivity (α_L), radius of influence (RI), shear stress on the surface of aquifer particles (σ), biofilm-shear-loss rate (R_s), and the ratio of volume occupied by cells grown to the original pore volume. At the tested site, the liquid/gas mixture spray injection method turned out to have several advantages compared to the traditional solution injection method: 1) transport of solute to a larger proportion of an aquifer by a factor of 1.3-1.7, 2) application of higher shear stress onto the surface of soil particles by a factor of 4.2-5.0, 3) faster biofilm sloughing rates by a factor of 2.3-2.6, 4) reduction in the ratio of the volume occupied by microorganisms to total pore volume (Vol_microbes/Vol_pore), and 5) efficient trichloroethylene (TCE) dechlorination for a period of 550 days without any injection problems. This new injection method showed positive effects on the hydrogeological and physical characteristics of the system, thus alleviating the biological clogging problem.

Comparative Life-Cycle Assessment of Aquifer Thermal Energy Storage Integrated with in Situ Bioremediation of Chlorinated Volatile Organic Compounds

Due to the increasing need for sustainable energy and environmental quality in urban areas, the combination of aquifer thermal energy storage (ATES) and in situ bioremediation (ISB) has drawn much attention as it can deliver an integrated contribution to fulfill both demands. Yet, little is known about the overall environmental impacts of ATES-ISB. Hence, we applied a life-cycle assessment (LCA) to evaluate the environmental performance of ATES-ISB, which is also compared with the conventional heating and cooling system plus ISB alone (CHC + ISB). Energy supply via electricity is revealed as the primary cause of the environmental impacts, contributing 61.26% impacts of ATES-ISB and 72.91% impacts of CHC + ISB. Specifically, electricity is responsible for over 95% of water use, global warming potential, acidification potential, and respiratory inorganics, whereas the production of the biological medium for bioremediation causes more than 85% of the eco- and human toxicity impacts in both cases. The overall environmental impact of ATES-ISB is two times smaller than that of CHC + ISB. Sensitivity analysis confirms the importance of electricity consumption and electron donor production to the environmental impacts in both energy supply and bioremediation. Thus, future studies and practical applications seeking possible optimization of the environmental performances of ATES-ISB are recommended to focus more on these two essential elements, e.g., electricity and electron donor, and their related parameters. With the comprehensive LCA, insight is obtained for better characterizing the crucial factors as well as the relevant direction for future optimization research of the ATES-ISB system.

A review on the sustainability of thermal treatment for contaminated soils

Sustainable remediation is a goal in the remediation industry. Thermal treatment can remediate contaminated sites quickly and reliably, but its energy-intensive nature and potential to damage soil properties make it seemingly not sustainable. This review evaluates the potential for thermal treatment to become a sustainable remediation technology based on a comprehensive analysis of the scientific literature. The fundamentals, advantages, and limitations of single thermal treatment technologies are summarized. The compatibility and advantages of thermal treatment coupled with thermal, physicochemical, or biological technologies are reviewed. The results suggest that ingeniously designed coupled technologies can improve the availability and removal efficiency of contaminant, suppress the production of toxic byproduct, and reduce the required heating temperature and energy input. The sustainability of thermal treatment is then discussed from the perspectives of energy efficiency and land reuse. Approaches for improving energy efficiency include applying solar energy-based technologies, smoldering-based technologies, and coupled technologies. For land reuse, heating below 250 °C has negligible adverse impacts on most soil properties, and can increase nutrient availability and release dissolved organic carbon to support the growth of microorganisms and plants. Heating above 250 °C can significantly reduce soil organic matter and clay content, which decreases the soil cation exchange capacity and water holding capacity, and consequently damages the soil fertility. Some restoration strategies are also proposed for the recovery of soil quality. In addition, thermally remediated soil is considered to be a good candidate as an engineering medium for construction. This review concludes with an outlook of future research efforts that will further shift thermal treatment toward sustainable remediation.

Potential impact of hydrodynamic shear force in aquifer thermal energy storage on dissolved organic matter releasement: A vigorous shaking batch study

The combination of bioremediation and aquifer thermal energy storage (ATES) has become attractive because of the possibility of solving environmental and energy problems simultaneously. While the impact of ATES on groundwater quality due to temperature change has received ample attention in literature, the effect of the greatly enhanced groundwater flow velocity on groundwater quality has not yet received sufficient scientific attention. To fill this gap in understanding, we conducted a simple yet straightforward experiment to illustrate the impact of hydrodynamic shear force due to the water flow by ATES on the release of dissolved organic matter, which can potentially be advantageous to bioremediation. Vigorous shaking conditions were applied to simulate the enhanced dynamics at the ATES well center and nearby. As the indicators of dissolved organic matter, COD and TOC concentrations were significantly impacted by shaking. COD increased from 5.4 mgO₂/L to 36.3 mgO₂/L during horizontal shaking. The maximum COD level was determined as 33.8 mgO₂/L during orbital shaking, while the TOC level was growing from 6.7 to 28.7 mg C/L. Meanwhile, redox potential (with initial level -100 mV) was decreasing to -450 mV synchronously with the elevating COD and TOC level. Temperature was also revealed as a significant factor in the organic matter releasement. Microbial iron reduction was deemed to occur, yet sulfate reduction was not initiated during the whole experiment. Eventually, the structure of the soil-water matrix has been changed due to the extensive hydraulic and particle collisions, resulting in blackish appearance and thicker layer of fine particles. Overall, the findings advance our understanding of the role of the ATES-induced water flow in the subsurface biogeochemistry and give insight into the perspective of the combination of bioremediation and ATES. In general, an increase in dissolved organic matter can be expected due to the increased shear force at high flow conditions in the ATES system.

Viral and bacterial community responses to stimulated Fe(III)-bioreduction during simulated subsurface bioremediation

The delivery of fermentable substrate(s) to subsurface environments stimulates Fe(III)-bioreduction and achieves detoxification of organic/inorganic contaminants. Although, much research has been conducted on the microbiology of such engineered systems at lab and field scales, little attention has been given to the phage-host interactions and virus community dynamics in these environments. The objective was to determine the responses of soil bacterial communities and viral assemblages to stimulated anaerobic Fe(III)-bioreduction following electron donor (e.g. acetate) addition. Microbial communities, including viral assemblages, were investigated after 60 days of Fe(III)-bioreduction in laboratory-scale columns continuously fed with acetate-amended artificial groundwater. Viral abundances were greatest in the influent section and decreased along the flow path. Acetate availability was important in influencing bacterial diversity, microbial interactions and viral abundance and community composition. The impact of acetate addition was most evident in the influent section of the columns. The increased relative abundance of Fe(III)-reducing bacteria coincided with an increase in viral abundance in areas of the columns exhibiting the most Fe(III) reduction. The genetic composition of viruses in these column sections also differed from the control column and distal sections of acetate-treated columns suggesting viral communities responded to biostimulated Fe(III)-bioreduction.

Effects of biological clogging on 1,1,1-TCA and its intermediates distribution and fate in heterogeneous saturated bio-augmented permeable reactive barriers

Biological clogging in porous media was an important concern in the design of bio-augmented permeable reactive barriers (Bio-PRBs) that were used to remediate groundwater with dense non-aqueous phase liquids (DNAPLs). Here, we used laboratory sandbox experiments to develop and calibrate reactive transport models (C1 and C2) simulating 1,1,1-trichloroethane (1,1,1-TCA) change in heterogeneous saturated porous media. The routine (1,1,1-TCA chain kinetic reactions) and subroutine (the relationship between hydraulic conductivity (K) and time (t)) were included in the model computer code. The simulation results suggested that the model C1 had the applicability for simulating contaminant transport and fate in bio-augmented flow field. By using the model C1 which was suitable for constant K condition, the performance of different types of Bio-PRBs was evaluated, and the regularity of contaminants chain kinetic reactions in different heterogeneous saturated porous media was obtained. The results demonstrated that Bio-PRBs in immobilized microorganism (IM) protocol were more superior to Bio-PRBs in free microorganism (FM) protocol. In addition, by using the model C2 (updated model C1) which was suitable for decreasing K condition, the different and optimized regularity of contaminants transport and transformation was obtained. The results showed that microbial growth which further decreased K was beneficial to preventing the transport of contaminants and accelerating the transformation of contaminants. However, the negative effects of biological clogging on hydraulic conductivity and relative hydraulic conductivity ratio in FM Bio-PRBs were significantly stronger than that in IM Bio-PRBs. Deploying IM Bio-PRBs for groundwater remediation would be much more efficient and meet the design criteria. The research work had guiding significance to engineering and provided consultation for designing and optimizing Bio-PRBs system. To make the design and optimization of Bio-PRBs system convenient, it was very essential to choose the suitable mathematical model (C1 or C2).