Development Of A Bench-scale Denitrification Wall For Nitrate Contaminated Groundwater In-Situ Remediationand Performance Evaluation

Nitrate contamination in groundwater is increasingly serious, which attracts widespread attention. Consuming nitrate-contaminated water may cause many diseases such as methe-moglobinemia and alimentary canal cancers. Therefore, it is necessary to develop efficient remediation technologies for nitrate-contaminated groundwater.In-situ biological denitrification is considered as an effective and economic process for nitrate removal from groundwater, and the main method for such in-situ groundwater treatment involves the use of a permeable reactive barrier. A biological denitrification grid is like the traditional permeable reactive barrier which is vertically installed underground and intercepts groundwater contaminants from migrat-ing into uncontaminated aquifers by using a chemical and/or biological treatment zone. This zone is often installed in slab or trench configurations that are backfilled with zero-valent iron, chelators, sorbents, and/or microbes. Denitrification walls(DWs) are traditional permeable reactive barriers that are inserted vertically into the ground to intercept ground-water flow; denitrification is then facilitated in the flow by adding electron donors,(such as organic matter) in the denitrification grid. Many pilot and field studies on denitrification walls have been conducted over the past decade. It is proved that DW is a long term and efficiently operated remediation technology.Before the PRB or DW is applied in the field, laboratory studies are considered to be essential for highlighting some negative problems that may arise under field conditions. Most commonly, batch and column experiments are used to select viable reagents for the PRB and to evaluate these agents’ capacity for contaminant removal. However, these column experiments only reflected one or two-dimensional flow conditions in the groundwater, while in practice, the actual seepage conditions in the aquifer or vadose zone are three-dimensional. Therefore, column experiments are unable to reflect practical operating conditions of DWs in the field. In contrast, sand-tank experiments may simulate the groundwater flow conditions more accurately.In this study, biodegradable plastic(BP), woodchips, and sulfur were used as sloid electorn donors, while zeolite was selected as biocarrier to study the application of heterotrophic denitrification and mixotrophic denitrification in the in situ remediation of nitrate contaminated groundwater. Batch and sandtank experiments were carried out to study the pollutants migration mechanisms druing the remedation process,and provided theoretical basis for applying the PRB or DW in actual contaminated sites. The results showed that:When the dosage ratio of BP to zeolite was 2:1(w/w), higher denitrification efficiencies, larger amount of biofilms, lower operation costs and less secondary pollution were observed in the heterotrophic denitrification process; The DW construction does not significantly influence average nitrate content of soil in the vadose zone. When nitrate loading was ≤ 157.68 mg N/(kg·d BP-zeolite), the nitrate removal efficiency of DW exceeded 97.7%. In the water tank and the upper grille in the aquifer denitrification were not observed in ammonia and nitrite accumulation. The permeability of DW increased and remained relatively stable after 35 days of operation, and no blocking was observed during the experimental period(80 days); Estimated longevities of the DW would be 27.3 years and 17.2 years when the nitrate loading rates of 86.66 mg N/(kg·d BP-zeolite), and 157.68 mg N/(kg·d BP-zeolite), respectively.Ammonium salts are needed for enhancing the denitrifying activity of the Thiobacillus bacteria, and the ammonium produced by the dissimilatory nitrate reduction to ammonium(DNRA) process could be utilized by the Thiobacillus bacteria in the woodchip-sulfur based heterotrophic and autotrophic denitrification(WSHAD) process; The denitrification performance of the WSHAD process is better than that of sulfur-based autotrophic denitrification, and the optimized ratio of woodchips to sulfur should be 1:1(w/w); No sulfate accumulation is observed in the WSHAD process, and the alkalinity generated by the heterotrophic denitrifiers can be consumed by the autotrophic denitrifiers; The autotrophic bacteria and the heterotrophic bacteria can coexist in the mixotrophic environment in the WSHAD process.The mixotrophic DW construction does not significantly influence on the average nitrate content and concentration distribution of soil in the vadose zone. When nitrate loading was ≤17.86 mg N/(kg·d Biocarrier), the nitrate removal efficiency of mixotrophic DW exceeded 86.6%. No nitrite and ammonium accumulation is observed in the WSHAD process, and the alkalinity generated by the heterotrophic denitrifiers can be consumed by the autotrophic denitrifiers; the autotrophic bacteria and the heterotrophic bacteria can coexist in the mixotrophic environment in the WSHAD process. The permeability of mixotrophic DW remained stable during the operation, and no blocking was observed during the experimental period.The developed bench-scale setup is valuable for design calculation of the denitrification wall construction in the field.