Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Co-Major Professor

Mahmood Nachabe, Ph.D.

Committee Member

Mauricio Arias, Ph.D.

Committee Member

Aydin Sunol, Ph.D.

Committee Member

Bina Nayak, Ph.D.

Keywords

biofilter, cation exchange capacity, denitrification, nitrification, surface area

Abstract

Excessive nitrogen and fecal indicator bacteria (FIB) in stormwater runoff from urban areas or dairy farm operations impair water bodies resulting algal blooms, gastrointestinal illness, and economic losses. Recent advancements in stormwater management, including low impact development (LID) techniques, have provided engineers with a variety of tools to use in place of traditional catch basins and retention ponds. One promising LID technology for runoff management is bioretention, which consists of a shallow depression containing engineered porous media and vegetation. Prior studies have shown that modified bioretention systems that include an internal water storage zone (IWSZ) to promote denitrification can improve total nitrogen (TN) and FIB removal. However, limited adsorption, nitrification, ammonification and denitrification lead to wide range of removals have been reported for total ammonia nitrogen (TAN), dissolved organic nitrogen (DON), dissolved organic carbon (DOC) and total nitrogen (TN) in sand based bioretention systems. Moreover, limited surface area (SA) and porous structure of sand hinder FIB attachment, resulting low removal efficiencies. Therefore, amending engineering media with an appropriate adsorbent can enhance bioretention performance.

Biochar is a promising low cost adsorbent that can be added to bioretention media to improve TN and FIB removal in urban and dairy farm runoff. It is a carbon rich by-product of waste materials pyrolyzed at high temperature under oxygen limited conditions. The type of feedstock, production process and pyrolysis temperatures are key factors that influence biochar properties. In general, biochar has a high cation exchange capacity (CEC), moisture content (MC), SA, porosity (n), pore size distribution (PSD), hydrophobicity, and ash content. The high nutrient retention and water holding capacity (WHC) of biochar aids plant growth, which can help inactivate pathogens and promote nitrification and denitrification by releasing root exudates. The overall goal of this research was to expand the current knowledge of biochar amended sand bioretention systems to manage TN and FIB removal from urban stormwater and agricultural runoff. This dissertation research was conducted in three phases, which included both laboratory-scale and pilot-scale studies with mathematical modeling.

Selection of media was initially carried out by characterizing both biochar and sand based on pH, hydraulic conductivity (K), bulk density (BD), electrical conductivity (EC), grain size distribution (GSD), n, MC, CEC, SA and PSD. Based on the physicochemical properties, masonry sand was used as the main medium for all three phases of research.

In Phase I, the influence of biochar properties on the fate of N-species and Escherichia coli (E. coli) removal in bioretention systems was investigated through batch and column studies using sand media, with and without biochar addition, for treating urban runoff. Two different commercial wood-chip based biochars were tested that were produced at different temperatures. In abiotic batch experiments, significantly higher TAN adsorption was observed for biochar (3.5 mg/g) than sand (0.05 mg/g) due to the higher CEC of biochar. Data also showed that, biochar had very low NOx (NO3−+ NO2−) adsorption capacity. TAN, DOC, and E. coli removals were significantly higher in biochar-amended columns due to biochar’s high CEC, pH, microporous structure, carbon content and SA. TAN adsorption resulted in increased nitrification during the antecedent dry periods (ADPs) when aerobic conditions developed. MC data revealed that saturated conditions prevailed toward the bottom of biochar-amended columns for several days after the storm event due to the high WHC of biochar, which favored denitrification and TN removal. E. coli removal was a strong function of SA and hydrophobicity; greater than 6 log E. coli removal was observed in the column amended with high SA biochar. Biochar amended columns also showed more stable TAN, DOC and E. coli effluent concentrations under varying hydraulic loading rates (HLRs) and ADPs. Therefore, biochar with higher SA was selected for the next phases of experiments.

In Phase II, the effect of biochar amendment rate on nitrogen species, DOC and FIB removals and hydraulic performance was evaluated in biofilter columns treating dairy farm runoff. One of the key differences between urban and dairy runoff are the compositions of runoff. Dairy runoff has a higher ionic strength, higher concentrations of DON, DOC, suspended and dissolved particles than urban runoff. Two biofilter columns with different biochar fractions (20% and 50% by volume) were operated at varying HLRs and ADPs. TAN, DON and DOC removals were significantly higher for the higher biochar fraction amended column. The higher biochar amendment rate increased the surface charge availability for TAN adsorption (71%), even with more complex influent compared to the column with the lower biochar amendment rate (34%). The high CEC of biochar increased TAN retention during the application period, allowing for nitrification during the ADPs when aerobic conditions developed in the media pores. However, low effluent NOx concentrations were observed from both systems. Biochar high SA also resulted in greater retention of DON and DOC by adsorption. The high WHC of biochar and presence of adsorbed DOC enhanced denitrification. Therefore, TN removal was significantly higher with the higher biochar amendment rate (65%) compared with the lower biochar amended column (39%). Significantly higher E. coli removals were observed compared with Enterococci in both columns, indicating a greater attachment affinity to the biochar surface for E. coli. However, there were no significant differences in E. coli or Enterococci removals between the two columns with different biochar fractions. Moreover, longer ADPs were found to enhance E. coli removal in the higher biochar fraction column.

A variable saturation flow model of biochar amended biofiltration was developed using HYDRUS-1D software. The model was calibrated using data from conservative tracer and moisture content studies. Model results showed that the high microporous structure of the biochar increases the time needed to reach full saturation, lowers the saturated conductivity and increases the hydraulic retention time in the medium.

In Phase III, pilot-scale studies were conducted with four modified bioretention systems that included an IWSZ. Experiments were designed to test TN and FIB removal performance with and without biochar and with and without plants (Muhlenbergia). Higher DOC adsorption in the IWSZ in systems with biochar favored denitrification, resulting in higher TN removal (>96%) in both biochar (B) and biochar with plant (BP) bioretention systems. Due to high moisture and nutrient retention, better plant growth was observed in BP compared with sand with plant (SP). The presence of plants also influenced N-species removal. The inclusion of plants, biochar and an IWSZ in pilot-scale systems resulted in the best E. coli removal (> 5 log E. coli removal). Plants can improve E. coli removal through predation and competition by rhizosphere microbes or inactivation by antimicrobial compounds from root exudates. Higher E. coli removals in pilot units with an IWSZ may have been due to the longer retention times and/or the anoxic conditions present in the IWSZ.

Future research should be carried out considering other pollutants i.e. phosphorous, metals, pesticides and viruses, which also cause water quality impairment. Pilot and field-scale research should also be carried to investigate maintenance requirements for biochar amended bioretention.

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