Journal of Environmental Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Abstract

Underground storage tanks (USTs) primarily store and dispense fuel products (petroleum hydrocarbons) such as gasoline and diesel as well as some hazardous substances. When a release occurs from an UST, it must be cleaned up and remediated as necessary. As part of the site characterization process, monitoring wells are installed to measure petroleum hydrocarbon concentrations, such as benzene, to help site managers make decisions on remediation strategies and effectiveness, determine site status, and whether sites pose a risk to local receptors—with implications to human health and the environment. Previous studies have used various approaches to estimate benzene plume lengths using these monitoring well data. Examining one state’s large database of monitoring well data from leaking underground storage tank (LUST) cleanup sites, and through statistical and spatial analysis, we find that studies that used existing monitoring well data may have underestimated plume lengths. Our study identified elevated benzene concentrations at the boundaries of monitoring well extents, including at the leading edge of plumes. For example, in 3,495 LUST sites, 83% exceeded a selected threshold concentration of $5\mathrm{ug}/\mathrm{L}$ in a monitoring well at the site boundary, and 76% of sites exceeded $5\mathrm{ug}/\mathrm{L}$ at the monitoring well furthest from a defined source, the point used to delineate length. Of the total 17,816 monitoring wells that delineated UST site boundaries, 47% had concentrations exceeding $5\mathrm{ug}/\mathrm{L}$. These data analyses and results are done over the lifetime of a monitored UST site, not targeted over time periods when decisions are being rendered. However, results suggest that studies to date using existing monitoring well data may have underestimated plume lengths and extents. Furthermore, results suggest monitoring wells in combination with other site information, such as modeling, should be used to guide site managers in their decisions regarding leaked UST products, the area and extent of contamination, risks to receptors, and remedial approaches.

Practical Applications

USTs, which store fuel at locations such as gas stations, are known to leak fuel products into soil and groundwater. Fuel products contain petroleum hydrocarbons like benzene, a cancer-causing chemical. Previous studies that have attempted to identify the maximum distance benzene plumes travel have been hampered by a bias in available data—specifically, that monitoring wells are often not placed at far enough distances to be able to map a plume to concentrations below concerning levels. This paper identifies this bias and provides insight into how it may be considered in the future to more effectively map plumes and identify potential biases that can lead to inconclusive results related to plume behavior. While traditional physics-based models remain useful, they can be augmented with data-driven models to more accurately estimate plume extents, especially when real-world data are lacking.

Acid mine drainage (AMD) is an environmental concern, and its discharge into the surrounding environment can lead to acidification of water bodies and soil. In this research, red mud (RM) with alkaline pH as aluminum industry waste was implemented as an adsorbent for sulfate removal, which can be available in AMD and adsorption processes as an influential method was applied for AMD treating. Seawater washing (BRM), acid treatment (HRM), and composition with chitosan (CRM) were employed as RM modification methods. Various analytical methods were applied, including x-ray fluorescence spectroscopy (XRF), Fourier-transform infrared spectroscopy (FTIR), Brunauer Emmett Teller (BET), inductively coupled plasma analysis, and zeta potential, to have a clear comprehension of the sulfate adsorption behaviors and performance of activated methods. These methods have had desirable effects on the sulfate adsorption and adsorption amount, which range from ${1.1\mathrm{mg}\mathrm{g}}^{-1}$ for RM reached to 13.4, 39.7, and ${67.4\mathrm{mg}\mathrm{g}}^{-1}$ for BRM, HRM, and CRM, respectively. XRF results demonstrated a decrease in calcium and sodium ion amounts that can occur due to their dissolution in the acidic solution. It can create the active surface areas for adsorption onto RM with acid washing. Based on FTIR results, sharp intensity vibration was observed in BRM, HRM, and CRM after sulfate adsorption in the ${\mathrm{1,100}\mathrm{cm}}^{-1}$ band that is approximately related to ${\mathrm{SO}}_4^{2-}$ vibration. BRM porosity decreased from 0.0686 to ${0.0345\mathrm{cm}}^3{\mathrm{g}}^{-1}$ due to various salts’ precipitation in seawater, but the porosity volume and specific surface area increased for HRM from 0.0686 to ${0.184\mathrm{cm}}^3{\mathrm{g}}^{-1}$ due to dissolution of ions in HCl. In addition, zeta potential increased to positive amounts from 2.7 to 3.4, 3.8, and 4.5 mv for BRM, HRM, and CRM, respectively. Langmuir isotherm indicated that the highest adsorption amount (${\mathrm{q}}_{\mathrm{m}}$) as a calculated parameter for BRM, HRM, and CRM was 0.88, 2.45, and ${18.6\mathrm{mg}\mathrm{g}}^{-1}$, respectively. Among modification methods, combination RM and chitosan had the highest impact and increases sulfate removal and adsorption amount at lower pH levels due to further positive charge on the CRM surface.

The influent of municipal wastewater treatment plants (WWTPs) in China is generally facing the problem of insufficient carbon sources. Pyrite-based autotrophic denitrification (PAD), which uses pyrite as an electron donor, has good advantages of less acid production, low sulfate generation, and efficient simultaneous nitrogen and phosphorus removal from wastewater. In this study, a novel type of mixotrophic sequencing batch biofilm reactor (SBBR) was constructed by combining PAD with SBBR, and the effect of reflux ratio on the treatment efficiency of pyrite-based SBBR was studied. The research showed that the addition of reflux improved the removal capability of nitrogen and phosphorus, and the optimal reflux ratio for the pyrite-based SBBR was 200% when the removal rates of ${\mathrm{COD}}_{\mathrm{Cr}}$, ${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$, TN, and ${\mathrm{PO}}_4^{3-}\text{-}\mathrm{P}$ reached $71.07\%\pm 5.33\%$, $80.94\%\pm 7.23\%$, $53.94\%\pm 3.34\%$, and $49.68\%\pm 11.85\%$, respectively in the anoxic environment with pH 7–8. The variation of pollutants in the typical operating cycle showed that most of the organic pollutants were removed in the aerobic stage. The analysis of extracellular polymeric substances (EPS) further verified that the addition of reflux was beneficial to the removal of nitrogen and phosphorus.

Up to now, only limited data are available on the influence of process-related variables [influent characteristics, solids retention time (SRT), hydraulic retention time (HRT), biomass recirculation rates, process configuration, biochemical reaction rates, etc.] on oxygen transfer, as stated in the aeration manuals. Recently, standards have been updated in regard to oxygen transfer testing in clean water as well as in in-process wastewater. Unfortunately, the situation has not improved since the publication of these documents. In particular, little information exists on alpha ($\alpha$) and oxygen transfer efficiency $({\mathrm{OTE})}_{\mathrm{f}}$ in biological nutrient removal (BNR) processes to study the impact of biochemical reactions on oxygen transfer under controlled process conditions. A new model is herewith proposed that would deal with the effect of oxygen consumption rate due to these substrate materials.

Solid ammonia dosing technology has been extensively researched in the last decade as an effective solution to address issues such as urea crystallization and carrier blockage in traditional selective catalyst reduction (SCR) systems. In this study, a comparison of nitrogen oxide (NOx) conversion efficiency between solid ammonia dosing (SAD) and Adblue dosing on a heavy-duty diesel engine SCR system was conducted. First, an engine emission test bench with SAD SCR and Adblue SCR systems was established. Subsequently, both dosing systems were tested and studied on the engine emission test rig. The results showed that the NOx conversion efficiency of SAD was slightly improved by approximately 2%–5% when the catalyst upstream temperature exceeded 250°C, compared with the Adblue system. This improvement can be attributed to the better mixing ability of SAD. Additionally, SAD demonstrated significant advantages in avoiding issues related to urea decomposition at low exhaust temperatures. Furthermore, similar results were observed in the emission cycle test results of the engine. Under the European steady-state cycle (ESC) and European transient cycle (ETC) conditions, the NOx conversion efficiency of SAD was only slightly higher than that of the Adblue system by about 2%. However, under the world harmonized transient cycle (WHTC) condition, where more low-temperature operating conditions are included, the beneficial weighted NOx conversion efficiency of SAD increased to approximately 7.4% at a release condition of 190°C.

This study evaluated the effectiveness of using iron (Fe) powder and ferrous chloride (${\mathrm{FeCl}}_2$) to decrease hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) concentration ([${\mathrm{H}}_2\mathrm{S}$]) from biogas production by anaerobic digestion (AD) of primary sludge, and to decrease phosphate (${\mathrm{PO}}_4\text{-}\mathrm{P}$) concentration ([${\mathrm{PO}}_4\text{-}\mathrm{P}$]) in the sludge. [${\mathrm{H}}_2\mathrm{S}$] and [${\mathrm{PO}}_4\text{-}\mathrm{P}$] removal increased as doses of Fe powder and ${\mathrm{FeCl}}_2$ increased to a certain point. Optimal supplementation was $5\mathrm{g}/\mathrm{L}$ for Fe powder (${\mathrm{H}}_2\mathrm{S}\text{reduction}=98\%$, ${\mathrm{PO}}_4\text{-}\mathrm{P}\text{removal}=95\%$) and $0.5\mathrm{g}/\mathrm{L}$ for ${\mathrm{FeCl}}_2$ (${\mathrm{H}}_2\mathrm{S}\text{reduction}=98\%$, ${\mathrm{PO}}_4\text{-}\mathrm{P}\text{removal}=97\%$). Appropriate dosage of Fe powder or ${\mathrm{FeCl}}_2$ can control [${\mathrm{H}}_2\mathrm{S}$] in biogas and decrease [${\mathrm{PO}}_4\text{-}\mathrm{P}$] in AD effluent, but excessive dosage can inhibit the digestion. The biogas production and methane content were higher with Fe powder compared with ${\mathrm{FeCl}}_2$, but neither treatment had a significant effect on methane yield. The measured biogas production was accurately captured by the first-order kinetic model, as evidenced by a strong fit with high $R^2$ values (0.957–0.980), and the deviations between the measured and predicted biogas production using the first-order kinetic model were $<10\%$.