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.

Abstract

Biocementation using the microbial-induced calcite precipitation (MICP) technique is an environmentally friendly method that imitates the natural cementation phenomenon to improve the behavior of soils. Urea hydrolysis performed by ureolytic microorganisms results in the immobilization of the calcium ions by electrostatic attraction, leading to the formation of calcite crystals. The Ganga River sand (GRS) used in the present study is a fine-grained sand possessing serious engineering problems and in need of such improvements, for which a detailed study is necessary beforehand. Hence, the present study has explored the competence of different soil bacterium of the Bacillus family i.e., Bacillus sphaericus, Bacillus sp., and Bacillus subtilis in hydrolyzing urea, and also for forming the desirable biofilm over GRS in India. The colloidal behavior of the prepared bacterial solution along with the hydrodynamic diameter for different bacteria and its importance in the biofilm formation over GRS particles have been examined. The study is extended to determine biofilm formation’s competence for urea hydrolysis and calcite precipitation at different intervals. It is revealed that the biofilm formed by B. subtilis was stronger but the precipitation by B. sphaericus was maximum with 23% precipitation. Further, the rate of precipitation over time and its effect on the morphology of calcite crystals for all the selected bacteria have been discussed.

Practical Applications

MICP is a natural method of soil improvement. The microorganisms used for this treatment are nonpathogenic which precipitates calcite in the sand pores and over the surface of sand particles. This phenomenon leads to the densification of the sand by calcite precipitation in the pores which is responsible for the enhanced geotechnical properties. By controlling the rate of precipitation, MICP can be useful in different applications like strengthening sand, bioremediation of toxic metals, surface erosion control, liquefaction resistance, permeability reduction, soil stabilization, seal fractures, carbon sequestration, and so forth. The treatment requires the least amount of energy input and only the specific metabolism pathway of bacteria is needed for this method. The technique is comparatively easier to apply in the fields and requires little to no additional skills for implementation. MICP also finds its usage in concrete technology alongside ground improvement techniques. This technique is useful for the production of self-healing concrete, bio-bricks, and so forth. Its application goes far beyond the boundaries of civil engineering projects and requires an interdisciplinary approach to understand its full potential.

Abstract

Soil contamination by chlorinated volatile organic compounds (CVOCs) such as tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (cDCE) has become apparent. When the contamination period increases, soil contaminated by CVOCs becomes more difficult to clean. One of the important reasons for this difficulty of remediation could be that CVOCs sorb into the deepest part of the soil particle pores over the contamination period, making desorption difficult. However, previous studies have not been able to confirm the long-term sorption behavior of CVOCs on soil because the increase in sorption was too small to measure after reaching a fast sorption equilibrium in a few hours or days. In this study, we investigated the long-term sorption behavior by measuring the difficulty of repeated solvent extraction from two types of soil that sorbed CVOCs via the gas phase over 5 months. The results showed that CVOCs become more difficult to extract from the soils as the contamination period increases. Upon comparing the two types of soils, we found that the smaller the particle size, the faster the increase in slow sorption due to the larger external surface area and shorter distance to penetrate to the deepest of pores. We also discovered that the greater the number of macropores, the easier it is for CVOCs to penetrate deeper into the soil pores. In addition, cDCE, rather than PCE and TCE, had lower absorptivity and a higher molecular diffusion coefficient, resulting in a larger diffusion coefficient for easier intragranular penetration into the deepest of pores. From the results, at the actual site, after several decades of sorption, CVOCs sorbed deep in the micropores and mesopores of the coarse particles are extremely difficult to desorption, and may be the cause of the prolonged cleanup process and rebound phenomenon.

Practical Applications

This study confirms that the longer soil is contaminated with CVOCs, the more difficult it is to elute CVOCs from soil particles. The relationship between the difficulty of extraction and the characteristic values of soil particles and material properties was also discussed. At an actual contamination site, much of the contamination occurs over a period of several decades, so it is needed to study the long-term sorption behavior of CVOCs on soil particles. However, in previous studies, sorption behavior was examined over a short period of time as several hours or days, but long-term sorption behavior could not be measured. This study measured long-term adsorption behavior, and the results and findings could explain one of the reasons for the difficulty of remediation at actual contaminated sites. It will be useful for studying which soils and which substances are more difficult to clean up, and for considering improvements in more efficient remediation methods.

The effective treatment of wastewater generated from households, communities, and industries is vital for preserving our environment. Chemical treatment, which involves adding coagulant chemicals directly into the wastewater, is the most popular treatment method. However, due to its high operating costs and the issue of secondary contamination, there is a need for an effective method to address these issues. In this study, we investigated the electrocoagulation (EC) system, which is currently undergoing a revival in research due to its promising features, such as low system and operating costs and its environmentally friendly process. We developed two-dimensional unsteady mathematical models supported by Langmuir isotherm theory to elucidate the underlying physics of the aluminum-based EC process and to understand key control parameters. The developed model was used to analyze the effects of EC cell geometry and operating conditions on the local distribution of intermediate ionic species and their impact on the adsorption behaviors of EC-generated coagulants on arsenic pollutants in the wastewater for the first time. It was found that a higher removal rate was achieved as the EC treatment time and current intensity increased, while the cell gap and inflow rate decreased. Applying the current-off condition during the EC process was found to be an efficient method for increasing the removal rate with less consumption of electrical energy.

Lactic acid (LA) and probiotic biomass production were evaluated from bovine manure supported with 12% v/v molasses using three probiotic bacteria, Lactobacillus acidophilus, Lactobacillus fermentum, and Bacillus subtilis, with three inoculum concentrations, 5%, 10%, and 15% v/v. Laboratory-scale tests were carried out in 250-mL flasks (useful volume of 200 mL) at 37°C and 120 rpm for 72 h. During this time, samples were taken every 4 h to determine (1) the LA content using titration, (2) cell growth using the poured plate method, and (3) consumption of carbohydrates using the anthrone-sulfuric method. The data obtained were adjusted to the Gompertz generalized four-parameter model to obtain the kinetic parameters of the growth of the bacteria in the residue. With the species L. acidophilus at 10% v/v, the highest LA production yield was obtained, $Y_{(P/S)}=0.7\mathrm{g}\mathrm{LA}/\mathrm{g}$ carbohydrates, with a concentration of $13.5\mathrm{g}\mathrm{LA}/\mathrm{L}$ ($p>0.05$) and values of ${\mu }_{\max }=0.161/\mathrm{h}$ and $K_s=0.46\mathrm{g}/\mathrm{L}$. With the same bacteria at 15% v/v, the biomass obtained had the highest concentration of proteins, 32.19% dry weight of proteins ($p>0.05$). The results showed that bovine manure has the potential to produce LA and biomass, which are products with high nutritional, economic, and energetic value.

In order to investigate the effect of compound antibiotics on anaerobic digestion of chicken manure, the inhibitory effect of three different combinations of oxytetracycline (OTC), sulfamethoxazole (SMZ), and enrofloxacin (ENR) on anaerobic digestion of chicken manure was studied by the sequencing batch method. Under the condition of adding three different combinations of antibiotics, the experiment was run for 50 days. The concentrations of soluble chemical oxygen demand (SCOD), total ammonia nitrogen (TAN), VFAs, and methane production in the reactor were measured, and the microbial diversity affecting the stability of the system was analyzed. The results showed that OTC and SMZ inhibited methane production, while SMZ and ENR promoted methane production, and the cumulative methane production was $\mathrm{R}4>\mathrm{R}3>\mathrm{R}1>\mathrm{R}2$. After the 25th day, the SCOD concentration in the R2 and R4 groups decreased, which was due to the existence of SMZ and ENR promoting the degradation of SCOD. TAN concentrations in the four groups were kept between 1,190 and $\mathrm{2,100}\mathrm{mg}/\mathrm{L}$. The results of 16S rRNA sequencing showed that the mixed antibiotics had different effects on the bacterial community, which was mainly composed of Firmicutes, Bacteroidota, unidentified bacteria, Cloacimonadota, Proteobacteria, and Spirochaetota. Bacteroidota was superior in the whole reaction process under the influence of OTC and SMZ because it stimulated the microbial flora and made its activity higher. The study provides relevant data for the proper management of livestock manure.

Herein we present the production of a two-stage wastewater treatment system for industrial azo-dye using an immobilized nitrogen-doped zinc oxide (ND-ZnO) nanoparticles and a microalgae bioreactor. The treatment system was found capable of reducing up to 95% of methylene blue wastewater in 48 h under a seminatural condition. In the first stage, the immobilized ND-ZnO chamber was found to have photocatalyzed up to 80% using natural sunlight in 4 h. In the second stage, the algae (Chlorella vulgaris) were found to be capable of repeatedly handling the semitreated wastewater and found remained active for at least 7 cycles (hydraulic retention time = 44 h, solid reaction time = 14 days). The algae not only take part in purifying the wastewater but also absorb and metabolize the toxic cleaved by-products therein. The presented system demonstrated a low-cost alternative, both in terms of production and operation, for treatment of recalcitrant azo-dye. Furthermore, unlike other pilot systems using suspended nanoparticles, expensive recovery of nanoparticles is not required and accidental discharge of nanoparticles is avoided. Toxicity assay using fish gill cell line demonstrated that the acute toxicity of the effluent was negligible. To conclude, this novel treatment system opened a new and safe way for effective yet simple treatment of azo-dye wastewater.

In this paper, an error-correcting ensemble model combining empirical mode decomposition and reconstruction combined with statistical methods is proposed to improve the prediction accuracy of a high concentration of particulate matter with diameters of 2.5 μm or less (PM2.5) around a tailings reservoir. The proposed prediction model consists of a prediction model of a high concentration of PM2.5 and an error correction model. The optimal prediction model is obtained by comparing the two indicators of fitting effect and result error, constantly adjusting the batch size and the number of network layers. The measured data of the tailings reservoir in Ma’anshan City, China, were used for validation. The results showed that the mean absolute error (MAE) of our long short-term memory–improved empirical mode decomposition–long short-term memory (LSTM-IEMD-LSTM) mode was 0.125, 0.661, and 4.372 lower than that of LSTM, multivariable linear regression–improve empirical mode decomposition–multivariable linear regression (MR-IEMD-MR), and autoregressive integrated moving average model–improve empirical mode decomposition–autoregressive integrated moving average model (ARIMA-IEMD-ARIMA), and the root-mean-square error (RMSE) of our LSTM-IEMD-LSTM compared with LSTM, MR-IEMD-MR, and ARIMA-IEMD-ARIMA decreased by 13.2%, 45.5%, and 84.2%. The model is also used to predict other high-concentration pollutants to test their generalization ability, which also has high prediction accuracy.

This study deals with how shifting (jhum) cultivation affects the physical, chemical, and hydraulic properties of soil-water resources in the vadose zone. This included analysis of changes in parameters such as organic matter concentration, pH, nutrient levels, and hydraulic conductivity, as well as soil texture and structure. For this, a microcatchment situated in the Aizawl District of Mizoram State in Northeast India was selected. Three sites were chosen in the catchment, each representing a different stage of cultivation viz. (1) Nonjhum land (NJL) or fallow land, (2) Newly burnt jhum land (NBJL), and (3) Cultivated jhum land (CJL). By selecting shifting (jhum) sites with similar topographical conditions, this study provides a better understanding of the effects of shifting (jhum) cultivation on the physicochemical properties of the local soil and water resources. To assess the effects of shifting (jhum) cultivation at three different stages of shifting (jhum), soil samples were collected from multiple depths, including the top surface, 5 cm, and 55 cm of the vadose zone. Water and sediment samples were also collected from three streams flowing downstream from these three shifting (jhum) sites. Findings reveal that both mean hydraulic conductivity and surface infiltration values were highest at NBJL. Furthermore, soil indices including the soil aggregate stability index (SASI), soil sealing index (SSI), and Forestier index (FI) were estimated to understand the properties of surface soils and their micronutrient and macronutrient availability. According to the SASI analysis, all soil samples from the shifting (jhum) sites exhibited a stable soil structure. The SSI results indicated that the surface soils had no risk of thrust or erosion due to shifting (jhum) practice. The FI assessment revealed that the vadose zone soils possessed abundant nutrient reserves. Soil texture in all three areas was found almost similar, with maximum percentages of medium to fine sand and the texture varying from sandy loam to loamy sand. Total organic component (TOC) (%) of surface samples were highest at CJL at 8.73% compared to 7.62% at NBJL and 7.47% at NJL. To study the seasonal variability of sediment quality, premonsoon, and monsoon comparison of all the stream sediment analysis was conducted. Macro and Micronutrients (phosphorus, potassium, calcium, magnesium, manganese, iron, sodium, and zinc) were quantified for soil and sediment samples to study the changes in their concentration at various stages of the shifting (jhum).