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.

Soil pH can effectively regulate phosphorus (P) transport. In this study, phosphogypsum (PG) leachate, with a high concentration of fluoride ions (${\mathrm{F}}^{-}$) and low pH, was applied to soils for the control of P release under different soil pH conditions. P transport in soil columns was investigated by monitoring tracer molecules in acidic, alkaline, and neutral soils. Saturated P-soil column experiments were conducted to determine the influence of soil pH on the release and retention of P derived from PG leachate in the soil. The results were compared with those obtained using phosphate fertilizer solution and a blank control, which showed that PG leachate with a high ${\mathrm{F}}^{-}$ concentration and low pH significantly reduced the P retention capacity of all three types of soil. The high ${\mathrm{F}}^{-}$ concentration and low pH level of PG leachate did not affect the conversion direction of P in the acidic soil environment (S01), which was dominated by iron (Fe) and aluminum (Al) elements. In contrast, the high ${\mathrm{F}}^{-}$ concentration and low pH level promoted the conversion of dicalcium phosphate (${\mathrm{Ca}}_2\text{-}\mathrm{P}$) and octacalcium phosphate (${\mathrm{Ca}}_8\text{-}\mathrm{P}$) to iron phosphate (Fe-P) and aluminum phosphate (Al-P) in alkaline soil (S02), which was dominated by calcium (Ca) systems. Varying pH conditions were found to change the fractions of various P forms in soil by affecting microbial biomass and activity related to soil P cycling, thus inhibiting the mutual conversion of PG leachate P between inorganic and organic forms in different types of soil. The findings of this study provide support for the effective control of soil and groundwater pollution caused by P contamination derived from PG leachate.

Methane recovery in up-flow anaerobic sludge blanket (UASB) reactors performing anaerobic digestion (AD) can be improved with adjustments in operational factors such as temperature, organic loading rate, and the addition of granular activated carbon (GAC). This study aims to perform a multiple operational factor analysis for their impacts on UASB microbial communities. Data collected from seven continuously operated UASB reactors and batch tests were analyzed using a range of bioinformatics and statistical tools. Temperature and reactor types were the most important factors in microbial communities in UASB reactors, although the addition of GAC also had a statically significant impact. The positive and negative correlations between classified phylotypes and performance indicators were determined. It was noted that more phylotypes were positively correlated with hydrogenotrophic specific methanogenic activity (SMA) than acetoclastic SMA. The occurrence network of the overall microbial communities from samples amended with GAC was modularized into eight main groups (occupying 92% of the nodes). Seven modules containing both methanogens and syntrophs were identified as potential functional communities for AD. These modules were mainly regulated by reactor types and driven by different combinations of operational factors using co-occurrence network analysis and generalized joint attribute modeling. Overall, the paper bridged operational configurations and reactor performance with microbial community dynamics.

Abstract

As a series of laws and regulations governing the water drainage in coal-fired power plants have been implemented in China, the discharge requirements of desulfurization wastewater from coal-fired power plants are becoming increasingly stringent. Conventional desulfurization wastewater treatment can no longer satisfy the needs of the current zero-discharge policy. It is imperative to develop advanced desulfurization wastewater treatment technology. Low-temperature flue gas waste heat concentration and desulfurization wastewater technology, a current low-cost zero-discharge technology, has attracted the wide attention of the industry. When the wastewater is concentrated nearly 8–15 times, the concentration of chloride ions in the concentrated wastewater can be increased to nearly $240\mathrm{g}/\mathrm{mL}$. Moreover, the wastewater has strong acidity, and the pH is nearly 1.0–2.0. The effective use of this concentrated waste liquid is the key to the success of low-temperature flue gas waste heat concentration desulfurization wastewater treatment technology. Accordingly, this study suggests that the compound water purifying agent of polymeric ferric chloride (PFC) and polymeric ferric sulfate (PFS) can be prepared through the reaction of concentrated high-chlorine wastewater with red mud, and PFC and PFS have a synergistic effect on water purification. The prepared water purifiers outperform PFS or polyaluminum ferric sulfate (PAFS) in selectivity, sensitivity, and cost-effectiveness. Water purifier preparation experiments and flocculating effect comparison experiments were performed. The resource utilization of salt was achieved in the concentrated waste liquid. This technical route provides a novel insight into the reasonable transfer of salt in desulfurization wastewater and the preparation of compound water-purifying agents.

Practical Applications

This study proposes a technical process for zero discharge of desulphurization wastewater coupled with the preparation of a water purifier which changes the conventional preparation method of water purifiers. It solves the problems of high concentrations of chlorine salt and the large use and cost of pharmaceuticals in high-salt wastewater from power plants. Moreover, it used red mud and industrial waste acid resources to turn waste into a valuable resource and solved the problem of misplaced and wasted resources. Low-temperature flue gas waste heat was adopted to concentrate desulfurized wastewater, such that the concentration of chloride ions in wastewater was concentrated into high-chlorine wastewater. The compound water purifier of polymeric ferric chloride and polymeric ferric sulfate was prepared by the reaction of concentrated low-pH and high-chlorine wastewater with red mud and waste acid. The compound water purifier was used for sewage pretreatment to reduce turbidity and chemical oxygen demand (COD) content of sewage.

Phthalic acid estes (PAEs) are typical organic contaminants in landfill leachate. Loess–bentonite cut-off walls were widely used to control the transport of organic contaminants. Bentonite modified by hexadecyltrimethylammonium chloride (HTMAC-B) mixed with loess material may play a good role in enhancing the adsorption of organic pollutants. The adsorption batch tests were used to analyze the adsorption characteristics and mechanisms of loess–HTMAC-B for single and mixed solutions of dimethyl phthalate (DMP) and diethyl phthalate (DEP). Contaminant transport simulations were then performed to investigate the performance of the modified bentonite–loess cut-off wall. The results show that the adsorption capacity of loess–bentonite for single DMP can be 1.7 times lower than that for mixed DMP. Results of numerical transport simulations based on the adsorption results show that the Tóth model performs better than the Langmuir model as it can well capture adsorption behavior induced by the heterogeneous surface morphology due to the presence of an HTMAC cation in the bentonite interlayer. We show that HTMAC amended loess–bentonite can effectively increase the breakthrough time of PAEs. In order to meet the designed service time of 50 years, $P_L$ should be controlled below 0.9 for DMP and 10.1 for DEP in the case of 1.2 m cut-off walls.

The onsite wastewater treatment system (OWTS) with a sand trench is an economical option for residents in rural areas or the countryside where a centralized sewer system is inaccessible. The 2A sand achieves improved filtration, microbial activity, and consistent long-term performance when compared with gravel- or scoria based-trench systems. Additionally, it has a reputation for premature blockage when overloaded or compacted. Although sand trenches overcome the failures reported on septic tank discharge, this could not be the only and the most sustainable solution. The aim of this research is to investigate and critically evaluate the performance of crushed glass (CG) with respect to 2A sand when treating primary treated effluent from a septic tank. A test rig was designed and constructed to simulate the real environment in a discharge control trench. The treatment efficiency was recorded and compared in this study. Overall, the CG loaded at $25\mathrm{mm}/\mathrm{day}$ (${\mathrm{CG}}_{25}$) and $50\mathrm{mm}/\mathrm{day}$ (${\mathrm{CG}}_{50}$) provided an average of 13% and 6% more total nitrogen (TN) reduction, respectively, than the sand filter. The ${\mathrm{CG}}_{50}$ media performed similarly as the 2A sand media in terms of total suspended solids (TSS) and biochemical oxygen demand (${\mathrm{BOD}}_5$) removal rate of 95% and 96%, respectively. The ${\mathrm{CG}}_{25}$ media exhibited low TSS and ${\mathrm{BOD}}_5$ removal rates of 92% and 91%, respectively. The rates have been improved over the sampling period and ultimately achieving similar results to the filters loaded at $50\mathrm{mm}/\mathrm{day}$. Life-cycle cost analyses and carbon balances were completed for the two media. It highlighted that the current price of CG is only half of 2A sand, yet it produces significantly less ${\mathrm{CO}}_2$ emissions than 2A sand. A three-bedroom dwelling could save up to $500 and reduce 200 kg of ${\mathrm{CO}}_2$ released to the environment annually when 2A sand is substituted for CG.

Abstract

China has pledged to peak its carbon emissions before 2030 and achieve the net-zero ambition in the 2060s. Reducing the national carbon emission intensity can help achieve these ambitions effectively. To explore the tradeoff between emission reduction and system health, a factorial hypothetical extraction method has been proposed. It was applied to identify key carbon emission sectors, and further help formulate countermeasures on reducing the national emission intensity. A seven-factor factorial analysis was developed to evaluate the effects of factors (i.e., 7 countermeasures) and the combinations (i.e., 128 scenarios) on responsive variables (i.e., system health). Main effects and interactions for response variables were also detected between these factors. Results show that the most effective combination, i.e., simultaneously enlarging the production scales of agriculture and other services, and lessening those of metallurgy sectors, can help reduce emission intensity by $-19.2\%$. Lessening the production scales of electricity-generation/metallurgy, and enlarging those of wholesale and retailing sectors, can help reduce national emission intensity, while these factors negatively impacted system sustainability and robustness. The mitigation effects of these countermeasures will be weakened if these countermeasures are implemented simultaneously. Enlarging the production scales of leasing and commercial services and other services sectors can help achieve a win-win outcome.

Practical Applications

China has pledged to reduce the national carbon emission intensity (NEI) by 60%–65% in 2030 compared to the level in 2015. However, emission reduction policies may come at the expense of system health. In this research, key carbon emission sectors are extracted and seven countermeasures targeting on these sectors are proposed. Then, there are 128 combinations of these countermeasures. Under all the combinations, the changing rates of NEI are evaluated under all the combinations. In addition, the contribution effects of all the combinations on system health (i.e., robustness and sustainability) are also assessed. Accordingly, enlarging the production scales of leasing and commercial services, and other service sectors can positively impact system health, as well as NEI. Lessening the production scales of electricity-generation/metallurgy, and enlarging those of wholesale and retailing sectors, can help reduce NEI, while these factors negatively impacted system sustainability and robustness.

There has been a recent increase of interest in sewer network water quality, both for pollutants and wastewater epidemiology. Of particular interest is the ability to perform cost-effective small-scale monitoring to understand the sewer network and perform source localization (the process of identifying the sources of materials of interest within the network), enabling prioritization of combined sewer overflow (CSO) interventions and targeted response to the detection of infectious diseases. Rhodamine WT fluorescent dye tracing was carried out in the combined sewer networks of four UK cities, for which network geometries were available. Over 100 dye concentration profiles were recorded, from which discharge, travel time (velocity), and dispersion were quantified. A simplified hydraulic and water quality (conservative solute transport) modeling approach was used to investigate dispersion further. A theoretical method for calculating dispersion over a reach with nonuniform properties was derived and used with the models and recorded data to develop a method for estimating the dispersion coefficient in sewers. Novel simultaneous injections into multiple manholes within one sewer network were conducted. Modeling of these injections validated the modeling approach and explained the measured concentration profiles, demonstrating both the potential of hydraulic and solute transport modeling and the new dispersion coefficient predictor for source localization. Such modeling can be used to develop sewer network “fingerprints” and source location probability plots based on residence time distribution (RTD) theory to maximize information from limited water quality monitoring. This will aid managers and operators in identifying potential intermittent sources of material within the network.

In nature, staggered structures, such as knitted baskets and brick patterns in the structure of buildings, always exhibit the feature of structural reinforcement. Can floating arrays become more dynamically stationary if floating modules of an array are interconnected in a staggered pattern rather than in an aligned pattern? This work examined the dynamic characteristics of staggered floating arrays. A floating array with 12 identical semisubmersible modules in a $3\times 4$ configuration was used to study the effects of staggered structure on the dynamic characteristics of floating systems. A network modeling method considering the geometric effect of connectors was used to establish the analysis model. The numerical analysis was conducted in three main aspects. First, the impact of the staggered deployment of modules on the stiffness feature of the floating structure was investigated. Subsequently, the variation of the stability of floating array with increasing staggered displacement was studied, and the corresponding dynamic mechanism is discussed in detail. Furthermore, the characteristic transition of the floating array evolving from aligned pattern to brick-pattern with different staggered displacements is studied. The numerical results demonstrate that the staggered deployment of modules could increase the floating system’s stiffness along the longitudinal direction and improve the stability of heave and pitch motions near the head wave. This work could provide guidelines for the configuration design of floating islands for optimal ocean space utilization.

Hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) is a primary cause of odor and corrosion in sewer systems. In drop structures, falling wastewater impinges the surface of bottom water pool as a jet or numerous drops, triggering a high level of turbulence and thus enhancing the emission of ${\mathrm{H}}_2\mathrm{S}$ and the absorption of oxygen (${\mathrm{O}}_2$) at the pool surface. In this study, laboratory experiments were conducted to study the mass transfer at the pool surface with falling water drops or a single jet of water. In the test range of falling flow rate of $49–223\mathrm{mm}/\mathrm{h}$ and falling velocity of $3.1–5.2\mathrm{m}/\mathrm{s}$ [kinetic energy flux $(\mathrm{KEF})=0.11–0.80\mathrm{J}/({\mathrm{m}}^2·\mathrm{s})]$, the mass transfer coefficient $K_L$ for the pool surface was found to be $2.6–14.8\times {10}^{-5}\mathrm{m}/\mathrm{s}$ for ${\mathrm{H}}_2\mathrm{S}$. In addition, $K_L$ was found to be 76% larger when the pool surface was impinged by water drops than by a single jet. $K_L$ was $27\%-47\%$ larger in the half of the water surface directly receiving the drops or jet than the other half. The increase of water depth in the pool promoted the mass transfer, especially for the scenario of falling jet. Equations were proposed to predict $K_L$ under drops or jet. Moreover, $K_L$ for ${\mathrm{H}}_2\mathrm{S}$ and for ${\mathrm{O}}_2$ was found almost the same, indicating ${\mathrm{O}}_2$ can be a safe surrogate gas for ${\mathrm{H}}_2\mathrm{S}$. Finally, the research results were applied to estimate ${\mathrm{H}}_2\mathrm{S}$ emissions at the pool surface of a prototype drop structure with a falling height of up to 10 m, which showed that turbulent wastewater surface can enhance mass transfer over 100 times compared with a quiescent surface.