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.

Multiphase flows in porous media are important in many natural and industrial processes. Pore-scale models for multiphase flows have seen rapid development in recent years and are becoming increasingly useful as predictive tools in both academic and industrial applications. However, quantitative comparisons between different pore-scale models, and between these models and experimental data, are lacking. Here, we perform an objective comparison of a variety of state-of-the-art pore-scale models, including lattice Boltzmann, stochastic rotation dynamics, volume-of-fluid, level-set, phase-field, and pore-network models. As the basis for this comparison, we use a dataset from recent microfluidic experiments with precisely controlled pore geometry and wettability conditions, which offers an unprecedented benchmarking opportunity. We compare the results of the 14 participating teams both qualitatively and quantitatively using several standard metrics, such as fractal dimension, finger width, and displacement efficiency. We find that no single method excels across all conditions and that thin films and corner flow present substantial modeling and computational challenges.

Traditional models of two-fluid flow through porous media at the macroscale have existed for nearly a century. These phenomenological models are not firmly connected to the microscale; thermodynamic constraints are not enforced; empirical closure relations are well known to be hysteretic; fluid pressures are typically assumed to be in a local equilibrium state with fluid saturations; and important quantities such as interfacial and curvilinear geometric extents, tensions, and curvatures, known to be important from microscale studies, do not explicitly appear in traditional macroscale models. Despite these shortcomings, the traditional model for two-fluid flow in porous media has been extensively studied to develop efficient numerical approximation methods, experimental and surrogate measure parameterization approaches, and convenient pre- and post-processing environments; and they have been applied in a large number of applications from a variety of fields. The thermodynamically constrained averaging theory (TCAT) was developed to overcome the limitations associated with traditional approaches, and we consider here issues associated with the closure of this new generation of models. It has been shown that a hysteretic-free state equation exists based upon integral geometry that relates changes in volume fractions, capillary pressure, interfacial areas, and the Euler characteristic. We show an analysis of how this state equation can be parameterized with a relatively small amount of data. We also formulate a state equation for resistance coefficients that we show to be hysteretic free, unlike traditional relative permeability models. Lastly, we comment on the open issues remaining for this new generation of models.

In electrochemical processes such as electrodialysis or redox flow batteries, where ion exchange membranes (IEMs) play a critical role in process performance, energy losses can be reduced by minimizing the permeability of IEMs to water and salt. In pure, homogeneous polymer membranes, water permeability is known to be controlled by the size of the free volume elements. However, there is very limited evidence concerning the extent to which this theory applies to practical, commercial IEMs, which frequently have more complex structures. We recently reported water and salt transport characteristics (i.e., permeability, partition, and diffusion coefficients) of 20 commercial IEMs, and demonstrated that water and salt transport were governed primarily by the microstructure of the membrane rather than the polymer chemistry. To further investigate the factors that determine water and salt transport in commercial IEMs, in this study we adopted a statistical approach informed by free volume theory and other literature to examine relationships between transport characteristics and water uptake (i.e., swelling) in addition to fixed charge concentration, ion exchange capacity (IEC), Manning parameter, and contact angle. Our analysis shows that water uptake had the strongest correlation with water and salt transport in commercial IEMs, which is consistent with the predictions of free volume theory for homogeneous polymers; however, the relationship observed between water uptake and permeability in commercial membranes was not as straightforward as that reported in the literature for homogeneous polymers. Membrane charge (IEC) was also correlated with permeability and diffusion coefficients, but to a more limited extent than water uptake, while the Manning parameter and contact angle did not appear to be correlated to any transport properties. Furthermore, there are indications that microstructural differences among membranes may significantly affect permeability. Therefore, further study of IEM microstructure, e.g., phase separation, is an important strategy for advancing the development of commercial IEMs.

A closure relation for capillary pressure plays an important role in the formulation of both traditional and evolving models of two‐fluid‐phase flow in porous medium systems. We review the traditional approaches to define capillary pressure, to describe it mathematically, to determine parameters for this relation, and to constrain the domain of applicability of this relation. In contrast to the traditional approach, we provide a rigorous, multiscale definition of capillary pressure, define the state domain of interest in practice, summarize computational and experimental approaches to investigate the system state, and apply the methods for two‐fluid states in a model ink bottle system, the classical Finney pack of spheres, and a synthetic sphere pack system. The results of these applications show that a state equation exists that describes capillary pressure without hysteresis. This state equation parameterizes a function that describes the nonwetting phase volume fraction in terms of the capillary pressure, the interfacial area, and the specific Euler characteristic of the nonwetting phase. Furthermore, this state equation applies over the complete range of conditions encountered in practice, and it applies under both equilibrium and dynamic conditions. This state equation involving capillary pressure forms an important foundation for the development of the next generation of macroscale two‐fluid‐phase flow models in porous medium systems.

Models of flow of two immiscible fluids in a porous medium overwhelmingly involve use of an equilibrium correlation between what is identified as capillary pressure and the saturation of one of the fluids. This correlation is said to be hysteretic with the functional relation depending on which of the two fluids has displaced the other in a flow scenario. This correlation was proposed when abilities to investigate porous medium systems experimentally for the distribution of fluids or using computer simulation were limited. Because of advances, we can now assert that the quantity called capillary pressure is ambiguously defined and the alleged hysteretic behaviour, even at equilibrium, is actually due to incompleteness in the functional dependence. We provide a path forward for a theoretically sound formulation that corrects the assertion that capillary pressure is a hysteretic function of saturation, and we advocate for moving beyond ingrained erroneous notions.

Greenhouse gas emissions (GHGs) from swine production systems are relatively well researched with the exception of emissions from land application of manure. GHGs inventories are needed for process-based modeling and science-based regulations. Thus, the objective of this observational study was to measure GHG fluxes from land application of swine manure on a typical corn field. Assessment of GHG emissions from deep injected land-applied swine manure, fall and reapplication in the spring, on a typical US Midwestern corn-on-corn farm was completed. Static chambers were used for flux measurement along with gas analysis on a GC-FID-ECD. Measured gas concentrations were used to estimate GHGs flux using four different models: linear regression, nonlinear regression, first order linear regression and the revised Hutchinson and Mosier (HMR) model, respectively for comparisons. Cumulative flux estimates after manure application of 5.85 × 105 g•ha–1 (1 ha = 0.01 km2) of CO2, 6.60 × 101 g•ha–1 of CH4, and 3.48 × 103 g•ha–1 N2O for the fall trial and 3.11 × 106 g•ha–1 of CO2, 2.95 × 103 g•ha–1 of CH4, and 1.47 × 104 g•ha–1 N2O after the spring reapplication trial were observed. The N2O net cumulative flux represents 0.595% of nitrogen applied in swine manure for the fall trial.

A sustained effort by a growing number of researchers over the last quarter century has been aimed at developing a new class of macroscale two-fluid-phase flow models that resolve and evolve not only fluid pressures, saturations, and velocities, but other important measures of the system state, such as interfacial areas. It has been posited that a more complete system state would enable reducing, or removing, hysteresis from the closure relations. Efforts have also been aimed at understanding the role of dynamics in the relaxation of such systems to an equilibrium state. The overall goal of this work is to report on recent theoretical, experimental, and computational advancements to evolve this next generation of model, and to outline the remaining challenges that must be overcome to complete this effort.

The swine industry, regulatory agencies, and the public are interested in farm-tested methods for controlling gaseous emissions from swine barns. In earlier lab- and pilot-scale studies, a renewable catalyst consisting of soybean peroxidase (SBP) mixed with calcium peroxide (CaO2) was found to be effective in mitigating gaseous emissions from swine manure. Thus, a farm-scale experiment was conducted at the university's 178-pig, shallow-pit, mechanically-ventilated swine barn to evaluate SBP/CaO2 as a surficial manure pit additive under field conditions. The SBP was applied once at the beginning of the 42-day experiment at an application rate of 2.28 kg m(-2) with 4.2% CaO2 added by weight. Gas samples were collected from the primary barn exhaust fans. As compared to the control, significant reductions in gaseous emissions were observed for ammonia (NH3, 21.7%), hydrogen sulfide (H2S, 79.7%), n-butyric acid (37.2%), valeric acid (47.7%), isovaleric acid (393%), indole (31.2%), and skatole (43.5%). Emissions of dimethyl disulfide/methanethiol (DMDS/MT) increased by 30.6%. Emissions of p-cresol were reduced by 14.4% but were not statistically significant. There were no significant changes to the greenhouse gas (GHG) emissions of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). The total (material + labor) treatment cost was $2.62 per marketed pig, equivalent to 1.5% of the pig market price. The cost of CaO2 catalyst was similar to 60% of materials cost. The cost of soybean hulls (SBP source) was $0.60 per marketed pig, i.e., only 40% of materials cost (C) 2017 Elsevier Ltd. All rights reserved.

Comprehensive control of odors, hydrogen sulfide (H2S), ammonia (NH3) and odorous volatile organic compound (VOC) emissions associated with animal production is a critical need. Current methods utilizing wind tunnels and flux chambers for measurements of gaseous emissions from area sources such as feedlots, lagoons, pastures and cropland are often criticized for potential bias. The bias is due to the temporal isolation of the emitting surface, affecting the air velocity and temperature at the surface, forcing controlled surface-to-gas-phase mass transfer, and therefore, affecting emission measurements. That bias can limit the applicability of these methods to relative and comparative measurements. In this research, a new approach is proposed to minimize these biases with a specially constructed wind tunnel. Pilot-scale experiments were conducted to evaluate an instrumented wind tunnel capable of reproducing ambient air and soil-air mass transfer conditions in real time. Matching the inside of the wind tunnel with ambient conditions is based on simultaneous measurements of wind velocity near the emitting surface, and real-time adjustment of a wind tunnel fan to minimize velocity differences. The wind tunnel’s wind speed matching parameters were first optimized in regards to wind velocity sampling rate and wind velocity match criteria. The optimum wind tunnel operating parameters chosen resulted in a 25% absolute difference in the wind velocities generated in the wind tunnel compared to the outside wind velocities. An instrumented wind tunnel capable of reproducing ambient air and soil-air mass transfer conditions in real time could be a promising tool for measuring NH3 and VOC emissions from land-applied livestock waste.

Comprehensive control of odors, hydrogen sulfide (H2S), ammonia (NH3), and greenhouse gas (GHG) emissions associated with swine production is a critical need. The objective of this paper is to review the use of soybean peroxidase (SBP) and peroxides as a manure additive to mitigate emissions of odorous volatile organic compounds (VOC), NH3, H2S, and GHGs. Soybean peroxidase plus peroxide (SBPP) was tested as a mitigation technology for swine manure emissions on three scales (lab, pilot and farm). Several laboratory scale experiments were completed to assess SBPP dosages and type of oxygen source mixed into swine manure and surface application. A pilot scale experiment was done with surface application of SBPP and multiple dosages to observe scale up effects. Finally, a farm scale trial was completed to assess the SBPP treatment to a swine manure surface under a fully slatted barn floor. The ‘gated’ approach to testing SBPP from labto pilotand finally the farm-scale was appropriate and allowed for controlled experiments with sufficient replication. This approach resulted in gradual decrease of the dose of SBP, decreasing the cost of treatment, increase of treatment longevity, inclusion of many key gases of concern to the experimental protocol, and finally testing the treatment on farm-scale. To date, the farm-scale results indicate that SBPP can be effective in mitigating many important odorous gas emissions without increasing GHGs. Specifically, a 2.28 kg m-2 SBP dose mixed with 4.2% CaO2 added by weight and added to manure surface resulted in significant reductions in gaseous emissions of NH3 (21.7%), H2S (79.7%), n-butyric acid (37.2%), valeric acid (47.7%), isovaleric acid (39.3%), indole (31.2%), and skatole (43.5%). Emissions of DMDS/MT increased by 30.6%. Emissions of p-cresol were reduced by 14.4% but were not statistically significant. There were no significant changes to the GHG emissions of CH4, CO2 and N2O. The treatment cost (SBP+CaO2) was $1.45 per marketed pig of which the cost of SBP was only ~40%. Thus, further research is needed to optimize the dose and the cost of catalysts. .

A new method was used at the Ag 450 Farm Iowa State University (41 .98N, 93.65W) from October 24, 2012 through December 14, 2012 to assess GHG emission from land-applied swine manure on crop land. Gas samples were collected daily from four static flux chambers. Gas method detection limits were 1 .99 ppm, 17 0 ppb, and 20.7 ppb for CO2, CH4 and N2O, respectively . Measured gas concentrations were used to estimate flux using four different models, i.e., (1) linear regression, (2) non-linear regression, (3) non-equilibrium, and (4) rev ised Hutchinson & Mosier (HMR). Sixteen day s of baseline measurements (before manure application) were followed by manure application with deep injection (at 41 .2 m3/ha), and thirty seven day s of measurements after manure application. Why Study Greenhouse Gases and Land Application of Swine Manure? Assessment of greenhouse gas (GHG) emissions from land-applied swine manure is needed for improved process-based modeling of nitrogen and carbon cy cles in animal-crop production sy stems.