Methane (CH4) emissions from landfills may vary diurnally and seasonally, influenced by cover type, atmospheric conditions, and the presence/absence of gas collection and control systems (GCCS). To evaluate how these factors influence CH4 flux, high-frequency eddy covariance (EC) data were collected from portions of three U.S. landfills - Southeastern US Landfill (SE), California Landfill 1 (CA 1), and California Landfill 2 (CA 2). Fluxes for each landfill cover for each measurement day were normalized by th e mean daytime flux (9am-9 pm). For all cover types and landfills, emissions varied with time of day, with peak fluxes between 12 PM - 6 PM. At SE, intermediate covers without GCCS showed average normalized peak fluxes (-) of 1.45 (2012) and 1.39 (2013), i.e., fluxes were 39-45% higher than the daily average during the most emissive period, with peak emissions between 12-3 PM on 49% of days. Similar results occurred at CA 1 and CA 2 landfills for landfill covers with or without GCCS. At the CA 1 landfill, EC measurements from the landfill covers were used to estimate the whole-landfill emissions between 9am and 9 pm. Between 12 PM and 6 PM, whole-landfill emissions were ∼ 20% higher than the daytime mean. At the SE landfill, where data were available to assess emissions over 24 h, measurements during peak emission periods could overestimate daily emissions by up to 70%. These findings underscore the need for diurnally resolved emission data, especially from uncovered or poorly controlled landfill sites, to support accurate emission inventories and optimal mitigation strategies.

Landfills are considered the third largest source of anthropogenic methane emissions in the U.S. and there is considerable interest in estimating emissions from individual landfills. Collection efficiency (CE) or methane collection index (MCI) is used in models to estimate emissions. The objective of this review is to critically analyze literature describing measured CEs and MCIs. Reported values ranged from 14 to 95% for intermediate covers and 53-100% for final covers. Many factors contribute to the ranges including: covers were classified as intermediate or final which is overly simplistic; variability in operational factors (e.g., well coverage, maintenance quality); waste characteristics such as age, composition, and mass in place which influence methane generation and likely collection; and uncertainty in emissions measurements and sometimes collected methane. A desired outcome of this review was to recommend values or ranges for intermediate and final covers that could be used in methane emissions models as applied to U.S. landfills. Such a recommendation is difficult considering the ranges reported in the literature and numerous site-specific factors that influence methane collection. Ultimately, each reported measurement is valid for a specific landfill at a specific time. Models that relate cover type and operational variables to MCI would best support estimation of a site-specific MCI, but such models are not available. Based on this review, the authors do not recommend a specific value for the MCI for an intermediate or final cover but rather expect the operator to select a value considering factors applicable to their landfill and the information presented here.

Decentralized sanitation systems such as eco-vapor toilets (EVTs) use hydrophobic laminate-lined containers to collect and dry fecal matter. Removing water in EVTs is critical to their success. While computational fluid dynamics (CFD) modeling of drying can guide EVT designs and site selection, it is expensive. To develop a simpler modeling approach, bench-scale experiments quantified the constant-rate drying period over a wide range of environmental conditions using deionized water as a surrogate for fecal sludge. Then, two modeling approaches were tested using these data: a CFD model, and a simple evapotranspiration model, the Food and Agriculture Organization of the United Nations (FAO) - Penman Monteith (PM) model. The model-predicted drying rates for 29 experiments resulted in a coefficient of efficiency (E, optimal value = 1.0) of 0.47 and -0.05 for the CFD and FAO-PM models, respectively. To reduce the level of effort for a complete CFD model but increase the accuracy of a FAO-PM model alone, a new CFD-Evaporation model is proposed. The FAO-PM and the CFD-Evaporation model applied to 40 L EVTs in an 11-day field test resulted in an E of -1.38 and 0.15, respectively. The CFD-Evaporation model is useful where computational efficiency is more critical than absolute accuracy.

Urban development often results in compacted soils, impairing soil structure and reducing the infiltration and retention of stormwater runoff from impervious features. Biochar is a promising organic soil amendment to improve infiltration and retention of stormwater runoff. Soil at the disconnection between impervious and pervious surfaces represents a critical biochar application point for stormwater management from urban impervious features. This study tested the hypothesis that biochar would significantly improve water retention and transmission at four sites, where varying percentages (0%, 2%, and 4% w/w) of biochar were amended to soils between impervious pavement, and pervious grassed slopes. Field-saturated hydraulic conductivity (Ksat) and easily drainable water storage capacity were monitored at these sites for five months (two sites) and 15 months (two sites). At the end of the monitoring periods, the physical, chemical, and biological properties of each site's soil were assessed to understand the impact of biochar on soil aggregation, which is critical for improved soil structure and water infiltration. Results indicated that the field Ksat, drainable water storage capacity, and plant available water content (AWC) were 7.1 ± 3.6 SE, 2.0 ± 0.3 SE, and 2.1 ± 0.3 SE times higher in soils amended with 4% biochar, respectively, compared to the undisturbed soil. Factor analysis elucidated that biochar amendment increased the organic matter content, aggregate mean weight diameter, organo-mineral content, and fungal hyphal length while decreasing the bulk density. Across the 12 biochar/soil combinations, the multiple linear regression models derived from factor analysis described the changes in Ksat and AWC reasonably well with R2 values of 0.51 and 0.71, respectively. Using soil and biochar properties measured before biochar addition, two recent models, developed from laboratory investigations, were found helpful as screening tools to predict biochar's effect on Ksat and AWC at the four field sites. Overall, the findings illustrate that biochar amendment to compacted urban soils can significantly improve soil structure and hydraulic function at impervious/pervious surface disconnections, and screening models help to predict biochar's effectiveness in this context.

Bioretention systems are planted media filters used in stormwater infrastructure. Maintaining plant growth and survival is challenging because most designs require significant sand. Conventional bioretention soil media (BSM) might be augmented with biochar to make the BSM more favorable to plants, to improve nutrient removal efficiency, and enhance plant survivability during drought while replacing compost/mulch components that have been linked to excess nutrient export. Pots with BSMs representing high and moderate sand content were amended with wood biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. After 20 weeks, 4% biochar amendment significantly increased stormwater infiltration (67%) and plant available water (52%) in the high sand content BSM (NC mix, which meets requirements for the state of North Carolina (US) and contains no compost/mulch), and these favorable hydraulic properties were not statistically different from a moderate sand content, biochar-free BSM with compost/mulch (DE mix, which meets requirements for state of Delaware (US)). While biochar amendment improved plant height (25%), the number of shoots (89%), and total biomass (70%) in the NC mix, these parameters were still less than those in the biochar-free DE mix containing compost/mulch. TN and NO3−1 removal were also improved (28–35%) by biochar amendment to NC mix, and the resulting TN and TP loadings to groundwater were 10 and 7 times less, respectively than biochar-free DE mix with compost/mulch. During the drought period, biochar amendment increased the time to switchgrass wilting by ∼8 days in the NC mix but remained 40% less than the biochar-free DE mix. A recalcitrant carbon-like biochar mitigates some of the deleterious effects of high sand content BSM on plants, and where nutrient pollution is a concern, replacement of compost/mulch with wood biochar in BSM may be desired.

Accidental releases of highly saline produced water (PW) to land can impact soil quality. The release of associated salts can clog soil pores, disperse soil clays, and inhibit plants and other soil biota. This study explores a novel remediation technique using ferrocyanide to enhance the evaporative flux of soil porewater to transport dissolved salts to the soil surface, where crystallization then occurs. The addition of ferrocyanide modifies crystal growth that enhances salt transport, allowing salt efflorescence on the soil surface and physical removal. Release sites were simulated through beaker sand column experiments using two PWs collected from the Permian Basin. PW composition altered efflorescence, with up to ten times as much ferrocyanide required in PWs than comparable concentrations of pure NaCl solutions. The addition of EDTA reduced dissolved cation competition for the ferrocyanide ion, improving PW salt recovery at the soil surface. The speciation model, PHREEQC, was used to predict the onset of salt precipitation as a function of evaporative water loss and model the effect of aqueous ferrocyanide and EDTA speciation on efflorescence. The results highlight the utility of predictive modeling for optimizing additive dosages for a given release of PW.

Bioretention systems are one example of green stormwater infrastructure that may mitigate the hydrologic impact of stormwater runoff. To improve water retention while maintaining rapid stormwater infiltration, conventional bioretention soil media (BSM) might be augmented with biochar. Biochar may improve the BSM's structure by increasing soil aggregation, which might improve water retention and increase stormwater infiltration while also improving root growth. Pots with BSMs representing high and moderate sand content media were amended with a wood-derived biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. In the high sand content medium (NC mix), biochar amendment increased hydraulic conductivity (Ksat), and this effect increased with time. At 0 weeks, 2% and 4% (w/w) biochar increased Ksat by 4 ± 2% and 10 ± 4%, respectively, while at 30 weeks the increase was 30 ± 10 and 70 ± 20%, respectively, above biochar-free media. Similar improvements were seen in plant available water (PAW) in NC mix. However, minimal improvements in Ksat and PAW from biochar amendment were found in the moderate sand content BSM that contained compost and mulch (DE mix). Where biochar promoted Ksat, this was correlated with increased water-stable aggregate size (r = 0.86), fine root volume (r = 0.88), and below ground biomass (r = 0.83). Important factors affecting Ksat and aggregation in the NC mix were biochar's influence on organo-mineral association, fungal hyphae length, and plant roots. Wood-derived biochar amendment to BSM may obviate the need for compost/mulch since biochar has similar effects on improving BSM hydrology and root growth without the risk of undesired nutrient leaching.

Diurnal patterns of methane flux are examined at a landfill in the Southeastern US. Methane fluxes are measured by an eddy covariance (EC) tower during representative one-week periods in three seasons: summer, fall, and winter. Measured methane fluxes are compared with atmospheric pressure, temporal variation of atmospheric pressure, wind shear velocity, and air temperature. Landfill methane flux varies significantly with shear velocity and temporal changes in atmospheric pressure when the atmosphere is neutral. Under unstable atmospheric conditions, air temperature correlates best with methane flux, which is corroborated with an independent dataset of tracer correlation method (TCM) measurements for similar measurement periods. These field data support a mathematical model previously proposed to describe atmospheric effects on methane flux from landfills. The field data also indicate significant diurnal methane flux variations, with daytime fluxes up to 23 times greater than nighttime fluxes. Because the majority of historical TCM measurements of whole landfill methane flux are between 12 pm and 6 pm at this landfill, when daily emissions are highest because of atmospheric effects, average diurnal fluxes might have been overestimated by as much as 73%. Methane emissions are most representative of diurnal average emissions when atmospheric stability is near-neutral, which occurs in the late morning (∼11 am) and in the early evening (∼5 pm) at this site.

How milldams alter riparian hydrologic and groundwater mixing regimes is not well understood. Understanding the effects of milldams and their legacies on riparian hydrology is key to assessing riparian pollution buffering potential and for making appropriate watershed management decisions. We examined the spatiotemporal effects of milldams on groundwater gradients, flow directions, and mixing regime for two dammed sites on Chiques Creek, Pennsylvania (2.4 m tall milldam), and Christina River, Delaware (4 m tall dam), USA. Riparian groundwater levels were recorded every 30 min for multiple wells and transects. Groundwater mixing regime was characterized using 30‐min specific conductance data and selected chemical tracers measured monthly for about 2 years. Three distinct regimes were identified for riparian groundwaters—wet, dry, and storm. Riparian groundwater gradients above the dam were low but were typically from the riparian zone to the stream. These flow directions were reversed (stream to riparian) during dry periods due to riparian evapotranspiration losses and during peak stream flows. Longitudinal (parallel to the stream) riparian flow gradients and directions also varied across the hydrologic regimes. Groundwater mixing varied spatially and temporally between storms and seasons. Near‐stream groundwater was poorly flushed or mixed during storms whereas that in the adjacent swales revealed greater mixing. This differential groundwater behavior was attributed to milldam legacies that include: berm and swale topography that influenced the routing of surface waters, varying riparian legacy sediment depths and hydraulic conductivities, evapotranspiration losses from riparian vegetation, and runoff input from adjoining roads.

Drying of fecal sludge (FS) enclosed in a breathable, hydrophobic membrane laminate was investigated in 208 and 40 L container-based toilet (CBS) systems referred to as Eco-Vapor toilets (EVTs). EVTs were constructed and pilot tested in four households in urban slums of Kanpur, India over a period of 2 years. The average moisture losses of 0.8 and 0.9 kg/day were observed in laminate-lined 208 L drums for Year 1 tests, and this in situ drying decreases disposal frequency by 8 days compared with CBS that do not allow FS drying. In Year 2, smaller EVTs with 40 L laminate-lined drums and waste segregation increased replacement time over conventional CBS by 45%, as opposed to the 19% increase observed in Year 1 tests. Despite its limitations, the stagnant film model using meteorological data predicted the mass-loss rate within 52 and 28% error for the 208 and 40 L drums, respectively.

While the use of biochar as a soil amendment is gaining popularity for environmental and agricultural purposes, spatial heterogeneity of biochar (segregation) in biochar-amended media and its underlying causes have been overlooked. In this study, for the first time particle segregation in biochar-amended media and its impact on the media's saturated hydraulic conductivity (Ksat) were investigated. Two uniformly graded media were amended with different sizes of a wood-based biochar under dry and wet conditions. While the intended biochar volume fraction (bf) was 17.5%, in dry-packed columns biochar was often segregated and the measured bf ranged from 7.5 ± 0.8 SE% (SE = standard error) to 23.6 ± 1.8 SE% across all spatial locations. If, however, 20% water (volume of water/bulk volume of packed media) was added to the mixtures during mixing, homogeneous packings were achieved. In dry-packing, segregation was governed by the difference in the physical properties of the media and the biochar: particle size, density, and shape. In wet-packing, segregation was prevented due to the inter-particle adhesion forces associated with water. Although X-ray computed tomography images showed that the presence of segregation altered particle distributions and pore morphologies, the Ksat for wet-packed and dry-packed columns were statistically identical. The results of this study suggest that laboratory methods for packing biochar-amended media should include moisturizing the mixture to inhibit particle segregation. Mixing under wet conditions is recommended for any type of soil and biochar and for any scale of application, in both the laboratory and field.

Laboratory soil column experiments have been frequently performed for investigating various soil-related processes. In recent years, the demand for using biochar as a soil amendment for environmental and agricultural purposes has increased significantly. To assess the beneficial impacts of biochar, laboratory column experiments may be conducted using repacked biochar-amended soil before large-scale biochar application. Biochar is a porous material that might have transient hydrophobicity, and particle density, size, and shape that often differ from native soil. These factors might cause several experimental problems in repacked laboratory columns, including unrealistic hydraulic and solute transport and transformation measurements, spatial variation of biochar content, and error in estimating the repacked biochar-amended soil properties. Therefore, it is necessary to modify standard repacked column packing procedures for biochar-amended soil. In this work, several modifications are described for preparing repacked biochar-amended soils. The modifications are rinsing and oven-drying biochar, determining the optimum moisture content to achieve a homogenous mixture, determining the desired bulk density before column packing, and mixing and packing under wet conditions. In addition, repacked columns should be characterized by their inter, intra, and total porosities and pore volume after column packing.•Steps are recommended prior to packing the repacked biochar-amended soil columns: rinsing biochar and pre-determining optimum moisture content and bulk density.•Columns are wet-packed in subsections at the optimum moisture content to the desired bulk density. Following packing, the inter, intra, and total porosities and pore volume should be determined.•These steps will reduce unrealistic transient results, inhibit nonuniform packing and heterogeneity of biochar content, and provide important information for interpreting the performance of biochar-amended media.

The dry end of the soil water retention curve (WRC) plays an important role in various hydrologic, solute transport, plant, and microbial processes. Despite increasing application of biochar as a soil amendment, knowledge about water retention in biochars and biochar‐amended soils under dry conditions is lacking. Mechanistic models are presented to predict the WRC for biochars and biochar‐amended soils at matric potential ( ψ ) < ~− 1 MPa. For biochars, the amount of water retained is linked to biochar surficial oxygen content and pore volume and surface area distributions. The WRC for soils at dry conditions is predicted using specific surface area. The WRC model for biochar‐amended soils is the sum of the contributions of models for biochar and soil. The model's utility was examined for three natural soils and a uniform sand, a wood‐based biochar, and 10 different combinations of these soils and biochar. The accuracy of the model for biochars was further tested for six other pyrogenic carbonaceous materials (PCMs). The models agreed well with experimental data: for the biochar and PCMs, soils, and biochar‐amended soils, the root mean square error normalized to the range of water content was almost always <10%. The line of best fit for predicted versus measured gravimetric water content at permanent wilting point had slope of 0.935 ± 0.013 and a coefficient of determination of 0.997. The applicability of these models for different biochars, soils, and their mixtures is discussed.

Biochar is black carbon produced from pyrolysis of biomass and may be added to soil to sequester carbon and improve soil water retention. To date models to predict changes in soil water retention with biochar amendment are still missing and therefore direct measurements are required for every biochar/soil combination, which can be time-consuming. Here, a predictive model for biochar's effect on soil water retention was developed and tested that includes water retained in biochar intrapores and biochar's impact on interpores between particles. The independently measured parameters needed for the model are the particle size distributions (PSDs) and particle densities for biochar and soil, water retention data for biochar-free soil, biochar intrapore volume distribution from mercury porosimetry, amount of biochar added, bulk density of the biochar/soil mixture, and dew point potentiometer measurements of biochar. The model was tested using poultry litter and wood biochars amended to two soils (sand and sandy loam) at 2 and 7% mass fraction. The model predicted changes in the soil water characteristic well for the biochar amendment, with RMSE decreasing by similar to 50% when the full model was used. Model predictions of the change in available water capacity with biochar amendment for eight biochar/soil combinations had an average absolute error of 0.017 +/- 0.006 and an average relative error of 1 (0) over bar0 +/- 40%. The model correctly predicted the increase in available water content when sandy loam was amended with wood biochar, and the decrease if amended with poultry litter biochar. The model provides an improved understanding of the mechanisms by which biochar alters water retention, and a means to estimate the initial change in available water capacity for a particular biochar/soil combination if necessary biochar and soil properties are measured.

Correction for ‘Understanding fecal sludge drying in membrane-lined container-based toilets for developing countries with CFD modeling’ by Babak Ebrazi Bakhshayesh et al., Environ. Sci.: Water Res. Technol., 2019, 5, 2219–2231.

Laminated hydrophobic membranes have been proposed as liners for container-based sanitation systems in developing countries. The laminate allows drying of fecal sludge, which might significantly reduce the frequency of container emptying, while containing liquids and solids. While previous laboratory tests demonstrated rapid drying of fecal sludge or water retained in laminates, experiments did not assess the effects of system dimension or scale on performance. In this study fecal sludge drying and water evaporation were evaluated in 3D laminate boxes (decimeter scale) or 3D laminate-lined 40 L and 55 gallon drums (meter scale) that arc prototypes of toilet containers for field application. A stagnant film model described fecal sludge drying and water evaporation in the laminate boxes and laminate-lined drums well. The effective diffusion length (lambda) for the laminate was fitted in all systems and increased with system dimension and scale: lambda increased by a factor of 1.4 from 1D decimeter-scale envelopes to 3D decimeter-scale boxes, and by a factor of 13-1.7 from 3D decimeter-scale boxes to 3D meter-scale drums. The longer lambda with increasing dimension and scale is likely due to nonuniform temperature and relative humidity in the air outside the laminate and nonuniform temperature within the laminate. Using best-fit lambda for the laminate-lined 40 L and 55 gallon drum experiments conducted in a controlled laboratory, drying was predicted for an 11-day field experiment. Although the air temperature and relative humidity varied significantly in the field tests from -1 T to 26 degrees C and 35% to 97%, respectively, the stagnant film model predicted drying over the 11-day period reasonably well with total error <= 13% using 24-h average air temperature and relative humidity. Drying of fecal sludge in laminate-lined drums in the field might be adequately described with a stagnant film model using daily-average weather conditions, if wind speeds are low. (C) 2019 The Authors. Published by Elsevier B.V.

Drying of faecal sludge enclosed in a breathable, hydrophobic membrane laminate was investigated for the potential application of breathable membranes in decentralized container-based sanitation systems for developing nations. Moisture loss from the membrane-enclosed faecal sludge was studied using membrane ‘envelopes’ filled with faecal sludge collected from random volunteers. A drying test with a new membrane envelope resulted in 71.2% mass reduction over a period of 7 days with an average moisture flux of 0.73 g/day-cm2. Slight decrease in the sludge drying rates was observed over five reuses of the same membrane envelope. A stagnant film model was used to predict drying rates of membrane-enclosed faecal sludge in ten developing countries with high urban populations. Based on a loading rate of 15 L/day into a 200-L (55-gallon) collection container, the predicted drying rates range from 7.1 to 12.4 L/day. The filling time of the membrane-lined container decreased due to in-situ drying, resulting in longer operation time and less frequent emptying of the container.