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This paper documents the existence of significant gender gaps in STEM occupations in Costa Rica, El Salvador, and Mexico and estimates the aggregate costs associated with these gaps in Mexico. For Mexico we calibrate and simulate a version of the general equilibrium occupational choice model of Hsieh et al. (2019) to estimate the output losses associated with these differences since 1992. We find that if barriers in STEM occupations were eliminated aggregate output would have been between 1% and 10% larger, depending on the year. If female-specific social norms were also eliminated, the rise in aggregate output would be between 1.4% and 14%. For comparison purposes, we also compute the gains of eliminating all the distortions in high-skilled occupations as well as in all occupations. We find that aggregate output would rise between 16.5% and 3.6% in the first case and between 36.7% and 12% in the latter.

Stricter environmental regulation policies and freshwater as an increasingly valuable resource have led to global growth of zero liquid discharge (ZLD) processes in recent years. During this development, in addition to water, the recovery of recyclable materials, e.g. salts, from industrial wastewater and brines is considered more frequently. Within the framework of the HighCon research project, the subject of this study, a new ZLD process with the goal of pure single-salt recovery from industrial wastewater has been developed and investigated in a demonstrational setup at an industrial site. With regard to pure salts recovery, separating organic components is of great importance during the treatment of the concentrate arising from used water recycling. The removal of COD and of ions responsible for scaling worked very well using nanofiltration. The nanofiltration permeate containing the monovalent ions was pre-concentrated using electrodialysis and membrane distillation before selective crystallization for single-salt recovery was performed. An example economic case study for the newly developed ZLD process – based on demonstration results and considering optimization measures for a full-scale design – indicates that the costs are equal to those of a conventional ZLD process, which, however, does not provide inter alia the aforementioned benefit of single-salt recovery. HIGHLIGHTS Stricter environmental regulation policies and freshwater as an increasingly valuable resource are leading to a global growth of zero liquid discharge (ZLD) processes in recent years.; In the study, the results of a demonstrational setup at an industrial site for a newly developed concentrate treatment process are presented. This new ZLD process, named HighCon, has the goal of pure single-salt recovery from industrial wastewater.; The demonstration results show that separating organic components and monovalent ions worked out very well with nanofiltration in the HighCon process and recovering an organic-free salt mixture or – using selective crystallization – the separation of an organic-free single salt with >90% purity has been enabled.; In a cost analysis, the total specific costs have been calculated for the newly developed HighCon process and a comparable ZLD process. With approximately 4 €/m³, the results show that the specific costs are in the same range for both processes.; The HighCon process has the advantage of pure (single-)salt recovery over conventional ZLD generating the mixture of solid wastes. Recovery of resources is becoming increasingly important – economically and with regard to the closing of recoverable substance cycles.

BACKGROUND In this study, the enhancement of propionic acid production from a model feedstock mimicking kitchen waste was investigated. For that purpose, two operational runs of a semicontinuous anaerobic hydrolysis reactor were carried out at pH 6.0 ± 0.1 and mesophilic (30 °C) temperature. Two different types of inocula, a mixed microbial culture selected over 24 months for growth on cellulose and a culture contained in goat cheese were compared. RESULTS The results show that the goat cheese inoculum was significantly more efficient for propionic acid (PA) production. The highest propionic acid concentration achieved amounted to 139 mmol L−1 at a yield of 23.3 mg g−1 volatile solids (VS), which was 55% greater than what was achieved with the mixed culture. Furthermore, it was observed that propionic acid production was enhanced by a combination of high hydraulic retention time (HRT) with low organic loading rate (OLR), ensuring sufficient time for complete processing of the complex organic substrates. The fermentation could be kept in a stable process of propionic acid production at HRT of 20 days and a rather low OLR of 11.1 g L−1 day−1 VS. CONCLUSION Our results give a better understanding of PA production in semicontinuous mode, applying optimized process parameters and selecting the adequate microbial community for inoculation. This study provides important information for the improvement of PA production from complex substrates for future industrial application. © 2020 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

The aim of this study is the utilization of membrane distillation (MD) in the treatment of hydrothermal liquefaction wastewater (HTL-WW) to recover ammonium in the condensate. Experiments were carried out using MD under air���gap configuration with HTL-WW pretreated via Ultrafiltration. The results showed membrane stability in long-term operations, up to 36 days and through a wide range of feed temperatures, from 30 ��C to 60 ��C (Coolant temperature was kept at 20 ��C). Feed temperatures, 50 ��C and 60 ��C provided the best condensate quality, defined by high ammonium concentrations, up to 12 g/L (for 60 ��C feed temperature), and low impurity (low contamination by TOC) based on the highest NH4+:TOC ratio of 13 (for 50 ��C feed temperature). Furthermore, since flux experienced an exponential growth with the increase of feed temperature, 60 ��C was chosen as the optimal temperature to expand the study on membrane/condensate recovery, which was performed until 80%. From observational and several analytical methods, wetting was unavoidable above 60% recovery and the cause was credited to organic fouling, mainly via surfactants��� adsorption on the membrane surface. This decreased the membrane hydrophobicity, and eventually led to the progressive wetting of the membrane at 80% recovery.

Fouling formation in membrane distillation limits process stability and could cause besides wetting a rapid flux decrease making the process inefficient. Process performance is conventionally assessed by flux monitoring. Optical Coherence Tomography (OCT) enables in-situ visualization and quantification of fouling layers dominated by scaling in a fully operated direct contact membrane distillation system (DCMD). This study presents an innovative methodology for 3D dataset analysis allowing for the detection and quantification of predominant scaling on the membrane surface. Defined fouling parameters permit a correlation between process performance indicators, especially flux decrease and quantified fouling layer coverage. Additionally, structural information about the fouling layer was derived from the fouling parameters calculated from OCT C-scans across the feed channel within the flat sheet membrane unit. The method was successfully applied in DCMD of hot spring water, showing a critical fouling ratio between RS = 50–60 µm3/µm2, resulting in an overall flux decrease of 80%. The consideration of covered membrane area in RC proved the correlation of increasing fouling layer coverage (up to 90%) inducing a flux decline (by 80%) verifying the hypothesis, that the share of covered membrane area is the limiting step for MD performance. The presented method was able to link scalable fouling parameters with macroscopic process parameters. Hence making performance monitoring possible while enabling an adapted process control.

Two-stage biogas systems consisting of a CSTR-acidification reactor (AR) and an anaerobic filter (AF) were frequently described for microbial conversion of food and agricultural wastes to biogas. The aim of this study is to investigate the integration of a membrane filtration step in two-stage systems to remove inert particles from hydrolysate produced in AR in order to increase the efficiency of the subsequent AF. Hydrolysates from vegetable waste (VW) and grass/maize silage (G/M) were treated in cross-flow ceramic membrane filtration system (pore size 0.2 µm). Organic acids were extracted efficiently through filtration of hydrolysate. For both the substrates, membrane permeability was stable and high (46.6-49.3 L m-2 h-1 bar-1). Filtration process effectively improved the specific methane yield of permeate by 40% (VW) and 24.5% (G/M) compared to hydrolysate. It could be shown that, the filtration-step increased hydrolysate's degradability, which lead to higher conversion efficiency in the following AF.

Polymeric multichannel hollow fiber membranes were developed to reduce fiber breakage and to increase the volume-to-membrane-surface ratio and consequently the efficiency of filtration processes. These membranes are commonly used in ultrafiltration and are operated in in-out dead-end mode. However, some of the filtration details are unknown. The filtration efficiency and flow in the multichannel membranes depend on filtration time and are expected to vary along spatial coordinates. In the current work, in-situ magnetic resonance imaging was used to answer these questions. Velocities were quantified in the feed channels to obtain a detailed understanding of the filtration process. Flow and deposits were measured in each of the seven channels during filtration of sodium alginate, which is a model substance for extracellular polymeric substances occurring in water treatment. Volume flow and flow profiles were calculated from phase contrast flow images. The flow in z-direction in the center channel was higher than in the surrounding channels. Flow profiles variate depending on the concentration of Ca2+, which changes the filtration mechanism of aqueous solutions of sodium alginate from concentration polarization to gel layer filtration.

In this study, lab-scale batch fermentation tests were carried out at mesophilic temperature (30 °C) to examine the influence of inoculum type, pH-value, and thermal pretreatment of substrate on propionic acid (PA) production from dog food. The selected inocula comprised a mixed bacterial culture, milk, and soft goat cheese. The batch tests were performed at pH 4, pH 6, and pH 8 for both, untreated and thermally pretreated food. Results show that the production of PA and volatile fatty acids (VFAs) in general were significantly dependent on the chosen inoculum and adjusted pH value. The maximum PA production rates and yields were determined for the cheese inoculum at pH 6 using untreated and pretreated dog food. PA concentration reached 10 gL-1and 26.5 gL-1, respectively. Our findings show that by selecting optimal process parameters, an efficient PA production from model food waste can be achieved.

The two-stage high-pressure fermentation (HPF) process enables the production of methane at high operating pressure. Pressure significantly reduces the energy needed for injecting the produced biogas into the gas grid by 45–60%. It also allows for incorporating large parts of the necessary biogas upgrading process into the synthesis step. As a result, the two-stage HPF process provides pressurized biogas with methane volume fraction ranging from 0.75 to 0.94. The pressure is not generated by energy intensive gas compression, but in-situ by microbial gas production. In comparison to conventional biomethane production, the overall costs could be reduced up to 20%. HPF is most beneficial when its operating pressure is adapted to that of the gas grid. The article presents briefly the development of the two-stage HPF beginning with tests in batch reactors, followed by experiments on gas solubility, and proof-of-concept in continuously operated methanogenesis reactors (MR) up to 9 bar. It also represents the effect of incorporating microfiltration (MF) of the feed stream, on improving the biogas quality and process stability of a continuously operated lab scale HPF process. By linking the MF with the HPF, methane volume fraction in the MR increases from 0.86 to 0.94 at 25 bar. Finally, the simulation and experimental results show good agreement with each other thereby making them a good basis for further optimization of the HPF process.

Heavy metal-ion pollution in water is a serious problem for the environment, and membrane filtration technologies such as nanofiltration (NF) have become viable treatment alternatives. However, it is necessary to understand the performance of NF as a function of effluent composition and membrane surface charge properties. This work evaluated first the impact of metal ions on the zeta potential using streaming potential measurements for a commercial NF membrane (NF270). Then, NF rejection of C a 2 + , C u 2 + , and A l 3 + in sulphate and chloride solutions was analyzed based on zeta potential results. Reductions, neutralizations, and inversions of the surface charge of the membrane were observed through streaming zeta potential analysis. Our results also revealed significant differences of the zeta potential between divalent ions and aluminum under all experimental conditions studied. Based on the zeta potential measurements, it was possible to explain the rejects observed for the different metallic ions, through an analysis of steric and electric exclusion effects. Adsorption of ions caused also changes of membrane properties that were analyzed by ATR-FTIR, AFM and contact angles.

In this work, the concentration polarization layer (CPL) of sulphate in a cross-flow membrane system was measured in-situ using Raman microspectroscopy (RM). The focus of this work is to introduce RM as a new tool for the study of mass transfer inside membrane channels in reverse osmosis (RO) and nanofiltration (NF) generally. Specifically, this work demonstrates how to use RM for locally resolved measurement of sulphate concentration in a cross-flow flat-sheet NF membrane flow cell with channel dimensions similar to commonly applied RO/NF spiral wound modules (channel height about 0.7 mm). Concentration polarization profiles of an aqueous magnesium sulphate solution of 10 gsulphate·L−1 were obtained at operating pressure of 10 bar and cross-flow velocities of 0.04 and 0.2 m·s−1. The ability of RM to provide accurate concentration profiles is discussed thoroughly. Optical effects due to refraction present one of the main challenges of the method by substantially affecting signal intensity and depth resolution. The concentration profiles obtained in this concept study are consistent with theory and show reduced CPL thickness and membrane wall concentration with increasing cross-flow velocity. The severity of CP was quantified to reach almost double the bulk concentration at the lower velocity.

The formation of an inorganic deposit layer (scaling) is one of the most critical challenges affecting the long-term stability of membrane distillation processes applied in desalination applications. The consequences are a reduced permeate flux and a lowered permeate quality e.g., throughout wetting. Conventionally, scale formation is assessed based on the detected flux reduction without online proof. Optical coherence tomography (OCT) allows for a non-invasive, in situ visualization and quantification of scale layers in a fully operated MD system. Therefore, OCT was used in this work for the in situ scale study. Scale parameters were derived as membrane-based process key parameters, facilitating the quantification and objective morphological assessment of the scale layer. Variation of the temperature (76 °C and 60 °C) showed a significant influence on the formation of calcite scale layers. A high temperature on the feed side lowered the ion solubility, which caused the massive and fast formation of big crystals on the membrane surface. However, the variation of Air Gap and Direct Contact configuration showed no difference in the formation of scaling. The application of different water matrixes caused the formation of different chemical and morphological scale layers. Both revealed the same flux reduction although the degree of coverage and the morphology of the crystals was different. The results emphasize the insufficient information supply based on single flux detection. It only allows a limited interpretation of scale formation, whereas the calculation of scale parameters determined the differences clearly and enables the calculation of scale formation rate and crystal morphology. Additionally, wetting phenomena were non-invasively visualized by means of OCT.

For an undisturbed operation of two-stage high-pressure fermentation up to 100 bar, a particle-free hydrolysate appears to be necessary. This is even more important if the second stage, i.e., the methane reactor, is designed as fixed bed. Here, we present the potential of microfiltration membranes as separation unit after the first stage, which is the hydrolysis. The study included the selection of membrane material, membrane performance investigations, and long-term-behavior during the filtration period. In a series of experiments, the optimum type of membrane material and the mode of operation [either crossflow (CF) or submerged (S)] were determined. Ceramic membranes proved to be the better option to treat the process stream due to their chemical and temperature resistance. The crossflow filtration achieved a sustainable flux of up to 33 L/(m2 h), while long-term experiments with the submerged membranes confirmed a critical flux of 7 L/(m2 h). Comparative analyses of hydrolysate and permeate showed that the rejected chemical oxygen demand (COD) as well as total organic carbon (TOC) fraction and thereby the loss of organic carbon in the permeate does not reduce the methane yield.

Application of nanofiltration (NF) membranes has increased during the last years: many industries from drinking water treatment and water re-use to the pharmaceutical and food industry deal with the application of NF and its drawbacks. One of the main problems of membrane technology is fouling. Fouling during NF is a complex problem because of the importance of the membrane surface properties (e.g. charge and structure) and the corresponding interactions between the water constituents and the membrane. In this work, the fouling layer composition of three membranes was investigated using different raw waters and it was correlated with the membrane performance. Rejection properties depended to a great extent on the water composition and the ionic strength of the raw water. Investigations of fouling layer compositions suggest that the membrane properties play a key role for flux decline and the related “amount” of fouling material on the membranes and its constituents.

The combination of ultrafiltration (UF) and powdered activated carbon (PAC) has gained in importance over the last 15 years. The aim of applying PAC-UF hybrid processes is to improve the rejection capacity and the performance of membrane processes. In this work the influence of PAC-addition on membrane flux decline and fouling formation was studied with a submerged module using different water matrices. The PACs used were characterized by measurements of the surface area, the particle size distribution and the surface charge to get a better understanding of the observed effects. Results showed that the membrane flux decline was dependent on the applied type of activated carbon. When adding Picachem 8P, the transmembrane pressure (TMP) increased very quickly and the membrane performance was even worse than that without PAC. However, when adding Norit SA-UF under identical filtration conditions, the TMP was stable and the membrane performance was better than that without PAC. The fouling layers of the two PACs applied showed entirely different structures. Therefore the selection of a suitable PAC is a main factor for the overall performance of PAC-UF hybrid systems with submerged membranes.

The application of membrane filtration in the treatment of spent filter backwash water (SFBW) permits efficient removal of microorganisms, suspended particles and organic substances, depending on the used membrane molecular weight cut-off. However, flux decline, due to deposits and adsorption of substances (salts, colloids, organics, particles, microorganisms, etc.) tends to limit the use of membranes. Characterization of SFBW samples from different waterworks showed that three major factors contribute to the SFBW properties: the raw water itself, the time interval of sand filter operation and additional treatment steps. The main differences between SFBW samples were found principally in DOC, TOC and turbidity. Experiments with submerged membranes (lab- and pilot- scale modules) showed that there was a clear correlation between DOC concentration of the feed and the flux decline: when the DOC-concentration increased, the flux decline increased. Additionally, the presence of calcium led not only to an important flux decline but to high adsorption of NOM on the membrane surface. Iron concentrations in the micromolar range resulted in a considerably decline of flux. Filtration of SFBW revealed that the decline of permeability is mainly determined by DOC, calcium and iron concentrations. A decisive effect of biofouling on membrane performance is expected for long term experiments.

Trihalomethanes (THM) are the most typical disinfection by-products (DBPs) found in public swimming pool water. DBPs are produced when organic and inorganic matter in water reacts with chemical disinfectants. The irregular contribution of substances from pool visitors and long contact time with disinfectant make the forecast of THM in pool water a challenge. In this work occurrence of THM in a public indoor swimming pool was investigated and correlated with the dissolved organic carbon (DOC). Daily sampling of pool water for 26 days showed a positive correlation between DOC and THM with a time delay of about two days, while THM and DOC didn't directly correlate with the number of visitors. Based on the results and mass-balance in the pool water, a simple simulation model for estimating THM concentration in indoor swimming pool water was proposed. Formation of THM from DOC, volatilization into air and elimination by pool water treatment were included in the simulation. Formation ratio of THM gained from laboratory analysis using native pool water and information from field study in an indoor swimming pool reduced the uncertainty of the simulation. The simulation was validated by measurements in the swimming pool for 50 days. The simulated results were in good compliance with measured results. This work provides a useful and simple method for predicting THM concentration and its accumulation trend for long term in indoor swimming pool water.

Interactions between dissolved organic substances (dissolved organic matter and two model substances), ions (Na + and Ca 2+ ) and the membrane surface were investigated using a submerged membrane module. The membrane permeability, the fouling formation and the rejection were significantly influenced by the solution’s ionic strength and especially by the presence of calcium ions. An increase of the ionic strength led to a decrease of the DOMrejection of the membrane from 75% to about 50% if Na + -ions were added and to about 15% if Ca 2+ -ions were added. This was due to neutralization of charges of the membrane surface and due to shielding of negative charges of DOM molecules. The ionic strength and pH of the cation solutions were kept at similar values. As the solution ionic strength increases, DOM molecules can permeate more easily through the membrane pores, since their shape and molecular size change. Adsorption of DOM on the membrane surface and in the membrane pores was only observed by filtration of solutions with Ca 2+ . The results showed that in the presence of Ca 2+ , adsorption played a mayor role in flux decline and fouling formation.

In this study natural organic matter (NOM) removal by ultra- and nanofiltration from four different raw waters was investigated. The experiments were carried out with a flat channel cross flow unit and with a pilot unit equipped with two parallel spiral wound modules. NOM rejection was characterized using 13CMAS-NMR spectroscopy, scanning electron microscopy (SEM) and size exclusion chromatography with online DOC detection (LC-OCD). Additionally measurements, such as DOC, UVA (254nm and 436nm), AOX- and THM-precursors, ion concentration, turbidity, etc. were made on raw waters, permeates and concentrates. The results indicate that NOM removal by nanofiltration for all water sources is almost complete (>90%), while NOM removal by ultrafiltration depends on raw water chemistry, especially ionic strength. Solid state 13C-NMR spectra of freeze-dried NOM deposits on membrane surfaces suggest the presence of structural polysaccharides (such as chitin and cellulose). SEM images confirm the NMR-results: after filtration, inorganic deposits and algae like structures were found on membrane surfaces.

In investigations concerning the photocatalytic degradation by TiO2 usually filter discs with a pore size of 0.22 μm and 0.45 μm are used for the removal of photocatalyst particles in aqueous suspensions. In this study the effective rejection of suspended particles by microfiltration in different types of membrane modules and with different membrane materials was investigated. Furthermore, Powdered Activated Carbon (PAC), which can be used to gain an increase in photocatalytic degradation rates, was investigated concerning its influence on the membrane performance. It is shown that by membrane filtration with a pore size above 0.1 μm, irrespective of the experimental conditions, no complete removal can be achieved. However, UV irradiation was found to improve the removal efficiency for all types of tested membrane materials. The addition of PAC also led to a higher performance of membrane filtration with regard to particle rejection. In long-term experiments with a hollow fibre membrane module in the presence of PAC a five-fold decrease of TiO2 particles in the permeate could be proven. Besides, it was shown that added PAC can shield the membrane regarding the abrasivity of TiO2, which could otherwise lead to the destruction of the membrane. Therewith PAC exhibits another crucial advantage besides its synergetic effect in photodegradation.

The transport of metal ions (Al, Fe, Zn, Pb) complexed by natural organic matter (NOM) was investigated by column experiments. Direct breakthrough of metal-NOM complexes was observed after elution of one bed volume for Al, Fe, and Pb, but not for Zn. This observation cannot be understood assuming local thermodynamic equilibrium in the columns. Hence, a model was developed, taking into account the kinetics of the interactions of metal ions with NOM and the solid phase. Of all possible reactions during passage through the column, the dissociation of the metal-NOM complexes was assumed to be the rate-determining step. Dissociation-rate constants were determined by cation-exchange experiments with a non-NOM-adsorbing cation-exchange resin (Chelex-100). These rate constants were used to predict the migration of metal-NOM complexes in the column experiments. Experimental and modeling results were in good agreement for the bivalent metal ions. Also, the model well predicted the pH dependence of breakthrough of trivalent metal ions and the differences between Al and Fe breakthrough on a qualitative basis. This leads to the conclusion that the dissociation kinetics of metal-NOM complexes is an essential parameter for the estimation of NOM-facilitated metal transport. However, for the trivalent metals, Al and Fe, the model overestimated the direct breakthrough, thus giving a worst case prediction of metal transport. With the help of coupling size-exclusion chromatography to inductively coupled plasma-mass spectrometry, the formation of Al-hydroxide and Fe-hydroxide colloids in addition to NOM complexes was detected. These colloids, which were not considered in the model, were partially filtered off in the column, thus leading to overestimation of metal breakthrough.

This study aims to evaluate the application of ultrafiltration technology for the separation of particles and oil droplets and the recovery of ammonia from hydrothermal liquefaction (HTL) wastewater. Real HTL wastewater from the hydrothermal liquefaction of municipal sewage sludge was used in this study. Experiments were carried out using a submerged polyethersulfone ultrafiltration membrane with molecular weight cutoff of 100 kDa in combination with air stripping and addition to acid and base traps for recovery of volatiles. Results showed, that the best operation mode of ultrafiltration is with backwash cycles of the permeate, maintaining a flux lower than the critical flux of 6 L/h·m2. The setup led to fast stripping of ammonia, which was successfully recovered by 88% in the acid trap. This application can be considered an adequate first stage treatment of the HTL wastewater. The importance of this work is that it proves that membrane technology can be successful in treating complex real HTL wastewater, and is not only limited for applications using model solutions.