Polysulfide Concentration and Chain Length in the Biological Desulfurization Process: Effect of Biomass Concentration and the Sulfide Loading Rate.

Removal of hydrogen sulfide (H2S) can be achieved using the sustainable biological desulfurization process, where H2S is converted to elemental sulfur using sulfide-oxidizing bacteria (SOB). A dual-bioreactor process was recently developed where an anaerobic (sulfidic) bioreactor was used between the absorber column and micro-oxic bioreactor. In the absorber column and sulfidic bioreactor, polysulfides (Sx2-) are formed due to the chemical equilibrium between H2S and sulfur (S8). Sx2- is thought to be the intermediate for SOB to produce sulfur via H2S oxidation. In this study, we quantify Sx2-, determine their chain-length distribution under high H2S loading rates, and elucidate the relationship between biomass and the observed biological removal of sulfides under anaerobic conditions. A linear relationship was observed between Sx2- concentration and H2S loading rates at a constant biomass concentration. Increasing biomass concentrations resulted in a lower measured Sx2- concentration at similar H2S loading rates in the sulfidic bioreactor. Sx2- of chain length 6 (S62-) showed a substantial decrease at higher biomass concentrations. Identifying Sx2- concentrations and their chain lengths as a function of biomass concentration and the sulfide loading rate is key in understanding and controlling sulfide uptake by the SOB. This knowledge will contribute to a better understanding of how to reach and maintain a high selectivity for S8 formation in the dual-reactor biological desulfurization process.

Increasing the Selectivity for Sulfur Formation in Biological Gas Desulfurization.

In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H2S) is removed from sour gas and oxidized to elemental sulfur (S8) by sulfide-oxidizing bacteria. Besides S8, the byproducts sulfate (SO42-) and thiosulfate (S2O32-) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S8 by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S8 formation was 96.6% on a molar H2S supply basis; selectivity for SO42- and S2O32- were 1.4 and 2.0% respectively. The selectivity for S8 formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO42- and S2O32- formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO42- formation further decreased. In addition, chemical formation of S2O32- was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S8 formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.

Nitrogen dynamics in flooded soil systems: an overview on concepts and performance of models.

Extensive modelling studies on nitrogen (N) dynamics in flooded soil systems have been published. Consequently, many N dynamics models are available for users to select from. With the current research trend, inclined towards multidisciplinary research, and with substantial progress in understanding of N dynamics in flooded soil systems, the objective of this paper is to provide an overview of the modelling concepts and performance of 14 models developed to simulate N dynamics in flooded soil systems. This overview provides breadth of knowledge on the models, and, therefore, is valuable as a first step in the selection of an appropriate model for a specific application. © 2017 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Effect of Methanethiol Concentration on Sulfur Production in Biological Desulfurization Systems under Haloalkaline Conditions.

Bioremoval of H2S from gas streams became popular in recent years because of high process efficiency and low operational costs. To expand the scope of these processes to gas streams containing volatile organic sulfur compounds, like thiols, it is necessary to provide new insights into their impact on overall biodesulfurization process. Published data on the effect of thiols on biodesulfurization processes are scarce. In this study, we investigated the effect of methanethiol on the selectivity for sulfur production in a bioreactor integrated with a gas absorber. This is the first time that the inhibition of biological sulfur formation by methanethiol is investigated. In our reactor system, inhibition of sulfur production started to occur at a methanethiol loading rate of 0.3 mmol L(-1) d(-1). The experimental results were also described by a mathematical model that includes recent findings on the mode of biomass inhibition by methanethiol. We also found that the negative effect of methanethiol can be mitigated by lowering the salinity of the bioreactor medium. Furthermore, we developed a novel approach to measure the biological activity by sulfide measurements using UV-spectrophotometry. On the basis of this measurement method, it is possible to accurately estimate the unknown kinetic parameters in the mathematical model.

Modelling anaerobic sulfide removal by sulfide shuttling bacteria.

Sulfide oxidizing bacteria are used in industrial biodesulfurization processes to convert sulfide to sulfur. These bacteria can spatially separate sulfide removal from terminal electron transfer, thereby acting as sulfide shuttles. The mechanisms underlying sulfide shuttling are not yet clear. In this work, newly obtained sulfide removal data were used to develop a new model for anaerobic sulfide removal and this model was shown to be an improvement over two previously published models. The new model describes a fast chemical step and a consecutive slow enzymatic step. The improved model includes the effect of pH, with higher total sulfide removal at increasing pH, as well as partial sulfide removal at higher sulfide concentrations. The two-stage model is supported by recent developments in anaerobic sulfide removal research and contributes to a better understanding of the underlying mechanisms. The model is a step toward accurately modelling anaerobic sulfide removal in industrial systems.

Influence of oxidation-reduction potential and pH on polysulfide concentrations and chain lengths in the biological desulfurization process under haloalkaline conditions.

Biological desulfurization under haloalkaline conditions has been applied worldwide to remove hydrogen sulfide (H2S) from sour gas steams. The process relies on sulfide-oxidizing bacteria (SOB) to oxidize H2S to elemental sulfur (S8), which can then be recovered and reused. Recently, a dual-reactor biological desulfurization system was implemented where an anaerobic (sulfidic) bioreactor was incorporated as an addition to a micro-oxic bioreactor, allowing for higher S8 selectivity by limiting by-product formation. The highly sulfidic bioreactor environment enabled the SOB to remove (poly)sulfides (Sx2-) in the absence of oxygen, with Sx2- speculated as a main substrate in the removal pathway, thus making it vital to understand its role in the process. The SOB are influenced by the oxidation-reduction potential (ORP) set-point of the micro-oxic bioreactor as it is used to control the product of oxidation (S8 vs. SO42-), while the uptake of Sx2- by SOB has been qualitatively linked to pH. Therefore, to quantify these effects, this work determined the concentration and speciation of Sx2- in the biological desulfurization process under various pH values and ORP set-points. The total Sx2- concentrations in the sulfidic zone increased at elevated pH (8.9) compared to low pH (< 8.0), with on average 3.3 ± 1.0 mM-S more Sx2-. Chain lengths varied, with S72- only doubling in concentration while S52- increased 9 fold, which is in contrast with observations from abiotic systems. Changes to the ORP set-point of the micro-oxic reactor did not produce substantial changes in Sx2- concentration in the sulfidic zone. This illustrates that the reduction degree of the SOB in the micro-oxic bioreactor does not enhance their ability to interact with Sx2- in the sulfidic bioreactor. This increased understanding of how both pH and ORP affect changes in Sx2- concentration and chain length can lead to improved efficiency and design of the dual-reactor biological desulfurization process.

Integrated hydroponics systems with anaerobic supernatant and aquaculture effluent in desert regions: Nutrient recovery and benefit analysis.

Hydroponics is a resource-efficient system that increases food production and enhances the overall sustainability of agricultural systems, particularly in arid zones with prevalent water scarcity and limited areas of arable land. This study investigated zero-waste hydroponics systems fed by agricultural waste streams as nutrient sources under desert conditions. Three pilot-scale systems were tested and compared. The first hydroponics system ("HPAP") received its nutrient source internally from an aquaponic system, including supernatant from the anaerobic digestion of fish sludge. The second system ("HPAD") was sourced by the supernatant of plant waste anaerobic digestion, and the third served as a control that was fed by commercial Hoagland solution ("HPHS"). Fresh weight production was similar in all treatments, ranging from 488 to 539 g per shoot, corresponding to 5.7 to 6.0 kg total wet weight per m2. The recovery of N and P from wastes and their subsequent uptake by plants was highly efficient, with rates of 77 % for N and 65 % for P. Plants that were fed using supernatants demonstrated slightly higher plant quality compared with those grown in Hoagland solution. Over the duration of the full study (3 months), water was only used to compensate for evapotranspiration, corresponding to ~10 L per kg of lettuce. The potential health risk for heavy metals was negligible, as assessed using the health-risk index (HRI < 1) and targeted hazardous quotient (THQ < 1). The results of this study demonstrate that careful management can significantly reduce pollution, increase the recovery of nutrients and water, and improve hydroponics production.

Adsorption of phosphonate antiscalant from reverse osmosis membrane concentrate onto granular ferric hydroxide.

Adsorptive removal of antiscalants offers a promising way to improve current reverse osmosis (RO) concentrate treatment processes and enables the reuse of the antiscalant in the RO desalination process. This work investigates the adsorption and desorption of the phosphonate antiscalant nitrilotris(methylenephosphonic acid) (NTMP) from RO membrane concentrate onto granular ferric hydroxide (GFH), a material that consists predominantly of akaganéite. The kinetics of the adsorption of NTMP onto GFH was predicted fairly well with two models that consider either combined film-pore or combined film-surface diffusion as the main mechanism for mass transport. It is also demonstrated that NTMP is preferentially adsorbed over sulfate by GFH at pH 7.85. The presence of calcium causes a transformation in the equilibrium adsorption isotherm from a Langmuir type to a Freundlich type with much higher adsorption capacities. Furthermore, calcium also increases the rate of adsorption substantially. GFH is reusable after regeneration with sodium hydroxide solution, indicating that NTMP can be potentially recovered from the RO concentrate. This work shows that GFH is a promising adsorbent for the removal and recovery of NTMP antiscalant from RO membrane concentrates.

Pathways of sulfide oxidation by haloalkaliphilic bacteria in limited-oxygen gas lift bioreactors.

Physicochemical processes, such as the Lo-cat and Amine-Claus process, are commonly used to remove hydrogen sulfide from hydrocarbon gas streams such as landfill gas, natural gas, and synthesis gas. Biodesulfurization offers environmental advantages, but still requires optimization and more insight in the reaction pathways and kinetics. We carried out experiments with gas lift bioreactors inoculated with haloalkaliphilic sulfide-oxidizing bacteria. At oxygen-limiting levels, that is, below an O(2)/H(2)S mole ratio of 1, sulfide was oxidized to elemental sulfur and sulfate. We propose that the bacteria reduce NAD(+) without direct transfer of electrons to oxygen and that this is most likely the main route for oxidizing sulfide to elemental sulfur which is subsequently oxidized to sulfate in oxygen-limited bioreactors. We call this pathway the limited oxygen route (LOR). Biomass growth under these conditions is significantly lower than at higher oxygen levels. These findings emphasize the importance of accurate process control. This work also identifies a need for studies exploring similar pathways in other sulfide oxidizers such as Thiobacillus bacteria.

Plants Dictate Root Microbial Composition in Hydroponics and Aquaponics.

The role of the microbial community in mediating fish and plant co-culture is often considered the black box of aquaponics. Despite widespread recognition regarding the dependency of plants on their rhizosphere, the extent to which upstream aquaculture influences downstream hydroponic root communities has been poorly described in the literature. In this study we performed a taxonomic survey (16S rRNA metabarcoding) of microbial communities originating in the facility water source, hydroponic nutrient solution (HNS) sump, nutrient supplemented biofilter effluent (BF) sump, and recirculating aquaculture system tanks stocked with Nile tilapia (Oreochromis niloticus). Lettuce (Lactuca sativa) was then grown using the HNS and BF effluent under sterilized or mature (prior aquaponics/hydroponics lettuce culture water) conditions, likewise, the influence of probiotic addition or inoculation with soil-grown lettuce rhizosphere was assessed. Compositional similarities across treatments suggest that under soil-less conditions, plants are able to exert a stronger discriminatory influence on their rhizosphere composition than is done by colonization from upstream sources. Furthermore, cluster dendrograms grouped the sterilized and unsterilized treatments more consistently together than hydroponics and aquaponics treatments. These findings contradict conventional beliefs that microbial communities in the water column colonize roots based on their presence alone, ignoring the role that plants play in rhizosphere community selection.

Clean and Safe Drinking Water Systems via Metagenomics Data and Artificial Intelligence: State-of-the-Art and Future Perspective.

The use of next-generation sequencing technologies in drinking water distribution systems (DWDS) has shed insight into the microbial communities' composition, and interaction in the drinking water microbiome. For the past two decades, various studies have been conducted in which metagenomics data have been collected over extended periods and analyzed spatially and temporally to understand the dynamics of microbial communities in DWDS. In this literature review, we outline the findings which were reported in the literature on what kind of occupancy-abundance patterns are exhibited in the drinking water microbiome, how the drinking water microbiome dynamically evolves spatially and temporally in the distribution networks, how different microbial communities co-exist, and what kind of clusters exist in the drinking water ecosystem. While data analysis in the current literature concerns mainly with confirmatory and exploratory questions pertaining to the use of metagenomics data for the analysis of DWDS microbiome, we present also future perspectives and the potential role of artificial intelligence (AI) and mechanistic models to address the predictive and mechanistic questions. The integration of meta-omics, AI, and mechanistic models transcends metagenomics into functional metagenomics, enabling deterministic understanding and control of DWDS for clean and safe drinking water systems of the future.

Carbon dynamics and energy recovery in a novel near-zero waste aquaponics system with onsite anaerobic treatment.

Aquaponics is gaining renewed interest to enhance food security. This study aimed to investigate the performance of a novel off-grid aquaponics system with near-zero water and waste discharge, focusing on the carbon cycle and energy recovery that was achieved by the addition of onsite anaerobic treatment of the solid waste streams. Following a stabilization stage, the system was closely monitored for four months. Fish tank water was recirculated via solid and nitrification reactors, from which 66% was recycled to the fish tank directly and 34% indirectly through the hydroponically grown plants. Fish solid waste was anaerobically treated, energy was recovered, and the nutrient-rich supernatant was recycled to the plants to enhance production. Plant waste was also digested anaerobically for further recovery of energy and nutrients. Fish stocking density was 15.3 and over time reached approximately 40 kg/m3 where it was maintained. Feed (45% protein content) was applied daily at 2% of body weight. Typical fish performance was observed with a survival rate >97% and feed conversion ratio of 1.33. Lettuce production was up to 5.65 kg/m2, significantly higher than previous reports, largely because of high nutrients reuse efficiency from the anaerobic supernatant that contained 130 and 34 mg/L N and P, respectively. Of the feed carbon, 24.5% was taken up by fish biomass. Fish solid wastes contained 38.2% carbon, of which 91.9% was recovered as biogas (74.5% CH4). Biogas production was 0.84 m3/kg for fish sludge and 0.67 m3/kg for dry plant material. CO2 sequestration was 1.4 higher than the feed carbon, which reduced the system's carbon footprint by 64%. This study is the first to demonstrate highly efficient fish and plant production with near-zero water and waste discharge and with energy recovery that can potentially supply the system's energy demand.

Removal of small elemental sulfur particles by polysulfide formation in a sulfidic reactor.

For over 30 years, biological gas desulfurization under halo-alkaline conditions has been studied and optimized. This technology is currently applied in already 270 commercial installations worldwide. Sulfur particle separation, however, remains a challenge; a fraction of sulfur particles is often too small for liquid-solid separation with conventional separation technology. In this article, we report the effects of a novel sulfidic reactor, inserted in the conventional process set-up, on sulfur particle size and morphology. In the sulfidic reactor polysulfide is produced by the reaction of elemental sulfur particles and sulfide, which is again converted to elemental sulfur in a gas-lift reactor. We analyzed sulfur particles produced in continuous, long term lab-scale reactor experiments under various sulfide concentrations and sulfidic retention times. The analyses were performed with laser diffraction particle size analysis and light microscopy. These show that the smallest particles (< 1 µm) have mostly disappeared under the highest sulfide concentration (4.1 mM) and sulfidic retention time (45 min). Under these conditions also agglomeration of sulfur particles was promoted. Model calculations with thermodynamic and previously derived kinetic data on polysulfide formation confirm the experimental data on the removal of the smallest particles. Under the 'highest sulfidic pressure', the model predicts that equilibrium conditions are reached between sulfur, sulfide and polysulfide and that 100% of the sulfur particles <1 µm are dissolved by the (autocatalytic) formation of polysulfides. These experiments and modeling results demonstrate that the insertion of a novel sulfidic reactor in the conventional process set-up promotes the removal of the smallest individual sulfur particles and promotes the production of sulfur agglomerates. The novel sulfidic reactor is therefore a promising process addition with the potential to improve process operation, sulfur separation and sulfur recovery.

Resource management as a key factor for sustainable urban planning.

Due to fast urbanization and increasing living standards, the environmental sustainability of our global society becomes more and more questionable. In this historical review we investigate the role of resources management (RM) and urban planning (UP) and propose ways for integration in sustainable development (SD). RM follows the principle of circular causation, and we reflect on to what extent RM has been an element for urban planning. Since the existence of the first settlements, a close relationship between RM, urbanization and technological development has been present. RM followed the demand for urban resources like water, energy, and food. In history, RM has been fostered by innovation and technology developments and has driven population growth and urbanization. Recent massive resource demand, especially in relation to energy and material flows, has altered natural ecosystems and has resulted in environmental degradation. UP has developed separately in response to different questions. UP followed the demand for improved living conditions, often associated to safety, good manufacturing and trading conditions and appropriate sanitation and waste management. In history UP has been a developing research area, especially since the industrial era and the related strong urbanization at the end of the 18th century. UP responded to new emerging problems in urban areas and became increasingly complex. Nowadays, UP has to address many objectives that are often conflicting, including, the urban sustainability. Our current urban un-sustainability is rooted in massive resource consumption and waste production beyond natural limits, and the absence of flows from waste to resources. Therefore, sustainable urban development requires integration of RM into UP. We propose new ways to this integration.

Biologically enhanced hydrogen sulfide absorption from sour gas under haloalkaline conditions.

We studied a biotechnological desulfurization process for removal of toxic hydrogen sulfide (H2S) from sour gas. The process consists of two steps: i) Selective absorption of H2S into a (bi)carbonate solution in the absorber column and ii) conversion of sulfide to sulfur by sulfide oxidizing bacteria (SOB) in the aerated bioreactor. In previous studies, several physico-chemical factors were assessed to explain the observed enhancement of H2S absorption in the absorber, but a full explanation was not provided. We investigated the relation between the metabolic activity of SOB and the enhancement factor. Two continuous experiments on pilot-scale were performed to determine H2S absorption efficiencies at different temperatures and biomass concentrations. The absorption efficiency improved at increasing temperatures, i.e. H2S concentration in the treated gas decreased from 715 ±â€¯265 ppmv at 25.4 °C to 69 ±â€¯25 ppmv at 39.4 °C. The opposite trend is expected when H2S absorption is solely determined by physico-chemical factors. Furthermore, increasing biomass concentrations to the absorber also resulted in decreased H2S concentrations in the treated gas, from approximately 6000 ppmv without biomass to 1664 ±â€¯126 ppmv at 44 mg N/L. From our studies it can be concluded that SOB activity enhances H2S absorption and leads to increased H2S removal efficiencies in biotechnological gas desulfurization.

Development and validation of a physiologically based kinetic model for starting up and operation of the biological gas desulfurization process under haloalkaline conditions.

Hydrogen sulfide is a toxic and corrosive gas that must be removed from gaseous hydrocarbon streams prior to combustion. This paper describes a gas biodesulfurization process where sulfur-oxidizing bacteria (SOB) facilitate sulfide conversion to both sulfur and sulfate. In order to optimize the formation of sulfur, it is crucial to understand the relations between the SOB microbial composition, kinetics of biological and abiotic sulfide oxidation and the effects on the biodesulfurization process efficiency. Hence, a physiologically based kinetic model was developed for four different inocula. The resulting model can be used as a tool to evaluate biodesulfurization process performance. The model relies on a ratio of two key enzymes involved in the sulfide oxidation process, i.e., flavocytochrome c and sulfide-quinone oxidoreductase (FCC and SQR). The model was calibrated by measuring biological sulfide oxidation rates for different inocula obtained from four full-scale biodesulfurization installations fed with gases from various industries. Experimentally obtained biological sulfide oxidation rates showed dissimilarities between the tested biomasses which could be explained by assuming distinctions in the key-enzyme ratios. Hence, we introduce a new model parameter α to whereby α describes the ratio between the relative expression levels of FCC and SQR enzymes. Our experiments show that sulfur production is the highest at low α values.

Glocal assessment of integrated wastewater treatment and recovery concepts using partial nitritation/Anammox and microalgae for environmental impacts.

This study explored the feasibility and estimated the environmental impacts of two novel wastewater treatment configurations. Both include combined bioflocculation and anaerobic digestion but apply different nutrient removal technologies, i.e. partial nitritation/Anammox or microalgae treatment. The feasibility of such configurations was investigated for 16 locations worldwide with respect to environmental impacts, such as net energy yield, nutrient recovery and effluent quality, CO2 emission, and area requirements. The results quantitatively support the applicability of partial nitritation/Anammox in tropical regions and some locations in temperate regions, whereas microalgae treatment is only applicable the whole year round in tropical regions that are close to the equator line. Microalgae treatment has an advantage over the configuration with partial nitritation/Anammox with respect to aeration energy and nutrient recovery, but not with area requirements. Differential sensitivity analysis points out the dominant influence of microalgal biomass yield and wastewater nutrient concentrations on area requirements and effluent quality. This study provides initial selection criteria for worldwide feasibility and corresponding environmental impacts of these novel municipal wastewater treatment plant configurations.

Inhibition of a biological sulfide oxidation under haloalkaline conditions by thiols and diorgano polysulfanes.

A novel approach has been developed for the simultaneous description of reaction kinetics to describe the formation of polysulfide and sulfate anions from the biological oxidation of hydrogen sulfide (H2S) using a quick, sulfide-dependent respiration test. Next to H2S, thiols are commonly present in sour gas streams. We investigated the inhibition mode and the corresponding inhibition constants of six thiols and the corresponding diorgano polysulfanes on the biological oxidation of H2S. A linear relationship was found between the calculated IC50 values and the lipophilicity of the inhibitors. Moreover, a mathematical model was proposed to estimate the biomass activity in the absence and presence of sulfurous inhibitors. The biomass used in the respiration tests originated from a full-scale biodesulfurization reactor. A microbial community analysis of this biomass revealed that two groups of microorganism are abundant, viz. Ectothiorhodospiraceae and Piscirickettsiaceae.

Volatile fatty acids production from sewage organic matter by combined bioflocculation and anaerobic fermentation.

This work aims at exploring the feasibility of a combined process bioflocculation to concentrate sewage organic matter and anaerobic fermentation to produce volatile fatty acids (VFA). Bioflocculation, using a high-loaded aerobic membrane bioreactor (HL-MBR), was operated at an HRT of 1h and an SRT of 1 day. The HL-MBR process removed on average 83% of sewage COD, while only 10% of nitrogen and phosphorus was removed. During anaerobic fermentation of HL-MBR concentrate at an SRT of 5 days and 35 °C, specific VFA production rate of 282 mg VFA-COD/g VSS could be reached and consisted of 50% acetate, 40% propionate and 10% butyrate. More than 75% of sewage COD was diverted to the concentrate, but only 15% sewage COD was recovered as VFA, due to incomplete VSS degradation at the short treatment time applied. This shows that combined process for the VFA production is technologically feasible and needs further optimization.