Implications in the production of defossilized methanol: A study on carbon sources.

The transition of the current fossil based chemical industry to a carbon-neutral industry can be done by the substitution of fossil carbon for defossilized carbon in the production of base chemicals. Methanol is one of the seven base chemicals, which could be used to produce other base chemicals (light olefins and aromatics). In this research, we evaluated the synthesis of methanol based on defossilized carbon sources (maize, waste biomass, direct air capture of CO2 (DAC), and CO2 from the cement industry) by considering carbon source availability, energy, water, and land demand. This evaluation was based on a carbon balance for each of the carbon sources. Our results show that maize, waste biomass, and CO2 cement could supply 0.7, 2, 15 times the carbon demand for methanol respectively. Regarding the energy demand maize, waste biomass, DAC, and CO2 from cement demand 25, 21, 48, and 45GJtonMeOH separately. The demand for water is 5300, 220, 8, and 8m3tonMeOH. And lastly, land demand was estimated to 1031, 36, 83, and 77m2tonMeOH per carbon source. The high-demanding-resource production of defossilized methanol is dependent on the availability of resources per location. Therefore, we analyzed the production of defossilized methanol in the Netherlands, Saudi Arabia, China, and the USA. China is the only country where CO2 from the cement industry could provide all the demand of carbon. But as we envision society becoming carbon neutral, CO2 from the cement industry would diminish in time, as a consequence, it would not be sufficient to supply the demand for carbon. DAC would be the only source able to provide the demand for defossilized carbon.

Enhancement of Ammonium Oxidation at Microoxic Bioanodes.

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m-3 d-1 under microoxic conditions (dissolved oxygen at 0.02-0.2 mg-O2/L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m-3 d-1). We found that this enhancement was related to the formation of hydroxylamine (NH2OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH2OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

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.

Effect of organic amendments obtained from different pretreatment technologies on soil microbial community.

The application of organic amendments (OAs) obtained from biological treatment technologies is a common agricultural practice to increase soil functionality and fertility. OAs and their respective pretreatment processes have been extensively studied. However, comparing the properties of OAs obtained from different pretreatment processes remains challenging. In most cases, the organic residues used to produce OAs exhibit intrinsic variability and differ in origin and composition. In addition, few studies have focused on comparing OAs from different pretreatment processes in the soil microbiome, and the extent to which OAs affect the soil microbial community remains unclear. This limits the design and implementation of effective pretreatments aimed at reusing organic residues and facilitating sustainable agricultural practices. In this study, we used the same model residues to produce OAs to enable meaningful comparisons among compost, digestate, and ferment. These three OAs contained different microbial communities. Compost had higher bacterial but lower fungal alpha diversity than ferment and digestate. Compost-associated microbes were more prevalent in the soil than ferment- and digestate-associated microbes. More than 80% of the bacterial ASVs and fungal OTUs from the compost were detected 3 months after incorporation into the soil. However, the addition of compost had less influence on the resulting soil microbial biomass and community composition than the addition of ferment or digestate. Specific native soil microbes, members from Chloroflexi, Acidobacteria, and Mortierellomycota, were absent after ferment and digestate application. The addition of OAs increased the soil pH, particularly in the compost-amended soil, whereas the addition of digestate enhanced the concentrations of dissolved organic carbon (DOC) and available nutrients (such as ammonium and potassium). These physicochemical variables were key factors that influenced soil microbial communities. This study furthers our understanding of the effective recycling of organic resources for the development of sustainable soils.

Potential reuse of domestic organic residues as soil organic amendment in the current waste management system in Australia, China, and The Netherlands.

Soil organic carbon (SOC) is essential for most soil functions. Changes in land use from natural land to cropland disrupt long-established SOC balances and reduce SOC levels. The intensive use of chemical fertilisers in modern agriculture accelerates the rate of SOC depletion. Domestic organic residues (DOR) are a valuable source of SOC replenishment with high carbon content. However, there is still a lack of knowledge and data regarding whether and to what extent DOR can contribute to replenishing SOC. This paper aims to unpack the potential of DOR as a SOC source. Total SOC demand and annual SOC loss are defined and calculated. The carbon flow within different DOR management systems is investigated in three countries (China, Australia, and The Netherlands). The results show that the total SOC demand is too large to be fulfilled by DOR in a short time. However, DOR still has a high potential as a source of SOC as it can mitigate the annual SOC loss by up to 100%. Achieving this 100% mitigation requires a shift to more circular management of DOR, in particular, more composting, and direct land application instead of landfilling and incineration (Australia and China), or a higher rate of source separation of DOR (The Netherlands). These findings form the basis for future research on DOR recycling as a SOC source.

Sulfur Reduction at Hyperthermoacidophilic Conditions with Mesophilic Anaerobic Sludge as the Inoculum.

Sulfur reduction at hyperthermoacidophilic conditions represents a promising opportunity for metal sulfide precipitation from hot acidic metallurgical streams, avoiding costly cooling down. The suitability of mesophilic anaerobic sludges as the inoculum for sulfur-reducing bioreactors operated at high temperature and low pH was explored. We examined sludges from full-scale anaerobic reactors for sulfur-reducing activity at pH 2.0-3.5 and 70 or 80 °C, with H2 as an electron donor. At pH 3.5 in batch experiments, sulfidogenesis started within 4 days, reaching up to 100-200 mg·L-1 of dissolved sulfide produced after 19-24 days, depending on the origin of the sludge. Sulfidogenesis resumed after removing H2S by flushing with nitrogen gas, indicating that sulfide was limiting the conversion. The best performing sludge was used to inoculate a 4 L gas-lift reactor fed with H2 as the electron donor, CO2 as the carbon source, and elemental sulfur as the electron acceptor. The reactor was operated in semibatch mode at a pH 3.5 and 80 °C, and stable sulfide production rates of 60-80 mg·L-1·d-1 were achieved for a period of 24 days, without formation of methane or acetate. Our results reveal the potential of mesophilic anaerobic sludges as seed material for sulfur-reducing bioprocesses operated at hyperthermoacidophilic conditions. The process needs further optimization of the volumetric sulfide production rate to gain relevance for practice.

Concurrent use of methanol and ethanol for chain-elongating short chain fatty acids into caproate and isobutyrate.

Microbial chain elongation (MCE) is a bioprocess that could utilise a mixed-culture fermentation to valorise organic waste. MCE converting ethanol and short chain fatty acids (SCFA; derived from organic waste) to caproate has been studied extensively and implemented. Recent studies demonstrated the conversion of SCFAs and methanol or ethanol into isomerised fatty acids as novel products, which may expand the MCE application and market. Integrating caproate and isomerised fatty acid production in one reactor system is theoretically feasible given the employment of a mixed culture and may increase the economic competence of MCE; however, the feasibility of such has never been demonstrated. This study investigated the feasibility of using two electron donors, i.e. methanol and ethanol, for upgrading SCFAs into isobutyrate and caproate concurrently in MCE Results show that supplying methanol and ethanol in MCE simultaneously converted acetate and/or butyrate into caproate and isobutyrate, by a mixed-culture microbiome. The butyrate supplement stimulated the caproate production rate from 1.5 to 2.6 g/L.day and induced isobutyrate production (1.5 g/L.day). Further increasing ethanol feeding rate from 140 to 280 mmol carbon per litre per day enhanced the direct use of butyrate for caproate production, which improved the caproate production rate to 5.9 g/L.day. Overall, the integration of two electron donors, i.e. ethanol and methanol, in one chain-elongation reactor system for upgrading SCFAs was demonstrated. As such, MCE could be applied to valorise organic waste (water) streams into a wider variety of value-added biochemical.

Calcium Carbonate Packed Electrochemical Precipitation Column: New Concept of Phosphate Removal and Recovery.

Phosphorus (P) is a vital micronutrient element for all life forms. Typically, P can be extracted from phosphate rock. Unfortunately, the phosphate rock is a nonrenewable resource with a limited reserve on the earth. High levels of P discharged to water bodies lead to eutrophication. Therefore, P needs to be removed and is preferably recovered as an additional P source. A possible way to achieve this goal is by electrochemically induced phosphate precipitation with coexisting calcium ions. Here, we report a new concept of phosphate removal and recovery, namely a CaCO3 packed electrochemical precipitation column, which achieved improved removal efficiency, shortened hydraulic retention time, and substantially enhanced stability, compared with our previous electrochemical system. The concept is based on the introduction of CaCO3 particles, which facilitates calcium phosphate precipitation by buffering the formed H+ at the anode, releases Ca2+, acts as seeds, and establishes a high pH environment in the bulk solution in addition to that in the vicinity of the cathode. It was found that the applied current, the CaCO3 particle size, and the feed rate affect the removal of phosphate. Under optimized conditions (particle size, <0.5 mm; feed rate, 0.4 L/d; current, 5 mA), in a continuous flow system, the CaCO3 packed electrochemical precipitation column achieved 90 ± 5% removal of phosphate in 40 days and >50% removal over 125 days with little maintenance. The specific energy consumptions of this system lie between 29 and 61 kWh/kg P. The experimental results demonstrate the promising potential of the CaCO3 packed electrochemical precipitation column for P removal and recovery from P-containing streams.

Branched Medium Chain Fatty Acids: Iso-Caproate Formation from Iso-Butyrate Broadens the Product Spectrum for Microbial Chain Elongation.

Chain elongation fermentation can be used to convert organic residues into biobased chemicals. This research aimed to develop a bioprocess for branched medium chain fatty acids (MCFAs) production. A long-term continuous reactor experiment showed that iso-caproate (4-methyl pentanoate, i-C6) can be produced via ethanol based chain elongation. The enriched microbiome formed iso-caproate from iso-butyrate at a rate of 44 ± 6 mmol C L-1 day-1 during the last phase. This amounted to 20% of all formed compounds based on carbon atoms. The main fermentation product was n-caproate (55% of all carbon), as a result of acetate and subsequent n-butyrate elongation. The microbiome preferred straight-chain elongation over branched-chain elongation. Lowering the acetate concentration in the influent led to an increase of excessive ethanol oxidation (EEO) into electron equivalents (e.g., H2) and acetate. The formed acetate in turn stimulated straight chain elongation, but the resulting lower net acetate supply rate towards straight chain elongation led to an increased selectivity towards and productivity of i-C6. The electrons produced via oxidation routes and chain elongation were apparently utilized by hydrogenotrophic methanogens, homoacetogens, and carboxylate-to-alcohol reducing bacteria. Further improvements could be achieved if the acetate-producing EEO was minimized and limitations of ethanol and CO2 were prevented.

Bulk pH and Carbon Source Are Key Factors for Calcium Phosphate Granulation.

Recovery of calcium phosphate granules (CaP granules) from high-strength wastewater is an opportunity to reduce the natural phosphorus (P) scarcity, geographic imbalances of P reserves, and eutrophication. Formation of CaP granules was previously observed in an upflow anaerobic sludge bed (UASB) reactor treating source separated black water and is enhanced by Ca2+ addition. However, the required operating conditions and influent composition for CaP granulation are still unknown. In this study, we have experimentally demonstrated that the carbon source and bulk pH are crucial parameters for the formation and growth of CaP granules in a UASB reactor, operating at relatively low upflow velocity (<1 cm h-1). Degradation of glucose yielded sufficient biomass (microbial cells and extracellular biopolymers) to cover crystal and amorphous calcium phosphate [Ca x(PO4) y], forming CaP granules. Influent only containing volatile fatty acids as the carbon source did not generate CaP granules. Moreover, bulk pH between 7.0 and 7.5 was crucial for the enrichment of Ca x(PO4) y in the granules over bulk precipitation. Bulk pH 8 reduced the Ca x(PO4) y enrichment in granules of >1.4 mm diameter from 9 to 5 wt % P. Moreover, for bulk pH 7.5, co-precipitation of CaCO3 with Ca x(PO4) y was reduced.

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.

Recovery of calcium phosphate granules from black water using a hybrid upflow anaerobic sludge bed and gas-lift reactor.

Adding calcium during anaerobic digestion of vacuum collected black water (BW) in an up-flow anaerobic sludge bed (UASB) reactor increased the retention of total phosphorus (P) in the reactor from 51% to 87%. However, the insufficient mixing in the reactor caused cementation and relatively high content of organics in the recovered calcium phosphate (CaP) granules, limiting the P recovery. In this study, the UASB reactor was mixed with an internal gas-lift (UASB-GL) to prevent cementation and to enhance the P content in CaP granules. The novel UASB-GL reactor operated for 300 days, treating concentrated BW. At steady state, the removal of total COD and P was 92% and 90%, respectively. The gas injection created a sludge bed with an average total suspended solids concentration of 73 ±â€¯16 g/L at the bottom and 31 ±â€¯5 g/L at the top of the reactor. The concentration of solid P at the bottom of the reactor was 4.58 ±â€¯1.34 gP/L, while at the top a much lower concentration was obtained (0.75 ±â€¯0.32 gP/L). 89% of the CaP granules was found at the bottom of the reactor. The harvested CaP granules (>0.4 mm diameter) contained on average 7.8 ±â€¯0.6 wt% of P. A potential recovery of 57% of P in BW as CaP granules was calculated, considering actual application of the UASB-GL reactor in source separated sanitation.

Performance and Long Distance Data Acquisition via LoRa Technology of a Tubular Plant Microbial Fuel Cell Located in a Paddy Field in West Kalimantan, Indonesia.

A Plant Microbial Fuel Cell (Plant-MFCs) has been studied both in the lab and in a field. So far, field studies were limited to a more conventional Plant-MFC design, which submerges the anode in the soil and places the cathode above the soil surface. However, for a large scale application a tubular Plant-MFC is considered more practical since it needs no topsoil excavation. In this study, 1 m length tubular design Plant-MFC was installed in triplicate in a paddy field located in West Kalimantan, Indonesia. The Plant-MFC reactors were operated for four growing seasons. The rice paddy was grown in a standard cultivation process without any additional treatment due to the reactor instalation. An online data acquisition using LoRa technology was developed to investigate the performance of the tubular Plant-MFC over the final whole rice paddy growing season. Overall, the four crop seasons, the Plant-MFC installation did not show a complete detrimental negative effect on rice paddy growth. Based on continuous data analysis during the fourth crop season, a continuous electricity generation was achieved during a wet period in the crop season. Electricity generation dynamics were observed before, during and after the wet periods that were explained by paddy field management. A maximum daily average density from the triplicate Plant-MFCs reached 9.6 mW/m2 plant growth area. In one crop season, 9.5-15 Wh/m2 electricity can be continuously generated at an average of 0.4 ± 0.1 mW per meter tube. The Plant-MFC also shows a potential to be used as a bio sensor, e.g., rain event indicator, during a dry period between the crop seasons.

Analysis of adoption rates for Needs Driven versus Value Driven innovation water technologies.

This paper analyzes six case studies of new water technology innovations in the last three decades and investigates the differences in timelines for moving through the various stages of water technology commercialization. The concept of two different types of innovation was explored: Crisis/Needs Driven and Value Driven. It was found that the case studies that mapped to the Crisis/Needs Driven innovation moved relatively quickly compared to Value Driven innovations and in most cases involved new entrants. New entrants refer to new companies or start-ups that have recently entered the water technology market. The case studies, which could be mapped to Value Driven innovation, had a slower rate of technology diffusion, and they involved a combination of existing companies as well as new entrants. PRACTITIONER POINTS: The paper identifies two key types of innovation: Crisis/Needs Driven and Value Driven. Legislation was observed to be a key driver for the adoption of new technology innovation in the water sector. The Crisis/Needs driven innovations studied were observed to diffuse through the Water Technology Diffusion model at up to twice the pace of Value driven innovation. Crisis/Needs driven innovation typically involves disruptive innovation offered by new entrants, whereas with Value driven innovation, the solutions are provided by both existing companies as well as new entrants. It is also observed that in most cases a technology that is adopted in order to meet a crisis or need in the market is more expensive at the outset compared with incumbent solutions. While value driven adoption has a slower cycle for adoption, it presents a lower risk as it is less dependent on external factors and timing of implementation of regulations or the occurrence of some public health related or environmental crisis.

Is There a Precipitation Sequence in Municipal Wastewater Induced by Electrolysis?

Electrochemical wastewater treatment can induce calcium phosphate precipitation on the cathode surface. This provides a simple yet efficient way for extracting phosphorus from municipal wastewater without dosing chemicals. However, the precipitation of amorphous calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO3) and brucite (Mg(OH)2). To increase the content of ACP in the products, it is essential to understand the precipitation sequence of ACP, calcite, and brucite in electrochemical wastewater treatment. Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic solubility product and highest saturation index in the wastewater, it has the potential to precipitate first. However, this is not observed in electrochemical phosphate recovery from raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high bicarbonate concentration in the wastewater resulting in formation of calcite. In the case of decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in the wastewater was used for ACP formation instead of calcite. The formation of of brucite, however, was only affected when the current density was decreased or the size of cathode was changed. Overall, the removal of Ca and Mg is much more affected by current density than the surface area of cathode, whereas for P removal, the reverse is true. Because of these dependencies, though there is no definite precipitation sequence among ACP, calcite, and brucite, it is still possible to influence the precipitation degree of these species by relatively low current density and high surface area or by targeting phosphorus-rich wastewaters.

The Effect of Bioinduced Increased pH on the Enrichment of Calcium Phosphate in Granules during Anaerobic Treatment of Black Water.

Simultaneous recovery of calcium phosphate granules (CaP granules) and methane in anaerobic treatment of source separated black water (BW) has been previously demonstrated. The exact mechanism behind the accumulation of calcium phosphate (Ca x(PO4) y) in CaP granules during black water treatment was investigated in this study by examination of the interface between the outer anaerobic biofilm and the core of CaP granules. A key factor in this process is the pH profile in CaP granules, which increases from the edge (7.4) to the center (7.9). The pH increase enhances supersaturation for Ca x(PO4) y phases, creating internal conditions preferable for Ca x(PO4) y precipitation. The pH profile can be explained by measured bioconversion of acetate and H2, HCO3- and H+ into CH4 in the outer biofilm and eventual stripping of CO2 and CH4 (biogas) from the granule. Phosphorus content and Ca x(PO4) y crystal mass quantity in the granules positively correlated with the granule size, in the reactor without Ca2+ addition, indicating that the phosphorus rich core matures with the granule growth. Adding Ca2+ increased the overall phosphorus content in granules >0.4 mm diameter, but not in fine particles (<0.4 mm). Additionally, H+ released from aqueous phosphate species during Ca x(PO4) y crystallization were buffered by internal hydrogenotrophic methanogenesis and stripping of biogas from the granule. These insights into the formation and growth of CaP granules are important for process optimization, enabling simultaneous Ca x(PO4) y and CH4 recovery in a single reactor. Moreover, the biological induction of Ca x(PO4) y crystallization resulting from biological increase of pH is relevant for stimulation and control of (bio)crystallization and (bio)mineralization in real environmental conditions.

Controlling Ethanol Use in Chain Elongation by CO2 Loading Rate.

Chain elongation is an open-culture biotechnological process which converts volatile fatty acids (VFAs) into medium chain fatty acids (MCFAs) using ethanol and other reduced substrates. The objective of this study was to investigate the quantitative effect of CO2 loading rate on ethanol usages in a chain elongation process. We supplied different rates of CO2 to a continuously stirred anaerobic reactor, fed with ethanol and propionate. Ethanol was used to upgrade ethanol itself into caproate and to upgrade the supplied VFA (propionate) into heptanoate. A high CO2 loading rate (2.5 LCO2·L-1·d-1) stimulated excessive ethanol oxidation (EEO; up to 29%) which resulted in a high caproate production (10.8 g·L-1·d-1). A low CO2 loading rate (0.5 LCO2·L-1·d-1) reduced EEO (16%) and caproate production (2.9 g·L-1·d-1). Heptanoate production by VFA upgrading remained constant (∼1.8 g·L-1·d-1) at CO2 loading rates higher than or equal to 1 LCO2·L-1·d-1. CO2 was likely essential for growth of chain elongating microorganisms while it also stimulated syntrophic ethanol oxidation. A high CO2 loading rate must be selected to upgrade ethanol (e.g., from lignocellulosic bioethanol) into MCFAs whereas lower CO2 loading rates must be selected to upgrade VFAs (e.g., from acidified organic residues) into MCFAs while minimizing use of costly ethanol.

Development and Application of a Model to Study Water Technology Adoption.

This paper develops a set of criteria that can be used to study industry adoption and dissemination of water technologies through various stages of a market adoption model. It tests the applicability of these criteria on a diverse array of over 488 water technologies. Based on case studies, it seeks to define the typical and reasonable time frames in which a water technology moves through these defined stages of industry adoption and dissemination. The development of these criteria, and the definition of reasonable industry average timelines to move through these stages, is an important contribution to the understanding of the process of water technology development. The criteria and defined timelines described in this paper are foundational, and will be used as the basis for subsequent research and analysis to examine the success rates for different water technologies, and common factors linked to why some technologies succeed and others fail.

Magnetite synthesis from ferrous iron solution at pH 6.8 in a continuous stirred tank reactor.

Partial oxidation of defined Fe2+ solutions is a well-known method for magnetite synthesis in batch systems. The partial oxidation method could serve as basis for an iron removal process in drinking water production, yielding magnetite (Fe3O4) as a compact and valuable product. As a first step toward such a process, a series of experiments was carried out, in which magnetite was synthesized from an Fe2+ solution in a 2 L continuous stirred tank reactor (CSTR) at atmospheric pressure and 32 °C. In four experiments, elevating the pH from an initial value of 5.5 or 6.0 to a final value of 6.8, 7.0 or 7.5 caused green rust to form, eventually leading to magnetite. Formation of NH4+ in the reactor indicated that NO3- and subsequently NO2- served as the oxidant. However, mass flow analysis revealed an influx of O2 to the reactor. In a subsequent experiment, magnetite formation was achieved in the absence of added nitrate. In another experiment, seeding with magnetite particles led to additional magnetite precipitation without the need for a pH elevation step. Our results show, for the first time, that continuous magnetite formation from an Fe2+ solution is possible under mild conditions, without the need for extensive addition of chemicals.

Effect of low concentrations of dissolved oxygen on the activity of denitrifying methanotrophic bacteria.

Chemical energy can be recovered from municipal wastewater as biogas through anaerobic treatment. Effluent from direct anaerobic wastewater treatment at low temperatures, however, still contains ammonium and considerable amounts of dissolved methane. After nitritation, methane can be used as electron donor for denitrification by the anaerobic bacterium 'Candidatus Methylomirabilis oxyfera'. It was shown that in the presence of 0.7% O2, denitrifying methanotrophic activity slightly increased and returned to its original level after oxygen had been removed. At 1.1% O2, methane consumption rate increased 118%, nitrite consumption rate increased 58%. After removal of oxygen, methane consumption rate fully recovered, and nitrite consumption rate returned to 88%. Therefore, traces of oxygen that bacteria are likely to be exposed to in wastewater treatment are not expected to negatively affect the denitrifying methanotrophic process. 2.0% O2 inhibited denitrifying activity. Nitrite consumption rate decreased 60% and did not recover after removal of oxygen. No clear effect on methane consumption was observed. Further studies should evaluate if intermittent addition of oxygen results in increased growth rates of the slow-growing 'Candidatus Methylomirabilis oxyfera'.