Biomethane Quality Research Articles

Enhancing biomethane quality via the activation of natural zeolite from Lombok Island

<p>This study presents the results of the biogas purification test using the fixed-bed column method. As an adsorbent, natural zeolite from the Lombok island Indonesia was used. Zeolite is used for the purpose of purifying biogas produced from the anaerobic digestion (AD) process. To enhance the adsorption capacity, the zeolite underwent activation using distinct strong base and acid solutions—potassium hydroxide (KOH) and hydrochloric acid (HCl)—with concentrations of 10%, 15%, and 20%, individually. The experiment was carried out until the adsorbent reached saturation, at which point the methane (CH4) concentration began to decline. It was revealed that the typical Lombok zeolite showed good performance in increasing the concentration of CH4 in biogas. Furthermore, zeolites work better when activated with strong base solutions and at high concentrations. This is indicated by the achievement of higher CH4 concentrations and a longer saturation time. The results from analyzing zeolite features and pore characteristics indicate that acidic and base compounds yield distinct outcomes in terms of the surface structure of zeolite pores.</p>

Optimization of Thermodynamic Parameters of the Biological Hydrogen Methanation in a Trickle-Bed Reactor for the Conditioning of Biogas to Biomethane

The increased demand for resources and energy that is developing with rising global consumption represents a key challenge for our generation. Biogas production can contribute to sustainable energy production and closing nutrient cycles using organic residues or as part of a utilization cascade in the case of energy crops. Compared to hydrogen (H2), biogas with a high methane (CH4) content can be fed into the gas grid without restrictions. For this purpose, the CH4 content of the biogas must be increased from 52 to 60% after anaerobic digestion to more than 96%. In this study, biological hydrogen methanation (BHM) in trickling-bed reactors (TBR) is used to upgrade biogas. Design of experiments (DoE) is used to determine the optimal process parameters. The performance of the reactors is stable under all given conditions, reaching a “low” gas grid quality of over 90%. The highest CH4 content of 95.626 ± 0.563% is achieved at 55 °C and 4 bar, with a methane formation rate (MFR) of 5.111 ± 0.167 m3/(m3·d). The process performance is highly dependent on the H2:CO2 ratio in the educts, which should be as close as possible to the stochiometric ratio of 4. In conclusion, BHM is a viable approach to upgrade biogas to biomethane quality and can contribute to a sustainable energy grid.

A comprehensive review on current trends and development of biomethane production from food waste: Circular economy and techno economic analysis

Biogas production from food waste with anaerobic digestion process and co-digestion process obtained high purity of biogas. Food waste consists of proteins, carbohydrates, inorganic compounds, fibre, sugars, and lipids. Fossil fuels have caused some environmental and human health problems, such as effects of global warming, greenhouse gas emissions. Biogas production is a simple process and is purified into methane and used in many applications, heat, electricity and vehicle fuel. Different types of food waste are collected and processed into biogas, which is essential for human use and as an alternative to fossil fuels. The collection of food waste, pre-treatment process, production of biogas, purification of biogas, upgradation of biomethane, and circular bioeconomy are clearly explained. Anaerobic digestion and co-digestion processes have increased biomethane yield, up gradation and purification of biogas have also been discussed. Five different types of upgradation processes are included, such as physical scrubbing, chemical scrubbing, pressure swing adsorption, membrane separation, and cryogenic separation. The upgradation process and the purity level also mentioned in this review, it represents the biomethane quality. Techno economic effects and circular bioeconomy levels are addressed in this review. Life cycle assessment, life cycle impact analysis and environmental impact level are discussed. However, Challenges and future perspectives of biogas production and to improve the biomethane purity level. 89% of the biogas production is obtained from fruit waste with anaerobic digestion, after the upgradation process, 99% purity of biomethane is separated.

Biogas/Biomethane Quality and Requirements for Combined Heat and Power (CHP) Units/Gas Grids with a Special Focus on Siloxanes-a Short Review

The presence of siloxanes in biogas and biomethane is a major barrier to use them as renewable energy sources in Combined Heat and Power (CHP) units and national grids systems. Siloxanes in the shape of methyl siloxanes (incl. L2, L3, L4, D3, D4, D5, D6), Trimethylsilanol (TMSOH), as well as other contaminants such as H2S, NH3, relative Humidity (rH), halogenated compounds (including organic chlorine and fluorine), and Volatile Organic Compounds (VOCs) presented in biogas upgraded to biomethane quality are detrimental to engines, turbines and gas grids, therefore it is necessary to remove them before its high-value utilization. Under the oxidation, process siloxanes are converted into microcrystalline silicon dioxide (SiO2) deposits that can shorten the lifetime of the engine and affect the gas grids. The review presents the actual requirements of biogas and biomethane quality in context to their utilization in CHP units and national gas grids. Moreover, the methods of siloxanes removal based on adsorption, absorption, cryogenic condensation, membranes, and biofiltration are described.

Prospects of state support of the development of the biomethane industry in Ukraine until 2040

The paper considers the prospects for the state support for the development of biomethane industry in Ukraine from 2025 to 2040. The main financial incentives for the use of biomass-derived energy are a special tariff for heat from sources other than natural gas, and a feed-in tariff (the auction price in the future). In the EU, biomethane production is gaining ground due to available financial incentives (premiums to the cost of natural gas, and feed-in premiums). The main obstacle to the large-scale spread of biogas (and, accordingly, biomethane) is the high cost of equipment. The amounts of state support for biogas production with its purification to biomethane and supply of the latter to the gas transmission and gas distribution networks under the conditions of biomethane production in the amounts provided by the draft Roadmap for Bioenergy Development in Ukraine until 2050 were assessed. While maintaining the price of natural gas at 2021 prices (EUR 0.24/m3), the need to subsidize biomethane production from 2025 to 2040 can reach EUR 0.263-3.5 billion, on average EUR 16.5-217 million per year. Infrastructure expenditures were not taken into account in the assessment. The possibility of electricity output from biomethane was not considered, as biogas refining to the quality of biomethane requires additional funds. The statutory auction price may be sufficient only for certain types of feedstock and for large biogas plants. The use of biomethane may be appropriate in the transport sector, as biomethane is an "advanced biofuel", and Ukraine already has a relatively extensive network of methane filling stations. Biomethane production in Ukraine will require state support, particularly in the form of direct subsidies to biomethane producers (in the form of premium to the price of natural gas), and in the form of a premium to the auction price. The use of biomethane will partially reduce dependence on imported fossil fuels, being also an important element in the decarbonization of sectors using natural gas, replacing up to 0.76 billion m3 of the latter in 2040, which is in line with the global leading decarbonization trends.

Multi-step biogas quality improving by adsorptive packed column system as application to biomethane upgrading

This study investigates the effect of the specially designed adsorptive packed column system (APCS) in improving the quality of biogas to biomethane purity. The APCS installed with the specially constructed “dispersal star” was operated on biogas from wastewater treatment plant. The four adsorbent packed columns were arranged based on two impregnated and two no-impregnated commercially available active carbons to remove siloxanes, halogenated compounds and volatile organic compounds. The removal performance of ACPS was tested using portable devices along with gas chromatography techniques coupled with different types of detectors. The removal efficiency of total non-silica impurities was obtained at the level of 99.76%, whereas siloxanes were removed entirely. The adsorption affinity of siloxanes in the shape of D4 and D5 was considered by adsorption on high BET surface area adsorbent, while the desulphurisation process was conducted on impregnated active carbons. This study describes the concentrations of silica and non-silica impurities in relation to CHP manufacturers’ limits and biomethane quality. Furthermore, the economical estimation of full-scale APCS was also conducted taking into account the three variants of biogas purity, as well as different types of iron-based adsorbents.

ПЕРСПЕКТИВИ ДЕРЖАВНОЇ ПІДТРИМКИ РОЗВИТКУ ГАЛУЗІ БІОМЕТАНУ В УКРАЇНІ ДО 2040 РОКУ

The paper considers the prospects for the state support for the development of biomethane industry in Ukraine from 2025 to 2040. The main financial incentives for the use of biomass-derived energy are a special tariff for heat from sources other than natural gas, and a feed-in tariff (the auction price in the future). In the EU, biomethane production is gaining ground due to available financial incentives (premiums to the cost of natural gas, and premiums to feed-in tariff). The main obstacle to the large-scale spread of biogas (and, accordingly, biomethane) is the high cost of equipment. The amounts of state support for biogas production with its purification to biomethane and supply of the latter to the gas transmission and gas distribution networks under the conditions of biomethane production in the amounts provided by the draft Roadmap for Bioenergy Development in Ukraine until 2050 were assessed. While maintaining the price of natural gas at the level of prices of 2021 (EUR 0.24/m3), the need to subsidize biomethane production from 2025 to 2040 can reach EUR 0.263-3.5 billion, on average EUR 16.5-217 million per year. Infrastructure expenditures were not taken into account in the assessment. The possibility of electricity output from biomethane was not considered, as biogas refining to the quality of biomethane requires additional funds. The statutory auction price may be sufficient only for certain types of feedstock and for large biogas plants. The use of biomethane may be appropriate in the transport sector, as biomethane is an "advanced biofuel", and Ukraine already has a relatively extensive network of methane filling stations. Biomethane production in Ukraine will require state support, particularly in the form of direct subsidies to biomethane producers (in the form of premium to the price of natural gas), and in the form of a premium to the auction price. The use of biomethane will partially reduce dependence on imported fossil fuels, being also an important element in the decarbonization of sectors using natural gas, replacing up to 0.76 billion m3 of the latter in 2040, which is in line with the global leading decarbonization trends.

Influence of biogas supply regime on photosynthetic biogas upgrading performance in an enclosed algal-bacterial photobioreactor

Photosynthetic biogas upgrading has recently emerged as an alternative to physical/chemical technologies in order to decrease the energy demand and environmental impact of biomethane production. Despite enclosed photobioreactors (PBRs) provide higher photosynthetic efficiency and CO2 capture capacity than open ponds, the number of studies of PBRs devoted to biogas upgrading is scarce. The effect of biogas supply regime (12 h feeding during the light period vs 24 h feeding during the light and dark periods) on biomethane quality and biochemical biomass composition was evaluated in a 132 L Tubular-PBR interconnected to a 2.5 L biogas scrubbing column operated at high alkalinity. Process operation at a biogas flow rate of 14.4 L d−1 during the light period supported a biomethane composition of 95.9 ± 1.3% v/v of CH4, 2.5 ± 0.8% v/v of CO2, 1.4 ± 0.6% v/v of N2, 0.1 ± 0.1% v/v of O2, and negligible H2S concentrations. On the contrary, CH4 and CO2 concentrations of 88.8 ± 1.6% v/v and 9.3 ± 1.1% v/v, respectively, were recorded when biogas was continuously supplied (28.8 L d−1). The oxygenation capacity of the PBR during the light period was sufficient to oxidize NH4+ to NO3−, which was further used during the dark period to oxidize H2S and prevent microalgae inhibition. Biomass productivity and composition remained constant regardless of the biogas supply regime, with a slight increase in carbohydrate content from 23.6 ± 5.7% (12 h supply) to 30.9 ± 3.5% (continuous supply).

Innovative operational strategies in photosynthetic biogas upgrading in an outdoors pilot scale algal-bacterial photobioreactor

Three innovative operational strategies were successfully evaluated to improve the quality of biomethane in an outdoors pilot scale photobioreactor interconnected to an external absorption unit: i) the use of a greenhouse during winter conditions, ii) a direct CO2 stripping in the photobioreactor via air stripping during winter conditions and iii) the use of digestate as make-up water during summer conditions. CO2 concentrations in the biomethane ranged from 0.4% to 6.1% using the greenhouse, from 0.3% to 2.6% when air was injected in the photobioreactor and from 0.4% to 0.9% using digestate as make up water. H2S was completely removed under all strategies tested. On the other hand, CH4 concentrations in biomethane ranged from 89.5% to 98.2%, from 93.0% to 98.2% and from 96.3% to 97.9%, when implementing strategies i), ii) and iii), respectively. The greenhouse was capable of maintaining microalgae productivities of 7.5 g m−2 d−1 during continental weather conditions, while mechanical CO2 stripping increased the pH in order to support an effective CO2 and H2S removal. Finally, the high evaporation rates during summer conditions allowed maintaining high inorganic carbon concentrations in the cultivation broth using centrate, which provided a cost-effective biogas upgrading.

Optimization of photosynthetic biogas upgrading in closed photobioreactors combined with algal biomass production

Photosynthetic biogas upgrading in microalgae photobioreactors constitutes a sustainable and environmentally friendly technology for the simultaneous removal of CO2 and H2S from raw biogas. The potential of algal-bacterial symbiosis for biogas purification has been already studied and demonstrated in open photobioreactors. However, few studies have focused on the implementation of this process in closed photobioreactors, which offer higher photosynthetic efficiencies, enhanced biomass productivities, a limited N2 contamination and improved CO2 mass transfer. In this study, the influence of alkalinity (inorganic carbon concentrations from ∼200 to ∼1700 mg L–1), and nitrogen (N) deprivation strategies (24 or 48 h limitation) on both biomethane quality and biomass composition has been evaluated and optimized in a closed tubular photobioreactor (TPBR) interconnected to a mixing chamber and a biogas absorption column (total working volume of 132 L). The increase in the alkalinity of the cultivation broth mediated an increase in the upgrading performance of the TPBR, reaching CO2 removals of 96.3 ± 0.8 %, and a complete H2S abatement, resulting in an enhanced biomethane composition (CH4 = 97.2 ± 0.9 %). Moreover, process operation under N deprivation did not affect the quality of biomethane, which complied with standards required for injection into natural gas grids or use as vehicle fuel. N-deprivation promoted the continuous storage of intracellular carbohydrates (29.0 ± 4.4 % compared with 14.3 ± 2.1 % obtained under N excess). A maximum biomass productivity obtained was of 15.8 ± 4.3 g m–2 d–1 during N excess vs. 11.2 ± 3.3 g m–2 d–1 during N-deprivation at 26.8 d of hydraulic retention time. During the N feast-famine cycles, the cyanobacterial species Pseudanabaena sp. was dominant over green algae.

How Operational Parameters Affect Electromethanogenesis in a Bioelectrochemical Power-to-Gas Prototype

Bioelectrochemical power-to-gas represents a novel solution for electrical energy storage, currently under development. It allows storing renewable energy surplus in the form of methane (CH4), while treating wastewater, therefore bridging the electricity and natural gas (and wastewater) grids. The technology can be coupled with membrane contactors for carbon dioxide (CO2) capture, dissolving the CO2 in wastewater before feeding it to the bioelectrochemical system. This way, the integrated system can achieve simultaneous carbon capture and energy storage objectives, in the scenario of a wastewater treatment plant application. In this study, such technology was developed in a medium-scale prototype (32 L volume), which was operated for 400 days in different conditions of temperature, voltage and CO2 capture rate. The prototype achieved the highest CH4 production rate (147 ± 33 L m-3 d-1) at the lowest specific energy consumption (1,0 ± 0,3 kWh m-3 CH4) when operated at 25 oC and applying a voltage of 0,7 V, while capturing and converting 22 L m-3 d-1 of CO2. The produced biogas was nearer to biomethane quality (CH4 > 90% v/v) when CO2 was not injected in the wastewater. Traces of hydrogen (H2) in the biogas, detectable during the periods of closed electrical circuit operation, indicated that hydrogenotrophic methanogenesis was taking place at the cathode. On the other hand, a relevant CH4 production during the periods of open electrical circuit operation confirmed the presence of acetoclastic methanogenic microorganisms in the microbial community, which was dominated by the archaeal genus Methanothrix (Euryarcheota). Different operational taxonomic units belonging to the bacterial Synergistes phylum were found at the anode and the cathode, having a potential role in organic matter degradation and H2 production, respectively. In the panorama of methanation technologies currently available for power-to-gas, the performances of this bioelectrochemical prototype are not yet competitive, especially in terms of volumetric CH4 production rate and power density demand. However, the possibility to obtain a high-quality biogas (almost reaching biomethane quality standards) at a minimal energy consumption represents a potentially favourable business scenario for this technology.

Influence of the diffuser type and liquid-to-biogas ratio on biogas upgrading performance in an outdoor pilot scale high rate algal pond

Four different types of biogas diffusers (metallic of 2 µm, porous stone, and two ceramic membranes of 0.2 and 0.4 µm) were evaluated to improve the quality of biomethane in an outdoor pilot scale photobioreactor interconnected to an external biogas absorption unit. Each type of diffuser was tested independently using three different liquid to biogas (L/G) ratios (0.5, 1 and 2). No significant difference was recorded in the CH4 concentrations of biomethane (i.e. > 93.0%) working with the different types of diffusers at L/G ratios > 1. Only the metallic biogas diffuser supported CH4 concentrations higher than 94.0% at a L/G ratio of 0.5. The increase in L/G ratio induced the stripping of the dissolved N2 and O2 into the biogas, which compensated the decrease in CO2 concentration mediated by the higher pH value of the scrubbing solution. The ANOVA of the results here obtained confirmed that both the type of biogas diffuser and the L/G ratio significantly determined the quality of the upgraded biogas.

Strategies for N2 and O2 removal during biogas upgrading in a pilot algal-bacterial photobioreactor

Photosynthetic biogas upgrading in algal-bacterial photobioreactors interconnected to external absorption columns has been proven as a feasible, low cost and environmentally friendly technology, ensuring a high removal performance of the main biogas contaminants (i.e., CO2 and H2S). However, despite the promising results, the upgraded biogas is commonly contaminated with N2 and O2 desorbed from the cultivation broth, which limits biomethane applications. In this work, two innovative strategies to improve the quality of biomethane were evaluated: degasification of the cultivation broth by means of a polydimethylsiloxane (PDMS) gas-liquid membrane, and biogas scrubbing at high pressure. The hollow fiber PDMS membrane demonstrated a superior degassing performance at 100–300 mbar of vacuum pressure, exhibiting dissolved oxygen removals of 61.7 ± 7.8% in the cultivation broth and reducing N2 and O2 concentrations in the upgraded biomethane to ~0.8 and ~0.1%, respectively. On the other hand, biogas scrubbing at 2 atm (over atmospheric pressure) promoted CO2 and H2S mass transfer from the biogas to the recirculating broth, achieving removals of ~98% and 100%, respectively, and N2 and O2 concentrations in the biomethane of ~1.7 and ~0.1%, respectively. Both strategies provided a biomethane composition complying with most international standards for utilization as vehicle fuel or injection into natural gas grids.

Performance evaluation of a control strategy for photosynthetic biogas upgrading in a semi-industrial scale photobioreactor

The validation of a control strategy for biogas upgrading via light-driven CO2 consumption by microalgae and H2S oxidation by oxidizing bacteria using the oxygen photosynthetically generated was performed in a semi-industrial scale (9.6 m3) photobioreactor. The control system was able to support CO2 concentrations lower than 2% with O2 contents ≤ 1% regardless of the pH in the cultivation broth (ranging from 9.05 to 9.50). Moreover, the control system was efficient to cope with variations in biogas flowrate from 143 to 420 L h−1, resulting in a biomethane composition of CO2 < 2.4%, CH4 > 95.5%, O2 < 1% and no H2S. Despite the poor robustness of this technology against failures in biogas and liquid supply (CH4 concentration of 67.5 and 70.9% after 2 h of biogas or liquid stoppage, respectively), the control system was capable of restoring biomethane quality in less than 2 h when biogas or liquid supply was resumed.

Modeling of syngas biomethanation and catabolic route control in mesophilic and thermophilic mixed microbial consortia

The syngas biomethanation process is a promising bioconversion route due to its high versatility, as it could be applied as a stand-alone technology, coupled to gasification plants, and integrated in anaerobic digestion or bioelectrochemical conversion systems. The biomethanation of syngas typically takes place through a rather complex network of interspecies metabolic interactions, which may vary significantly depending on the operating conditions applied and the diversity of microbial groups present. Despite there are several benefits derived from using microbial consortia, these also present challenges associated with limited process control and low product selectivity. To address the latter, the syngas biomethanation process carried out by mesophilic and thermophilic microbial consortia was modelled with the ultimate goal of studying possible catabolic route control strategies through the modulation of key operating parameters. The results showed that the thermophilic microbial consortium presented much higher apparent specific methane productivity (18.8 mmol/g VSS/d) than the mesophilic (4.6 mmol/g VSS/d) at an initial PCO of 0.2 atm, and that the difference increased with increasing initial PCO. This difference in productivity was found to derive from the catabolic routes used rather than the kinetic parameters of each microbial consortium. Additionally, the thermodynamic considerations included in the models revealed the possibility of controlling the catabolic routes used by each consortium through the modulation of the mass transfer and PCO2. Our results strongly indicate that modulating the PCO2 is a promising operational strategy for boosting the product selectivity towards CH4, the productivity of the system and the biomethane quality simultaneously.

Development of a control strategy to cope with biogas flowrate variations during photosynthetic biogas upgrading

The design and evaluation of a control system for a photosynthetic biogas upgrading unit was successfully carried out in this study. This control system ensured a specific biomethane quality under any disturbance in the biogas flowrate. The recycling liquid flowrate, and indirectly the liquid to biogas (L/G) ratio, was selected as the manipulated variable in order to maintain the CO2 and O2 content of biomethane, and therefore comply with the requirements for its use as natural gas substitute (≤2.5% and ≤1.0%, respectively). The control system was able to maintain the biomethane CO2 content below the set point value under a stepwise increase in the biogas flowrate from 60 to 150 ml min−1, together with negligible H2S concentrations and an O2 stripping from the recycling liquid to the biomethane lower than 1%, thus obtaining a consistent biomethane quality over time. On the contrary, the biomethane CO2 content increased up to 13.2% under this stepwise increase in the biogas flowrate without control system. Successful results were also obtained when the control system was challenged with stepwise surges in the biogas flowrate between 60 and 120 ml min−1 under different temperatures (15 and 35 °C) and inorganic carbon concentrations (1500, 500 and 100 mg L−1) when the recycling liquid entering the absorption column presented a pH = 10. However, the high liquid flowrates required at a cultivation broth pH of 8.5 as a result of the low CO2 mass transfer led to an excessive O2 desorption to the biomethane, resulting in biomethane O2 contents >1%.

CO2/CH4 separation by hot potassium carbonate absorption for biogas upgrading

In biogas upgrading to biomethane, the release of CO2 off-gas into the atmosphere is generally regarded as a carbon-neutral emission, but a significant loss of CH4 can occur in this step: considering the global warming potential of this latter compound, methane slip can worsen or even nullify the CO2 savings associated to biomethane. This study investigates a novel approach for biogas upgrading to biomethane, aimed at reducing the methane loss. A plant based on hot potassium carbonate was fed with 150–200 Nm3 h−1 of biogas from municipal waste. CO2 is removed in a K2CO3 absorption column, with negligible CH4 absorption. An assessment of biomethane quality was performed to check its compliance with recent National and European standard specifications. Results show that a methane slip below 0.1% can be achieved with this technology, thus significantly reducing the greenhouse gas emissions associated to biomethane industry. This leads to a lower capital expenditure because no off-gas post-treatment is required. Heat and electricity consumption were monitored, and operational expense resulted to be lower than membrane separation in the specific case study, by applying life cycle cost (LCC) methodology.

Assessing the potential of purple phototrophic bacteria for the simultaneous treatment of piggery wastewater and upgrading of biogas

The potential of purple phototrophic bacteria (PPB) for the simultaneous treatment of piggery wastewater (PWW) and biogas upgrading was evaluated batchwise in gas-tight photobioreactors. PWW dilution was identified as a key parameter determining the efficiency of wastewater treatment and biomethane quality in PPB photobioreactors. Four times diluted PWW supported the most efficient total organic carbon (TOC) and total nitrogen removals (78% and 13%, respectively), with CH4 concentrations of 90.8%. The influence of phosphorous concentration (supplementation of 50 mg L−1 of P-PO43−) on PPB-based PWW treatment coupled to biogas upgrading was investigated. TOC removals of ≈60% and CH4 concentrations of ≈90.0% were obtained regardless of phosphorus supplementation. Finally, the use of PPB and algal-bacterial consortia supported CH4 concentrations in the upgraded biogas of 93.3% and 73.6%, respectively, which confirmed the potential PPB for biogas upgrading coupled to PWW treatment.

Determination of linear and cyclic volatile methyl siloxanes in biogas and biomethane by solid-phase microextraction and gas chromatography-mass spectrometry

A new method based on solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS) was developed for the analysis of seven linear (L2 – L5) and cyclic (D3 – D5) volatile methyl siloxanes (VMS) in biogas and biomethane, directly collected into Tedlar® bags (Tedlar SPME) from anaerobic digesters and wastewater treatment plants. The method was employed to monitor VMS content in biomethane produced by biogas upgrading with a pilot-plant membrane unit and provided adequate limits of quantification (< 0.05 mg m−3) to detect trace siloxane impurities. Tedlar SPME was validated against a standard procedure based on indirect sampling of gas streams with sorbent tubes followed by solvent extraction and GC-MS. Method precision (RSD) on total and individual VMS concentrations was lower than 10%, while RSD values of the standard procedure were higher than 20%. Tedlar SPME suitably revealed high VMS levels, expressed as total volatile silicon (> 1 mgSim−3), in wastewater biogas and provided a more efficient sampling of heavier VMS in comparison to the sorbent tubes method. At low values (< 0.1 mgSim−3) typical of wood waste biogas and biomethane, no statistically significant differences were observed between the two methods. Overall, Tedlar SPME simplified the analytical procedure by reducing the procedural steps, avoiding the use of solvents and demonstrated its applicability for testing the quality of biomethane as advanced biofuel.

Integral (VOCs, CO2, mercaptans and H2S) photosynthetic biogas upgrading using innovative biogas and digestate supply strategies

The performance of a pilot high rate algal pond (HRAP) interconnected with a biogas absorption column during the simultaneous upgrading of biogas and treatment of digestate was evaluated under two innovative biogas and nutrient supply strategies. Process operation with biogas supply during the night at a liquid recirculation/biogas ratio of 0.5 to prevent N2 and O2 stripping resulted in a biomethane complying with most international regulations for injection into natural gas grids (99.1 ± 1% CH4, 0.5 ± 0.2% CO2, 0.6 ± 0.5% N2 and 0.07 ± 0.08% O2). The potential of this technology to remove methyl mercaptan (MeSH), toluene and hexane from biogas (typically present at trace levels) was assessed, for the first time, with removal efficiencies under steady-state correlating with pollutant hydrophobicity (7 ± 7% for hexane, 66 ± 4% for MeSH and 98 ± 1% for toluene). Finally, the supply of digestate during the dark period shifted both microalgae population structure and biomass composition in the HRAP without a significant impact on biomethane quality. Overall, the removal of nitrogen and phosphorous from digestate in the HRAP was almost complete (96–99%) regardless of the nutrient supply strategy.