Environmental Injustices of Leaks from Urban Natural Gas Distribution Systems: Patterns among and within 13 U.S. Metro Areas.

Natural gas leaks in local distribution systems can develop as underground pipeline infrastructure degrades over time. These leaks lead to safety, economic, and climate change burdens on society. We develop an environmental justice analysis of natural gas leaks discovered using advanced leak detection in 13 U.S. metropolitan areas. We use Bayesian spatial regression models to study the relationship between the density of leak indications and sociodemographic indicators in census tracts. Across all metro areas combined, we found that leak densities increase with increasing percent people of color and with decreasing median household income. These patterns of infrastructure injustice also existed within most metro areas, even after accounting for housing age and the spatial structure of the data. Considering the injustices described here, we identify actions available to utilities, regulators, and advocacy groups that can be taken to improve the equity of local natural gas distribution systems.

Continuous production of high-concentrated ammonia broth through fermentation.

Being considered as a valuable resource and energy carrier, extensive research is going on to efficiently extract ammonia (NH3) from anaerobic digestate. However, due to the well-known NH3 inhibition on methanogens, the total NH3 nitrogen (TAN) concentration is typically limited to 1-4 g N/L in digestate, making the NH3 extraction process energy-consumptive. Here, NH3 fermentation, specifically targeting augmented NH3 production through biological reaction, was performed in a continuous mode. With the increase of gelatin input (10 to 150 g COD/L), NH3 concentration and volumetric productivity gradually increased, reaching 12.0 g TAN-N/L and 36.0 g NH3-N/L/d, which were the highest values ever reported. The stepwise increase in NH3 exposure prompted a shift in microbial dominance towards Hathewaya (from 1 % to 68 %), a critical factor for having high NH3 tolerance. Finally, NH3 stripping results suggested that highly concentrated broth could reduce the specific energy consumption for NH3 extraction to 1/3.

Effects of particle size on the pretreatment efficiency and subsequent biogas potential of polylactic acid.

The fragmentation of bioplastics (BPs) before pretreatment and anaerobic digestion is conducted for higher efficiency; however, based on the literature, the size reduction varies widely. In this study, initially, various combinations of thermal-alkaline pretreatments were applied at different strengths to the polylactic acid (PLA) in three groups (<0.5, 0.5 < size < 1.0, and 1.0 < size < 2.0 mm). After pretreatment, the solubilization of PLA was increased to 11.5-40.0 % using alkaline dosage and temperature ranging from 50 to 200 g OH-/kg BP, 60-100 °C, respectively, in a 1-10 h timeframe. The results were statistically proved using a 3D response surface graph, where the pretreatment was more effective for smaller particle sizes. The reduction in particle size also increased the CH4 production, which was more pronounced at the strong pretreatment (24 % increment vs. 10-15 %). Computed results indicated 44-86 % conversion of pretreated PLA particles to CH4, supported by Fourier transform infrared spectroscopy analysis, especially focusing on the intensity of -OH bands.

Use of reverse osmosis concentrate for mitigating greenhouse gas emissions from pig slurry.

Due to the high global warming potential (GWP) in a short time scale (GWP100 = 28 vs. GWP20 = 86), mitigating CH4 emissions could have an early impact on reducing current global warming effects. The manure storage tank emits a significant amount of CH4, which can diminish the environmental benefit resulting from the anaerobic digestion of manure that can generate renewable energy. In the present study, we added the reverse osmosis concentrate (ROC) rich in salt to the pig slurry (PS) storage tank to reduce CH4 emissions. Simultaneously, pure NaCl was tested at the same concentration to compare and verify the performance of ROC addition. During 40 days of storage, 1.83 kg CH4/ton PS was emitted, which was reduced by 7-75% by the addition of ROC at 1-9 g Na+/L. This decrease was found to be more intensive than that found upon adding pure sodium, which was caused by the presence of sulfate rich in ROC, resulting in synergistic inhibition. The results of the microbial community and activity test showed that sodium directly inhibited methanogenic activity rather than acidogenic activity. In the subsequent biogas production from the stored PS, more CH4 was obtained by ROC addition due to the preservation of organic matter during storage. Overall, 51.2 kg CO2 eq./ton PS was emitted during the storage, while 8 kg CO2 eq./ton PS was reduced by biogas production in the case of control, resulting in a total of 43.2 kg CO2 eq./ton PS. This amount of greenhouse gas emissions was reduced by ROC addition at 5 g Na+/L by 22 and 65 kg CO2 eq./ton PS, considering GWP100 and GWP20 of CH4, respectively, where most of the reduction was achieved during the storage process. To the best of our knowledge, this was the first report using salty waste to reduce GHG emissions in a proper place, e.g., a manure storage tank.

Enhanced anaerobic treatment of sulfate-rich wastewater by electrical voltage application.

Treatment of sulfate-rich wastewater with high methane recovery is a major concern due to sulfide inhibition. Here, an electrical voltage (EV) aims to enhance methanogenesis and sulfidogenesis to treat sulfate-rich wastewater. Two (control and EV-applied) reactors were operated with a gradual decrease in chemical oxygen demand (COD)/SO42- ratios (CSR). EV-applied reactor (EVR) demonstrated an increase of ∼30 % in methane production and ∼40 % in sulfate removal, compared to the control till CSR of 2.0. At CSR 1.0, the control failed, while EVR still exhibited a stable performance of 50 % COD-methane recovery. Microbial community results showed that the relative abundance of sulfate-reducing bacteria in EVR was 1.5 times higher than the control. Furthermore, higher relative abundance of dissimilatory sulfate reductase (>50 %) and Ni/Fe hydrogenase (x15) genes demonstrated an improved tolerance against H2S toxicity. This study highlights the importance of EV application by minimizing the byproduct inhibition in sulfate-rich wastewater.

Upflow anaerobic sludge blanket reactor operation under high pressure for energy-rich biogas production.

Autogenerative high-pressure digestion has an advantage of producing CH4-rich biogas directly from the reactor. However, its continuous operation has rarely been reported, and has never been attempted in an upflow anaerobic sludge blanket reactor (UASB). Here, UASB was continuously operated at 10 g COD/L/d with increasing pressure from 1 to 8 bar. As the pressure increased, the CH4 content in the biogas increased gradually, reaching 96.7 ± 0.8% at 8 bar (309 MJ/m3 biogas). The pH was dropped from 8.2 to 7.2 with pressure increase, but COD removal efficiency was maintained > 90%. The high pressure up to 8 bar did not adversely impact the physicochemical properties of granules, which was due to the increased production of extracellular polymeric substances (EPS), particularly, tightly bound EPS (34% increase). With pressure increase, there was no changes in the microbial community and ATPase gene expression, but 41% increase in carbonic anhydrase gene expression was observed.

Electron bifurcation reactions in dark fermentation: An overview for better understanding and improvement.

Electron bifurcation (EB) is the most recently found mode of energy conservation, which involves both exergonic and endergonic electron transfer reactions to minimize energy loss. Several works have been devoted on EB reactions (EBRs) in anaerobic digestion but limited in dark fermentative hydrogen production (DF). Two main electron carriers in DF are ferredoxin (Fd) and reduced nicotinamide adenine dinucleotide (NADH), complicatedly involved in EB. Here, i) the importance of EB involvement in DF, ii) all EBRs possible to present in DF, as well as iii) the limitation of previous studies that tried incorporating any of EBRs in DF metabolic model, were highlighted. In addition, the concept of using metagenomic analysis for estimating the share of each EB reaction in the metabolic model, was proposed. This review is expected to initiate a new wave for studying EB, as a tool for explaining and predicting DF products.

Electrical voltage application as a novel approach for facilitating methanogenic granulation.

Despite having high-rate methanogenic performance, up-flow anaerobic sludge blanket reactor still has challenges regarding long-start up period (3-8 months) for granulation. In this study, "electrical voltage (EV, 0.3 V) application" was attempted for facilitating granulation in the continuous operation with increased organic loading rates (0.5-11.0 kg COD/m3/d). Up to 11.0 kg COD/m3/d, EV-reactor exhibited the stable performance, while the control failed. After 49 days of operation (at 7 kg COD/m3/d), the granules collected from EV-reactor had larger diameter (2.3 vs 1.6 mm), higher settling velocity (2.6 vs 1.9 cm/s), and higher hydrophobicity (52.1 % vs 34.5 %), compared to the control. EV application also increased the specific methanogenic activity for propionate and hydrogen almost by two times. The relative abundance of Pseudomonas sp. (quorum sensing (QS)-related microbe) in EV-reactor was 17 % higher than that in the control. In addition, EV application increased the expression of QS genes significantly by 27 times.

Automatic temperature rise in the manure storage tank increases methane emissions: Worth to cool down!

A significant amount of CH4 is emitting from livestock manure (LM) storage tank, which is being counted according to the guidelines provided by the Intergovernmental Panel on Climate Change (IPCC). Among various parameters affecting CH4 conversion factor (MCF) of LM, temperature is known as the most influential factor. As a degree of temperature, atmospheric temperature (Ta), not the manure temperature (Tm), is used for determining the MCF. Currently, the closed-type tank is more common than open-type tank, which would cause the substantial difference between Ta and Tm, probably due to the automatic temperature rise (ATR). Here, we repeatedly observed the ATR by storing pig slurry (PS) in a pilot-scale tank (30 m3, surface/volume ratio of 1.9), and its consequent impact on the increased CH4 emissions by comparing with the results from a lab-scale tank (1 L, surface/volume ratio of 72.2) controlled at 30 °C. As storage began, the Tm increased gradually from 16 to 23 °C to above 30 °C even in winter (-5 °C < Ta < 15 °C). During 30 d of storage, the CH4 emissions of 1.3-2.5 kg CH4/ton PS (MCF 26-29%) was observed in the lab-scale tank, while the emissions was increased to 2.6-4.2 kg CH4/ton PS (MCF 40-50%) in the pilot-scale tank (Two-Tail test, |tt|<|tc|). For the first time, a detailed heat energy balance considering the waste heat from organic degradation, the heat requirement for warm up, and the heat loss by convection, was conducted, proving that the waste heat generated during storage was enough to reach above 30 °C. Cooling-down of LM at 20 °C was found to be effective for reducing CH4 emissions by 90%, which sufficiently offset the greenhouse gas emissions in power consumption for cooling. Our findings strongly suggest that more CH4 is emitting from LM storage tank than expected, and therefore, the IPCC needs to develop guidelines more accurately in determining MCF.

Use of citric acid for reducing CH4 and H2S emissions during storage of pig slurry and increasing biogas production: Lab- and pilot-scale test, and assessment.

The use of sulfuric acid (SA) for reducing greenhouse gases (GHGs, mainly CH4) emissions in manure management encounters with problems related with safety issue and increased H2S emissions. In the present study, citric acid (CA) as an alternative to SA was assessed in the lab-scale experiment at various dosages (pH 5.0-7.0), and then confirmed in the pilot-scale tank (effective volume of 30 ton). During 35 d of pig slurry (PS) storage at 30 °C, it was found that the CA addition to initial pH down to 6.5 could lead negligible reduction, while 85-99% and 48-72% reduction of CH4 and H2S emissions were achieved at pH ≤ 6.0, respectively. The similar reduction performance was confirmed (control vs. pH 6.0) in the pilot-scale test, but, interestingly, two times higher CH4 emissions of 123.7 kg CO2 eq./ton PS was detected caused by the automatic temperature increase (≥35 °C). The pH of acidified PS did not exceed 6.5 during the whole storage period, while it was maintained 7.3-7.7 in the control. A continuous AD reactor fed with acidified PS exhibited a higher CH4 yield of 10.0 m3 CH4/ton PS, compared to the control (5.7 m3 CH4/ton PS), due to the preservation of organic matters and added CA. In overall, about 8.5 [(4.4, storage) + (4.1, biogas)] kg of CH4/ton PS was generated from raw PS and it was reduced to 7.8 [(0.7, storage) + (7.1, biogas)] kg of CH4/ton PS by CA-acidification. Despite the carbon footprint for manufacturing CA, it was calculated that GHG reduction of 107 kg CO2 eq./ton PS could be attained by CA-acidification. In terms of economic profit, it was estimated that 6.3 USD/ton PS can be gained by CA-acidification, while it was 2.4 USD/ton PS in case of control.

Production of high-calorific biogas from food waste by integrating two approaches: Autogenerative high-pressure and hydrogen injection.

Auto-generative high pressure digestion (AHPD) and hydrogen-injecting digestion (HID) have been introduced to directly produce high CH4-content biogas from anaerobic digester. However, each approach has its own technical difficulties (pH changes), and practical issues (high cost of H2) to obtain > 90% CH4 containing biogas, particularly, from the high-strength waste like food waste (FW). To overcome this problem, in this study, AHPD and HID were integrated, which can offset each drawback but maximize its benefit. Substrate concentration of FW tested here was 200 g COD/L, the highest ever applied in AHPD and HID studies. At first, the reactor was operated by elevating the autogenerative pressure from 1 to 3, 5, and 7 bar without H2 injection. With the pressure increase, the CH4 content in the biogas gradually increased from 52.4% at 1 bar to 77.4% at 7 bar. However, a drop of CH4 production yield (MPY) was observed at 7 bar, due to the pH drop down to 6.7 by excess CO2 dissolution. At further operation, H2 injection began at 5 bar, with increasing its amount. The injection was effective to increase the CH4 content to 82.8%, 87.2%, and 90.6% at 0.09, 0.13, and 0.18 L H2/g CODFW.fed of H2 injection amount, respectively. At 0.25 L H2/g CODFW.fed, there was a further increase of CH4 content to 92.1%, but the MPY was dropped with pH increase to 8.7 with residual H2 being detected (4% in the biogas). Microbial community analysis showed the increased abundance of piezo-tolerant microbe with pressure increase, and direct interspecies electron transfer contributors after H2 injection. In conclusion, the integration of two approaches enabled to directly produce high calorific biogas (90% > CH4, 180 MJ/m3 biogas) from high-strength FW at the lowest requirement of H2 (0.18 L H2/g CODFW.fed) ever reported.

Effects of storage temperature on CH4 emissions from cattle manure and subsequent biogas production potential.

CH4 is one of the main greenhouse gases (GHGs) generated from agricultural sector, and a significant amount of it is emitted during the storage of livestock manure. To mitigate the CH4 emissions, strong acid addition to the manure was attempted, which is only applicable to slurry-type manure. On the other hand, lowering the storage temperature could be an effective method to reduce the CH4 emissions, particularly applicable to solid-type manure. In this study, cattle manure (CM) with a high-solid content (TS > 30%) was stored at different temperatures (15-35 °C) for 80 d. The highest CH4 emissions of 375.1 kg CO2 eq./ton VS was observed at 35 °C, and this was reduced to less than half at ≤20 °C. Like the difference in CH4 emissions, the degradation of organic matter showed a similar trend. The maximum VS reduction of 29% was observed at 35 °C, while only 8% reduction was observed at 15 °C. Results from microbial community analyses and specific methanogenic activity tests indicated that hydrogenotrophic methanogens were the dominant indigenous CH4-producers, and the abundance of psychrophilic methanogens increased with decreasing temperature. The conservation of organic matter at low temperature led to an increase in biogas production potential from 25 to 43 L CH4/kg CM. It was calculated that the GHGs emissions from electricity consumption for cooling CM below 25 °C can be offset by mitigating CH4 emissions during storage but increasing in subsequent biogas production potential of CM. Compared at 35 °C, 91.6 kg CO2 eq./ton CM of GHGs reduction can be attained at 15 °C.

Combination of H2SO4-acidification and temperature-decrease for eco-friendly storage of pig slurry.

Owing to the economic benefit and efficiency, H2SO4-acidification is often applied for reducing CH4 emissions during storage of pig slurry (PS). However, it encounters with several problems related with safety and the concomitant H2S emissions. To reduce the required amount of H2SO4, in this study, the storage at low temperature (20-35 °C) was applied to the mild-acidified PS (pH 6.5 and 7.0). 55.1 kg CO2 eq./ton PS of CH4 was emitted from the control (non-acidified at 35 °C), which was reduced to 14.4-40.2 kg CO2 eq./ton PS at 20-30 °C. Temperature-decrease led to the increase of the abundance of methanogens (Methanobrevibacter and Methanolobus) that can grow at low temperature and the drop of specific methanogenic activity value. To achieve 70 % CH4 reduction, 1.6 kg H2SO4/ton PS was needed in PS acidification, which was decreased to 0.5 kg H2SO4/ton PS by decreasing temperature from 35 °C to 25 °C. CH4 production potential of the PS stored at 35 °C-pH 6.5 and 25 °C-pH 7.0 was increased by 21-33 % compared to the control. The GHG reduction of 33.6-41.9 kg CO2 eq./ton PS and the profit of 6.6 USD/ton PS could be attained by applying acidification or combined storage, indicating that the temperature-decrease can be effectively combined with H2SO4-acidification.

Enhanced Anaerobic Digestion of Long Chain Fatty Acid by Adding Magnetite and Carbon Nanotubes.

This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe3O4, 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10 - 4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe3O4 and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 ± 0.5 (control), 12.5 ± 0.8 (nFe3O4-added), and 15.7 ± 1.1 µS/cm (CNT-supplemented). Dissolved iron concentration, released from nFe3O4, seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe3O4 and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe3O4 and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.

Effects of pig slurry acidification on methane emissions during storage and subsequent biogas production.

In addition to undesirable odorous gases, substantial amounts of greenhouse gases (GHG), particularly methane (CH4), are generated during the storage of livestock manure. To reduce the CH4 emissions, first, pig slurry (PS) was stored for 40 d at 30 °C after adjusting the pH at 5.0-7.0 using H2SO4 solution. In the control (non-acidified PS), 3.7 kg CO2 eq./ton PS of CH4 emissions was detected, which was reduced to 1.8, 0.9, 0.4, 0.2, and 0.1 kg CO2 eq./ton PS at pH 7.0, 6.5, 6.0, 5.5, and 5.0, respectively. Methanosarcina was found to be the dominant genus (67% of the total archaeal sequence) in the control, whose dominance was reduced as storage pH decreased. The results of ribonucleic acid analysis and specific methanogenic activity test further confirmed the inhibition of indigenous methanogens by acidification. Later, the biochemical CH4 potential of stored PS was tested. Compared to the control (10.6 L CH4/L PS), the acidified PS showed higher CH4 yields of 12.7-14.6 L CH4/L PS, presumably by keeping degradable organic matters in PS under acidic condition. Among different acidification pHs tested, the maximum amount of GHG reduction was achieved at pH 6.0 by reducing CH4 emission to +0.4 kg CO2 eq./ton PS during storage while increasing biogas production potential equivalent to 48.3 kWh/ton PS (-22.5 kg CO2 eq./ton PS), resulting in a further reduction of (-)9.6 kg CO2 eq./ton PS compared to the control.

Biohydrogen production from food waste: Current status, limitations, and future perspectives.

Among the various biological routes for H2 production, dark fermentation is considered the most practically applicable owing to its capability to degrade organic wastes and high H2 production rate. Food waste (FW) has high carbohydrate content and easily hydrolysable in nature, exhibiting higher H2 production potential than that of other organic wastes. In this review article, first, the current status of H2 production from FW by dark fermentation and the strategies applied for enhanced performance are briefly summarized. Then, the technical and economic limitations of dark fermentation of FW are thoroughly discussed. Economic assessment revealed that the economic feasibility of H2 production from FW by dark fermentation is questionable. Current efforts to further increase H2 yield and waste removal efficiency are also introduced. Finally, future perspectives along with possible routes converting dark fermentation effluent to valuable fuels and chemicals are discussed.