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

Responses of CH4, N2O, and NH3 emissions to different slurry pH values of 5.5-10.0: Characteristics and mechanisms

Animal slurry storage is a significant source of greenhouse gas (GHG) and ammonia (NH3) emissions. pH is a basic but key factor that could pose great influence on gas emissions, but the simultaneous evaluation of its influence on GHG and NH3 emissions and the understanding of its underlying mechanism are not enough. In this work, pH was adjusted between 5.5 and 10.0 by a step of 0.5 unit by adding lactic acid and sodium hydroxide (NaOH) properly and frequently to the stored slurry during a 43-day storage period. The cumulative NH3 emissions were linearly correlated with the slurry pH, with R2 being 0.982. Maintaining the slurry pH at 5.5-6.0 could reduce NH3 emissions by 69.4%-85.1% compared with the non-treated group (CK). The pH ranges for maximum methane (CH4) and nitrous oxide (N2O) emissions were 7.5-8.5 and 6.5-8.5, respectively, and the slurry under pH 7.5-8.5 showed the highest GHG emissions. Acidification to pH 5.5 helped reduce the CH4, N2O, and total GHG emissions by 98.0%, 29.3%, and 81.7%, respectively; while alkalinization to pH 10.0 helped achieve the mitigation effects of 74.1%, 24.9%, and 30.6%, respectively. The Pearson's correlation factor between CH4 and the gene copy of mcrA under different pH values was 0.744 (p < 0.05). Meanwhile, the correlation factors between N2O and the gene copies of amoA, narG, and nirS were 0.644 (p < 0.05), 0.719 (p < 0.05), and 0.576 (p = 0.081), respectively. The gene copies of mcrA, amoA, narG, and nirS were maintained at the lowest level under pH 5.5. These results recommended keeping slurry pH lower than 5.5 with lactic acid can help control GHG and NH3 emissions simultaneously and effectively.

Bioelectrochemical system for enhancing anaerobic digestion of pharmaceutical-containing domestic wastewater

The unprecedented recent expansion in usage of paracetamol (AAP) has increased the need for suitable wastewater treatment technology. Furthermore, direct interspecies electron transfer promotion (DIET) offers simple and efficient approach for enhancing anaerobic digestion (AD). In this work, using AAP-containing domestic wastewater as feed, control AD reactor (RC) was operated, besides three DIET-promoted AD reactors (REV, RMC and REVMC, referring to electrical voltage "EV"-applied, nFe3O4-multiwall carbon nanotube (MCNT)-supplemented, and "EV applied + MCNT supplemented" reactor, respectively). Maximal treatable organic loading rates by RC, REV, RMC and REVMC were 3.9, 3.9, 7.8 and 15.6 g COD/L/d, corresponding to AAP loading rate of 26, 78, 156 and 312 μg/L/d, respectively. Methane production rate generated by RC, REV, RMC and REVMC reached 0.80 ± 0.01, 0.86 ± 0.04, 1.40 ± 0.07, and 3.01 ± 0.17 L/L/d, respectively. AAP expectedly followed hydroquinone degradation pathway, causing AD failure by acetate accumulation. However, this performance deterioration could be mitigated by DIET-promoted microbes with higher methanogenic activity and advanced electric conductivity. Economic evaluation revealed the favourability of MCNT addition over EV application, since payback periods for RC, REV, RMC and REVMC were 6.2, 7.7, 4.2 and 5.0 yr, respectively.

Frequent export of pig slurry for outside storage reduced methane but not ammonia emissions in cold and warm seasons

Manure management is a significant source of methane (CH4) and ammonia (NH3), and there is an urgent need for strategies to reduce these emissions. More frequent export of manure for outside storage can lower gaseous emissions from housing facilities, but the longer residence time may then increase emissions during outside storage. This study examined CH4 and NH3 emissions from liquid pig manure (pig slurry) removed from the in-house slurry collection pits at three different frequencies, i.e., three times per week (T2.3), once per week (T7), or once after 40 days (T40, reference). The slurry from treatments T2.3 and T7 was transferred for outside storage weekly over four weeks, and slurry from treatment T40 once after 40 days, in connection with summer and winter production cycles with growing-finishing pigs. The slurry was stored in pilot-scale storage tanks with solid cover and continuous ventilation. Compared to T40, the treatments T2.3 and T7 increased CH4 emissions during outside storage, but in-house emissions were reduced even more, and the net effects on total CH4 emissions from manure management (housing unit and outside storage) were reductions of 18-41% in summer and 53-83% in winter. The frequent slurry export for outside storage led to more NH3 emissions, except for the treatment T2.3, which has slurry funnel inserts beneath the slatted floor. Measurements of in-vitro CH4 production rates suggested that shorter residence time for slurry in pig houses delayed the development of active methanogenic populations, and that this contributed to the reduction of CH4 emissions.

Uncertainty in non-CO2 greenhouse gas mitigation contributes to ambiguity in global climate policy feasibility

Despite its projected crucial role in stringent, future global climate policy, non-CO2 greenhouse gas (NCGG) mitigation remains a large uncertain factor in climate research. A revision of the estimated mitigation potential has implications for the feasibility of global climate policy to reach the Paris Agreement climate goals. Here, we provide a systematic bottom-up estimate of the total uncertainty in NCGG mitigation, by developing 'optimistic', 'default' and 'pessimistic' long-term NCGG marginal abatement cost (MAC) curves, based on a comprehensive literature review of mitigation options. The global 1.5-degree climate target is found to be out of reach under pessimistic MAC assumptions, as is the 2-degree target under high emission assumptions. In a 2-degree scenario, MAC uncertainty translates into a large projected range in relative NCGG reduction (40-58%), carbon budget (±120 Gt CO2) and policy costs (±16%). Partly, the MAC uncertainty signifies a gap that could be bridged by human efforts, but largely it indicates uncertainty in technical limitations.

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.

Developing a biogas centralised circular bioeconomy using agricultural residues - Challenges and opportunities

Anaerobic digestion (AD) can be used as a stand-alone process or integrated as part of a larger biorefining process to produce biofuels, biochemicals and fertiliser, and has the potential to play a central role in the emerging circular bioeconomy (CBE). Agricultural residues, such as animal slurry, straw, and grass silage, represent an important resource and have a huge potential to boost biogas and methane yields. Under the CBE concept, there is a need to assess the long-term impact and investigate the potential accumulation of specific unwanted substances. Thus, a comprehensive literature review to summarise the benefits and environmental impacts of using agricultural residues for AD is needed. This review analyses the benefits and potential adverse effects related to developing biogas-centred CBE. The identified potential risks/challenges for developing biogas CBE include GHG emission, nutrient management, pollutants, etc. In general, the environmental risks are highly dependent on the input feedstocks and resulting digestate. Integrated treatment processes should be developed as these could both minimise risks and improve the economic perspective.

Acidification of slurry to reduce ammonia and methane emissions: Deployment of a retrofittable system in fattening pig barns

Livestock farming, and in particular slurry management, is a major contributor to ammonia (NH₃) and methane (CH₄) emissions in Europe. Furthermore, reduced NH₃ and CH₄ emissions are also relevant in licensing procedures and the management of livestock buildings. Therefore, the aim is to keep emissions from the barn as low as possible. Acidification of slurry in the barn can reduce these environmental and climate-relevant emissions by a pH value of 5.5. In this study, an acidification technology was retrofitted in an existing fattening pig barn equipped with a partially slatted floor. The slurry in a compartment with 32 animals was acidified. An identical compartment was used for reference investigations (case-control approach). Several times a week slurry was pumped for acidification in a process tank outside the barn compartment in a central corridor, where sulphuric acid (H₂SO₄) was added. Then the slurry was pumped back into the barn. In contrast to other systems, where acidified slurry was stored mainly in external storage tanks, in this study the slurry was completely stored in the slurry channels under the slatted floor, during the entire fattening period. The emission mass flow of NH₃ and CH₄ was measured continuously over three fattening periods, with one period in spring and two periods in summer. On average 17.1 kg H₂SO₄ (96%) (m³ slurry)^-1 were used for acidification during the three fattening periods. NH₃ and CH₄ emissions were reduced by 39 and 67%, respectively. The hydrogen sulphide (H₂S) concentration in the barn air of the acidification compartment was harmlessly low (0.02 ppm). Thus, despite the storage of the acidified slurry in the barn, the system leads to a lower concentration of detrimental gases, which is beneficial for the animals' as well as for the workers' health. The study shows that it is possible to retrofit acidification technology into existing pig barns. Further investigations shall identify possible measures to reduce the amount of H₂SO₄ used and thus minimise the sulphur input into the slurry.

Highly efficient reduction of ammonia emissions from livestock waste by the synergy of novel manure acidification and inhibition of ureolytic bacteria

The global livestock system is one of the largest sources of ammonia emissions and there is an urgent need for ammonia mitigation. Here, we designed and constructed a novel strategy to abate ammonia emissions via livestock manure acidification based on a synthetic lactic acid bacteria community (LAB SynCom). The LAB SynCom possessed a wide carbon source spectrum and pH profile, high adaptability to the manure environment, and a high capability of generating lactic acid. The mitigation strategy was optimized based on the test and performance by adjusting the LAB SynCom inoculation ratio and the adding frequency of carbon source, which contributed to a total ammonia reduction efficiency of 95.5 %. Furthermore, 16S rDNA amplicon sequencing analysis revealed that the LAB SynCom treatment reshaped the manure microbial community structure. Importantly, 22 manure ureolytic microbial genera and urea hydrolysis were notably inhibited by the LAB SynCom treatment during the treatment process. These findings provide new insight into manure acidification that the conversion from ammonia to ammonium ions and the inhibition of ureolytic bacteria exerted a synergistic effect on ammonia mitigation. This work systematically developed a novel strategy to mitigate ammonia emissions from livestock waste, which is a crucial step forward from traditional manure acidification to novel and environmental-friendly acidification.

Effects of different treatments of manure on mitigating methane emissions during storage and preserving the methane potential for anaerobic digestion

Current agricultural practices in regards to storage of manure come with a significant GHG contribution, due, to a big extent, to CH₄ emissions. For example, in Denmark, the agricultural sector is responsible for about 11.1 metric tons of CO₂ equivalents; only about 0.2 metric tons come directly from CO₂, while 6.0 tons come from CH₄. The present study aims at evaluating and comparing two methods based on their effect on suppressing CH₄ emissions during storage as well as on preserving and enhancing CH₄ yield in a subsequent anaerobic digestion step: the commonly applied acidification with H₂SO₄ as acidifying agent and thermal treatment at the mild temperatures of 70 and 90 °C (pasteurization). Although both treatments effectively suppressed CH₄ emissions during storage, they exhibited a significant difference in preserving and/or enhancing the CH₄ potential of manure. Specifically, thermal treatment resulted in 16-35% enhancement of CH₄ potential, while acidification resulted in decreasing the CH₄ yield by 6-23% compared to non-treated manure. Further investigation showed that storage itself positively affected the CH₄ potential of treated manure in a subsequent anaerobic digestion step; this was attributed to microbial activity other than biomethanation during storage. In overall and based on the results obtained regarding suppression of CH₄ emissions during storage as well as CH₄ potential enhancement, pasteurization at the temperatures tested is a promising alternative to the broadly applied acidification of manure.

Impact of slurry removal frequency on CH4 emission and subsequent biogas production; a one-year case study

Anaerobic digestion of animal slurry to produce biogas is the dominated treatment approach and a storage period is normally applied prior to digestion. Pre-storage, however, contributes to CH₄ emissions and results in loss of biogas potential. Manure management was found to be an efficient approach to reduce not only the on-site CH₄ emission but may also have extended influence on CH₄ emission/losses for storage and subsequent biogas process, while the connection remains unclear. The objective of this study was therefore to evaluate the impact of slurry management (e.g. removal frequency) on CH₄ emission (both on-site and storage process prior to biogas) and biogas yield. An experimental pig house for growing-finishing pigs (30-110 kg) and the relevant CH₄ emission was monitored for one year. In addition, the specific CH₄ activity (SMA) test was conducted and used as an alternative indicator to reflect the impact. Results showed that the manure management affected both on-site and subsequent methane emission; with increased manure removal frequencies, the methane emission became less dependent on variation of temperatures and the specific methanogenesis activity was significantly lower. The highest SMA (100 mL CH₄ gVS^-1), for instance, was observed from the slurries with limited emptied times, which was 10 times of that from the slurries being emptied three times a week. These findings could enlighten the development of environmentally friendly strategies for animal slurry management and biogas production.

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 (T_a), not the manure temperature (T_m), 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 T_a and T_m, 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 T_m increased gradually from 16 to 23 °C to above 30 °C even in winter (-5 °C < T_a < 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, |t_t|<|t_c|). 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.

Techno-economic analysis of livestock urine and manure as a microalgal growth medium

Microalgae have the potential to utilize the nutrients in livestock urine and manure (LUM) for the production of useful biomass, which can be used as a source of bioindustry. This study aims to evaluate the economic benefits of LUM feedstock that have not been clearly discussed before. Two types of photobioreactors were designed with a capacity of 200 m³ d^-1. Using the experimental data, the economic feasibility of the suggested processes was evaluated via techno-economic analysis. Itemized cost estimation indicated that the submerged membrane photobioreactor has a lower unit production cost (5.4 $ to 5.1 $ kg^-1) than the conventional photobioreactor system (14.6 $ to 13.8 $ kg^-1). In addition, LUM-based growth is another good option for reducing the unit production cost of biomass. The revenues from lowering the cost of LUM treatment significantly contribute to enhancing the economic profitability, where the break-even prices were 1.18 $ m^-3 (photobioreactor) and 0.98 $ m^-3 (submerged membrane photobioreactor). Finally, this study provides several emerging suggestions to reduce microalgal biomass production costs.

Dairy manure acidification reduces CH4 emissions over short and long-term

Acidification with sulphuric acid and cleaning residual manure in tanks are promising practices for reducing methane (CH₄), which is a potent greenhouse gas. To date, no data are available on CH₄ reductions from acidifying only residual manure (rather than all manure). Moreover, long-term effects of manure acidification (i.e. inoculating ability of previously acidified residual manure in the subsequent storages) are not known. To address these gaps, fresh manure (FM; 150 mL) combined with treated or untreated inoculum (30 mL) were anaerobically incubated at 17°C, 20°C, and 23°C for 116 d. Acidified treatments, regardless of location of acid addition, reduced CH₄ production by 81% at 17°C, 78% at 20°C, and 19% at 23°C compared to the control (untreated FM and untreated inoculum). To test long-term acidification effects, FM was inoculated with manure that had been acidified 6-months prior. This created comparable CH₄ production to FM with no inoculum and reduced CH₄ production by 99% at 17°C and 20°C, and 49% at 23°C compared to the control. Results indicate that residual slurries of acidified manure become poor inoculants in subsequent storage, hence manure acidification has a long-term treatment effect in reducing CH₄ production. This could reduce how often acidification is needed in dairy manure tanks and also increasing its cost-effectiveness for farmers.

Corn cobs efficiently reduced ammonia volatilization and improved nutrient value of stored dairy effluents

Dairy farms produce considerable quantities of nutrient-rich effluent, which is generally stored before use as a soil amendment. Unfortunately, a portion of the dairy effluent N can be lost through volatilization during open pond storage to the atmosphere. Adding of covering materials to effluent during storage could increase contact with NH₄⁺ and modify effluent pH, thereby reducing NH₃ volatilization and retaining the effluent N as fertilizer for crop application. Here the mitigation effect of cover materials on ammonia (NH₃) volatilization from open stored effluents was measured. A pilot-scale study was conducted using effluent collected at the Youran Dairy Farm Company Limited, Luhe County, Jiangsu, China, from 15 June to 15 August 2019. The study included seven treatments: control without amendment (Control), 30-mm × 25-mm corn cob pieces (CC), light expanded clay aggregate - LECA (CP), lactic acid (LA) and lactic acid plus CC (CCL), CP (CPL) or 20-mm plastic balls (PBL). The NH₃ emission from the Control treatment was 120.1 g N m^-2, which was increased by 38.1% in the CP treatment, possibly due to increased effluent pH. The application of CC reduced NH₃ loss by 69.2%, compared with the Control, possibly due to high physical resistance, adsorption of NH₄⁺ and effluent pH reduction. The lactic acid amendment alone and in combination with other materials also reduced NH₃ volatilization by 27.4% and 31.0-46.7%, respectively. After 62 days of storage, effluent N conserved in the CC and CCL treatments were 21.0% and 22.0% higher than that in the Control (P < 0.05). Our results suggest that application of corn cob pieces, alone or in combination with lactic acid, as effluent cover could effectively mitigate NH₃ volatilization and retain N, thereby enhancing the fertilizer value of the stored dairy effluent and co-applied as a soil amendment after two months open storage.

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 COD_FW.fed of H₂ injection amount, respectively. At 0.25 L H₂/g COD_FW.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 COD_FW.fed) ever reported.

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.

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 by Stimulating DIET Reaction

Since the observation of direct interspecies electron transfer (DIET) in anaerobic mixed cultures in 2010s, the topic “DIET-stimulation” has been the main route to enhance the performance of anaerobic digestion (AD) under harsh conditions, such as high organic loading rate (OLR) and the toxicants’ presence. In this review article, we tried to answer three main questions: (i) What are the merits and strategies for DIET stimulation? (ii) What are the consequences of stimulation? (iii) What is the mechanism of action behind the impact of this stimulation? Therefore, we introduced DIET history and recent relevant findings with a focus on the theoretical advantages. Then, we reviewed the most recent articles by categorizing how DIET reaction was stimulated by adding conductive material (CM) and/or applying external voltage (EV). The emphasis was made on the enhanced performance (yield and/or production rate), CM type, applied EV, and mechanism of action for each stimulation strategy. In addition, we explained DIET-caused changes in microbial community structure. Finally, future perspectives and practical limitations/chances were explored in detail. We expect this review article will provide a better understanding for DIET pathway in AD and encourage further research development in a right direction.

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.