Anaerobic Mesophilic Codigestion of Rice Straw and Chicken Manure: Effects of Organic Loading Rate on Process Stability and Performance

Plug-flow hydrolysis with lignocellulosic residues: effect of hydraulic retention time and thin-sludge recirculation

BACKGROUND

Two parallel plug-flow reactors were successfully applied as a hydrolysis stage for the anaerobic pre-digestion of maize silage and recalcitrant bedding straw (30% and 66% w/w) under variations of the hydraulic retention time (HRT) and thin-sludge recirculation.

RESULTS

The study proved that the hydrolysis rate profits from shorter HRTs while the hydrolysis yield remained similar and was limited by a low pH-value with values of 264-310 and 180-200 g_O2 kg_VS^-1 for 30% and 66% of bedding straw correspondingly. Longer HRT led to metabolite accumulation, significantly increased gas production, a higher acid production rate and a 10-18% higher acid yield of 78 g_SCCA kg_VS^-1 for 66% of straw. Thin-sludge recirculation increased the acid yield and stabilized the process, especially at a short HRT. Hydrolysis efficiency can thus be improved by shorter HRT, whereas the acidogenic process performance is increased by longer HRT and thin-sludge recirculation. Two main fermentation patterns of the acidogenic community were found: above a pH-value of 3.8, butyric and acetic acid were the main products, while below a pH-value of 3.5, lactic, acetic and succinic acid were mainly accumulating. During plug-flow digestion with recirculation, at low pH-values, butyric acid remained high compared to all other acids. Both fermentation patterns had virtually equal yields of hydrolysis and acidogenesis and showed good reproducibility among the parallel reactor operation.

CONCLUSIONS

The suitable combination of HRT and thin-sludge recirculation proved to be useful in a plug-flow hydrolysis as primary stage in biorefinery systems with the benefits of a wider feedstock spectrum including feedstock with cellulolytic components at an increased process robustness against changes in the feedstock composition.

Comparison of GHG emissions from open field burning and anaerobic digestion of rice straw

Rice straw (RS) has been burned in open fields in many countries, which is a growing environmental concern. Anaerobic digestion (AD) of RS is one of the sustainable alternatives to open burning of RS for converting the biomass into methane, carbon dioxide, and digestate for use as agricultural manure. The present study evaluates the RS generation, use patterns, and greenhouse gas emissions from open field burning and the AD of RS. Results indicate the AD of RS offers greater environmental benefits than that by RS open field burning. As a result of the open burning of RS in India emitted 592032.52 kg of CO2 equivalents/kg per year, whereas AD of RS contributes 122.86 kg of CO2 equivalents/kg per year. If RS were burned in an open field, its Global Warming Potential would be 493,873 times greater than that of AD. By minimizing GHG emissions and creating sustainable energy from waste, sustainable development, and circular economies are achieved.

Review on anaerobic digestion of rice straw for biogas production

India is an agrarian country producing a large amount of rice straw as an agricultural residue. These residues are burnt openly leading to severe environmental pollution and health hazards. Among several options available, anaerobic digestion of rice straw into biomethane gas and digestate is a promising technology. The current paper reviews the characteristics, principles of rice straw and the process variables (temperature, volatile fatty acids, and pH, carbon to nitrogen ratio, metal elements and organic loading rate) that affect the performance of the rice straw digestion and process strategies which may alleviate the barriers and may improve the biomethane yield. Co-digestion of rice straw with nitrogen-rich substrates is proven to be an effective way to balance the carbon to nitrogen ratio, in turn, leads to nutrient balance and enhance the biomethane yields of anaerobic co-digestion system. Moreover, pretreatment is another effective strategy; physical, chemical and biological pretreatments are reviewed in the article which improved the performance of digester. The utilisation of rice straw along with other co-substrates and appropriate pretreatment may be a recommended sustainable solution for preventing environmental and health hazards.

Comparison of microbial community structures between mesophilic and thermophilic anaerobic digestion of vegetable waste

The anaerobic digestion performance correlates with the functional microbial community. Mesophilic and thermophilic digestions of vegetable waste were conducted, and dynamics of the microbial community were investigated. The mesophilic and thermophilic collapsed stages occurred at organic loading rates of 1.5 and 2.0 g VS/(L d) due to the accumulation of volatile fatty acids with final concentrations of 2276 and 6476 mg/L, respectively. A high concentration of volatile fatty acids caused the severe inhibition of methanogens, which finally led to the imbalance between acetogenesis and methanogenesis. The mesophilic digestion exhibited a higher microbial diversity and richness than the thermophilic digestion. Syntrophic acetate-oxidizing coupled with hydrogenotrophic methanogenesis was the dominant pathway in the thermophilic stable system, and acetoclastic methanogenesis in the mesophilic stable system. The dominant acidogens, syntrophus, and methanogens were unclassified_f__Anaerolineaceae (8.68%), Candidatus_Cloacamonas (19.70%), Methanosaeta (6.10%), and Methanosarcina (4.08%) in the mesophilic stable stage, and Anaerobaculum (12.59%), Syntrophaceticus (4.84%), Methanosarcina (30.58%), and Methanothermobacter (3.17%) in thermophilic stable stage. Spirochaetae and Thermotogae phyla were the characteristic microorganisms in the mesophilic and thermophilic collapsed stages, respectively. These findings provided valuable information for the deep understanding of the difference of the microbial community and methane-producing mechanism between mesophilic and thermophilic digestion of vegetable waste.

Anaerobic Digestion for Producing Renewable Energy-The Evolution of This Technology in a New Uncertain Scenario

Anaerobic digestion is a well-known technology with wide application in the treatment of high-strength organic wastes. The economic feasibility of this type of installation is usually attained thanks to the availability of fiscal incentives. In this review, an analysis of the different factors associated with this biological treatment and a description of alternatives available in literature for increasing performance of the process were provided. The possible integration of this process into a biorefinery as a way for producing energy and chemical products from the conversion of wastes and biomass also analyzed. The future outlook of anaerobic digestion will be closely linked to circular economy principles. Therefore, this technology should be properly integrated into any production system where energy can be recovered from organics. Digestion can play a major role in any transformation process where by-products need further stabilization or it can be the central core of any waste treatment process, modifying the current scheme by a concatenation of several activities with the aim of increasing the efficiency of the conversion. Thus, current plants dedicated to the treatment of wastewaters, animal manures, or food wastes can become specialized centers for producing bio-energy and green chemicals. However, high installation costs, feedstock dispersion and market distortions were recognized as the main parameters negatively affecting these alternatives.

A Review on the Fate of Nutrients and Enhancement of Energy Recovery from Rice Straw through Anaerobic Digestion

Open field burning and tilling the rice straw (RS) back into the fields causes environmental threats by contributing to the increased greenhouse gas emissions. Energy and nutrient recovery from RS through anaerobic digestion (AD) is an effective solution for its utilization. Although RS has good methane potential, its characteristics make it a difficult substrate for AD. This paper reviews the characteristics of RS, mass balance, and distribution of nutrients into liquid and solid digestate in the AD. The present review also discusses the effect of temperature, co-digestion, mixing, inoculum, organic loading rate, recycling liquid digestate, the addition of trace elements, and their bioavailability on the enhancement of biogas/methane yield in the AD of RS. In addition, the digestion of RS at various scales is also covered in the review.

Effect of viscosity on process stability and microbial community composition during anaerobic mesophilic digestion of Maotai-flavored distiller's grains

To investigate the effects of viscosity on the mesophilic digestion of Maotai-flavored distiller's grains, a continuous experiment was conducted in a 70 L reactor at organic loading rates of 3, 4, 5, and 6 g VS/(L·d) with and without effluent recirculation. High organic loading rates and continuous effluent recirculation increased the digestate viscosity, and high viscosity caused severe foaming, which blocked the biogas outlet pipe. Moreover, a viscosity above 782 mPa·s was proposed as an early warning indicator for foaming. A maximum volumetric biogas production rate of 1.72 L/(L·d) was accomplished by diluting the feed without effluent recirculation at a recommended organic loading rate of 5 g VS/(L·d). Proteiniphilum, Ruminococcus_2, norank_f_Synergistaceae, norank_o__DTU014, Syntrophomonas, Methanosarcina, Methanobacterium, and Methanosaeta were the dominant acidogens, syntrophic bacteria, and methanogens existed in both low and high viscosity groups. Candidatus_Methanofastidiosum capable of employing the methylated thiol reduction pathway was found only in the high viscosity system.

Performance of Anaerobic Digestion of Chicken Manure Under Gradually Elevated Organic Loading Rates

Poultry manure is the main source of agricultural and rural non-point source pollution, and its effective disposal through anaerobic digestion (AD) is of great significance; meanwhile, the high nitrogen content of chicken manure makes it a typical feedstock for anaerobic digestion. The performance of chicken-manure-based AD at gradient organic loading rates (OLRs) in a continuous stirred tank reactor (CSTR) was investigated herein. The whole AD process was divided into five stages according to different OLRs, and it lasted for 150 days. The results showed that the biogas yield increased with increasing OLR, which was based on the volatile solids (VS), before reaching up to 11.5 g VS/(L·d), while the methane content was kept relatively stable and maintained at approximately 60%. However, when the VS was further increased to 11.5 g VS/(L·d), the total ammonia nitrogen (TAN), pH, and alkalinity (CaCO₃) rose to 2560 mg·L^-1, 8.2, and 15,000 mg·L^-1, respectively, while the volumetric biogas production rate (VBPR), methane content, and VS removal efficiency decreased to 0.30 L·(L·d)^-1, 45%, and 40%, respectively. Therefore, the AD performance immediately deteriorated and ammonia inhibition occurred. Further analysis demonstrated that the microbial biomass yield and concentrations dropped dramatically in this period. These results indicated that the AD stayed steady when the OLR was lower than 11.5 g VS/(L·d); this also provides valuable information for improving the efficiency and stability of AD of a nitrogen-rich substrate.

Effect of organic loading rate on anaerobic co-digestion of rice straw and pig manure with or without biological pretreatment

In this study, rice straw (RS) and pig manure (PM) mixtures with or without bio-pretreatment were used as the substrates and digested in a 9 L of anaerobic reactor at Organic loading rates (OLRs) of 0.4-3.1 kg COD/(m³ d). The volumetric methane production rate (VMPR), methane yield and anaerobic stability were comparatively investigated. The results showed the co-anaerobic digestion processes of RS and PM mixture after biological pretreatment were very stable at OLRs of 0.4-2.5 kg COD/(m³ d), and its optimal VMPR and methane yield could reach 0.64 L CH₄/(L d) and 0.4557 L CH₄/g COD_removed at OLR of 2.5 kg COD/(m³ d), which were 62.4% and 37.8% higher than those of the control under the same OLR condition. This study indicated the biological pretreatment with a cellulolytic microbial consortium own great potential in improving the methane yield and productivity of RS and PM wastes.