• Energy Research

L'inhibition du sulfure par les bactéries nitrifiantes a empêché l'intégration de la nitrification des digestats et de la désulfuration du biogaz pour simplifier les systèmes de digestion anaérobie. Dans cette étude, le digestat liquide avec une solution de NaHS a été traité en utilisant des boues nitrifiantes dans un réacteur séquentiel avec une longue période de remplissage, avec un taux de charge en ammonium de 293 mg-N L-1 j-1 et une augmentation progressive du taux de charge en sulfure de 0 à 32, 64, 128 et 256 mg-S L-1 j-1. Des bioessais par lots et des analyses de la communauté microbienne ont également été effectués avec des boues de réacteur sous chaque taux de charge en sulfure pour quantifier l'acclimatation microbienne en sulfure. Dans le réacteur, le sulfure a été complètement éliminé. La nitrification complète a été maintenue jusqu'à une charge en sulfure de 128 mg-S L-1 j-1, ce qui est supérieur à celui des rapports précédents et suffisant pour le traitement du biogaz. Dans les essais biologiques par lots, la tolérance au sulfure de l'activité oxydante du NH4+ (la concentration de sulfure inhibitrice de 50 %) a quadruplé au fil du temps avec le déplacement de la composition des bactéries nitrifiantes vers Nitrosomonas nitrosa et Nitrobacter spp. Cependant, le taux d'élimination du soufre des boues a légèrement diminué, bien que l'abondance des bactéries oxydantes du soufre Hyphomicrobium ait augmenté de 30%. Par conséquent, les boues nitrifiantes ont probablement été acclimatées au sulfure non pas par l'augmentation du taux d'élimination des sulfures, mais plutôt par l'augmentation des bactéries nitrifiantes, qui ont une tolérance élevée aux sulfures. La nitrification et la désulfuration simultanées réussies ont été obtenues à l'aide d'un réacteur séquentiel avec une longue période de remplissage, qui a été efficace pour faciliter l'acclimatation actuelle. La inhibición del sulfuro a las bacterias nitrificantes ha impedido la integración de la nitrificación del digestato y la desulfuración del biogás para simplificar los sistemas de digestión anaeróbica. En este estudio, el digestato líquido con solución de NaHS se trató utilizando lodo de nitrificación en un reactor de lotes secuenciales con un largo período de llenado, con una tasa de carga de amonio de 293 mg-N L-1 d-1 y un aumento gradual en la tasa de carga de sulfuro de 0 a 32, 64, 128 y 256 mg-S L-1 d-1. También se realizaron bioensayos por lotes y análisis de la comunidad microbiana con lodo del reactor bajo cada tasa de carga de sulfuro para cuantificar la aclimatación microbiana a sulfuro. En el reactor, el sulfuro se eliminó por completo. La nitrificación completa se mantuvo hasta una carga de sulfuro de 128 mg-S L-1 d-1, que es mayor que la de informes anteriores y suficiente para el tratamiento de biogás. En los bioensayos por lotes, la tolerancia al sulfuro de la actividad oxidante de NH4+ (la concentración de sulfuro inhibidora del 50%) se cuadruplicó con el tiempo con el cambio de composición de las bacterias nitrificantes a Nitrosomonas nitrosa y Nitrobacter spp. Sin embargo, la tasa de eliminación de azufre del lodo disminuyó ligeramente, aunque la abundancia de la bacteria oxidante de azufre Hyphomicrobium aumentó en un 30%. Por lo tanto, los lodos de nitrificación probablemente se aclimataron al sulfuro no por la creciente tasa de eliminación de sulfuro, sino más bien por el aumento de las bacterias nitrificantes, que tienen una alta tolerancia al sulfuro. La nitrificación y desulfuración simultáneas exitosas se lograron utilizando un reactor de lotes secuenciales con un largo período de llenado, que fue eficaz para facilitar la presente aclimatación. Sulfide inhibition to nitrifying bacteria has prevented the integration of digestate nitrification and biogas desulfurization to simplify anaerobic digestion systems. In this study, liquid digestate with NaHS solution was treated using nitrifying sludge in a sequential-batch reactor with a long fill period, with an ammonium loading rate of 293 mg-N L-1 d-1 and a stepwise increase in the sulfide loading rate from 0 to 32, 64, 128, and 256 mg-S L-1 d-1. Batch bioassays and microbial community analysis were also conducted with reactor sludge under each sulfide loading rate to quantify the microbial acclimatization to sulfide. In the reactor, sulfide was completely removed. Complete nitrification was maintained up to a sulfide load of 128 mg-S L-1 d-1, which is higher than that in previous reports and sufficient for biogas treatment. In the batch bioassays, the sulfide tolerance of NH4+ oxidizing activity (the 50% inhibitory sulfide concentration) increased fourfold over time with the compositional shift of nitrifying bacteria to Nitrosomonas nitrosa and Nitrobacter spp. However, the sulfur removal rate of the sludge slightly decreased, although the abundance of the sulfur-oxidizing bacteria Hyphomicrobium increased by 30%. Therefore, nitrifying sludge was probably acclimatized to sulfide not by the increasing sulfide removal rate but rather by the increasing nitrifying bacteria, which have high sulfide tolerance. Successful simultaneous nitrification and desulfurization were achieved using a sequential-batch reactor with a long fill period, which was effective in facilitating the present acclimatization. منع تثبيط الكبريتيد للبكتيريا النترجة من دمج نترجة الهضم وإزالة الكبريت من الغاز الحيوي لتبسيط أنظمة الهضم اللاهوائية. في هذه الدراسة، تمت معالجة هضم السائل بمحلول NaHS باستخدام حمأة النترجة في مفاعل على دفعات متتابعة مع فترة تعبئة طويلة، مع معدل تحميل أمونيوم يبلغ 293 مجم - N L -1 d -1 وزيادة تدريجية في معدل تحميل الكبريتيد من 0 إلى 32 و 64 و 128 و 256 مجم - S L -1 d -1. كما تم إجراء المقايسات الحيوية الدفعية وتحليل المجتمع الميكروبي مع حمأة المفاعل تحت كل معدل تحميل كبريتيد لقياس التأقلم الميكروبي مع الكبريتيد. في المفاعل، تمت إزالة الكبريتيد بالكامل. تم الحفاظ على النترجة الكاملة حتى حمل كبريتيد قدره 128 مجم - S L -1 d -1، وهو أعلى من ذلك في التقارير السابقة وكافٍ لمعالجة الغاز الحيوي. في المقايسات الحيوية للدفعة، زاد تحمل الكبريتيد لنشاط أكسدة NH4 + (تركيز الكبريتيد المثبط بنسبة 50 ٪) أربعة أضعاف مع مرور الوقت مع التحول التركيبي للبكتيريا النترجة إلى Nitrosomonas nitrosa و Nitrobacter spp. ومع ذلك، انخفض معدل إزالة الكبريت من الحمأة بشكل طفيف، على الرغم من أن وفرة البكتيريا المؤكسدة للكبريت Hyphomicrobium زادت بنسبة 30 ٪. لذلك، ربما تأقلمت الحمأة النترتية مع الكبريتيد ليس من خلال زيادة معدل إزالة الكبريتيد ولكن بالأحرى من خلال زيادة البكتيريا النترتية، التي تتمتع بتحمل عالي للكبريتيد. تم تحقيق النترجة وإزالة الكبريت بنجاح في وقت واحد باستخدام مفاعل دفعة متتابعة مع فترة ملء طويلة، والتي كانت فعالة في تسهيل التأقلم الحالي.

Anaerobic digestion is an effective method for treating excessive submerged macrophytes, which are causing severe environmental issues worldwide. The biomethane potential (BMP) of submerged macrophytes varies depending on the seasonal changes in the lignin content of each species and the species composition of harvested submerged macrophytes. In this study, the seasonality of the chemical composition and BMP of three dominant submerged macrophytes species, i.e., Egeria densa, Elodea nuttallii, and Potamogeton maackianus, were elucidated. The theoretical monthly methane yield (TMMY) and theoretical annual methane yield (TAMY) of the submerged macrophytes harvested from Lake Biwa were then estimated. The methane yields of E. densa and E. nuttallii were 212–252 and 189–284 mL g-VS−1, respectively, while that of P. maackianus was lower, at 140–165 mL g-VS−1. Although chemical composition parameters, such as the lignin content, significantly changed between different seasons (p < 0.05), they range from only 7.8–14.6%. Therefore, the seasonal variations in the methane yield of the harvested submerged macrophytes depend on the species composition. The calculated TMMY of submerged macrophytes harvested from Lake Biwa was lower from autumn to spring (171–186 mL g-VS−1) than that in summer (213–231 mL g-VS−1) due to the predominance of P. maackianus. The estimated TAMYs for several years revealed that a constant volume of methane gas could be obtained annually from the harvested submerged macrophyte.

Abstract Overgrowth of water primrose (Ludwigia grandiflora) causes environmental problems worldwide. Anaerobic digestion of L. grandiflora is a low-cost treatment. However, lignin in the lignocellulose matrix of the biomass limits the anaerobic digestion process. Therefore, steam explosion was performed at 11 different severity factors with different combinations of temperature and retention time before anaerobic digestion to enhance the microbiological conversion of the lignocellulosic biomass. Increasing severity of the pre-treatment caused biomass solubilisation, which resulted in the production of inhibitors such as dissolved lignin and phenolic compounds. The maximum methane yield was 272.4 mL g-VS−1 at 165 °C for 30 min at a severity factor of 3.3, achieving a 3.2-fold increase compared to that of untreated biomass. At SF higher than 3.3 and temperature higher than 165 °C, the methane yield started decreasing, indicating the effect of inhibitors. Thus, steam explosion pre-treatment effectively enhanced the methane production of L. grandiflora despite its high lignin content and woody structure.

Influence of the labile organic fraction (LOF) on anaerobic digestion of food waste was investigated in different S/I ratio of 0.33, 0.5, 1.0, 2.0 and 4.0g-VSsubstrate/g-VSinoculum. Two types of substrate, standard food waste (Substrate 1) and standard food waste with the supernatant (containing LOF) removed (Substrate 2) were used. Highest methane yield of 435ml-CH4g-VS(-1) in Substrate 1 was observed in the lowest S/I ratio, while the methane yield of the other S/I ratios were 38-73% lower than the highest yield due to acidification. The methane yields in Substrate 2 were relatively stable in all S/I conditions, although the maximum methane yield was low compared with Substrate 1. These results showed that LOF in food waste causes acidification, but also contributes to high methane yields, suggesting that low S/I ratio (<0.33) is required to obtain a reliable methane yield from food waste compared to other organic substrates.

A consortium of microalgae and nitrifiers has attracted attention as an alternative to the expensive traditional nitrification process. A possible obstacle to achieving this is the inhibition of nitrifiers under strong light irradiation. This study evaluated the effect of moving bed carriers on anaerobic digestate nitrification in an open photobioreactor inoculated with microalgae and nitrifiers under an incident light intensity of 1000 μmol photons m-2 s-1. The results showed higher specific nitrification activity in the carrier-added photobioreactor (103.6 mg-N g-TSS day-1) than in one in which no carrier was added (11.7 mg-N g-TSS day-1). The empirical equations for determining the light intensity at different depths in the photobioreactor showed a significant contribution by carriers in attenuating the incident light intensity. This is due to the large light attenuation caused by the carrier (1.09 cm-1). The average light intensity inside of the photobioreactor decreased considerably in the carrier-added photobioreactor (342 μmol photons m-2 s-1), whereas it did not decrease in the one with no added carrier. It was found that specific nitrification activity was significantly negatively affected by average light intensity inside of the reactor, and not by incident light intensity, by combining the results from different studies including ours. This study demonstrated, for the first time, the effectiveness of adding moving bed carriers in photobioreactors to mitigate light inhibition of nitrifiers in a consortium of microalgae and nitrifiers.

Abstract Biological treatment of organic waste is environmentally friendly and a wide range of treatment methods exists. Integrated biological treatment systems with additional equipments, such as pre-treatment, wastewater treatment and deodorisation processes are currently in use. To promote and spread the application of biological waste treatment, a life cycle assessment (LCA) study was conducted on six biological treatment methods: integrated wet anaerobic digestion (AD), integrated dry AD, simple wet AD, simple dry AD, integrated composting and simple composting systems. The impacts of operating rate and wastewater treatment, which affect GHG emissions, were also quantitatively analysed. Integrated wet AD showed the highest total GHG emissions due to the high energy consumption by additional equipments which occupy 80% of the whole process. Integrated composting also presented higher GHG emissions than simple composting because of the higher electricity consumption. Additional equipments are necessary for integrated systems installed in urban areas, and this study suggests that the reduction of energy consumption for these additional equipments is an important issue. Among the additional equipments for AD, wastewater treatment largely affected the GHG emissions. Dry AD normally generates less wastewater due to low moisture content in the waste. Thus, effective treatment of wastes with low environmental loads can be achieved by dry AD, where energy consumption from wastewater treatment is low. On the other hand, methane yield from food waste by dry AD is generally smaller than wet AD. Installing an advanced dry AD reactor with additional functions such as long solid retention time, and adjusting the moisture content of input waste by mixing paper waste will contribute to the efficient treatment of organic waste in urban areas.

Abstract The objective of this study was to investigate the effects of particle size reduction and solubilization on biogas production from food waste (FW). To clarify the effects of volatile fatty acids (VFAs) in the digestion process, the relationship between particle size and VFA accumulation was investigated in detail. For this purpose, substrates of various particle sizes were prepared by bead milling to support hydrolysis. Batch anaerobic digestion experiments were carried out using these pretreated substrates at mesophilic temperature for a period of 16 days. The results of pretreatment showed that the mean particle size (MPS) of substrates ground with a bead mill decreased from 0.843 to 0.391 mm, and solubilization accounted for approximately 40% of the total chemical oxygen demand (total COD) for grinding pretreatment by bead milling. Anaerobic digestion batch experiments revealed that MPS reduced by bead milling at 1000 rpm improved methane yield by 28% compared with disposer treatment. Moreover, this may have increased microbial degradation during the VFA production process with increasing total number of revolutions (operation time × revolutions per minute). However, excessive reduction of the particle size of the substrate resulted in VFA accumulation, decreased methane production, and decreased solubilization in the anaerobic digestion process. These results suggest that optimized reduction of the particle size of the substrate in conjunction with optimized microbial growth could improve the methane yield in anaerobic digestion processes.

Anaerobic digestion of food waste was conducted at high OLR from 3.7 to 12.9 kg-VS m(-3) day(-1) for 225 days. Periods without organic loading were arranged between the each loading period. Stable operation at an OLR of 9.2 kg-VS (15.0 kg-COD) m(-3) day(-1) was achieved with a high VS reduction (91.8%) and high methane yield (455 mL g-VS-1). The cell density increased in the periods without organic loading, and reached to 10.9×10(10) cells mL(-1) on day 187, which was around 15 times higher than that of the seed sludge. There was a significant correlation between OLR and saturated TSS in the sludge (y=17.3e(0.1679×), r(2)=0.996, P<0.05). A theoretical maximum OLR of 10.5 kg-VS (17.0 kg-COD) m(-3) day(-1) was obtained for mesophilic single-stage wet anaerobic digestion that is able to maintain a stable operation with high methane yield and VS reduction.

Aquatic plant biomass is characterised by high moisture content and a lignocellulose structure. To apply the anaerobic digestion (AD) treatment to aquatic plants, the simultaneous achievement of high methane (CH4) recovery per biomass volume and high biodegradability have been a challenge owing to these characteristics. Herein, we propose a novel two-stage serial wet- and solid-state AD (SS-AD) system that quickly digests the labile cytoplasm fraction in the first wet AD reactor in a short retention time while slowly digesting the lignocellulosic fraction in the later SS-AD with long retention time. In this study, the effect of this serial AD on CH4 recovery and chemical oxygen demand (COD) balance from aquatic plant biomass was examined in a semi-continuous operation. Elodea nuttallii, which grows excessively in the southern basin of Lake Biwa, Japan, was used as the substrate. For comparison, single-stage AD with different hydraulic retention times (HRTs) (30 d and 15 d) was performed. The CH4 conversion efficiency in single-stage AD deteriorated from 47.6 to 33.1% COD with shortened HRT, probably owing to the low degradability of slowly degrading fraction (i.e. lignocellulose) in the short retention time. In contrast, the serial AD under the same HRT (15 d) as a single-stage AD exhibited higher CH4 conversion efficiency of 65.1% COD, mainly owing to the enhanced degradation of slowly degrading fraction because of the prolonged solid retention time (52.2 d) of the entire system. The CH4 recovery from the wet AD alone in the serial AD system surpassed that from the 30 d-HRT of the single-stage AD, possibly due to the appropriate HRT for labile fraction and/or the microbial recirculation. The serial wet and SS-AD was suggested as a suitable technology for the treatment of aquatic plant biomass with recalcitrant cell walls and a labile cytoplasm.