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.

Response surface optimization of substrates for thermophilic anaerobic codigestion of sewage sludge and food waste.

This study investigated the effects of food waste constituents on thermophilic (55 degrees C) anaerobic codigestion of sewage sludge and food waste by using statistical techniques based on biochemical methane potential tests. Various combinations of grain, vegetable, and meat as cosubstrate were tested, and then the data of methane potential (MP), methane production rate (MPR), and first-order kinetic constant of hydrolysis (kH) were collected for further analyses. Response surface methodology by the Box-Behnken design can verify the effects and their interactions of three variables on responses efficiently. MP was mainly affected by grain, whereas MPR and kH were affected by both vegetable and meat. Estimated polynomial regression models can properly explain the variability of experimental data with a high-adjusted R2 of 0.727, 0.836, and 0.915, respectively. By applying a series of optimization techniques, it was possible to find the proper criteria of cosubstrate. The optimal cosubstrate region was suggested based on overlay contours of overall mean responses. With the desirability contour plots, it was found that optimal conditions of cosubstrate for the maximum MPR (56.6 mL of CH4/g of chemical oxygen demand [COD]/day) were 0.71 g of COD/L of grain, 0.18 g of COD/L of vegetable, and 0.38 g of COD/L of meat by the simultaneous consideration of MP, MPR, and kH. Within the range of each factor examined, the corresponding optimal ratio of sewage sludge to cosubstrate was 71:29 as the COD basis. Elaborate discussions could yield practical operational strategies for the enhanced thermophilic anaerobic codigestion of sewage sludge and food waste.

Low-strength ultrasonication positively affects methanogenic granules toward higher AD performance: Implications from microbial community shift.

To elucidate the enhanced methane yield from organic wastes, the effects of low-strength ultrasonication on the microbial community structures in upflow anaerobic sludge blanket reactors were for the first time analyzed using pyrosequencing. Interestingly, a more even microbial community was observed in the ultrasonicated granules than in the control, which could compensate for the decreased richness and resulted in comparable (archaea) or even higher (bacteria) diversity. The ultrasonicated granules contained higher levels of δ-Proteobacteria, of which many are reportedly potential syntrophs, as well as methanogenic genera Methanosaeta, Methanotorris, and Methanococcus. The increased presence of syntrophic bacteria with their methanogenic partners was discussed with respect to hydrogen flux; their selective proliferation seems to be responsible for the enhanced anaerobic performance. This study is the first research shedding light on the novel function of low-strength ultrasound shifting the microbial structure towards better biogas production performance, and will facilitate application of low-strength ultrasound to other bioprocesses.

Effect of hydraulic retention time on lactic acid production and granulation in an up-flow anaerobic sludge blanket reactor.

In the present work, lactic acid (LA) production performance with granulation was investigated at various hydraulic retention times (HRTs), 8-0.5h. Glucose was used as a feedstock, and anaerobic mixed cultures were inoculated in an up-flow anaerobic sludge blanket reactor. As HRT decreased, the average diameter and hydrophobicity of the granules increased from 0.31 to 3.4mm and from 17.5% to 38.3%, respectively, suggesting the successful formation of granules. With decreasing HRT, LA productivity increased up to 16.7gLA/L-fermenter/h at HRT 0.5h. The existence of rod-shaped organisms with pores and internal channels at granule surface was observed by scanning electron microscope. Next generation sequencing revealed that Lactobacillus was the dominant microorganism, accounting for 96.7% of total sequences, comprising LA-producing granules.

Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester.

In this study, the positive effects of low-strength ultrasonication (LS-ultrasonication) on the anaerobic digestion (AD) performance were investigated by continuously operating an upflow anaerobic sludge blanket reactor (UASBr) and a dry digester. In the ultrasonicated UASBr system (1 s per min, 0.05 W/mL), ultrasonication enhanced the CH4 production by 38% and 19% in an ambient and a mesophilic condition, respectively. In addition, a different sludge yield and a changed electron flow were observed after ultrasonication. In the ultrasonicated dry digestion system (2 s per 30 s, 0.0025 W/mL), a 40% increase in the production of CH4 was observed after lowering the total solid content of the reactor from 12% to 10%, implying that a high solid content diminished the ultrasonic stimulation effect. Moreover, the ultrasonication strength itself appeared to be a more crucial factor than the ultrasonication density during the application of LS-ultrasonication in the AD system.

Dry anaerobic digestion of food waste under mesophilic conditions: performance and methanogenic community analysis.

The performance of dry anaerobic digestion (AD) of food waste was investigated under mesophilic conditions and the methanogenic community was investigated using 454 pyrosequencing. Stable dry AD was achieved by hydraulic retention time (HRT) control without the addition of alkali agents. The average CH4 production rate, CH4 content, and volatile solid reduction rate were 2.51±0.17m(3)/m(3)/d, 66±2.1%, and 65.8±1.22%, respectively, at an HRT of 40d. The methanogenic community of the seed sludge experienced a significant reduction in genus diversity from 18 to 4 and a dominant methanogenic shift from hydrogenotrophic to acetoclastic groups after the acclimation under dry condition. Almost all sequences of the dry anaerobic digester were closely related with those of Methanosarcina thermophila with similarity of 96.4-99.1%. The experimental results would serve as useful information to understand the dry AD system.

Low strength ultrasonication positively affects the methanogenic granules toward higher AD performance. Part I: physico-chemical characteristics.

To elucidate the correlation between enhanced biogas production and changed physico-chemical properties of methanogenic granules after low strength ultrasonication, in this study, the effects of low strength ultrasonication on the settling velocity, permeability, porosity, and fluid collection efficiency of the methanogenic granules were investigated. In addition, their morphological changes were visualized using a scanning electron microscopic technique. The experimental results indicate that low strength ultrasonication increased both the permeability (37%) and specific surface area (230%) of the granules through the generation of greater craters and cracks on the granular surface compared to the control granules. The penetration of nutrients and substrate into the granules was thereby enhanced, and more favorable conditions for achieving higher anaerobic performance were provided to the ultrasonicated granules. The microbial community shift caused by the changed physico-chemical properties of the methanogenic granules will be further analyzed in part II of this study.

Enhanced activity of methanogenic granules by low-strength ultrasonication.

In this study, low-strength ultrasonication was applied at various ultrasonication densities (UDs) (0-0.1 W/mL) and ultrasonication time (UT) (0-30 min) to methanogenic granules on the purpose of increasing their activity, and eventually, enhancing the performance of upflow anaerobic sludge blanket reactor (UASBr). Batch test results showed that 5 min of ultrasonication at 0.05 W/mL was found to be the optimal conditions, resulting in the increase of dehydrogenase activity and adenosine triphosphate content by 257%, and 374%, respectively, compared to the control. These increments were confirmed by specific methanogenic activity test. When ultrasonication (UD 0.05 W/mL, UT 5 min) was irradiated every 8h during the continuous operation of UASBr, it caused a gradual drop of methanogenic activity, complete loss after 20 days. At further operation, UT was decreased to 1s but irradiated every 1 min, which resulted in a 43% higher specific CH(4) production rate.

Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions.

With increasing concerns over the limited capacity of landfills, conservation of resources, and reduction of CO(2) emissions, high-solids (dry) anaerobic digestion of organic solid waste (OSW) is attracting a great deal of attention these days. In the present work, two dry anaerobic co-digestion systems fed with different mixtures of OSW were continuously operated under mesophilic conditions. Dewatered sludge cake was used as a main seeding source. In reactor (I), which was fed with food waste (FW) and paper waste (PW), hydraulic retention time (HRT) and solid content were controlled to find the maximum treatability. At a fixed solid content of 30% total solids (TS), stable performance was maintained up to an HRT decrease to 40 d. However, the stable performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again. In further operation, instead of decreasing HRT, solid content was increased to 40% TS, which was found to be a better option to increase the treatability. The biogas production rate (BPR), CH(4) production yield (MPY) and VS reduction achieved in this condition were 5.0m(3)/m(3)/d, 0.25 m(3) CH(4)/g COD(added), and 80%, respectively. Reactor (II) was fed with FW and livestock waste (LW), and LW content was increased during the operation. Until a 40% LW content increase, reactor (II) exhibited a stable performance. A BPR of 1.7 m(3)/m(3)/d, MPY of 0.26 m(3) CH(4)/g COD(added), and VS reduction of 72% was achieved at 40% LW content. However, when the LW content was increased to 60%, there was a significant performance drop, which was attributed to free ammonia inhibition. The performances in these two reactors were comparable to the ones achieved in the conventional wet digestion and thermophilic dry digestion processes.

Combined (alkaline+ultrasonic) pretreatment effect on sewage sludge disintegration.

The individual effects of alkaline (pH 8-13) and ultrasonic (3750-45,000kJ/kg TS) pretreatments on the disintegration of sewage sludge were separately tested, and then the effect of combining these two methods at different intensity levels was investigated using response surface methodology (RSM). In the combined pretreatment, ultrasonic treatment was applied to the alkali-pretreated sludge. While the solubilization (SCOD/TCOD) increase was limited to 50% in individual pretreatments, it reached 70% in combined pretreatment, and the results clearly showed that preconditioning of sludge at high pH levels played a crucial role in enhancing the disintegration efficiency of the subsequent ultrasonic pretreatment. By applying regression analysis, the disintegration degree (DD) was fitted based on the actual value to a second order polynomial equation: Y=-172.44+29.82X(1)+5.30x10(-3)X(2)-7.53x10(-5)X(1)X(2)-1.10X(1)(2)-1.043x10(-7)X(2)(2), where X(1), X(2), and Y are pH, specific energy input (kJ/kg TS), and DD, respectively. In a 2D contour plot describing the tendency of DD with respect to pH and specific energy input, it was clear that DD increased as pH increased, but it seemed that DD decreased when the specific energy input exceeded about 20,000kJ/kg TS. This phenomenon tells us that there exists a certain point where additional energy input is ineffective in achieving further disintegration. A synergetic disintegration effect was also found in the combined pretreatment, with lower specific energy input in ultrasonic pretreatment yielding higher synergetic effect. Finally, in order to see the combined pretreatment effect in continuous operation, the sludge pretreated with low intensity alkaline (pH 9)/ultrasonic (7500kJ/kg TS) treatment was fed to a 3 L of anaerobic sequencing batch reactor after 70 days of control operation. CH(4) production yield significantly increased from 81.9+/-4.5mL CH(4)/g COD(added) to 127.3+/-5.0mL CH(4)/g COD(added) by pretreatment, and this enhanced performance was closely related to the solubilization increase of the sludge by pretreatment. However, enhanced anaerobic digestion resulted in 20% higher soluble N concentration in the reactor, which would be an additional burden in the subsequent nitrogen removal system.