Anaerobic Granular System Research Articles

Deciphering the formation of sludge blanket structure in anaerobic granular systems from the perspective of bubble-entrapment assumption

Granular sludge is the basic unit of anaerobic granular systems, and the hydrodynamics of the sludge blanket is significantly governed by the granule characteristics. Although trapped bubble was one of the reasons to trigger granule floatation, until now, consensus has not been reached about the impact of bubble-entrapment on granule behavior and their spatial distribution in granular system. Besides, anaerobic granular sludge subjected to calcium-rich wastewater usually faced serious operational problems, leading to granule deactivation and sludge bed cementation. Bubble entrapment and calcium precipitation in granule change particle characteristics in opposite ways. In this study, a bubble entrapment model was proposed to infer the granule floatability based on the force balance between bubble-induced buoyancy and calcium precipitation-induced gravity. The results of the stratified sludge bed characterization conducted in two full-scale anaerobic reactors clearly present their diversified granule features in the vertical direction. The result shows that the lower boundary density for calcified granule was 1.10 × 103 kg/m3 and the most dynamic granules had a density ranged from 1.10 to 1.20 × 103 kg/m3. The comparative analysis of AnGS fluidization behavior and spatial distribution confirmed the reliability of the probabilistic prediction model of theoretical granule interception. Overall, these results contribute to an educated assumption into the understanding of the impact of granule density variation caused by biogas production and calcification on the formation of stratified sludge bed structure in anaerobic granular system.

The diversity, distribution and function of N-acyl-homoserine lactone (AHL) in industrial anaerobic granular sludge

Although AHL-mediated quorum sensing (QS) signaling has been proved to be ecologically important in biofilm formation and aerobic granulation process, the biological role of AHL in anaerobic granule has not been experimentally investigated. In this paper, we explored the AHL level in 10 full-scale industrial anaerobic granular bioreactors and detected a total of 4 kinds of AHLs. C8-HSL and C10-HSL were indicated to be the potentially universal QS signal molecules in anaerobic granules and involved in extracellular polymeric substance (EPS) production and granulation process. The add-back experiments further supported the hypothesis that C8-HSL and C10-HSL might play an important role in facilitating anaerobic granulation through regulation of EPS synthesis. 12 Microorganisms might be regulated by AHL to play an important role in EPS production. This study provides a foundation for exploring the function of AHL in anaerobic granular systems, which may shed light on advanced AHL-based anaerobic granulation strategy.

Granular biomass floatation: A simple kinetic/stoichiometric explanation

Floatation events are commonly observed in anammox, denitrifying and anaerobic granular systems mostly subjected to overloading conditions. Although several operational strategies have been proposed to avoid floatation of granular biomass, until now, there is no consensus about the conditions responsible for this phenomenon. In the present study, a simple explanation based on kinetic and stoichiometric principles defining the aforementioned processes is provided. The operational zones corresponding to evaluated parameters where risk of floatation exists are defined as a function of substrate concentration in the bulk liquid and the radius of the granule. Moreover, the possible control of biomass floatation by changing the operating temperature was analyzed. Defined operational zones and profiles fit data reported in literature for granular biomass floatation events. From the study the most influencing parameter on floatation occurrence has been identified as the substrate concentration in the bulk media.

Zero valent iron simultaneously enhances methane production and sulfate reduction in anaerobic granular sludge reactors

Zero valent iron (ZVI) packed anaerobic granular sludge reactors have been developed for improved anaerobic wastewater treatment. In this work, a mathematical model is developed to describe the enhanced methane production and sulfate reduction in anaerobic granular sludge reactors with the addition of ZVI. The model is successfully calibrated and validated using long-term experimental data sets from two independent ZVI-enhanced anaerobic granular sludge reactors with different operational conditions. The model satisfactorily describes the chemical oxygen demand (COD) removal, sulfate reduction and methane production data from both systems. Results show ZVI directly promotes propionate degradation and methanogenesis to enhance methane production. Simultaneously, ZVI alleviates the inhibition of un-dissociated H2S on acetogens, methanogens and sulfate reducing bacteria (SRB) through buffering pH (Fe0 + 2H+ = Fe2+ + H2) and iron sulfide precipitation, which improve the sulfate reduction capacity, especially under deterioration conditions. In addition, the enhancement of ZVI on methane production and sulfate reduction occurs mainly at relatively low COD/SO42− ratio (e.g., 2–4.5) rather than high COD/SO42− ratio (e.g., 16.7) compared to the reactor without ZVI addition. The model proposed in this work is expected to provide support for further development of a more efficient ZVI-based anaerobic granular system.

Calculation of energy recovery and greenhouse gas emission reduction from palm oil mill effluent treatment by an anaerobic granular-sludge process

Conventional aerobic and low-rate anaerobic processes such as pond and open-tank systems have been widely used in wastewater treatment. In order to improve treatment efficacy and to avoid greenhouse gas emissions, conventional treatment can be upgraded to a high performance anaerobic granular-sludge system. The anaerobic granular-sludge systems are designed to capture the biogas produced, rendering a potential for claims of carbon credits under the Kyoto Protocol for reducing emissions of greenhouse gases. Certified Emission Reductions (CERs) would be issued, which can be exchanged between businesses or bought and sold in international markets at the prevailing market prices. As the advanced anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they render more carbon credits than other conventional anaerobic systems. In addition to efficient waste degradation, the carbon credits can be used to generate revenue and to finance the project. This paper presents a scenario on emission avoidance based on a methane recovery and utilization project. An example analysis on emission reduction and an overview of the global emission market are also outlined.

Quantification of the inhibitory effect of methyl fluoride on methanogenesis in mesophilic anaerobic granular systems

The inhibitory effect of CH 3F on methanogenesis in mesophilic anaerobic granules was tested at different concentrations (0–10% v/v, in the gas phase) and verified by the stable carbon isotopic signatures of CH 4 and CO 2. The results showed that the inhibitory effect increased with the initial CH 3F concentration up to 5%. The CH 3F concentration causing 50% metabolic inhibition was 0.32%. Complete inhibition of acetoclastic methanogenesis with a 91% reduction in total methanogenic activity was achieved when 5% CH 3F was initially added to the headspace, which resulted in 870 μM dissolved CH 3F in the liquid. It was much higher than that applied in other natural anoxic non-granular systems, indicating that the layered granular structure influenced the inhibitory effect. The obvious increase in hydrogen content indicated that high concentrations of CH 3F (⩾5%) suppressed hydrogenotrophic methanogenesis as well. The stable inhibition lasted for at least 6 d as the CH 3F concentration decreased slowly with incubation time. These results suggested that CH 3F could be used for investigating methanogenic processes in anaerobic granular systems after the CH 3F concentration and incubation time for specific inhibition of acetoclastic methanogenesis were carefully determined. In the present system, CH 3F concentration of 5% was suggested to be optimal.

Carbon balance of anaerobic granulation process: Carbon credit

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. This study investigated the potential carbon credit derived from laboratory-scale upflow anaerobic sludge blanket (UASB) reactors based on a carbon balance analysis. Methane emission reduction could be calculated by calculating the difference of UASB reactors and open lagoon treatment systems. Based on the 2.5 l bench-scale reactor, the total CH 4 emissions reduction was calculated as 29 kg CO 2/year. On scaling up to a typical full-scale anaerobic digester, the total CH 4 emissions reduction could achieve 46,420 tons CO 2 reduction/year. The estimated carbon credits would amount to 278,500 US$ per year by assuming a carbon price of 6 US$ per metric ton CO 2 reduction. The analysis postulated that it is financially viable to invest in advanced anaerobic granular treatment system from the revenue generated from carbon credits.

Carbon credit and emission trading: Anaerobic wastewater treatment

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto Protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. Granular anaerobic processes have become an attractive choice of treatment technology especially for high strength wastewaters, considering the fact that in addition to efficient waste degradation, the carbon credits can be used to generate revenue and to finance the project. This paper presents a scenario on emission reduction based on a methane recovery and utilisation project. An example analysis on emission reduction and the future trend is also outlined.