Article pubs.acs.org/est
Microscale Investigation of Anaerobic Biogas Production under Various Hydrodynamic Conditions Jin B. Zhang,† J. Wu,‡ S. Poncin,† M. Hamelin,§ and Huai Z. Li*,† †
Laboratory of Reactions and Process Engineering, University of Lorraine, CNRS, 1, rue Grandville, BP 20451, 54001 Nancy cedex, France ‡ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P.R. China § Direction of International Action, ADEME, 27 rue Louis Vicat 75737 Paris cedex 15, France S Supporting Information *
ABSTRACT: This work presents an investigation at microscale of various mechanisms affecting anaerobic reactions within the microdevices. In particular, the effect of different hydrodynamic conditions associated with the granular particles’ size and density on the biogas production was studied in order to intensify the performance of the anaerobic reactor. The image analysis techniques offer an opportunity to observe and quantify the nucleation and growth of biogas bubble at microscale on a single granule. In addition, the flow conditions were perfectly controlled in the microdevice, and the liquid flow fields were measured by a microparticle image velocimetry (micro-PIV) system. The effect of real hydrodynamic conditions exerted directly on granules onto the maximum biogas production rate was described for the first time. Finally, the role of hydrodynamic conditions on the biogas production at microscale is discussed through a straightforward relationship between the shear rates exerted on the granule and the optimal biogas production rate. The investigation reveals that big granules could have satisfactory biogas production ability under relatively weak hydrodynamic conditions. Thus they would be priority selection for industrial reactors.
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INTRODUCTION In the last two decades, considerable attention has been paid to alternate energy sources due to both the increasing demand and price for energy. The anaerobic biogas production is gaining attractive interests within this context, and the anaerobic digestion has been considered as one of the most suitable alternatives to produce the biogas, which can be recovered and then converted into heat or electricity as a renewable energy source.1,2 In recent years, anaerobic digestion technology was significantly improved by the development of digesters based on granular sludge. The related study focused mainly on the granulation process, especially, the formation, structure, and metabolism of granular sludge in the anaerobic reactors3−11 or aerobic reactors such as SBR;12,13 other studies were devoted to the dynamical effects of operational or environmental variation on the performance of the anaerobic reactor, such as hydraulic and organic load variation, temperature, nutrients, inhibition, pH, alkalinity, pretreatment.14−18 These works reveal the importance of monitoring various parameters during anaerobic digestion from a point of view of process control. Until now, little is known about the effect of hydrodynamic conditions, e.g. shear rate, on the efficiency of treatment, although it is well acknowledged that the hydrodynamics shear force is a key factor that affects the performance of an anaerobic process.8,19−23 © 2012 American Chemical Society
In practice, the hydrodynamic conditions in reactors are denoted by superficial liquid and gas velocities for many years.5,8 When the high-strength organic wastewater is treated, the hydrodynamic force is expressed by superficial gas velocity and when the low-strength organic wastewater is treated, the hydrodynamic force is expressed by superficial liquid velocity.5 As global parameters, the superficial velocities are significantly different from the real conditions exerted on sludge due to channel flow and dead zones. The Reynolds number could be a better global parameter to represent the hydrodynamic conditions because it contains the effect of the substrate and sludge through the introduction of the viscosity, even the latter is not easy to be determined in an industrial reactor. On the other hand, a local parameter like the shear rate could be a judicious choice as a scaling parameter to guide the scale-up between a pilot and an industrial reactor. Till now little is known about the real conditions exerted on sludge - the aggregate of the microorganisms. The present study aims at developing an original microscale approach to investigate the hydrodynamic conditions around Received: Revised: Accepted: Published: 8698
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dx.doi.org/10.1021/es3013722 | Environ. Sci. Technol. 2012, 46, 8698−8704
Environmental Science & Technology
Article
granules used in our experiments were selected for their proven methanogenic activity. Analytical Methods and Experimental Procedure. A micro-PIV system (Dantec Dynamics, Denmark) was used to measure the instantaneous liquid velocity field in the microdevices by seeding the carrier fluid with calibrated 0.88 μm diameter and 1056 kg·m−3 Latex particles (Merck, France) and monitoring the flow through a ×5 objective mounted on an inverted microscope. The flow was illuminated by a laser stroboscope, and the images of the flow were taken by a double image digital camera through the microscope. The pair of images was then recorded on a computer for further analysis. The FlowMap software (Dantec Dynamics, Denmark) was used to analyze these raw image pairs. A cross-correlation was then performed and gave the instantaneous flow field. The random movement due to the Brownian motion was 1.3 μm·s−1 for the seeding particles, and the sedimentation velocity was 0.03 μm·s−1.24 As the lowest liquid velocity used in our study was 1000 μm·s−1 in the microdevices, the experimental errors do not exceed 0.14% and are then negligible. The microdevices were horizontally placed under the ×4 objective of a second microscope (Motic Group, China) equipped with a digital camera. The size of microbubbles was determined using a quantitative image analysis program (Motic Images Advanced 3.2), and the shape of produced microbubbles was fairly spherical due to their small size (diameter
