Two‐phase anaerobic digestion processes: a review

This review provides a detailed comparative summary of the recent and current research activities in the area of two‐phase anaerobic digestion processes. The acid phase and the methane phase are first evaluated, individually, from microbiological, kinetic and modelling, process optimization, operation and control, inhibition, and toxicity points of view. The overall process performance is subsequently evaluated as a whole. Finally, areas requiring further research are determined. © 2002 Society of Chemical Industry     

Anaerobic acidogenesis of dairy wastewater: the effects of variations in hydraulic retention time with no pH control

The performance of a laboratory‐scale mesophilic acidogenic reactor was evaluated in this study, in terms of volatile fatty acid production and distribution, with respect to variations in hydraulic retention time (HRT). The continuous flow‐completely mixed anaerobic reactor, coupled with a conventional gravity settling tank and a continuous recycling system, was operated in a hydraulic retention time ranging between 24 and 12 h, and up to an organic loading rate of about 9.3 kg COD m^?3 d^?1, without pH control. The acid production gradually increased proportionally to the organic loading rate, with decrease in hydraulic retention time. The highest degree of acidification and the rate of acid production were 56% and 3.1 g dm^?3 d^?1 at 12 h of HRT. Variations in hydraulic retention time affected volatile fatty acid production and distribution substantially, for the range investigated. Acetic, propionic, butyric and valeric acids were commonly produced during acidogenesis of dairy wastewater. Copyright © 2004 Society of Chemical Industry     

Changes to bacterial community make‐up in a two‐phase anaerobic digestion system

Changes to microbial populations in a two‐phase anaerobic digestion system were studied over 34 weeks. Numbers of autofluorescent methanogenic and non‐methanogenic bacteria decreased significantly during start‐up, but did not change markedly either in the acid reactor or the upflow anaerobic filter for the remainder of the study. Although the proportion of autofluorescent methanogens increased in the acid reactor, the numbers of viable methanogens decreased 590‐fold. The numbers of viable methanogens increased 10‐fold in the port, decreased 10‐fold in the effluent and there was almost no change in the drain of the upflow anaerobic filter. The data indicated that bacterial attachment in the upflow anaerobic filter gave a 90% COD removal and a methane yield of 0.33 m³ CH₄ kg⁻¹ COD removed at an organic loading rate of 7 kg COD m⁻³day⁻¹. Epifluorescence microscopy of the seed sludge revealed a diverse methanogenic population of equally dominant groups of medium rods and filaments with Methanococcus, short rods, long rods and Methanosarcina also present. The medium rod‐shaped species remained the most dominant group in the acid reactor. As the volatile fatty acid concentration increased in the acid reactor the number of Methanosarcina and filament species decreased, becoming the least dominant groups. At the end of the operation, Methanococcus species were the dominant group in the upflow anaerobic filter having been washed from the biofilm. © 2000 Society of Chemical Industry     

Spray Characteristics of Two‐phase Feed Nozzles

The present study focuses on understanding the spray characteristics of a turbulent gas‐liquid jet (Re_liq = 24,000). Air and water are used as the test fluids. The angles of injection of the two phases upstream of the nozzle are varied (θ = 20°, 45° and 90°) and the effect of carrier gas on the droplet characteristics is are also investigated. The droplet size and velocity are non‐intrusively measured using a Phase‐Doppler Particle Analyzer (PDPA). In some respects, the characteristics of the present two‐phase jet are similar to those noticed in previous studies, while revealing some important differences. The centreline mean droplet velocities (15 ～ 20 m/s) increase in the initial region of the jet, attain a maximum and then decrease at larger distances from the nozzle exit. Most of the entrainment occurs at the tip of the nozzle and the jet expansion rate decreases significantly at distances where the spray velocity profiles become self‐similar. A Lorentz‐type fit has been used to model the normalized radial velocity profiles. The results indicate that the test configuration with θ = 45° may be beneficial for the scenario discussed.     

Hydrodynamic Model for Horizontal Two‐phase Flow Through Porous Media

A unidirectional, two‐fluid model based on the volume‐average mass and momentum balance equations was developed for the prediction of two‐phase pressure drop and external liquid hold‐up in horizontally positioned packed beds experiencing stratified, annular and dispersed bubble flow regimes. The so‐called slit model drag force closures were used for the stratified and annular flow regimes. In the case of dispersed bubble flow regime, the liquid‐solid interaction force was formulated on the basis of the Kozeny‐Carman equation by taking into account the presence of bubbles in reducing the available volume for the flowing liquid. The gas‐liquid interaction force was evaluated by using the respective solutions of drag coefficient for an isolated bubble in viscous and turbulent flows. The proposed drag force expressions for the different flow patterns occurring in the bed associated with the two‐fluid model resulted in a predictive method requiring no adjustable parameter to describe the hydrodynamics for horizontal two‐phase flow in packed beds.     

Treatment of high‐fat‐containing dairy wastewater in a sequential UASBR system: influence of recycle

BACKGROUND: Raw cheese whey originating from white cheese production results in a strong and complex wastewater excessively rich in organic matter (chemical oxygen demand, COD = 28–65 g L^?1), fatty matter (14–24.5 g L^?1) and acidity (3.9–6.1 g L^?1). It was treated in a three‐stage configuration consisting of a pre‐acidification (PA) tank and sequential upflow anaerobic sludge bed reactors (UASBRs) at 2.8–7 g COD L^?1 day^?1 organic loading rates, during which the effects of effluent recycling at low rates and promoted SRB activity were investigated. Acidification, volatile fatty acids (VFA), COD and fatty matter removal and volatile solids were monitored throughout the system during the study. RESULTS: Recycling of the effluent promoted VFA and COD removal as well as pH stability in both stages of the UASBRs and the effluent where high alkalinity levels were recovered reducing alkali requirement to 0.05 g OH g^?1 COD_applied. Higher removal rates of 71–100 and 50–92% for VFA and COD were obtained by use of recycling. Fatty matter was removed at 63–89% throughout the study. Volatile solids build‐up was significant in the inlet zones of the UASBRs. CONCLUSIONS: The system produced efficient acidification in the PA tank, balanced pH levels and an effluent high in alkalinity and BOD/COD ratio. Efficient VFA removal and solids immobilization was obtained in both stages up to the highest loading rate. Recycling improved the system performance under high fatty matter loading conditions. A major advantage of the sequential system was that the second stage UASBR compensated for reduced performance in the first stage. Copyright © 2010 Society of Chemical Industry     

Multi‐Stage Aqueous Two‐Phase Extraction for the Purification of Monoclonal Antibodies

Using monoclonal antibodies in a cell culture harvest as an example for complex biomolecules, a mini‐plant‐scale aqueous two‐phase purification process was studied. The results of this study indicate that antibodies can be concentrated and purified in a single extraction step employing a small phase ratio. Following the extraction step, a multi‐stage wash‐extraction for further purification was investigated. Starting at a test tube‐scale cross‐current wash, the process was advanced towards a continuous counter‐current mixer‐settler and column wash process. It was shown that a test tube cross‐current extraction operation can predict the multi‐stage purity reasonably well. The hydrodynamic process performance for the multi‐stage wash column was evaluated and related to the separation performance.     

A kinetic evaluation of the anaerobic digestion of two‐phase olive mill effluent in batch reactors

A comparative kinetic study was carried out on the anaerobic digestion of two‐phase olive mill effluent (TPOME) using three 1‐dm³ volume stirred tank reactors, one with freely suspended biomass (control), and the other two with biomass supported on polyvinyl chloride (PVC) and bentonite (aluminium silicate), respectively. The reactors were batch fed at mesophilic temperature (35 °C) using volumes of TPOME of between 50 and 600 cm³, corresponding to chemical oxygen demand (COD) loadings in the range of 1.02–14.22 g, respectively. The process followed first‐order kinetics and the specific rate constants, K₀, were calculated. The K₀ values decreased considerably from 2.59 to 0.14 d^?1, from 1.93 to 0.23 d^?1 and from 1.52 to 0.17 d^?1 for the reactors with suspended biomass (control) and biomass immobilized on PVC and bentonite, respectively, when the COD loadings increased from 1.02 to 14.22 g; this showed an inhibition phenomenon in the three reactors studied. The values of the critical inhibitory substrate concentration (S*), theoretical kinetic constant without inhibition (K_A) and the inhibition coefficient or inhibitory parameter for each reactor (n) were determined using the Levenspiel model. Copyright © 2004 Society of Chemical Industry     

A Hydration Shell‐Based Thermodynamic Model for Aqueous Two‐Phase Systems

In this work a Flory‐Huggins model modified to account for some unique features of Aqueous Two‐Phase Systems is presented. The model takes into account observed solvation between water and polymer molecules through the incorporation of a hydration shell to express the number of water molecules bonded to each polymer molecule. The parameters of the modified equation were determined using experimental data of ATPS containing poly (ethylene glycol) and dextran. The results revealed remarkable improvement in the correlation ability of the model. A general expression that defines the number of water molecules in the hydration shell was also obtained.     

Volatile fatty acid production during anaerobic mesophilic digestion of solid potato waste

The production of volatile fatty acids by anaerobic digestion of solid potato waste was investigated using a batch solid waste reactor with a working capacity of 2 dm^?3 at 37°C. Solid potato waste was packed into the digester and the organic content of the waste was released by microbial activity by circulating water over the bed, using batch loads of 500 g or 1000 g potato waste. The sequence of appearance of the volatile fatty acids was (acetic, propionic); (n‐butyric); (n‐valeric, iso‐valeric, caproic); (iso‐butyric). After 300 h digestion of potato waste on a small scale, the fermentation products were chiefly (mg g^?1 total VFAs): acetic acid (420), butyric acid (310), propionic acid (140) and caproic acid (90), with insignificant amounts of iso‐butyric acid, n‐valeric and iso‐valeric acids. When the load of potato solids was increased, the volatile fatty acid content was similar, but butyric acid constituted 110 mg g^?1 and lactic acid 400 mg g^?1 of the total volatile fatty acids. The maximum soluble chemical oxygen demand (COD) achieved under the experimental conditions used was 27 and 37 g COD dm^?3 at low and high loadings of potato solids, respectively. The total volatile fatty acids reached 19 g dm^?3 of leachate at both loads of potato solid waste. Gas production was negligible, indicating that methanogenic activity was effectively inhibited. Copyright © 2004 Society of Chemical Industry     

Two‐dimensional simulation of hollow fiber membrane fabricated by phase inversion method

In the steady fabricating process, two‐dimensional hollow fiber membrane near the spinneret was numerically simulated using the finite element method (FEM). The unknown positions of free surface and moving interface were calculated simultaneously by the velocity and pressure fields. The effects of seven relevant parameters, i.e., inertia term, gravity term, dope flow rate, bore flow rate, dope viscosity, tensile force, end velocity and non‐Newtonian on the velocity and diameter profile were studied. On the basis of the simulated results, the inertia term in hollow fiber‐spinning process was safely neglected in low speed, while the effect of gravity was not be neglected. Besides, the outer diameter of the fibers increased with an increase of dope flow rate and bore flow rate; Large tensile force or large end velocity could cause large deformation in the air gap; larger viscous dope solution tended to make less deformation in the air gap. It was found that an increase of the dope flow rate at small dope flow rate resulted in an increase of the inner diameter, while at large dope flow rate, it decreased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2067–2074, 2006     

Two‐phase flow behavior in microtube reactors during biodiesel production from waste cooking oil

Flow patterns in the course of transesterification of waste cooking oil (WCO), sunflower oil (SFO) with water and/or oleic acid as a model of WCO, and pure SFO in the presence of a KOH catalyst in microtubes were investigated. FAME yield for the transesterification of WCO reached more than 89% in the microtube reactors with a residence time of 252 s at 333 K. The flow patterns when using WCO were changed from a liquid–liquid slug flow at the inlet region to a parallel flow at the middle region, and then to a homogeneous liquid flow at the outlet region as the reaction proceeded at 333 K. Fine droplets containing glycerol and methanol generally formed in oil slugs when using pure SFO, but were almost unobservable when using WCO. The soap produced from free fatty acids was considered to be the main factor affecting the flow patterns of WCO and SFO. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Energy Efficiency of Two‐Phase Mixing in a Modified Bubble Column

Energy efficiency for gas liquid mixing in a modified downflow bubble column reactor has been analyzed in this paper. Efficiencies of the different parts of the bubble column have been assessed on the basis of energy dissipation. Prediction of the energy dissipation coefficient as well as energy utilization efficiency due to gas‐liquid mixing as a function of different physical, geometric and dynamic variables of the system has been done by correlation method. The distribution of energy utilization in the different zones of the column has also been analyzed. Experiments were carried out with air‐water and air‐aqueous solutions of carboxy methyl cellulose with different concentrations.