Cristiano Nicolella

Wastewater treatment with particulate biofilm reactors

The review presented in this paper focuses on applications of particulate biofilm reactors (e.g. Upflow Sludge Blanket, Biofilm Fluidized Bed, Expanded Granular Sludge Blanket, Biofilm Airlift Suspension, Internal Circulation reactors). Several full-scale applications for municipal and industrial wastewater treatment are presented and illustrated, and their most important design and operation aspects (e.g. biofilm formation, hydrodynamics, mass transfer, mixing) are analysed and discussed. It is clear from the review that this technology can be considered a grown up technology for which good design and scale-up guidelines are available.

Particle-based biofilm reactor technology.

Particle-based biofilm reactors provide the potential to develop compact and high-rate processes. In these reactors, a large biomass content can be maintained (up to 30 g l-1), and the large specific surface area (up to 3000 m2 m-3) ensures that the conversions are not strongly limited by the biofilm liquid mass-transfer rate. Engineered design and control of particle-based biofilm reactors are established, and reliable correlations exist for the estimation of the design parameters. As a result, a new generation of high-load, efficient biofilm reactors are operating throughout the world with several full-scale applications for industrial and municipal waste-water treatment.

Gasification of pelletized biomass in a pilot scale downdraft gasifier

This work presents a pilot-scale investigation aimed at assessing the feasibility and reliability of biomass pellet gasification. Wood sawdust and sunflower seeds pellets were tested in a 200 kW downdraft gasifier operating with air as gasifying agent. The gasification of pelletized biomass led to rather high and unstable pressure drops, reducing the gasifier productivity and stability. Furthermore the generation of fine residues compromised the operation of wet ash removal systems. On the other hand, good syngas compositions (H(2) 17.2%, N(2) 46.0%, CH(4) 2.5%, CO 21.2%, CO(2) 12.6%, and C(2)H(4) 0.4%), specific gas production (2.2-2.4 N m(3) kg(-1)) and cold gas efficiency (67.7-70.0%) were achieved. For these reasons pelletized biomass should be considered only as complementary fuel in co-gasification with other feedstock.

Substrate counterdiffusion and reaction in membrane-attached biofilms : mathematical analysis of rate limiting mechanisms

Abstract A mechanistic model of organic substrate biodegradation in membrane-attached biofilms growing in extractive membrane bioreactors is presented and analysed to establish the rate-limiting steps. The model accounts for counterdiffusion and reaction of oxygen and organic substrate within the biofilm, and predicts detailed substrate concentration profiles and the evolution over time of biofilm thickness. Good agreement was found between model predictions and organic substrate flux and biofilm thickness measured experimentally in a lab-scale single-tube extractive membrane bioreactor. Analysis using this model showed that, due to oxygen diffusion limitations, the reaction zone within the biofilm is located at the biofilm/biomedium boundary and constitutes a small fraction of the entire biofilm volume. This allows a considerable simplification of biofilm modelling. A simple diffusion model was formulated as an alternative to the more complex full diffusion–reaction model for the calculation of organic substrate flux. This simple model is based on the insight that the organic compound flux is limited primarily by the biofilm diffusion resistance. The diffusion model was combined to a yield-based expression for biofilm accumulation to give the evolution over time of biofilm thickness. The simplified model predicts, as accurately as the full mechanistic model, the biofilm thickness and organic substrate flux.

Mass transfer and reaction in a biofilm airlift suspension reactor

Mass transfer and reaction were studied in a lab-scale biofilm airlift suspension reactor. A new approach for the determination of liquid–solid mass transfer coefficients in biofilm systems under reacting conditions is presented. The method is based on the analysis of oxygen consumption in a biofilm airlift suspension reactor and a biological oxygen–monitoring system and allows the independent estimation of gas–liquid and liquid–solid mass transfer coefficient and biofilm reaction rate. The influence of some operating conditions (solid loading, particle size and gas flow rate) on the liquid–solid mass transfer coefficient in biofilm airlift suspension reactors was investigated and a correlation for the mass transfer to biofilm-coated particles is proposed. Liquid–solid mass transfer to biofilm particles was proven to be mainly influenced by gas velocity, whilst particle size and solid loading had minor effect. The liquid–solid mass transfer coefficient measured for biofilm-coated particles was found to be smaller (by a factor varying from 5 to 25%) than the values reported for rigid particles.

Terminal settling velocity and bed-expansion characteristics of biofilm-coated particles.

Fluid dynamic behavior of biofilm-coated particles in ambient water has been investigated. New experimental results are presented and compared with published data. From experimental measurements of the single particle terminal settling velocity the corresponding drag coefficient was found to be larger (by a factor of 1.6) than that for a smooth, rigid sphere at the same Reynolds number. A new simple correlation describing this finding is suggested. For multiparticle systems the Richardson-Zaki equation, derived empirically for rigid particles, provided a satisfactory description of biological beds. Of the two numerical parameters characterizing the expansion law, i. e. the slope n and the extrapolation to voidage equal one ui, the first was found to be similar to that suggested by Richardson and Zaki (1954), whereas ui gave results smaller than the single-particle terminal settling velocity, in contrast with the mentioned work but in agreement with more recently published behavior.

Hydrodynamic characteristics and gas–liquid mass transfer in a biofilm airlift suspension reactor

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold‐up and volumetric mass transfer coefficient, were studied on a lab‐scale airlift reactor with internal draft tube. Basalt particles and biofilm‐coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold‐up. The ratio between gas hold‐up and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems.

Assessment of syngas composition variability in a pilot-scale downdraft biomass gasifier by an extended equilibrium model

A new simplified approach based on equilibrium modeling is proposed in this work to describe the correlations among syngas species experimentally observed in a pilot scale downdraft biomass gasifier operated with different feedstocks (biomass pellets and vine prunings). The modeling approach is based on experimental evidence on the presence of devolatilization products in the syngas and fluctuations of syngas composition during stationary operation, accounted for by introducing two empirical parameters, a by-pass index and a permeability index. The simplified model correctly reproduces the correlations among the main syngas species (including methane and ethylene) resulting from experimental data of pilot tests with different feedstocks and under a wide range of operating conditions.

Numerical and experimental investigation of downdraft gasification of woody residues.

A pilot scale throated downdraft gasifier was operated with vine prunings as feedstock to assess the effect of biomass loading rate on process performance. A distributed 1D model of mass and heat transfer and reactions was applied to aid the interpretation of experimental evidence. The model takes into account peculiar gasifier design features (air inlets and throat) and it reproduces satisfactorily the temperature profiles and the mass fluxes of gaseous species at different biomass loading rates. The integration of pilot-scale experiments and numerical simulations provides sound indications for the gasifier operation. In particular, simulations performed at different loading rates and feedstock humidity show that steady state operation and stable performance of the gasifier rely on the thermal balance between the enthalpy of cold biomass moving downward and the counter-current radiative heat fluxes moving upward from the oxidation zone. This balance can be destabilized by high loading rate and moisture contents.

Countercurrent transport of organic and water molecules through thin film composite membranes in aqueous–aqueous extractive membrane processes. Part I: experimental characterisation

Abstract This paper presents an experimental study on organic and water mass transfer through thin film composite (TFC) membranes, made of a 2 μm thick polydimethylsiloxane (PDMS) layer coated onto a microporous support of polyethersulfone. These were used for the extraction of various organic molecules from aqueous solutions with different ionic strengths. Independent measurements of organic and water transfer rates were performed using lab-scale single- and multi-tube mass exchangers. The rate of organic transfer across TFC membranes is shown to be independent of the affinity of the organic molecules for the PDMS layer, owing to negligible mass transfer resistance in this thin layer. As a consequence, the overall mass transfer coefficient measured for aqueous–aqueous extraction of hydrophilic compounds (i.e. with low affinity for PDMS) was up to one order of magnitude larger for TFC membranes than for 350 μm thick PDMS membranes operated under the same hydrodynamic conditions. The effect of osmotic pressure on water transport through TFC membranes was assessed by liquid–liquid extraction experiments using saline solutions. Water flux increases with increasing difference in salt concentration across the membrane, due to increased activity gradient. For the range of salt concentration and PDMS layer thickness used in this work, the osmotic water flux across the membrane has only a marginal effect on the rate of organic extraction. On the other hand, the ionic strength of feed and extractive phases has a direct influence on the organic transfer, due to the dependence of transport and equilibrium properties (i.e. diffusion and partition coefficients) on salt concentration. As a result, TFC membranes exhibit an asymmetric behaviour depending on operating mode. For the same hydrodynamic conditions and ionic strength, organic flux is higher when a saline solution is on the coated side than when it is on the porous side of the membrane. This has obvious implications in the design and operation of TFC membrane processes for the extraction of organics from saline solutions.