Andrew Hoadley

Dewatering of microalgal cultures: A major bottleneck to algae-based fuels

Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel prodn. The dil. nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.

Electrical field: A historical review of its application and contributions in wastewater sludge dewatering

Electric field-assisted dewatering, also called electro-dewatering, is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. Electro-dewatering is not a new idea, but the practical industrial applications have been limited to niche areas in soil mechanics, civil engineering, and the ceramics industry. Recently, it has received great attention, specially, in the fields of fine-particle sludge, gelatinous sludge, sewage sludge, pharmaceutical industries, food waste and bull kelp, which could not be successfully dewatered with conventional mechanical methods. This review focuses on the scientific and practical aspects of the application of an electrical field in laboratory/industrial dewatering, and discusses this in relation to conventional dewatering techniques. A comprehensive bibliography of research in the electro-dewatering of wastewater sludges is included. As the fine-particle suspensions possess a surface charge, usually negative, they are surrounded by a layer with a higher density of positive charges, the electric double layer. When an electric field is applied, the usually negative charged particles move towards the electrode of the opposite charge. The water, commonly with cations, is driven towards the negative electrode. Electro-dewatering thus involves the well-known phenomena of electrophoresis, electro-osmosis, and electromigration. Following a detailed outline of the role of the electric double layer and electrokinetic phenomena, an analysis of the components of applied voltage and their significance is presented from an electrochemical viewpoint. The aim of this elementary analysis is to provide a fundamental understanding of the different process variables and configurations in order to identify potential improvements. Also discussed herein is the investigation of the electrical behaviour of a porous medium, with particular emphasis on porous medium conductivity determination.

Electro-dewatering of wastewater sludge: influence of the operating conditions and their interactions effects.

Electric field-assisted dewatering, also called electro-dewatering (EDW), is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. The application of these additional fields can be applied to either or both dewatering stages (filtration and/or compression), or as a pre-or post-treatment of the dewatering process. In this study, the performance of the EDW on wastewater sludge was investigated. Experiments were carried out on a laboratory filtration/compression cell, provided with electrodes, in order to apply an electrical field. The chosen operating conditions pressure (200-1200 kPa) and voltage (10-50 V) are sufficient to remove a significant proportion of the water that cannot be removed using mechanical dewatering technologies alone. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of EDW on (i) the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of EDW and (ii) the energy consumption calculated for each additional mass of water removed. A two-factor central composite design was used to establish the optimum conditions for the EDW of wastewater sludge. Experiments showed that the use of an electric field combined with mechanical compression requires less than 10 and 25% of the theoretical thermal drying energy for the low and moderate voltages cases, respectively.

An evaluation of a hybrid ion exchange electrodialysis process in the recovery of heavy metals from simulated dilute industrial wastewater

Hybrid ion exchange electrodialysis, also called electrodeionization (IXED), is a technology in which a conventional ion exchange (IX) is combined with electrodialysis (ED) to intensify mass transfer and to increase the limiting current density and therefore to carry out the treatment process more effectively. It allows the purification of metal-containing waters, as well as the production of concentrated metal salt solutions, which could be recycled. The objective of this paper was to investigate the ability of the IXED technique for the treatment of acidified copper sulphate solutions simulating rinsing water of copper plating lines. A single-stage IXED process at lab-scale with a small bed of ion exchanger resin with a uniform composition was evaluated, and the treatment performance of the process was thoroughly investigated. The IXED stack was assembled as a bed layered with the ion exchanger resin (strong acid cation-exchange Dowex™) and inert materials. The stack configuration was designed to prevent a non-uniform distribution of the current in the bed and to allow faster establishment of steady-state in the cell for IXED operation. The influence of operating conditions (e.g. ion exchanger resin with a cross-linking degree from 2 to 8% DVB, and current density) on IXED performance was examined. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of IXED on (i) the abatement yield of the metal cation, which is a fundamental purification parameter and an excellent indicator of the extent of IXED, (ii) the current yield or the efficiency of copper transport induced by the electrical field and (iii) the energy consumption. The experimental results showed that the performance at steady-state of the IXED operation with a layered bed remained modest, because of the small dimension of the bed and notably the current efficiency varied from 25 to 47% depending on the conditions applied. The feasibility of using the IXED in operations for removal of heavy metals from moderately dilute rinsing waters was successfully demonstrated.

Lignite aided dewatering of digested sewage sludge.

Mechanical dewatering is commonly used to increase the solids content of municipal sludge prior to its disposal. However, if the rate of filtration is slow, mechanical dewatering can be expensive. In this study, the use of lignite to improve the sludge dewatering is investigated. The effectiveness of lignite conditioning of polyelectrolyte-flocculated sludge is examined using mechanical compression tests. Results show that lignite conditioning in conjunction with polyelectrolyte flocculation gives much better dewatering than the polyelectrolyte flocculation alone. Using Darcys filtration theory, the specific cake resistance and permeability of the compressed cakes are obtained. Both of these parameters are significantly improved after lignite conditioning. Mercury porosimetry tests on compressed cakes show that the porosity of the lignite-conditioned sludge cake is much higher than that of the polyelectrolyte-flocculated sludge and it increases with increasing doses of lignite. The mercury porosimetry results show that the lignite pore volume of pores greater than 0.5 microm are reduced with increasing sludge ratio indicating that sludge is trapped within these pores, whereas smaller pores are unaffected. The yield stress curves for sludge, lignite and sludge-lignite mixtures show that the sludge filter cake is very compressible, but the lignite-conditioned cake has a range of compressibility which although more than lignite indicate that the cake is relatively incompressible at low pressures. Thus, lignite conditioning acts to maintain the permeability of the filter cake during compression dewatering by resisting cake compression. This leads to a trade-off between the rate of dewatering and the solids content of the compressed cake. With lignite conditioning, the dewatering rate can be increased by a factor of five for the same degree of water removal.

Dewatering of biomaterials by mechanical thermal expression

Dewatering by mechanical thermal expression (MTE) for a range of materials is explored using a laboratory-scale MTE compression-permeability cell. It is shown that MTE can be used to effectively dewater a range of biomaterials including lignite, biosolids, and bagasse. The underlying dewatering mechanisms relevant to MTE, namely (1) filtration of water expelled due to thermal dewatering, (2) consolidation, and (3) flash evaporation, are discussed. At lower temperatures, the dominating dewatering mechanism is consolidation, but with increasing temperature, thermal dewatering becomes more important. A major focus is an investigation of the effects of processing parameters, including temperature (20 to 200°C) and pressure (1.5 to 24 MPa), on material permeability, a fundamental dewatering parameter. It is illustrated that permeability is particularly dependent on the processing temperature, owing to changes in both the material structure and the water properties. In addition, a comparison of permeability in the direction of applied force (axial) and perpendicular to the direction of applied force (radial) is presented. It is shown that, due to alignment of particles under the applied force, the permeability and, hence, rate of water removal in the radial direction is greater than in the axial direction. SEM micrographs are presented to illustrate the particle alignment.

Pore destruction resulting from mechanical thermal expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.

Critical review and exergy analysis of formaldehyde production processes

Abstract Formaldehyde is one of the most important intermediate chemicals and has been produced industrially since 1889. Formaldehyde is a key feedstock in several industries like resins, polymers, adhesives, and paints, making it one of the most valuable chemicals in the world. However, not many studies have been dedicated to reviewing the production of this economically important product. In this review paper, we study the leading commercial processes for formaldehyde production and compare them with recent advancements in catalysis and novel processes. This paper compares, in extensive detail, the reaction mechanisms and kinetics of water ballast process (or BASF process), methanol ballast process, and Formox process. The thermodynamics of the reactions involved in the formaldehyde production process was investigated using HSC Chemistry™ software (Outotec Oyj, Espoo, Finland). Exergy analysis was carried out for the natural gas to methanol process and the methanol ballast process for formaldehyde production. The former process was simulated using Aspen HYSYS™ and the latter using Aspen Plus™ software (Aspen technology, Burlington, MA, USA). The yield and product specifications from the simulation results closely matched with published experimental data. The exergy efficiencies of the natural gas to synthesis gas via steam reforming, methanol synthesis, and formaldehyde synthesis processes were calculated as 60.8%, 61.6%, and 66%, respectively. The overall exergy efficiency of natural gas conversion into formaldehyde was found to be only 43.2%. The main sources of exergy losses were the steam reformer and methanol loss in formaldehyde synthesis process. Despite high conversions and selectivities of these processes, the low exergy efficiency suggests that innovations in formaldehyde production processes could give a more sustainable product. Novel methods of direct conversion of natural gas or synthesis gas into formaldehyde will improve the exergy efficiency, but the conversion rate must also be increased with advancements in catalysis.

Formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase

Formaldehyde (HCHO) is an essential building block in many industries for producing value-added chemicals like resins, polymers and adhesives. Industrially, formaldehyde is produced via partial oxidation and/or dehydrogenation of methanol. Methanol is produced from natural gas in a series of processes, with synthesis gas as an intermediate. This study presents for the first time, formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase, which eliminates the need for methanol synthesis, which may potentially save capital costs and reduce energy consumption. Gas phase hydrogenation of CO into formaldehyde is thermodynamically limited and therefore, resulted in a low CO conversion of only 1.02 × 10−4%. However, the aqueous phase hydrogenation of CO into formaldehyde was found to be thermodynamically favourable and kinetically limited. The highest CO conversion of 19.14% and selectivity of 100% were achieved by using a Ru–Ni/Al2O3 catalyst at 353 K and 100 bar. The rapid hydration of formaldehyde in the aqueous phase to form methylene glycol shifts the CO hydrogenation reaction equilibrium towards formaldehyde formation. Increasing the pressure and stirring speed increased the yield of formaldehyde, whereas increasing the temperature above 353 K resulted in a lower yield.

Advances in Mechanical Dewatering of Wastewater Sludge Treatment

Dewatering of wastewater sludge is a difficult process. The difficulty has been attributed mainly to the fact that particles are very fine, colloidal in nature and possess a gel-like structure due to polymeric flocculation. In order to tackle the limitations in wastewater sludge dewatering, new technologies have been developed in recent years. Some technologies, such as wastewater sludge digestion, wastewater sludge mineralisation or peroxidation, allow to reduce the amount of wastewater sludge to be dewatered, or the dewaterability of the sludge, by changing the biochemical composition. Nevertheless, wastewater sludge remains hard to dewater, and therefore, an improvement in the conventional dewatering equipments is desirable. Therefore, current research tends to propose potential alternatives to enhance the dewatering ability of conventional processes, to increase the final dry solids content, and to accelerate the dewatering process with low energy consumption compared to thermal drying.