An ultrasonically enhanced inclined settler for microalgae harvesting

Microalgae have vast potential as a sustainable and scalable source of biofuels and bioproducts. However, algae dewatering is a critical challenge that must be addressed. Ultrasonic settling has already been exploited for concentrating various biological cells at relatively small batch volumes and/or low throughput. Typically, these designs are operated in batch or semicontinuous mode, wherein the flow is interrupted and the cells are subsequently harvested. These batch techniques are not well suited for scaleup to the throughput levels required for harvesting microalgae from the large‐scale cultivation operations necessary for a viable algal biofuel industry. This article introduces a novel device for the acoustic harvesting of microalgae. The design is based on the coupling of the acoustophoretic force, acoustic transparent materials, and inclined settling. A filtration efficiency of 70 ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL·min^?1 and an energy consumption of 3.6 ± 0.9 kWh·m^?3. The effects of the applied power, flow rate, inlet cell concentration, and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at 100 W L^?1 of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50 ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:414–423, 2015     

Prediction and verification of centrifugal dewatering of P. pastoris fermentation cultures using an ultra scale-down approach

Recent years have seen a dramatic rise in fermentation broth cell densities and a shift to extracellular product expression in microbial cells. As a result, dewatering characteristics during cell separation is of importance, as any liquor trapped in the sediment results in loss of product, and thus a decrease in product recovery. In this study, an ultra scale-down (USD) approach was developed to enable the rapid assessment of dewatering performance of pilot-scale centrifuges with intermittent solids discharge. The results were then verified at scale for two types of pilot-scale centrifuges: a tubular bowl equipment and a disk-stack centrifuge. Initial experiments showed that employing a laboratory-scale centrifugal mimic based on using a comparable feed concentration to that of the pilot-scale centrifuge, does not successfully predict the dewatering performance at scale (P-value <0.05). However, successful prediction of dewatering levels was achieved using the USD method (P-value ≥0.05), based on using a feed concentration at small-scale that mimicked the same height of solids as that in the pilot-scale centrifuge. Initial experiments used Baker's yeast feed suspensions followed by fresh Pichia pastoris fermentation cultures. This work presents a simple and novel USD approach to predict dewatering levels in two types of pilot-scale centrifuges using small quantities of feedstock (<50 mL). It is a useful tool to determine optimal conditions under which the pilot-scale centrifuge needs to be operated, reducing the need for repeated pilot-scale runs during early stages of process development.     

Competition between brown trout and Atlantic salmon parr over pool refuges during rapid dewatering

Variations in distributions and behaviours of Atlantic salmon Salmo salar in allopatry (homogeneous) and in sympatry with brown trout Salmo trutta (mixed) were observed before, during and after 2 day periods of dewatering in a large glass-sided indoor stream at densities typical of Scottish upland streams. Brown trout utilized pools more than Atlantic salmon at normal flows and in both species the majority of fishes moved into pools during dewatering. There was no significant effect of brown trout, which was the more dominant species, on the overall ability of Atlantic salmon to use pool habitat as a refuge during dewatering. Within mixed and homogeneous groups, average feeding levels decreased during dewatering. The highest ranking fish, which was always a brown trout in mixed groups, predominantly monopolized the pool and other individuals in pools adopted a more cryptic, stationary behaviour. Dewatering effectively increased local population density with the result that dominance status became much more important in maintaining food intake, and polarization between the top ranking fish and others increased. During the first day of dewatering, there was extreme behavioural polarization such that the dominant fish exhibited most aggression and least feeding within the group. Among dominant fish on the second day of dewatering, aggression had largely abated and feeding had returned to pretreatment levels despite the reduced average feeding within the group. The main difference between mixed and homogeneous groups was in the behaviour of the most dominant Atlantic salmon, which was near-despotic in allopatry and subordinate to brown trout in sympatry.     

A method for concentrating dilute solutions of macromolecules

A simple, inexpensive acrylate polymer which has a capacity to absorb 170 ml of water per g has been developed. It can be used to concentrate dilute solutions of macromolecules such as proteins, nucleic acids, and carbohydrates. The polymer absorbs only low-molecular-weight substances such as glucose, sucrose, and inorganic salts. It can replace the various conventional concentration methods. No special device or electricity is needed for the concentration. The inexpensive polymer, molded in the form of rods, can be very conveniently used as “disposable concentration sticks.”     

Evaluation of electro‐coagulation–flocculation for harvesting marine and freshwater microalgae

Although microalgae are considered as a promising feedstock for biofuels, the energy efficiency of the production process needs to be significantly improved. Due to their small size and low concentration in the culture medium, cost‐efficient harvesting of microalgae is a major challenge. In this study, the use of electro‐coagulation–flocculation (ECF) as a method for harvesting a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum) microalgal species is evaluated. ECF was shown to be more efficient using an aluminum anode than using an iron anode. Furthermore, it could be concluded that the efficiency of the ECF process can be substantially improved by reducing the initial pH and by increasing the turbulence in the microalgal suspension. Although higher current densities resulted in a more rapid flocculation of the microalgal suspension, power consumption, expressed per kg of microalgae harvested, and release of aluminum were lower when a lower current density was used. The aluminum content of the harvested microalgal biomass was less than 1% while the aluminum concentration in the process water was below 2 mg L⁻¹. Under optimal conditions, power consumption of the ECF process was around 2 kWh kg⁻¹ of microalgal biomass harvested for Chlorella vulgaris and ca. 0.3 kWh kg⁻¹ for Phaeodactylum tricornutum. Compared to centrifugation, ECF is thus more energy efficient. Because of the lower power consumption of ECF in seawater, ECF is a particularly attractive method for harvesting marine microalgae. Biotechnol. Bioeng. 2011;108: 2320–2329. © 2011 Wiley Periodicals, Inc.     

Challenges and opportunities in the use of iron in water and wastewater treatment

Even though iron salts are widely used in waterand wastewater treatment, many questionssurrounding the use of this element remain.Issues surrounding the impact of mixingconditions on floc structure and strength, theeffect of reduction of ferric iron species onanaerobic sludge properties (includingdewatering) and the possible options for choiceof ligand in the modified photo-Fenton processfor contaminant oxidation are discussed andaspects requiring further attention in theseand other areas involving iron transformationspresented.     

Dewatering of algal suspension using microfiltration with cross flow in the presence of magnetite as a filter aid

Dewatering algal suspensions is an important step in the extraction of oil and other useful substances from algae. In this study, spherical Nannochloropsis sp. and ellipsoidal Monoraphidium sp. suspensions were dewatered in the presence of different amounts of 350-nm magnetite particles using a microfiltration membrane with 360-nm pores in cross-flow mode. Magnetite functions as a filter aid by reducing the deformation of the cake of filtered algae on the membrane and providing paths for water to flow through the filtration cake of algae. In the case of Nannochloropsis sp., the highest dewatering rate was obtained when the number ratio, defined based on the size and ideal density, between Nannochloropsis sp. and magnetite was 1:12.5, but the addition of magnetite had no observable effect on the filtration of ellipsoidal Monoraphidium sp. suspensions through the membrane. After dewatering, magnetite was effectively separated from the concentrated algal suspension by the application of a magnetic field in an open flow system. Magnetite has the potential to enhance dewatering performance using a cross-flow membrane system.     

Ultra scale‐down device to predict dewatering levels of solids recovered in a continuous scroll decanter centrifuge

During centrifugation operation, the major challenge in the recovery of extracellular proteins is the removal of the maximum liquid entrapped within the spaces between the settled solids–dewatering level. The ability of the scroll decanter centrifuge (SDC) to process continuously large amounts of feed material with high concentration of solids without the need for resuspension of feeds, and also to achieve relatively high dewatering, could be of great benefit for future use in the biopharmaceutical industry. However, for reliable prediction of dewatering in such a centrifuge, tests using the same kind of equipment at pilot‐scale are required, which are time consuming and costly. To alleviate the need of pilot‐scale trials, a novel USD device, with reduced amounts of feed (2 mL) and to be used in the laboratory, was developed to predict the dewatering levels of a SDC. To verify USD device, dewatering levels achieved were plotted against equivalent compression (Gt_comp) and decanting (Gt_dec) times, obtained from scroll rates and feed flow rates operated at pilot‐scale, respectively. The USD device was able to successfully match dewatering trends of the pilot‐scale as a function of both Gt_comp and Gt_dec, particularly for high cell density feeds, hence accounting for all key variables that influenced dewatering in a SDC. In addition, it accurately mimicked the maximum dewatering performance of the pilot‐scale equipment. Therefore the USD device has the potential to be a useful tool at early stages of process development to gather performance data in the laboratory thus minimizing lengthy and costly runs with pilot‐scale SDC. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1494–1502, 2013