Mass Transfer Features of Wavy-Bottomed Cascade Photobioreactors

It has been suggested that the energy-efficient production of microalgae biomass can be more easily obtained in short light path photobioreactors that can be operated at high biomass concentration. On the downside, however, high biomass concentrations also require an efficient gas exchange rate to avoid metabolic growth limitation or inhibition. A cascade photobioreactor featuring a thin liquid layer flowing down a sloping, wavy-bottomed surface can be operated at a biomass concentration that is much higher compared to most usual open-type equipment. Liquid flow, upon investigation, proved to exhibit peculiar “local recirculation” hydrodynamics, potentially conducive to the mixing of superficial and deep zones of the photobioreactor. Mass transfer coefficient represents a useful parameter to optimize the performance of a microalgal photobioreactor and its scale-up. The aim of the present article is to discuss the experimental mass transfer features of this novel type of photobioreactor and highlight expected opportunities and issues entailed by different ways of installing and operating such novel types of photobioreactors.

Effect of magnetic field on biomass properties and their role in biodegradation under condition of low dissolved oxygen

Low condition of dissolved oxygen (DO) is commonly associated with sludge bulking problem that was able to disrupt the efficiency of wastewater treatment performances. Relatively, very little attention was paid to the possibility of applying magnetic field in controlling the bulking problem. Hence, this study aims to investigate the performance of magnetic field on biomass properties and its effect on biodegradation under low condition of DO. Two continuous laboratory-scale sequencing batch reactors—Reactor A (SBRA) and Reactor B (SBRB)—were setup. SBRA was equipped with the magnetic device to exhibit magnetic field of 88 mT, while SBRB acted as a control system. The results showed that the biomass concentration in SBRA was higher compared to SBRB. High biomass concentration in SBRA resulted to better settleability with mean SVI of less than 30 mL/g. SBRA also showed consistently high removal performances of organic and inorganic contents compared to SBRB. These observations confirmed that the magnetic field was able to enhance the biomass properties, which further enhance the biodegradation ability of the aerobic bacteria under low DO condition. This also indicates that under the sludge bulking circumstances, the use of magnetic field stands a great chance in maintaining high biodegradation of the treatment system.

Second generation bioethanol production at high gravity of pilot-scale pretreated wheat straw employing newly isolated thermotolerant yeast Kluyveromyces marxianus DBTIOC-35

Application of thermotolerant yeast Kluyveromyces marxianus DBTIOC-35 in SSF decreases overall process time, and increases productivity and yield by allowing elimination of presaccharification step and use of high biomass concentration, respectively.

Investigation of the Effect of Growth From Low to High Biomass Concentration Inside a Photobioreactor on Hydrodynamic Properties of Scenedesmus obliquus

Most of the current production cost in algae biodiesel plants utilizing photobioreactors comes from the high energy required for pumping, CO2 transfer, mixing, and harvesting. Since pumping affects the mixing and CO2 transfer, which are the main factors in algae productivities, solutions to reduce the required energy for pumps can significantly make algae biodiesel production more economically feasible. An investigation on the effect of Scenedesmus obliquus’s growth from low to high biomass concentration inside a horizontal tubular photobioreactor to determine the impact that it has on hydrodynamic performances, which will affect cost and production efficiency, was performed. As the biomass concentration increased, the algal culture was found to remain Newtonian. Additionally, the biomass concentration (expressed in cell density) was found to have lower viscosity even at the highest concentrations evaluated at 2.48 × 108 cell/ml (1.372 × 10−3 ± 1.32 × 10−4 Pa s) compared to the Modified Bold’s 3N medium (1.408 × 10−3 ± 9.41 × 10−5 Pa s). Furthermore, the total energy consumption does not appear to depend on the S. obliquus biomass concentrations, but rather on the medium the algae grows in. The rheological properties of autotrophic algae will not have significant impact on energy requirements until technology improves so that the concentrations reach those of heterotrophic algae.

Apparent hydrogen consumption in acid reactors: observations and implications

Investigations of hydrogen production by dark fermentation have received increasing attention as a green fuel production process. Research focus is mainly on yields and rates of hydrogen production under different operation conditions. The importance of hydrogen consumption is addressed here, based on results from lab-scale reactors. Experiments were run using mixed cultures and a variety of operating conditions: HRT 6-40 hours; temperature 25–55°C. Initial hydrogen yields between 0.8–1.5 mol H2/mol glucose and ≈50% H2 in headspace was observed, followed by a decrease in hydrogen production as the culture matures, resulting in hydrogen yields down to 0.02 mol H2/mol glucose. It is concluded that hydrogen or “hydrogen equivalents” consumption is significant, especially in reactors with high biomass concentration and/or high sludge age. Sustainable H2 production by dark fermentation alone is therefore not likely to be developed. The results suggest that it is possible to control and avoid significant H2 production in dark fermentation. Minimizing H2 production can be useful in preparation of organic feed for other bio-fuel production processes, such as methanogenic processes and bio-electrochemical H2 production.

High rate treatment by aerobic upflow sludge blanket (AUSB) with external oxygenation

A 3-year study was conducted using an aerobic upflow sludge blanket (AUSB) reactor to achieve high-rate biological treatment through maintenance of a high biomass concentration (7–10 g VSS/L) together with a high oxygen flux. The AUSB reactor was not aerated directly; instead, oxygenation was provided in an external chamber with flow recirculation. The oxygenation was provided at four different ressurizations: 0, 15, 20 and 25 psig. The AUSB reactor was also pressurized to avoid the flotation of biomass. The flow recirculation rate was varied from 400% to 1,500% in order to ensure adequate oxygen supply. It was found that the AUSB system was able to handle a volumetric loading of as high as 10 kg COD/m3-day with a removal efficiency of 92%. Despite a high upflow velocity through AUSB, the effluent suspended solid concentration was mostly below 60 mg/L. The active fraction of biomass in the AUSB sludge was about 3 times higher than that of the regular activated sludge. This was indicated by a very high specific oxygen uptake rate (SOUR), up to 180-250 mg O2/g VSS-hr. The sludge yield in the entire system was only 0.09 to 0.13 g VSS/g COD removed. This was mainly caused by additional auto-oxidation of biosolids in the oxygenation chamber due to flow recirculation.