On the Reduction of Power Consumption in Vortexing Unbaffled Bioslurry Reactors

Abstract

Bioremediation of polluted soils via bioslurry reactors is an interesting option among those available nowadays, especially when recalcitrant pollutants are present. Vortexing unbaffled stirred tanks may be a valuable choice to this purpose as they were recently found to be more efficient than baffled vessels for solid suspension processes where mixing time is not a controlling factor. When operated at sufficiently high agitation speeds, the central vortex bottom reaches the impeller and air bubbles start to be distributed throughout the system, thus avoiding any sparger and related clogging issues. In the present work, a vortexing unbaffled stirred tank with solid loadings ranging from 2.5% w/w (weight of the solid/weight of the liquid) up to the very high 160% w/w was studied. Different turbine types including Rushton turbine, up- and down-pumping pitched blade turbines, and A310 were investigated. The minimum impeller speeds for complete particles’ suspension (N js) and system aeration (N cr) along with the relevant power (P js , P cr) and specific power (per mass unit) consumptions (εjs, εcr) were assessed, in order to identify the geometrical configuration and operating condition providing the lowest power consumption. Results showed that the Rushton turbine and a solid concentration B of about 30% may be the most economically convenient arrangement to achieve system aeration and complete particles’ suspension at the same time inside the reactor.

Research progress of bio-slurry remediation technology for organic contaminated soil

“…80 Aer the pretreatment process of adding nutrients, inoculating bacteria, and adjusting pH, the contaminated soil enters the bioslurry reactor. 81 The bio-slurry remediation process uses mechanical or pneumatic mixing to keep the soil in suspension, and the soil-to-water ratio is usually maintained at 10-30% w/v. 63 Compared with other bioremediation technologies, bioslurry remediation technology has some unique advantages, mainly reected in: (1) high mass transfer rate, which can signicantly improve the contact efficiency of microorganisms/ pollutants/nutrients; 82 (2) the biodegradation rate of organic pollutants is signicantly higher than in situ bioremediation; (3) multiple electron acceptors can be used: O 2 , SO 4 2− , CO 2 , NO 3 − ; 83 (4) controllable and adjustable environmental parameters, such as temperature, pH, etc.…”

“… 80 After the pretreatment process of adding nutrients, inoculating bacteria, and adjusting pH, the contaminated soil enters the bio-slurry reactor. 81 The bio-slurry remediation process uses mechanical or pneumatic mixing to keep the soil in suspension, and the soil-to-water ratio is usually maintained at 10–30% w/v. 63 …”

“…Mixing strength is another key factor in the design of the bio-slurry remediation processes, which directly affects the degradation performance of organic matter. 81 Continuous mixing during remediation not only maintains the homogeneity of the slurry during the remediation process but also enhances turbulence and improves mass transfer rates. 75 Appropriate mixing strength can keep solid particle suspension and slurry homogeneity.…”

“…Especially for hydrophobic organic pollutants such as PAHs and PCBs, adequate aeration of the aerobic slurry bioremediation process can significantly improve the biodegradation rate. 81 There are various types of mixing devices, and the aerobic bio-slurry remediation device is mainly mechanical mixing and pneumatic mixing device. 90 Hybrid modes include batch and continuous.…”

An experimental and numerical analysis of the fluid flow in a mechanically agitated vessel

“…F. Scargiali et al [4] analyse the influence of Reynolds and Froude numbers on power consumption characteristics in the case of unbaffled stirred tanks. Authors found that the most critical factor in decreasing power consumption and mixing quality is the shape of the impeller.…”