Academic journal article Journal of Environmental Engineering and Science

Effect of Media Volume on Mixing of Biological Aerated Filters

Academic journal article Journal of Environmental Engineering and Science

Effect of Media Volume on Mixing of Biological Aerated Filters

Article excerpt

Abstract: Effect of reduced media volume on the mixing of biological aerated filters (BAF) was quantitatively determined. The degree of mixing was evaluated based on Reynolds numbers (Re) while the nonideality of the flow was assessed from information of residence time distribution. At HRTs of 1411, 1111, 781 and 581 min and their, respectively, approximate OLRs of 2, 3, 4, and 6 kg COD x [m.sup.-3] x [d.sup.-1], the Re in the partial-bed were 40300, 36100, 25500, and 40400, respectively, whilst those of the full-bed were 19500, 20500, 11700, and 26400. Porosity of the BAF is a bed characteristic resulting from the balance between the effect of biomass accumulation due to growth and biomass loss due to shear stress, arising from increased organic loadings, gas flow, and increased upflow velocity. The number of tanks, calculated from the residence time distribution data, is either 1 or 2 (rounded up to an integer). This indicates the occurrence of a completely mixed pattern inside both reactors.

Key words: mixing, biological aerated filters, Reynolds number, residence time distribution, dead volumes.

Resume : L'effet de la reduction du volume du milieu de fixation sur le melange des filtres biologiques aeres (BAF) a ete determine de maniere quantitative. Le niveau de melange a ete evalue en se basant sur le nombre de Reynolds (Re) et le caractere non ideal de l'ecoulement a ete evalue a partir d'information sur la distribution du temps de residence. A des temps de retention hydraulique (HRT) de 1411, 1111, 781 et 581 minutes et leur taux de charge organique (OLR) de 2, 3, 4 et 6 kg DCO x [m.sup.-3] x [j.sup.-1], le Re dans le lit partiel etait de 40 300, 36 100, 25 500 et 40 400 respectivement, alors que le Re dans le lit complet etait de 19 500, 20 500, 11 700 et 26 400 respectivement. La porosite des BAF est une caracteristique des lits resultant de l'equilibre entre l'effet de l'accumulation de la biomasse en raison de la croissance et la perte de biomasse causee par le cisaillement. Ce cisaillement decoule de l'augmentation des charges organiques, de l'ecoulement des gaz et de la vitesse accrue du courant ascendant. Le nombre de reservoirs, calcule en se basant sur les donnees de distribution du temps de residence, est de 1 ou de 2 (arrondi au nombre entier superieur). Cela indique un patron de melange complet a l'interieur des deux reacteurs.

Mots-cles : melange, filtres biologiques aeres, nombre de Reynolds, distribution du temps de residence, volumes morts.

[Traduit par la Redaction]

Introduction

Uniformity and effective contact between substrate and biomass in a fixed bed reactor system play an important role in its performance and behaviour (Nabizadeh et al. 2000). This has been well established over a range of scales and types of bioreactors (Smith et al. 1996; Murphy 1971; Monteith and Stephenson 1981; Guillard and Tragardh 1999; Levenspiel 1999). In other words, knowledge of the degree of mixing and the flow pattern within a reactor is vital before evaluation on the reactor's performance be made.

Quantification on the degree of mixing inside biological aerated filters is rather complex. Nabizadeh et al. (2000) for example stressed the effects of water flow, air supply, shape and morphological characteristics of packed media and filter porosity in aerated submerged fixed film reactors. Almost all of the parameters mentioned above change in sympathy with organic loading rate (OLR), i.e., kg COD/[m.sup.3]. Properties of a fluid phase such as viscosity and density influence the flow velocity and eddy patterns within a liquid stream, as do liquid-solid and liquid-gas interfaces. Friction occurs and free movement of eddies is hindered at the interfaces (Wilderer et al. 1995).

To account for all the inertial and viscous forces in the full- and partial-bed reactors, the Reynolds number (Re), a dimensionless parameter was selected. Although there is an abundance of literature on the topic of packed bed reactor hydraulics (Muslu 1986; Smith et al. …

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