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In conventional activated sludge (CAS) systems and membrane bioreactors (MBR), aeration, mixing and filtration are affected by various properties of activated sludge (AS) and especially viscosity and morphological properties of flocs. The aim of the present study was to understand the relationship between hydrodynamic conditions and AS floc size properties. Floc size distribution (FSD) measurements were performed in four systems showing different hydrodynamics: (i) a standard stirred reactor (SR), (ii) a pilot bubble column (BC), (iii) a full-scale aeration zone equipped with fine bubble diffusers, (iv) a full-scale filtration zone equipped with coarse bubble diffusers. The measurements were carried out on-site, using the mixed liquor from a full-scale MBR. The FSD of the given activated sludge mainly depends on the average shear rate ( γ ) and on the circulation time in the SR. To a lesser extent, local properties, such as maximal shear rate, also influence the FSD. In the same way, the main operating parameter governing the FSD in the BC was γ . For a similar γ , the flocs were slightly larger in the BC than in the SR. In addition, in the full-scale systems, for a similar γ estimated, the FSD characteristic diameters were lower in the filtration zone (with coarse bubbles) than in the aeration zone (with fine bubbles). The FSD in the aeration zone was well correlated with the FSD in the BC whereas the flocs were slightly smaller in the filtration zone than in the BC.

Oxygen transfer in wastewater treatment is significantly influenced by the mixed liquor suspended solids (MLSS). The effect is more pronounced at MLSS concentrations higher than 20 g L−1 when supplying air by conventional diffused aeration systems. The oxygen transfer performance of a supersaturated oxygenation technology (i.e., the supersaturated dissolved oxygen (SDOX) system) was evaluated in clean water and in activated sludge with MLSS concentrations from 4 to 40 g L−1 as a promising technology for uncapping such limitation. The evaluation was carried out at the laboratory facilities of the faculty of food technology and biotechnology at the University of Zagreb. The sludge was collected from a full-scale conventional activated sludge (CAS) wastewater treatment plant (WWTP) operated at a solid retention time (SRT) of approximately 5 days. The evaluation was carried out using a laboratory-scale setup consisting of a bench-scale SDOX system (2.75 L) supplying pure oxygen to a 5 L biological reactor. The SDOX exhibited oxygen mass transfer rate coefficient (KLa) values (2.6 h−1) in clean water lower than for fine bubble diffusers (11 h−1). However, higher oxygen transfer rate (OTR) values and alpha factors (mass transfer ratio of process-water to clean-water) as a function of the MLSS concentration were observed. A standard oxygen transfer efficiency (SOTE) of approximately 100 % in clean water was reported. The SDOX technology can be presented as a promising alternative for supplying dissolved oxygen (DO) into mixed liquor solutions; particularly, at the high MLSS concentrations required by high-loaded membrane bioreactor (HL-MBR) systems and aerobic digesters.

International audience; Mixing and aeration characteristics obtained on three full scale loop reactors for 28 configurations (different mixers' location and rotation speed, air flow rates and diffuser grids' location) were analyzed. The propellers' thrust was measured in situ. Observed values vary with the tank geometry, the location of the mixers and the induced axial velocity. In situ thrust values are lower than the nominal ones obtained during shop tests, by 10 to 30 %. For the nominal rotation speed of the propellers, observed thrust-to-power ratios are comprised between 685 and 875 N kW-1. In agreement with previous results, an increase in the oxygen transfer rate with the axial velocity was observed, due to a partial neutralization of the spiral flows formed at the edge of the aerated zones. Aeration efficiency is not decreased, as the additional power consumption due to mixing is offset by the oxygen transfer enhancement. SAE values are comprised between 2.0 and 2.6 kg O2 kWh-1. Such on site results are especially important to be used as input and output data in CFD models.

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 24 full scale annular ditches, were interpreted using dimensional analysis. Two relationships have been developed between the oxygen transfer efficiency per meter of submergence (SOTEs) and the parameters affecting oxygen transfer: air flow rate (QG), depth of submergence (h), surface area (S), membrane surface (Sp), aerated surface (Sa), width of the basin (W), angle between mixers and diffusers (Ang) and horizontal velocity (UC). These relationships differentiate annular ditches according to the diffuser placement: tank coverage higher than 50 % of the surface area (Type I) or at the maximum 50 % of the area (Type II). They can be used to predict oxygen transfer performances, at a design stage.; Les performances d`oxygénation standard en eau claire des systèmes d`insufflation d`air en fines bulles équipant 24 chenaux d`aération annulaires ont été interprétées à l`aide de l`analyse dimensionnelle. Deux relations ont été développées entre le rendement d`oxygénation spécifique standard (SOTEs) et les paramètres influençant le transfert d`oxygène : le débit d`air (QG), la hauteur d`immersion des diffuseurs (h), la surface du bassin (S), la surface totale de membrane perforée (Sp), la surface totale des zones occupées par les diffuseurs (Sa), la largeur du chenal (W), l`angle entre les agitateurs et la première rampe de diffuseurs (Ang) et la vitesse horizontale de circulation (UC). Ces relations différencient les chenaux selon la disposition des diffuseurs : surface de bassin couverte supérieure à 50 % du radier (Type I) ou au plus 50 % du radier (Type II). Elles peuvent être utilisées pour prédire les performances d`oxygénation du couple chenal d`aération/système d`aération, au moment du dimensionnement de l`installation.

[Departement_IRSTEA]GMA [TR1_IRSTEA]21 - TECHNEAU / EPURE; On the basis of full-scale data from 58 clean water tests performed in 26 activated sludge tanks equipped with fine bubble diffusers and of a theoretical approach, it can be stated that fine bubble aeration systems with total floor coverage arrangement provide higher k(L) values and the lowest spiral liquid circulation. An efficiency criterion for oxygen transfer (N(T)) was defined on the basis of the dimensional analysis. The transfer number N(T) allows to take account of the impact of vertical liquid circulation movements on oxygen transfer. The values of N(T) calculated from the results of full scale nonsteady-state clean water tests vary from 5.3 10(-5) to 9.1 10(-5) and are directly dependent upon the arrangement of air diffusers. It has been evidenced that the highest transfer numbers corresponded to the total floor coverage arrangement and the average calculated N(T) values is 7.7 10(-5), independently of the diffuser density and of the gas velocity, over the ranges studied. The lowest transfer numbers are obtained when the diffusers are located in separate grids, and the transfer number is reduced with increasing air flow rate.; A partir des résultats de 58 mesures de performances d'oxygénation de systèmes d'insufflation d'air en fines bulles effectuées sur 26 sites réels et d'une approche théorique, il a été montré que les couvertures plancher de diffuseurs conduisent aux kLa les plus élevés et aux vitesses de circulation verticales du liquide les plus faibles. Un critère d'efficacité du transfert d'oxygène (N(T)) a été défini à partir de l'analyse dimensionnelle. Le nombre de transfert N(T) permet de rendre compte de l'impact des mouvements de circulation liquide verticale sur le transfert d'oxygène. Les valeurs de N(T) calculées à partir des résultats des essais de réoxygénation varient de 5.3 10(-5) à 9.1 10(-5) et dépendent directement de la disposition des diffuseurs. Il a été montré que les nombres de transfert les plus élevés correspondaient aux couverture plancher et la moyenne des N(T) calculés est de 7.7 10(-5), indépendamment de la densité de diffuseurs et des vitesses de gaz dans les gammes étudiées. Les nombres de transfert les plus faibles sont obtenus pour des configurations modulaires et sont d'autant plus faibles que le débit d'air est élevé.