Detachment is one of the major processes determining the physical structure and microbial functionalities of biofilms. To predict detachment, it is necessary to take the mechanical properties of the biofilm and the effect of both hydrodynamic and growth conditions into account. In this work, experiments were conducted with biofilms developed under various shear stresses and with various substrate natures. In addition, two cases were considered in order to differentiate between the effect of hydrodynamic factors and growth factors: the biofilms were directly grown under the targeted shear stress (τ) condition or they were precultivated under very low shear stress (0.01 Pa) and then exposed to high shear stress in the range of 0.1–13 Pa. An exponential and asymptotic decrease of the biofilm thickness and mass with increasing τ was observed in both cases. On contrary density, expressed as the biofilm dry mass on a known substratum divided by the average thickness increased with τ. Denitrifying biofilms always showed greater thickness and density than oxic biofilms. These results showed the presence of a compact basal layer that resisted shear stresses as high as 13 Pa whatever the culture conditions. Above this basal layer, the cohesion was lower and depended on the shear stress applied during biofilm development. The application of shear stress to the biofilms resulted in both detachment and compression, but detachment prevailed for the upper part of the biofilms and compression prevailed for the basal layers. A model of biofilm structure underlying the stratified character of this aggregate is given in terms of density and cohesion.

Effect of shear stress and growth conditions on detachment and physical properties of biofilms

A mathematical model of a nitrifying expanded-bed reactor for the pretreatment of drinking water

An Alternative Method to Estimate Influent Concentration Using On-site Lab-scale Activated Sludge Reactors

Trickling filter process is focused on developing Asian countries in recent years as an alternative energy-saving wastewater treatment to conventional activated sludge systems. This paper investigated influential hydraulic operational parameters on the process performance using a nitrification reactor. The trickling filter process was expressed as 2-dimensional biofilm layers where the influent flowed to the bottom of the filter bed while oxygen was dissolved from the ambient air. The 4 m-height pilotscale trickling filter filled with tubular plastic media (1.5 cm × 1.5 cm, 371 m2/m3-reactor volume) was installed at a municipal wastewater treatment plant in Da Nang, Vietnam. During the 2-day hydraulic stress test, the nitrogen and hydraulic loadings were changed in a step-wise manner. Focusing on the nitrification which was a dominant oxygen uptake, the process performance was dynamically simulated with a modification of IWA Activated Sludge Model (International Water Association). The effluent concentrations of dissolved oxygen and inorganic soluble nitrogenous compounds were reasonably calculated under the hydraulic loading ranging between 2.4 and 18 m/h. The reactive wetted surface area of the filter seemed to increase when high linear velocity of the fluid was applied, which resulted in the increase of volumetric biological reaction rates.

Tran Van Quang

Papers

Dynamic Simulation of Trickling Filter Process with Hydraulic Stress Tests