26.27 A biofilm reactor with a well-mixed liquid phase shown below will be used to treat wastewater contaminated with trichloroethylene (TCE) at a concentration 0.25 mg/L (1.9 mmole/m³, MTCE 131.4 g/gmole). The available sur- face area of the biofilm in the reactor is 800 m², and the desired outlet concentration of TCE is 0.05 mg/L (0.05 g/m³). It can be assumed that in a well-mixed, continuous- ow reactor at steady state, the concentration of the solute of the liquid phase of inside the reactor is equal to the concen- tration of the solute in the liquid ow exiting the reactor. It may also be assumed that the TCE degradation in the biofilm, which of 8 = 100 µm thickness, proceeds by homogeneous first- order reaction kinetics. = a. What volumetric ow rate of wastewater is allowed to enter the reactor? The temperature of the process is constant at 20°C. b. What is the concentration of TCE in the biofilm at the point where the biofilm is attached to the surface? What fraction of 100 µm thickness of the biofilm is utilized? Potentially useful data*: KTCE = 4.31 s¯¹ (first-order rate constant for TCE in biofilm); DTCE-biofilm = 9.03 × 10-¹0 m²/s (diffusion coefficient TCE in biofilm). *J.P. Arcangeli, E. Arvin, Environ. Sci. Technol., 31, 3044 (1997). Waste water feed steam CAi = 0.25 g TCE/m³ V = ? Biofilm Well-mixed contactor Treated waste water CAo = 0.05 g TCE/m³

Liquid is flowing through a circular pipe in a laminar phase. Now, the fluid through the pipe is replace with a more viscous fluid and passed through the pipe again with the same velocity. What will be the nature of flow of the fluid? Eddies will star to form B The flow will become turbulent Insufficient data to define The fluid will remain laminar The flow will start to be transitional

1. Water (p= 1000 kg m-³ and μ = 0.0011 kg m-¹ s-1) flows at 5 m s-1 over a 0.3 m long flat plat. Determine if the flow in the boundary layer is laminar over the entire length of the plat or it becomes turbulent at a downstream location. If latter is the case, determine the location of transition from laminar to turbulent. [Ans. BL flow is Laminar & turbulent, x = 0.11 m]

Several different classifications of fluid flows were outlined - match each of the class pairs listed. Viscous A. Inviscid Extrernal B. Turbulent Incompressible C. Internal Laminar D. Compressible