General Question Test format: What types of questions should we expect? 7 multiple choice questions (5 points each) Short problems (similar to previous prelims) How far should we be able to extrapolate beyond what is in the notes? You should understand the fundamental concepts and be able to talk about those concepts with your friends Algebra and arithmetic are expected

Design of the float valve The float valve has an orifice that restricts the flow of the chemical and that affects the head loss. Different float valves have different sizes of orifices. Chemical Feed Tank Why are we concerned with the orifice head loss? Where else is there head loss? Is this a minimum or a maximum orifice diameter? Why not increase  h?

H Open channel supercritical flow Drop tube Dosing tubes Stock Tank of coagulant Lever LFOM Float Constant head tank Float valve Slider Purge valves

H Decrease dose

Increase dose H

H Half flow

Plant off

Dosing tubes What is the purpose of What is the purpose of the dosing tubes? What is the design constraint for the maximum flow rate in the dosing tubes? Why does the flow through the dose controller increase if the plant flow rate increases?

What are examples of major and minor losses? Major: Caused by shear with the solid surface Pipe walls Flocculator baffle surfaces (insignificant) Minor: Flow expansions (analogous to pressure drag) Orifice, elbow, valve, any place where streamlines are diverging! Velocity Shear (wall on fluid)

Origin of How is this equation used for rapid mix? How is this equation used for flocculator design? Draw a picture. What are the parameters in each case?

Orifice Diameter to Obtain Target Mixing (Energy Dissipation Rate, Kolmogorov Length Scale) Substitute for D Jet and solve for D Orifice The orifice must be smaller than this to achieve the target energy dissipation rate

Energy dissipation rate for uniform nanoglob application We need an energy dissipation rate of approximately 3 W/kg to ensure uniform application of nanoglobs to colloids! Below 10 NTU the mixing in the flocculator should be adequate!

How does aggregation occur? Coat flocs with nanoglobs of coagulant precipitate to make them sticky, then provide energy to create velocity gradients Collisions!!!!

Collision time dependence on floc diameter Why does one of them have a (d/do) term and the other doesn’t?

Successful Collision Models viscous inertia Time for one collision Number of successful collisions  is fractional surface coverage of colloids with adhesive nanoglobs d Floc is perhaps mean floc size of flocs that are capturing colloids For completeness we should probably include a correction for hydrodynamic effects that make it difficult for non porous particles to approach closely. This may increase the time for the first few collisions when flocs aren’t very porous Number of collisions is equal to time over collision time

When does the first order reaction rate apply to flocculation? The big question! What is C? The bigger question! Why this disappointing result?

Flocculation Model: Integrating and tracking residual turbidity The change in colloid concentration with respect to the potential for a successful collision is proportional to the colloid concentration (the fraction of the colloids swept up is constant for a given number of collisions) Integrate from initial colloid concentration to current colloid concentration Separate variables Classic first order reaction with number of successful collisions replacing time

Proposed Turbulent Flocculation Sedimentation Model (missing phase 1) Energy dissipation rate Flocculation time Fractional surface coverage of colloid by coagulant Initial floc volume fraction Sedimentation tank capture velocity Sed imentation velocity of ??? -log(fraction remaining) Characteristic colloid size Lambert W Function What does the plant operator control? ________ What does the engineer control? __________  What changes with the raw water? __________

Review Why is it that doubling the residence time in a flocculator doesn’t double pC* for the flocculator? Why does increasing floc volume fraction decrease the time between collisions? Which terms in the model are determined by the flocculator design?

Reflection Questions What are some alternate geometries? How else could you generate head loss to create flocs? 0.6S 0.4S

Design steps: Given H,W, Q,  Max, and  Calculate S Calculate   Calculate  B, N B, and h e Total For H/S>5 For H/S<5 Why only one  B ?

Cool Equations Ultimately I get to pick three values when designing a flocculator. I’m willing to use 40 cm of head loss and would like to provide 35 m 2/3 of collision potential. I’ll design an efficient flocculator. What is the required residence time and the maximum energy dissipation rate? What do you expect for the residence time?

Cool Equations I’m willing to use 40 cm of head loss and would like to provide 35 m 2/3 of collision potential. I’ll design an efficient flocculator. What is the required residence time and the maximum energy dissipation rate?

What is the maximum energy dissipation rate?

Reflection Questions How does the collision potential in a flocculator change with flow rate? What is the average energy dissipation rate in a flocculator? What is the maximum energy dissipation rate for small H/S?

A few Reflections The energy dissipation rate that we are using in design is the maximum (in the core of the energy dissipation zone right after the expansion) This implicitly suggests that the break up of flocs is the most important constraint It is also possible that total head loss (average energy dissipation rate) or maintaining the flocs in suspension is more important

A few Design Guidelines for High High head loss Small flocs Keeps flocs in suspension Slightly lower required residence time Perhaps goal is to produce flocs that have sedimentation velocities similar to upflow velocity in sedimentation tank for rapid formation of floc blanket Tradeoffs without clear guidance (yet)

Reflection Questions What are some alternate geometries? How else could you generate head loss to create flocs? 0.6S 0.4S

Estimate Residence Time Directly Use the collision potential equation to solve for residence time Need to know H/S to do this! If H/S<5

Reflection Questions What is the relationship between potential energy loss and the average energy dissipation rate in a flocculator? How did AguaClara get around the 45 cm limitation?

Review What is the symbol for collision potential? Approximately how much collision potential is required for a flocculator? What is  ? How does the non uniformity of  influence efficiency of energy use?

Two changes to for nonuniformity We currently design based on

What controls ɛ in a flocculator flocculator? What is ɛ controlled by? What change in geometry is required to change ɛ ? Flocculator design! What controls collision potential? What controls energy dissipation rate? Energy dissipation rate, residence time, number of baffles Baffle spacing

Flocculator Efficiency H/S = 4 H/S = 10  Can you clarify the H/S ratio limit?  Why does a transition happen at H/S=5?  Transition marks the geometry where the jet has the opportunity to fully expand before going around the next bend Energy dissipation zone limited by H Energy dissipation zone limited by S