CHEE 370 Waste Treatment Processes

1. CHEE 370Waste Treatment Processes Lecture #7 Sedimentation

2. ReviewScreening • First unit operation in WWTP • Device with uniform openings (usually) that is used to retain coarse solids and debris • Coarse screens (termed “Bar racks”) typically have uniform openings in the 6 to 150 mm size range • Can be cleaned either manually or mechanically

3. Headloss • hL = headloss (m) • C = empirical discharge coefficient to account for turbulence and eddy losses (clean screen = 0.7; clogged screen = 0.6) • V = velocity through the openings (m/s) • v = approach velocity in upstream channel (m/s) • g = acceleration due to gravity (9.81 m/s2)

4. Flow Equalization Figure 5-20, Metcalf and Eddy

5. Sedimentation • Removal of solid particles from a suspension by gravity settling • Sedimentation = settling • Separation based on density

6. Sedimentation • In WW treatment, sedimentation is used to separate solids from liquids • In most cases, the main purpose of sedimentation is to produce a clarified effluent • Sedimentation is also important in producing sludgewith a solids concentration that can be easily handled and treated

7. Application of Sedimentation in WW Treatment

8. Application of Sedimentation in WW Treatment • Grit Tanks: • Settling of sand, gravel, and other heavy solids • Typically assume a specific gravity of 2.65 for the solids • Objectives for removal: • Protect pumps and other mechanical equipment • Reduce the formation of deposits in the pipes • Remove inert material that would accumulate unnecessarily during the biological treatment phase (minimizes the required bioreactor size)

9. Application of Sedimentation in WW Treatment • Primary Clarifier: • Sedimentation is used to remove suspended solids and settleable organic material • Two streams: • Primary sludge: rich in solids • Primary effluent: only ~ 40% of the solids content of unprocessed WW • Decreases the load of organic material that needs to be processed in the aeration tank (bioreactor) • Produces an organically-rich primary sludge that can be stabilized through anaerobic digestion

10. Application of Sedimentation in WW Treatment • Secondary Clarifier: • Following the aeration tank (bioreactor) • Sedimentation of the biomass (i.e. bacteria) to produce an effluent with a low solids concentration • Two streams: • Effluent: low in solids < 25 mg/L • Return activated sludge: high in solids ~ 5000 mg/L

11. Application of Sedimentation in WW Treatment • Sludge Thickening: • Performed on the biomass that is wasted from the aeration tank • Further separates water from the biomass • Objective is to produce a highly concentrated stream of wasted biomass to minimize the volume sent to the anaerobic digester • Reduces the size requirement for the anaerobic digester by minimizing the volume of the wasted biomass that must be processed

12. Types of Settling • Type I: Discrete Settling • Type II: Flocculent Settling • Type III: Hindered or Zone Settling

13. Type I Settling Discrete Settling • Settling of discrete, non-flocculating particles • Particles settle as individual entities at a constant velocity • Minimal interaction between particles • Applies only to particles in a suspension with a low solids concentration

14. Type I Settling Discrete Settling • Where it occurs in WWTP: • Grit tanks • Top zone of the primary clarifiers • In order to design a settling tank, the settling velocities must be determined • The physical properties of a particle determine its settling velocity

15. Frictional Drag Force FD Bouyant Force Fb Gravitational Force Fg Type I Settling Discrete Settling

16. Modelling Discrete SettlingAssumptions Using Newton and Stoke’s laws: • Particle is discrete • Particle size and shape do not change • Infinite sized vessel • Viscous fluid • Single particle • DERIVATION IN CLASS

17. Reynold’s Number • The Reynold’s number for settling particles is defined as: • Where: •  = dynamic viscosity (N•s/m2) Eq’n 4

18. Dependence of the Drag Coefficient on the Reynold’s Number Figure 5-20, Metcalf and Eddy

19. Modelling Discrete SettlingReynold’s Number • For particles that are approximately spherical • When Re < 1 (laminar flow range) • Viscosity is the predominant force governing the settling process Eq’n 5 Eq’n 6

20. Modelling Discrete SettlingSettling in the Laminar Region • When Re < 1 (laminar flow range) • Substituting Eq’n 6 into Eq’n 3: • If the size distribution profile of the particles is known, you can solve for the settling velocity Eq’n 7

21. Modelling Discrete SettlingSettling in the Transitional to Turbulent Region • When 1 < Re < 10000 • For spherical particles, the complete form of Eq’n 5 is used to describe the coefficient of drag • The settling velocity distribution of the particles can be solved using an iterative method with Eq’n 3 and Eq’n 5

22. Modelling Discrete SettlingIterative Approach • Assume the settling velocity (you could start by calculating it from Eq’n 7 as a first guess) • Calculate NR from Eq’n 4 and confirm Re >1 • Calculate CD from Eq’n 5 (Or Figure), then calculate new vp(t) from Eq’n 3 • Compare the new vp(t) with the assumed value in step 1 • If they are equal, you have the answer. • Otherwise, use the new vp(t) as the assumed value and go back to step 1.

23. Sedimentation Tanks • WW containing particles enters the clarifier at one end and the effluent (cleaner WW, overflow) flows out the other end • The WW must be in the basin for sufficient time period to remove particles of a desired size • Smaller particles require longer settling times

24. Clarifier Design • Select a particle size that will have a terminal velocity of vo • vp(t) ≥ vo • Larger particles • 100% removal of particles (i.e. the particles will remain in the tank) • vp(t) ≤ vo • Smaller particles will be partially removed

25. Practice Problem • Determine the settling velocity for a sand particle with an average diameter of 0.4 mm and a specific gravity of 2.65, settling in water at 25 °C (dynamic viscosity value is 0.890 x 10-6 N•s/m2).