Fundamental Concepts: Sedimentation and Flow Equalization

1. Fundamental Concepts: Sedimentation and Flow Equalization

2. Sedimentation • Sedimentation is the gravitational accumulation of solids at the bottom of a fluid (air or water)

3. Uses for Sedimentation • Removal of solids from drinking water • Removal of solids from waste waters • Settling of solids from air emissions • Removal of solids from runoff water

4. Types of Settling Four types of sedimentation occur: • Discrete settling • Flocculant settling • Hindered settling • Compression

5. Examples of Settling Types Discrete Flocculant Hindered

6. Types of Sedimentation • In discrete settling individual particles settle independently • It occurs when there is a relatively low solids concentration

7. Types of Sedimentation • In flocculant settling, individual particles stick together into clumps called flocs • This occurs when there is a greater solids concentration and chemical or biological reactions alter particle surfaces to enhance attachment

8. Types of Sedimentation • In hindered settling, particle concentration is great enough to inhibit water movement • Water must move in spaces between particles

9. Types of Sedimentation • Compression settling occurs when particles settle by compressing the mass below

10. Factors Influencing Sedimentation • Particles • size • density • shape • charge • Fluid • velocity • density • viscosity

11. Sedimentation Rate • Stoke’s Law • Used for spherical particles • Assumes no fluid mixing, so usually will not work for gasses

12. Sedimentation Rate vp = particle settling velocity (m/s or ft/s) p = particle density (kg/m3 or lbm/ft3) w = fluid density (kg/m3 or lbm/ft3) d = particle diameter (m or ft) g = gravitational acceleration (9.81 m/s2 or 32.2 ft/s2) μ = dynamic viscosity (kg/m·s or lbm/ft·s)

13. Properties of Water • http://www.thermexcel.com/english/tables/eau_atm.htm • 1 lbm/ft·s = 1.48816 kg/m·s

14. Applications • Stoke’s Law can be used to determine the surface area of a settling tank • The critical velocity for settling is equal to the settling velocity of the smallest particle to be completely removed • The overflow rate is equal to the volumetric flow rate into the tank divided by the surface area OFR = Q/A • Setting the overflow rate equal to the critical settling velocity allows time to capture the smallest particles of interest

15. Applications OFR = over flow rate (m/s or ft/s ) vc = critical settling velocity (m/s or ft/s) Q = the flow rate into the basin (m3/s or ft3/s) A = the surface area of the basin (m2 or ft2)

16. Flow Equalization (BAEN 465) • Flow rates to treatment plants vary throughout the day • Treatment processes are more stable if the flow rate is consistent • Equalization tanks store wastewater during peak flow periods and are drawn down during low flow to keep downstream flow constant • These also help smooth out fluctuations in BOD concentration to downstream processes

17. Equalization Tank Design • Need information on daily flow cycle (input): • average flow rate for each time interval • BOD concentration for each time interval • Assume wastewater leaves the tank at a constant flow rate (average rate) • Tank must be large enough to store all input in excess of daily average flow (output) and may allow for additional storage • Mass balances on BOD determine changes in concentration in tank effluent going to downstream processes

18. Equalization Tank Design Time when flow exceeds avg flow

19. Equalization Tank Design • Rearrange table so first row is time when flow first exceeds the average flow rate:

20. Equalization Tank Design • Now calculate the difference between flow into and out of tank (ΔV) and the cumulative volume in the tank (Σ(ΔV)) for each time period:

21. Equalization Tank Design • Minimum storage volume required • Maximum value of Σ(ΔV) • In preceding example this is 5897 m3 • May design for additional storage, e.g., for downstream processes being shut down for a period of time.

22. Equalization Tank Design

23. Equalization Tank Design • Mass balance on BOD5 for example: