Sedimentation

Question 1

Solid flux

Answer 1

• batch solid flux = product of settling velocity and biomass concentration X
• continuous solid flux = sum of batch solid flux and underflow (or bulk) flux
• proper operation requires that:
  > incoming flux G’ of solids are removed via the underflow (= Gb (flux due to settling) + Gu (flux due to drain))
  > incoming flux G’ (horizontal line) has to be situated below the (Gb + Gu) curve

Question 2

Settling Zones

Answer 2

Picture

Question 3

Settler Design: Sludge Loading/Thickening Criterion

Answer 3

• the sludge loading rate of a settler has to be such that a settler is able to:
  > treat a sufficient amount of suspended solids = remove the incoming solids via the underflow
  > deliver a sludge at a certain (pre-defined high) sludge concentration)
• to determine the area needed, the limiting solids Gmin (approximately based of field experiments) of the clarifier is taken from G
• equation
• Yoshlokas graphic mehtod:
  > for a selected value of XR and starting from the point (XR,0) a tangent is drawn to the batch solid flux curve
  > the intersection with the ordinate = Gmin
• both Xr and X are control variable to be chosen properly during the dimensioning of activated sludge tanks
• recycle ration can be approximated by: r = Qr/Q = Cx/Cx,r - Cx where Cx,r is the total ss conversion to organic SS needed (Cx,r = 1000/SVI)

Question 4

Settler Design: Hydraulic Loading/Effluent Criterion

Answer 4

• high clarification degree
• no solids dragged away with the overflow
  q0 = Qc/A = Qc*(1-w)/A

Question 5

Secondary Parameters of Settler Design

Answer 5

– for the hydraulic stability of the water volume
– to eliminate dead zones
> rectangular tanks (primary settling) L/H = 20 – 35
> circular tanks (secondary settling) diameter/H = 15 – 25
– sludge retention times in the clarifiers should be limited from 2 to 6 h to avoid denitrification (if there are still organic components and nitrates in the water) or P-release (if there are still organic components in the water that could induce P-release in anaerobic conditions (= first phase of “P-removal”)