1. Keynote 4.2 and 4.3: Settling Tanks and Drying Beds
Online Course Faecal Sludge Management
Developed by UNESCO-IHE and SANDEC
January 2016
Keynote 4.2 and 4.3: Settling Tanks and Drying Beds
Key note speaker Mbaye Mbégueré
Project Coordinator at Office National de l'Assainissement du Sénégal

2. FSM treatment technologies
Overview

4. Settling-thickening tanks
FSM treatment technologies
Settling-thickening tanks

5. Settling-thickening pond in Achimota, Accra, Ghana
Rufisque faecal sludge
management plant
Dakar, Senegal

6. Fundamental mechanisms
Settling
Thickening
Flotation
Anaerobic digestion

7. Settling thickening tanks
Separation into 4 layers:
Scum 3. Separation layer
Supernatant 4. Thickened layer

8. Settling thickening tanks
Prerequisites:
No turbulent flow
Width : length ratio of 1 : 5-10
Not fresh sludge alone (water too much bound)

9. Desludging

10. Desludging Desludging / sludge emptying:
manually by front loaders (after removal of the liquid and a period of drying)
mechanically through draw-off pipes
every months

11. Settling thickening tanks
Always aftertreatment required:
liquids + solids still high in pathogens
sludge not yet stabilised / fully dewatered
STs especially beneficial:
when treating FS with low solids
in temperate or rainy climates
can reduce required area of following treatment steps

12. Settling thickening tanks
Sedimentation pond or settling tank?
Depends on:
sludge consistency
space available
means of sludge removal
Undigested (public) toilet sludge may be diluted with more stabilised sludge in order to be treatable in settling tanks

13. Design of settling-thickening tanks
Laboratory tests and FS characteristics
Sludge Volume Index
Empirical method to determine settleability of sludge
Imhoff cone is filled; settling for min; settled FS volume recorded in mL/L
SVI (mL/g) = volume (mL/L) / SS concentration (g/L)
Example:
SS concentration: 6.6 g/L
Settled volume: 198 mL/L
SVI = 198/6.6 = 30 mL/g

14. Design of settling-thickening tank
Tank surface and length
Tank volume
Screening

15. To summarize Sufficient volume for sludge accumulation
Sufficient depth (> 1.5 m)
Loading/resting cycle - much shorter than in sedimentation ponds
Baffles -> favourable hydraulic conditions:
good settling
retain floating scum
Design: accumulating sludge of 5 to 9 L/kg TS can be considered

16. Take care of the scum too!

17. Removal rates Sedimentation pond Settling tank Suspended solids 60%
90%
BOD and COD (unfiltered)
30 – 50% %
BOD (filtered)
18%
45%

18. Removal rates Concentration inlet (g/L) Thickening duration (days)
Concentration thickened zone (g/L)
Dakar FSTP 5 10
60-70
Accra FSTP
15-20 9
60-85 30
>100 50
140
Accra laboratory 40 7
100

19. Removal rates Q 50 m3/day Ci 30 g/L e 50% N 5 days Ct L 20 m W 2 m
H 1.5 m

20. Removal rates Q 50 m3/day Ci 30 g/L e 50% N 5 days Ct
= (50 * 30 *0.5 * 5) / 60
= 62.5 g/L
L 20 m
W 2 m
H 1.5 m
V = 60 m3

21. Cambérène FSTP example

22. Constraints of S-T tanks
Constraints of ST include:
lack of experience operating with FS, and lack of empirical data and results on which to base designs on;
settled sludge still has relatively high water content and requires further dewatering;
the liquid fraction remains highly concentrated in SS and organics; and
pathogen removal is not significant, and the endproducts of ST therefore cannot be discharged into water bodies or directly used in agriculture.