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 mbaye.mbeguere@onas.sn www.fsm-e-learning.net

FSM treatment technologies Overview

Settling-thickening tanks FSM treatment technologies Settling-thickening tanks

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

Fundamental mechanisms Settling Thickening Flotation Anaerobic digestion

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

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

Desludging

Desludging Desludging / sludge emptying: manually by front loaders (after removal of the liquid and a period of drying) mechanically through draw-off pipes every 2 - 4 months

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

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

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 30 - 60 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

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

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

Take care of the scum too!

Removal rates Sedimentation pond Settling tank Suspended solids 60% 90% BOD and COD (unfiltered) 30 – 50% 70 - 95% BOD (filtered) 18% 45%

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

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

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

Cambérène FSTP example

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.