1. SLUDGE DEWATERING: THE CAPE TOWN PERSPECTIVE
APPROPRIATE TECHNOLOGY CONFERENCE
Presented by: KEVIN FAWCETT
Date: Tuesday, 4th September 2007

2. INTRODUCTION
Past experience was that sludge disposal would look after itself
The reality that sludge handling could possibly account for as much as 50% of operating costs was ignored
Many WWTW in Cape Town relied entirely on sludge lagoons and sludge drying beds as the primary option for sludge dewatering
In 1995 the then Cape Town City Council approved the installation of plant to effect sludge thickening, dewatering and thermal drying of the sludge at Cape Flats WWTW, some 20 years after the WWTW was commissioned

3. Cape Flats WWTW: 200 Ml/day

4. Original drying beds

5. CFWWTW: Sacrificial land disposal

7. CFWWTW: Sludge Management Options

8. CFWWTW: Sludge solids mass balance

9. Estimate of operating costs for each option: 1995 Costs
Process Description
Rand/ton # 1
Disposal(Vissershok)
2229 2
Dewatering/composting/disposal(Vissershok)
854 3
Dewatering/drying(60%DS)/composting/disposal(Swartklip)
741 4
Dewatering/drying(60%DS)/disposal(Swartklip)
683 5
Dewatering/drying(90%DS)/disposal(Swartklip)
622 6
Digestion/dewatering/composting/disposal(Swartklip)
534 7
Pre-thickening/digestion/dewatering/composting/disposal(Swartklip)
506 8
Dewatering/disposal(Vissershok)
448 9
Dewatering/private disposal
382 10
Digestion/dewatering/drying(90%DS)/disposal(Swartklip)
327 11
Digestion/dewatering/drying(60%)/composting/disposal(Swartklip)
315 12
Pre-thickening/digestion/dewatering/drying(60%DS)/composting/disposal(Swartklip)
294 13
Pre-thickening/digestion/dewatering/drying(90%DS)/disposal(Swartklip)
295* 14
Digestion/dewatering/disposal(Swartklip)
273 15
Digestion/dewatering/drying(60%DS)/disposal(Swartklip)
269 16
Pre-thickening/digestion/dewatering/disposal(Swartklip)
260 17
Pre-thickening/digestion/dewatering/drying(60%DS)/disposal(Swartklip)
250

10. CENTRIFUGES SLUDGE THICKENING & DEWATERING

11. DEWATERED SLUDGE INLET
PELLETS OUTLET
DRYING DRUM
DEWATERED SLUDGE INLET
THERMAL DRYING PLANT

12. Process Considerations when selecting centrifuges
Anaerobic digestion of waste activated sludge prior to dewatering could present problems with respect to Struvite precipitation
Screening of sludge is necessary to prevent accumulation of fibre in the centrifuge bowl
Installation of tungsten carbide tiles should be a necessity if the feed sludge is in any way abrasive

13. Struvite

15. ANAEROBIC DIGESTED SLUDGE
4 to 5% Dry Solids

16. STRAINPRESSES - REMOVAL OF FIBRE AND FOREIGN OBLECTS FROM SLUDGE

17. Strainer Boxes
Lifting chain
6mm perforations

18. Centrifuges can be problematic
Poor operation, grit and fibre in sludge
Not following the suppliers operating and maintenance instructions

19. Tungsten Carbide Wear Tiles

20. Centrate & sludge cake inspection Points
Regular wear check profiles of the scroll are essential

22. Macassar WWTW

23. Wildevoelvlei WWTW

24. Incorrect belt tensioning

25. Foreign objects between belts

26. Roller failure

27. Roller failure due to poor manufacture

28. In a number of instances old established technologies are still appropriate
Biofilter Plant

29. However the same does not apply to activated sludge plants- high sludge production rates
Winter sludge disposal costs = R0,75m

30. Conveyance of dewatered sludge cake: What is the appropriate technology?
Screw Conveyors: Centreless or Shafted
Progressive Cavity Pumps
The following slides indicate some short comings and problems and where conveyors are appropriate

31. Where sludge storage silos are not required, conveyors and skips are appropriate

32. Accessibility for maintenance
Failure of silo screw conveyors= empty the silo

33. Old versus upgrade version

35. Sludge cake transfer to disposal skip

37. Where do we go from here?

38. Athlone WWTW: Anaerobic digesters/ drying beds

39. Possible Solution? Progressive Cavity Pump

40. Dewatered cake and silo pumps
Can insert a knife gate valve for installation at the base of sludge storage silo
Advantage = In the event of repairs to pump can isolate silo without having to empty it

42. Level sensors
Pumped discharge

44. Proposal for ATHLONE WWTW
Progressive cavity pumps
Centrifuges

45. Conclusion Have we made the correct decision – time will tell
What ever dewatering technology is selected it is essential that the appropriate operating and maintenance skills are in place before the plant is commissioned
Consider the possibility of entering into a maintenance agreement with the equipment supplier to ensure sustainability of the equipment
All Metro’s and Municipalities have between them a multitude of varied experiences in the wastewater industry, both good and bad – it is time that we engaged in open discussions with each other to learn from each others experiences and hopefully the selection of the “appropriate technology” will become easier.