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

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Presentation on theme: "SLUDGE DEWATERING: THE CAPE TOWN PERSPECTIVE APPROPRIATE TECHNOLOGY CONFERENCE Presented by: KEVIN FAWCETT Date: Tuesday, 4 th September 2007."— Presentation transcript:

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

6

7 CFWWTW: Sludge Management Options

8 CFWWTW: Sludge solids mass balance

9 Estimate of operating costs for each option: 1995 Costs OptionProcess DescriptionRand/ton # 1Disposal(Vissershok)2229 2Dewatering/composting/disposal(Vissershok)854 3Dewatering/drying(60%DS)/composting/disposal(Swartklip)741 4Dewatering/drying(60%DS)/disposal(Swartklip)683 5Dewatering/drying(90%DS)/disposal(Swartklip)622 6Digestion/dewatering/composting/disposal(Swartklip)534 7Pre-thickening/digestion/dewatering/composting/disposal(Swartklip)506 8Dewatering/disposal(Vissershok)448 9Dewatering/private disposal382 10Digestion/dewatering/drying(90%DS)/disposal(Swartklip)327 11Digestion/dewatering/drying(60%)/composting/disposal(Swartklip)315 12Pre-thickening/digestion/dewatering/drying(60%DS)/composting/disposal(Swartklip)294 13Pre-thickening/digestion/dewatering/drying(90%DS)/disposal(Swartklip)295* 14Digestion/dewatering/disposal(Swartklip)273 15Digestion/dewatering/drying(60%DS)/disposal(Swartklip)269 16Pre-thickening/digestion/dewatering/disposal(Swartklip)260 17Pre-thickening/digestion/dewatering/drying(60%DS)/disposal(Swartklip)250

10 CENTRIFUGES SLUDGE THICKENING & DEWATERING DEWATERING THICKENING

11 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

14

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

21

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

34

35 Sludge cake transfer to disposal skip

36

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

41

42 Level sensors Pumped discharge

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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 Metros 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.


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