SLUDGE DEWATERING: THE CAPE TOWN PERSPECTIVE
APPROPRIATE TECHNOLOGY CONFERENCE
Presented by: KEVIN FAWCETT
Date: Tuesday, 4th September 2007
INTRODUCTIONPast 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
Cape Flats WWTW: 200 Ml/day
Original drying beds
CFWWTW: Sacrificial land disposal
CFWWTW: Sludge Management Options
CFWWTW: Sludge solids mass balance
Estimate of operating costs for each option: 1995 Costs
Option 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
CENTRIFUGES SLUDGE THICKENING & DEWATERING
DEWATERING
THICKENING
PELLETS OUTLET
DRYING DRUM
DEWATERED SLUDGE INLET
THERMAL DRYING PLANT
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
Struvite
ANAEROBICDIGESTED SLUDGE
4 to 5% Dry Solids
STRAINPRESSES - REMOVAL OF FIBRE AND FOREIGN OBLECTS FROM SLUDGE
Strainer Boxes
Lifting chain
6mm perforations
Centrifuges can be problematic
Poor operation, grit and fibre in sludge
Not following the suppliers operating and maintenance instructions
Tungsten Carbide Wear Tiles
Centrate & sludge cake inspection Points
Regular wear check profiles of the scroll are essential
WWTW DEWATERING EQUIPMENT
Athlone Centrifuges being consideredBellville Belt PressBorcherds Quarry Centrifuge and Belt PressCape Flats CentrifugeKraaifontein Belt PressMacassar Belt PressMitchells Plain Centrifuge and Belt PressPotsdam Belt PressWildevoelvlei Belt PressZandvliet Belt Press
Macassar WWTW
Wildevoelvlei WWTW
Incorrect belt tensioning
Foreign objects between belts
Roller failure
Roller failure due to poor manufacture
In a number of instances old established technologies are still appropriate
Biofilter Plant
However the same does not apply to activated sludge plants- high sludge production rates
Winter sludge disposal costs = R0,75m
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
Where sludge storage silos are not required, conveyors and skips are appropriate
Accessibility for maintenance
Failure of silo screw conveyors= empty the silo
Old versus upgrade version
Sludge cake transfer to disposal skip
Where do we go from here?
Athlone WWTW: Anaerobic digesters/ drying beds
Possible Solution?Progressive Cavity Pump
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
Level sensors
Pumped discharge
Proposal for ATHLONE WWTW
Progressive cavity pumps
Centrifuges
ConclusionHave we made the correct decision – time will tellWhat ever dewatering technology is selected it is essential that the appropriate operating and maintenance skills are in place before the plant is commissionedConsider the possibility of entering into a maintenance agreement with the equipment supplier to ensure sustainability of the equipmentAll 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.