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.