Abstract
M.Ing.
The Rietvlei Water Treatment Plant was extended with a granular activated carbon (GAC)
filtration system after an exhaustive series of tests, which were started in 1994. Upon
commissioning towards the middle of 1999, a year of close monitoring followed to measure the
GAC performance at full-scale. After verification that the GAC does indeed ensure a high
quality product under all conditions, the emphasis shifted to the optimisation of the GAC
handling and regeneration system. Frequently moving the entire GAC inventory from the filters
to an off-site regeneration plant and back requires significant operational effort and contributes a
major part of the total cost of the GAC system. A number of systematic investigations were
carried out in response to a number of practical questions that arose at Rietvlei.
The first part of the study was directed towards tracking and quantifying the GAC on and off
site. The main findings were that 10.0% of the GAC is lost from the filter during backwashing
(0.3%) and removal of GAC from the filter for regeneration (9.7%). The sump traps not all this
GAC and 2.3% of the total inventory is lost to the river. Inserting a sieve at the outlet of the
sump can eliminate this loss. A further 80.3% of the GAC in a filter is removed for regeneration,
of which 18.7% is lost during the regeneration process. The minimising off this loss can only be
achieved through the optimisation of the regeneration process, which falls within the domain of
the regeneration contractor.
The second part of the study was directed at the behaviour of the GAC whilst within the filter
bed. The porosity and sphericity was determined by laboratory tests and calculations. The
porosity was found to be 0.69 for the 12 x 40 size carbon and 0.66 for the 8 x 30 size carbon and
the sphericity was found to be 0.67 for the 12 x 40 size carbon and 0.66 for the 8 x 30 size
carbon. By using a calibrated bed expansion model, the bed expansion could be calculated at 9°C
and 23°C for the two carbons gradings; the maximum temperature range experienced at Rietvlei.
The main finding of this part of the study was that the average available freeboard is 650 mm for
the 12 x 40 grading and 430 mm for the 8 x 30 grading, and therefore no GAC should wash over
the weir at all during backwashing.
The third part of the study measured the physical changes of the GAC found at different points in
the GAC cycle. The main findings were that the small fraction of GAC washed out of the bed
during backwashing and removal has a finer grading, higher apparent density and lower
adsorption capacity than the GAC in the filter bed. There seems to be no marked attrition of the
carbon or generation of fines during the removal and transport of the GAC to the regeneration
plant. After regeneration, there was a 7% decrease in apparent density and a 30% increase in
adsorption capacity.
The final part of the study correlated the adsorption capacity of the GAC with its time in use as
well as UV254 removal. After regeneration, UV254 removal begins at approximately 20% and
declines to 14% after 400 days of operation, and to 10% after 600 days. After regeneration, the
iodine number begins at approximately 800, declines to 600 after 400 days of operation,
and to 500 after 600 days.