Colour of wastewater was assessed through absorbency at
436, 525 and 620 nm. Removal efficiencies varied between 30
and 99.5% corresponding to effluent levels between 0.2 and
22 m1. Greatest deviations were examined at 436 nm, whereas
colour removal at 525 and 620 nm were found to be between 46
and 98.5% and between 57 and 99.8%, respectively.
As shown in Fig. 8, effluent colour does not follow the
feeding pattern as expected. It is assumed that the main
mechanism inducing colour removal is adsorption on biomass.
Biodegradation seemed to play a minor role in colour removal
as the persistent nature of textile dyes in activated sludge
systems is extensively reported in literature [11]. In fact, there is
certain evidence that adsorption is dominating colour removal
for the present case. In Fig. 9, the colour removal–sludge
growth relationship is depicted for all wavelengths examined. A
distinct relationship between sludge growth and colour removal
can be observed. An increasing sludge growth rate yielded
higher colour removal efficiencies as more biomass to adsorb
incoming colour was generated. This effect seems to be more
pronounced for colour components detected at 436 nm,
whereas colour at 525 and 620 nm showed good removal
efficiencies even at low sludge growth rates.
Above an sludge growth rate of 0.3 g/(l day), colour removal
was above 87% for all wavelengths. This results even exceeds
colour removal efficiencies obtained by Ref. [12] (75%), and
[7] (80%). It seems to be recommendable to concentrate ones
attention to sludge growth rate if maximum colour removal is
objected. For the present case sludge growth rates above 0.3 g/
(l day), corresponding to a sludge age of 50 days at a sludge
concentration of 15 g/l, are suggested to attain maximum
removal efficiencies.