Page 284 - Membranes for Industrial Wastewater Recovery and Re-Use

Case sludies 2 5 3

which consists of a sodium hydroxide soak at pH 10.5 resulting in a spent
chemical flow of 40 m3 d-I.
The major challenge with the plant was connected to concentrate disposal
which had to remove both colour and chromium from the concentrated brine
prior to discharge. A number of techniques were explored including electrolytic
treatment, chemical oxidation, chemical reduction and flocculation. Both the
electrolytic and chemical reduction techniques were not successfully
implemented at this site probably due to other contaminates in the flow.
Ultimately, the most successful option was the use of chlorine dioxide as a
chemical oxidant. The chemical destroys the chromophore and hence
decolourises the water whilst simultaneously releasing the Cr(II1). Colfloc
flocculent was then used to aid the physical separation of the insolublc
chromium from the flow. In total the plant produces 20 m3 d-l of concentrate
which requires 10 kg d-’ of chlorine dioxide (0.5 g I-]) to effectively treat it prior
to transport to the storage tank and ultimately disposal.
Once the plant was operational and the discharge volume was significantly
reduced Forsell and Sons were able to re-negotiate the consent limits based on
a reduce flow of 30 m3 d-l (Table 5.9). The changes had considerable impact in
terms of COD, Cr and Zn as it greatly reduced the need to treat the concentrate.
Most importantly concentrate levels of heavy metals were below the new
discharge consents and so no treatment of these components was necessary. The
impact on the process was to reduce the chemical demand of the plant by
removing the need for a flocculent.

5.7.3 Performance
The mill is no longer operational but the wastewater recovery plant was
successfully operated for over two years recovering 95% of the water as RO
permeate which was then reused within the plant. It produced an overall
removal of 80.5% COD and 59% total dissolved solids with complete removal of
suspended solids. Colour removal was almost complete with absorbance below

Table 5.9 Change in consent limit at Forsell and Sons Ltd
Parameter Original coriseril New consent

Flow (m3 d-’) 227 30
Rate(m’ h-’) 7 7
PH 6-10 6-10
SS (mg I-’) 1000 700
COD (mg 1-I) 3000 9000
Cr (mg I-’) 10 15
Zn (mg 1-I) 5 10
Colour at 500 nm 0.057 0.431
Colour at 5 50 nm 0.053 0.401
Colour at 600 nm 0.047 0.356
Colour at 650 nm 0.047 0.356