Page 282 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studirs 25 1

Table 5.8 Colour consents
Wavelength (nm) Colour absorbency
Consent Typical effluent
500 <0.057 0.094
5 50 <0.053 0.084
600 < 0.047 0.070
650 < 0.047 0.055

individual textile companies including T. Forsell and Son Ltd together with four
other discharges in the area.
The textile plant processes 10 Mg of yarn per day producing a total of 400 m3
of effluent. The effluent typically exceeds the colour consent by between 20 and
80% and contains high concentrations of chromium. The cost to meet the
consent is sufficiently high that recovery and reuse of the water becomes a viable
economic consideration. The company then embarked on a pilot study to
establish the most appropriate technology. The criteria outlined were the ability
to produce reusable water (based on colour and chrome) and the robustness to
withstand the natural fluctuations in water quality. Four options where tested:

0 Flocculation
0 Biological treatment
0 Advanced oxidation
0 Reverse osmosis

Flocculation is probably the most popular form of effluent treatment in the
textile industry. The system trialled at T. Forsell and Son utilised a synthetic
organic clay with a high anion exchange capacity. The system was relatively
simple and suitable for colour removal but was unable to reduce the chromium
concentration below 3 ppm. The biological system comprised an aerobic reactor
coupled with UV and hydrogen peroxide capable of removing 80% of the COD.
The plant was relatively inexpensive at €1 SO 000 ($232 650) but was unable to
remove chromium. The third option was chemical oxidation with ferrous
sulphate, hydrogen peroxide, sulphuric acid and lime. The process effectively
removed chromium but retained a high salt concentration in the product water.
In all the options the final product water was unsuitable for reuse and so not
economically feasible. Ultimately a membrane-based technology was selected as
it enabled reuse of the water and hence recovery of the spent capital.

5.7.2 Description of plant
The process train is relatively simple with minimal pretreatment required ahead
of the reverse osmosis unit (Fig. 5.1 5). The flow initially passes through a 20 pm
screen prior to being pumped to the RO plant. Concentrate from the process is
treated with appropriate chemicals and the generated sludge separated out from