Page 178 - Membranes for Industrial Wastewater Recovery and Re-Use

150 Membranes for lndiistrial Wastewater Recovery and Re-use

Recycling
The textile processes providing the most propitious conditions for application of
membranes are fabric coating, desizing, scouring and dyeing. They have been
successfully employed at full scale in all these areas for the last 20 years in South
Africa, and more recently in other countries.
For two of these operations both the water and the concentrate are reusable.
Fabric coating makes use of latex, a relatively expensive chemical, which arises
in low concentrations in the effluent as an emulsion, with droplet sizes down to
0.05 pm. The use of ultrafiltration for latex and water recovery allows
substantial recovery of the emulsified latex (Groves et al., 1978). The main
limitation appears to be from the stability of the latex. Coagulation of the latex on
the membrane produces a deposit that can only be removed using appropriate
organic solvents and is then no longer recoverable. On the other hand, latex
stability can be improved through dosing with appropriate surfactants
(Cheryan, 1998). Recovery of synthetic size (polyvinyl alcohol and polyacrylate)
is also possible using UF (Porter, 1998), with original reports of successful pilot-
scale trials dating back to 1980 (Groves and Buckley, 1980). In this instance the
elevated effluent temperature (75°C) both increases the permeate flux and also
increases the cost benefit through recovering energy as heat.
Wool scouring produces a high-BOD effluent (Table 3.20) containing
detergents, suspended solids, fats, oils and grease (FOG) which can be treated
biologically and/or by dissolved air flotation. The use of ultrafiltration, operating
at 8-10 bar, permits substantial concentration of the major pollutants in the
retentate stream. The concentration factor is typically -7, producing a retentate
very high in organics (10-35 wt% COD). The longest established wool scouring
UF plant in Europe is in Norway (Bilstad et al.. 1994). This UF plant has been
operational since 1989 and achieves > 80% COD removal, nonetheless leaving a
filtrate still too high in COD for recycling. An interesting recent development is
the emergence of membrane bioreactors for this duty. A zenon hollow fibre
submerged MRR has been installed for treating a 13 m3 h-l flow of 3 500-5000
mg I-’ COD wool scouring wastewater in the UK (Bennett, 2000). Polishing of
the effluent by a two-stage reverse osmosis concentrator process, which includes
interstage pumping, which is designed to attain an overall recovery of 85% (75%
from the first stage and 50% from the second). No performance data has been
reported for this system, however.
Reviews of the use of membranes for dyewaste treatment have been presented
by a number of authors (Buckley, 1992: Cooper, 1993; Diaper et al., 1996).
Although not all membrane applications relate specifically to recycling, it is
apparent from the performance attainable in terms of rejection and product
water quality (Tables 3.33 and 3.34) that recycling of the permeate is possible in
many cases. At least two of the more recent studies (Marcucci et al., 2001; Sojka-
Ledakowicza et al., 1998) listed in Table 3.34 have incorporated testing of the
recovered water for dyeing. In both cases, it was concluded that the more
selective reverse osmosis process was required for a reusable water product.
Dyewaste is characteristically high in dissolved solids and low in suspended
solids, with colour arising from exhausted dye (Table 3.20). It is of interest to