Page 143 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 11 7

as concentration of coating colour. The filter normally operates at low pressures
-as low as 0.8 bar - with the main energy demand being mechanical, between 3
and 5 KWh m-3. As a stacked disc system, the module imposes a lower footprint
than tubular systems whilst achieving higher fluxes.
Four CR ultrafilters were successfully used in 1988-1992 at MODO Husum
sulphate pulp mill in Sweden to treat the E1/0 bleaching effluent (Jonsson and
Tragirdh, 1990). The membrane area of one filter was 50 m2 and its capacity
50 m3 h-', giving a flux of 1000 LMH. The concentration factor was 15,
the pressure 0.2-0.8 bar and temperature 50-60°C. Hoechst PES2 5
polyethersulphone UF membranes with a MWCO of 25 kDa were used. The
concentrate was incinerated and the permeate was directed to the external
biological treatment plant.
CR filters were also installed in 1989 at Rottneros Board Svaneholm in Sweden
to treat board machine white water (Jonsson, 1990; Gavelin, 1991; Anon.,
1996). Two CRlOOO filters (membrane area 100 m2 and membrane cut-off 100
kDa) are used to treat the white water removing 3 t d-l of suspended solids. The
permeate is directed back to the board machine and the retentate is first
concentrated in an evaporator and then burned together with the stripped bark.
The content of suspended solids in the white water has been reduced from 5 to
2% and the downtime associated with machine cleaning and maintenance
reduced. Also, the efficiency of the secondary treatment plant has been increased
due to the lower organic loading. The energy consumption is 6-8 kWh m-3 of
permeate. Successful pilot tests with UF for white water treatment have also been
carried out at Stora Kabel (Kothe and Schroth, 1999) and StoraEnso Uetersen
(Kluter, 1999; Huhtamaki et al., 2002). For the UF plant the payback time
calculated on the basis of the pilot trials in Stora Kabel is 4.9 years and the
internal rate of return 21%. The cost for fresh water and wastewater is about
$0.5 per m3. Through recycling, and thus the reduction in freshwater
consumption, about $0.15 per m3 is saved. A recent example of the mill scale
application of a CR filter to the treatment of paper mill circulation water (Teppler
etal., 1998) is detailedinsection 5.6.
As with other industrial sectors, the cost benefit of the water recycling plant is
substantially improved if other resources are simultaneously recovered for reuse.
In paper mills producing, for example, coated high-quality printing papers, such
as lightweight coated LWC paper grades, a significant amount of coating colour
is discharged in the effluent. Moreover, the waste coating colour adheres to
equipment, which then requires frequent cleaning. This represents a significant
economical loss, as well as the colour in the effluent being an environmental
issue. In many mills coating colour effluents are still treated with precipitation
and the solid waste then landfilled. By employing ultrafiltration colour
components in the coating colour kitchen effluent stream can be recycled, along
with the water. The dilute effluents are concentrated with ultrafiltration
membranes to an appropriate total solids content after which the retentate is
used in the preparation of fresh coating colour. The permeate can be used for
dilution or washing purposes in the coating colour kitchen. The payback time of
such membrane plants is less than a year depending on the price of the coating