Page 280 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studies 249

Screen

To bolcgtcal
Treatment plant
I NF filtrate

tank
To biological
Treatment olant
Figure 5.13 Processflow diagram of Kirkniemi white water recyclingplant

at a cross-flow velocity of 13 m s-'. Fouling control in the cross-rotational
membrane unit is achieved by rotors between all membrane stacks thus
generating shear directly at the membrane's surface. Membrane cleaning is
conducted by a combination of acid and alkaline detergents every 5 days for a
duration of 60 minutes. Concentrate from the stacks is combined and sent to the
biological treatment plant prior to discharge.
Permeate from the UF plant then is split into two streams with 30-4096 of the
flow being sent for further treatment in a nanofiltration plant. The plant contains
a total membrane area of 900 m2 and is designed to process 30-40 m3 h-l at a
temperature of 58-60°C and an overall recovery of 80%. The membranes are
polyaramide/polysulphone spiral-wound modules and are rated at a salt
rejection of 50%. The membranes operate at a TMP of 9 bar which delivers a
mean flux of 30-40 LMH. Membrane cleaning in conducted every 3 days with an
alkaline detergent and every week with an acid detergent. The membrane life of
the plant is in excess of 2 years.

5.6.3 Performance
The UF plant effectively removes all suspended solids and colloidal materials
from the feed (Table 5.20). COD removal is only 15% but the final water quality is
suitable for dilution water for paper machine chemicals. The use of the
ultrafiltration plant has not only reduced freshwater intake but has improved
maintenance and repair of the paper machines which may be due to the fact that
the water is free from suspended solids and microorganisms. The cross-rotational
UF plant incurs an energy demand of 1.5 kW m-3 which equates to an operating
costof€0.12 m-3 ($0.12 mp3).
The white water contains sugars, fatty acids and other wood extracts which
all exhibit a high fouling propensity. A key aspect of the scheme has been the
identification and development of a membrane system that can effectively
operate with such a feed flow. The use of highly hydrophilic, almost uncharged
regenerated cellulose membranes has had a significant influence on the efficacy
of the technology to this application. Operation of the system has been below the
critical flux such that only gradual fouling of dissolved species rather than rapid