Norway's largest yarn factory, Sandnes Uldvarefabrikk, produces knitting wool from 1,000 tons of raw wool a year. The raw wool is washed in 55°C water with high pH detergents. The resulting wastewater has a chemical oxygen demand, COD, of up to 100,000 mg/l and was previously piped directly to the public sewer. The Norwegian Environmental Protection Agency ordered the factory to reduce the COD loading by 75%, effective at the end of 1989.
The winter and spring of 1989 was used to test membranes and gather design information for a fullscale separation plant for treating the wool washwater. The results from the pilot testing were encouraging and a fullscale membrane plant was designed, manufactured and installed during the fall of 1989. The wool factory gradually increased the wool scouring capacity, however, and two years later the capacity of the full scale membrane plant was doubled simply by doubling the membrane area of the original plant.
The membrane separation plant is tubular ultrafiltration (UF) and the treatment is on a batch basis with a volume reduction of ten. The 31 m2 UF membrane area has an average permeate production capacity of 2 m3/h at 8 kg/cm2 inlet pressure and 3.8 m/s fluid velocity. The feed temperature is never allowed below 40°C due to the high concentration of fat which tends to solidify at lower temperatures and would plug the membranes.
The retentate from UF separation is returned to the feed tank whereas the permeate is routed through a heat exchanger to the public sewer. The start volume of each batch is between 15 m3 and 25 m3 with the UF plant manually started. The shutoff is automatic by a level switch set at 2 m3 batch volume. The retentate or sludge is pumped to a second holding tank and the accumulated sludge is once a week transported and dewatered in a lagoon at the local sanitary landfill.
The UF membrane plant has a 10 kW power consumption and has more than 700 operating days since 1989. The operator spends two hours a day managing the UF plant including cleaning the membranes after each daily batch, pumping the sludge, logging the data and reporting to the plant management. Membrane replacement is performed once a year at a cost of ₤3,800. The COD, fat and solids reductions have consistently been above 80%.