Ultrafiltration has proven to be a useful technology for treating alkaline
bleach plant filtrates in sulphite pulp mills. In this investigation the tr
eatment of seven alkaline filtrates from kraft pulp mills were studied. The
results include high fluxes and fair, or even good, removal of organic, as
well as inorganic, substances. The flux seems to depend on the concentrati
on of the bleach plant filtrate rather than its origin. The factor with the
largest impact on flux is conductivity, i.e., the concentration of the low
-molecular-weight inorganic substances. The retention of organic substances
varies depending on the type of filtrate and the water management system o
f the mill. The retention is highest when treating filtrates from the first
alkaline stage from traditional ECF mills. Filtrates from modem ECF and TC
F mills contain a larger fraction of low-molecular-weight substances, which
are not retained as efficiently. The treatment not only reduces the organi
c content of the filtrates but also increases the fraction that is biodegra
dable. The membranes retain a rather large fraction of the multi-valent ion
s, while the monovalent ions are let through.