Fate of Lignin in Activated Sludge Treatment
of Kraft Effluents
GANCZARCZYK, Associate Professor
Department of Civil Engineering
University of Toronto
Toronto, Canada
INTRODUCTION
There are two main objectives of treatment of Kraft mill effluents: removal of the
wastewater- BOD and colour. The first aim can be relatively easily achieved by various
means of biological treatment. The removal of colour, however, calls quite often for
subsequent chemical treatment.
The colour of Kraft mill effluents is attributed mostly to lignin content in this
wastewater. The exact nature of the colour-causing structures of Kraft lignin is uncertain
(1), although it seems most likely that double bonds conjugated with the aromatic ring and
quinonemethides and quinones are contributing substantially to the colour. The two latter
structures may also serve as oxidative species creating further chromophoric structures.
The light absorption curve of Kraft lignin shows the aromatic maximum at 280 m/Li, a
shoulder around 340 m/i (which may be attributed to phenolic hydroxyls, phenolicaryl-a-
carbonyl groups and conjugated double bond stilbene structures), and a relatively steady
reduction of adsorption in the visible region. The latter is likely to be caused by a number of
chromophores (1).
Available information on lignin removal from Kraft mill effluents in activated sludge
process treatment is very scarce. Schmidt and Weigt (2) observed in pilot-plant studies the
decrease of the wastewater methoxyl content of 42 percent at loadings below 2000 g
BOD/m3/day. Anderson and Ganczarczyk (3) in laboratory experiments and using
simulated wastewater, reported 54 percent of lignin removal measured by nitroso-lignin
reaction at loadings of 1230 gCOD/m3/day and 0.3 gCOD/g MLSS/day.
More data were presented on the wastewater colour removal by biological treatment.
Edde (4) in bench scale activated sludge studies, carried out at sludge loadings varying from
0.25 to 0.97 g BOD/g MLSS/day, observed no significant colour reduction. However, in
the pilot-plant experiments Ganczarczyk (5) showed the possibility of the wastewater
colour decreases at loadings below 1500 g BOD/ m3/day. Similar results were received also
by Schmidt and Weigt (2) who for loadings below 2000 g BOD/m3/day observed colour
decreases in the range from 34 to 44 percent. Full scale plant performance studies (6) of the
activated sludge treatment of unbleached Kraft mill effluent mixed with a small amount of
town sewage, under aeration tank BOD loadings of 1430 g/m3/day and 1030 g/m3/day,
showed colour decreases of 39 and 23 percent, respectively, from starting average values of
1900 and II70 mg/1 Pt.
Some light on the possible lignin removal may be also received by the study of the
wastewater COD decreases. Schmidt and Weigt (2) reported in the cited studies at least 65
percent COD elimination, (6) in the full-scale operation noted 66.5 and 58.6 percent COD
decreases, a nd Anderson and Ganczarczyk (3) in laboratory scale studies observed the 57.9
percent wastewater COD uptake. All these values surpass the COD fractions attributed to
the non-lignin content of the discussed wastewater.
It seems that the relative stability of lignin to biological degradation does not
necessarily prevent an effective removal of this material during activated sludge treatment
of the respective wastewater.
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