Clarifiers for Waste Water
Treatment in the Swedish
Pulp Industry
PETER ULLMAN, Research Associate
Swedish Water and Air Pollution Research Laboratory
Stockholm, Sweden
INTRODUCTION
Mill surveys have shown that the discharges of suspended solids, largely consisting of fibres, from Swedish pulp and paper mills earlier exceeded 50 kg (110 lb)
of SS per ton of pulp in many cases. The upper limit now set by the Swedish
authorities is 5 kg per ton, that is 0.5 per cent of the production. To meet this limit
several measures for internal fibre recovery are necessary, but of course attention
must also be paid to clarifiers for treating the final effluent. The authorities now
regularly specify the use of such clarifiers.
This paper describes some field studies and experiments from different types of
waste water clarifiers at Swedish pulp mills. The main object of the studies was to
evaluate load curves (e.g. SS in effluent versus surface load), but other factors have
been studied too, such as the influence of tank depth, the effect of chemical floccula-
tion, clarifier performance compared with results from a commonly used clarifier
dimensioning method (cylinder test), and sludge problems.
LOAD VARIATIONS
Methods
The load experiments were carried out as follows. Each of the mills investigated treats its fibre containing waste water in two or more clarifiers. The distribution
of flow between the clarifiers was varied and the flow measured with weirs. Composite samples, usually from two hr periods, were taken with automatic samplers
from influent and effluent. The sampling frequency was 20 samples per hr.
The samples were analysed for suspended solids, SS (not in the case of Mill A)
and settleable solids. SS were analysed as mg/l by filtration technique. Settleable
solids were first analysed by one hr sedimentation in an Imhoff cone and reported
as ml/l. Later settleable solids were analysed as mg/l by decanting the clear liquid in
the Imhoff cone after one hr, followed by filtering and weighing of the sediment. The
latter quantity, settleable solids as mg/l, should provide more information than the
filterable suspended solids in studies of sedimentation equipment, as it more directly
measures the solids which in fact can be removed by the clarifier under ideal conditions.
Experimental and results
Load experiments were carried out at three pulp mills denoted A, B and C. The
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