Treatment of Chrome-Tanning
Wastes for Acceptance by an Activated
Sludge Plant
F. J. WIMS, Mechanical Superintendent and Plant Manager
Collis Leather Company, Limited
Aurora, Ontario
INTRODUCTION
The leather making process varies quite considerably from plant to plant.
There are many reasons for this, but in the main, it is because development in
this industry has traditionally been along empirical, rather than closely scientific
lines. The manufacture of leather is frequently - and correctly - described as
being more of an art than a science.
In consequence, a paper dealing with the treatment of waste from such a
process will be more specific than general. Our particular reference will be to the
treatment of the tanning wastes of the Collis Leather Company for acceptance by
the Town of Aurora Sewage Treatment Plant.
Located at Aurora, Ontario, approximately 15 miles due north of Metropolitan
Toronto, Collis Leather processes calfskins only. (The chrome-tanning method is
used exclusively and this, in passing, should be distinguished from the vegetable-
tanning process which produces a waste effluent of decidedly different characteristics. ) Through the plant's premises meanders a small stream having an average
flow of approximately 4.5 cfs. (This is a branch of the Holland Creek, a tributary of the Holland River, eventually emptying into Lake Simcoe.) The stream
has long provided the only means of disposal, after treatment, of domestic and
industrial effluents in the Town of Aurora. In addition to the tannery, the latter
presently include the wastes from a chicken processing plant, a large drug manufacturer, an anodizing process, and a dairy. Domestic wastes derive from Aurora's
population of 9200 as of March 1963.
All animal skins consist of three layers, namely, the epidermis (i.e., the
cuticle or outer layer), a layer of fatty tissue, and the inner layer, or corium. It
is the latter from which leather is made and the objective of the tanning process is
to strip off the two outer layers and to subject the corium to the action of agents
which convert it from a semi-soluble protein to the tough insoluble mass known as
leather. The exact nature of this transformation is unknown. A typical flow sheet
(1) of the chrome tanning process is shown in Figure 1.
Qualitative analysis of the composite wastes of a typical chrome tannery effluent will show that it has a high pH, contains dissolved organic material in large
amounts, with calcium, sodium, and chromium salts, together with various sulphides also in solution. The wastes will also be heavy in suspended matter including such constituents as hair, bits of flesh, and particles of lime and calcium
carbonate.
When such a waste is discharged to a relatively small stream, the action of
aeration in passing down the stream results in liberation of volatile ammoniacal
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