Disposal of Tannery Wastes
R. R. PARKER,  Consulting Engineer
Reid Crowther and Partners, Limited
Toronto, Ontario, Canada
INTRODUCTION
In considering the treatment of tannery waste and the selection of the optimum method, one is immediately faced with a situation wherein the type of receiving medium,  rather than the effluent,  determines the method of treatment.
This situation is not confined to the tanning industry but it appears to be particularly applicable to other industries as well. Frequently one is told that this or
that treatment system has been found by its operator to be completely satisfactory.
Undoubtedly it is, but no thought has been given to the fact that a method of
treatment, or rather the resultant effluent quality acceptable in one area, would
be judged as completely unacceptable in other places.
There are basically five methods of disp sal for effluent after some form of
treatment: 1) Municipal sewers; 2) Tide water; 3) Classified rivers; 4) Controlled
fresh water streams or lakes; or 5) On the land. The permitted levels of contaminants for each of these methods of disposal vary greatly.
NATURE OF TANNERY WASTE
In present tannery practice, there are two principal tanning methods used:
1) Vegetable tanning and 2) Chrome tanning. Other methods are in use in some
special instances, but for the purposes of this paper only these two systems, or a
combination of them, will be considered.
The term "vegetable" as applied to tanning is used in its broadest sense and
includes all tanning operations wherein one uses extracts of wood, bark, or other
plant parts, including some by-products from the paper industry.
In chrome tanning the hide is tanned with a solution of an acid chromium
sulfate. The chromium compound is reduced so that the chromium is present only
in the trivalent form.
In general, vegetable tanning is used for the production of sole leather and
chrome tanning for upper leather,  but combinations of the system are common.
The preparation of the cattle hide for either process is essentially the same.
Hides are first washed to remove manure, blood and salt from the curing process.
Within the last few years the use of brine, rather than dry sodium chloride, for
curing has increased. The brined hide is somewhat cleaner as most of the manure
and blood has been removed in the brining operation, consequently the BOD of the
wash effluent is somewhat lower. The chloride content is approximately the same,
as frequently some dry salt is used to supplement the brine. The content of sodium
chloride normally is 14 to 15 per cent of the hide weight, as received.
After washing,   the hides are immersed in a thin slurry of calcium hydroxide,
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