Effect of Flocculants on the Sedimentation of
Organic Solids in Tannery Unhairing Effluents
J.E. COOPER, Research Chemist
W.F. HAPPICH, Research Chemist
E.H. BITCOVER, Research Chemist
E.F. MELLON, Research Chemist
E.M. FILACHIONE, Research Chemist
Hides and Leather Laboratory
Eastern Regional Research Center
Agricultural Research Service
U.S. Department of Agriculture
Philadelphia, Pennsylvania 19118
INTRODUCTION
The primary treatment processes of sanitary (domestic) and industrial wastes are
normally divided into unit operations, for example: a) grit removal, b) screening, c)
comminution or grinding, d) sedimentation and/ or flocculation, and e) chemical
precipitation. Sedimentation is the removal of suspended solids by gravitational settling of
naturally aggregated particles that are heavier than water. When chemicals are added to aid
the coalescence of the suspended particles, the process is called coagulation or flocculation.
According to Rideal (1) the concentration of the coagulant, in milligrams of ions per
liter necessary to cause coagulation, is called the liminal concentration and may vary with
the valence of the coagulator in the proportion 1:2:3. The coagulative concentrations for the
mono, di, and trivalent ions are in the proportions of 1:1 / x: 1 / x2 where x is approximately
30. Substituting the value 30 in the proportion and taking the reciprocals yields 1:30:900,
the coagulative powers. The polyvalent ions are much more effective as coagulators, than
the monovalent ions. Colloidal suspensions have an electrical charge because of preferential
adsorption of cations or anions. A positive suspension will be precipitated when an
electrolyte containing a readily adsorbable anion is added. When the electrolyte contains a
cation that tends to offset the effect of the anion, then the quantity of electrolyte added will
vary with the nature of the cation. The quantities of various electrolytes that contain
common ions necessary for flocculation will also vary with the nature of the cation or anion.
Tannery lime-sulfide unhairing wastes are complex mixtures: sulfide (S=), bisulfide
(HS"), calcium ions (Ca++), dissolved albumin, mucoids, mucopolysaccharides, keratin,
dissolved and emulsified fats, and insoluble organic and inorganic compounds. The size of
particles ranges from a few microns to millimeters, more or less. The net charge of the
mixture is positive, possibly due to the entrainment of impurities by a particular ion present.
The removal of suspended lime-solids from beam house wastes of a sole-leather
tannery has been reported by Eye (2), Del Pezzo (3) and Sproul (4). They separated the lime-
wastes from the bating-wastes and wash waters, and then treated with a polyelectrolyte and
alum. Schaffer (5) used nonionic, anionic, and cationic polyelectrolytes on meat plant
wastes and in digesters for sludge thickening, and on metal wastes and combined sewages
for settling of suspended solids. The lime-sulfide unhairing unit process contributes 75-80
percent (6) of the biochemical oxygen demand (BOD) and chemical oxygen demand (COD)
and a high suspended solids concentration to the beam house effluent. Cooper, et al (7)
determined that the loading to be applied to a bio-aeration unit would cause a large oxygen
deficit due to the high concentration of suspended organic and inorganic solids.
This paper reports the use of inorganic and organic polyelectrolytes for the reduction
of organic solids and COD at pH 8 or greater.
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