Dewatering of Tannery Sludge
HANS W. FORSTER, Consultant
Passavant Corporation
Birmingham, Alabama 35201
INTRODUCTION
When the S.B. Foote Company tannery in Redwing, Minnesota was confronted with
the necessity of dewatering their sludges in 1966, various methods and systems were
investigated. Vacuum filtration was used at a number of tanneries, however, the cake solids
concentration from this process was at a level which made landfllling, and for that matter
incineration, economically unattractive.
A review of other processes revealed the success of the C. Freudenberg Tannery at
Schoenau, Germany. This tannery uses the "Pressure Filtration Process," resulting in
higher cake solids concentration than experienced before and, in addition, produces a
filtrate extremely low in suspended solids.
PILOT PLANT OPERATION
To explore the suitability of the pressure filtration process in depth, a pilot plant was
installed and operated throughout the summer of 1968.
No sludge thickening facilities existed at that time, thus, two gravity thickeners were
fabricated and installed. These units were 4' x 4' and permitted the generation of various
sludge concentrations as well as the optimizing of sludge solids for the pilot filter feed.
The pilot plant consisted of: I) a 300-gallon mix tank; 2) a sludge transfer pump; 3) a
precoat tank; 4) a feed tank; 5) a pressure filter with 6 chambers, having 36 sq ft filtration
area and a 2.25 cu ft cake holding capacity; 6) an air compressor; and 7) a motor control
panel.
SLUDGE CONDITIONING STUDIES
Practically all sludges require some type of conditioning, the cost of which contributes
greatly to the over-all operating cost of the plant. The utilization of waste materials was
foremost in the mind of the investigators and the following materials were explored: 1) Fly
ash from Wisconsin Power and Light Co.; 2) Fly ash from the S.B. Foote Co. power boilers;
3) Buffing dust from the tannery; 4) Chrome dust from the tannery; 5) Ferric chloride; 6)
Combinations of fly ash and ferric chloride and lime; 7) Polymers A-1 and B-35-A; and 8)
Ash generated from the incinerated tannery sludge. The latter was investigated in depth as
Incineration for final disposal was seriously considered. However, this approach was later
abandoned because of the possibility of tri-valent chrome converting to hexavalent chrome
salts which are highly toxic. The full scale plant was finally designed to operate with
chemicals; i.e., ferric chloride and lime. Polymer conditioning was thought to be too
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