41    KINETICS OF BIOLOGICAL CYANIDE AND
THIOCYANATE REMOVAL
Radh V. Chinnaswamy, Environmental Engineer
Aqua Terra Consultants
Mountain View, California 94043
Wendell H. Hovey, Associate Professor
Civil Engineering Department
South Dakota School of Mines and Technology
Rapid City, South Dakota 57701
INTRODUCTION
Cyanide is widely found in industrial wastewaters. For example: Wastewaters containing cyanide
result from ore mining and extracting, photographic processing, coke making, synthetic fiber manufacturing, steel case hardening, and gas scrubbing. Cyanide is produced in catalytic cracking of
petroleum. A source of waste cyanide is the electroplating industry, where cyanide baths are used lo
hold metallic ions in solution.
There are several methods of treating wastewaters containing cyanide. Chlorination is frequently
used to partially oxidize cyanide lo cyanate, or to completely oxidize it to carbon dioxide and
nitrogen. Other oxidizers, such as hydrogen peroxide or ozone are also used. Physical processes such
as ion exchange and reverse osmosis are used. The biological oxidation of cyanide to carbon dioxide
and nitrogen by bacteria has been observed since the mid 1950s, but the phenomenon has never been
widely exploited to treat wastewaters containing cyanide.
The Homestake Mining Company in Lead, South Dakota, is successfully treating an average of 5.5
million gallons per day of wastewater containing cyanide, thiocyanate, and metal complexed cyanides
using a biological process. The treatment plant, in operation since August 1984, removes or degrades
90% to 98% of the cyanide and thiocyanide present in cyanidation mill and mine discharge
wastewaters.
The Homestake wastewater treatment process uses fixed film biological reactors known as rotating
biological contactors (RBCs). Each RBC consists of corrugated plastic disks 3.5 meters in diameter
arranged in a 7 meter long stack along a horizontal shaft. The disks rotate in the wastewater, at 40%
disk submergence. The surface area of each RBC ranges from 100,000 to 150,000 square feet. A mixed
culture of bacteria containing Pseudomonas paucimobitis mudlock (ATCC accession number 39294)
attach themselves to this large surface area and oxidize the cyanide and thiocyanate (U.S. Patent
4,440,644, "Method for the Biological Removal of Free and Complex Cyanides and Thiocyanates
from Water," and U.S. Patent 4,461,834, "Strain of Pseudomonas paucimobitis," are held by the
South Dakota School of Mines and Technology).
It is significant that cyanide, thiocyanate, and complexed metal cyanides can be removed biologically. Time and time again, biological treatment of hazardous wastes has been proven to be less
expensive than chemical treatment. For instance, the capital cost of Homestake Mines' biological
wastewater treatment plant was two-thirds the capital cost of a hydrogen peroxide plant.'
The operating costs of biological treatment systems are also considerably lower. The only chemical
addition required at the Homestake treatment plant is 11 milliliters of phosphoric acid per minute.
Because of the relationship between growth and utilization kinetics, biological systems are self-
regulating. This means that the entire system can be controlled by a relatively small operating staff.
Operating costs in 1989 were $3.46 per kilogram of cyanide removed, or $0.22 per cubic meter of
water treated.1
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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