69    RESPIROMETRIC INVESTIGATION OF ACTIVATED
SLUDGE BIOINHIBITION BY COBALT/MANGANESE
CATALYST
Stephen W. C. Constable, Consulting Engineer
The Du Pont Company
Newark, Delaware 19711
Alan F. Rozich, Program Manager
Environmental Resources Management, Inc.
Exton, Pennsylvania 19341
Robert DeHaas, Senior Engineer
The Du Pont Company
Wilmington, North Carolina 28401
Richard J. Colvin, Project Engineer
Environmental Resources Management, Inc.
Exton, Pennsylvania 19341
INTRODUCTION
The Du Pont Company is a diversified chemical and synthetic fibers producer. Wastewater discharges from one of its integrated production facilities are treated on-site in a two-stage extended
aeration activated sludge system. The wastewaters from a polymer intermediate production facility
are discharged after equalization into the first stage treatment system. After typically removing 95%
as chemical oxygen demand (COD) of the organic acid-based wastewater load, the discharges are
further treated along with other site manufacturing wastewaters in the second stage activated sludge
system. The first stage system typically treats 0.5 mgd of a moderately low pH (4-5) wastewater of
high organic strength (feed COD varying from 1 to 1.5%).
A decline in organic treatment efficiency in the first stage activated sludge system was noted after
completion of process modifications that were required for air permit compliance. Wastewater feed
samples were analyzed for possible inhibitory agents and elevated concentrations of cobalt and
manganese were discovered. The most likely source of these metals is a soluble cobalt/manganese
organic acid production catalyst. Although a number of production area sources may routinely
contribute a cumulative total of up to 1 ppm of catalyst (as cobalt) to the wastewater treatment plant
feed, the major source of catalyst discharge to the waste treatment system is believed to be a solids
slurry, which was diverted to waste treatment to maintain production while the air permit process
modifications were performed. Soluble cobalt concentrations in the equalization basins were measured at levels ranging from 10 to 40 ppm after the slurry was introduced into waste treatment.
The inhibitory effect of metals on activated sludge bacteria has been well documented, with some
pertinent data summarized in Table 1. However, little information is available in the technical literature concerning cobalt-induced inhibition. The most conclusive study available' indicates that soluble
cobalt (as CoCl2) concentrations as low as 1 ppm could be inhibitory. Since soluble cobalt concentrations in the site's first stage waste treatment feed were measured at levels in excess of I ppm,
respirometric studies were undertaken to determine the effect of the catalyst discharges. These studies
were undertaken at Du Pont's Engineering Test Center (ETC) in Newark, Delaware and by Bioprocess
Engineering Inc.(BEI), at Bioscience Management, Inc. laboratories in Allentown, Pennsylvania.
TEST METHODOLOGY
A sample of the first stage activated sludge aeration basin influent wastewater was analyzed by ETC
for a variety of metals and other cations known to be present in the wastewater. Influent wastewater
sample metals concentrations are tabulated against literature values in Table I. A comparison of the
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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