Section One
TOXIC AND HAZARDOUS WASTES
1    TOXICITY REDUCTION-HAVE THE BUGS HAD IT?
W. Wesley Eckenfelder, Jr., Board Chairman
AWARE Incorporated
Nashville, Tennessee 37228
INTRODUCTION
For many years the design and operation of the activated sludge process was directed toward the
removal of BOD. A variety of relationships have been developed to describe the kinetics of the process
in terms of sludge age, hydraulic detention time, and biomass concentration. Biomass concentration,
in most cases, has been related to the mixed liquor volatile suspended solids (MLVSS).
There are several factors which must be considered in interpreting process kinetics as related to
BOD. These are:
1. BOD is nonspecific and measures all of the biodegradable carbon present in the
wastewater.
2. Residual BOD's after biological treatment may be expected to range from 3 to 10 mg/L.
The accuracy of the BOD test causes considerable variation in these effluent values.
3. In most cases, the BOD remaining after biological oxidation is no longer the original
organics present in the wastewater but rather oxidation by-products. Because of these
factors, kinetic relationships as related to BOD are frequently of questionable value.
Recent legislation has changed the emphasis of biological wastewater treatment process performance. Consideration must now be given to residual priority pollutants in (ig/L levels and aquatic
toxicity as defined by a bioassay. It becomes obvious that kinetic relationships in terms of BOD are no
longer applicable in these cases.
Let us consider the mechanisms of removal of specific priority pollutants in the activated sludge
process. These are: stripping, sorption on the biological floes, and biological oxidation. In most cases,
sorption of organics in the biological floes is not a major factor in the process. Exceptions to this are
Lindane and PCBs which have been reported to accumulate on the biological floes.
Most volatile organics will be both air stripped and biodegraded as shown in Table I. Recent
legislation limiting the emission of volatile organic carbon to the atmosphere requires careful consideration of the fate of volatile compounds in the activated sludge process.
Considering chlorinated hydrocarbons, as the number of chlorine atoms on the molecule increases,
the biodegradation rate decreases and a greater percentage of the compound is stripped as shown in
Figure 1. It is because of this phenomena that permissible influent concentrations of VOC should be
related to the biodegradability of the compound, i.e., what fraction will be biodegraded and what
fractions stripped.
There are several factors which must be considered with respect to biodegradation and achievable
effluent concentrations for toxic organics in the activated sludge process.
Acclimation
Tabak, et al.2 have shown that a majority of the priority pollutants will acclimate resulting in
complete degradation in seven days using a static culture flask procedure. In this procedure, yeast
extract is used as a synthetic medium with the test compound in a 7-day static incubation at 25°C
followed by three weekly subcultures using settled domestic wastewater as the microbial innoculum.1
A number of the organics, however, required considerably longer acclimation periods as shown in
Figure 2. It should be noted that the static culture flask procedure is purely a screening technique and
that under process operating conditions with a variety of wastewater compositions, acclimation may
43rd Purdue Industrial Waste Conference Proceedings, & 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.