USE OF LONG SOLIDS RETENTION TIMES TO
IMPROVE THE ACTIVATED SLUDGE BIODEGRADATION
OF A COMPLEX WASTE
Karen Hagelstein, Assistant Professor
South Dakota School of Mines and Technology
Rapid City, South Dakota 57701
Richard R. Dague, Professor
Civil and Environmental Eng.
University of Iowa
Iowa City, Iowa 52242
INTRODUCTION
The biodegradation of a complex organic waste from the pyrolysis of refuse was the research challenge. The contributions were improvements in the biodegradation of a complex waste and the application of toxicity testing to treatment efficiency monitoring.
The characteristics of the pyrolysis waste dictated the nature of the treatment approach evaluated.
The pyrolysis waste is a refuse-derived fuel product containing high concentrations of polynuclear
aromatic hydrocarbons (PAHs) and consequently, a very high organic oxygen demand (BOD5 =
20,000 mg/1). The ubiquitous sources of PAHs in the environment and biocidal, refractory properties
of PAHs in the pyrolysis waste [1] offered the biodegradation challenge. The PAHs are highly adsorb-
able, volatile, refractory, and toxic [2]. These properties are conducive to volatilization, long retention times, and high dilution treatment methods.
Biomass adaptation to industrial wastewaters and the use of a long retention time has been
reported to affect efficient decomposition of problematic, hazardous organic in a biological system
[3]. Studies have shown that microorganisms consume the least toxic fraction of petroleum (normal
alkanes) in a few days or months, depending on temperature and nutrient supply. The fraction containing aromatics and naphthalenes is more toxic than the alkanes and degrades more slowly [2].
Therefore, for complete biodegradation, time is necessary.
High sludge age activated sludge process operational data have been rare. Grutsch [4] reported
results from municipal, refinery, and chemical plants which provided insight into the biological oxidation by activated sludge. Not only is the amount of biomass produced minimized in high sludge age
systems due to low cell maintenance requirements, but the biomass properties from high and low
sludge age units are contrasting. The high sludge age biomass is stable, has a minimal respiration rate,
and requires no stabilization of waste sludge [4].
The second research objective was to examine the treatment effectiveness of the long solids retention times by means of a bioassay to determine the residual toxicity of the pyrolysis waste. Toxicologi-
cal data suggest that Daphnia species are among the most sensitive species tested [5,6]. Because of
their sensitivity to industrial effluents, daphnids have been suggested for screening toxicity tests for
industries to monitor the toxicity of treated effluents and the efficiency of waste treatment facilities
[7]. Gehrs et al. [8], applied the short-term bioassay using Daphnia to test the toxicity of different
fractions of a complex waste. This research utilized toxicity testing of this complex wastewater effluent, rather than an individual chemical, as a monitoring method to be compared to the standard measurements.
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