CONSIDERATIONS IN BIOLOGICAL TREATABILITY
STUDIES FOR INDUSTRIAL WASTEWATERS
Robert G. Kunz, Senior Environmental Engineer
John G. Landis, Scientific Systems Analyst
Air Products and Chemicals, Inc.
AHentown, Pennsylvania   18105
INTRODUCTION
The purpose of a treatability test is to obtain design information on contaminant
removal rates, sludge production and oxygen consumption.   Over the life of an activated
sludge plant, the loading conditions for the plant vary as a result of daily, seasonal or
yearly influent variations.   To test every plant operating condition in the laboratory is
usually impractical; therefore, a mathematical model is used to translate the experimental
results into predictions of plant performance under different operating conditions.
Many conventional models are often inadequate because as operating conditions change
the biokinetic "constants" must change in order to describe properly substrate removals,
sludge production and oxygen consumption.   Generally, the failure of the model predictions is a result of assumptions made to simplify the mathematical model for each
application, in particular, the assumption that the active biological mass is equal to the
mixed liquor volatile suspended solids (MLVSS) in the aeration basin.   Aside from obvious experimental errors, scatter is frequently caused by the use of these simple models
when evaluating treatability test data obtained under apparently identical conditions.
This chapter explores, by way of several examples, the effect of sludge age on the rate
of substrate removal and outlines a design model which distinguishes between active biological solids and suspended solids.   The usefulness of this model is illustrated by comparing its predictions with experimental treatability data.
SUBSTRATE REMOVAL EQUATIONS AS A BASIS FOR MODELING
An equation for substrate removal as a function of substrate and microorganism concentration forms a realistic description of the biochemical reactions involved in waste
treatment.   The mathematical model allows for prediction of the performance of the
waste treatment process beyond the actual conditions investigated.   To accomplish this,
the mathematical model must be detailed enough to reflect, however empirical, the results of the processes occurring on a microscopic level.
By way of review, one model in general use is the Michaelis-Menten equation, written
as follows:
ii -        *SX (1)
dt Ks + S
where S and X are the concentrations of substrate and microorganisms respectively, t is
the time, and k and Ks are constants representative of each particular system.   The k
represents the maximum rate of substrate utilization per mass of microorganisms and Ks,
the Michaelis-Menten or half-velocity constant, is equal to that substrate concentration
at which the rate of substrate removal is one-half its maximum value.   In many cases, the
microorganism concentration changes are small during an experimental run so that the
concentration can be considered to be a constant.
Equation 1 attempts to represent in a single continuous formulation two separate
regimes in waste stabilization:   one at high substrate concentrations where substrate is
not limiting, Equation 2; and the other at low substrate levels where removal is dependent on substrate concentration, Equation 3, i.e..
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