Process Technology for the Biological Treatment of Toxic Organic Wastes
Authors: Rozich AF , Gaudy AF 
Published: December 2008

The problem of toxic waste disposal is one which has global implications regarding the preservation of the integrity of the environment. Concomitantly, it has become increasingly obvious that, because of the economics of water pollution control, biological treatment processes will be expected to incur an ever-increasing share of the task of purifying waste streams. With increasing industrialization, waste streams will more frequently contain organic compounds that are classified as toxic or inhibitory. The purpose of this report is to present and collate the results of several year's work in our laboratories that concerned the aerobic biological treatment of toxic organic wastes. Based on this work, it is feasible to suggest a number of engineering strategies applicable to the biological treatment of inhibitory organics. The thrust of this paper is to present this information in such a way that it can be readily utilized by pollution control professionals for designing and operating aerobic biological systems for treating biodegradable toxic organics.

In the first portion of the paper, the kinetics of microbial growth on toxic, or inhibitory, wastes and on nontoxic, or “conventional,” wastes are compared and contrasted by illustrating both the differences and similarities with the biodegradation kinetics of these different types of wastes. In order to have the ability to recommend relevant treatment models for the purpose of designing and operating effective treatment facilities, it is essential to characterize the nature, that is, inhibitory or noninhibitory, of microbial growth kinetics on a particular waste. Consequently, this portion of the paper delineates the criteria that are utilized to categorize a waste as toxic or nontoxic.

A key requirement for the design and operation of biological treatment facilities is to have a relatively accurate assessment of both the range and variability of the biokinetic constants that characterize the biomass that is responsible for purification of a particular waste. These constants can be classified as the biokinetic growth constants and the maintenance constants. The latter group is responsible for quantifying sludge production in a biological treatment system while the former group quantifies the relationship between biomass growth rate and exogenous substrate (waste) concentration. Methods for evaluating the maintenance constants are relatively straightforward and are the same for both inhibitory and noninhibitory wastes. The situation is different, however, for the biokinetic growth constants. For noninhibitory wastes, only two biokinetic constants, μmax and Ks, are utilized in the growth rate function (Monod equation), whereas for inhibitory wastes three biokinetic growth constants, μmax, Ks, and Ki, are contained in the inhibition growth function (Haldane equation). This makes the kinetic analysis and the task of biokinetic constant evaluation for inhibitory compounds more difficult. In this section of the paper, a number of methodologies are presented that can be used for collecting growth data and for evaluating the biokinetic constants for acclimated population growing on inhibitory compounds.

The final portion of the paper concerns the development and application of design and operating equations that can be used for activated sludge processes treating toxic wastes. These are developed by inserting an inhibition growth function into the mass balance equations for an activated sludge reactor. The effect of the growth kinetics of toxic organics on activated sludge design and operation policies are illustrated via the utilization of dilute-out curves. Additionally, the genesis and application of critical point curves which quantify the operational location of the critical, or peak, growth rate μ*, which characterizes activated sludge reactors treating inhibitory wastes, are shown; the use of these curves can maximize treatment efficiency while preventing sudden effluent deterioration and washout in activated sludge systems treating toxic or inhibitory wastes.