DYNAMICS OF PRODUCT FORMATION IN ACTIVATED SLUDGE SYSTEMS

ELIYAHU MILLER, Purdue University

Abstract

Research during the past decade has suggested that much of the organic matter in the effluent from the activated sludge process is of microbial origin rather than being residual substrate from the influent. Consequently if operational control could be exerted over effluent quality through manipulation of the environment in which the sludge is grown it might be possible to control the treatability of the activated sludge effluent in downstream treatment operations. Conversely, if uncontrollable factors exert a greater effect on effluent quality than the controllable ones, engineering control of effluent quality may not be possible. This research was undertaken to assess the potential for exerting engineering control over the nature of the organic matter in activated sludge effluents. To accomplish this goal lab-scale activated sludge reactors were operated under eight experimental conditions in a two-level, three-factor experimental design. Two of the factors, pH and dissolved oxygen (DO) concentration represented controllable variables while the third, the nature of the influent carbon source, represented a variable not generally under engineering control. The pH levels studied were 6 and 8, and the DO concentrations were 1.0 and 7.0 mg/l, and the types of substrate were simple and complex. The reactors were operated at a solids retention time at 9.0 days for a period of approximately 135 days so that the effects of a fourth variable, the time-dependent changes in the microbial culture, could also be studied. In addition to routine performance sampling, five long-term (9 day) composite samples were collected at evenly-spaced intervals to allow assessment of effluent quality. The organic matter in each of those samples was subjected to a series of specialized tests which included: nonbiodegradable fraction (NBF), non-adsorbable fraction (NAF) on activated carbon, molecular weight distribution (MWD), adsorption isotherm on activated carbon, rate of carbon adsorption, adsorbability on an XAD-2 column, NBF and MWD following adsorption. All of the reactors exhibited similar cyclical phenomena in performance which suggested that the cycling was a fundamental characteristic of the microbial population. Cycling also occurred in the NBF, NAF, and low molecular weight fraction (LMF) and the magnitudes of the time-dependent variations were greater than the variations associated with the other independent variables. This suggests that it would be impossible to totally control effluent quality. Taken within the constraints of the time-dependent variations, however, both pH and DO concentration exerted a stronger effect on adsorbability than the feed composition. This confirmed that the majority of the organic matter was of microbial origin and suggested that proper manipulation of the pH and DO concentration in the reactor can influence the treatability of effluent organic matter by adsorption processes. The MWDs varied with time and their variation appeared to be associated with the formation and breakup of high molecular weight materials. The low molecular weight materials were generally present in greatest amount and exhibited the least variability. The adsorption isotherms could be characterized reasonably well by the Freundlich equation although the parameters varied with time. Some, but not all of the Freundlich parameters were similar to those of humic materials. This, together with the fact that there was little correlation between the quality parameters, suggests that no single class of compounds dominated the character of the effluent. Finally it was noted that the biodegradability of the organic matter as well as its MWD was altered by contact with activated carbon, suggesting that chemical reactions occurred during passage through the activated carbon columns.

Degree

Ph.D.

Subject Area

Civil engineering

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