18    ANAEROBIC TREATMENT OF HIGH STRENGTH, HIGH
SULFATE WASTES
Barry L. Hilton, Research Assistant
Jan A. Oleszkiewicz, Associate Professor
Department of Civil Engineering
The University of Manitoba
Winnipeg, Manitoba
Canada R3T 2N2
INTRODUCTION
The high rate of accumulation of sulfates constitutes a growing environmental concern. Large
quantities of sulfates are generated by the electric power industry from flue gas desulfurization
sludges. The pulp and paper industry, pharmaceutical industry, and segments of the food industry
produce effluents which are high in sulfates and other species of oxidized sulfur compounds. The
application of methanogenic treatment processes to these wastes may be hindered by the resulting
reduction of these oxidized sulfur species by sulfate reducing bacteria (SRB) to sulfides. The threshold
of inhibition of sulfides, generally acknowledged to be toxic to biomass, in particular the methane
producing bacteria (MPB), has been assumed to be 200-300 mg S2"/L [1].
The selectivity of various sulfate reducing bacteria and methane producing bacteria for energy
substrates is quite varied. As shown in several recent studies, the spectrum of organics utilized by
SRB's is much wider than in the case of methanogens [2-4]. This explains early successes in utilizing
SRB's to accelerate anaerobic stabilization of sewage sludges [5,6]. More recently, the mixed-culture
work of Middleton and Lawrence [7], DLA Inc. [8], Hilton et al. [9], Oleszkiewicz and Hilton [10,11],
and Olthof et al. [12] have indicated substantial advantages of sulfidogenic pathways in application to
high sulfate complex industrial waste streams. In the sulfidogenic pathways, organic matter is oxidized using sulfate as an electron acceptor in contrast to the conventional methanogenic pathway
where C02 acts as an electron acceptor and where the volatile fatty acids are broken down to methane
as an end product.
Middleton and Lawrence [7], and Obayashi and Cork [13], using seed cultures obtained from
anaerobic digesters, grew SRB's to the exclusion of methanogens, using acetate as the carbon source.
In another study [14], the acetate consumption rate was shown to be 15-fold higher for SRB than for
MPB's. Lovley et al. [15] found that the SRB's usually outcompete MPB due to lowering the partial
pressure of hydrogen below levels that could effectively be utilized by MPB. Based upon those
studies, one would expect that in the presence of fermentative bacteria forming short-chain fatty
acids, especially lactate and acetate, SRB's would oxidize the lactate and acetate to carbon dioxide and
water and would reduce sulfate to sulfide. In the process, one would expect that the methanogenic
population either would be outcompeted for substrates or would be inhibited by the generated sulfide.
PURPOSE AND SCOPE
The purpose of this study was to maximize sulfate reduction using a minimal quantity of carbon. It
was expected that the lactate formed during acidogenesis would first be incompletely oxidized to
acetate and carbon dioxide by incomplete oxidizing SRB's then the remaining acetate would be
completely oxidized to carbon dioxide by the complete oxidizing SRB's. In other words, it was
expected that the mixed culture would consist of acidogenic anaerobes, incomplete oxidizing SRB's,
and complete oxidizing SRB's. Continuous stripping of residual sulfides was operated to eliminate
problems of transient inhibition. High sulfate loads, in excess of 1.0 g S6 + /L/d were accompanied by
high carbon loads in excess of 1.5 g TOC/L/d in order to define the optimum conditions for sulfate
reduction in terms of the carbon to sulfur (C/S) ratio in the feed and in terms of maximum organic
loading (Bv) and sulfate-sulfur loading (L,) to the system.
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