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Rinzema, A. (1988)
Publisher: Rinzema
Languages: English
Types: Doctoral thesis
<p>This thesis describes research on the application of granular sludge bed upflow reactors for anaerobic treatment of wastewaters contaminated with lipids and sulfate, two contaminants that have so far seriously hampered the application of anaerobic treatment in several branches of industry. The Upflow Anaerobic Sludge Bed reactor is the most popular anaerobic treatment system at this moment. However, it is mainly applied to wastewaters with readily biodegradable dissolved contaminants, and hardly to more complex wastewaters.<p>Two problems can occur in anaerobic treatment of lipid containing wastewater, viz. (1) inhibition of anaerobic bacteria by long chain fatty acids, and (2) flotation of the biomass. Sulfate may cause direct and indirect inhibition of methanogenic and acetogenic bacteria.<p>The first part of the thesis deals with the inhibitory effect of long chain fatty acids (LCFA) in anaerobic digestion. Inhibition can occur after overloading, i.e. during the start-up period of the digester, or as a result of a shock load. LCFA affect especially the acetotrophic methanogens, above a critical threshold concentration they exert a bactericidal effect. The methanogens do not adapt to LCFA. The threshold concentration for capric acid - one of the most toxic saturated acids - is approximately 1 kg/m <sup><font size="-2">3</font></SUP>. The precise value of the threshold concentration depends upon the mass transfer characteristics of the anaerobic reactor, and upon the particle size and specific activity of the biomass aggregates. Furthermore, the presence of phospholipids may enhance the inhibitory effect of LCFA. Inhibition can be prevented by addition of soluble calcium salts to the wastewater. However, the addition of calcium cannot eliminate the second deleterious effect of a shock load of LCFA, viz. flotation and subsequent wash-out of biomass aggregates.<p>The second part of this thesis describes the anaerobic treatment of solutions of LCFA and emulsions of triglycerides in the Expanded Granular Sludge Bed reactor. With LCFA solutions this modified upflow reactor can achieve a mineralization efficiency of at least 85-90% at space loading rates of ca. 30 kg COD/m <sup><font size="-2">3</font></SUP>.day. Modification of the sludge separation system is required to reduce sludge wash-out during treatment of triglyceride emulsions. A novel sieve-drum separator was developed, which allows stable operation. Although the treatment capacity is significantly lower with triglyceride emulsions than with LCFA solutions, the EGSB reactor with sieve-drum separator can accommodate higher organic and hydraulic loading rates than previously described anaerobic filter reactors. Upscaling of the EGSB system and flotation of lipids require further research.<p>The third part of the thesis deals with the inhibitory effect of sulfide and sodium sulfate. From pH 6.4 to 7.2 approximately 250 mg H <sub><font size="-2">2</font></sub> S per litre causes a 50% decrease of the maximum specific activity of acetotrophic methanogens. The inhibitory effect of a given H <sub><font size="-2">2</font></sub> S concentration increases significantly when the pH approaches 8. Consequently, an increase of the pH level in the anaerobic digester above ca. 7.2 is not beneficial. Immobilization in biomass aggregates or films may provide protection against H <sub><font size="-2">2</font></sub> S inhibition. Propionate degradation may be the rate limiting step during treatment of sulfate containing wastewater, because it is affected more severely by sulfide than acetotrophic methanogenesis. At extremely high sulfate concentrations, also inhibition by cations has to be considered. At neutral pH levels, sodium concentrations up to 5 g/l cause no inhibition of acetotrophic methanogens. A sodium concentration of 10 g/l causes a 50% decrease of the maximum specific acetotrophic methanogenic activity, 14 g/l causes complete inhibition. Acetotrophic methanogens do not adapt to high sodium concentrations.
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