Conversion of methanogenic substrates in anaerobic reactors : metals, mass transport, and toxicity

The EGSB systems represents an attractive option to extend further the use of anaerobic technology for wastewater treatment, particularly with respect to waste streams originating from chemical industries. Frequently chemical waste streams are unbalanced with respect to nutrients and/or micronutrients and furthermore these streams may contain toxic-biodegradable compounds. To reduce toxicity high recycle ratios may be applied as in the case of EGSB reactors however, this at the same time may adversely affect the substrate conversion rates due to mass transport limitations. These aspects were considered in this research. The main objectives of the work described in the thesis were to assess the kinetic impact of (i) nickel and cobalt limitations on the methanogenic degradation of methanol, (ii) the relative importance of mass transport phenomena in methanogenic granular sludge, and (iii) to characterize the toxicity and biodegradation of formaldehyde in the anaerobic conversion of methanogenic substrates. Particularly in the case of anaerobic systems, the ubiquitous presence of sulfides resulting from sulfate reduction and organic matter mineralization will lead to strong metal precipitation as metal sulfides. These precipitated metals are not directly available for the biomass. It is shown here that precipitation-dissolution kinetics of metal sulfides may play a key role in the bioavailability of essential metals.We furthermore showed that nutrient limitations can be overcome if the essential metals are added continuously at a proper rate so that their availability in solution can fulfill the requirement for biomass activity and growth. The metal dosing rates utilized range from 0.05 to 0.2μmol/h corresponding to metal to methanol ratios of 0.1-0.4 (μmol/g methanol-COD) and these values agree well with the calculated metal requirements based on the biomass yield and Ni and Co content of methylotrophic methanogens ( Methanosarcina sp.) grown on methanol. With respect to mass transport phenomena, it was found that at liquid upflow velocities exceeding 1 m/h liquid-film (external) mass transfer limitations normally can be neglected for acetate degrading methanogenic granular sludge. On the contrary, a clear increase in apparent K S -value was found at increasing mean granule diameters. Herewith we have clearly shown that anaerobic biofilms can be internally transport limited. In addition we also demonstrated that substrate transport in the biofilm can be described by diffusion, and that there was no evidence of convective flow due to biogas production in the anaerobic granules. Regarding formaldehyde conversion and toxicity, we demonstrated that methanogenesis from formaldehyde mainly occurred after intermediate formation of methanol and formate.Furthermore it was shown that the characteristics of formaldehyde toxicity were independent of the methanotrophic substrate used (methanol or acetate). Formaldehyde toxicity was in part reversible since once the formaldehyde concentration was extremely low or virtually removed from the system, the methane production rate was partially recovered. Since the degree of this recovery was not complete, we conclude that formaldehyde toxicity was also irreversible. The irreversible toxicity likely can be attributed to biomass formaldehyde-related decay. Independent of the mode of formaldehyde addition (slug or continuous), a certain amount of formaldehyde irrevocably reduced the methane production rate to a certain extent, hence the irreversible toxicity was dependent on the total amount of formaldehyde added to the system. This finding suggest that in order to treat formaldehyde containing waste streams a balance between formaldehyde-related decay and biomass growth should be attained. We furthermore showed that the biomass diversity may play a key role in the outcome of toxicity tests. Therefore we recommend that metabolic characterization of methanogenic sludge should be an integral component of toxicity studies and we describe a newly developed methodology that can be used for that purpose.