Look W. Hulshoff Pol

Thermophilic sulfate reduction and methanogenesis with methanol in a high rate anaerobic reactor

Sulfate reduction outcompeted methanogenesis at 65 degrees C and pH 7.5 in methanol and sulfate-fed expanded granular sludge bed reactors operated at hydraulic retention times (HRT) of 14 and 3.5 h, both under methanol-limiting and methanol-overloading conditions. After 100 and 50 days for the reactors operated at 14 and 3.5 h, respectively, sulfide production accounted for 80% of the methanol-COD consumed by the sludge. The specific methanogenic activity on methanol of the sludge from a reactor operated at HRTs of down to 3.5 h for a period of 4 months gradually decreased from 0. 83 gCOD. gVSS(-1). day(-1) at the start to a value of less than 0.05 gCOD. gVSS(-1). day(-1), showing that the relative number of methanogens decreased and eventually became very low. By contrast, the increase of the specific sulfidogenic activity of sludge from 0. 22 gCOD. gVSS(-1). day(-1) to a final value of 1.05 gCOD. gVSS(-1). day(-1) showed that sulfate reducing bacteria were enriched. Methanol degradation by a methanogenic culture obtained from a reactor by serial dilution of the sludge was inhibited in the presence of vancomycin, indicating that methanogenesis directly from methanol was not important. H(2)/CO(2) and formate, but not acetate, were degraded to methane in the presence of vancomycin. These results indicated that methanol degradation to methane occurs via the intermediates H(2)/CO(2) and formate. The high and low specific methanogenic activity of sludge on H(2)/CO(2) and formate, respectively, indicated that the former substrate probably acts as the main electron donor for the methanogens during methanol degradation. As sulfate reduction in the sludge was also strongly supported by hydrogen, competition between sulfate reducing bacteria and methanogens in the sludge seemed to be mainly for this substrate. Sulfate elimination rates of up to 15 gSO(4)(2-)/L per day were achieved in the reactors. Biomass retention limited the sulfate elimination rate.

Control of the sulfide (S2-) concentration for optimal zinc removal by sulfide precipitation in a continuously stirred tank reactor

Precipitation of Zn2+ with S2- was studied at room temperature in a continuously stirred tank reactor of 0.5l to which solutions of ZnSO4 (800-5800 mgl(-1) Zn2+) and Na2S were supplied. The pH was controlled at 6.5 and S2- concentration in the reactor was controlled at set point values ranging from 3.2x10(-19) to 3.2x10(-4) mgl(-1), making use of an ion-selective S2- electrode. In steady state, the mean particle size of the ZnS precipitate decreased linearly from 22 to 1 microm for S2- levels dropping from 3.2x10(-4) to 3.2x10(-18) mgl(-1). At 3.2x10(-11) mgl(-1) of S2-, the supplies of ZnSO4 and Na2S solutions were stoichiometric for ZnS precipitation. At this S2- level, removal of dissolved zinc was optimal with effluent zinc concentration <0.03 mgl(-1) while ZnS particles formed with a mean geometric diameter of about 10 microm. Below 3.2x10(-11) mgl(-1) of S2- insufficient sulfide was added for complete zinc precipitation. At S2- levels higher than 3.2x10(-11) mgl(-1) the effluent zinc concentration increased due to the formation of soluble zinc sulfide complexes as confirmed by chemical equilibrium model calculations.

Anaerobic biodegradability of phthalic acid isomers and related compounds.

All three phthalic acid isomers ( ortho, meta and para benzene dicarboxylic acid) are produced in massive amounts, and used in the chemical industry as plasticizers or for the production of polyester. Wastestreams generated during the production of phthalate isomers generally contain high concentrations of aromatic acids. To study the potential biodegradability of these primarily anthropogenic compounds in anaerobic bioreactors, biodegradability studies were performed. Compounds tested were benzoate, ortho-phthalate, isophthalate, terephthalate, dimethyl phthalate, dimethyl terephthalate, para-toluate and para-xylene. Seed materials tested were two types of granular sludge and digested sewage sludge. It was found that all phthalate isomers and their corresponding dimethyl-esters, could be completely mineralized by all seed materials studied. Lag phases required for 50% degradation of these compounds, ranged from 17 to 156 days. The observed degradation curves could be explained by growth of an initially small amount of organisms in the inoculum with the specific ability to degrade one phthalate isomer. The observed order in the length of the lag phases for the phthalate isomers is: phthalate < terephthalate < isophthalate. This order appears to be related to the environmental abundancy of the different phthalate isomers. The initial step in the degradation pathway of both dimethyl phthalate esters was hydrolysis of the ester sidechain, resulting in the formation of the corresponding mono-methyl-phthalate isomer and phthalate isomer. The rate limiting step in mineralization of both dimethyl phthalate and dimethyl terephthalate was found to be fermentation of the phthalate isomer. Para-toluate was degraded only by digested sewage sludge after a lag phase of 425 days. The observed degradation rates of this compound were very low. No mineralization of para-xylene was observed. In general, the differences in the lag phases between different seed materials were relatively small. These results indicate that the time needed for the start-up of anaerobic bioreactors treating wastewaters containing phthalic acid isomers, depends little on the microbial composition of the seed material applied, but may take several months.

Mathematical modelling as a tool to study population dynamics between sulfate reducing and methanogenic bacteria

The existing mathematical models of sulphate fed anaerobic reactors are reviewed. Special attention was put on pecularities of the description of sulphide inhibition and competition between sulphate reduction and methanogenesis in such systems. The paper also presents an integrated mathematical model of the functioning of a sulphate fed granular sludge reactor taking into account concentration gradients on substrates, intermediates, products and bacteria inside the reactor as well as multiple-reaction stoichiometry and kinetics. The developed model includes the following blocks: a) hydrodynamic block describing liquid flow as well as transport and distribution of the components along the reactor height; b) kinetic block including growth, metabolism, inhibition and competition of acidogenic, acetogenic, methanogenic and sulphate reducing bacteria in the system; c) physico-chemical block for calculation of pH in each compartment of the liquid phase; d) transfer block describing a mass transfer of gaseous components from the liquid to the gas phase. The integrated model was calibrated and validated using laboratory studies on the functioning of sulphidogenic granular sludge reactors, i.e. their start-up and the maximisation of sulphide yield in these reactors. The modelling of the reactor operation is supplemented with hypothetical computer simulations to illustrate the influence of engineering parameters on the operation performance and sulphate conversion of sulphidogenic reactors.

Biological sulfate reduction using synthesis gas as energy and carbon source.

Biological sulfate reduction was studied in laboratory‐scale gas‐lift reactors. Synthesis gas (gas mixtures of H2/CO/CO2) was used as energy and carbon source. The required biomass retention was obtained by aggregation and immobilization on pumice particles. Special attention was paid to the effect of CO addition on the sulfate conversion rate, aggregation, and aggregate composition.

Optimisation of sulphate reduction in a methanol-fed thermophilic bioreactor

Several methods were tested to optimise sulphate reduction and minimise methane formation in thermophilic (65 degrees) expanded granular sludge bed reactors fed with a medium containing sulphate and methanol. Lowering the pH from 7.5 to 6.75 resulted in a rapid decrease of methane formation and a concomitant increase in sulphate reduction. The inhibition of methane formation was irreversible on the short-term. Lowering the COD/SO4(2-) ratio (COD: chemical oxygen demand) from 6 to 0.34 (g/g) rapidly favoured sulphate reduction over methanogenesis. Continuous addition of 2 g L(-1) 2-bromoethanesulphonate was ineffective as complete inhibition of methanogenesis was obtained only for two days. Inhibition of methanogens by sulphide at pH 7.5 was only effective when the total sulphide concentration was above 1200 mg S L(-1). For practical applications, a relatively short exposure to a slightly acidic pH in combination with operating the reactor at a volumetric methanol-COD loading rate close to the maximum volumetric sulphide-COD formation rate.

The effect of sulphate on methanol conversion in mesophilic upflow anaerobic sludge bed reactors

Mesophilic (30 °C) upflow anaerobic sludge bed reactors were fed with an influent containing sulphate (2 g l-1) and methanol (1.33 g l-1). More than 90% of the methanol was mineralised to methane, while only ˜5–10% of the methanol was used for sulphate reduction. This pattern was independent of short-term pH variations in the range from 5 to 8, addition of acetate as co-substrate and type of granular seed sludge (methanogenic and sulphidogenic). On average 0.4 gSO42- lreactor-1 per day was reduced under these conditions. Also applying 1-day temperature shocks of 65 or 80 °C did not stimulate sulphate reduction. Sulphite, added as an alternative acceptor, appeared to be disproportionated to sulphate and sulphide. Results show that methanol conversion to methane in upflow sludge bed reactors is very stable in the presence of sulphate. This suggests that under mesophilic conditions, methanol is not a suitable feedstock for sulphate-reducing processes in such reactors.

Cultivation of well adapted pelletized methanogenic sludge

SummaryTreating soluble wastewaters anaerobically in UASB-reactors, the development of sludge-granules can be expected, provided that there are optimal growing conditions. Two types of granules were obtained, both with high specific activities and excellent settling properties. Type 1 mainly consists of short fragments of multicellular filaments of a bacterium showing much resemblance with a recently isolated acetate-degrading methanogenic bacterium (Zehnder et al, 1980). Type 2 is mainly made up by long filaments of presumably the same bacterium. The development of anaerobic bulking sludge must be prevented.

Use of 1H NMR to study transport processes in sulfidogenic granular sludge

Pulsed field gradient nuclear magnetic resonance (NMR) techniques have been applied to study diffusion and flow in a sulfidogenic granular sludge bed. When sulfidogenic granular sludge is exposed to a 20 MHz magnetic field, a multi-exponential spin-spin relaxation (T 2 ) with at least 5 populations is observed. One of these populations (T 2 ≈ 30 ms) is intracellular water. Diffusion measurements at 22°C with 1 H-water as tracer indicated that sulfidogenic granular sludge contains a distribution of diffusion coefficients between 1.0 × 10 −9 m 2 /s and 2.1 × 10 −9 m 2 /s. Analysing the data set using a monoexponential fit gives a general parameter that can be used to describe the apparent diffusion coefficient in granular sludge. This approach showed that sulfidogenic granular sludge cultivated in different reactor configurations (UASB, USSB and baffled reactors) has comparable diffusional characteristics. Finally, the use of flow and imaging measurements in sulfidogenic granular sludge beds is discussed.