Role of nickel in high rate methanol degradation in anaerobic granular sludge bioreactors

The effect of nickel deprivation from the influent of a mesophilic (30°C) methanol fed upflow anaerobic sludge bed (UASB) reactor was investigated by coupling the reactor performance to the evolution of the Methanosarcina population of the bioreactor sludge. The reactor was operated at pH 7.0 and an organic loading rate (OLR) of 5–15 g COD l−1 day−1 for 191 days. A clear limitation of the specific methanogenic activity (SMA) on methanol due to the absence of nickel was observed after 129 days of bioreactor operation: the SMA of the sludge in medium with the complete trace metal solution except nickel amounted to 1.164 (±0.167) g CH4-COD g VSS−1 day−1 compared to 2.027 (±0.111) g CH4-COD g VSS−1 day−1 in a medium with the complete (including nickel) trace metal solution. The methanol removal efficiency during these 129 days was 99%, no volatile fatty acid (VFA) accumulation was observed and the size of the Methanosarcina population increased compared to the seed sludge. Continuation of the UASB reactor operation with the nickel limited sludge lead to incomplete methanol removal, and thus methanol accumulation in the reactor effluent from day 142 onwards. This methanol accumulation subsequently induced an increase of the acetogenic activity in the UASB reactor on day 160. On day 165, 77% of the methanol fed to the system was converted to acetate and the Methanosarcina population size had substantially decreased. Inclusion of 0.5 μM Ni (dosed as NiCl2) to the influent from day 165 onwards lead to the recovery of the methanol removal efficiency to 99% without VFA accumulation within 2 days of bioreactor operation

Effect of methanogenic substrates on anaerobic oxidation of methane and sulfate reduction by an anaerobic methanotrophic enrichment

Anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) is assumed to be a syntrophic process, in which methanotrophic archaea produce an interspecies electron carrier (IEC), which is subsequently utilized by sulfate-reducing bacteria. In this paper, six methanogenic substrates are tested as candidate-IECs by assessing their effect on AOM and SR by an anaerobic methanotrophic enrichment. The presence of acetate, formate or hydrogen enhanced SR, but did not inhibit AOM, nor did these substrates trigger methanogenesis. Carbon monoxide also enhanced SR but slightly inhibited AOM. Methanol did not enhance SR nor did it inhibit AOM, and methanethiol inhibited both SR and AOM completely. Subsequently, it was calculated at which candidate-IEC concentrations no more Gibbs free energy can be conserved from their production from methane at the applied conditions. These concentrations were at least 1,000 times lower can the final candidate-IEC concentration in the bulk liquid. Therefore, the tested candidate-IECs could not have been produced from methane during the incubations. Hence, acetate, formate, methanol, carbon monoxide, and hydrogen can be excluded as sole IEC in AOM coupled to SR. Methanethiol did inhibit AOM and can therefore not be excluded as IEC by this study

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 0.5 l to which solutions of ZnSO4 (800–5800 mg Zn2+/l) 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 mg l−1, making use of an ion-selective S2− electrodePrecipitation of Zn2+ with S2- was studied at room temperature in a continuously stirred tank reactor of 0.51 to which solutions of ZnSO4 (800-5800 mg(-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 3.2 x 10(-19) to 3.2 x 10(-4) mg l(-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 mum for S2- levels dropping from 3.2 x 10(-4) to 3.2 x 10(-18) mg l(-1). At 3.2 x 10(-11) Mg l(-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 mg l(-1) while ZnS particles formed with a mean geometric diameter of about 10 mum. Below 3.2 x 10(-11) mg l(-1) of S2- insufficient sulfide was added for complete zinc precipitation. At S2- levels higher than 3.2 x 10(-11) mg l(-1) the effluent zinc concentration increased due to the formation of soluble zinc sulfide complexes as confirmed by chemical equilibrium model calculations. (C) 2003 Elsevier Science Ltd. All rights reserved

Use of Biogenic Sulfide for ZnS Precipitation

A 600 ml continuously stirred tank reactor was used to assess the performance of a zinc sulfide precipitation process using a biogenic sulfide solution (the effluent of a sulfate-reducing bioreactor) as sulfide source. In all experiments, a proportional-integral (PI) control algorithm was used to control the pH and the sulfide (S2−) concentration at the desired level in the precipitator. The pS (defined as: −log [S2−]) and pH were optimised using a chemical Na2S solution as sulfide source. A S2− concentration of 10−15M (i.e. pS 15) was found to be optimal for zinc sulfide precipitation, resulting in a residual zinc concentration of 0.07 mg/l from a 3 g/l Zn2+ influent, for both chemical Na2S and biogenic sulfide solutions. The mean particle size of the ZnS precipitates at pS 15 and pH 6.3 was 7.5 and 10.2 micron when using biogenic sulfide and chemical Na2S, respectively, indicating that both sulfide sources are adequate for solid–liquid separation by sedimentation. When biogenic sulfide instead of chemical Na2S was used, the efficiency of the ZnS precipitation process slightly decreased both in terms of zinc effluent concentration (at pS 10 and 20) and particle size of the precipitate (at pS 10, 15 and 20). This was shown to be attributed to the presence of various substances (phosphate, micro-nutrients, acetate, EDTA) present in the sulfate-reducing reactor effluent

Starch hydrolysis in autogenerative high pressure digestion: Gelatinisation and saccharification as rate limiting steps

Autogenerative high pressure digestion (AHPD) provides an integrated biogas upgrading technology, capable of producing biogas with a CH4 content exceeding 95% at pressures up to 90 bar. Hydrolysis is generally regarded as the rate-limiting step in the anaerobic digestion of complex organic matter, governing the volatile fatty acid (VFA) production rate for subsequent conversion to methane. Starch hydrolysis rates in AHPD systems were studied and the potential risk for VFA accumulation was assessed. Under the anticipated practical moderate pressure conditions at 30 °C, experimental CH4-content of the biogas improved from 49 to 73 ± 2% at atmospheric and elevated pressure, respectively. Furthermore, no significant effect of pressure on the hydrolysis was found. Like under atmospheric pressure, gelatinisation was the rate-limiting step for particulate starch (0.05 d-1) and saccharification for gelatinised starch (0.1 d-1). Because no effect was observed on starch, an effect on the hydrolysis rate of more complex organic matter like (ligno-)cellulose is also not anticipated