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In this thesis, the development of a newbiotechnological process for the treatment of undiluted sulfidic spent caustics (SSC’s) using soda lake bacteria is described. SSC’s are waste solutions that are formed in the oil and gas industry due to the caustic (NaOH) scrubbing of hydrocarbon streams for the removal of sulfur compounds.Without treatment, SSC’s may impose serious environmental problems because of their alkalinity (pH>12), salinity (Na+ 5-12 wt%) and high sulfide (HS- and S2-) levels. Depending on the hydrocarbon stream that is treated, SSC’s may also contain organic sulfur compounds and monoaromatic hydrocarbons. Biological treatment of undiluted SSC’s would be a cheaper and safer alternative to the currently applied physico-chemical treatment methods (e.g., wet air oxidation or deep well disposal) since no additional chemicals are needed and the process works at ambient pressure and temperature conditions.
In chapter 2 the biological treatment of refinery SSC’s is described in continuously fed systems under haloalkaline conditions (i.e. pH 9.5; Na+ 0.8 M). The experiments were performed in gas-lift bioreactors operated under aerobic conditions at 35 oC. Sulfide removal was complete up to 27 mmol L-1 day-1 by conversion to sulfate (SO42-). The sulfide conversion was accomplished by haloalkaliphilic sulfide-oxidizing bacteria (HA-SOB) belonging to the genus Thioalkalivibrio. Members of the this genus are extremophiles that are able to oxidize sulfide under a broad range of haloalkaline conditions (0.3 - 4.3 M Na+ and up to pH 10.6). In this chapter, it wasalso shown that benzene, at influent concentrations ranging from 100 to 600 µM, was removed by 93% due to air-stripping and biodegradation. Microbial community analysis revealed the presence of haloalkaliphilic heterotrophic bacteria belonging to the genera Marinobacter, Halomonas andIdiomarina which might have been involved in the observed benzene removal.
Sour gases and SSC’s may also contain elevated amounts of methanethiol (MT; CH3SH). Hence, knowledge on the potential toxic effects of these type of compounds on the performance of this biotechnological process is required. Under sulfur (S0) forming conditions, MT reacts with biologically produced S0 particles resulting in a mixture of inorganic polysulfides (Sx2-), dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS). Respiration experiments with HA-SOB (Thioalkalivibrio mix) show in chapter 3 that biological oxidation of sulfide to S0 is inhibited by 50% (Ki value) at 0.05 mM MT. The measured Ki values for DMDS and DMTS were 1.5 and 1.0 mM, respectively.As DMDS and DMTS are products from the reaction between MT and S0, this reaction results in a partial detoxification of MT in a S0-producing bioreactor.The results from the respiration experiments as shown in chapter 3 indicate that the application of the biotechnological process for the treatment of H2S and MT containing gases and SSC’s is feasible as long as MT, DMDS and DMTS do not accumulate in the bioreactor.Accumulation of MT can be prevented by auto-oxidation of MT to DMDS, by the reaction between MT and biosulfur particles or biodegradation.
Chapter 4 discusses the biological treatment of synthetically prepared SSC’s containing both sulfide and DMDS.Continuously fed gas-lift bioreactor experiments showed that biological sulfide oxidation (4-10 mmol L-1 day-1) is possible in the presence of low concentrations of DMDSunder haloalkaline conditions (i.e., pH 9.5; Na+ 0.8 M).Sulfide was completely oxidized to SO42- by members of the genus Thioalkalivibrio (closely related to Thioalkalivibriosp.K90-mix). It was also shown that severe inhibition of thebiological sulfide oxidation capacity and process deterioration occurs at DMDS effluent concentrations between 0.1 and 0.9 mM. The measured DMDS removal efficiency amounted up to 40-70% (0.05-0.37 DMDS-S L-1 day-1), of which 25% could be attributed to air stripping. It is yet unclear what other processes contributed to the total DMDS removal and it can only be speculated that the remainder was removed by biological conversion and/or adsorption. Results from respiration experiments presented in chapter 4 reveal that pure cultures of HA-SOB (Thioalkalivibrio sp.K90-mix and Thioalkalivibrio sulfidophilus) as well as biosludge taken from a full-scale installation for H2S removal (Thiopaq) are more severely inhibited by MT than DMDS. Furthermore, the Ki values for DMDS and MT were lower for Thioalkalivibrio sp. K90-mix and Thioalkalivibrio sulfidophilus compared to Thiopaq sludge. From bioreactor and respiration experiments it follows that, to ensure stable process conditions, MT and DMDS concentrations need to be below 0.02 and 0.1 mM, respectively. This clearly demonstrates that treating SSC’s with elevated MT and DMDS concentrations will easily inhibit the sulfide oxidation capacity of the process. Although auto-oxidation of MT will result in (partial) detoxification due to the formation of DMDS, the effluent levels still need to be kept very low. Successful biological treatment of MT and DMDS containing SSC’s will depend on the biological degradation of these compounds. When rapid biodegradation of organic VSC’s can be achieved, the concentrations in the reactor will remain below the critical levels.
Chapter 5 shows that the application of a newly developed 2-step process for the biological treatments of SSC’s using HA-SOB allows significantly higher sulfide removal efficiencies compared to a 1-step process. The detoxification of sulfide by the abiotic oxidation to thiosulfate (S2O32-) in the first chemical oxidation step and the subsequent complete biological oxidation in the second step allowed total-S loading rates up to 33 mmol L-1 day-1. Experiments with synthetically prepared solutions were performed in a continuously fed system consisting of two gas-lift reactors in series. These reactors were operated at haloalkaline (pH 9.5; Na+ 0.8M) and aerobic conditions at 35oC.Mathematical modelling of the 2-step process shows that under the prevailing conditions an optimal reactor configuration consists of 40% ‘abiotic’ and 60% ‘biological’ volume, whilst the total reactor volume is 22% smaller than for the 1-step process. The major advantages of a 2-step process are the improved anticipation to shock loads of sulfideand lower investment and operational costs due to downsizing of the total reactor volume.
Further research regarding the biological treatment of SSC’s may involve the potential of heterotrophic soda lake bacteria for the degradation of organic VSC’s as well as the mechanisms of toxicity of these compounds.
|Qualification||Doctor of Philosophy|
|Award date||18 Dec 2012|
|Place of Publication||S.l.|
|Publication status||Published - 2012|
- thiobacillus denitrificans
- microbial degradation
- biological treatment
- waste treatment