Application of spent sulfidic caustics for autotrophic denitrification in a MLE process and their microbial characteristics by fluorescence in situ hybridization

Abstract

Spent sulfidic caustics (SSCs) produced from petrochemical plants contain a high concentration of hydrogen sulfide and alkalinity, and some organic matter. Most of the SSCs are incinerated with the auxiliary fuel causing secondary pollution problems. The reuse of this waste is becoming increasingly important in terms of economical and environmental viewpoints. To denitrify wastewater with a low COD/N ratio, additional carbon sources are required. Therefore, autotrophic denitrification has received increasing attention. In this research, SSCs were injected as electron donors for sulfur-based autotrophic denitrification in a modified Ludzack-Ettinger (MLE) process. According to the variations in the SSCs dosage, the efficiencies of COD, nitrification and TN removal were evaluated. Heterotrophic denitrification by organic matter and autotrophic denitrification by SSCs were also investigated. As a result, adequate injection of SSCs showed stable autotrophic denitrification. To investigate some of the harmful effects of SSCs, fluorescence in situ hybridization (FISH) for nitrifying bacteria and Thiobacillus denitrificans was performed. Ammoniaoxidizing bacteria (AOB) and Nitrospira genus showed a similar pattern. Excessive injection of SSCs made nitrifying bacteria decrease and nitrification failure occur because of the high pH caused by the SSCs. The distribution of T. denitrificans was relatively uniform as SSCs were injected. This result means that T. denitrificans are available at high pH.

Autotrophic denitrification and inhibitory effect caused by the injection of spent sulfidic caustic in a modified Ludzack-Ettinger process

“…Park et al [29] reported that the optimum dosage of SSC was below an S/N ratio of 2.5 and excess SSC injection induced nitrification failure because the caustic content is higher than required. Based on these results, the pH of SSC was neutralized from approximately 13.3 to 12.0 with the addition of sulphuric acid (H 2 SO 4 ), after which the neutralized SSC was injected into the anoxic tank.…”

“…As the SSC dosage increased, SOUR decreased. Park et al [29] reported that 4 mL/L of SSC dosage was the optimum dosage for autotrophic denitrification. The slight reduction in SOUR was observed until the injection of 4 mL/L of SSC.…”

“…Therefore, autotrophic microorganisms must spend a greater proportion of their energy for synthesis than do heterotrophs, thus resulting in generally lower yields of cell mass and growth rates [28]. Whereas the study of Park et al [29] focused on the optimal SSC dosage, we focused on the denitrification rate in the anoxic zone via changing the volume of the anoxic tank of modified Ludzack-Ettinger (MLE) process in this study. Park et al [29] also previous reported that excess SSC dosage caused nitrification failure due to high pH, ranging from 8.4 to 9.9 in the aerobic zone.…”

“…Whereas the study of Park et al [29] focused on the optimal SSC dosage, we focused on the denitrification rate in the anoxic zone via changing the volume of the anoxic tank of modified Ludzack-Ettinger (MLE) process in this study. Park et al [29] also previous reported that excess SSC dosage caused nitrification failure due to high pH, ranging from 8.4 to 9.9 in the aerobic zone. Thus, the pH of SSC was neutralized from approximately 13.3 to 12.0 with addition of sulphuric acid (H 2 SO 4 ) and SSC was injected into the anoxic tank of the MLE process.…”

Study on Haloalkaliphilic Sulfur-Oxidizing Bacterium for Thiosulfate Removal in Treatment of Sulfidic Spent Caustic

“…Nowadays physicochemical methods such as wet air oxidation (WAO), advanced oxidation and the MEROX process, as well as biological methods are proposed for the treatment of sulfidic spent caustic [17][18][19][20]. among which biological systems could be advantageous due to lower cost and higher safety [21].The limited number of researches conducted on the biotreatment of sulfidic spent caustic mostly focused on the application of neutrophilic sulfur bacteria belonging to the genus Thiobacillus, where the stream needs to be diluted in order to decrease the pH and salinity levels [16,[22][23][24]. On the other hand, at pH levels below 9, most of the dissolved sulfide would be released as toxic hydrogen sulfide into the atmosphere [25,26].…”