Treatment technologies for chemical wastewater containing hydrogen sulfide

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I. Sources and forms of existence of hydrogen sulfide Hydrogen sulfide is a colorless gas with the odor of rotten eggs; it is soluble in water, ethanol, and glycerin. The hydrogen sulfide-containing wastewater in our company mainly comes from the distillation units and the dehydration process in the crude oil tank area (with dehydration in the crude oil tank area accounting for only a small portion). In nature, hydrogen sulfide is produced by the reduction of sulfates, and there are typically two pathways for this process: assimilative sulfate reduction and heterotrophic sulfate reduction. In aqueous solution, hydrogen sulfide exists in three different forms: H2S, HS-, and S2-. Its existence pattern is directly influenced by the pH value of the water. When the pH is 6–6.8, 75–90% of the sulfides exist in the form of H2S ； At pH=7, sulfides dissociate almost equally into H2S and HS-, with S2- accounting for only one part in a million; at pH=8, sulfur exists mainly in the S2- form. II. Hazards of hydrogen sulfide: In oil refining, four types of wastewater are generally generated: saline wastewater, sulfur-containing wastewater, oil-containing wastewater, and alkaline residue wastewater. Among these, sulfur-containing wastewater poses the greatest hazards and is the most difficult to treat. 1. Hydrogen sulfide can react with iron in water, along with Fe2+ produced by chemical oxidation, to form FeS and Fe(OH)2; this is the main cause of rusting in iron pipes. This process is known as anaerobic corrosion of iron. 2. Sulfides in water also have an impact on the health of humans and animals and plants. Studies show that even a hydrogen sulfide concentration as low as 0.07 mg/m3 in drinking water can affect its taste. Since hydrogen sulfide is a toxic substance with the same level of toxicity as hydrocyanic acid, a hydrogen sulfide concentration in water of 0.15 mg/m3 already affects the growth of fish in that water. 3. Hydrogen sulfide in wastewater is released into the air; the maximum allowable concentration of hydrogen sulfide in the air is 10 mg/m3 (7 ppm), whereas in some treatment units of wastewater treatment plants, the hydrogen sulfide concentration can reach several dozen to several hundred ppm. When the concentration is ≥760 mg/m3 (502 ppm), people will quickly suffer from acute poisoning and die due to respiratory paralysis ； Concentrations between 300 and 760 mg/m3 (198–502 ppm) can cause pulmonary edema, bronchitis and pneumonia, headache, dizziness, nausea, vomiting, and difficulty urinating ； At concentrations of 10–300 mg/m3 (6.6–198 ppm), acute eye irritation symptoms can occur, while prolonged exposure can lead to pulmonary edema. Yi Jing Shui Wang summarizes the treatment techniques for sulfur-containing wastewater as follows: III. Methods for dealing with hydrogen sulfide 1. Closed-collection and disposal method: Closed collection devices can be installed at the locations where hydrogen sulfide is emitted in large quantities, and the hydrogen sulfide can then be collected and treated using exhaust fans. However, this method requires strict conditions for the sealed systems – no leaks are allowed – and it is not possible to carry out normal operation, maintenance, or repair of the equipment inside these sealed systems. In my workshop’s wastewater treatment facility, it is necessary to use sealed systems to collect hydrogen sulfide gas from the sump pits, buffer tanks, horizontal oil separation tanks, and vortex air flotation tanks. If that is the case, not only will the initial investment be too high, but it will also be impossible to carry out routine operations on the aforementioned units, affecting the proper functioning of the wastewater treatment system. Even when sealed collection is possible, the hydrogen sulfide gas collected can be handled in one of the following ways: it can be released directly into the atmosphere in an open area; doing so not only pollutes the atmosphere but may also cause poisoning in people ； The second method is to use an alkaline solution for absorption; this requires the installation of a separate alkaline washing system, and such a system cannot absorb 100% of the hydrogen sulfide gas – the remaining hydrogen sulfide will still be released into the atmosphere ； Third is precipitation using heavy metal salts, but it is too costly and also causes heavy metal pollution ； Fourth is the installation of sulfur recovery units to convert hydrogen sulfide into elemental sulfur, but both the investment and maintenance costs are too high, making it unsuitable for use in small-scale installations. In summary, the method of sealed collection and disposal of hydrogen sulfide is not suitable for our company’s wastewater treatment plant to address the issue of high hydrogen sulfide concentrations. 2. The support gas membrane method utilizes the support gas membrane technology, also known as membrane permeation desorption-chemical absorption technology. By adjusting the pH to maintain it below 5, over 95% of H2S exists in its free form in water. The wastewater is passed through the tube side of a polypropylene hydrophobic microporous hollow fiber membrane module, while dilute sodium hydroxide solution (with a pH greater than 11) flows counter-currently on the shell side; as a result, hydrogen sulfide is irreversibly absorbed by the membrane. If the pH of the wastewater remains at 5 throughout, or even below 4, 95% or even 99% of the hydrogen sulfide can be removed and concentrated in the absorption phase (by dozens to hundreds of times). Alkaline sodium sulfide aqueous solutions are collected from various dispersed production sites, acidified, and then stripped to produce high-concentration hydrogen sulfide, which is used in the Claus process to manufacture elemental sulfur. This requires the installation of stripping units and sulfur recovery systems, with initial investment costs of at least 1.5–2 million yuan, in addition to high ongoing maintenance expenses. 3. In the treatment plant using the stripping recovery method, the main source of hydrogen sulfide is the wastewater generated by the distillation units; stripping and alkaline washing can be carried out in those unit areas. The sulfur-containing wastewater is first stripped using a wastewater stripping unit, which removes hydrogen sulfide from the wastewater and sends it to the alkaline washing system. The remaining hydrogen sulfide from this process is fed into a heating furnace for combustion; due to its low flow rate, it has no impact on the combustion process in the heating furnace ； The remaining hydrogen sulfide in the wastewater can be discharged to the wastewater treatment plant, thereby significantly reducing the hydrogen sulfide concentration there. The process is as follows: There is already one alkali washing system in the distillation unit area; adding just one stripping system will suffice to carry out the recovery and treatment of hydrogen sulfide. It is recommended to adopt this solution. Please indicate the source when reproducing: Yijingshui Network