I. Introduction
The malodorous gases are widely generated from the agricultural and industrial production, municipal sewage and sludge treatment and waste incineration process (generating dioxin, a strong cancer-causing substance). The public hazards of malodorous gases do harm to the surrounding environment and some of them are categorized as toxic pollutants and their emission is restrained by the relevant Air Pollution Laws and Regulations.
II. Source of Malodorous Gases
The agricultural and industrial production, municipal sewage and sludge treatment and waste treatment facilities are important sources for the malodorous gases. With the expanding outward of the urban area, the treatment facilities previously far away from the urban area are getting closer and closer to the new urban area and to people’s working and living place, thus more and more people are deeply disturbed by the malodorous gases.
III. Gas Types
Different treatment facilities and processes will produce different malodorous gases. Major malodorous gases produced in the influent water lifting pump house is hydrogen sulphide, major malodorous gases produced during anaerobic digestion process in the primary sedimentation tank are hydrogen sulphide and other sulphur gases, while ammonia gas and other volatile substances will be produced during the sludge alkalinization and stabilization process. The ammonia gases such as amine，sulphur compounds，fatty acid，aromatic and dimethyl sulphide will be produced during the waste composting process.And aerobic digestion and air drying process may mainly produce mercaptan and dimethyl sulphide as well as a small quantity of hydrogen sulphide.
IV. Process Flow
The malodorous gases are collected with collection cap and enter into high-pressure water mist absorptiontower 1 through the induced draft fan for odor removal. Several layers of high-pressure water mist combined spray heads are placed in the tower. The absorption solution with certain concentration is pumped into the combined spray heads, forming high-pressure water mist which will be sprayed out and contact with the malodorous gases sufficiently. The super-fine dust in the malodorous gases is moistened and agglomerated into large granule which will fall under its own gravity action and flows into the ash water tank. The ammonia gas and hydrogen chloride of the malodorous gases fully contact and react with the water mist formed by the absorption solution to produce the corresponding salt which will fall and flows into the ash water tank. After the solution is clarificated, add sodium hydroxide into it to increase its PH value. Then filter the solution and make it flow into the clear solution tank. Add oxidant (sodium hypochlorite) into the clear solution tank. After that the solution will be filtered and for cyclic utilization by the high pressure pump set. The small quantity of malodorous gases like hydrogen sulphide, mercaptan and dimethyl sulphide, if encountering the high-pressure water mist sprayed in the form of counter-flow in the absorption tower 2, will be absorbed and oxidized into clear gas without odor. The purified gas reaching emission standard provision will be discharged from the chimney after its water of supersaturation deprived.
V. Main Technical Indexes
1. Hydrogen Chloride: ≦100mg/Nm3
2. Fluor Complex: ≦90mg/Nm3
3. Chlorine: ≦65mg/Nm3
4. Benzene: ≦12mg/Nm3
VI. Technical Characteristics
1. Many malodorous gases can be removed through two-stage or three-stage absorption system with high removal efficiency.
2. The system can adapt to the change of gas flow and concentration through adjusting dose and solution circulation flow, so it has strong operating flexibility.
3. This process applying unit operation theory and practical experience in the chemical industry has the advantages of maturity, reliability and effectiveness. It will still be the mainstream for the malodor control technology in a long time in the future, especially for the malodor removal in the old plant modification and new plant with limited land.
4. Applicable for the emission places for malodorous gas with toxin and hazard such as chemical plant, plastics plant, feedstuff factory, non-ferrous smelting, waste incineration for agricultural and industrial production.
