H2S removal from polluted gas streams

Sulphide in wastewater originates from biological sulphate reduction under anaerobic conditions in the presence of organic biodegradable substrates and from several industrial processes such as leather tanning, papermaking, oil and gas industry, pharmaceutical industries, food processing, livestock farming, slaughterhouse, etc…
 
In leather tanning process, sulphide-containing effluents are produced during the de-hairing process and, depending on environmental conditions (pH, concentration, temperature and turbulence), H2S can be desorbed from liquid to gas phase.
 
As a matter of fact, in leather tanning and other industrial areas sulphide management represents a major problem and gaseous streams containing hydrogen sulphide (H2S) need to be treated not only in wastewater treatment plants, but also in each single manufacturing plant.
 
Tanneries and tannery wastewater treatment plants are usually equipped with H2S removal systems consisting of wet scrubbers where sodium hydroxide is used as reagent. However, due to high number of sources, the control of air pollution is complex in large tannery industrial district such as the area of S. Croce sull’Arno (Pisa, Italy).
 
The discharge of H2S effluents also causes serious problems to the surroundings, which strongly impacts the population life quality. The concentration of H2S in the effluent streams is usually brought to 1-2 ppm, but given the high number of sources and the very low olfactory threshold of H2S, the interested areas are usually characterized by odour problems. This fact has many social and economical consequences: decrease of land value, lack of recreational areas, etc… especially close to WWTPs.
 
Chemical scrubbing is the most established technology for H2S removal in WWTPsfor relatively highly loaded effluents (from 20 to 400 ppm of H2S) and, with respect to alternative technologies, it presents several advantages: short contact time (1.3–2 s), low capital cost, easy operation and maintenance, ability to cope with highly variable loads and low surface requirements. However, chemical scrubbing has important drawbacks such as high energy requirements, secondary pollutants production and the consumption of large amounts of chemicals, primarily NaOH, that increase both the operating costs and carbon footprint of the treatment.
 
The environmental problem targeted are closely related to the current procedure applied in the Cuoiodepur WWTP (San Romano – San Miniato, Pisa, Italy) to remove H2S. In this plant H2S gas streams are collected to a centralized facility which treats about 45000 m3/h of gaseous effluents in 8 chemical scrubbers. The scrubbers are fed with a NaOH solution which is recirculated. The system consumes about 2 ton/d of NaOH, 1600 KWh/d for gas and water pumping and 220 m3/d of well water. Moreover, as the costs of chemical scrubbing are very high (12 €/Kg of H2S removed), pure oxygen and hydrogen peroxide in the wastewater equalization tanks are used, in order to minimize potential sulphide production due to the reduction of sulphate.
 
Energy requirement for the production of oxygen to control sulphide in the Cuoiodepur WWTP (15 tons/d) is very high (about 15 Mwh/d). Considering the two main WWTPs in the Tuscany tannery district, about 30 tons of pure oxygen are dosed (with a corresponding consumption of about 30 MWh of electric energy per day). The production of NaOH is also highly energy-consuming and causes CO2 emission. Only in the Tuscan tannery district, accounting for about 400 tanneries and 4 centralized waste and wastewater treatment facilities the yearly carbon footprint due to NaOH consumption for scrubbing can be estimated in 40.000  tons of CO2 emitted.