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Treatment of Odors in Ventilation Ducts in Wastewater Tunnels

2021-10-05 09:32:55

Odour treatment in sewage tunnel ventilation ducts
Case Study: Izmir Wastewater Plant (Turkey)

Project Description

BION's distributor for municipal, biogas and odor treatment applications in Turkey, Biotek, has recently won a contract to supply equipment and filter media for odor abatement from ventilation ducts in wastewater tunnels in the municipality of Izmir, managed by IZSU.

These tunnels are used to remove stale air in subway areas and replace it with fresh air, and are common in mines and wastewater plants.

A wide variety of malodorous contaminants are present in these gases. Depending on the O2 content and the turnover rate, the compounds present can range from aldehydes, ketones or inorganic compounds such as NH3 or H2S, to the really problematic ones from anoxic processes such as dimethylsulfide, dimethyldisulfide, mercaptans, especially problematic because of low detection levels.

The ventilation duct is 3.6 m in diameter. There are two aeration shafts in the tunnel, whose dimensions are 2.5x4 m and are closed with AISI 316L lids. The gas stream is passed through a tank packed with filter media in order to release contaminant-free air.
Odour problems generally appear between August and September.
Characterization of the gas stream 

Biotek analyzed the composition of the vent effluent in order to find the most accurate solution in terms of filter media selection.

The results revealed H2S concentrations and NH3 concentrations in the range of 1 to 5 ppmv, O2 content around 20% v v-1, while CO2 was measured at lower levels than in air. Interestingly, CH4 was measured at concentrations of about 0.5% v v-1, which may indicate a lack of O2 in certain areas of the duct.
After studying the composition and concentration of the odorous stream, as well as the design of the installation, Biotek with advice from BION, suggested the following proposal for tanks and filter media:
- BION ACPA in the bottom layer to remove NH3.
- Mixture of BIONSIGMA and BIONISORB to remove H2S and low molecular weight volatile organic compounds.
It is important to note that the proposed solution has a higher capacity due to the presence of O2, which helps the BIONSIGMA to self-regenerate, prolonging the autonomy of the unit.
Descriptions of the filter media 

BION ACPA is an adsorbent media impregnated with phosphoric acid. It has been widely used in applications involving odors and irritating gases associated with printers, fertilizers, cleaning products, urine and fish processing.

BION ISORB is designed to successfully target a wide range of gases. It is especially recommended for the control of acid gases, nitrogen containing compounds, sulfur compounds and low molecular weight gases. BIONISORB uses a combination of two processes for contaminant removal. One of a physical nature, trapping the molecules inside the pellet due to its large internal surface area and a second by chemical oxidation. The oxidized gases are converted into harmless products. A particularity of BIONISORB is that it maintains its effectiveness even in the treatment of streams with relative humidity contents above 95%.
This filter media is commonly used in applications such as wastewater treatment plants, pulp and paper plants, airports, chemical plants, refineries or laser cutting and engraving plants.

BION SIGMA is a filter media especially suitable for H2S adsorption, consisting of an extruded pellet composed of a porous material and Fe(OH)3 iron hydroxide.
Hydrogen sulfide is removed from the gas stream according to the following reaction:  

                                             2 Fe(OH)3 + 3 H2S ® 2 FeS + 1/8 S8 + 6 H2O
The generation of ferrous sulfide, FeS, changes the original color of the pellet from yellowish to blackish, indicating that the media has been consumed. Once exhausted, the media can be regenerated with air and the following reaction takes place:

                                               2 FeS + 3/2 O2 + 3 H2O ® 2 Fe (OH)3 + 1/4 S8.

No H2S is released during the regeneration process. Considering that one cycle is defined as the completion of these two reactions, BIONSIGMA can undergo at least 10 cycles before its total exhaustion, each cycle results in the removal of 45 mg H2S per gram of BIONSIGMA which means that the total removal capacity is approximately 450 mg H2S / g of filter media.

In the absence of oxygen, BIONSIGMA shows a higher H2S adsorption capacity than a caustic impregnated activated carbon. On the other hand, the spent activated carbon cannot be regenerated and must therefore be discarded after a single cycle.

The main application of BIONSIGMA is the removal of H2S in gas streams under anaerobic conditions, e.g. for biogas desulfurization. However, depending on the O2 content, BIONSIGMA can be used for the treatment of low H2S concentrations, as in this application, where the depletion and regeneration processes take place at the same time.

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