Case study - Odor treatment in a ventilation shaft of a sewage tunnel

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Courtesy of Bioconservacion SA

Bioconservacion’s distributor for municipal biogas and odor applications in Turkey, Biotek Ltd, has recently won a tender for Izmir’s municipal authority IZSU to supply equipment and media for odor abatement coming from a ventilation shaft of a sewage tunnel.

These shafts, also known as airshafts or vent shafts, are passages used in different applications such as mines or sewage ducts among others, to move fresh air underground, and to remove stale air.

Thus, a wide variety of smelly pollutants will be present in gas phase. Depending on the O2 content and the renovation rate, these compounds might vary from aldehydes, ketones or inorganic compounds such as NH3 or H2S, to more problematic ones coming mostly from anoxic processes such as dimethylsulfide, dimethyldisulfide, mercaptans, which are especially challenging due to their low threshold detection limits.

The ventilation shaft has a diameter of 3.600 mm. There are two aeration manholes on the tunnel measuring 2500x4000mm. Those aeration manholes have been sealed with AISI 316L covers and a vessel packed with dry media has been placed in the stream’s exhaust in order to release pollutant-free air to the environment.

In this case, odors are produced especially in the summer months of August and September.

Gas phase characterization

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

Results revealed H2S and NH3 concentrations in a range between 1-5 ppmv, O2 content in the range of 20% v v-1, while CO2 is at lower levels than in regular air. It is interesting to point out that CH4 was measured at concentrations around 0,5% v v-1, which may indicate a lack of O2 in certain zones of the shaft.

Solution

After studying the composition and concentration of odorous stream as well as the layout of the installation, Bioconservacion assisted Biotek in the design of the vessels and provided media suggested configuration the following:

  • Bottom Layer: BiOn ACPA to eliminate NH3
  • Blend of Bi-On SIGMA and Bi-On ISORB: H2S removal and volatile organic compounds of low molecular weight.

It is important to highlight that the solution suggested has an extended capacity because presence of O2 helps the Bi-On SIGMA to self-regenerate, which prolongs the autonomy of the unit.

Media description

Bi-On ACPA is an adsorbent media impregnated with phosphoric acid. It has been widely employed at applications related with odours and irritants associated to printers, fertilizers, cleaners, urine and fish process.

Bi-On ISORB is designed to target successfully a wide range of gases. It is especially recommended for the control of acid gases, N-containing compounds, sulphur compound and low molecular weight gases. Bi-On ISORB uses the combination of two processes for pollutants elimination. One of physical nature by trapping the molecules inside the pellet due to its large internal surface area and a second by chemical oxidation. Oxidized gases are converted into innocuous products. Bi-On ISORB’s unique properties allow it to perform successfully including in environments with up to 95% relative humidity.

This media has been designed to be used in applications such as sewage treatment plants, pulp and paper plants, airports, chemical plants, refineries or laser cutting and engraving among others.

In turn, Bi-On SIGMA is a H2S scavenger, consisting of iron hydroxide, Fe(OH)3, blended into a proprietary pelletized support.

Hydrogen sulphide is stripped 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 sulphide, FeS, turns the media black. Once the media is spent, it may be regenerated by blowing ambient air through the bed, as it can be seen in the following chemical reaction:

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

No H2S is released while the regenerating process takes place. Taking into consideration that one cycle is defined as the completion of these two reactions, Bi-On SIGMA may undergo at least 10 cycles before exhaustion and disposal. Each cycle results in the abatement of 45 mg H2S per gram of Bi-On SIGMA, which means that the total removal capacity is about 450 mg H2S / g media.

In the absence of oxygen, Bi-On SIGMA displays a higher H2S breakthrough capacity than a caustic impregnated activated carbon. Furthermore, spent activated carbon cannot be regenerated and hence must be disposed of after just one cycle.

Bi-On SIGMA ’s main application is the removal of H2S from anaerobic gas streams, e.g. biogas. However, depending on the O2 content, Bi-On SIGMA can be employed for the treatment of low H2S loads such as in this application, since exhaustion and regeneration processes take place at the same time.

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