Suction Air from sewers
Odorous substances can be produced by all living things, but also by chemical reactions with inorganic compounds. There are so many natural and technical chemical reactions which produce odorous substances that it is virtually impossible to count them all.
Several thousand individual substances can be identified as an odour from known chemical compounds. These can be organic and inorganic substances. Humans are capable of recognizing over 10,000 different smells. The chemical structure and the functional group are key factors which determine the qualitative effect of odorous substances. Unpleasant smells arise in particular during biochemical decomposition processes, such as the degradation, decay, and rotting of organic hydrocarbons (carbohydrates, protein and fats). Short-chain, volatile fatty acids are formed, for example formic acid, acetic acid or butyric acid, along with heterocyclic nitrogen compounds such as ammonia, skatole, indole and sulphurous organic compounds such as hydrogen sulphide, sulphur dioxide or mercaptan. The synthetic production of many chemical products and food processing both give rise to smells which are categorised as a nuisance.
Treatment of unpleasant outlet air from sewers
In some sewers, the biological decomposition processes in the sewage produce hydrogen sulphide, dimethyl sulphide and mercaptans, among others. In parts, these can be eliminated by adding expensive doses of iron compounds (e.g. iron hydroxide, iron II-chloride) or calcium nitrate. Another cheaper method for solving odour problems is to extract the air and treat it with biofilters or simple air activated carbon. Biofilters, however, can be tricky to operate and their adaptation to the concentrations of pollutants and odours to be treated is sluggish. In most cases, normal air activated carbon cannot adsorb the unpleasant odorous substances. For this reason Harbauer has been looking for suitable adsorption material to adsorb the bothersome odours at minimal cost. In some experiments Harbauer has been able to test different types of special activated carbon, which were exposed to odour-intensive sewage smells under real conditions. This activated carbon differs from normal air activated carbon, as it is specially treated or produced. Different impregnated and treated types of active carbon are used. These types of active carbon do not only allow normal physical adsorption (physisorption) to take place, but also allow for chemisorption. Chemisorption is a special form of adsorption, which in contrast to physisorption binds the adsorbate to the adsorbent (substrate) with stronger chemical bonds. Due to chemisorption, the adsorbate and/or the adsorbent is chemically altered. With physisorption however, the adsorbate molecule bonds to the substrate through physical forces (Van-der-Waal-force) and its chemical properties remain unchanged.
Extraction plants project
Harbauer built two versions of air extraction plants for a Berlin firm. The malodorous air discharged from the sewer is extracted directly above a special manhole cover. One version consists of an 8-foot container (2.44m x 2.20m) with a 0.5m³ filter and can extract and treat up to 600m³/h exit air. The larger version of the plant is in a 10-foot container (3 x 2.44m) with a 1.5m³ filter and can extract up to 2000m³/h of air. In order to operate near a residential area, the containers are completely soundproofed and are operated with pipe sound absorbers.
- Chemisorption and
- Physical adsorption using treated special form activated carbon
- Hydrogen sulphide
- Dimethyl sulphide