Making sense of smell
Gas molecules locked up in a solute do not smell. Surfactant Induced Absorption Technology (SIAT), by harnessing the principles of acid rain, achieves unprecedented results in the removal of odorous gases from air. Dave Stanley, E3 Consultants, explains the technology, now available on wheels.
SIAT succeeds by employing engineering and sound science that works with nature. The principle is well demonstrated by the phenomenon of acid rain – the result of components of sulphur dioxide and oxides of nitrogen, for instance from power stations, being absorbed into water and falling as rain. Both of these acids are extremely water-soluble. Hydrogen sulphide and many of the other odorous gases produced by sewage treatment are, on the other hand, only slightly so. SIAT enhances this effect by using the piggyback effect of one surfactant on another to achieve a broad spectrum hydrophile-lipophile balance that is not chemically nor pH selective, but is extremely effective in promoting the absorption of many different compounds. SIAT alters the solubility of compounds it comes into contact with by having its hydrophilic (water loving) end in the water and its hydrophobic (water hating) end in the air. This can increase the absorbency of the water droplets by a factor of something in the order of 500,000. Immediately the droplets are formed, they can simultaneously absorb acidic, alkaline and neutral odorous gases, effectively locking up the offending odour molecules.
Achieving the correct droplet size is fundamental to the efficiency of SIAT. With any scrubbing or extraction system, to achieve the maximum extraction rate it is extremely important to have the largest practical surface area or interface area in relationship to the mass or volume of the agent. In other words, achieving maximum extraction with minimum liquid. For the same volume there are 216 x 50-micron water droplets for one 300-micron droplet, and a 600% increase in surface area and efficiency. A litre of water misted at 50 microns produces some 426 million droplets.
Multiply this figure by the number of litres per minute times the number of minutes of operation per day and some very large numbers result. Compared with conventional scrubbing techniques, when one appreciates this remarkable increase in physical area available for absorbing gases by correctly engineered misting, it is then easier to understand how, with the further benefit of no back pressure, SIAT can reduce resource and energy requirements by 90% and do the same, if not superior, job.
The mass of the water absorbent is huge in comparison to a gas – just about floating in the air. Consider the size of the pollution molecules. Hydrogen Sulphide, for example, has an atomic mass of 34.08; this means that it is less than 1,000,000,000th of the size of a 50 micron droplet. Even a very big pollutant molecule such as Skatole (C9H9N – a white/brown crystalline solid with a strong faecal odour, found in beetroot) with a molecular weight of 131.17, is still very small by comparison. When the gas molecules collide with the water droplet they are captured, forming a solute. As the absorbent droplets become saturated, they become more dense and drop to the ground where they are degraded by the natural bacteria present.
SIAT is engineered to deliver the mist appropriate to a particular problem (it is also very effective in absorbing dust and particulates). Similarly, the concentration of the surfactant, from 1% down to 0.25%, is matched to the type and strength of the polluting gases. It is most effectively delivered at the point of emission.
There are, however, limitations. Stable gases such as pure methane are not absorbed and, as SIAT employs water, high temperatures need to be controlled to prevent evaporation.
The solute may be recovered, or, if non-toxic, emitted to atmosphere where it will fall and biodegrade naturally using bacteria present in the surrounding environment, and avoiding the costs normally associated with the disposal of spent filters and chemicals.
The technology has been evaluated by the UK WRc (Water Research Centre), and by the US Environmental Protection Agency, who identified it as up to 90% more efficient than available US technologies, including biofiltration. Surfactant Induced Absorption Technology has also been granted Millennium Product Status by the Design Council. Processes tackled to date range from food processing, landfill sites, sewage works, sludge holding, gas bottle cleaning, waste management and composting.
Advanced Clean Air Technology (ACAT) Ltd of Peterborough continued throughout last year to innovate and develop further applications for this technology. A leading water company had major odour problems with moving and spreading sewage sludge at numerous remote locations, which were causing nuisance complaints. ACAT worked with the water company to produce Bloodhound, a trailer-mounted system capable of being deployed to a remote problem location and giving a minimum of 24 hours of odour control operations, independent of utilities. For reliability and safety, the system is completely air operated by an onboard diesel compressor. Other novel features include pumped refilling of diesel, water, and chemical to both facilitate one-man operation and to minimise environmental risks. A delayed switch-off capability tackles the decaying odours after cease work and saves continuous operation. The Bloodhound trailer can be deployed by one man and be operational within 20 minutes.