Odour removal with multi-bed systems based on activated alumina adsorbents concentrates the benefits of a train ofchemical scrubber towers with in a compact unit without the need to handle hazardous substances. Moreover, the use of inorganic media with a high degree of selectivity and efficiency regardless of variations in the flow and concentration of malodorous compounds makes it an option with many benefits over biological treatment.
For some time now, odour control systems with activated alumina-based adsorbents have proved to be a more than attractive option for tackling odor problems at wastewater treatment facilities.
The implementation of small compact units with activated alumina is accompanied by innovative criteria for the confinement of odour sources. In this sense, it is important to emphasize that the odour control system used in a given area is not designed by calculating a certain number of air changes depending on the volume of the area, but rather by exclusively treating the source of the odour and enclosing this source insofar as possible to prevent odour dispersal throughout the rest of the plant/room.
In this way, the foul air captured is forced through the chemical adsorbents, which have the capacity to remove the gases that cause the odours with an efficiency of over 99.99%.
In accordance with these criteria, odour control solutions have been developed for each of the operations carried out at waste water treatment plants or pumping stations that might give rise to odours. The areas affected include inlet chambers, pumping wells, filtering zones, settling tanks, bioreactors, thickeners, sludge dewatering units, sludge silos, etc.
Odour control units
Alphachem, founded in 2012 and staffed by pioneering professionals in the use of this technology, designs and builds chemical filtration equipment in Spain.
These units incorporate the products of Alphasorb, a leading manufacturer of impregnated activated alumina adsorbents.
Alphachem odour control systems normally incorporate from 2 to 4 different chemical adsorbents, which makes it possible to remove all habitual malodorous compounds at municipal wastewater treatment plants and wastewater pumping stations.
Odour control solutions
The company has a comprehensive range of odour control units, which house the adsorbents. The units generally consist of a blower, a demister, particle filter and chemical adsorbent. The size of the unit is far smaller than those used for the same purpose in traditional odour control systems. This is mainly due to two factors:
• Confinement of odour sources and reduction of the flow to be treated to that which is absolutely necessary. This gives rise to great savings in ducts and energy costs associated with gas pumping.
• High-capacity, high-performance adsorbents for the elimination of typical contaminants, with very low retention times, thereby enabling operation with low adsorbent loads at high throughput rates.
The most appropriate unit for each case will depend on:
• Flow of foul air to be treated.
• Concentration of gases.
• Characteristics of the odour source.
• Sludge thickeners. Odour control can be carried out with a small unit from the SF range. There are different odour confinement and treatment solutions for the sludge line using these units.
• Covered primary settling tanks. In this case, a unit from the SFM range is required. The adsorbent is housed in modules and the unit has the capacity to treat a greater flow. This solution can be used for any closed operation in the water line.
Benefits over traditional odour control methods
The dry odour control system with impregnated activated alumina has a number of advantages over traditional odour control systems in wastewater treatment, regardless of whether such traditional systems use activated carbon (normally impregnated with NaOH), bio-filters, or chemical gas scrubbers (between 1 and 3 stages, acid, basic and oxidant):
• Traditional systems usually have large gas collection and duct infrastructures to take the gases to the centralised treatment system, which leads to high piping and pumping costs.
• Alumina adsorbents can be used for a wide range of inlet flows and concentrations of contaminating gases. Their performance is not affected by variations in these parameters, unlike chemical scrubber towers and bio-filters.
• The loss in capacity of the adsorbents is low and they are 100% active from the moment air begins to pass through them.
• Their high efficiency ensures the absence of odours at the outlet of the unit (traditional bio-filters do not offer high efficiency and activated carbon is specifically suited to the treatment of hydrogen sulphide).
• Temperature and relative humidity variations can give rise to considerable agglomeration problems in the activated carbon bed, thereby notably increasing loss of capacity of the system and consequently reducing odour removal efficiency.
• It is possible to change or adjust the capacity and type of chemical adsorbent to tackle new or unexpected contaminants.
• It is possible to determine the degree to which the adsorbent has been consumed by means of chemical analysis, which facilitates monitoring and replacement. In the case of the adsorbent impregnated with potassium permanganate, it is also possible to estimate activity based on the color of the spheres.
• Very cost-effective for medium and low flows and/or concentrations.
• The odour control system using small independent units is very flexible. It requires very little maintenance and provides an individual solution for each odour source, while at the same time optimizing adsorbent performance.
Most common malodorous compounds in municipal wastewater
• Hydrogen sulphide (H2S) is the most common compound and generates a rotten-egg type odour. It also creates environmental corrosion problems that affect electronic control equipment. Loss of odour perception occurs at high concentrations and minimal exposure to the compound can be lethal.
• Mercaptans, normally methyl or ethyl mercaptans, which have a very low odour threshold, meaning that at low concentrations, they can cause considerable disturbance.
• Volatile organic compounds (VOCs). A wide group encompassing compounds that produce great disturbance due to their odours. Sometimes, synergistic effects occur in such a way that the odour arising from blends of such compounds can be considerably more potent than the sum of the effects of each individual compound.
• Ammonia (NH3). Its odour impact is often overstated. In order to cause disturbance, it must be present in concentrations of over 1,5ppm. It also has a considerable corrosive effect on electronic equipment.
• Aminos. These include pyrroles, skatoles and other compounds that can cause considerable odour pollution.
• Sulphur derivatives. Thiocresols, thiophenols and carbonyl sulphite are the most common malodorous compounds.
Most frequently used chemical adsorbents
Alphasorb 8. Activated alumina spheres impregnated with potassium permanganate, capable of eliminating hydrogen sulphide, mercaptans, ethylene, formaldehyde, SOx, NOx, etc. Widely used in odour control, corrosion control in industrial environments, elimination of ethylene in the control of fruit and vegetable ripening, air purification in white rooms, cleans rooms and work tables.
Alphasorb 5. Impregnated activated alumina spheres for the removal of chlorine, fluoride, bromine, iodine and acidic gases (e.g. SO2). Particularly useful in industry to control leakages of such gases.
Alphasorb 15. Caustic carbon impregnated activated alumina spheres for the removal of acidic and corrosive gases, especially hydrogen sulphide. The main application is corrosion control in industrial environments and odour removal in wastewater treatment.
Alphasorb 3. Granules of activated zeolite, particularly suited to the removal of ammonia and ammonia derivatives in different industries and applications.
Alphacarb. High-quality virgin or impregnated activated carbon pellets, depending on purification requirements. Specifically developed for the removal of VOCs, ammonia and derivatives, mercury vapours and hydrogen sulphide.
Alphablend 8A. Blend of activated alumina impregnated with potassium permanganate and virgin activated carbon pellets. Capable of eliminating a wide range of gases. Extensively used in odour removal, VOC control and internal air quality enhancement in offices, hotels and public buildings.