By Michael Levey, CEO, Global Advantech Resources Ltd.
Anaerobic digestion is a practical and efficient technology to utilize considerable amounts of organic waste from intensive animal production and food processing and wastage during distribution/consumption, municipal waste, etc. to produce methane gas for electricity generation, local/process heating, and direct injection into the gas supply network.
Degradation of organic waste by anaerobic organisms not only produces useful methane, highly odorous gases and other compounds are also produced, including hydrogen sulfide (H2S), ammonia, volatile fatty acids, etc. Release of any of these compounds, especially hydrogen sulfide, in the local environment has the potential to cause considerable nuisance and result in complaints from residents and businesses nearby; hydrogen sulfide can be detected by at least half the population at a concentration in air as low as 0.47 ppb. This means that considerable care and effort must be undertaken in the design and management of anaerobic digester plants — whether mesophilic (lower temperature) or thermophilic (higher temperature) — as increasing numbers are built, often close to towns and villages, to ensure that unpleasant odors are not released during any stage of their operation. Considerations include:
There are several proven methods to control and prevent the release of the odorous gases and compounds that can be deployed so that anaerobic digesters remain a benefit to the local community and do not become an unpleasant nuisance. The plant should be designed so that all processes related to the anaerobic digestion are carried out in enclosed buildings/containers. Forced extraction of air is carried out to maintain a negative air pressure inside at all times and the air is then vented to atmosphere via either:
The anaerobic digestion process is completely contained, because it is necessary to capture the biogas produced for use. However, it is extremely important to manage the reception and handling of feedstock material, especially food/meat byproducts and animal waste from intensive production — cattle, pig, chicken, etc. — to prevent local nuisance from the odors released as this material decomposes. The digestate discharged from the digesters, especially from mesophilic digesters, which are typically used for the production of biogas from cattle and pig waste, is disposed of by spraying on fields as an agricultural fertilizer. Unless this digestate has been through an additional aerobic digestion process to remove all ammonia, hydrogen sulfide, etc., and is kept well aerated prior to spraying, the sprayed digestate will release considerable unpleasant odors for a time until aerobic biological activity takes over and it is absorbed into the soil. Control of the release of odors during the secondary aerobic digestion can only be achieved by carrying this out in enclosed vessels, where the venting to atmosphere is controlled by one of the means given above.
Except for anaerobic digesters built in isolated locations away from towns and villages, additional, active odor control measures will be necessary to prevent nuisance to the local communities. For active odor control, it is necessary to set up a network of atomizing nozzles around the main odor-producing parts of the site, which are fed under pressure with a dilute, aqueous solution of an effective odor control chemical formulation. The smaller the atomized droplet size, the larger the surface area to droplet volume, therefore the more effective the droplets are at capturing/neutralizing odors. High-pressure (60 to 70 bar) atomizing systems are more effective than low-pressure atomizing systems (5 to 15 bar), since these high-pressure systems produce smaller droplets, typically 10 to 15 microns in diameter, than the lower-pressure systems, which produce droplets typically 20 to 50 microns in diameter. It is often advantageous and improved odor control can be obtained with fewer nozzles by combining the nozzles with fed-driven venturis, which ensure an effective spread of the droplets and mixing with air. The layout of the nozzles and venturis across the site is very important in ensuring effective control of odors; due consideration must be taken of proximity of local communities, wind direction, potential amount/strength of release of odorous gases from particular items of digester plant, etc.
The selection of odor control chemical is very important, as there are many odor control products on the market that only contain perfumes and have some surfactants added. This type of product merely masks bad odors and does not neutralize them, leaving the odor still present. A suitable chemical formulation for odor control will be highly concentrated and will contain a number of active ingredients that specifically react with and neutralize the problematic odors — ammonia, hydrogen sulfide, etc. — to ensure that it is both effective and cost-efficient in use. In addition to a pleasant perfume and essential oils, an odor control product should contain a number of other chemical compounds including:
Depending upon the anaerobic digester feedstock (especially animal wastes) and digestate handling procedures, aerosols of feedstock/digestate that contain pathogenic bacteria/fungi might be released into air and could present a hazard to plant operators and people living nearby. In the case of a risk that such aerosols might be produced at an anaerobic digester plant, then this risk can be mitigated through the use of an odor control product that contains a synergistic blend of cationic biocides, which will kill most gram-positive and gram-negative bacteria and fungi upon contact. (It is important to note that for safety, cationic biocides selected should be permitted for use in indirect food contact applications.)
To minimize odor control operational costs, a super concentrate odor control product should be capable of being diluted to at least 1 part in 500 parts of clean water, rather than most conventional odor control products that are used diluted at 1 part in 100 parts or 200 parts of water. These super concentrate odor control products offer further advantages:
In conclusion, a combination of treating gases produced by the process and active odor control will ensure that the population in nearby towns and villages are not troubled by the release of unpleasant and obnoxious odors by anaerobic digester plants.
Image credit: "Ciudad Juarez South Wastewater Treatment and Cogeneration Plant 16" Chuck Moravec © 2016 used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by-nc-nd/2.0/