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:
- Receiving feedstock material that has already begun to anaerobically degrade (rot) inside tanks and containers. These are not pressure vessels and therefore are not sealed.
- Sorting and loading the anaerobic digesters on site.
- Collection and treatment of the gas produced to separate the methane from the other gases.
- Discharge of the anaerobic digesters at the end of the digestion process.
- Treatment digestate, e.g., dewatering to collect solids, aerobic oxidation prior to spreading on fields as a fertilizer, etc.
- Maintenance, plant modification, and cleaning.
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:
- A series of scrubber towers, packed with an open plastic media, where the air is drawn upwards through the media against a counter flow of water containing a solution of chemicals to absorb and neutralize the ammonia, hydrogen sulfide, and other odorous compounds. The chemical solution will need treatment to remove the dissolved salts, either by a treatment system installed onsite or taken away by a specialist waste management company for treatment offsite.
- Activated carbon filters to absorb the ammonia, hydrogen sulfide, etc. The activated carbon filter media needs to be changed regularly to avoid it becoming saturated with absorbed gases and ceasing to work effectively. The media may be sent away for heat treatment so that it can be reused.
- Air biofilters. These are towers or other containers packed with chipped wood or similar media, kept moist with water so that biofilm of aerobic bacteria and fungi develop, which biologically oxidize the ammonia, hydrogen sulfide, etc. to water and soluble, non-odorous compounds. The air being vented is drawn upwards through the media, which is kept moist by a downward trickle flow of water and dissolved nutrients (to maintain the biofilm). The main disadvantages of air biofilters is that they only become fully effective once the biofilm has become established on the media and they must be kept moist and aerobic at all times by maintaining a flow of air through them. In cold climates, the biofilters may need to be insulated/heated depending upon the temperature of the air being vented through them.
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:
- Non-ionic and cationic surfactants to solubilize the perfumes and essential oils in the formulation, and to solubilize volatile organic compounds released into air.
- Organic acids to neutralize ammonia and other ammoniacal compounds.
- Specific esters of long-chain fatty acids that react with and neutralize hydrogen sulfides and other volatile sulfides.
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:
- Transport costs reduced by 60 percent or more, with less storage required compared to conventional VOC deodorizer formulations.
- Maintenance operations to replenish odor control concentrate reduced by at least 60 percent.
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.