Food waste accounts for approximately 12.5 percent of total municipal solid waste (MSW) generated and occupies 22 percent of landfills in the U.S., according to the Environmental Protection Agency (EPA). When organic material decomposes in a landfill, methane emissions are released as greenhouse gases. In order to reduce waste and convert organics into valuable products such as renewable natural gas (RNG) or nutrient-filled fertilizer, separating these organics from the rest of the MSW waste stream is needed. Following diversion, food waste may be converted by composting or standalone anaerobic digestion, farm-based use or by being utilized at wastewater treatment plants.
With an increase in food disposal regulations promoting the growth of the organic waste processing industry, organics processing facilities are being faced with elevated volumes. This translates to a critical need for controlling resultant air pollution at these sites.
What is the source of odor?
Odor is made up of chemical compounds and is detected by people due to its smell and intensity. It is difficult to measure odor since it is subject to the sensitivity of an individual’s olfactory system, however, prepared odor samples can be analyzed by a panel to determine the odor threshold, and thus, concentration in odor units. Based on the known concentration in the sample and the subjects’ reactions, the sample is given a detection threshold and calculated in odor units per cubic meter (OU/m3). Detection threshold is a significant focal point when discussing waste facilities since it is likely the facility will be limited by the odor units at nearby sensitive receptors. Odor is generally perceived as a nuisance and is mainly regulated and monitored by local authorities through odor complaints of adjacent land users.
The first step in addressing odor issues at organics processing facilities is determining the food waste type and specific resultant compounds contributing to the smell. Common odor-causing compounds include ammonia and other nitrogen-based compounds, reduced sulfur, volatile organic compounds (VOC) and volatile fatty acids (VFA).
To determine achievable destruction efficiency, one must first estimate the input of different contaminants with a focus on likely contributors to odor that are quantifiable, such as with VOCs. This can be accomplished by conducting air sampling campaigns at the facility; however, this is a challenging step especially during pre-construction where testing is not possible. An efficient odor-reducing design must be focused on the compounds themselves to find the best available control technology (BACT). Categories of treatment types will be compared herein to present the reader options of one’s best fit solution.
How to treat odor?
When selecting an odor-control technology, the site’s designer should consider site restrictions as well as facility-specific considerations, whether it be capital cost, operating cost, destruction efficiency, footprint, environmental impact, required utilities and other parameters distinct to each individual location. The approaches to treatment include biological, chemical, thermal and physical solutions, all of which have their own implications.
The table below summarizes the key aspects of each proven and commonly used technology.
Biological processes treat odor by fostering nutrient-rich environments for microorganisms to thrive on the compound-laden process air. These technologies include bio-scrubbers, bio-trickling filters and biofilters (open or covered) that employ microorganisms to decompose the contaminants. They are limited because microorganisms need a consistent input of food waste, so if there is a big day-to-day variation in the waste stream, this would not be the best option. These also require a large footprint. Operators should keep in mind they will need to address intermittent operation with nutrient additives when the facility is not receiving organics. Depending on the construction and design, biofilters using organic media represent a low-cost option for odor control. More involved vessel-type designs using inorganic media are comparable in cost to other technologies described in the following sections. Biological treatment technologies require large quantities of water for irrigation of media, which must be disposed of or treated if local regulations do not allow for sewer disposal.
Chemical wet scrubbers remove contaminants from the process stream by absorption of a liquid, usually water, with efficiencies greater than 90 percent and as high as 99.9 percent, per EPA. Absorption scrubbers include packed towers, plate (or tray) columns, venturi scrubbers and spray chambers. Packed towers are the most common due to their high removal efficiency, ability to process higher liquid rates, and lower water consumption compared to other absorbing equipment. Limitations of packed towers include potential for high pressure drops, clogging and fouling; the high maintenance costs attributed to required packing materials and chemicals; and disposal of the contaminated liquid.
Adsorption (not to be confused with absorption) systems are implemented to remove and recover VOCs from low to moderately concentrated air streams. Adsorption is a physical process whereby gaseous molecules flow through a solid bed of adsorptive media and are trapped there by attractive forces known as bonding sites. Since this technology recovers rather than destroys the contaminants, the carbon needs to be regenerated by a desorption cycle or new media must replace the carbon once it reaches capacity. Considerations include disposal of recovered contaminants, access to filter media and media changes. Carbon media is known to easily adsorb alcohols, a category of compounds usually found in high concentrations in organics processing facilities, leading to an early saturation of media and decreased effectiveness in the capture and treatment of odors.
Thermal oxidizers are among the most effective devices for neutralizing odors. They destroy pollutants through combustion. Oxidizers use an input fuel to combust the organics in the air stream. The air remains in the combustion chamber for the designed residence time to sufficiently destroy the contaminants before being released through a stack, mostly in the form of carbon dioxide and water. Oxidizers can be employed to efficiently destroy VOCs, some particulate matter and hydrocarbons by 98 to 99.99 percent, per EPA. Although a limitation with this technology is supplemental fuel consumption and associated maintenance costs, heat recovery can be achieved by regenerative and recuperative oxidizers. Thermal oxidizers have the advantage of faster reaction times to unexpected spikes in pollutant concentration compared to other technologies.
What else can operators do?
While instituting a well-designed air pollution control device will treat a facility’s odor issue, there are also operational and infrastructure design approaches to help minimize the release of odors.
One method is operating buildings under negative pressures by maintaining the flow of air into the building slightly lower than the exhaust flow rate. Negative air pressure monitoring systems can be employed to continuously monitor pressure and prevent odorous air from escaping into the environment. Operations where the building doors are left open for long periods of time accomplish the opposite in odor management. High odorous areas, such as reception and tipping floors, can be operated with an airlock. In addition, a well-designed ventilation system will ensure the minimum air changes per hour for operators’ safety and comfort while capturing the process air from the most odorous areas of the building.
Regular maintenance, cleaning and washing of floors and equipment is equally important in minimizing and controlling odors. Other measures to consider in site operation and design include water misting systems to suppress airborne particles, enclosing equipment and direct ventilation to a treatment device, and increasing the height of the exhaust stack for dispersion of the process plume.
In order to promote the successful growth of organic processing facilities across the country, odor from process air must be addressed. With so many opportunities for organics recovery and beneficial use, operators cannot give neighbors and regulators an excuse to deter the industry.