FUMIGATION CO2 Considerations for a controlled atmosphere for pest control
by Blaine Timlick, Canadian Grain Commission, Canada
The fumigation of bulk commodities, such as grains, has been performed for a century and the requirements for such treatments are increasing. Changing demographics of insect pests into new host ranges, increased commodity production, climate change, increased demand for reduced pesticide use and evolution of insects that are tolerant or resistant to specific pest control products has resulted in the investigation and use of alternate gases.
Of the naturally occurring gases abundant in the environment, CO2 has been the most investigated and is now registered in a number of countries as an alternate fumigant for pest control.
CO2 was discovered to impact insect pests at low levels in grain as early as 1972, where it was discovered that a CO2 concentration increase of as little as one percent could induce a spiracle (breathing tubes) opening response and most insect species responded at concentration changes of between two-to- five percent. At concentrations of CO2 where spiracles remain open for elongated periods, insects perish from lack of oxygen and/or desiccation due to water loss.
Through the 1970"s and 80"s, investigations studied application methods, as there were requirements to determine appropriate methods for generating the gas, the appropriate concentrations to cause effective mortality and techniques for effective application.
Methods such as application of dry ice, purging with gas and blending with air or nitrogen all proved effective. Of these methods, purging the bin from the bottom that forces atmospheric gases out of the bin has proven to be one of the most efficient methods and is what is employed currently in many situations where CO2 is used in pest control.
In using this method, bin preparation and sealing is crucial. Appropriate sealing is an obvious requirement, as the structure enclosing the grain needs to be able to retain the gas with minimal leakage. This can be partially accomplished by using sealant products (foams, membrane and sealing paints, tapes, etc) and applying them to areas where potential leakage can occur (vents, cracks, seams).
Performing this with diligence can assist in improving the maintenance of the gas concentration. However, gas concentrations require constant monitoring, as the effects of wind on larger structures will deplete the concentration. Once sealing is completed, performing a pressure test of the bin will provide information of how well the bin was sealed and, therefore, how effective the fumigation might be.
Pressure testing is performed by sealing the bin as required and then applying air pressure to the sealed structure. In general, 500 Pa is a suitable limit for pressure, but 100-200 Pa may be the limit of some structures or sealing methods. Therefore, it is suggested to initially attempt a pressure test at 100 pa and then measure the time of pressure decay from the upper limit of the pressure applied to half of that pressure.
Slightly higher pressures can then be used as a mechanism for determining the fumigation potential of the structure. An approximate time of about 300 seconds from maximum pressure to half of that pressure should be observed to provide assurance that the gas concentration required for an effective fumigation can be achieved and maintained.
Other considerations are required for effective CO2 fumigation: wind speed and structural composition and integrity are two of the most important. Wind movement, especially across the top of grain storage, can influence the gas loss from a bin and concrete, if not sealed, can allow for CO2 to be absorbed.
Gas loss from a concrete storage bin is strongly correlated to the ambient wind speed and barometric pressure. The absorptive uptake rate of CO2 by concrete is dependent upon the gas concentration and at rates of 60-70 percent CO2, up to five percent of the CO2 can be absorbed within 24 hours of application.
Without proper sealing, gas losses attributable to wind and natural leakage will generally average 16 percent per day. Therefore, it is very important to have the capability to add more gas to maintain CO2 levels.
In order to perform an effective fumigation with CO2, a number of considerations are required. Before the fumigation, knowledge of the temperature and moisture content of the grain to be fumigated is needed to determine both the application rate and the length of time the grain needs to be fumigated.
Pearman and Jay found that mortality of stored product insects, due to CO2 fumigation, was greater when the grain moisture content (and therefore the relative humidity in the grain mass) was lower. Therefore, consider aerating cereals prior to CO2 fumigation to a level below 14% moisture content to improve the efficacy.
Like utilising any gas for purposes of fumigation, CO2 requires an even distribution at the desired concentration. With CO2 fumigation, this can be achieved in a number of ways. Using dry ice or generation of CO2 via combustion has proven effective but is somewhat awkward. Using dry ice or generation of CO2 via combustion has proven effective but is somewhat awkward. Direct application of CO2 gas, which can be controlled, is a more efficient means for large bulk fumigation of products such as grains.
With advances in sensor technology and gas recirculation techniques, gas can be added at a known rate and can easily be distributed within a well-sealed storage structure, by purging through a full floor aeration system, or by using extended application pipes in addition to a recirculation system. One of the easiest ways to perform a highly effective CO2 fumigation is by using sensors that can automatically add more gas when concentrations drop below desired levels.
While this method is efficient and effective, it also illustrates an indices of leakage, so a manager can observe how much CO2 is being used over time and compare this with predetermined calculations. The results can demonstrate appropriate or ineffective sealing.
In sensor-monitored systems, gas is typically stored on site in a large gas cylinder (usually 2000lb or 908 kg). To achieve greater than a 60 percent concentration approximately 5 lb/100 cubic feet is required. Therefore, to treat large concrete grain storage (bins of approximately 37,000 cubic feet) close to 2000 lb is required (this includes initial purging, application and top up over the course of the fumigation).
The gas stored is typically in liquid phase. Therefore, a vaporiser is required to change the state from a liquid to a gas. Flow meters and pressure regulators are then used to manage the application rates. The control panels of these systems can then be set to required limits (a high point where the system stops and low point where the system engages to add more gas). The gas is usually applied from the bottom of the bin as purging the bin of oxygen is most effective in this way as CO2 is 1.47 times heavier than air.
The control panel can be set for 60-70 percent and the lower limit can be set for 20-30 percent. The initial purge forces the air out, and when the gas front reaches the top of the bin the desired concentration is reached throughout the bin and the upper limit sensor stops the addition of gas.
Given aspects of leakage and sorption, in addition to the delivery demands of clients for grains, the fumigation time of using any fumigant needs to be as short as possible. CO2, like other fumigant gases, works well at higher temperatures. At a grain temperature of 25°C, a successful fumigation with CO2 can be completed in five days and compares well to fumigants, such as phosphine.
However, when grain temperature reaches 20°C, 14 days is required, and when the temperature is 10°C, 56 days are required. Thus, given vessel loading requirements and export demands, fumigation in port locations when grain is below 20°C is not viable and thus would force export fumigations to occur on vessels.
While this in itself is not a problem, performing a fumigation in the last stage of a process may result in problems that can cause import challenges or rejection of the commodity by the buyer. Additionally, maintaining gas concentrations over longer periods can be considerably more expensive given leakage and sorption.
The use of the type of system described would be for a large production operation, a large storage facility or a processing enterprise as the cost requirements for the aforementioned system can be high to install. However, after installation, costs are only for the product used.
Training requirements are also encouraged in order to understand the safety aspects of the gas. A leaky bin may allow for the gas to collect in low-lying areas in unventilated spaces (e.g. hopper valve areas below ground). If the concentration reaches levels above 8.5 percent, human health ramifications can occur within 60 minutes. As mentioned, CO2 is heavier than air so in an undisturbed lower area, concentrations can increase in the proximity of a CO2 fumigation. The gas is odourless and therefore a properly calibrated gas sensor badge for CO2 (like those that should be worn for any fumigation) should be worn by those working in the area.
Integrating CO2 into a pest management plan can be a very effective part of mitigating insects in the commodities being stored. As with any fumigation, care is required to ensure that the fumigation is successful. The control that is observed is only as good as the weakest part of the system being implemented, so it is vitally important to review the pest management plan. Aspects of cleaning, inspecting, monitoring pest presence, and ensuring proper applications of reactive measures such as CO2 fumigation can assist in assuring product quality is maintained and that customers are satisfied.