Paper Presented at
7th International Working Conference
on Stored-product
Protection
Beijing, China, 14-19 October 1998
Beijing Central Grain Depot, China
1 Beijing Central Grain Depot, Chang Ping, Beijing, Peoples Republic of China
2 Grain Tech Systems, #33Jia, Juerhutong, Dongcheng District, Beijing, P.R.China.
3 CSIRO Div. of Entomology (retired), PO Box 5145, Sunshine Coast Mail Centre, Qld Australia 4560,
4 CSIRO Div. of Entomology, Stored Grain Research Lab., GPO Box 1700, Canberra, ACT 2Australia 2601
5 BOC Gases Australia Limited, PO Box 288, Chatswood NSW 2067, Australia
Abstract
SIROCIRC® is a new phosphine recirculation technology for stored grain disinfestation, developed by CSIRO Division of Entomology, Australia. SIROCIRC® is a development of the well-proven SIROFLO® technology that uses phosphine dispensed in gaseous form from a pressurised container, in which it is mixed with liquid carbon dioxide. The gas mixture is manufactured by BOC Gases Australia Limited and marketed under the trade name ECO2FUME®.
SIROFLO® and SIROCIRC® technologies offer technical, environmental, entomological and operational advantages over traditional tablet-based fumigation methods, by uniformly distributing and maintaining low concentrations of phosphine over extended periods of time in storages that are not gas-tight. They achieve this by providing a continuous flow of gas and air into the storage, and by maintaining a positive pressure within the storage. In the case of SIROFLO®, the gas is lost through leaks in the structure; with SIROCIRC® some gas is recovered from the headspace above the grain and recirculated back through the grain, thereby effecting savings in gas and reducing cost.
The paper reports on the results of tests conducted on the first SIROCIRC® system installed in China (the first outside Australia, and only the fourth in the world) and compares the results with systems installed on different types and sizes of storage in Australia.
Introduction
The fundamental requirement of fumigation is to provide an adequate concentration of fumigant for a sufficient period of time that will kill all stages of insect species present. The fumigant dose needs to be distributed evenly throughout the storage to achieve an effective fumigation . While all recommendations for fumigation specify the storage be gastight most are conducted in structures that would fail the minimum gastight standard required to ensure dosages sufficient to kill all life-stages of insect present in the infested commodity. In leaky storage fumigation with tablets or pellets produces a peak phosphine concentration in about 1.5 days that decays to zero in about 5 to 7 days. These typical concentration profiles give the an illusion of success as the susceptible adults and larvae may be killed, but the more tolerant eggs and pupae survive because the two components of dose "Concentration & Time" are not achieved and the infestation returns. At this stage the fumigator, farmer or storage manager fumigates the commodity again and in the process may select the population for resistance to phosphine which constitutes the greatest threat to the continued use of this valuable fumigant (Winks, 1986; Winks and Ryan, 1990).
The most obvious approach to sound fumigation practices is to achieve a standard of gastightness that will retain the gas long enough ie the "Time" component of the dose, to kill all stages of insects including the more tolerant eggs and pupae. Forces that gives rise to air ingress and resultant fumigant dilution in silos are the chimney effect and wind. The chimney effect occurs as a result of temperature differences between the grain and the atmosphere surrounding the silo (Winks and Russell, 1994). In a silo in which the intergranular air is at higher temperature than the air surrounding the silo, the lower density of the intergranular air causes the air within the silo to rise and to escape through leaks in the silo top and roof structure, taking fumigant gas with it. At the same time, cold air is drawn in through leaks in the base of the silo, diluting the fumigant remaining in the silo. Conversely, where the air in a silo is colder than the surrounding air, a positive pressure is generated at the base of the silo causing air and gas to escape through the base while drawing air in to dilute the gas at the top of the silo. The temperature difference between the grain and the atmosphere outside will determine the chimney "potential", and the size of the cracks will determine the amount of air-flow into and out of the silo - ie the amount of air ingress, and hence the size of the pockets of fumigant dilution where insect survival can occur.
Active fumigation systems overcome the problem of ingress of air, and at the same time they ensure distribution of the fumigant throughout the grain mass.
SIROFLO®, the active fumigation flow-through fumigation method developed by Stored Grain Research Laboratory, CSIRO Division of Entomology, Canberra ACT Australia (Winks, 1992), is effective in poorly sealed and unsealed grain storage.
SIROCIRC® (a recirculation method which employs the principles of SIROFLO®) is a multiple-bin, low-flow, recirculation method that provides a balanced flow of fumigant simultaneously through several bins for the longer exposure periods required for effective phosphine fumigation. Unlike conventional fumigant recirculation methods, it is designed to be used in bins that are capped but are not sealed to the gastightness standard necessary for "sealed storage" fumigation methods.
The SIROCIRC® method was initially evaluated in 50 tonne farm bins built to a sealed standard but in which systematic leaks (148mm hole in the top and a 100mm hole in the bottom of the silo) were created. The SIROCIRC® phosphine usage rate varied from ~70% to ~9% of the SIROFLO® rate, i.e. as low as 3¢ / tonne (Winks and Russell, 1997). ECO2FUME®, a non-flammable mixture of 2 wt% phosphine in liquid carbon dioxide (Ryan, 1997) was used as the source of phosphine and was introduced into the air stream to obtain the required input concentration of ~35 ppm.
Workspace and environmental phosphine concentrations measurements at the first commercial SIROCIRC® fumigation (Gladstone, Qld, Australia) involved monitoring ambient phosphine levels in and around bulk grain vertical storage with capacities from 400 tonne to 2700 tonne (Pratt and Waterford, 1997). Phosphine levels outside the building were generally less than 0.05 ppb. The average levels inside the building ranged from 4ppb to 70 ppb near the old cells and 0.10ppb to 28 ppb near the new cells.
Phosphine Dosage Recommendations
Phosfume [ECO2FUME®] is a registered pesticide in Australia, New Zealand, South Africa and Cyprus. Registration applications are being reviewed in North America, Europe and China. Approved Phosfume [ECO2FUME®] application rates were the result of an extensive review of phosphine toxicity on stored product insects carried out by an expert committee. The recommendations of this committee are listed in a draft document of the National Registration Authority, Australia (NRA, 1996). Examples of minimum application rates and exposure times applicable to SIROCIRC® and detailed on the Phosfume [ECO2FUME®] registered label, are given in Table 1.
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15 day exposure g/m3 (ppm) |
28 day exposure g/m3 (ppm) |
Group1:
eg Sitophilus spp |
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Group 2:
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The fumigation enclosure should be ventilated after fumigation so that concentrations of phosphine measured in the work space when grain is handled do not exceed OH&S standards. The short term exposure limit for exposures not exceeding 15 minutes, providing that there are not more than 4 exposures per day and there is more than 60 minutes between exposures, is 1 ppm. The time-weighted average (TWA) exposure standard for phosphine is 0.3 ppm. This is the average airborne concentration of phosphine when calculated over a normal eight-hour working day, for a five-day working week.
In SIROCIRC® facilities it may be practical and economically feasible to use higher concentrations than those normally recommended for 15-day and 28-day treatment times. For example, minimum concentrations of 200 ppm may be possible which would allow a minimum treatment time of 10 days (plus the time required to distribute the dose throughout the grain mass). Workspace levels of phosphine should be carefully monitored if higher concentrations are used.
Trial Protocol
The first SIROCIRC® trial outside Australia was conducted at the Changping Grain Depot, next door to the new Beijing Central Grain Depot, Peoples Republic of China. ECO2FUME® (2% w/w PH3 in CO2) was used as the source of phosphine and was introduced into the airstream to obtain the required concentration. At the base of the bins standard SIROFLO® inlet ductwork was installed connected to existing aeration ducting inside the silos. PVC manifold and pipe-work was installed. Pipe connections at the top and base of each silo were fitted with off/on ball valves to isolate individual bins, and flanges containing orifice plates to control and balance the air-flow into each bin. A small fan was provided at the base of the silos to generate a positive air pressure in the ducting connected to each bin base.
Air recirculation ducts were connected to the top of each silo and brought to a single return duct which was connected to the inlet of the fan.
The initial SIROCIRC® trial was operated at an inlet concentration of ~50 ppm to ensure a short exposure fumigation. The phosphine concentrations were monitored using portable electrochemical analysers (Canary & Bedfont).
The Phosfume cylinder was placed on a scale and the weight was recorded at intervals to monitor the amount of phosphine actually used. The weight was used to compare the SIROCIRC® rates of usage of ECO2FUME® with SIROFLO® rates at similar concentrations. In addition, the weight was used to derive an estimate of the maximum possible phosphine emission rates.
Results of Evaluation Trials
The inlet concentration was set at to achieve a high ~50 ppm phosphine [PH3] level throughout the bulk of the storage and this was achieved in 3 days (Table 2). The results in Table 2 show some variations in concentration profiles due to the experiments using the manual control mode [option also of an Automatic Phosphine Controller which maintains the input phosphine gas concentration].
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Note: Expected Fumigation Days is the total expected fumigation period based on time taken to date, current usage rate and amount of gas remaining in the bottle
Conclusion
The SIROCIRC® recirculation technology treatment cost for the 600 tonne storage is less than USD $0.30/tonne which is equivalent to ~$0.22/tonne for the standard 28 day exposure period. Lower fumigation costs are achieved in taller vertical silos and in storage which have improved level of gastightness eg the silos at the Grainco terminal at Gladstone, Qld. Australia (the first commercial SIROCIRC® installation) are being fumigated for less than USD$0.03/tonne.
This method achieves the two requirements of effective fumigation ie uniform distribution and appropriate dose. The use of low phosphine levels has the additional advantage of satisfying Occupational Health and Safety (OH&S) concern. The levels of less than 0.070ppm inside buildings which is 25% of the Threshold Limit Value (TLV) overcomes the OH&S concerns. The ambient phosphine levels outside the buildings of less than 0.00005ppm more than satisfies any environmental concerns.
The ability to treat all existing storage (including unsealed silos) with SIROFLO® technology and the low cost and OH&S advantages of SIROCIRC® particularly with capped storage has longterm benefit of achieving "nil insect" and "nil residue" status in all existing storage with minimum capital expenditure.
REFERENCES
Pratt, S. and Waterford, C. (1997) CSIRO Div. of Entomology,
Technical Report #74, Aug. 1997.
Ryan, R.F. (1997) Gaseous Phosphine - A Revitalised Fumigant. Proc. Int. Conf.
of Controlled Atmospheres and Fumigation in Grain Storage, Eds Navarro, S.
and Donahaye, E. and Varnava, A., Nicosia, Cyprus, 21-26 April 1996. Printco
Ltd., Nicosia, Cyprus, pp305-310.
NRA, (1996) National Registration Authority [Agricultural & Veterinary
Chemicals] Phosphine-Generating Products; Guidelines for Labelling and Use,
Nov. 1996
Winks, R.G. (1986) Strategies for effective use of phosphine as a grain fumigant
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Winks, R.G. and Russell, G.F. (1994) Effectiveness of SIROFLO™ in vertical
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Winks, R.G. and Russell, G.F. (1997) Active Fumigation Systems: Better Ways
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