Article published in

World Grain Magazine

April 1998 Issue



Grain Fumigation - a New Look
by Chris Newman, Director of GTS

Introduction

Fumigation remains one of the most important weapons in the armoury of grain storage managers in their battle against insect infestations. Chemical pesticides are fast becoming redundant in many countries because of their high cost, the development of insect resistance to them, and the increasing disaffection of consumers who do not want poisons mixed with their foodstuffs. Aeration remains a useful tool for controlling insects, but it cannot be relied upon to kill them. Controlled atmospheres of Carbon Dioxide and Nitrogen now provide a means of supplying the market with "organic" grain that is free from all chemical intrusion, but they are mostly too expensive for general use and are only able to be used in extremely well sealed storage. The importance of fumigation can therefore be expected to increase in the world's market-place where insect-free grain is becoming the accepted standard for export and trading purposes.

Only two fumigants, methyl bromide (CH3Br) and Phosphine (PH3) remain in common use in the grain industry, and of these, the future of methyl bromide is expected to be short-lived because of its harmful effects on the atmosphere. While alternatives such as carbonyl sulphide (COS) are being studied by researchers in Australia, it can be expected that it will take many years before they are registered for use or manufactured in commercial quantities. The world will therefore increase its reliance on phosphine as the main fumigant for control of insects in its grain stocks.

Phosphine

Despite the widespread use of phosphine throughout the world, there is surprisingly little understanding within the grain industry of way in which it should be used to control insects. Research conducted by the CSIRO in Australia and others over the last 20 years, has conclusively demonstrated that not only does phosphine have to be applied at low concentrations for long periods of time to ensure insect mortality, but also that insects can develop resistance to it if it is not administered properly. Ineffective fumigations will select resistant insects through the survival of the "fittest". The survivors of repeated fumigations will, over a period of time, produce resistant strains of insect which become increasingly difficult to kill.

Insects develop over a four-stage life-cycle. Eggs hatch into larvae, which become immobile pupae before finally developing into adults. The eggs and pupae are "immobile" phases; only the larvae and adults provide visual evidence of an infestation. The mobile larvae and adults are easy to kill with phosphine, which is why the belief remains so common that a "quick" fumigation will "do the job". However, the immobile phases are much more difficult to kill and can hatch or emerge, and within a month or two after fumigation, infestation of the grain will become visible again. Effective fumigation must aim at killing all the developmental stages of the insects.

Effective fumigation can only be achieved by administering low doses of phosphine over long periods of time, and ensuring that a minimum dosage is distributed and maintained throughout the grain mass over the full fumigation period. Some adult insect species take several days before they begin to absorb phosphine and will survive short fumigations even at high concentrations. Excessively high dosages can cause insects to become narcotised to the extent that they become comatose and absorb no phosphine at all. Applying low dosage rates over a long periods (15 to 28 days) ensures that all eggs and pupae are given time to develop into larvae and adults respectively, which are most easily killed. Application rates and minimum concentration requirements are temperature dependent; recommended figures for a range of fumigation periods are tabled as follows (for tolerant species of insect):

Grain Temperature Deg C

10 Days

15 Days

21 Days

Initial Dose
g/cu m

Minimum Concentration ppm

Initial Dose
g/cu m

Minimum Concentration ppm

Initial Dose
g/cu m

Minimum Concentration ppm

>20

1

200

0.5

50

0.3

25

15 - 20

not recommended

0.75

100

0.5

50

Note: 1. The minimum concentrations listed should be present at the end of the fumigation period.
2. Fumigating grain at temperatures below 15 deg C is not recommended.

Sealed Storage

Phosphine is normally generated by applying aluminium or magnesium phosphide tablets either by mixing them with the grain, or placing them inside the grain storage enclosure. The gas is released through chemical reaction with the moisture in the air; the reaction time may take one to three days (or more) depending on the grain moisture content and temperature.

Sufficient phosphine must be applied to achieve the required minimum dosage rate at the end of the fumigation period, with the result that a high initial dose has to be applied in order to allow for losses that will occur (through sorption into the grain and leakage from the storage). A lot of the fumigant is lost, and effectively wasted, through these effects. If loss of fumigant is so great that the concentration of gas is too low at the end of the fumigation period, then the fumigation will be ineffective.

The only way to ensure that enough gas is retained in the grain during a fumigation period, is to seal the storage to an acceptable degree of gas-tightness. No storage can be completely gas-tight, and some loss of gas is inevitable. The degree of gas-tightness is best determined by "pressure-testing" the storage, which is done by applying a positive or negative air-pressure to it, and measuring the time it takes for the pressure to drop to half its starting value. The starting pressure should suit the type of storage so that it does not cause structural damage (in the order of 0.5 kPa or less for horizontal steel storages, and perhaps 1.0 to 1.5 kPa for concrete silos). An acceptable pressure test is one where the half-life pressure decay is in excess of 8 minutes if the storage is full, or 15 minutes if it is empty. Any type of storage can be sealed to achieve this target, but some are easier and less costly to seal than others. Fumigating with tablets in storages which do not meet this standard are likely to be ineffective.

Of particular importance is sealing the tops and bottoms of vertical silos, since temperature differentials between the grain and the outside air can cause massive air-movements through any top and bottom openings, through what is called the "chimney effect". Cold air leaking into the bottom of a silo full of warm grain and escaping thought the top (or warm air can leaking into the top of a silo full of cold grain), can result in a complete air-change within the silo in a matter of one or two days (hours in some instances), which may remove all traces of phosphine from the silo.

Current Methods of Tablet Application

Phosphide tablets are normally applied by placing them inside the storage, either by mixing them with the grain or by placing them in trays or in "blankets" above the grain mass.

Admixture: Mixing the tablets with the grain requires the grain to be moved so that the tablets can be dropped onto the grain as it is carried on a conveyor or flows down a chute. This method has many disadvantages:

  • it requires infested grain to be moved through the plant, spreading the infestation through the plant;
  • generation of phosphine may begin to take place while the grain is on the conveying system, with risk to worker safety, especially if the conveying system stops for any reason;
  • a lot of the phosphine that is released during the time the storage is being filled will be lost through the displacement of air from the silo; some of it may find its way into working areas;
  • it is not possible to control the concentration of phosphine: if the concentration is found to be too low (eg because of losses through sorption or air leaks), it is difficult or impossible to add more tablets;
  • there is a risk to operators if the grain is removed from the silo before all the phosphine is released and ventilated from the storage;
  • phosphine residues will remain in the grain: up to 5% of unreacted phosphide may be detectable. Deaths from phosphide poisoning have been recorded where grain containing phosphine residues have been fed to animals. In addition, less harmful aluminium (or magnesium) oxide residues will be left in the grain.

For all these reasons, fumigation by the addition of phosphine to the grain is no longer a recommended practice.

Placement: Placement of tablets into the storage is a preferred method of fumigation, since it is more easy to control the fumigation (eg more tablets can be added if required) and no residues are left in the grain. In flat storages or "squat" silos (in which the height to width ratio is less than 1.5) natural air convection will distribute the gas through the grain mass and there is no need for forced recirculation. In tall vertical silos, a small fan and recirculation ducting are needed to recirculate the gas and to ensure that the gas is distributed throughout the grain mass.

Tablets may be spread out in a thin layer on aluminium trays which are placed in the head-space of the storage. Alternatively, "sachet" formulations may be used where the tablets are supplied in fabric sachets which may be joined together to form "blankets" which can be easily spread over the surface of the grain.

Care must be taken to prevent the formation of a high concentration of phosphine around or within the tablets, since this may cause spontaneous combustion of the gas. The tablets must be spread out in a thin layer with plenty of space for air movement around them.

In all cases, the storage must be gas-tight to ensure that the phosphine is retained long enough to kill all stages of all insects in the grain mass.

New Approaches

New methods of phosphine fumigation have been developed in Australia over the last 10 years which overcome all of the problems associated with tablet fumigation. These methods have been developed by Dr Bob Winks of the Stored Grain Research Laboratory of CSIRO (Australia's principal government research organisation), and are marketed under the trade names of SIROFLO® and SIROCIRC®.

SIROFLO® and its derivative SIROCIRC® have been specifically developed to allow leaky silos to be successfully fumigated by maintaining sufficient concentration of phosphine throughout the entire grain mass inside the silo for a long-enough time to kill all stages of all insect species (eggs, larvae, pupae and adults). In addition, both systems offer other major benefits as follows:

  • they allow complete control of the fumigation process by allowing the fumigator full control of the gas concentration in the grain, and the fumigation time;
  • they ensure correct and even distribution of phosphine throughout the grain mass;
  • they reduce risks to operators and installations because only very low gas concentrations that are used, and the phosphine is supplied in a non-flammable form;
  • reduced amounts of phosphine are absorbed by the grain because the concentrations are so low;
  • they leave no residues (such as aluminium oxide or unreacted aluminium phosphide) in the grain mass.

They achieve these results by applying phosphine in a gaseous, rather than a solid, formulation. The gas formulation was developed by BOC Australia and marketed under the brand name Phosfume®. It is now being marketed world-wide by the BOC Group (previously known as the British Oxygen Corporation) under the name ECO2FUME®.

The phosphine is supplied in pressurised cylinders mixed with liquid carbon dioxide. Each cylinder contains 31 kg of CO2 and 620 grams of PH3 (98% CO2: 2% PH3). The CO2 acts as a dilutant to keep the phosphine below its flammability limit, hence there is thus no risk of the phosphine catching fire or exploding. The amount of CO2 in the mixture has no affect on the grain or on live insects, and once released into a storage it becomes undetectable against the much larger "background" concentration of CO2 that is naturally generated by the grain itself.

SIROFLO® uses this gaseous phosphine mixture by controlling its rate of its discharge from the cylinder (or cylinders) such that it is released slowly and continuously into the grain mass over the entire fumigation period. The gas is mixed with air which is blown at low pressure (by means of a small fan and ducting) into the bottom of the grain mass. The fan is sized to maintain a slight positive air-pressure within the storage such the air/gas mixture leaks out through any holes in the silo structure; this prevents any air from leaking in and diluting the gas concentration. The gas and air flows are separately controlled so as to maintain the required minimum concentration of phosphine throughout the storage for any desired period. The system is so flexible that any number of silos, and any combination of sizes and shapes of silo, can be fumigated at the same time. Control of the system is very simple and very safe, and training requirements are minimal. Typically, a SIROFLO® fumigation maintains a minimum phosphine concentration of 20 ppm in a storage for a 28 day period, or where shorter fumigations are required, a 35 ppm minimum concentration is maintained for 15 days.

SIROCIRC® is a recent development of the earlier SIROFLO® system, and differs from it in that phosphine is collected from the head-space above the grain mass, recirculated back through the fan and pumped back into the storage. SIROCIRC® can achieve a significant reduction in gas consumption depending on the "leaky-ness" of the storages and the amount of gas that is lost through their structures. An automatic control system monitors the concentration of gas that is delivered to the storages, and adjusts the flow of gas from the cylinders to maintain the required inlet concentration at the base of each storage. Re-use of the gas in a SIROCIRC® system may reach 90% in well sealed storages.

A third and simpler method of fumigating with Ecofume, called SIROFUME®, is a direct periodic treatment, and involves adding gas from cylinders to maintain gas concentration in a storage which is left sealed for the fumigation period. Like tablet fumigations, it can only be properly carried out in an effectively sealed storage, however its major advantages are (a) that maximum gas concentration is achieved instantaneously, reducing the overall time required to fumigate, (b) extra fumigant can be added safely and at any time to top-up the concentration, and (c) there are no residues left in the grain.

SIROFLO® is well accepted in Australia where it widely used throughout the grain industry. SIROCIRC® has only recently been introduced to the market, and only three systems have so far been installed in Australia.

The fourth and most recent SIROCIRC® installation has just been installed in Beijing as an introductory demonstration of the technology to China. The installation was carried out by Grain Tech Systems Pty Ltd with the assistance of a representative from CSIRO, and in cooperation with BOC Australia, who provided the pipework, control equipment and gas, and the Beijing Central Grain Depot who provided the site for the demonstration. Grain Tech Systems (GTS) is an Australian company with a registered office in Beijing and is specialising in the marketing of new technologies to the grain industry in China. GTS is working with BOC to obtain registration for use of ECO2FUME in China, and sees a big future for SIROFLO® family of technologies in China. BOC has recently introduced SIROFLO® technology to Cyprus and is currently pursuing registration of the technology in the USA where the first plant for the manufacture of the gas outside of Australia is being established.

SIROCIRC® Fumigant Recirculation Ducting - Beijing 1997

SIROCIRC® Recirculation and Control Cabinet - Beijing 1997

SIROCIRC® Control Cabinet - Beijing 1997