The effectiveness of phosphine fumigation of wheat in a farm-scale thermosyphon silo

Fumigation of pests is essential for maintaining the quality of stored grain. Rapid and effective pest control is the key to minimise damage to the valuable grain. The current study investigated the efficiency of a farm-scale thermosyphon silo to passively circulate phosphine fumigant gas by measuring the phosphine concentrations at a series of points throughout the silo. A 50 tonne sealable grain silo with an external thermosyphon was fitted with gas sampling lines at three different levels in the grain body, and also in the headspace. The grain was fumigated with phosphine gas 12 times over a 12 month period to contrast the thermosyphon system efficiency under varied external environmental conditions. The minimum fumigation standards of 200 ppm for 10 days or 300 ppm for 7 days were reached and exceeded in all fumigation cycles throughout the year in 12.5–22 days, demonstrating the effectiveness of the thermosyphon system. In addition, the final fumigation was held until the phosphine concentration dropped to 200 ppm. The silo reached 200 ppm only four days after fumigation, reached a maximum of approximately 600 ppm after 30 days, and did not fall below 200 ppm until 106 days after fumigation. Linear air velocity in the was also measured in the thermosyphon pipe and showed that airflow could reach as high as 2.7 m/s, but flow was sporadic and not consistent, most likely due to pressure build up on the headspace of the silo due to resistance to airflow by the bulk grain body. Temperature and humidity results indicated that while both parameters are fundamental in the production of phosphine from aluminium phosphide tablets, they work against each other. This provides an effective control mechanism that prevents the rapid production of phosphine, thereby reducing the buildup of phosphine and reducing the risk of an explosion.