A Composting System to Decompose Radiocesium Contaminated Baled Grass Silage

Due to the Fukushima Daiichi nuclear power plant accident, a tremendous amount of organic waste (e.g., baled grass silage) contaminated with radioactivity was generated in Tohoku region, northeastern Japan. To establish a safe and efficient way to treat cesium contaminated silage, we investigated the use of aerobic, high temperature composting. Radiocesium (137Cs and 134Cs) contaminated silage (2000 kg, approximately 2700 Bq/kg), water (4000 kg) and matured compost soil (as inoculum, 16,000 kg) were mixed by a wheel loader, and then the mixture was piled up. Air was supplied from the bottom of a compost pile continuously, and the fermentation continued for 7 weeks. The temperature at 100 cm below the surface reached approximately 100 °C. The water content decreased to less than 30% after 7 weeks. The level of radioactive cesium in the final product (18,000 kg) was 265 Bq/kg, which was below the tolerance value for fertilizer (400 Bq/kg) suggested by the Japanese government. The radioactive cesium within silage remained in the final products. We cultivated tomato (fruit), soybean (seed), carrot (root), Italian ryegrass (leaf feed for livestock), Swiss chard (leaf), cosmos (flower) and field mustard (seed) in an experimental farm fertilized with the matured compost made from the radiocesium contaminated silage, for 3 months. Radiocesium levels of edible parts and non-edible parts of each crop were lower than 20 Bq/kg, which was less than one-fifth of the Japanese government value for food (100 Bq/kg). This research demonstrated that the final product can be used safely as an organic fertilizer.