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.
KeywordsAerobic High temperature composting Compost Cultivation of crops Fertilizer Radiocesium Silage Waste treatment
6.1 Composting Organic Waste Contaminated with Radioactive Cesium
Due to radiation fallout at the Fukushima Daiichi nuclear power plant accident caused by the giant tsunami associated with the Tohoku earthquake on 11th March 2011, vegetation (e.g., baled grass silage) and carcasses of animals (e.g., livestock, wild boars) were contaminated with radioactive cesium. This contaminated organic matter has mostly remained untouched since 2011. Although the final treatment of organic waste from farms has yet to be been decided, it is essential to investigate the most optimum and safest way to do this to accelerate the recovery process.
Since radioactive cesium-contaminated waste is too large to remove and store, it is essential to reduce its physical weight and volume. Incineration is one solution, but this requires expensive equipment and is undesirable from the standpoint of the environment.
Instead, we have proposed to treat organic waste with microorganisms, that is, to decompose by composting, especially using aerobic, high temperature composting devised by Sanyu limited company (Kagoshima, Japan) (Oshima and Moriya 2008). We have already succeeded to treat contaminated carcasses of cows, pigs and wild boars in towns in Fukushima prefecture. In short, we could demonstrate the conversion of dead animal bodies into a smaller amount of compost soil.
In this article, we converted radioactive contaminated baled grass silage into compost soil which is less bulky. We also showed that the final product is usable as a fertilizer, with the resulting vegetables having a radioactivity level below 100 Bg/kg.
6.2 Reduction in the Volume and Weight of Silage Contaminated with Radiocesium by an Aerobic, High-Temperature Composting System
6.3 Dynamics of Radiocesium in Crops Grown with Radioactive Contaminated Silage Compost
In our earlier report, we cultivated seven different crops including soybean (seed), sweet corn (seed), eggplant (fruit), bitter gourd (fruit), potato (rhizome), cabbage (leaf) and ginger (root) on cubic holes filled with the radiocesium contaminated compost soil in a field of the experimental ranch of the University of Tokyo (Manabe et al. 2014a, b, c, 2016). The radiocesium levels in the roots, stems, leaves, and fruits of each crop were less than 20 Bq/kg (the Japanese government radiocesium limit in food is 100 Bq/kg).
The compost produced by composting of radiocesium contaminated silage in Kurihara city was 265 Bq/kg and can sell as commercial organic fertilizer. The results of the chemical composition of radiocesium contaminated silage compost indicated that amounts per weight of nitrogen (N) and phosphate (P2O5) were 2.6 and 4.6% respectively, which is higher than the compost of non-contaminated cattle feces in Kurihara city. However, the amount of potassium (K2O) in the silage compost was 0.5%, which is one-quarter the amount of the compost made from non-contaminated cattle feces in Kurihara city (2.2%). Surplus potassium in soil is considered to compete against radiocesium crop absorption. In other words, radioactive cesium will be more easily transferred to plants from potassium deficient soil. We examined the movement of radiocesium into crops from soil when the contaminated silage compost was applied.
Low levels of radioactive cesium contaminated silage were decomposed by aerobic, high-temperature composting system. The final product contained radioactivity of 265 Bq/kg, which was lower than the Japanese government maximum tolerable level in fertilizers (400 Bq/kg). Radiocesium in the silage compost might have changed into a more insoluble form during the composting process because radiocesium in the compost was less soluble in water and citrate solution than before. We cultivated crops fertilized with radiocesium-contaminated silage compost. Radiocesium levels in the edible and non-edible parts of each crop were less than 20 Bq/kg, which is below the Japanese government maximum tolerable level in food (i.e., 100 Bq/kg). These results strongly suggested that aerobic, high-temperature composting system was an extremely useful and safe way to treat low levels of radiocesium-contaminated organic waste. In addition, because the contaminated silage compost can be reused as organic fertilizer or soil conditioner safely, a final landfill site is not required. For reconstruction of sustainable agriculture in radiocesium-contaminated areas of Japan, it can be helpful to treat the organic waste contaminated with radioactive cesium by aerobic, high-temperature composting.
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