Abstract
The purpose of this study was to explore the processing capacity of phosphogypsum in the composting system and improve the resources utilization intensity of phosphogypsum. The rice husk was used as the main raw material, the chicken manure was used as auxiliary organic material, and the phosphogypsum was used as a compost conditioner. The rice husk and chicken manure were thoroughly mixed in a certain proportion to obtain organic raw materials for composting, which C/N ratio of the raw material was 30. According to the different amount of phosphogypsum added in the composting system, 5 composting treatments were set, including added without phosphogypsum (CK), added 10% phosphogypsum (P10), added 20% phosphogypsum (P20), added 30% phosphogypsum (P30) and added 40% phosphogypsum (P40), which were based on the proportion of organic materials (dry weight) of composting. The moisture content of the composting mixture of 5treatments was adjusted to 55%, and the composting was carried out. The physical, chemical and biological indicators during the composting process and organic matter content, nutrient properties (N, P2O5 and K2O), heavy metal content (Hg, As, Cd, Pb and Cr) of compost at the end of composting were studied. The results showed that compared with CK, 10%~30% phosphogypsum addition significantly promoted the rising temperature of composting and maintaining the high-temperature time, increased the fermentation strength of the composting. With the increasing of the addition amount of phospgypsum, the pH value of the composting significantly decreased, and the electrical conductivity value of the composting significantly increased. At the end of composting, the pH value and electrical conductivity value of P40 treatment were 5.02 and 3.59 ms/cm, respectively. After the end of composting, the total organic carbon reduction of treatments with phosphogypsum were generally higher than that of CK (the total organic carbon content of CK, P10, P20, P30 and P40 decreased by 11.32%, 12.78%, 12.53%, 12.19% and 11.61%, respectively), which further indicated that the addition of phosphogypsum was beneficial to increase the fermentation strength of the composting. But the dilution effect of 40% phosphogypsum (P40) was too obvious, resulting in the organic matter content of the compost product not meeting the NY525-2012 standard, which the organic matter content of P40 was 34.3%. Phosphogypsum addition could increase the germination index value of the compost, by the end of composting, the germination index values of CK, P10, P20, P30 and P40 were 65.43%, 86.54%, 97.52%, 81.35% and 71.40%, respectively. However, when P40 was processed to the end of composting, the water-soluble NH4+-N content was still up to 528.2 mg/kg. Compared with CK, the nutrient content of P10, P20 and P30 treatment increased significantly, and all of them met requirements of the NY525-2012 standard. With the increase of the addition amount of phosphogypsum, the contents of Hg, As, Cd, Pb and Cr in composting products increased significantly, and the contents of heavy metals in each treatment did not exceed requirements of the NY525-2012 standard, indicating that the addition of phosphogypsum still had the risk of increasing the contents of heavy metals during the composting, therefore, the heavy metal content of phosphogypsum should be taken into account when it was used as composting conditioner. Thus, the composting system with rice husk as the main raw material, adding 30% phosphogypsum as the dry weight of organic material was the maximum consumption of phosphogypsum. This study explored the processing capacity of phosphogypsum in the rice husk-chicken manure composting system, and the results provided a scientific basis for phosphogypsum promoting the composting efficiency of rice-husk as the main raw material and maximizing the utilization of phosphogypsum in some areas of China in which rice husk production was high and livestock waste was scarce as composting auxiliary materials.
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