Public, animal, and environmental health risks associated with inadequate compost processing on small-scale pig farms

Abstract

The proper management of manure from pig production represents both an environmental and a productive challenge for the agricultural sector. When stabilized through processes like composting, this waste can transform potentially polluting and hazardous materials into valuable resources for agriculture, livestock farming, and soil improvement, thereby establishing a circular economy within the farms. In Tandil county (Buenos Aires, Argentina), some small-scale pig farms pile a mixture of manure and bedding materials for composting. This study evaluated the quality of supposedly stabilized materials from "old piles" (longer stabilization time) and "fresh piles" (shorter composting time) across three small-scale pig farms named A, B, and C. Physicochemical parameters such as pH, electrical conductivity (EC), organic matter (OM), total nitrogen (N), the nitrogen-to-carbon ratio (C:N), N as nitrate (N-NO₃⁻), N as ammonium (N-NH₄⁺) and the N-NH₄⁺/N-NO₃⁻ ratio were analysed. Microbiological characterization and parasitological examination included the detection of faecal coliforms, nematodes (eggs/larvae), cestodes (or tapeworms) eggs, and oocysts of Toxoplasma gondii. None of the intended stabilised material met the regulatory standards concerning the N-NH₄⁺/N-NO₃⁻ ratio. The OM values ranged from 31.37% to 45.04% in the fresh piles and from 7.73% to 15.11% in the old piles. Only two samples from the old piles at farms A and B showed values compatible with maturity (C:N ratios of 10.6 and 15, respectively). All samples were within the acceptable pH ranges (6.28 to 7.56), and exhibited a N-NH₄⁺/N-NO₃ ratio of more than 0.3, and absolute N-NH₄⁺ values less than 400 ppm. In all farms, fresh piles showed EC values ranging from 5.1 to 11.5 mS/cm while in old piles the EC values ranged from 0.36 to 1.14 mS/cm, classifying the compost as ´saline´ and ´non-saline, respectively. Fecal coliform levels in all samples were below 1000 MPN per gram of total solids dry weight. Ascaris lumbricoides and S. stercolaris eggs were detected in some samples from farms A and B, while tapeworms were present in samples from farms A and C. Toxoplasma gondii (2-5 oocysts/field) was found in the stabilized material across all three farms. Based on these results, the use of stabilized material as animal bedding or as a soil amendment is not recommended. While the raw materials (pig manure and crop hay) possess good physical properties for composting, the high N-NH₄⁺/N-NO₃⁻ values of the compost indicate non-stable and immature materials, and the N-NH₄⁺ content could cause phytotoxicity. Elevated EC values, particularly with the continuous use of fresh compost, pose a significant risk of soil salinization. Despite composting´s known ability to reduce pathogens, the detection of parasitic eggs and T. gondii oocysts shows that current post-composting handling is inadequate. Consequently, the material remains unsafe for direct agricultural application and presents a zoonotic infection risk. Effective composting requires careful management, particularly with sufficient heat generation and stabilization, to significantly reduce pathogen levels.