Analysis of salinity inventory variation before and after piggery wastewater treatment and cumulative characteristics of land application

Salt components have posed a serious risk to the piggery wastewater for land application. This study aims to explore the salinity inventory and cumulative variation in the land application before/after piggery wastewater treatment. The piggery wastewater samples were also collected from 25 livestock farms. A systematic investigation was made on the profiles of the salt component in the piggery wastewater at different treatment stages. Critical salt components were identified in the soil accumulation. The application thresholds were then determined for the manure land application. Quantitative analysis was conducted on 16 key salt components in the piggery wastewater at three stages (pre-, post-, and pre-land application) from 25 farms using modern chemical analysis, such as inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography. A nitrogen-based model was also established to accumulate the soil salt components under piggery wastewater application. The elemental balance method was utilized to determine the threshold for the liquid manure application rates. The soluble salt elements/ions in the piggery wastewater primarily included nine major metal elements (Na, K, Ca, Mg, Cu, Zn, Fe, Mn, and Cr), one non-metal element (As), and six ions (NH₄⁺, HCO₃⁻, Cl⁻, SO₄²⁻, PO₄³⁻, and NO₃⁻). These 16 elements/ions accounted for 80.0%–105.3% of the total salt content in the piggery wastewater, with 74.7% of the samples. There was a detection rate of over 90% for total salt components. Among piggery wastewater, six elements/ions (K, Na, Ca, Mg, SO₄²⁻, and Cl⁻) each contributed over 5% to the total salt content, collectively accounting for 78.13% of the total. The average volumetric concentration of NH₄⁺ was 922.6 mg/L, thus representing 25.96% of the total salt content. The most abundant ion was primarily derived from the urea conversion in the piggery wastewater. Overall, the liquid manure treatment and stabilization shared no significant impact on the variations in the salt content of the manure. There were no significant changes in the salt components before and after treatment, as well as prior to the field application (P<0.05). The anaerobic fermentation - advanced treatment effectively removed 27% of the total salt content in the piggery wastewater. The optimal salt reduction was obtained in field trials with corn. Furthermore, 50% and 100% of nitrogen fertilizer were replaced with the piggery wastewater, resulting in the soil salt accumulation increments of 197.0 and 670.2 kg/ hm² at a season, respectively. Six elements (K, Na, Cu, Zn, As, and Cl) were identified as the major contributors to the soil salt accumulation under liquid manure. Once the background soil salinity was measured at 0.5 g/kg, the liquid manure with 50% nitrogen substitution resulted in the saline soil after only 7 seasons of application, leading to moderately saline soil properties after 32 seasons of application. In contrast, the liquid manure with 100% nitrogen substitution caused soil salinization after only 2 seasons, and the moderately saline soil after 9 seasons of application. The total soil salt balance was achieved to apply the 99.9 m³/hectare/season of piggery wastewater (equivalent to a 20% nitrogen substitution ratio) from a salt balance perspective. The residual accumulation of Na, Cu, Zn, As, Cr, Cl, and HCO₃⁻ remained on a total of 49.8 kg/hectare /season, indicating the prioritized removal during piggery wastewater treatment. Some suggestions were recommended to reduce the intake of the key salt components in liquid manure at the source. The effective desalination fully met the agronomic requirements of the corn nitrogen (N) fertilizer in practice. The total salt content was reduced in liquid manure, particularly for the risks in field application. The high accuracy and strong replicability can be extended to mitigate the salt accumulation risks in agricultural soils. The finding can also provide a strong reference to promote the resource utilization of piggery wastewater.