Effect of centrifugal microfiltration on solid-liquid separation of pig farm wastewater

Large amounts of livestock waste are discharged owing to the rapid development of the livestock industry, and they cause serious environmental pollution if not effectively treated. Livestock waste has high pollutant concentrations and complex compositions; hence, it requires effective pretreatment to avoid high post-treatment difficulties and low treatment effects. Solid-liquid separation has been reported to be a key technology for livestock waste treatment. This technology could produce a solid fraction that can be used as a high-nutrient fertilizer and reduce pollutants in the waste, lowering the loads for subsequent treatments. However, the effect and efficiency of the traditional solid-liquid separation process for treating livestock waste are relatively low and need to be improved. In this study, a new centrifugal microfiltration separator, used for the reduction of pollutants in livestock waste, was systematically evaluated under different conditions. This study monitored the correlation between total solid (TS) concentrations in pig farm wastewater and other related water quality parameters. The effects of different TS concentrations and mesh sizes on the rate and treatment costs of the separator were also studied. The TS was set to 1%, 2%, 3%, 4%, and 5%, and the mesh sizes were set to 15, 25, and 50 µm. The results showed that TS concentrations were negatively correlated with pH and electrical conductivity (EC), and positively correlated with chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH₄ ⁺-N) and total phosphorus (TP) in the wastewater. The correlation between TS and pH, COD, and TP was higher, with correlation coefficients (R²) of 0.57, 0.53, and 0.66, respectively. TS had no obvious correlation with turbidity, EC, TN, or NH₄ ⁺-N with R² of 0.33, 0.02, 0.10 and 0.03, respectively. The separator effectively removed TS from pig farm wastewater with a removal rate of 17%−68%. The removal rate of turbidity, COD, TN, TP and NH₄ ⁺-N were 3%–39%, 17%–59%, 4%–43%, 18%–54%, and 2%–17%, respectively. The removal rate of pollutants from pig farm wastewater increased with an increase in TS and a decrease in mesh size. The removal rate increased with increasing mesh size. The mesh size of 15 µm had the highest removal rates of 68% for TS, 40% for turbidity, 59% for COD, 42% for TN, and 54% for TP. There was a significant difference in treatment capacity between all mesh sizes (P<0.01). The treatment capacity of 50 µm mesh size was 14‒19 m³∙h⁻¹ and that of 15 and 25 µm mesh size was 2‒7 m³∙h⁻¹. The operational costs of centrifugal microfiltration machine using the screen sizes of 15, 25, and 50 µm in a pig farm having stock of 10 000 pigs as an example were estimated to be 2.44, 2.06, and 1.08 ¥∙m⁻³, respectively. The optimal mesh size and TS for treating pig farm wastewater were 50 µm and 5%, respectively, when considering removal rate and treatment capacity. Compared with traditional solid-liquid separators, the new separator has good application prospects because of its high separation effect and low energy consumption.