From the results of the life cycle assessment, the treatment of swine manure by solid‒liquid separation can effectively reduce the environmental pollution caused by large-scale pig breeding. Compared with Scenario 1 without solid‒liquid separation treatment, Scenario 2 with a screw extruder showed lower environmental impact potential in terms of global warming, eutrophication, acidification and human toxicity, which decreased by 56%, 81%, 83% and 273%, respectively. At the same time, Scenario 3 used centrifugal microfiltration for solid‒liquid separation of swine manure, which also showed a low environmental impact potential, which shows that solid‒liquid separation can significantly reduce the environmental load of swine manure treatment. First, solid‒liquid separation can effectively improve the efficiency of subsequent anaerobic digestion of swine manure. Compared with swine manure without solid‒liquid separation, the COD content of the separated liquid fraction is approximately 40%, and the potential of methane production is enhanced, which can produce more biogas for power generation, replace the electricity purchased by farms from the power grid, and reduce the pollution emissions caused by coal-fired power generation51. Second, solid‒liquid separation can effectively reduce the COD content in the liquid fraction of swine manure, reduce the conductivity, and effectively solve the loss of nitrogen and phosphorus53. IPCC research shows that the composting process of manure is an important source of NH3, CH4 and N2O emissions. Studies by Guilayn52 show that the total greenhouse gas emissions produced by the separated solid fraction in the composting process are greatly reduced compared with the feces without solid‒liquid separation treatment. Solid‒liquid separation can remove as much water as possible and reduce the volume of the solid fraction, which is beneficial to the subsequent composting treatment. Solid‒liquid separation can also effectively reduce NH3 emissions in the process of farmland application. Nyord53 proved by experiments that compared with swine manure, the solids in the separated liquid fraction are significantly reduced, which is easier to transport to farmland by water pumps and can quickly penetrate into the soil, thus reducing NH3 emissions in farmland applications.
In addition, different solid‒liquid separation technologies will have different impacts on the environment. Compared with scenario 2 of solid‒liquid separation by screw extruder, scenario 3 of solid‒liquid separation by centrifugal microfiltration shows higher environmental impact potential, which is mainly related to energy consumption caused by centrifugal microfiltration technology and subsequent anaerobic fermentation treatment. The following section will describe in detail the individual environmental impact categories. Environmental impact potential for three scenarios in detail, as shown in Table. 9.
Table 9
Environmental impact potential for three scenarios.
scenario | Environmental impact potential |
Global warming | Eutrophication | Acidification | Abiotic depletion | Human toxicity |
Scenario 1 | 240.311 | 28.511 | 118.196 | -0.462 | 6.916 |
Scenario 2 | 104.850 | 5.415 | 20.524 | -0.201 | -11.988 |
Scenario 3 | 153.905 | 6.222 | 27.388 | -0.156 | -12.799 |
3.1 Global warming
Of the three scenarios evaluated, scenario 1, representing conventional manure management practices, had the highest net total GWP of 240.311 kg CO2-eq. Compared with scenario 1, scenario 2 and scenario 3 showed lower global warming potential, and solid‒liquid separation substantially reduced GWP from manure management, with gross totals of 135.461 kg CO2-eq and 86.406 kg CO2-eq, respectively. In the process of swine manure treatment, storage and composting produce many greenhouse gases, such as CO2, CH4 and NO, which will increase the risk of global warming. Solid‒liquid separation treatment of swine manure can effectively reduce the emission of CO2 produced in the process of manure composting. This view has also been confirmed by previous studies. Qi54 investigated anaerobic digestion of screened liquid manure and diluted manure in semicontinuous stirred tank reactors. The results showed that solid‒liquid separation had a beneficial effect on the process performance and digestate fertilizer characteristics of anaerobic digestion, and compared with dairy manure, the emissions of CO2, N2O and NH3 of the solid fraction treated with solid‒liquid separation were significantly reduced during composting. Vanotti51 developed a new treatment system combining high-rate solid‒liquid separation with nitrogen and phosphorus removal processes. These results suggested that the impact of the new treatment system on NH3 emission reduction was equivalent to closing conventional swine lagoons while actively growing 5145 pigs with minimal ammonia emissions from the farm. The separated swine manure reduced CO2 emissions by 96.9% in the process of composting and land utilization.
Furthermore, the implementation of solid‒liquid separation techniques has the potential to effectively mitigate methane (CH4) emissions during swine manure treatment. By means of solid‒liquid separation, the concentration of chemical oxygen demand (COD) in the liquid fraction can be significantly decreased, leading to a reduction in methane emissions and facilitating the attainment of greenhouse gas emission reduction targets. Chen55 compared liquid manure stored in a container after solid‒liquid separation with swine manure stored directly and found that the total greenhouse gas emissions of swine manure stored after solid‒liquid separation decreased by 79%. Additionally, solid‒liquid separation can significantly improve the efficiency of anaerobic digestion. Guan56 studied the effect of solid‒liquid separation on the anaerobic digestion of dairy manure. The results showed that anaerobic digestion with separated liquid manure could improve the methane production rate and shorten the hydraulic retention time compared with raw dairy manure, thus improving the equipment utilization rate.
In contrast to Scenario 2, Scenario 3 has a greater global warming potential. This is primarily attributed to the utilization of UASB technology and integrated wastewater treatment equipment in Scenario 3, which results in more power consumption. Simultaneously, the treatment of biogas slurry results in the release of extra greenhouse gases. Specifically, the emissions of CH4 and N2O produced during this treatment process surpass those emitted when biogas slurry is directly put to agricultural land, thereby leading to a greater global warming potential. Therefore, in the process of recycling swine manure, the remaining biogas slurry after anaerobic digestion should be properly treated to reduce gas emissions. Global warming potential in 3 scenarios is shown in Fig. 5.
3.2 Eutrophication
Compared with Scenario 1, Scenario 2 and Scenario 3 show lower eutrophication potential. Eutrophication impacts are caused by emissions of N and P to water, and many studies report a high contribution to eutrophication from manure compost, specifically methane from lagoons. In this study, composting contributes 77%-86% to water eutrophication in all scenarios. Solid‒liquid separation is a viable method for efficiently eliminating the moisture content in the solid fraction. This process results in a reduction in compost volume and minimizes the loss of nitrogen and phosphorus during later treatment procedures. This perspective is further supported by prior research. In conclusion, this methodology improves the appropriateness of compost for utilization in agricultural settings. Jorgensen57 compared the phosphorus distribution between swine manure after solid‒liquid separation and swine manure during composting. The findings of the study indicate that the use of solid‒liquid separation in swine manure effectively preserves the phosphorus content and minimizes the loss of water-soluble phosphorus. After separating swine manure with a screw extruder or centrifugal microfilter, the majority of the nitrogen and phosphorus in the manure are concentrated in the solid fraction, which maximizes the retention of nutrients in swine manure for fertilizer production, thereby enhancing nutrient utilization and bringing about positive environmental effects. Ellison58 found that solid‒liquid separation can better match farmland and organic fertilizer and effectively ensure the nitrogen content of organic fertilizer.
In addition, the eutrophication potential of scenario 3 is greater than that of scenario 2 because, in scenario 3, after biogas slurry separation, the liquid fraction enters the integrated wastewater treatment apparatus for harmless wastewater treatment. Although the discharge of NH3, NO, N2O and NO3 can be avoided by using mechanical equipment, the nitrogen and phosphorus content entering the water body is increased, so scenario 3 has a greater eutrophication effect than scenario 2. Eutrophication potential in 3 scenarios is shown in Fig. 6.
3.3 Acidification
The deposition of pollutants such as SO2, NOx, NH3 and N2O in soil and water will lead to acidification. Scenarios 2 and 3, which include the solid‒liquid separation of swine manure, have a lower environmental acidification potential than Scenario 1, and the acid gas in the three scenarios comes primarily from composting, contributing between 82% and 94% to each scenario's environmental acidification. Compared with Scenario 1 without solid‒liquid separation, solid‒liquid separation reduces the environmental acidification potential of Scenario 3 by 77%. This reduction can be attributed to the efficient mitigation of acid gas formation throughout the composting process facilitated by solid‒liquid separation. Holly59 treated dairy manure with solid‒liquid separation, and the liquid and solid fractions were separately composted and then applied to farmland. Compared to the original bovine dung slurry, the solid‒liquid separation system reduced the emission of NH3 and the total emission of greenhouse gases by 31%. In addition, the environmental acidification potential of scenario 2 is lower than that of scenario 3, which is 25% lower than that of scenario 3. This is primarily because the biogas slurry treatment in Scenario 3 involves effluent treatment facilities, chemical agents, and the addition of PAC coagulant. The inclusion of chemicals will increase the emissions of acid gases and nitrogen oxides. After solid‒liquid separation, the majority of phosphorus is concentrated in the solid fraction, thereby increasing the phosphorus content of the organic fertilizer in Scenario 2 and allowing it to more effectively supplant the industrial chemical fertilizer. Combining the benefits of the solid‒liquid separation process and the farmland application process, Scenario 2 demonstrates the lowest acidification potential. Acidification potential for 3 scenarios is shown in Fig. 7.
3.4 Abiotic depletion
The major contributors to abiotic depletion in this study are the use of coagulants in biogas sediment treatment and the energy consumption caused by the operation of machinery. Scenario 1 has the lowest potential for abiotic depletion when compared to alternative scenarios. This can be primarily attributed to the absence of mechanical equipment for treatment and the consequent avoidance of energy consumption, resulting in a reduced environmental impact. In addition, in Scenario 1, both the solid and liquid fractions of swine manure are applied as organic fertilizer to farmland, which replaces more chemical fertilizers and reduces the pollution discharge during the production of chemical fertilizer. Scenarios 2 and 3 use a screw extruder, centrifugal microfilter, and waste water treatment equipment for subsequent treatment, which will utilize additional electric energy; consequently, the consumption potential of abiotic resources is greater for these two scenarios. Abiotic depletion in 3 scenarios is shown in Fig. 8.
3.5 Human toxicity
The contribution of human toxicity potential mainly comes from the emission of NH3. This study demonstrates that both Scenario 2 and Scenario 3 exhibit a negative human toxicity potential. Compared with Scenario 2 and Scenario 3, Scenario 1 has the greatest impact on the human toxicity environment. This is related to the anaerobic fermentation of swine manure. The process of solid‒liquid separation has the ability to efficiently eliminate the overall solid content present in manure while also significantly decreasing the chemical oxygen demand (COD) content. This creates advantageous circumstances for the occurrence of anaerobic fermentation [60]. Using solid‒liquid separation technology, Scenario 2 and Scenario 3 can produce biogas for power generation during the anaerobic digestion process, thereby reducing emissions generated during external energy production. Furthermore, it can be shown that scenario 3 exhibits a reduced risk for human toxicity compared to scenario 2. This is because the separation efficiency of centrifugal microfiltration technology is higher, and the separated liquid fraction can maintain 40% COD content, which has a higher biogas production potential and can produce more biogas, thus reducing the human toxicity potential of Scenario 3. Human toxicity potential in 3 scenarios is shown in Fig. 9.
In general, it is evident that the solid‒liquid separation treatment scenario has less environmental impact potential compared to the nonsolid‒liquid separation treatment scenario. Solid‒liquid separation can reduce the emission of acid gas and greenhouse gas during swine manure treatment61, which is mainly reflected in composting and farmland application. The results showed that solid‒liquid separation reduced the emissions of CH4, CO2, NH3 and N2O during composting and farmland application. Therefore, Scenario 2 and Scenario 3 show lower global warming potential, eutrophication potential, acidification potential and human toxicity potential. For abiotic depletion, Scenario 1 shows a lower potential of abiotic depletion because no mechanical equipment is used in the treatment process, resulting in additional power consumption, and at the same time, organic fertilizer is generated by composting to replace chemical fertilizer for farmland application, which reduces the environmental emission in the chemical fertilizer production process. Aguirre-Villegas62 tested the effects of anaerobic digestion and solid‒liquid separation of dairy manure on emission reduction. The research shows that both anaerobic digestion and solid‒liquid separation can reduce greenhouse gas emissions, and the combined use of anaerobic fermentation and solid‒liquid separation can achieve a greenhouse gas emission reduction rate of 41%. In addition, solid‒liquid separation can effectively improve the efficiency of anaerobic digestion, effectively avoid the production of chemical fertilizer and electric energy and have a positive impact on the environment. Kaparaju63 found that solid‒liquid separation of dairy manure can refine the particles in the liquid fraction, which can promote the efficiency of anaerobic digestion and gas production. In addition, solid‒liquid separation is beneficial for improving composting efficiency and reducing greenhouse gas emissions. The research of Sáez64 shows that solid swine manure stored for one month after solid‒liquid separation has the best composting effect with corn stalks, and the organic matter of the manure obtained after composting has a high humus degree and no phytotoxicity.
In addition, the mechanical equipment selected for solid‒liquid separation and the treatment method after separation will also impact environmental emissions. The separation efficiency of centrifugal microfiltration is better than that of screw extruder, which can effectively remove the COD content in liquid swine manure, facilitate subsequent anaerobic digestion, help to produce more biogas, and better replace coal for combustion and power generation. Compared with Scenario 2, Scenario 3 uses chemical coagulation in the biogas slurry treatment stage, which will not only improve the treatment efficiency but also bring environmental pollution. Therefore, when adding chemical coagulation, attention should be given to the dosage and addition sequence to avoid pollution.