Two-stage separation and acidification of pig slurry - Nutrient separation efficiency and agronomical implications

Hotspots and bottlenecks for the enhancement of the environmental sustainability of pig systems, with emphasis on European pig systems

Although pig systems start from a favourable baseline of environmental impact compared to other livestock systems, there is still scope to reduce their emissions and further mitigate associated impacts, especially in relation to nitrogen and phosphorous emissions. Key environmental impact hotspots of pig production systems are activities associated with feed production and manure management, as well as direct emissions (such as methane) from the animals and energy use. A major contributor to the environmental impacts associated with pig feed is the inclusion of soya in pig diets, especially since European pig systems rely heavily on soya imported from areas of the globe where crop production is associated with significant impacts of land use change, deforestation, carbon emissions, and loss of biodiversity. The "finishing" pig production stage contributes most to these environmental impacts, due to the amount of feed consumed, the efficiency with which feed is utilised, and the amount of manure produced during this stage. By definition therefore, any substantial improvements pig system environmental impact would arise from changes in feed production and manure management. In this paper, we consider potential solutions towards system environmental sustainability at these pig system components, as well as the bottlenecks that inhibit their effective implementation at the desired pace and magnitude. Examples include the quest for alternative protein sources to soya, the limits (perceived or real) to the genetic improvement of pigs, and the implementation of alternative manure management strategies, such as production of biogas through anaerobic digestion. The review identifies and discusses areas that future efforts can focus on, to further advance understanding around the potential sustainability benefits of modifications at various pig system components, and key sustainability trade-offs across the environment-economy-society pillars associated with synergistic and antagonistic effects when joint implementation of multiple solutions is considered. In this way, the review opens a discussion to facilitate the development of holistic decision support tools for pig farm management that account for interactions between the "feed * animal * manure" system components and trade-offs between sustainability priorities (e.g., environmental vs economic performance of pig system; welfare improvements vs environmental impacts).

Acidification and solar drying of manure-based digestate to produce improved fertilizing products

The increase in energy and fertilizer consumption makes it necessary to develop sustainable alternatives for agriculture. Anaerobic digestion and digestates appeared to be suitable options. However, untreated digestates still have high water content and can increase greenhouse gas emissions during storage and land application. In this study, manure-derived digestate and solid fraction of digestate after separation were treated with a novel solar drying technology to reduce their water content, combined with acidification to reduce the gaseous emissions. The acidified digestate and acidified solid fraction of digestate recovered more nitrogen and ammonia nitrogen than their respective non-acidified products (1.5-1.3 times for TN; 14 times for TAN). Ammonia and methane emissions were reduced up to 94% and 72% respectively, compared to the non-acidified ones, while N₂O increased more than 3 times. Dried digestate and dried acidified digestate can be labeled as NPK organic fertilizer regarding the European regulation, and the dried solid fraction and the improved dried acidified solid fraction can be labeled as N or P organic fertilizer. Moreover, plant tests showed that N concentrations in fresh lettuce leaves were within the EU limit with all products in all the cases. However, zinc concentration appeared to be a limitation in some of the products as their concentration exceeded the European legal limits.

Life cycle inventory for an organic swine waste treatment system

The aim of this study was to analyze the efficiency of a system of treatment of organic swine waste as a management tool in the transformation of organic waste into products of value in the swine industry. The residues from the pig farm and the products obtained (compost, biol and biogas) were quantified and characterized, as were the energy used within the process and the distribution of the products. The negative impacts on the soil and adjacent river, as well as the efficiency of the compost as fertilizers and biol in grass and corn crops, were evaluated. The subsystems were: S1-slurry separation, S2-anaerobic digestion, S3-composting solid fraction of slurry, and S4-composting of dead tissues. S2 was not efficient in obtaining biol, with COD and TSS required. The process requires 31.1 kW/d of electrical energy and 3.22 L/d of diesel. The biogas (35,486.0 m³/d) is used for cooking food and heating houses, whilst the compost (82 kg/d) and biol (7.72 m³/d) replace inorganic fertilizers in crops. The system was adequate for the transformation of 38,109.0 kg/d of waste into valuable products. The biol needs further treatment time or to couple biodigesters-another treatment. The pig farm can be considered eco-efficient.

Balance of total mass and nitrogen fluxes through consecutive digestate processing steps: Two application cases

The high water content and low nutrient concentration of digestate complicate its storage, transportation, and utilization. Subsequent digestate processing can effectively remove water and influence nutrient partitioning among digestate fractions and final products. The current study was carried out to evaluate the performance of two typical digestate processing chains, solid and liquid ones, respectively, and to give practical recommendations for optimization. Two fully operating biogas plants with advanced heat utilization were considered as data sources. The digestate mass flow balance of dry matter (DM), water, total N (TN), and ammonium-N mass flows was performed and the efficiency of the examined processing units was calculated. It was found that solid-liquid separation of raw digestate shifted 73-87% of TN and 60-93% of NH4-N to the liquid phase. Subsequent drying of separated solid fraction removed about 6% of the initial water and required 84% less thermal energy per kg N recovered than the processing of separated liquid. The final product, pellets, contained 14% of initial TN, but only 2% of initial NH4-N as a result of microbial conversion of inorganic N during drying. Vacuum evaporation of separated liquid fraction removed 34% of the initial water and left a DM-rich concentrate. At the same time, an ammonium sulfate solution (ASS) containing 21% of initial TN and 34% of initial NH4-N was produced. Both evaluated processing chains showed specific advantages and challenges. Solid products were characterized by a high share of recalcitrant organic compounds and could serve as a soil improver. Liquid processing concentrated plant-available N in ASS, which could be used as valuable inorganic fertilizer.

Hazards of swine slurry: Heavy metals, bacteriology, and overdosing-Physicochemical models to predict the nutrient value

In this work, 124 samples of slurry from 32 commercial farms of three animal categories (lactating sows, nursery piglets, and growing pigs) were studied. The samples were collected in summer and winter over two consecutive years and analyzed for physicochemical properties, macronutrient and micronutrient, heavy metals, and major microbiological indicators. The results were found to be influenced by farm type and to deviate especially markedly in nursery piglets, probably as a consequence of differences in pig age, diet, and management. The main potential hazards of the slurries can be expected to arise from their high contents in heavy metals (Cu and Zn), especially in the nursery piglet group, and from the high proportion of samples testing positive for Salmonella spp. (66%). Linear and nonlinear predictive equations were developed for each animal category and the three as a whole. Dry matter, which was highly correlated with N, CaO, and MgO contents, proved the best predictor of fertilizer value. Using an additional predictor failed to improve the results but nonlinear and farm-specific equations did. Rapid on-site measurements can improve the accuracy of fertilizer value estimates and help optimize the use of swine slurry as a result.

Assessment of environmental and farm business impacts of phosphorus policies in two European regions

This paper presents analyses of the economic and environmental impact of relevant phosphorus management policy scenarios, using representative pig and poultry farms in the Emilia-Romagna region (Po River basin, Italy) and Niedersachsen region (Ems River basin, Germany) as case studies. The analysis was done by using a farm-level linear programming bioeconomic model developed for different farm and animal types—sows, fattening pigs, laying hens and broilers. The baseline for the assessed scenarios involved farms situated within the Nitrate Vulnerable Zones (NVZs) and therefore, were subject to indirect phosphorus fertilization limitations via the Nitrates Directive (ND). The analyzed phosphorus management policy scenarios included the implementation of two different balances of 8.7 kg and 4.35 kg of P per ha per year, inspired by the German Fertilizer Ordinance for phosphorus fertilization implemented in 2007 and later updated from 2017 up to 2023, respectively. We also included a more rigorous zero P balance scenario. The results of the scenario simulations, based on model assumptions, reveal that the introduction of direct phosphorus management policies for pig and poultry farms situated in high livestock dense regions can be done without causing any significant impact on farm gross margin (around 2% of reduction) for both regions and all four animal types, except for sows and broiler production in the Ems River Basin (up to 12% of reduction). Selected technologies and methods, inspired by current practices, have been analyzed for their cost efficiency to achieve the target P balance of the individual scenarios, including export of slurry out of the farm or export of separation solids. Results also highlight that the Nitrates Directive alone is not enough to handle the P issue in monogastric livestock farms.

Separation efficiency of different solid-liquid separation technologies for slurry and gas emissions of liquid and solid fractions: A meta-analysis

Solid-liquid separation (SLS) technology is widely used in the slurry management in animal farms. This study conducted a comprehensive meta-analysis of 45 published articles to evaluate the differences in separation efficiencies (SE_X-SF) of various SLSs and the changes of gas emissions before and after the separation during on-farm slurry storage. The results indicated that the SE_X-SF of the untreated raw slurry and acidified slurry were consistently greater than those of the digested slurry, and centrifugation resulted in greater SE_X-SF than the other mechanical methods. Both measured and simulated data showed that the centrifuge technology had greater reductions in greenhouse gas (GHG) emissions relative to the screw press (56.1-58.0% vs. 38.9-40.2% for untreated slurry, and 29.7-30.2% vs. 22.5-23.2% for digested slurry), mainly due to CH₄ reduction. Additionally, we identify the need for further assessment of the environmental risks that are associated with SLSs for the development of an optimal slurry management chain.

Intermittent air mixing system for anaerobic digestion of animal wastewater: Operating conditions and full-scale validation

An air mixing system for anaerobic digestion has been proved to be beneficial for methane production. The aim of the present study was to further investigate the appropriate conditions for air mixing. The effective methane production time (EMPT) was defined to determine the air mixing time in the article. The results indicated that the appropriate aeration intensity was 66.7 mL air per volume of reactor per min and mixing time was 1.5 min. When air mixing time exceeded 3 min on each occasion, total CH₄ production was less than that achieved under the no mixing condition due to a decrease in the EMPT. In addition, the possibility of air mixing was evaluated in an anaerobic full-scale plant comprising a continuous stirred tank reactor. One year of operating data validated the feasibility of air mixing during the anaerobic digestion of swine wastewater.