Pig Manure Management in Large-Scale Farms

Due to increasing consumer demand for meat in recent years, the global pig farming industry has rapidly expanded. Global annual pig production has increased over the past 50 years, reaching approximately 120 million tons in 2018. Total pork consumption is projected to grow by 13% by 2030 and by 22% by 2050. Currently, the main global pork producers are China (accounting for 45% of global production), followed by the United States, Germany, Spain, and Vietnam. These five countries account for nearly 65% of the world's pork output. The rapid expansion of the livestock industry has led to increased waste from farms, especially pig manure. The large quantities of manure generated by large-scale farms have severe negative environmental impacts. Firstly, accumulated manure produces unpleasant odors, primarily due to the anaerobic decomposition of proteins. Secondly, nitrogen and phosphorus compounds in the manure can pollute soil and water bodies. In pig husbandry management, the chemical composition of pig manure depends on many factors, including the animal's growth stage and feeding management practices. The manure produced on pig farms is a mixture of urine, feces, and water. The liquid component primarily contains nitrogen compounds (including ammonia, ammonium compounds, nitrates) and organic matter. The solid portion mainly contains phosphorus compounds, existing predominantly in inorganic forms (74% to 87% of total P content) and as organic compounds.

1 Hazards of Pig Manure

1.1 Gases

Livestock farming is a major source of greenhouse gas emissions, accounting for about 66% of all agricultural emissions. Furthermore, handling pig manure contributes to terrestrial acidification and eutrophication in freshwater systems, while gases released during storage and transport exacerbate these environmental problems. Therefore, improper management of pig farm manure can have significant impacts on the environment, human and animal health, and climate change.

Pig manure emits gases such as ammonia (NH₃), carbon dioxide (CO₂), methane (CH₄), and hydrogen sulfide (H₂S). These gases are produced by microbial respiration in the manure (where bacteria utilize inorganic sources like nitrogen and sulfur instead of oxygen for respiration). NH₃ can cause eye irritation and severe respiratory deterioration. While usually not fatal, long-term exposure has serious effects on the respiratory health of humans and animals. Just as human inhalation of NH₃ affects the respiratory system, other livestock are also susceptible. In pigs, NH₃ concentrations reaching 50 ppm reduce expected performance and health status. Prolonged exposure of pigs to NH₃ concentrations around 300 ppm can cause convulsions. CO₂ may seem less threatening than other manure gases, but it is dangerous due to its ability to displace oxygen in the blood. Moderate concentrations of CO₂ can lead to rapid breathing and dizziness. Notably, humans can tolerate CO₂ concentrations up to 260,000 ppm before death, while pigs can only tolerate 200,000 ppm. CH₄ is not a serious issue from a short-term human respiration perspective, but in poorly ventilated buildings with manure storage, it can cause headaches and asphyxiation. Moreover, CH₃ can accumulate significantly in foam layers on top of liquid manure and is highly flammable. H₂S is the gas most likely to cause fatalities among humans and animals on farms and is considered the most dangerous. This gas spreads easily along the ground and in confined spaces (like manure storage areas). It paralyzes nerve cells in the nose, weakening the sense of smell. At concentrations of 700-1,000 ppm, it causes rapid loss of consciousness and death within minutes.

1.2 Heavy Metals

Animal manure is commonly applied to agricultural land to improve soil fertility and organic matter content. However, this practice can also lead to serious environmental problems, such as nitrate and phosphate pollution of surface waters. Another significant issue arising from animal manure application is heavy metal contamination, as animal manure contains high concentrations of metals (Cu, Zn, As, Cd). Due to long-term application on farmland, heavy metal residues in manure accumulate in the topsoil. The accumulation of heavy metals not only affects soil fertility and product quality but also promotes their migration into deeper groundwater via rainwater and runoff. Research assessing the heavy metal hazard in pig farm manure by examining levels in animal feed, manure, and sediments within farm water supply and drainage systems indicates that pig manure poses a higher risk of heavy metal pollution compared to chicken or cattle manure. Sager reported an average copper content of 282 mg/kg in Austrian pig manure. Similarly, surveys of manure in England and Wales commonly found Cu levels of 350 mg/kg in pig slurry. However, Dong et al. found much higher average copper concentrations (1,018 mg/kg) in pig manure from large-scale pig farms in Hangzhou, a major city in China's Yangtze River Delta. Mineral additives in animal feed contribute to this heavy metal accumulation in manure, increasing the risk of environmental pollution.

1.3 Antibiotics

The increasing consumer demand for animal-derived food has led to intensive and large-scale livestock production, generating vast quantities of manure. Manure excreted by animals like pigs has various negative environmental impacts. On one hand, antibiotics are frequently detected in livestock manure due to significant excretion rates after absorption by the animal. Tetracyclines, penicillins, and sulfonamides are the most used antibiotic classes in animals, accounting for 32%, 26%, and 12% of total veterinary antibiotic sales, respectively. Regarding pig manure excretions, while the pig gastrointestinal microbiota contains diverse bacteria known to support host health, they can also be a source of resistance genes. Intensive pig farms are characterized by high bacterial loads, and the presence of high levels of antimicrobials promotes the occurrence of antimicrobial-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Furthermore, disturbances in the gut microbiota may enhance ARG transfer or increase the abundance of ARB shed by the animals. Typically, more than half of the administered antibiotic dose is excreted unchanged in urine and feces. Only a few antibiotics are partially metabolized by the host animal, producing active metabolites or inactive compounds. In finishing pigs fed sulfamerazine, sulfachloropyridazine, sulfaquinoxaline, and sulfadimethoxine, the excretion rate increased with decreasing polarity of the respective sulfonamide. Sulfaquinoxaline (85%) and sulfachloropyridazine (62%) had the highest excretion rates after 13 days. Sulfadimethoxine and sulfamethazine had excretion rates of 38% and 43%, respectively. Based on the physicochemical properties of antibiotics and analysis of soil components, these antibiotic compounds may persist in manure-amended soil or be transferred to groundwater and surface water via leaching or runoff. Moreover, contaminants can be carried long distances by water and affect entire aquatic ecosystems. Degradation rates also vary depending on antibiotic type and can be very low in many cases. When present in soil for up to 30 days, these compounds may be absorbed by crops, accumulate, and enter the food chain.

2 Classification of Pig Farms

Based on long-term research experience related to technological solutions for pig farms, they can be classified into three types.

2.1 Closed Large-Scale Pig Farms

A closed large-scale pig farm is a self-sufficient "single system" where production and processing occur directly on the farm without involving external participants. Its characteristics include sufficient on-site manure processing facilities and storage capacity; adequate agricultural land within a cost-effective transport distance for applying all the organic fertilizer produced; ownership of feed mills and meat processing plants; cultivation of crops for own feed use and sale; and direct sales of pork and by-products.

2.2 Combined Large-Scale Pig Farms

A combined large-scale pig farm is a "single system" with minimal involvement of external participants. Compared to closed farms, it does not have its own feed mill but purchases feed. The available agricultural land is insufficient to apply all the organic fertilizer produced. Therefore, such farms rent land for fertilization, sometimes resulting in uneconomical transport distances.

2.3 Open Pig Farms

An open pig farm is a "single system" involving many external participants. It differs from combined farms in that both manure processing and the field application of organic fertilizer are outsourced to third parties.

3 Manure Treatment Methods

Due to its high nutrient content, pig manure has long been considered a valuable agricultural resource. The most common method of utilizing pig manure is through the application of Liquid Pig Manure (LPM): LPM is rich in nitrogen, phosphorus, and potassium, essential nutrients for plant growth. When applied properly, LPM can help enhance soil fertility and crop yields while reducing the need for chemical fertilizers.

3.1 Direct Land Application of Fresh Manure

Fresh pig manure contains various organic substances, including 0.5% nitrogen, 0.25% phosphorus, and 0.5% potassium. Additionally, it contains colloidal humic acid, which can improve soil structure, enhance soil water-holding capacity and aeration, and promote crop growth. Statistics show that one pig excretes approximately 2-2.5 tons of manure per year. If converted into fertilizer, its nutrient content is equivalent to about 50 kg of ammonium sulfate, 27.5 kg of superphosphate, and 20 kg of potassium sulfate. Applying fresh pig manure to farmland can increase grain yield by about 50 kg per hectare while improving soil fertility, achieving resource utilization. However, pathogenic microorganisms in pig manure also pose hazards to the environment and human health. Furthermore, the environment's self-cleaning capacity is limited; excessive application can easily cause pollution, hence the application rate of pig manure is also quite limited.

3.2 Feed Processing

Utilizing pig manure for feed processing is an important pathway for the comprehensive utilization of livestock manure and a significant measure for conserving grain resources. The total protein value in pig manure is almost higher than that in the feed consumed. Additionally, pig manure contains other essential nutrients such as crude fiber, calcium, phosphorus, minerals, trace elements, and various vitamins. The vitamin content in pig manure is mostly higher than in their base feed. Therefore, accumulated pig manure should be processed promptly to avoid rapid decomposition and loss of nutritional value. Current primary methods for processing pig manure into feed include drying, ammoniation, and fermentation for silage. Processed pig manure can be used as feed for ruminants, pigs, fish, etc. Particularly for fish feed, 10-23.61 kg of pig manure can produce about 1 kg of feed for herbivorous and omnivorous fish. The manure from one pig can produce approximately 50 kg of fish feed.

3.3 Biogas Fermentation

The series of processes where microorganisms decompose organic matter under anaerobic conditions, ultimately producing biogas and sludge, is called biogas fermentation. Biogas fermentation significantly reduces the odor of pig manure and kills most parasites, eggs, and some pathogens present in the manure through anaerobic digestion. Biogas fermentation transforms pig manure from waste into an asset. The methane produced after fermentation can be used as fuel. The digestate residue, containing substantial organic matter, serves as an excellent organic fertilizer. It can improve soil fertility and structure, prevent soil compaction, and reduce the actual use of chemical fertilizers.

3.4 Animal Breeding (Vermicomposting & Maggot Farming)

Pig manure can be used to breed earthworms and maggots (fly larvae). Earthworms have a strong capacity to process waste, decomposing biological proteins and other nutrients in pig manure, offering both environmental value and economic benefits. The castings (vermicompost) produced by earthworms not only have superior fertilizing effects but also avoid the environmental pollution caused by the direct application of large amounts of manure to farmland. Maggots are rich in protein and contain high-quality protein. They can be used as live bait for poultry, livestock, and fish, especially during juvenile stages (e.g., fry, chicks) and for animals that prefer live prey (e.g., bullfrogs, ornamental birds). They also serve as an animal protein feed alternative to fishmeal. Fresh manure from pigs, chickens, ducks, etc., is first fermented in a pool using effective microorganisms (like Nongfukang manure decomposer) to reduce odor. After fermentation, it is transferred to the maggot breeding facility. Omnivorous maggots grow rapidly, their breeding methods are simple, and the economic benefits are high. Moreover, fly maggot protein active powder possesses functions such as anti-fatigue, anti-radiation, anti-aging, liver protection, and immunity enhancement, making it a relatively ideal health product.

3.5 Aerobic Composting

Pig manure aerobic composting refers to the process where aerobic microorganisms decompose organic matter to produce humus, which promotes plant growth. Simultaneously, aerobic composting inactivates pathogenic microorganisms. This method converts pig manure into organic fertilizer to improve soil conditions and aid crop growth. Using well-decomposed pig manure as organic fertilizer not only increases crop yield and quality but also improves the physical and chemical properties of the soil. Currently, aerobic composting is widely used for decomposing poultry manure. In commercial composting facilities, compost turners are commonly employed for large-scale pig manure composting. Turning introduces oxygen into the compost pile, helping to accelerate the decomposition process. Pig manure is a nitrogen-rich organic material, indispensable for balancing the Carbon-to-Nitrogen (C:N) ratio in compost. The management and use of pig manure require scientific planning and management to ensure its effective utilization without causing environmental harm. Application on farmland should avoid periods of heavy rainfall or locations near surface water sources to prevent runoff into nearby water bodies. Furthermore, regular soil testing is essential to monitor soil nutrient levels and prevent over-application.

4 Summary

While the livestock farming industry rapidly develops to meet increasing consumer demand, it faces challenges related to environmental sustainability. Achieving scientific and resource-oriented treatment of farm manure requires, on one hand, strengthening the environmental protection awareness of farms (and farmers). On the other hand, it necessitates joint efforts between the government, universities, and leading enterprises to promote technological innovation and jointly create a green farming environment. This is paramount for the development of the pig farming industry. Through scientifically sound management and utilization of pig production waste (especially manure), these wastes can be incorporated into reusable and recyclable options, moving closer to a circular economy.