Manure treatment technologies

Manure management practices are critical for controlling the populations of bacteria, viruses, and protozoa found in animal manures. Farms use diverse manure handling methods (composting, lagoons, storage tanks) which in turn determines the survival of pathogens within manure. Survival of pathogens is affected by pH, temperature, aeration, and dry matter content during storage (Manyi-Loh et al., 2016; Alegbeleye and Sant'Ana, 2020).

Treatment systems for manure are critical for environmental management, operation profitability, and food safety (Manyi-Loh et al., 2016; Vanotti et al., 2007, 2008). When manure storage is coupled with managed treatment processes, the result of pathogens essentially acts as a multi-barrier system. Some treatment systems address several of these requirements, whereas some are specialized and only address individual factors (USDA, 2007).

The details of manure treatment options are further discussed in this section. Composting is likely the most recommended processing technique for manure in the world. The simple method of stacking and stockpiling solid manure is convenient and inexpensive but will not satisfactorily eliminate enteric pathogens (Manyi-Loh et al., 2016). Composting, stacking, and stockpiling are not equivalent treatment technologies. Chemical manure treatment methods using hydrogen peroxide or lime although effective may have potential negative impact on the environment and humans and are generally not publicly accepted (Alegbeleye and Sant'Ana, 2020).

Some of the biological approaches to treatment include thermophilic composting, vermicomposting, anaerobic digestion, thermophilic digestion, autothermal thermophilic aerobic digestion, sequencing batch reactors, and constructed wetlands (Aitken et al., 2007; Cirelli et al., 2007; Karim et al., 2008; Layden et al., 2007). In general thermophilic processes, particularly those operated in-vessel are designed to expose all treated material to extremely lethal temperatures (60–65°C), while still maintaining sufficient metabolic activity by the nonpathogenic bacteria to sustain the process heat (Gurtler et al., 2018). Thermophilic composting remains one of the most cost-effective treatment technologies for manure solids; it functions well in a variety of environments. Initial capital as well as operations and maintenance costs are minimal compared with other treatment technologies.

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Chapter

Manure treatment and utilization in production systems

2020, Animal Agriculture

Zong Liu, Xiao Wang

Manure utilization

Methods of manure utilization include land-application, pelletizing, biofuel production, nutrient extraction, and feed-stock for various value-added products, etc. One of the main goals of utilizing manure is to recycle its nitrogen and phosphorus nutrients. It is essential for manure utilization to take into account all of the waste, all of the time, and all the way. For example, utilization of slurry manure should take into account both solid and liquid fractions produced by solid-liquid separation processes. A major barrier for manure utilization is high transportation cost of fresh manure and its products. Conventionally, a local solution is preferred. However, as surplus manure is becoming a pressing problem for many animal production regions, there is growing interest in developing manure utilization methods, such as pelletizing and nutrient extraction, that can cost-effectively export excessive manure to the manure-deficient regions.

Land application

Manure from different chains of management has different fertilizer characteristics. For N, liquid manure usually has a relatively high proportion of highly-available N for plants. Solid manure from cattle and swine usually has a relatively high proportion of organic N, which is unavailable to plants in the short term. A large fraction of the organic N in poultry manure is in the form of uric acid, which will be mineralized shortly after land application. The P in manure is predominantly in a slow-release inorganic form. Chelated or complexed organics of micronutrients such as Fe, Mn, B, Zn, and Cu are present in manure. Manure can also indirectly affect the availability of soil elements by altering the pH of the soil.

Manure has a nearly fixed nutrient ratio between N and P. Applying manure to meet the nitrogen needs can lead to the over application of P. To overcome this problem, manure is often applied to fulfill the plant P requirement, and mineral N fertilizer is supplemented to fill the gap for the proper ratio of P and N. The nutrient imbalance problem can be improved by solid-liquid separation, which provides flexibility in establishing the nutrient ratio by separating N and P in the manure into the solid and liquid fractions, respectively.

Unlike most mineral fertilizers designed to deliver immediately available nutrients to plants, the N and P released from manure are less immediate and less predictable. Manure application often requires careful assessment of fertilizer value to account for the slow-release of the nutrients. The timing of application needs to be considered for the nutrient release characteristics, as well as the storage capacity of the manure management system. This often limits the application of manure to plants with a short growing season.

The slow-release nature of manure fertilizer can help reduce nutrient loss for some farming systems. However, manure is also associated with a great risk of nutrient-loss because of the slow-availability and unpredictability of nutrients content, results in over-application of nutrients. The nutrient availability in manure is soil and climate dependent. There is a significant state-to-state difference in the formula for predicting nutrient availability in manure; hence, the amount of manure applied to lands drastically different among different regions.

Fresh manure has a higher fertilizer value than treated manure such as compost, pellets, and digestate, because there are significant potential losses of nutrients associated with treatment and storage. Land application of fresh manure is more restrictive because of the potential to spread pathogens, especially when manure is applied to edibles plants. The USDA's National Organic Program guideline suggests that fresh manure may be applied to soil for a certain period of time before harvesting even if the crop may not come into contact with the soil.

Pelletizing

Transportation and land-application costs limit the utilization of manure as a substitute for chemical fertilizers. At the current costs of loading, hauling, and spreading, it is not economically feasible to transport manure and compost over dozens of miles in most cases. Manure handling and transportation costs can be reduced by pelletizing. During this process, compression and heat are applied to solid manure to granulate or mold it into pellets of uniform size. Manure is often concurrently dried during pelletizing or pre-dried using a solar field or thermal methods. The combination of pressure and heat can chemically stabilize manure, while preventing loss of N. As a result, manure pellets preserve and concentrate most of the nutrients, making them easier and cheaper to be transported to the end-user. For handling, manure pellets are usually dry, stable, and odorless products that can be packaged and land-applied like mineral fertilizer pellets.

Extracting nutrients from liquid and slurry manure

There are increasing commercial interests in extracting nutrients from liquid manure. One method is to harvest struvite (MgNH4PO4·6H2O) by precipitation, which is capable of separating more than 50% of phosphorus in the manure liquid.14 Manure-derived struvite is a high-value fertilizer that can deliver slow release nitrogen and phosphorus to the soil. It is also possible to obtain concentrated liquid fertilizer by removing water from liquid or slurry manure using methods such as ultrafiltration, evaporation, and reverse osmosis.

Biofuel

Manure has a considerable energy value. The higher heating value of the dry matter in manure is ∼20 MJ/kg, which is comparable to brown coal and firewood.15 Historically, dried manure is widely used as a fuel source for cooking and home heating in many countries. Direct combustion of manure in a generator is impractical and inefficient because manure contains high levels of incombustible residuals. It can be burned in a power plant using the co-firing method by mixing it with other solid fuels.

The most efficient way to produce energy from manure is to convert it into a fuel that can drive a conventional internal combustion engine. For liquid and slurry manure, anaerobic digestion can be implemented to produce a biogas that usually comprises 55–65% CH4, which can be used in a commercial generator after dewatering and removal of H2S. For solid manure, pyrolysis can be used to produce syngas, bio-oil, and biochar. Depending on the temperature, heating rate, and resident time, pyrolysis can yield different amounts of those products. The syngas, rich in H2, CO, and CH4, can be burned in a gas turbine for energy; the bio-oil, after refining, can be blended with conventional liquid fuels; and biochar can be used for direct combustion.

Emerging value-added products

Manure-derived biochar

Manure can be converted to biochar by pyrolysis when it is heated between 300 and 700 °C in a low oxygen environment. Manure biochar consists of highly porous polycyclic aromatic hydrocarbons. Most of the plant nutrients in the manure are unvolatilised and retained in the biochar. Manure biochar has a low-density porous structure with a large active surface area which make it a unique soil-conditioning agent capable of reducing soil bulk density and enhancing aeration and water/nutrient-holding capacity of the soil.16 With a large active surface, manure biochar can be used as an absorbing agent to sequester pollutants in soil and agricultural waste.17 It is also used as a bulking agent to enhance the efficiency of manure composting.18

Substrate for microbial culture

Besides being a soil amendment and fertilizer, animal manure also has great potential as a nutrient source for microbial growth because of its rich nitrogen and phosphorus content which is a key nutrient in microbial culture media, as well as most other essential nutrients.19 In recent decades, the demand for microbial growth media from biotechnological fermentation and biofuel production has significantly increased, which leads an increasing demand for cost-effective nutrient sources.20 Therefore, abundant and cheap microbial culture media such as animal manure for large scale biomass production is desired. In addition, animal manure is a rich complex medium likely to contain more easily convertible nitrogen, such as proteins or peptides, with smaller molecular weight, compared with other available organic nutrients. Previous results suggest that animal manure can be a great source of protein-rich material for many purposes.21

Microalgae are photosynthetic organisms with relatively simple nutrient requirements for growth. They can be used as human feed, animal feed, biofuels, and a source of valuable components used in the cosmetic and pharmaceutics industries. Microalgae such as Spirulina platensis, also known as Arthrospira platensis, has a high protein content and it is also a great source of vitamins, minerals, and polyunsaturated fatty acids. If treated properly and managed under optimal conditions, a tremendous amount of manure generated from large animal farms could be turned into high-value biomass. The desired growth conditions for microalgae include appropriate dilutions of manure that allow sufficient light for their photosynthesis, proper manure feeding amount and frequency, ideal agitation, temperature, and aeration, etc.