**1. Introduction**

According to Food Agriculture Organization, the world's meat consumption in 2020 was estimated to be 328 Mt and is expected to reach 374 Mt by 2030 [1]. As the meat consumption increases worldwide, the environmental impacts of livestock agricultural wastes are becoming serious issues. Insufficient disposal or management of the wastes can not only result in environmental contaminations, but also create human health hazards. Inadequate manure management has been causing

eutrophication in rivers, lakes, and bays through runoff of excess nutrients as well as groundwater contamination by leaching of manure through the soil. As a result, livestock manure alone is responsible for about 100 TgNy<sup>1</sup> of the nitrogen (N) input on the earth in 2005, twice as much as the N input of roughly 50 TgNy<sup>1</sup> in 1950 [2].

Anaerobic digestion of livestock manure is becoming popular, driven by an incentive to receive the renewable identification number by selling biomethane. An anaerobic digester (AD) generates the digestate as the discharge which is often treated by a solid-liquid separation. The solid is often used as low-valued solid fertilizers, while the liquid is sprayed on croplands. This N-rich liquid needs to be sprayed over a wide area to keep the nitrogen level in the soil at a certain level, currently regulated at the state levels. As the consolidation of livestock operations progresses, the livestock headcount/farm grows rapidly, generating ever more manure per ft2 . As a result, regulations as to how much N can be sprayed per ft2 of croplands are also being tightened. As a consequence, it is becoming increasingly difficult for livestock farmers to apply manure liquid economically without exceeding the N nutrient required for growing crops in the nearby lands in order to avoid the build-up of excess N in the soil.

The current practice of applying the digestate (to be referred to as digestate) solid and liquid can cause another problem: greenhouse gas (GHG) emissions. Nitrous oxide (N2O), almost 300 times as potent as the global warming potential of CO2, is generated by anaerobic denitrifying bacteria in the soil from ammonia after fertilizer applications. More than half (54.8%) of the total GHG emissions from agricultural activities in the U.S. are due to the N2O emissions from fertilizer applications to the soil. Of those N2O emissions, 30–50% originate from applications of animal manure, which includes organic nitrogen [3, 4]. Application of liquid manure or digestate to the soil provides the available N and carbon, which in turn promote heterotrophic activity, depleting the oxygen availability in the soil, and thus favor the creation of anaerobic microbes that release N2O via denitrification [5, 6]. The current manure management practice causes the disruption of the N cycle on Earth. However, there is potentially a considerable opportunity to reduce the N2O emissions from the soil without spraying manure as is, but instead by applying clean inorganic nitrogen recovered from animal manure without creating anaerobic conditions in the soil. This approach will be the first focus of our report.

**Table 1** lists the average concentrations of various nitrogen sources determined by daily samplings by one of the authors over a week from the digestate liquid after a solid-liquid separation on a dairy farm with 5000 heads in a Midwestern state. As **Table 1** indicates, the concentration of NH4 <sup>+</sup> is the highest among nitrogen sources. Since NH4 <sup>+</sup> can be easily converted to NH3, depending on the temperature and pH, a significant volume of NH3 can be lost through emissions into the atmosphere during the storage in a lagoon, though some go through the natural transformation of


#### **Table 1.**

*Nitrogen concentrations in dairy digestate liquid (mg/L).*

*Mitigation of Environmental Impact of Intensive Animal Farming through Conversion… DOI: http://dx.doi.org/10.5772/intechopen.105131*

nitrification or denitrification during the storage as well. Therefore, it is preferable to recover NH4 <sup>+</sup> before storage in a lagoon.

The nutrient runoff or leaching of nutrients in manure into the soil is also a waste of agricultural resources. Livestock manure is an important source of nutrients for crops and grains. For example, the U.S. annual consumption of N for crop production was 13 Mt in 2015 [7]. On the other hand, the estimated N produced from livestock animal manure in 2007 in the U.S. was 6.2 Mt [8]. Almost half the N fertilizer consumption for crop productions could be replaced by N recovered from animal manure. Accordingly, the recycling of N from manure is a key to the efficient utilization of agricultural resources and the protection of the environment.

Stripping/scrubbing processes are common for the NH3 recovery. There are many NH3 stripping and scrubbing processes such as AMFER [9], Dorset LGL [10], and BIOCAST Process [11]. AMFER can have a relatively high NH3 recovery rate of 80%; yet it uses heat for the NH3 stripping which increases the operation cost. What is common among these ammonia stripping processes is that the initial and operational costs of ammonia recovery using a stripping tower and a scrubber tower are high in general. The CAPEX has been estimated to be up to \$17.5 million for 800 m3 day<sup>1</sup> of flow rate with 2500 NH4-Nmg L<sup>1</sup> of the NH4 <sup>+</sup> concentration [9]. Further, to be cost-competitive with other recovery technologies, the NH4 <sup>+</sup> concentration in the influent must be higher than 2000 mg L<sup>1</sup> which limits the application of these processes. In addition, when the digestate liquid is used as the feed, it always contains CO2 which needs to be removed by a CO2 stripper prior to the NH3 stripping, since CO2 is acidic, raising the caustic soda consumption to increase the pH for the NH3 stripping. Moreover, the NH4 <sup>+</sup> concentration in the recovered solution is determined by the initial concentration of NH4 <sup>+</sup> in the feed which limits how high the N concentration in the recovered solution can go. As we will explain later, our process can produce high N-concentration fertilizers.

Membranes are often used for NH3 recovery from manure. Membrane filtrations include ultrafiltration, nanofiltration, reverse osmosis (RO), electrodialysis, and membrane distillation [9]. For example, Riaño et al. have developed a gas permeable membrane to recover NH3 from manure in a lagoon, controlling the pH gradient between inside and outside of the membrane as the mass transfer driving force [12]. Though high total ammonia removal rates, 79–99%, were obtained, the NH4 <sup>+</sup> concentration in the recovered tank was low, below 2% [12]. Furthermore, all membrane technologies have a fundamental problem of membrane fouling, especially treating manure which has a large content of organic matters, a main cause of fouling.

A challenge of recovering ammonia from manure/digestate liquid is removing undesirable materials in manure slurries without serious membrane fouling or high energy costs and then concentrating ammonia without using high-energy processes such as RO. On the other hand, once NH4 <sup>+</sup> in the manure liquid is transferred to the NH3 gas, the NH3 recovery becomes less problematic, leaving behind undesirable materials in the liquid phase. We have developed a simple process to recover NH3 from manure/digestate liquid by applying acid-base reactions for the NH3 stripping and dissolving the stripped NH3 gas into the water.

As to the digestate solid, it contains a considerable amount of protein, from 12 to 48 wt.%, depending on the animal and the growth period, according to the report by Pacific Northwest National Laboratories [13]. There is a substantially large volume of protein in livestock manure that could be potentially recovered, as is shown in **Table 2**.

Currently, such a protein is often being wasted. When the digestate solid is sprayed on croplands, protein in manure tends to stay in the soil longer since protein is not available to plants immediately as a nutrient; hence, it can be subject to environmental


*a The protein content in manure on a dry matter basis [13].*

*b The weight of manure discharged a day per head of an animal [14].*

*c The solid content in manure [14].*

*d The weight of dry manure discharged a day per head of an animal [14].*

*e The weight of protein discharged a day per head of an animal.*

*f The global head counts of each livestock animal [15].*

*g The annual weight of protein discharged by livestock animals globally in million.*

#### **Table 2.**

*The estimated global volume of protein generated by livestock manure.*

contamination before the complete breakdown of protein, if not properly treated. From a point of view of biological wastewater treatments, protein belongs to what is called biologically non-degradable organic nitrogen compounds which are difficult to treat by conventional treatment processes [16]. It would be beneficial if the protein is recovered from manure before it causes environmental problems. The recovered protein can be converted to various value-added products. We propose one application using the recovered protein: an antioxidant feed additive. The protein recovery from manure will be the second focus of this review.

A process of protein extraction from manure solid has been patented, using solvent extraction [17]. Their approach applies a high concentration, 1 M, of an alkali to the extraction. Such an approach not only requires separation of the alkali after the extraction, but recycling or disposal of the spent alkali as well. We have developed alternative extraction process, using the thermal hydrolysis process (THP) without the use of any chemical. THP has been applied to the extraction of bioactive compounds from plants successfully [18, 19]. Very few reports have been published on the application of THP for protein recovery from manure/digestate solid in the literature. We will examine the efficacy of the protein recovered from the digestate solid for its antioxidant activity.

Our objective is to discuss the two novel processes to recover value-added products from both digestate solid and liquid not only to mitigate eutrophication and GHG emissions associated with livestock manure, but also to bring in extra revenues from the products, some of which can be a renewable N fertilizer or non-carbon renewable energy, bioammonia, and an antioxidant feed additive.
