*3.1.1 NH3 removal and recovery rates*

**Figure 2** shows the NH4 <sup>+</sup> concentrations in the stripping column as a function of time: (a) without and (b) with Na2CO3. When Na2CO3 was not used, the concentration decreased about by half and then became a plateau. Since the formulated sample had a pH of 8, NH3 was stripped in the beginning; hence, the NH4 <sup>+</sup> concentration was reduced. As NH3 was stripped, H<sup>+</sup> was released by NH4 + , raising the pH of the formulated sample which slowed down the NH3 stripping which eventually came to

**Figure 2.** *The NH4 <sup>+</sup> concentrations in the stripping column as a function of time: (a) without Na2CO3 and (b) with Na2CO3.*

an end. When Na2CO3 was added, however, the aeration kept stripping NH3, decreasing the NH4 <sup>+</sup> concentration to nearly zero, driven by eq. 3.

The values of *ηNH*<sup>4</sup> *remove* were 96 and 61% with and without Na2CO3, respectively. How fast the NH4 <sup>+</sup> concentration decreases depends on the reaction kinetics and the diffusion of the NH3 gas through the formulated solution to reach air bubbles for the initial NH4 <sup>+</sup> concentration and the volume of the formulated sample.

**Figure 3** displays the NH4 <sup>+</sup> concentrations in the recovery column (a) with and (b) without sulfuric acid. A significant difference between the two cases was observed. Without sulfuric acid, the NH4 <sup>+</sup> concentration quickly reached a plateau and did not increase much. CO2 generated by eq. 3 decreased the pH of water in the recovery column somewhat, dissolving the NH3 gas into the water to a point; however, the NH3 dissolution is limited, determined by thermodynamics through pH and the temperature. With sulfuric acid added, the NH4 <sup>+</sup> concentration kept increasing, driven by eq. 5, NH3 stripped by aeration in the stripping column was mostly recovered in the recovery column. The values of *ηNH*<sup>4</sup> *recovery* were 90 and 26.9% with and without sulfuric acid, respectively. The efficiency of using chemicals for the NH3 recovery is clear.

While it took about 2 hours to remove most of NH4 <sup>+</sup> in the stripping column, it took almost 8 hours to dissolve the same amount of NH4 <sup>+</sup> in the recovery column when the volume of water was 1 L for both columns. The reason for the slow process of the NH3 recovery, relative to the rate of NH3 removal, is due to the limited amount of the NH3 gas going into the recovery column available for eq. 3. That is, the flow rate of the NH3 gas going into the recovery column is smaller than the rate of the NH3 gas generated in the stripping column through eq. 3. Eq. 3 occurs almost instantly, while eq. 5 only undergoes as the stripped NH3 gas has a contact with the acidic solution in the recovery column. The rate of eq. 5 is determined by the airflow rate of the air pump, the air flux associated with the air diffusers used, the size of air bubbles, and others. Namely, the faster the rate of the NH3 gas flow into the recovery column becomes, the more rapidly the dissolving process gets. For example, employing multiple pipes going from the stripping column to the recovery column can speed up the

**Figure 3.** *The NH4 <sup>+</sup> concentrations in the recovery column (a) with and (b) without sulfuric acid.*

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

NH3 dissolution process. An earlier study has reported that the shape and the behavior of air bubbles also affect the gas solubility in water [22].

As **Figures 2** and **3** show, our process ensures a high recovery of NH3 from manure digestate liquid by taking advantage of efficient chemical reactions, eqs. 3 and 5. The chemicals used are abundant and affordable. Still, the results shown in **Figures 2** and **3** are simply a proof of concept. The process can be improved by adjusting the kinetic parameters such as the mass transfer and the retention time of the air bubbles inside the columns. It should be noted, however, that eq. 3 produces sodium as a by-product which should be removed before spraying on croplands.
