**3. Result**

### **3.1 Fly ash and bottom ash**

### *3.1.1 Particle size*

Ash generated from sewage treatment facilities is largely divided into fly ash and bottom ash, and in particular, fly ash accounts for more than 97%. Fly ash and bottom ash generated from the incinerator were collected and the distribution by particle size was analyzed with a particle size analyzer.

**Figures 4** and **5** shows the particle size distribution of fly ash and bottom ash, respectively. X50 is Fly ash and bottom ash were 31.02 μm and 42.61 μm, respectively,

**Figure 4.** *Grain size distribution of fly ash.*

*Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated… DOI: http://dx.doi.org/10.5772/intechopen.109981*

**Figure 5.** *Grain size distribution of bottom ash.*

**Figure 6.** *SEM photographs(×50) of ash; a) Fly ash, b)bottom ash.*

and a large number of relatively small particles were distributed in the fly ash. In addition, as a result of specific surface area(Sv) analysis, fly ash and bottom ash were 0.46 m2 /cm3 respectively and 0.17 m2 /cm3 , the specific surface area of the fly ash was more than 2.5 times larger. Considering the experimental results of Roy [8] and the amount of incinerated ash, fly ash is judged to be more suitable for P elution. Phosphorus (P) contained in incineration ash is mostly in the form of Whitlockite and is known to be thermally stable and not volatilized even at high temperatures of 800 to 900°C [9].

**Figure 6** is a photograph (magnification ratio: 50 times) of the surface of fly ash and incineration ash using a Scanning Electron Microscope (SEM), and it was confirmed that a large number of relatively large particles were distributed in the bottom ash.

#### *3.1.2 Chemical composition*

Incineration ash, a by-product of incineration, is mostly emitted in the form of metal oxides along with combustion gases. **Table 2** shows the test results of fly ash and bottom ash analyzed using X- ray fluorescence spectrometry (XRF). First of all, the P2O5 of the fly ash was 13.65 ± 2.35%, indicating that it contained more P than the bottom ash (10.23 ± 2.38%). Merino et al. [10] reported that P2O5 of sewage sludge incineration ash in Spain was 14.2%, and Donatello et al. [11] reported P2O5 of incineration ash as 14.8%. Fly ash is also believed to contain similar levels of


#### **Table 2.**

*Chemical composition of ash by XRF. (unit: %).*

phosphorus. In addition, Al2O3 of fly ash was 16.90 ± 1.19%, and Al2O3 of bottom ash was 11.50 ± 1.90%, indicating that fly ash contained more Al. According to literature, Donatello et al. [11] reported 13.1% Al2O3 in incineration ash, and Adam et al. [12] reported 10.8% Al2O3 in incineration ash. P contained in incineration ash is mostly a Whitlockite type(Ca3(PO4)2) orthophosphate.

Incineration ash As a result of analysis by X- ray fluorescence spectrometry (XRF), the P2 O5 of the fly ash was 13.65 ± 2.35% and contained more phosphorus (P) than the bottom ash (10.23 ± 2.38%). In addition, MgO, a component of MAP precipitation, was 3.40 ± 0.27% and 3.83 ± 0.61% for fly ash and bottom ash, respectively, and there was no significant difference.

SiO2(24.15 ± 2.95%) was the oxide that occupied the largest proportion in fly ash, which was attributed to the large amount of soil contained in sewage sludge. SiO2 in incineration ash reported by other researchers Looking at the content, Huacheng Xu et al. [13] and Donatello et al. [11] reported that SiO 2 was the most present in incineration ash, with 43.1 ~ 49.1% and 31.6%, respectively. On the other hand, Fe2 O3 in the bottom ash accounted for the largest proportion at 42.45 ± 9.45% due to the ironbased coagulant mainly used in the return water treatment process of the S sewage treatment facility. Comparing the CaO content, fly ash was 16.45 ± 1.55% and bottom ash was 12.73 ± 5.38%, similar to the 15.7% CaO content reported by Adam et al. [12].

#### **3.2 Jar-test**

#### *3.2.1 Eluted phosphorus*

P elution efficiency was compared by adding sulfuric acid and sodium hydroxide to the fly ash. For the Jar-test, 0.4 N and 1 N sulfuric acid solutions and sodium hydroxide solutions were prepared as extraction liquids, and the stirring time was It was set to 120 min. **Figure 7** shows P eluted from 1 N sulfuric acid solution and sodium hydroxide solution.

*Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated… DOI: http://dx.doi.org/10.5772/intechopen.109981*

#### **Figure 7.**

*Comparison of H2SO4 and NaOH as extraction liquid.*

1 N sulfuric acid and 1 N sodium hydroxide were used, the eluted P was 9362.2 mg/L and 2462.7 mg/L, respectively, and the elution efficiency of sulfuric acid was about 3.8 times higher than that of sodium hydroxide. Through the above experimental results, when using sulfuric acid The maximum recovery of phosphorus that can be eluted is It was more than 0.094 g of PO4 3−-P/g ash. P eluted when 0.4 N sulfuric acid and 0.4 N sodium hydroxide were used under the same conditions were 3747.4 mg/L and 1986.1 mg/L, respectively, and P eluted when 1 N sulfuric acid was used as the eluent. P increased 2.5 times in proportion to 0.4 N sulfuric acid. Biswas, B et al. reported that when sodium hydroxide is used for phosphorus elution from incineration ash, about 40% of phosphorus elution is possible compared to acid [14].

#### *3.2.2 Stirring speed*

**Figure 8** shows P eluted at stirring speeds of 150 rpm and 200 rpm when 1 N sulfuric acid solution was used as the eluent. In order to select the optimal stirring speed for elution of incineration ash, It was fixed at 120 min. Stirring speed PO4 3−-P

**Figure 8.** *Variation of PO4 3−-P with different stirring speed.*

eluted at 200 rpm was 9141.0 mg/L, showing stirring speed It was slightly higher than P(8,634.9 mg/L) eluted at 150 rpm.

Based on the above experimental results, the phosphorus elution efficiency was determined by the stirring speed. At 200 rpm, Agitation speed through the superiority of contact area and the number of collisions It is judged to be better than 150 rpm.

#### *3.2.3 Heavy metal*

When using 1 N sulfuric acid solution Al, Ca and Mg concentrations are shown in **Figure 9**. After 120 minutes, the Al concentration was 2790.1 mg/L and Mg concentration was 1781.4 mg/L. Al and Mg concentrations increased with stirring time, but Ca concentration continuously decreased from 2404.1 mg/L(5 min) to 1211.3 mg/L(120 min).

**Figure 9.** *Variations of heavy metals with H2SO4.*

**Figure 10.** *Variations of heavy metals with NaOH.*

*Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated… DOI: http://dx.doi.org/10.5772/intechopen.109981*

This phenomenon is thought to be due to chemical precipitation of sulfate and calcium contained in the eluate as calcium sulfate (CaSO4). In addition, K and Na concentrations were produced below 400 mg/L, and As, Cr, Cd, Pb and Se were detected below the limit of quantification.

On the other hand, when using 1 N sodium hydroxide solution Al, Ca and Mg concentrations are shown in **Figure 10**. The Al concentration appeared constant at a level of 1800 mg/L, and Mg and Ca concentrations were maintained at a low concentration of 20 mg/L or less compared to when sulfuric acid was used as an eluent. Schaum et al. argued that phosphorus extraction efficiency is as low as 30% or less, but heavy metal elution is low when phosphorus is separated by applying alkali extraction method from sewage sludge incineration ash [15]. When sulfuric acid is used, phosphorus elution efficiency is excellent, but a large amount of other heavy metals are generated as by-products, and phosphorus crystallization. It is necessary to select an effective phosphorus recovery method in consideration of the process of adjusting above pH 9.5 and recovery cost.

#### **3.3 Operating result of PRS**

#### *3.3.1 Phosphorus*

**Figure 11** shows PO4 3−-P according to ultrasonic power in 1 N sodium hydroxide. Same as 12. L: S ratio of 10 without ultrasonic irradiation When stirred for 30 min, PO4 3−-P was 2117.7 mg/L, and when ultrasonic waves were irradiated at 100 W, PO4 3−-P was 3194.8 mg/L at same stirring time during ultrasonic irradiation.

It was found that the elution concentration of phosphorus increased by more than 50%. when ultrasonic power was increased to 500 W, PO4 3−-P was 4104.6 mg/L, indicating that phosphorus elution amount Improved over 94%. This is judged to be the result of an increase in the mass transfer rate of phosphorus adsorbed on fly ash or bonded in the form of a compound by the shear force of ultrasonic waves. The reason why the phosphorus elution concentration is high according to ultrasonic output after

**Figure 11.** *Variations of PO4 3−-P at different ultrasonic power with NaOH.*

**Figure 12.** *Extracted PO4 3−-P at different ultrasonic power with NaOH.*

ultrasonic irradiation time of 10 min is that the P adsorbed on the surface of the fly ash is easily separated in the beginning, but P bonded in the form of P compound is thought to be because elution is possible through a strong shear force such as ultrasound and long exposure.

The amount of PO4 3−-P dissolution by time according to the ultrasonic output was **Figure 12**. In the 0–5 min section, P adsorbed on the surface of the fly ash is easily separated, and it is found that a lot of P(phosphorus) over 900 mg/L (0.9 g P/100 g Ash) is eluted. On the other hand, ultrasonic waves were irradiated at 500 W, phosphorus elution amount It was significantly higher than 1100 mg/L (1.1 g P/100 g Ash) in the intervals of 10 ~ 15 min and 15 ~ 20 min.

#### *3.3.2 Dissolution rate*

The dissolution rate of PO4 3−-P per unit time according to the ultrasonic power in 1 N sodium hydroxide is shown in **Figure 13**. When stirring for 5 min without ultrasonic irradiation, PO4 3−-P dissolution rate was 238.4 mg/L-min, and when the ultrasonic wave was irradiated at 100 W, PO4 3−-P dissolution rate was 183.2 mg/L-min. Appeared as On the other hand, when ultrasonic waves were not irradiated (0 W), the dissolution rate decreased to less than 100 mg/L-min, but when ultrasonic waves were irradiated at 100 W, it was maintained constant at 100 mg/L-min.

Compared to when ultrasonic waves were not irradiated, P elution concentrations increased by 2.1 times and 2.7 times when ultrasonic power was irradiated with 100 W and 500 W, respectively. The effect was found to be large. Based on these results, it is judged that the phosphorus elution effect can be maximized if the operating conditions are derived in consideration of the phosphorus elution amount and the ultrasonic irradiation cost when sodium hydroxide is used for phosphorus recovery from fly ash.

#### *3.3.3 Heavy metal*

The concentration of heavy metals eluted from 1 N sodium hydroxide according to the ultrasonic power is shown in **Figure 14**. Without ultrasound irradiation When *Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated… DOI: http://dx.doi.org/10.5772/intechopen.109981*

**Figure 13.** *Variations of extraction rate at different ultrasonic power with NaOH.*

**Figure 14.** *Variations of heavy metals at different ultrasonic power with NaOH.*

stirring for 5 min (0 W), Ca concentration was 3.5 mg/L, and when ultrasonic waves were irradiated at 100 W, the Ca concentration was 2.6 mg/L. Also, the Mg concentration was measured without ultrasonic irradiation.

When stirred for 5 min and when irradiated with ultrasonic waves at 100 W, 0.2 mg/L and non-detection were found, respectively, indicating that the Ca and Mg concentrations were insignificantly affected by ultrasonic irradiation. On the other hand, the K concentration was measured without ultrasonic irradiation. Concentrations were 108.5 mg/L and 224.9 mg/L, respectively, when stirring and when ultrasonic waves were irradiated at 500 W, confirming that the K concentration increased as the ultrasonic power increased.

**Figure 15.** *Variations of heavy metals at different ultrasonic power with H2SO4.*

The heavy metal concentration according to the ultrasonic power in 1 N sulfuric acid is shown in **Figure 15**. Mg concentration was measured without ultrasonic irradiation. When stirring and ultrasonic irradiation at 100 W were 691.8 mg/L and 754.9 mg/L, respectively, effect of ultrasonic irradiation was not significant. On the other hand K without ultrasonic irradiation When stirred and when irradiated with 500 W of ultrasonic waves, it was 209.7 mg/L and 269.0 mg/L, respectively, and the K concentration slightly increased with the increase in ultrasonic power. On the other hand, Ca and Mg concentrations showed a significant increase in heavy metal elution concentration in sulfuric acid eluate compared to sodium hydroxide.

When sulfuric acid is used as eluent, Cu concentration more than 10 times higher than that of sodium hydroxide is detected. It is expected that the purity of struvite will be lowered or secondary contamination will occur. On the other hand, when sodium hydroxide was used, As, Ba, Cd, and Ni were not detected, and Cr, Pb, Se, and Zn concentrations were detected at low concentrations of less than 1 mg/L. When sulfuric acid is used as an eluent, phosphorus elution efficiency is high, but heavy metals such as Cu, Zn, and As are eluted at high concentrations, causing other side reactions.

#### *3.3.4 Optimization of operating condition*

In order to determine the elution conditions of P in fly ash, an experiment was conducted by setting the stirring time and irradiation time to 3 hr. On the other hand, without ultrasound Stirring time when only stirring at 120 min, PO4 3−-P was 4360.3 mg/L, compared to P(4,402.0 mg/L) eluted at 30 min at 500 W ultrasonic irradiation. It could be drastically reduced to 1/4. Considering this fact, fly ash When irradiated with 500 W ultrasound for 30 min in 1 N sodium hydroxide, P elution of up to 0.044 g P/g ash is possible. Compared to experimental results of recovering more than 0.094 g of P/g ash in 1 N sulfuric acid, about 46.8% or more of phosphorus *Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated… DOI: http://dx.doi.org/10.5772/intechopen.109981*


**Table 3.**

*Main chemical composition of precipitated struvite by XRF.*

contained in incineration ash can be recovered by using ultrasonic waves in 1 N sodium hydroxide. Based on the above experimental results, the optimal operating condition of ultrasonic elution tank is to minimize the concentration of harmful heavy metals and recovery cost when irradiating for 30 min at ultrasonic output of 500 W using 1 N sodium hydroxide at L/S ratio of 10.
