**2. Materials and methods**

#### **2.1 Materials**

Sodium dihydrogen phosphate, magnesium chloride and sodium hydroxide were obtained from the Kanto Chemical Co. Inc. (Tokyo/Japan), potassium chloride was obtained from Wako Pure Chemical Industries Ltd. All chemicals and reagents were of analytical grade and used without further purification.

The soil was collected at 0–20 cm of depth in the field center of Prefectural University of Hiroshima. The soil was analyzed in the laboratory at the Department of Environmental Science, Prefectural University of Hiroshima, Japan. Elemental characteristics of soil and compost were described previous work [16]. Radish (*Raphanus sativus L.*) and komatsuna (*Brassica rapa var. perviridis*) were collected from market store, which is in the Shobara city, Hiroshima Prefecture.

#### **2.2 Experimental design and treatments.**

#### *2.2.1 Bubble column reactor for struvite-K precipitation*

The use of different magnesium additives to recover MgKPO4.6H2O (MPP) was investigated. The experimental conditions are summarized in **Table 1**. Illustrates the experimental setup of the bubble column used in this study which has capacity of 10 L (**Figure 1**). There was a draught tube structure inside the bubble column. Air was fed using an air pump from the bottom of the tower as instead of mixing for homogeneous of solution. A pH probe was placed directly into the reactor just below the liquid surface. The bubble column was first filled with synthetic wastewater, and then potassium chloride, magnesium chloride and sodium dihydrogen phosphate solutions were fed from the outer reactor to the inner tubes. The pH


**Table 1.** *Characteristic of raw water.*

was adjusted to 12.9 with 0.1 M NaOH solutions. Afterward the solution of potassium chloride, magnesium chloride and sodium dihydrogen phosphate were added continuously with had pH of 3.4 by dose pump until pH of 11 with retention time of 1.98 (defined Eq. (2)).

$$\text{HRT} = \text{Total volume reactor} \,(\text{L}) / \,\text{Influent flow rate} \,(\text{L}) \tag{2}$$

Set points for minimum and maximum pH values defined a narrow band of 0.3 pH units. After filtering the reactor solution, a white precipitation was obtained; it was desiccated at 60°C for 24 h. Samples were taken directly from the precipitation zone. All experiments were done at room temperature.

## *2.2.2 Pot treatment designs*

The experimental design was completely randomized design, with five treatments and three replications were presented by pots. Seeds were soaked in water for 24 h before sowing at a maximum depth of 1.2 cm. The equal proportions of compost samples (150 g and 10 seeds) i,e Radish (*Raphanus sativus L.*) and komatsuna (*Brassica rapa var. perviridis*) were filled and installed in a greenhouse. The LED model (PF15-S5WT8-D with power 5 W) was used as a light source with free space between lamps and a pot about 37 cm. Treatments consisted of C: 100% (control), A1: 0.1% of compost, A2: 0.3% of compost, B1: 0.1% of struvite-K and B2: 0.3% of struvite-K. The pots were watered periodically to prevent drought stress of the plants. The experimental were conducted for 9 days.

*Simultaneously Recovery of Phosphorus and Potassium Using Bubble Column Reactor… DOI: http://dx.doi.org/10.5772/intechopen.100103*

#### *2.2.3 Analytical methods of struvite-K precipitates*

The concentrations of PO4 3− were determined by standard method (Japan Industrial Standard method JIS KO 102). The concentrations of potassium and magnesium ions were measured using atomic absorption spectrophotometer (AA-6300, Shimadzu Kyoto, Japan). The white precipitation was dissolved in 0.5 M HNO3 in 1 h for determination of the crystalline components. Microscope images (Olympus CX-32) was used to observe the surface morphology of the crystals. The percent P removal and P recovery are in Eqs. (3) and (4), respectively.

$$P\,\,removal\,\%=\frac{P\,\,Initial-P\,\,equilibrium}{P\,\,Initial}\,\,100\,\tag{3}$$

$$P\,\,recovery \%= \frac{P\,\,in\,\,white\,\,precision}{P\,\,decrement} \,100\,\tag{4}$$

#### *2.2.4 Analytical methods of soil*

The sample was ground by using a coffee mill to pass through a sieve <2-mm before analysis [16]. Cation exchange capacity (CEC) were extracted 1 M NH4OAc pH 7 via NH4 with the procedure in below:

$$\text{CEC} \left( \text{meq}/\text{100 g} \right) = \text{13.7} \ast \text{NH}\_4 \left( \text{mg}/\text{L} \right) \ast \text{dilution} / \text{3} \tag{5}$$

The ammonium (NH4) was measured by the phenate spectrophotometric with UV–Vis Spectrophotometer (Jasco V-530) at 640 nm. The detailed procedure in below:

Solution A

```
1.Phenol 5 gram
```
2.Sodium nitriferricyonide dehydrate 0.1 gram

3.Homogenized and adjusted with dilution water 500 mL

Solution B

1. 200 ml of NaCLO (assay 5%) = 200/5 = 40 mL

2.NaOH = 15 gram

3.Homogenized and adjusted with dilution water 1000 mL

Procedure:

10 mL from the liquid sample and 5 ml from solution A and B, respectively. Afterward, waiting for 30 minutes before analysis. Standard solution was used NH4Cl with calibration curve, y = 0.1747x – 0.1465. R2 = 0.9999.
