1. Introduce
Indonesia has many palm oil mills where biomass waste is still not fully utilized. Empty fruit bunch (EFB), fibers, and shell of oil palm are the primary solid waste which each ton of fresh fruit bunch can produce 14% fibers, 7% shells, and 23% EFBs [1]. Indonesian Palm Oil Mill industries produce about 13 million dried EFB per year [2]; this amount can provide the possibility for further utilization of EFB. Moreover, without proper handling, large quantities of EFB can cause some trouble in palm oil mills since the burning method can cause air pollution. Some studies show that EFB has a relatively high content of potassium. Based on the proximate analysis, EFB ash composition contains 55.48% potassium oxide (K2O) [3–4]. The high composition of potassium in EFB ash can be utilized to synthesize NPK fertilizer since potassium is one of the main components in NPK fertilizer, which is also a macronutrient needed by red onion plants. Red onion requires an intensive supply of nitrogen (N), phosphorus (P), and potassium (K) to attain maximum yield of bulbs because the plants have shallow, sparsely branched root system [5].
Red onion (Allium cepa L.) is one of Indonesia's important horticultural commodities, where itsneeds continue to increase [6]. One of the problems in the cultivation of red onion is low productivity so that it is necessary to develop the proper fertilization. Moreover, the consumption of NPK fertilizer in Indonesia tends to increase from year to year. Utilization of EFB ash for fertilizer raw material can be a solution to overcome the problem of EFB solid waste in a palm oil mill and an alternative source of raw materials in the synthesis of NPK fertilizer.
In this research, EFB ash is used as the potassium source in the synthesis of NPK fertilizer. The EFB ash is combined and mixed with other raw materials to fulfill the NPK fertilizer formula, suitable for the red onion plant. The fertilizer formula is also enhanced with the addition of micronutrients to support plant growth.
2.1. Materials
The NPK fertilizer was made by using some materials including Urea fertilizer to provide the nitrogen need, Diammonium Phosphate (DAP) for supplying the phosphate requirement, EFB ash as potassium source, Potassium Chloride (KCl) as supporting source of potassium need, zeolite as a matrix, micronutrient Mg from Magnesium Sulphate fertilizer and S from ZA fertilizer, and also molasses as the binding agent.
Ideally, the EFB ash was produced as waste from the gasification process of EFB. Since the existing gasification plant facility at our laboratory has a large capacity, the gasification is carried out using small scale gasifiers for this research. The composition of ash and clay and the gasifier temperature were varied, as shown in Table 1.
Table 1. Parameter of gasification for ash – clay mixture.
Ash (%w)
Clay (%w)
Temperature (ºC)
5
95
500
10
90
500
15
85
500
5
95
600
10
90
600
15
85
600
5
95
700
10
90
700
15
85
700
To study the effect of gasification on the potassium content of EFB ash, EFB ash and clay was then undergo the gasification process at the temperature of 500, 600, dan 700 ºC with the variation of ash and clay content as shown in Table 1. The clay used was crushed and screened to obtain the particle size of 60 mesh.
The gasification process was conducted using the small-scale gasifier at the Laboratory of Center for Technology Energy Resources and Chemical Industry (PTSEIK), BPPT, South Tangerang, Indonesia. The gasifier is equipped with a gas compressor, thermocouple, temperature control panel, and flow meter as showed in Figure 1.
After gasification, the mixture of ash and clay was characterized to obtain the composition of K2O. The sample with the highest potassium content then used as a basis for developing a formula and calculating the need for raw materials for the synthesis of NPK fertilizer refers to procedures done by [7]. After that, the fertilizer product was tested to study its properties.
The fertilizer's performance test was conducted by doing a small-scale field test for the red onion plant by using a polybag. This field test consists of 5 (five) treatments: without fertilizer, using commercial NPK, using NPK char fertilizer first formula (without micronutrient), NPK char fertilizer second formula (using micronutrient), and NPK ash – clay fertilizer (using micronutrient). Each treatment was tested using ten polybags.
The recommended dosage of the fertilizer for red onion was 600 kg/ha, so the dosage for each polybag was 0.47 g. The first fertilization was done after ten days of planting by giving half of the fertilizer dosage (0.236 g for each polybag), while the other half (0.236 g for each polybag) was given after 30 days of planting. The field test steps and the red onion growth are shown in Figure 8 and Figure 9. Soil preparation was done by adding some zeolite and compost in order to improve soil condition. The yield of red onion crop by using NPK ash fertilizer is then compared with the yield of NPK char fertilizer, commercial NPK, and without fertilizer treatments studied by [8]. The crop yield comparison is shown in Figure 10. The use of NPK char fertilizer (20-10-10) succeeded in increasing the red onion product by 76.9% compared to the commercial NPK. In comparison, the addition of micronutrients of Mg and S increased the red onion product by more than 110% (NPK 15-10-11-5-3). Furthermore, the use of NPK ash fertilizer showed a more significant increase in red onion products to more than 50% compared to NPK char fertilizer. It more than doubled compared to NPK char fertilizer without Mg and S micronutrient, and more than tripled when compared to without fertilizer treatment.
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