**3. Materials and method**

rate may induces physical drying [40]. It is shown that forced aeration during sewage sludge bio-drying controlled the matrix temperature and improved evaporation, establishing it as a vital parameter influencing water loss [18]. In effect, an increase in the air-flow rate increases the amount of water carried, improving the water loss and an output with high calorific value. Likewise, low air-flow rates result in decomposition without significant moisture removal.

It is well established that the supplied of air during bio-drying in one direction contributes to the appearance of temperature gradients, resulting in a lack of homogeneity in the moisture and energy content of the final product [26, 41]. However, it was suggested in another study that daily inversion of airflow in bio-drying by means of reactors eliminates marked temperature differences and leads to a homogeneous final product [41]. An increase in air flow rate at the inlet had positive contribution to moisture loss from the waste but had no effect on

Frei et al. [23] and Navaee-Ardeh et al. [31] indicated that high temperatures (>55°C) during biodrying process enhance the conversion of moisture to vapor and also facilitate the vapor pressure of the air-flow passing through the matrix to carry more moisture out. Accordingly, the biodegradation potential of a bulking agent (BA) would significantly influence the bio-drying process by the biogenerated heat. Additionally, the physical structure and moisture content of the materials are influenced by the decay of bulking agents. A study to investigate the effect of BA particle and controlled temperature on sludge bio-drying concluded that small-particlesized bulking agent coupled with high matrix temperature was more beneficial for volatile solid degradation whereas large-particle-sized bulking agent resulted in poor biodegradation [42].

Additionally, the use of bulking agent (BA) plays a crucial role in bio-drying process. The use of BA adjusts the initial moisture content and facilitates air movement due to the increase in voids ratio. It effects on bio-drying has been demonstrated by some authors. A number of different materials as bulking agents have been used by different researches including bark to bio-dry sewage sludge [23], and sawdust and/or straw [43, 44]. Yang et al. [34] revealed that air-dried sludge possesses a more suitable biodegradation potential than shredded rubber and sawdust when used as BA due to its porous nature and high water holding capacity. In short, the smaller or finer the particles, the stronger the water holding capacity of the substrate. Moreover, BA is important for regulating the matrix porosity and enabling air flow to carry away the water vapor passing through the matrix. For effective bio-drying, it is important to consider the physical structure as well as biodegradability of the bulking agent. In another study, rice straw of different sizes as BA was used in sludge bio-drying and it was reported that small-particle size BA reduced the water content by 0.3% more compared to the large particle size BA [42]. It is revealed that straw has substantial biodegradation potential in bio-drying process while sawdust has poor capacity to be degraded [44]. In order to improve the efficiency of bio-drying, it is important to consider the physical structure as well as the biodegradability when selecting a material as BA. Colomer-Mendoza et al. [40] observed that adding 15% of BA

*2.1.3. Temperature*

94 Agricultural Waste and Residues

*2.1.4. Bulking agents*

temperature and calorific values [25].

Different waste compositions obtained from bio-drying process (i.e. bio-dried material) consisting of biogenic and non-biogenic materials were used to assess the biogenic carbon and energy content of the bio-dried materials. The biogenic materials included food waste, paper and pruning waste, while plastic (light density polyethylene – LDPE) was considered as a non-biogenic material. These materials were varied at different proportions by weight in the bio-drying experiment and their impact on biogenic and calorific value was determined. **Tables 2** and **3** show the composition and physico-chemical properties of the different waste materials. The proportion of the waste components varied in the range of 30–90, 20–80, 5–50 and 30–60% for food waste, paper, plastic and pruning waste respectively. To further test more extreme conditions, two additional (T10 and T11) experiments were conducted with only biogenic and non-biogenic materials as the waste materials, respectively. Prior to mixing, the materials were separately shredded into 15×35, 2×14, 5×10 and 15 mm in diameter for food waste, paper, plastic and pruning waste respectively. The bio-drying experiments were carried out for a period of 7 days. A constant and uninterrupted air-flow rate (15 m3 h−1) was used in all the trials using a whirlpool pump connected to the bottom of the reactor with an air-flow meter. After the bio-drying process, bio-dried samples were analyzed for the moisture, biogenic and energy content. The moisture content of the substrate was analyzed following the


in different proportions was analyzed by SDM. The latter and former were determined based on Eqs. (1) and (2). The basic principle of this method is that the biogenic in bio-dried material

the inert material remains in the residue. Furthermore, the relationship between the biogenic

*mresidue* <sup>−</sup> *mresidue*−*ash* \_\_\_\_\_\_\_\_\_\_\_\_\_ *mS*

*XNB* = 100 − *XB* − *AS* (2)

where XB = Biogenic content (%); XNB = Non-biogenic content (%); *m*residue = Mass of residue (g); *m*residue-ash = Mass of residue and ash (g); AS = Ash content of sample (%); *m*<sup>S</sup> = Mass of dry

It is an established fact that combustible non-biogenic materials are characterized by higher heat content per unit weight than combustible biogenic materials. Consequently, the ratio of biogenic to non-biogenic material proportion can have a considerable effect on the heat content of a waste material intended for combustion purpose [40, 49]. **Figure 1** shows the relationship between moisture content and calorific value. The moisture content is a key parameter, as it affects both the biogenic carbon content and the effective heating value of the combustible waste. The moisture content of the bio-dried material varied between 8.59 and 50.93%, whereas that of the extreme conditions was 91.48 and 0.94% for biogenic (food waste) and non-biogenic materials, respectively (**Table 3**). It should be pointed out that the two extreme conditions were just raw materials without been subjected to bio-drying process. It can be seen that as the share of biogenic waste in the waste matrix gradually decrease, a corresponding trend in moisture content could be expected. Additionally, depending on the amount of food waste in the waste mix of the biogenic material, a decrease or increase in moisture content could be envisaged since the food waste contributes the highest initial moisture

The results revealed a positive correlation between moisture content and calorific value (R<sup>2</sup> = 0.85). As indicated earlier, the amount of biogenic waste had a significant impact on the former and latter. A discrepancy was observed in T3 and T5 in terms of moisture content and calorific value. Even though T3 had the lowest moisture content, T5 had the highest calorific value. The possibly reason was that the difference in food waste in both trials versus the other waste types in the biogenic mix was high enough to induce significant difference in the observed levels of calorific value, with approximately same non-biogenic mix. This suggest that, depending on the amount of food waste in the biogenic mix, the moisture content and calorific value of the bio-dried material could be significantly affected, regardless of the

+ \_\_\_ *AS*

Bio-Drying of Biodegradable Waste for Use as Solid Fuel: A Sustainable Approach for Green…

, while the non-biogenic (fossil material) and

<sup>100</sup>}] × 100 (1)

http://dx.doi.org/10.5772/intechopen.77957

97

SO<sup>4</sup> /H<sup>2</sup> O2

selectively dissolves and oxidizes in H<sup>2</sup>

*XB* = [1 − {

**4. Results and discussion**

content to the biogenic waste mix.

amount of non-biogenic waste in the waste matrix.

sample (g).

and energy content of the bio-dried were established.

**Table 2.** Composition of waste.


**Table 3.** Physico-chemical properties of raw material.

ASTM–D 3173 standard (105°C) using moisture analyzer (Precisa, XM 50), whereas the heat value of the bio-dried material was determined using IKA C-7000 model calorimeter (IKA Laboratory Equipment, Werke Staufen, Germany), in accordance with EN 15400 standard. It is worth mentioning that, due to the heterogeneous nature of the waste, the weighted average method was employed in determining the initial moisture content of the waste matrix, since it was impossible to get a typical sample from the heterogonous mixture of the waste, a similar procedure employed by Shuqing et al. [48]. Elemental analysis was analyzed with Thermo Scientific Flash 2000 Elemental Analyzer (Thermo Fisher Scientific Inc., Bremen, Germany).

Three different methods are employed for the determination of biogenic content of solid recovered fuels/bio-dried materials according to the technical specifications CEN/TS 15440:2006 (CEN, 2006). These include Selective Dissolution Method (SDM), Manual Sorting Method (MSM) and 14C Method. In the present study, the biogenic and non-biogenic content of the waste matrix in different proportions was analyzed by SDM. The latter and former were determined based on Eqs. (1) and (2). The basic principle of this method is that the biogenic in bio-dried material selectively dissolves and oxidizes in H<sup>2</sup> SO<sup>4</sup> /H<sup>2</sup> O2 , while the non-biogenic (fossil material) and the inert material remains in the residue. Furthermore, the relationship between the biogenic and energy content of the bio-dried were established.

$$X\_{\rm B} = \left[1 - \left\{\frac{m\_{\rm wind} - m\_{\rm wind\,work}}{m\_{\rm s}} + \frac{A\_{\rm s}}{100}\right\}\right] \times 100\tag{1}$$

$$X\_{\rm nB} = 100 - X\_{\rm g} - A\_{\rm s} \tag{2}$$

where XB = Biogenic content (%); XNB = Non-biogenic content (%); *m*residue = Mass of residue (g); *m*residue-ash = Mass of residue and ash (g); AS = Ash content of sample (%); *m*<sup>S</sup> = Mass of dry sample (g).
