*2.1.3. Temperature*

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 temperature and calorific values [25].

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].

to gardening waste resulted in 25% moisture reduction. It is proposed that BA of small particle size is preferred due to its adequate porosity and internal homogeneous porous size distribution within the matrix. These features enhance effective waste absorption. However, it should be pointed out that, the use of small particle size BA can cause compaction during bio-drying which can have adverse effect on moisture removal [45]. **Table 1** shows a summary of waste

**(days)**

Agricultural harvest + gardening waste 12–30 <50% na [40] Garden waste 20 <40–57% <40–60% [46] MSW 14 41% na [24] Sewage sludge + bio-dried + sawdust 20 <20% >35.5% [18] MSW 13 49.16 32.65% [27] Food waste + pruning waste 7 36.7–56.8% 10.32–48.9% [47] Sludge + MSW + harvest waste 8-9 na na [26]

**Weight loss (%)**

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

10 na 50% [25]

**Moisture loss** 

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

**Reference**

95

**(%)**

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

materials used in bio-drying process and their effect on weight and moisture loss.

**Substrate Residence time** 

**Table 1.** Summary of bio-drying of different waste materials.

Household waste + plant materials (straw, grass,

branches, −shrubs)

na, not available.

**3. Materials and method**

#### *2.1.4. Bulking agents*

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


**Table 1.** Summary of bio-drying of different waste materials.

to gardening waste resulted in 25% moisture reduction. It is proposed that BA of small particle size is preferred due to its adequate porosity and internal homogeneous porous size distribution within the matrix. These features enhance effective waste absorption. However, it should be pointed out that, the use of small particle size BA can cause compaction during bio-drying which can have adverse effect on moisture removal [45]. **Table 1** shows a summary of waste materials used in bio-drying process and their effect on weight and moisture loss.
