*2.3.2. Biochemical conversion method*

energy source, or it can be in the form of direct conversion of sludge to energy in the form of

**Figure 2.** Treated industrial sludge in Jababeka's wastewater treatment facility. (Source: Kurniawan et al.).

Organic sludge has the potential to be an alternative sustainable energy source if managed with the proper and efficient method. What needs to be realized is, to convert sludge into an energy source, a certain amount of energy is needed in the conversion process. In **Table 2**, we can observe several methods of converting feedstock into energy sources through various types of process. The sludge to energy conversion discussed in this paper covers the physical,

What is discussed in this section is the method of compacting sludge into a form of pellets or briquettes for later use as solid fuel known as refuse-derived fuel (RDF). It is done to facilitate storage and transport compared to the original form. The compacting process will directly affect the water content. In the direct combustion process, the low water content will increase the ease of solids to burn. It will affect the combustion temperature and heat value generated from combustion, because the heat produced is only used for the oxidation of organic matter rather than vaporizing the water content. Besides being burned directly, RDF sludge can also be applied to the pyrolysis process or gasification to produce synthesis gas

biochemical, thermochemical, and transesterification conversion methods.

heat released from combustion.

106 Renewable Resources and Biorefineries

*2.3.1. Physical conversion method*

or pyrolysis oil (**Figure 3**).

The decomposition of organic compounds using biological processes is one method that can be done to produce alternative energy sources. Biogas which has the main content of methane gas is produced from the decomposition process under controlled anaerobic conditions. Several stages in the biogas production process include the stages of hydrolysis, liquefaction, and fermentation; the formation of hydrogen and acetic acid; and the last is the methane gas formation stage [25]. Each stage of the process that the sludge goes through will involve different types of enzymes produced from the metabolism of anaerobic bacteria. Previous research has reported on critical review along with the biogas production process from wastewater sludge [25].

**Figure 3.** Pathway of sludge **c**ombustion process (Source: Kurniawan et al.).
