*2.3.3. Thermochemical method*

In the thermochemical method, combustion process is one part that cannot be separated. Heat is produced from chemical reactions that occur due to the decomposition of organic compounds through oxidation process. Some types of treatments classified as thermochemical methods include direct combustion, incineration, pyrolysis, and gasification. The combustion process takes place in a compartment, which is usually known as combustion chamber. In this compartment, several types of configuration are known based on the type of working fluid flow pattern (updraft and downdraft) and the type of solid bed (fixed and fluidized bed) (**Figure 4**).

The most fundamental difference between direct combustion with pyrolysis/gasification is the amount of oxygen used in the oxidation process. A certain amount of oxygen with sufficient stoichiometric concentration (even in excessive condition usually) is required in direct combustion, so that carbonaceous compound will be completely oxidized to carbon dioxide and water vapor. An imbalance in the amount of stoichiometric oxygen will result in incomplete combustion, indicated by the production of volatile compounds other than carbon dioxide. This phenomenon occurs in the gasification process, the supply of oxygen to the combustion chamber is intentionally limited in such a way that it does not meet the stoichiometric equilibrium as occurs in the complete combustion of carbonaceous compounds. In this process, flammable synthesis gas will be produced. In the context of the shortness of the duration and stages of the mass reduction process, and the flexibility of renewable energy that can be produced, gasifica-

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**Figure 4.** Sludge-energy conversion pathway (Source: Kurniawan et al. with modification).

tion is a viable option compared to direct combustion or anaerobic digestion method .

Typical oxidation medium in combustion is air, whereas in gasification, it can be air, pureoxygen, steam, or other substances [29]. In gasification, the energy released from decomposed solid fuel is packed into chemical energy in the form of gas fuel . On the contrary, combustion decomposes solid fuel and releases as much as possible the energy content in the form of heat. There are four stages of gasification process, heating and drying, devolatilization, gas-gas

In the fixed bed type, solid bed is on the screen or perforated plate in fixed position. As for the type of fluidized bed, solids are suspended in the working fluid flow. Updraft and downdraft flow show the pattern of working fluid in the bed reactor. Combination of solid bed types and the types of working fluid flow pattern can be applied to pyrolysis and gasification reactor according to needs.

**Figure 4.** Sludge-energy conversion pathway (Source: Kurniawan et al. with modification).

*2.3.3. Thermochemical method*

108 Renewable Resources and Biorefineries

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

according to needs.

In the thermochemical method, combustion process is one part that cannot be separated. Heat is produced from chemical reactions that occur due to the decomposition of organic compounds through oxidation process. Some types of treatments classified as thermochemical methods include direct combustion, incineration, pyrolysis, and gasification. The combustion process takes place in a compartment, which is usually known as combustion chamber. In this compartment, several types of configuration are known based on the type of working fluid flow pattern (updraft and downdraft) and the type of solid bed (fixed and fluidized bed) (**Figure 4**). In the fixed bed type, solid bed is on the screen or perforated plate in fixed position. As for the type of fluidized bed, solids are suspended in the working fluid flow. Updraft and downdraft flow show the pattern of working fluid in the bed reactor. Combination of solid bed types and the types of working fluid flow pattern can be applied to pyrolysis and gasification reactor The most fundamental difference between direct combustion with pyrolysis/gasification is the amount of oxygen used in the oxidation process. A certain amount of oxygen with sufficient stoichiometric concentration (even in excessive condition usually) is required in direct combustion, so that carbonaceous compound will be completely oxidized to carbon dioxide and water vapor. An imbalance in the amount of stoichiometric oxygen will result in incomplete combustion, indicated by the production of volatile compounds other than carbon dioxide. This phenomenon occurs in the gasification process, the supply of oxygen to the combustion chamber is intentionally limited in such a way that it does not meet the stoichiometric equilibrium as occurs in the complete combustion of carbonaceous compounds. In this process, flammable synthesis gas will be produced. In the context of the shortness of the duration and stages of the mass reduction process, and the flexibility of renewable energy that can be produced, gasification is a viable option compared to direct combustion or anaerobic digestion method .

Typical oxidation medium in combustion is air, whereas in gasification, it can be air, pureoxygen, steam, or other substances [29]. In gasification, the energy released from decomposed solid fuel is packed into chemical energy in the form of gas fuel . On the contrary, combustion decomposes solid fuel and releases as much as possible the energy content in the form of heat. There are four stages of gasification process, heating and drying, devolatilization, gas-gas reaction, and char gas reaction [3]. At the drying stage, there is an increase in temperature of carbonaceous sludge which results in evaporation of sludge-water content. At the devolatilization stage, thermal cracking process occurs that produces light gas products such as H2 , CO, CO2 , CH4 , H2 O, and NH3 . This devolatilization stage is also known as pyrolysis stage. In the gasification process, a gasification medium is required in the form of air, oxygen, or steam. In contrast to pyrolysis, no medium is required, but heat is required as a driving force to release the volatile compounds. Based on the explanation, it can be said that gasification is a pyrolysis process that is optimized to convert solid fuel into synthesis gas. The next two stages are the chemical reaction stage between gas-gas and char-gas which will eventually produce synthesis gas (syngas).

for 2 hours to be separated and then analyzed by gas chromatography. Another research has analyzed challenges in terms of process commercialization, including sludge collection, optimization of biodiesel production process, maintaining quality of product, formation of soap as a by-product and its purification method, proper design of bioreactor, pharmaceutical chemi-

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This section is an extended article from a paper entitled "The prospect of hazardous sludge reduction trough gasification process," which was presented in scientific forum. The study of the topic originated from consideration of disposal cost of biological sludge to secure landfill. The energy produced is a by-product of the process of reducing the organic sludge mass which, if optimized, can also be an alternative energy source and it is hoped that more or less

From several options of sludge to energy conversion method discussed in the previous section (see part 2.3), it can be observed that conversion technology is not a major problem, considering that up to now there are quite a number of conversion method options that have been developed. However, in the context of its implementation, it is necessary to consider several aspects including technical, economic, environmental, and other aspects that are interrelated with this. Technically, the selection of the conversion method should conform to the characteristics of the sludge. In sludge with high oil/fat content, the extraction options for oil/fat content followed by transesterification (extraction-transesterification) are possible. The oil extracted from sludge still has carbonaceous solid, which can be treated by thermal or biochemical processing methods, whereas for sludge with low oil/fat content, the extraction-transesterification method is certainly not the proper option considering the yield will also be very low. Another technical aspect that needs to be considered is the ease of operation. The complexity of the technology used will directly affect operational ease and will also affect the operational and energy costs. Environmental impacts that may result from the advanced treatment of sludge carried out also need to be considered. In the thermal conversion method that involves the combustion process, it must be ensured that the resulting flue gas is treated first before being released into the atmosphere. Improper treatment of flue gas can be a source of pollutants for the earth's atmosphere and may even be a contributor of gaseous pollutants, which in large quantities can exacerbate the greenhouse effect. In addition, reviews of economic aspects are also an aspect that needs to be considered, including investment cost, operational

Experiments of the pyrolysis process are carried out on a laboratory scale using pyrolysis reactor in the form of a closed stainless steel vessel equipped with a gas-fired heating system. The reactor is also equipped with a gas-liquid separator made of polypropylene and several

valves to regulate the fluid flow involved in the process, as illustrated in **Figure 5**.

cal in sludge, regulatory concerns, and economics of biodiesel production [15].

**2.4. Pyrolysis of treated wastewater sludge from Jababeka's treatment facility**

can be utilized as a substitute for fossil fuels in the area of treatment facilities.

cost, energy cost, and other costs that are related to this process.

*2.4.2. Simulation based on experimental data*

*2.4.1. General considerations*

Gasification process for pellets made from sewage sludge (sludge RDF) has been carried out by several previous researchers [22, 26, 28]. Research shows that the use of fixed bed and fluidized bed gasifiers is quite promising. The results of this study were obtained from several types of sludge which have different characteristics, so they cannot be compared with each other. The total heating value generated in the use of a fixed bed-type gasifier is around 4 MJ/m3 with hydrogen concentration around 10–11% v/v [22]. Other studies on uncompressed sewage sludge gasification showed that the higher the oxygen content and the temperature of compressed air, the higher heating value produced [6, 33]. In addition, it can be observed that the higher the sludge humidity, the higher the hydrogen concentration and its heating value [33]. This phenomenon occurs because the water content in the form of moisture on the sludge evaporates into steam which involves a water-gas shift reaction, in this case, the equilibrium of the reaction shifts to the right due to excess concentration of carbon components and water molecule in the form of steam on the side of reactant. This is in line with the previous research which shows that high-quality syngas can be obtained from steam-gasification of wet sludge which results in higher hydrogen-carbon ratio than air-gasification or oxygen-gasification [24]. Other studies on the type of gasification using water in supercritical conditions indicate that the higher and the longer detention time will increase the yield of hydrogen and methane [1].

#### *2.3.4. Transesterification*

In addition to anaerobic digestion, direct combustion, pyrolysis, and gasification, another option for organic sludge energy recovery is the transesterification process. It involves the reaction between triglycerides and methanol under controlled conditions, either with or without the presence of a catalyst. The use of homogeneous catalyst is relatively cheap compared to the use of heterogeneous catalysts. However, homogeneous catalysts are very sensitive to free fatty acids and the water content in oil, and can trigger saponification and hydrolysis reactions [17]. The first stage of the process is to extract the oil contained in the sludge that will be reacted with methanol to produce crude biodiesel; then the refining process is carried out by separating the biodiesel fraction from the glycerin layer. Several studies on sludge energy recovery through transesterification process have been carried out in lab-scale experiments or only in the form of simulation or process review. Research about transesterification on lab scale has been carried out in a quart-tubing tubular reactor equipped with gas sensors, heaters, and other equipment to produce controlled process conditions [17]. The products resulting from esterification reaction in the form of biodiesel, which is still mixed with glycerin, are then allowed to settle for 2 hours to be separated and then analyzed by gas chromatography. Another research has analyzed challenges in terms of process commercialization, including sludge collection, optimization of biodiesel production process, maintaining quality of product, formation of soap as a by-product and its purification method, proper design of bioreactor, pharmaceutical chemical in sludge, regulatory concerns, and economics of biodiesel production [15].

### **2.4. Pyrolysis of treated wastewater sludge from Jababeka's treatment facility**

This section is an extended article from a paper entitled "The prospect of hazardous sludge reduction trough gasification process," which was presented in scientific forum. The study of the topic originated from consideration of disposal cost of biological sludge to secure landfill. The energy produced is a by-product of the process of reducing the organic sludge mass which, if optimized, can also be an alternative energy source and it is hoped that more or less can be utilized as a substitute for fossil fuels in the area of treatment facilities.

## *2.4.1. General considerations*

reaction, and char gas reaction [3]. At the drying stage, there is an increase in temperature of carbonaceous sludge which results in evaporation of sludge-water content. At the devolatilization stage, thermal cracking process occurs that produces light gas products such as H2

In the gasification process, a gasification medium is required in the form of air, oxygen, or steam. In contrast to pyrolysis, no medium is required, but heat is required as a driving force to release the volatile compounds. Based on the explanation, it can be said that gasification is a pyrolysis process that is optimized to convert solid fuel into synthesis gas. The next two stages are the chemical reaction stage between gas-gas and char-gas which will eventually

Gasification process for pellets made from sewage sludge (sludge RDF) has been carried out by several previous researchers [22, 26, 28]. Research shows that the use of fixed bed and fluidized bed gasifiers is quite promising. The results of this study were obtained from several types of sludge which have different characteristics, so they cannot be compared with each other. The total heating value generated in the use of a fixed bed-type gasifier

uncompressed sewage sludge gasification showed that the higher the oxygen content and the temperature of compressed air, the higher heating value produced [6, 33]. In addition, it can be observed that the higher the sludge humidity, the higher the hydrogen concentration and its heating value [33]. This phenomenon occurs because the water content in the form of moisture on the sludge evaporates into steam which involves a water-gas shift reaction, in this case, the equilibrium of the reaction shifts to the right due to excess concentration of carbon components and water molecule in the form of steam on the side of reactant. This is in line with the previous research which shows that high-quality syngas can be obtained from steam-gasification of wet sludge which results in higher hydrogen-carbon ratio than air-gasification or oxygen-gasification [24]. Other studies on the type of gasification using water in supercritical conditions indicate that the higher and the longer detention time will

In addition to anaerobic digestion, direct combustion, pyrolysis, and gasification, another option for organic sludge energy recovery is the transesterification process. It involves the reaction between triglycerides and methanol under controlled conditions, either with or without the presence of a catalyst. The use of homogeneous catalyst is relatively cheap compared to the use of heterogeneous catalysts. However, homogeneous catalysts are very sensitive to free fatty acids and the water content in oil, and can trigger saponification and hydrolysis reactions [17]. The first stage of the process is to extract the oil contained in the sludge that will be reacted with methanol to produce crude biodiesel; then the refining process is carried out by separating the biodiesel fraction from the glycerin layer. Several studies on sludge energy recovery through transesterification process have been carried out in lab-scale experiments or only in the form of simulation or process review. Research about transesterification on lab scale has been carried out in a quart-tubing tubular reactor equipped with gas sensors, heaters, and other equipment to produce controlled process conditions [17]. The products resulting from esterification reaction in the form of biodiesel, which is still mixed with glycerin, are then allowed to settle

with hydrogen concentration around 10–11% v/v [22]. Other studies on

. This devolatilization stage is also known as pyrolysis stage.

CO, CO2

, CH4 , H2

110 Renewable Resources and Biorefineries

is around 4 MJ/m3

*2.3.4. Transesterification*

produce synthesis gas (syngas).

O, and NH3

increase the yield of hydrogen and methane [1].

,

From several options of sludge to energy conversion method discussed in the previous section (see part 2.3), it can be observed that conversion technology is not a major problem, considering that up to now there are quite a number of conversion method options that have been developed. However, in the context of its implementation, it is necessary to consider several aspects including technical, economic, environmental, and other aspects that are interrelated with this.

Technically, the selection of the conversion method should conform to the characteristics of the sludge. In sludge with high oil/fat content, the extraction options for oil/fat content followed by transesterification (extraction-transesterification) are possible. The oil extracted from sludge still has carbonaceous solid, which can be treated by thermal or biochemical processing methods, whereas for sludge with low oil/fat content, the extraction-transesterification method is certainly not the proper option considering the yield will also be very low.

Another technical aspect that needs to be considered is the ease of operation. The complexity of the technology used will directly affect operational ease and will also affect the operational and energy costs. Environmental impacts that may result from the advanced treatment of sludge carried out also need to be considered. In the thermal conversion method that involves the combustion process, it must be ensured that the resulting flue gas is treated first before being released into the atmosphere. Improper treatment of flue gas can be a source of pollutants for the earth's atmosphere and may even be a contributor of gaseous pollutants, which in large quantities can exacerbate the greenhouse effect. In addition, reviews of economic aspects are also an aspect that needs to be considered, including investment cost, operational cost, energy cost, and other costs that are related to this process.

### *2.4.2. Simulation based on experimental data*

Experiments of the pyrolysis process are carried out on a laboratory scale using pyrolysis reactor in the form of a closed stainless steel vessel equipped with a gas-fired heating system. The reactor is also equipped with a gas-liquid separator made of polypropylene and several valves to regulate the fluid flow involved in the process, as illustrated in **Figure 5**.

10% condensate (see **Figure 5**). Another parameter analyzed is the particle size of the sludge that is treated. The initial treatment carried out in laboratory experiments is the drying process with an oven, and then reducing the lump of sludge into particle which has an average size of 11 mm. Sludge, which has been reduced in solid size, is then processed thermally so that it produces char, the remaining solid sludge that does not decompose and remains in a solid form with an average size of 4 mm (see **Figure 6**). The results of these observations indicate that the heat treatment given results in decrease in sludge particle size by about 36%, this is directly proportional to the decrease in sludge mass up to around 40%. The decrease in sludge mass and the decrease in sludge particle size indicate that decomposition of the sludge component has occurred. Components classified as volatile heat-sensitive matter are evaporated, while fixed carbon and other unevaporated components remain in the solid form (**Table 3**). As mentioned in the previous paragraph, the decomposition process of solid sludge is indicated by a decrease in sludge particle size. And the decrease in sludge particle size also affects the total mass of sludge. The cost of managing treated wastewater sludge comes from the energy cost used to reduce the water content, in this case using a belt filter press unit. Besides that, there are sludge disposal costs involving third parties who have permits. In connection with this, a simulation of economic feasibility is carried out as one consideration to expand

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The economic feasibility simulation was carried out to compare solid sludge management by secured landfill (existing) methods with the thermal process method. In its existing condition, the industrial estate manager issues around US \$ 7020/month for sludge disposal to secured landfill through third-party services [11]. Also, about US \$ 26,492/month is issued for the use of electrical energy in Jababeka's wastewater treatment facilities [11], as illustrated in **Figures 7 and 8**. In the proposed business model, the cost incurred for the disposal of sludge is estimated to decrease to US \$ 2808/month, while the cost of electricity consumption fell to US \$ 25,560/month [11]. It is related to the calculation of green energy resulted from the process is assumed to be able to substitute energy requirements as much as that produced from the gasification process [11]. In total, based on the simulation results, it is estimated that there will be a reduction in costs of up to 40% if the proposed business model is actually implemented.

**Figure 6.** Average sludge particle size (a) & (b) size before pyrolysis: about 11 mm;(c) & (d) size after pyrolysis: about

this research to the pilot scale.

4 mm (Source: Hakiki et al. [11]).

**Figure 5.** Simplified schematic diagram of organic sludge pyrolysis process.

Treated wastewater sludge which contain 66% of carbonaceous compound has the potential that can be used as an alternative energy source if it is managed by the appropriate method. The amount of water contained in the wet sludge (43% of the total wet basis) is removed through the belt filter press unit and then dried with sunlight in the sludge drying area, until the water content is around 10% (dry basis).

In addition to organic matter and water, there are also contents of sulfur and other minerals which amount to 4% of the dry weight.

Sludge which has reduced in its water content by about 10% is then put into a fixed-bed pyrolysis reactor so that the mass of solids is reduced by 40%. Mass loss occurs due to the decomposition process of organic matter into simpler matter. The measurement results show that the decomposition process occurs at reactor temperature up to 550°C. The fraction that is converted to gas will then flow to the gas-liquid separator which is equipped with a carbon filter layer to reduce the amount of gas impurities carried to the gas collector.

Not all gas produced from thermal decomposition process flows into the gas collector, the heavier fraction will condense in the separator because of the difference in temperature and pressure inside the reactor with those on the inside of the separator.

Differences in temperature and pressure occur due to decomposition of some solid sludge into gas accumulated in pyrolysis reactor. The wet gas fraction as a top product from pyrolysis reactor will be flow to the gas-liquid separator that allows the decrease in gas velocity so that the separation process of the gas fraction and heavier fractions can be occur. The top-most product of pyrolysis reactor that enters the separator will be separated into 90% dry gas and 10% condensate (see **Figure 5**). Another parameter analyzed is the particle size of the sludge that is treated. The initial treatment carried out in laboratory experiments is the drying process with an oven, and then reducing the lump of sludge into particle which has an average size of 11 mm. Sludge, which has been reduced in solid size, is then processed thermally so that it produces char, the remaining solid sludge that does not decompose and remains in a solid form with an average size of 4 mm (see **Figure 6**). The results of these observations indicate that the heat treatment given results in decrease in sludge particle size by about 36%, this is directly proportional to the decrease in sludge mass up to around 40%. The decrease in sludge mass and the decrease in sludge particle size indicate that decomposition of the sludge component has occurred. Components classified as volatile heat-sensitive matter are evaporated, while fixed carbon and other unevaporated components remain in the solid form (**Table 3**).

As mentioned in the previous paragraph, the decomposition process of solid sludge is indicated by a decrease in sludge particle size. And the decrease in sludge particle size also affects the total mass of sludge. The cost of managing treated wastewater sludge comes from the energy cost used to reduce the water content, in this case using a belt filter press unit. Besides that, there are sludge disposal costs involving third parties who have permits. In connection with this, a simulation of economic feasibility is carried out as one consideration to expand this research to the pilot scale.

The economic feasibility simulation was carried out to compare solid sludge management by secured landfill (existing) methods with the thermal process method. In its existing condition, the industrial estate manager issues around US \$ 7020/month for sludge disposal to secured landfill through third-party services [11]. Also, about US \$ 26,492/month is issued for the use of electrical energy in Jababeka's wastewater treatment facilities [11], as illustrated in **Figures 7 and 8**. In the proposed business model, the cost incurred for the disposal of sludge is estimated to decrease to US \$ 2808/month, while the cost of electricity consumption fell to US \$ 25,560/month [11]. It is related to the calculation of green energy resulted from the process is assumed to be able to substitute energy requirements as much as that produced from the gasification process [11]. In total, based on the simulation results, it is estimated that there will be a reduction in costs of up to 40% if the proposed business model is actually implemented.

Treated wastewater sludge which contain 66% of carbonaceous compound has the potential that can be used as an alternative energy source if it is managed by the appropriate method. The amount of water contained in the wet sludge (43% of the total wet basis) is removed through the belt filter press unit and then dried with sunlight in the sludge drying area, until

In addition to organic matter and water, there are also contents of sulfur and other minerals

Sludge which has reduced in its water content by about 10% is then put into a fixed-bed pyrolysis reactor so that the mass of solids is reduced by 40%. Mass loss occurs due to the decomposition process of organic matter into simpler matter. The measurement results show that the decomposition process occurs at reactor temperature up to 550°C. The fraction that is converted to gas will then flow to the gas-liquid separator which is equipped with a carbon

Not all gas produced from thermal decomposition process flows into the gas collector, the heavier fraction will condense in the separator because of the difference in temperature and

Differences in temperature and pressure occur due to decomposition of some solid sludge into gas accumulated in pyrolysis reactor. The wet gas fraction as a top product from pyrolysis reactor will be flow to the gas-liquid separator that allows the decrease in gas velocity so that the separation process of the gas fraction and heavier fractions can be occur. The top-most product of pyrolysis reactor that enters the separator will be separated into 90% dry gas and

filter layer to reduce the amount of gas impurities carried to the gas collector.

pressure inside the reactor with those on the inside of the separator.

the water content is around 10% (dry basis).

**Figure 5.** Simplified schematic diagram of organic sludge pyrolysis process.

which amount to 4% of the dry weight.

112 Renewable Resources and Biorefineries

**Figure 6.** Average sludge particle size (a) & (b) size before pyrolysis: about 11 mm;(c) & (d) size after pyrolysis: about 4 mm (Source: Hakiki et al. [11]).


**Table 3.** Benefit cost analysis of the proposed business model (Source: Hakiki et al. [11] with modification).


**Figure 7.** Existing business model (Source: Hakiki et al. [11] with modification).

In calculating and analyzing economic feasibility in the simulation, the following assumptions are made[11]: (1) The disposal costs of sludge will increase by 5% per year; (2) electricity consumption costs will increase by 3% per year; (3) employee salary costs will increase by 10% per year; (4) fuel costs will increase by 10% per year; (5) maintenance costs will increase by 10% per year; (6) miscellaneous costs will increase by 10% per year; (7) inflation rate is 6% per year. All assumptions are based on data valid in March 2017. The summary of economic

**Table 4.** Summary of economic simulation based on the proposed business model (Source: Hakiki et al. [11] with

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Management of wastewater sludge originating from wastewater treatment facilities can be done in several ways, including physical process (compacted into briquettes), biochemical process (anaerobic digestion), thermochemical process (pyrolysis/gasification), and

simulation can be seen in **Table 4**.

**Figure 8.** Proposed business model (Source: Hakiki et al. [11] with modification).

**Items Units Value** Capacity (70% dry solid) tons/month 75 Investment US \$ 62,080.0 Lifetime of gasification unit years 5 Profit to revenue % 30% Payback period years 2.8 IRR % 42% NPV US \$ 32,625,4 BC ratio — 2.1

**3. Conclusion**

modification).


**Figure 8.** Proposed business model (Source: Hakiki et al. [11] with modification).


**Table 4.** Summary of economic simulation based on the proposed business model (Source: Hakiki et al. [11] with modification).

per year. All assumptions are based on data valid in March 2017. The summary of economic simulation can be seen in **Table 4**.
