**2.2. Sewage sludge and treated sewage sludge properties**

Previously, research has been carried out by several researchers regarding the analysis of the characteristics of sludge collected from sewer, as well as sludge generated from wastewater treatment process in wastewater treatment facilities. There are two major groups of sludge types whose characteristics are analyzed, which are municipal and industrial sludge. Municipal sludge is sludge originating from domestic activities in connection with activities in a residential area, while industrial sludge is produced from production processes in various fields of industry.

flammable gas and/or flammable oil, it means that volatile matter can be expressed as volatile carbon and can be used as a benchmark in determining the energy content. Furthermore, it can be expressed to determine the calorific value of the flammable gas and/or flammable oil produced from thermal conversion process. In fact, volatile matter contains not only carbon compounds but also volatile components such as nitrogen compounds, sulfur, and other components varying in number depending on the process characteristics and complex compounds involved. It shows that gas produced from the thermal conversion still requires a purification process to minimize the negative impact of emission from combustion (**Figure 2**).

Dried sewage sludge [9] 36.45 5.93 25.74 7.03 0.77 59.06 9.36 24.08 Urban sewage plant [18] 36.11 5.25 — 6.50 1.03 57.22 6.09 31.27

Primary treatment [20] 51.50 7.00 35.50 4.50 1.50 65.00 — — Biological treatment (low) [20] 52.50 6.00 33.00 7.50 1.00 67.00 — —

Primary and biological (mix) [20] 51.00 7.40 33.00 7.10 1.50 72.00 Digested [20] 49.00 7.70 35.00 6.20 2.10 50.00

**The origin of sludge (feedstock) C H O N S Volatile** 

Pretreated pulp industrial sludge [5] 18.48 1.78 78.82 0.83 — Pretreated textile industrial sludge [5] 32.15 5.73 59.04 1.36 1.64

Dry treated sewage sludge, digested

Recycling of organic sludge from TFT-LCD manufacturing process [4]

Dry treated sludge from urban wastewater treatment [7]

Dry treated sludge from urban wastewater treatment [8]

Dried sludge from wastewater treatment plant of thermal power

Biological treatment (low and mid)

plants [19]

[20]

(municipal) [2]

**matter**

**% % % % % % % %**

50.00 — — 9.00 2.10 — — —

38.82 6.19 — 5.78 1.17 64.90 7.90 27.20

28.50 4.30 22.40 4.10 0.80 47.00 6.40 39.90

32.30 4.90 24.90 5.30 0.57 64.70 — —

53.00 6.70 33.00 6.30 1.00 77.00 — —

29.50 4.67 20.20 5.27 1.31 — — 39.04

Energy Prospects of Hazardous Sludge from Wastewater Treatment Facilities

**Fix carbon**

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**Ash**

105

Before going further, it is necessary to clarify the terms that will be discussed in this section. What is meant by conversion of sludge to energy, in this context, can be the decomposition process of carbonaceous (organic) sludge into gas/fuel oil which will then be used as an

**2.3. Conversion method of sludge to energy**

**Table 1.** Characteristics of sludge from previous research.

Most of the previous research studies summarized in **Table 1** are regarding characteristics of sludge from urban/municipal activities [2, 7, 8]. Several reports concerning industrial sludge treatment have also been summarized [4, 5, 19]. Not all of the research studies summarized in **Table 1** are using treated sludge as the feedstock for energy recovery process. Some researchers use organic sludge which is the by-product of the manufacturing process of TFT-LCD and sludge from pulp and textile industries [4]. In addition, there is a review report of various sludge characteristics of each wastewater treatment stage [20].

When compared carefully based on the summary from **Table 1**, it can be observed that organic sludge which has the highest amount of volatile matter is sludge originating from a centralized biological wastewater treatment process. It can be observed that the volatile matter content in the sludge analyzed is in the range of 47.00–77.00%. If it is assumed that the main component of volatile matter is carbon compounds, which in the process will be converted into


**Table 1.** Characteristics of sludge from previous research.

biological floc which is then deposited in the secondary settling tank unit. A part of the sludge produced in this process is recirculated to maintain the continuity of the process, while other

Referring to the process description, it can be identified that there are two types of sludge produced from this treatment facility. Sludge produced from primary settling process mainly is inorganic material (about 55–65%) that comes from heavy suspended solid settled by physical gravity process. The total sludge production from primary treatment was about 500 kg/

The second type of sludge is produced by secondary settling process that mainly contains 60–70% of organic material from organic floc formed in the aeration tank. The quantity of organic sludge is much bigger than inorganic sludge due its relation with biological process control. The management of activated sludge concentration in aeration tank and sludge concentration in recirculation flow by distribution box unit is very significant for maintaining wastewater treatment process properly. In this case, the production of large amounts of organic sludge cannot be avoided. The organic sludge production is about 3200 kg/day (55% dry

of 100% dry sludge. This quantity contributes a very significant amount in the wastewater treatment operational cost. Reducing quantity or converting the organic sludge to the alterna-

Previously, research has been carried out by several researchers regarding the analysis of the characteristics of sludge collected from sewer, as well as sludge generated from wastewater treatment process in wastewater treatment facilities. There are two major groups of sludge types whose characteristics are analyzed, which are municipal and industrial sludge. Municipal sludge is sludge originating from domestic activities in connection with activities in a residential area, while industrial sludge is produced from production processes in vari-

Most of the previous research studies summarized in **Table 1** are regarding characteristics of sludge from urban/municipal activities [2, 7, 8]. Several reports concerning industrial sludge treatment have also been summarized [4, 5, 19]. Not all of the research studies summarized in **Table 1** are using treated sludge as the feedstock for energy recovery process. Some researchers use organic sludge which is the by-product of the manufacturing process of TFT-LCD and sludge from pulp and textile industries [4]. In addition, there is a review report of various

When compared carefully based on the summary from **Table 1**, it can be observed that organic sludge which has the highest amount of volatile matter is sludge originating from a centralized biological wastewater treatment process. It can be observed that the volatile matter content in the sludge analyzed is in the range of 47.00–77.00%. If it is assumed that the main component of volatile matter is carbon compounds, which in the process will be converted into

tive materials or energy is very helpful in terms of reducing operational cost.

**2.2. Sewage sludge and treated sewage sludge properties**

sludge characteristics of each wastewater treatment stage [20].

/day of wastewater with its calorie about 2000–2500 kcal/kg

/day of wastewater.

parts are channeled to the sludge treatment unit.

day for a flow rate of 13,300 m<sup>3</sup>

104 Renewable Resources and Biorefineries

solid) for a flow rate of 13,300 m<sup>3</sup>

ous fields of industry.

flammable gas and/or flammable oil, it means that volatile matter can be expressed as volatile carbon and can be used as a benchmark in determining the energy content. Furthermore, it can be expressed to determine the calorific value of the flammable gas and/or flammable oil produced from thermal conversion process. In fact, volatile matter contains not only carbon compounds but also volatile components such as nitrogen compounds, sulfur, and other components varying in number depending on the process characteristics and complex compounds involved. It shows that gas produced from the thermal conversion still requires a purification process to minimize the negative impact of emission from combustion (**Figure 2**).

#### **2.3. Conversion method of sludge to energy**

Before going further, it is necessary to clarify the terms that will be discussed in this section. What is meant by conversion of sludge to energy, in this context, can be the decomposition process of carbonaceous (organic) sludge into gas/fuel oil which will then be used as an

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

energy source, or it can be in the form of direct conversion of sludge to energy in the form of heat released from combustion.

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, biochemical, thermochemical, and transesterification conversion methods.

#### *2.3.1. Physical conversion method*

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

*2.3.2. Biochemical conversion method*

**Conversion method Feedstock Main process**

LCD manufacturing process [4].

from domestic, commercial and industrial activities [13].

industry; oily sludge from primary

dewatered digested sludge, and dried excessive activated sludge [27].

Pyrolysis Wastewater biosolids [21]. Pyrolysis product characterization of wastewater

Fermentation using a clostridium strain to produce

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107

Characterization of sludge refuse-derived fuel (RDF) and its combustion behavior and properties

Combustion characteristics of pure biomass RDF and RDF from sludge-biomass mixture. Comparative study of the energy consumption for

Characterization of fundamental properties of the wastewater sludge pyrolysis product in a fixed bed

Pyrolysis product characterization of wastewater

biosolid in a fixed bed pyrolysis reactor and its energy comparison for required and resulting

To produce bio-oil in fluidized-bed reactor through

Syngas production in semi-batch steam gasification

Gasification of feedstock pellets in fixed bed

production in tubular glass transesterification

In situ transesterification of sludge in subcritical

mixture of methanol and acetic acid.

Flash pyrolysis to produce pyrolysis oil in fixed

sludge in a fixed bed pyrolysis reactor.

hydrogen and methane.

Energy Prospects of Hazardous Sludge from Wastewater Treatment Facilities

pelletization process.

pyrolysis reactor.

energy content.

fast pyrolysis.

bed reactor.

reactor.

reactor.

downdraft reactor.

processing industry [32].

biological treatment [13].

Anaerobic digestion Wastewater sludge from food-

Pelletization Urban wastewater sludge from

Pelletization Recycling of organic sludge from TFT-

Pyrolysis Treated wastewater sludge originating

Pyrolysis Wastewater sludge from petrochemical

decanter [14]

Pyrolysis Combination of rice waste and treated sewage sludge [30]

Gasification Solar dried-treated wastewater sludge [29].

Gasification Undigested and dried-treated sewage sludge pellets [22].

Transesterification Dried sludge of food processing plant [15].

**Table 2.** Various conversion methods of sludge to energy source.

Pyrolysis Thickened excess activated sludge,

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].

Transesterification Wet activated sludge [17]. Hexane-lipid extraction and non-catalytic biodiesel


**Table 2.** Various conversion methods of sludge to energy source.
