**2.1. The origin of sludge: case study of Jababeka's wastewater treatment facilities**

In a centralized wastewater treatment facility, especially in an industrial estate, wastewater from each factory located within the industrial area will be drained through sewerage. Wastewater generated from the production process should be treated in advance so that it is in accordance with the wastewater quality standards determined by the industrial estate manager. In addition to its characteristics, the volume of wastewater discharge through the sewer should also follow the regulations determined by the industrial estate manager. It is required to ensure that the wastewater that flows to the treatment facility has characteristics that are in accordance with the processing capability and capacity of the existing treatment facilities. The treatment process is generally divided into several stages, among others primary treatment, secondary treatment, tertiary treatment, and other treatment options according to the wastewater characteristics to be treated.

waves, biomass, and other renewable energy sources have also been widely researched and developed as an alternative energy source. Nevertheless, until now fossil fuels are still one of the primary energy sources that cannot yet be completely replaced by other types of energy sources. The decrease in availability of fossil fuels as non-renewable energy sources is inversely proportional to the increasing population of the world. Raising awareness of the limited reserves of fossil fuel and of the need for another energy source as alternative energy source is needed. One alternative energy source that can be considered is the biomass content of sludge produced from biological wastewater treatment. Carbon compounds contained in biological sludge (bio-sludge) can be converted into energy through thermal process, biodigestion, lipid/oil extraction, or other methods that are in accordance with its characteristics [1, 2, 12, 15, 25, 34]. Therefore, the main objective of this research is to identify the prospect of several technologies to treat organic sludge in terms of mass reduction and energy recovery as a basis for feasibility study and further development. The thermal process of converting biomass to energy is also accompanied by a decomposition process of complex carbon into a simpler form of carbon compounds. So that, in this case, not only the conversion of biomass into energy, but also the mass reduction of sludge should be processed and managed to minimize the negative impacts that can be generated from the sludge generated by the wastewater

Before discussing further the next section, it is necessary to clarify in advance some terms in this paper. What is meant by sewage is liquid waste (in this case, it is wastewater) that flows in the sewer [31]; so, the terms sewage and wastewater will be used interchangeably according to the context of the sentence. Sewage sludge is mud that originated from a sewer, whereas treated sewage sludge means sludge that is produced from the treatment process in

The list of hazardous waste from non-specific sources found in Annex 1 of the Republic of Indonesia government regulation No. 101/2014 concerning the management of hazardous and toxic waste stated that sludge produced from integrated wastewater treatment facilities in industrial estate was classified as hazardous waste category 2 [10]. In this category, the waste in question is declared as toxic waste which is harmful to the environment and living

In a centralized wastewater treatment facility, especially in an industrial estate, wastewater from each factory located within the industrial area will be drained through sewerage. Wastewater generated from the production process should be treated in advance so that it is in accordance with the wastewater quality standards determined by the industrial estate manager. In addition to its characteristics, the volume of wastewater discharge through the sewer should also follow the regulations determined by the industrial estate manager. It is

**2.1. The origin of sludge: case study of Jababeka's wastewater treatment facilities**

treatment process [11].

102 Renewable Resources and Biorefineries

things.

wastewater treatment facilities.

**2. Hazardous sludge: properties and its potency**

Referring to the regulation of Republic Indonesia environment minister No. 5 year 2014 concerning wastewater quality standards, stated that each type of industry that produces wastewater from the results of its activities and conducts types of treatment activities, it is a mandatory to comply with the applicable quality standards [23]. Likewise, with centralized wastewater treatment facilities located in industrial estates, it is mandatory to comply with the applicable quality standards. One of the centralized wastewater treatment facilities in the industrial estate in Indonesia is the Jababeka's wastewater treatment facility located in Bekasi Regency, West Java. The main process in this treatment facility is a biological treatment process equipped with other treatment process units.

As can be seen in **Figure 1**, there are several treatment stages at the Jababeka's wastewater treatment facilities. Among them are the removal stages of sand, gravel, and rough mud on the grit chamber unit. Furthermore, wastewater will flow due to gravity toward the primary settling tank unit. In this unit, suspended solids will settle, while floating material will be separated with the scum collector and then flow to the sludge treatment unit. The primary settling tank effluent is flowed to the oxidation ditch which utilizes biological process (activated sludge) to decompose pollutants contained in wastewater. This process will produce

**Figure 1.** Schematic flow diagram of Jababeka's wastewater treatment facilities.

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 parts are channeled to the sludge treatment unit.

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

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 solid) for a flow rate of 13,300 m<sup>3</sup> /day of wastewater with its calorie about 2000–2500 kcal/kg 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 alternative materials or energy is very helpful in terms of reducing operational cost.
