Wastewater Sludge as an Alternative Energy Resource: a Review

In recent years, concerns regarding to wastewater sludge disposal have increased globally. Productionof sludge has increased recently due to the growth of population. Wastewater sludge classified as a hazardoussubstance, it is not easy to dispose because of certain treatment is required. Typically, sludge is treated atsecured landfill which its limited in availability and expensive. On the other hand, wastewater sludge originatedfrom biological treatment contains organic substance that can be converted into alternative energy resources. Atechnology is needed that is able to reduce the volume of sludge and convert sludge into energy source. Wepresent an overview of various technologies that can be used for conversion of sludge into energy resources.Those technologies are anaerobic digestion, pelletization, combustion, pyrolysis and gasification. Progress andchallenges of each technology is presented in detail. A summary of sludge characteristic originated fromdifferent source will be discussed as well. Emissions and residues that determines the environmental impact isalso considered. Referring to some previous research, it known that wastewater sludge, as unwanted product,has the potential to become future energy resource. This potential can only be used properly if the method ofconversion are effective and efficient.


INTRODUCTION
Bio-solid, could be defined as wastewater sludge resulted from WWTP that utilize biological process as the main treatment. This kind of sludge is classified as hazardous waste according to Indonesia's Government Regulation No. 101/2014. Figure 1 shows wastewater coming out from Jababeka Industrial Park (Indonesia) and the sludge resulted from separation process of solid particles from the wastewater. It was dried before being delivered to disposal facility. Daily production in Jababeka WWTP may reach more than 3 ton dry sludge / day. Gao et al. (2014) described that the amount of sludge production is growing due to the rapid urbanization and industrialization.
Disposal of wastewater sludge is crucial process because of its hazardous characteristics. It is not allowed to be disposed at just any landfill; rather it must be disposed at typical secured landfill which is limited and expensive. Johnson et al. (2008) reported that the cost of disposing sludge can be very high so that it become major part of WWTP operational budget. Alternatively, industries can turn dry sludge that contains high organic content as an alternative raw materials for cement production process. On the other hand, organic contents of wastewater sludge has the potential to be used as alternative energy sources (Karayildirim et al., 2006); (Hakiki et al., 2018). It can contribute towards energy supplies in heat, power or in combined Heat and Power (CHP) if properly exploited.
In the light of sludge disposal problem and its energy potential, this topic has drawn interest from various researcher all around the world. Various solutions were reported that aims to reduce the amount of sludge volume and its negative environmental impact by converting sludge into energy resource. This paper aims to investigate those various solutions and present the progress of this topic in systematic manner. A review of various sludge characteristic is presented as well in this paper in order to give overview of sludge content.
There were various industrial sludge based on its origin. Chen et al. (2011) conducted characterization of sludge from Thin Film Transitor-Liquid Crystal Display (TFL-LCD) industries. Chiou et al. (2014) used sludge originated from pulp and textile industries. Magdziarz and Werle (2014) performed characterization on thermal power plant originated sludge. The list of various type of sludge and its characteristic is shown in Table 1.
Almost all reported characterization activities are based on the use of -endtreatment sludge‖ that is the sludge resulted from the final process of wastewater treatment in WWTP. However, wastewater treatment consisted of many stages and each stage produces different sludge composition (Syed-Hassan et al., 2017). Manara and Zabaniotou (2012) conducted a review and summarized the characteristics of various type of sludge on each treatment stage. Those types are: are 1) Primary sludge produced from the primary treatment, 2) biological sludge produced from secondary treatment, 3) mixed sludge that is a mixture of primary and biological sludge, and 4) tertiary sludge produced from the tertiary or advanced wastewater treatment.

ENERGY RECOVERY TECHNOLOGIES
There are several options to recover sludge energy content (see Figure 2). Sludge may be digested anaerobically, pelletized or used directly in thermochemical process. Anaerobic digestion process involve microorganisms activities in certain condition to achieve the optimum substrate degradation. Pelletization of sludge produces sludge-Refuse Derived Fuel (RDF) that can be use as fuel in either thermochemical options (combustion, pyrolisis and gasification). Each method will be discussed in detail below. Review on various researches result based on type of conversion technology is shown in Table 2. Pyrolysis. Fix-bed reactor.

Industrial sewage sludge and Oil sludge
The characteristic of sludge before and after pyrolysis.

Wastewater biosolids
Energy content of pyrolysis resulted gas (pygas) and pyrolysis resulted oil (py-oil) and its comparison with energy requirement for biosolids drying. Xiong et al., 2013 Pyrolysis. Fix-bed reactor.
Influence of sludge moisture content to pyrolysis product. Pyrolysis was done in fix temperature of 1000 o C. Higher moisture content gives better results: increase H 2 production and reduces tar & light aromatics (toxic) production. Salleh et al., 2011 Fast pyrolysis. Fluidized-bed.

Combination of rice waste and sewage sludge
Focus on the production bio-oil by fast pyrolysis process. Pokorna et al., 2009 Fast pyrolysis. Fixebed reactor.

Activated waste sludge, dewatered sludge
Fast pyrolysis to maximize the yield of oil product. Maximum oil yield is about 40%Wt.
Sewage sludge Influence of composition and air temperature to the calorific value of syngas. The higher oxygen content and temperature of air, the higher the calorific value of syngas. Acelas et al., 2014 Supercritical water gasification. Batch process inside an autoclave.

Dewatered sewage sludge
Influence of Supercritical water temperature and its residence time to the syngas production. Higher temperature and longer residence time cause better production of H 2 and CH 4 . Roche et al., 2014 Air and Air-steam gasification. Dolomite was added in the process. Fluidized bed gasifier.
Influence of throughput, steam and dolomite to the sludge gasification products. Higher throughput causes increases in tar production and decreases H 2 content. The use of air mixed with steam and dolomite promote H 2 production. Xie  Influence of three primary catalyst: dolomite, olivine and alumina in gasification process. Dolomite performed the best tar reduction. The presence of water vapor and catalysts increases H 2 production by 60%. Reed et al., 2005 Continuous process fluidized bed gasifier. Air gasification. Air-N 2 gasification Air-steam-N 2 gasification.
Influence of gasification bed temperature and type of sewage sludge to the concentration of various trace elements in final solid residues: bariums, copper, mercury, lead and zinc. Petersen and Wether, 2004 Circulating Fluidized bed gasifier Air Gasification CO 2 /N 2 gasification. N 2 Gasification.
Influence of air-fuel ratio, gasification temperature, feeding height and fluidization velocity to the gasification product. Authors found air-fuel ratio was the most important parameters affecting the quality of syngas. Midili et al., 2002 Air gasification. Fixed bed downdraft gasifier.
Undigested and dried sewage sludge pellets.
Authors have successfully uses downdraft gasifier for gasification of sewage sludge pellets.

Anaerobic Digestion
Anaerobic digestion (AD) is one type of bio-chemical process that decomposes organic solid substance into gas phase by the activity of anaerobic microorganisms. Wastewater sludge act as a substrate undergoes processes such as hydrolysis, acidogenesis and methanogenesis step in the absence of free oxygen. Anaerobic miroorganisms will convert carbon compound to produce gases such as CH 4 , H 2 and CO 2 . The resulted gas mixture is usually called as biogas. Ting and Lee (2007) reported a production of hydrogen and methane from wastewater sludge by using clostridium strain. The sludge sample was obtained from food-processing wastewater. Yang and Wang (2017) provide the critical review on sludge AD process.

Pelletization
Pelletization is a process in which solid waste is pressurized and extruded to form pellet, known as Refuse Derived Fuel (RDF). Typical solid waste formed RDF is 5 -10 mm in diameter. The reason for making RDF from sludge was it offer easiness for storage and transportation, compared to sludge in its original form. Sludge RDF can be used as additional industrial fuel or combusted in incinerator. Alternatively, pyrolysis or gasification can be performed on sludge RDF in order to produce other kind of fuel, as shown in Figure 2.  Chiou et al. (2014) performed pelletization of pulp & textile sludge mixture to produce RDF which size was 10 mm in diameter and 10 -50 mm in length. They created and compared four types of RDF by combining variation of pulp sludge moisture content and proportion of textile sludge. They also studied the combustion behavior for each type of RDF. Chen et al. (2011) have created sludge RDF by using five different sludge resulted from TFT-LCD production. They mixed their sludge with sawdust made from waste wooden pallets in order to increase the carbon content of RDF and reduce ash after combustion. The mixing ratio of sludge to sawdust was 10:1.
Asphalt also was added to the sludge mixture as binding agent of RDF. Jiang et al. (2016) investigated various pellet using pure biomass and biomass-sludge mixture. The biomasses were Chinese fir, camphor and rice straw. The sludge was obtained from urban sewage plant. Pellet size was 7 mm and 70 mm in diameter and height, respectively. There are two types of energy consumption for pelletization process: compression and extrusion. Interestingly, it was reported that the energy consumption for making pellet of sludgebiomass mixture was lower compared to the one in pure biomass. While the compression energy reduced to about half, the extrusion energy reduced was much lower, about one fifth of the energy required for making pure biomass pellet. It means that making pellet from sludgebiomass mixture can be conducted at low pressure and temperature.

Combustion
Combustion of solid substance is a process in which solid matter is burned with excessive amount of oxygen (oxidation agent). Its main function is to convert chemical energy of solid fuel into heat. Figure 3 shows the combustion process of sludge. In the beginning sludge temperature increases and the water content evaporates. As temperature gets higher volatile matter was released and some of it reacts with oxygen, producing heat. After devolatilization, sludge has become char. Then, char react with various gases, forming other kind of gases. The char-gas reaction is also known as gasification. Heat produced from volatile combustion and subsequent processes are enough for continuous gasification process without the need of external heat souce. This is known as auto-gasification. Oxydation process continues during ash melting and chars combustion. When there is no combustible substance left, ash agglomerated.
Combustion yields emissions and ash. Combustion of sludge usually occurs by using incineration.  There are four configuration of combustion chamber based on the flow of working fluid (oxidation medium such as air & oxygen) and the behavior of solid waste (bed): Fixed-bed with updraft flow, Fixed-bed with downdraft flow, Fluidizedbed, and entraned flow. Fixed bed is a reactor that solid fuel is located in fixed position usually above a screen or perforated plate. Updraft and downdraft flow defines the direction of working fluid. Fluidized-bed is condition where solid fuel particle is suspended by the working fluid. As the result of fluidizing, the solid fuel behaves like a fluid. Entrained flow is condition where solid fuel was crushed into very small particle (smaller than the case of fluidized-bed) so that it can be injected to the combustion chamber by using nozzle. These different configurations of combustion chamber were applied as well to gasification and pyrolysis reactors.

Gasification
Gasification is a process, which solid substances are combusted in limited amount of oxidation agent to produce gas fuel that is known as synthesis gas (syngas). Gasification is a better option compared to direct combustion (e.g. incineration) and bio-chemical process because it offers faster route for volume reduction, flexibility usage of produced fuel gas and better control of environmental impact (Arena et al., 2012;Murphy et al., 2004;Young et al., 2010). Gasification is similar to combustion process in a way that both processes use heat (thermal energy) to decompose solid fuel into gas phase.
Typical oxidation medium in combustion is air whereas in gasification can be air, pure-oxygen, steam or other substance. 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. Basu et al. (2013) shows that gasification process can be divided into these stages (see Figure 4): 1) Heating and Drying. In this first stage, cold sludge receives heat and its temperature rises. The water starts to evaporate when the temperature reaches water saturated condition. 2) Devolatilization (or pyrolysis stage). In this second stage, sludge undergoes thermal cracking process which releases various light permanent gases. This process occurs from 160 °C until up to 700 °C. For biomass case, it releases gases such as H 2 , CO, CO 2 , CH 4 , H 2 O, and NH 3 . This stage is also known as thermal decomposition or pyrolysis. 3) Some chemical reactions (or gasification stage). At this stage, there are various chemical reactions occurred. Gasgas phase reaction is reactions between different volatile gases or between volatile gases and oxidation medium. Char-Gas reactions are a reaction between char and gasifying medium to form gases product. The chargas reactions is the reactions where solids of sludge are converted into gas (that is gasification process). Because of the chargas reactions, this stage is usually called as gasification stage. Midilli et al. (2002); Petersen and Werther (2005) and Reed et al. (2005) reported gasification process using sewage sludge pellet (sludge RDF). While the first author uses fixed-bed type gasifier, the last two authors uses fluidized bed gasifier. All authors have shown promising results of sludge RDF gasification. Although their experiments were not comparable because of different sludge characteristics, it is still worth to point out some of similarities and differences of their experimental results. Fixed-bed type gasifier (Midilli et al., 2002) yield syngas with overall calorific value of 4 MJ/m 3 with hydrogen concentration about 10-11% (V/V). (Petersen and Werther, 2005) reported about the same value of calorific value (average calorific value obtained from various experiments of (Reed et al., 2005) Petersen and Werther (2004) was 4.7 MJ/m 3 ). However, with higher reactor temperature and air-fuel ratio 0.3, syngas calorific value and hydrogen content was much higher, 5.5 MJ/m 3 and 18%, respectively. (Reed et al., 2005) didn't discuss their gasification experiment from energetic point of view, rather than they focuses on the analysis of various heavy metals in the final solid residues. Xie et al. (2010), de Andrés et al. (2011) and Werle (2015 reported air gasification of sewage sludge (not in pellet form). de Andrés et al. (2011) focuses on the influence of adding catalyst: dolomite, olivine and alumina on gasification process. Xie et al. (2010) has shown interesting finding: gasification of higher sludge moisture yields higher syngas quality that is higher hydrogen concentration and calorific value. Werle (2015) investigated the effect of air composition and temperature to the resulted syngas. It was shown that the higher oxygen content and temperature of air, the higher calorific value of the resulted syngas. Gasification experiment of wet sludge by using steam as the gasifying medium was reported by Nipattummakul et al. (2010). The syngas produced from this process has high concentration of hydrogen, which is considered as high-quality syngas. It is known that steam gasification yields gas fuel with hydrogen to carbon ratio the highest compared to air-gasification or oxygen-gasification. Roche et al. (2014) performed experiment of air-steam gasification and air gasification. It was reported that air-steam gasification promotes hydrogen production compared to air gasification. Acelas et al. (2014) reported typical gasification process using supercritical water. Their experiment results shown that higher temperature and longer residence time enhances the production of hydrogen and methane.

Pyrolysis
Pyrolysis is partial combustion that occurred without any oxygen or oxidating medium. Gasifying medium and heat are the requirements for gasification process, whereas only heat is required for pyrolysis process. One might get confused because pyrolysis which equal to devolatilization is one of the processes in gasification. The process of releasing gases from solid fuel is pyrolysis. In other words, gasification is a special type of pyrolysis which was designed to optimize solid to gas fuel conversion. Hossain et al. (2009) made comprehensive report of wastewater sludge pyrolysis originated from domestic, commercial and industrial wastewater.
Characterization of sludges and its pyrolysis product was reported. Energy balance analysis for successful pyrolysis process was also performed. Karayildirim et al. (2016) studied the characteristic and pyrolysis of industrial sewage sludge and oil sludge. The characterization of pyrolysis results: synthesis gas, liquid tar and char were also reported. McNamara et al. (2016) uses fix-bed type reactor for pyrolysis of wastewater biosolids. He also compared the energy generated from pygas (pyrolysis resulted gas) and py-oil (pyrolysis resulted oil) to energy required for drying the biosolids.
Since above reports are mostly about pyrolysis experiment, it didn't address on how to optimize production of syngas out from sludge. The data presented in those report still very useful for theoretical prediction. Xiong et al. (2013) performed pyrolysis of sewage sludge at 1000°C. It was found that higher moisture of sludge cause increase hydrogen fuel production and reduces tar & light aromatic production. Salleh et al. (2011) and Pokorna et al. (2009) reported their experiment about fast pyrolysis of sludge.
Fast pyrolysis is a rapid heating of solidfuel which followed by cooling process that cause the gas condensed into liquidfuel. Different from gasification, the main focus of fast pyrolysis is to produce liquid fuel.

FUEL TYPES, EMISSION AND RESIDUES COMPARISON Fuel Component from Different Recovery Technology
The main component fuel varies according to different characteristic of wastewater sludge, and feedstock. Some of fuel comparison can be seen in Table 3. Salleh et al., 2011 describes the majority chemical compounds of bio-oil from fast pyrolysis were phenol, aromatic, nitrogenated compound, alkenes and alkanes. The biomass and bio-oil, includes alkanes, alkenes, ketones, aldehydes, esters, nitrogenous compounds, alcohols, phenols, aromatic/heterocyclic and other compounds (Arazo et al., 2017). Utilization of bio-oil produced from pyrolysis of sewage sludge as feed material seems possible. High distribution of fatty acid can lead to extraction of these acids and their utilization in chemical industry. High presence of nitrogenous compounds can improve added value of bio-oils by isolation of these compounds from the bio-oil. Esterification is also another option. (Pokorna et al., 2009).  Table 3) due to process key parameters, such as the technology process used, temperature, steam to carbon (S/C) ratio, water content, organic matter, characteristic of raw materials, etc. Prior researches are also focus on optimization for pyrolysis/gasification products to gain higher amount of energy. Calorific values of the sludges and the pyrolysis products from three different sample of sewage sludges was conducted comprehensively, with the best results, concerning to pyrolysis products, are obtained with thickened excess activated sludge: 57.5% of organic matter is converted to bio-oil with calorific value of 24.7 MJ/kg and also the water content was the lowest: 10.3% (Gao et al., 2014). The energy content of py-gas and pyoil was always greater than the energy required for pyrolysis. The enthalpy of pyrolysis for biosolids was calculated as the difference between the energy outputs (heating values of char, py-oil, py-gas plus sensible and latent heat losses) and the energy input (heating value of the biosolids feed). The enthalpy of pyrolysis ranged from 2.1 MJ/kg-feed biosolids to 3.0 MJ/kg-feed biosolids; this variability is due to experimental variation (McNamara et al., 2016) Comparison between steam gasification and pyrolysis had conducted, where the steam gasification yielded more syngas, hydrogen, energy and higher apparent thermal efficiency (ratio of syngas energy to energy in solid sewage material) as compared to that obtained from pyrolysis at the same temperature of 1173 K. Peak value of syngas yield, energy yield, and hydrogen yield was obtained at S/C ratio of 5.62. The results show that HHV obtained from pyrolysis was higher than that from gasification. This is attributed to higher contents of methane and hydrocarbons, and lower content of carbon dioxide. In case of gasification, the increase in S/C ratio slightly increased the HHV value of the syngas (Nipattummakul et al., 2010). Based on this review, by using pyrolysis or gasification technology, there are high potentials of fuel (energy) from wastewater sludge that can be recovered. Gasification and pyrolysis can reduced the amount of wastewater sludge, and gain fuel as energy resources.

Emissions/Residues
Different type of technology will give different emissions and residues. The solid residue of wastewater sludge pyrolysis is a charcoal-like product rich in carbon and mineral matter. The potential use of the char depends on its fundamental characteristics (Karayildirim et al., 2006) Bio-char has recently attracted attention as a soil amendment for improving the quality of agricultural soils for the increase of crop yield and soil properties (Hossain et al., 2009). The potential use of solids depends on their chemical characteristics. Some properties of pyrolytic solids are therefore determined, such as ash content (TGA), (Pokorna et al., 2009). Emissions gases (some of them are greenhouse gases) and residues (some of them has heavy metal substances) from pyrolysis/gasification can be treated in a controlled system by using purification technology to eliminate the negative impacts to health and the environment. Instead, thermochemical process can be combined with technologies to eliminate toxic/harmful substances should be implemented, such as air pollution control, and also carbon capture storage (CCS)/biosorption, to eliminate the negative impacts.

CONCLUSION
Characterization of various sludge from different origin presented here have proved the significant level of organic content in wastewater sludge. It means that sludge, as unwanted product, has the potential to become future energy resource. This potential can only be used properly if the method of conversion are efficient. There is no single superior conversion method above the other method. However gasification and pyrolisis have shown promising path of conversion because of their process flexibility. Depending on the sludge conditions, one can choose different configuration of gasification / pyrolisis process in order to produces fuel in the most efficient way. It was shown that high calorific fuel can be produced from gasification and pyrolisis process. For domestic waste, char resulted by gasification and pyrolisis can be used in agriculture. Further research is needed to provide the best options for converting sludge that was derived from different stages of wastewater treament, not only sludge that resulted from the end of treatment process.