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Article

Characterization and Recovery of In Situ Transesterifiable Lipids (TLs) as Potential Biofuel Feedstock from Sewage Sludge Obtained from Various Sewage Treatment Plants (STPs)

by
Oh Kyung Choi
1,2,
Zachary Hendren
2,
Ki Young Park
3,
Jae-Kon Kim
4,
Jo Yong Park
4,
Ahjeong Son
5 and
Jae Woo Lee
1,*
1
Department of Environmental Engineering, Korea University, Sejong 30019, Korea
2
Center of Technology for Energy, Environment & Engineering, RTI International, Research Triangle Park, Durham, NC 27709, USA
3
Department of Civil and Environmental System Engineering, Konkuk University, Seoul 05029, Korea
4
Research Institute of Petroleum Technology, Korea Petroleum Quality & Distribution Authority, Chungbuk 28115, Korea
5
Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760, Korea
*
Author to whom correspondence should be addressed.
Energies 2019, 12(20), 3952; https://doi.org/10.3390/en12203952
Submission received: 9 September 2019 / Revised: 7 October 2019 / Accepted: 15 October 2019 / Published: 17 October 2019
Browse Figures
Versions Notes

Abstract

:
This study purposed to characterize the sewage sludge from various sewage treatment plants (STPs) as a biodiesel feedstock. Crude biodiesel was produced from each dried primary sludge (PS) and waste activated sludge (WAS) via in situ transesterification process. The average yield of transesterifiable lipid (TL) was 77.8% and 60.4% of the total lipid content from PS and WAS, respectively. The TL yield had a greater margin among WAS than PS samples due to differences in the biological processes adopted in each treatment plant. The TL recovered from PS and WAS contained 54.2% and 40.1% fatty acid methyl esters (FAMEs), respectively, which were mostly made up of palmitic acid (C16:0) and stearic acid (C18:0). The FAME composition of the biodiesel in the WAS sample was highly associated with a microbial community that grows otherwise, depending on the purpose of the biological treatment process. In particular, the increase in the proportion of nitrifying bacteria that grow predominantly under a relatively longer solid retention time (SRT) contributed significantly to the improvement in FAME content.

Graphical Abstract

1. Introduction

Sewage sludge production steadily increased over the last several decades, and this trend is expected to continue due to population increase and urbanization. For example, sewage sludge production doubled from 7.2 to 13.8 million dry tons from 2004 to 2015 in the United States (US) [1,2]. In addition, the annual production from the European Union (EU) and Korea was estimated to be 10.1 and 0.7 million dry tons, respectively, which correspond to 13% and 50% increases relative to the 2004 production numbers, respectively [3,4,5]. As the management of sewage sludge disposal is becoming more stringent each year, many countries are focusing on the energy potential of sewage sludge. Biodiesel generation from sewage sludge received attention as the most valuable and practical fuel with potential for use in vehicles [6,7,8,9]. Sewage sludge, represented by primary sludge (PS) and waste activated sludge (WAS), has many benefits as a biodiesel feedstock; it is a low-cost feedstock, with abundant and consistent generation during sewage treatment, and the lipid content (necessary for conversion to fuel) in both types of sewage sludge is significant [9]. Sewage sludge as an alternative feedstock could contribute to drastically reducing the material costs of biodiesel production which account for 70–85% of the overall cost [10]. In addition, the generation of sewage sludge is plentiful and consistent on a yearly basis; thus, it can stably supplement the lack of feedstock for biodiesel. Assuming that biodiesel is produced from the total amount of sewage sludge, biodiesel production is estimated to be more than three times the current capacity in the US [11]. Various lipids, including fats, oil, and fatty acids, present in the sewage sludge are biodiesel precursors. The triglycerides (TG) and phospholipid fatty acids (PLFAs), which are the predominant lipids of sewage sludge, can be converted to biodiesel via transesterification (Equations (1) and (2)). Furthermore, PS and WAS contain 5–36% and 2–20% free fatty acids (FFAs), respectively [12,13]. The sewage sludge FFAs are also convertible to fatty acid methyl esters (FAMEs) via acid-catalyzed esterification [14].
Triglyceride (TG) + R’OH (Alcohol) → Fatty acid methyl ester (FAME) + Glycerol.
Fatty acids (FA) + R’OH (Alcohol) → Fatty acid methyl ester (FAME) + Water.
Many studies demonstrated the possibility of biodiesel recovery from sewage sludge [6,7,8]. Furthermore, many engineering techniques were adopted to overcome the quality limitations and improve the productivity of sewage sludge-derived biodiesel: development of an in situ transesterification process for high-water feedstock [8]; a sequential biodiesel extraction using the binary catalyst application [15]; a study of the high yield recovery process using an enzymatic catalyst for sewage sludge [9]. Understanding the quantitative and qualitative characteristics of a biodiesel feedstock is important in optimizing the technologies, as well as estimating commercial feasibility. The biodiesel potential of feedstocks, such as crops, plants, and microalgae, was already generalized and characterized [16,17,18]. Nonetheless, the lipid fractions of sewage sludge may significantly depend on the state of the lipid source. Lipids in PS, originating from fats, oils, and greases (FOGs) directly received from a sewer [19,20], are likely to be differentiated by the inclusion of industrial sewerage and by the type of sewer system, i.e., combined/separated sewer. Lipids in WAS, originating from cellular membrane phospholipids, are highly dependent on the microbial type and population, closely linked to the plant configuration and operation of the biological treatment process [21,22,23]. Previous studies demonstrated that lipids that can be converted from sewage sludge to biodiesel varied from 2% to 15% depending on the source of sewage sludge (i.e., PS or WAS), as well as the configuration of the biological process (i.e., membrane bioreactor (MBR), anaerobic–anoxic–oxic (A2O)) and operating condition (i.e., biological oxygen demand (BOD) loading) [6,12]. To our knowledge, however, there is still a lack of basic information to clearly understand the impact of these factors on biodiesel production potential. Thus, it is worthy to investigate the dependence on the characteristics of sewer source and microbial community distribution, subjecting them to the configuration and operational characteristics of the biological process as a feedstock, and correlating them with crude biodiesel production potential.
The overarching goal of this study was to investigate the potential of biodiesel of PS and WAS obtained from various sewage treatment plants (STPs). Since the biodiesel potential is substantially dependent on the sludge properties, it is crucial to understand the main factors for each STP. The details investigated were (1) a comparison of the lipid contents between the PS and WAS, (2) the fractionation of the lipids from recovered products via an in situ transesterification process, and (3) the evaluation of STP operation factors affecting the transesterification products. In terms of biodiesel feedstock, the lipids of sewage sludge were characterized as three types (transesterifiable Lipids (TLs), non-TLs, and FAMEs). Variations of these three types of lipid content depending on sewage sludge could provide practical information about biodiesel recovery through the in situ transesterification process. In addition to production yield, the quality of the biodiesel was evaluated in terms of FAME composition analyzed by gas chromatography (GC). Due to the complexity of these FAME peaks, the differences among sludge samples were readily compared by statistical methods, i.e., principal component analysis (PCA). Finally, a microbial community structure analysis of WAS was carried out to determine the correlation between the operational conditions and biodiesel production potential. The correlations could be used as fundamental scientific information to determine suitable operating conditions of a biological process for enhancing biodiesel production.

2. Materials and Methods

2.1. Sludge Sample Preparation

PS and WAS samples were collected from three municipal STPs in Korea. “J” is the largest municipal STP located in the capital city (Seoul, Korea), with a service population of about 2.5 million people. “H” is located in Gangwon province (Hoengseong, Korea), where about 46,000 people live. “D” is in Daejeon, a moderately sized city, which services about 1.5 million people. The three cities not only have different sizes and plant configurations, but they are also separated by at least 100 km. PS and WAS samples were collected from the discharge ports of the primary and secondary clarifiers, respectively. However, “D” STP samples were taken from the individual thickener, since collecting them at the same point was not allowed. The sludge samples were immediately transferred to the laboratory refrigerator, and then analyzed for water content (WC), total solids (TS), and volatile solids (VS) according to standard methods [24]. Prior to transesterification experiments, the sludge samples were dried at 65 °C for 90 h and stored in a desiccator [25]. The dried samples were stored in a desiccator to keep the water content below 5%. Prior to biodiesel conversion, the lipid content of each dried sludge sample was estimated in terms of biodiesel potential. In addition, WAS pellets centrifuged at 5000 rpm were preserved at −80 °C until microbial community analysis.

2.2. Analysis of Lipid Content

The total lipid of the sewage sludge was extracted from a solvent mixture of chloroform and methanol according to the method proposed by Bligh and Dyer [26,27,28]. Firstly, 20 mL of a chloroform–methanol mixture (2:1 volume ratio) per gram of dried sludge sample was homogenized for 20 min. The homogenized sample was centrifuged (2000 rpm, 5 min) to separate the solution. The water-soluble impurities contained in the solvent were separated by washing with 0.9% NaCl solution and centrifuging again (2000 rpm, 2 min). After the upper phase (NaCl solution) was discarded, the lower phase (lipids contained in chloroform) was taken separately. Finally, the lipids were separated from the chloroform using a rotary evaporator [26].

2.3. In Situ Transesterification

The in situ transesterification process for extracting crude biodiesel from dried sewage sludge was adopted from our previous study [8]. Firstly, 25 g of sludge sample (dried weight) was placed into a volumetric flask together with 250 mL of methanol solution. Then, n-hexane (100 mL) was also added as co-solvent. The methanol solution contained 5 vol.% sulfuric acid as an acid catalyst. During the 8-h reaction, the mixture was heated at 55 °C and the solvent was recirculated through a condenser on top of the flask. After the reaction ended, the cooled mixture was carefully taken through a separatory funnel. The settled mixture in the separatory funnel could be separated into three phases based on density. The top layer represented nonpolar miscible transesterifiable lipids (TLs) including fatty acid methyl esters (FAMEs), the middle layer included water-soluble by-products, such as glycerol and residual methanol, and the bottom layer represented insoluble solids. The top layer (solvent containing TLs) was carefully taken and treated with a 2% (w/v) NaCl solution to remove water-miscible substances. After three repetitions of the washing step, the solvent was evaporated using a rotary evaporator (R-124, Buchi, Switzerland) at 50 °C under vacuum conditions to collect the final product (crude biodiesel).

2.4. DNA Extraction, PCR Amplification, and Pyrosequencing Analysis

The microbial community structure of the WAS samples taken from the biological treatment process of each STP was analyzed via pyrosequencing. Total DNA was extracted from the homogenized 1-g WAS sample using a PowerSoil DNA kit (MoBio Laboratories, Inc., Carlsbad, CA, USA). PCR amplification was performed with 1 μL of DNA extracted through the following condition: 5 min at 94 °C, 30 s with 30 cycles at 94 °C, 55 °C for 45 s, and 90 s of final extension at 72 °C. The amplified PCR products were used for pyrosequencing analysis. Pyrosequencing analysis was performed by the analytical laboratory (Chun’s Lab, Seoul, Korea) using a Roche/454 GC Junior system. Raw sequence reads were sorted by unique barcodes. The primer sequences were as follows: bacterial universal (27F: AGA GTT TGA TCM TGG CTC AG, 518r: WTT ACC GCG GCT GCT GG) and archaeal universal (arc112F: GCT CAG TAA CAC GTG G, arc516r: GGT DTT ACC GCG GCK GCT G) for bacterial and archaeal gene amplification, respectively. Sorting of each pyrosequencing read was performed using the EzTaxon-e database (http://eztaxon-e.org) after discarding chimeric sequences containing low-quality scores (average < 25) and read lengths shorter than 300 base pairs [29].

2.5. Analytical Methods

The extraction yield can be defined as the fraction of nonpolar miscible TLs recovered from in situ transesterification.
TL   yield   ( % ) = Total mass of the TLs ( g ) Total mass of dried sludge ( g ) × 100
The FAME content in nonpolar TL sample was determined using the EN14103 method. This method involves performing the analysis using a gas chromatograph (Agilent 7890A, Agilent, Santa Clara, CA, USA) equipped with a flame ionization detector (FID). The analytical conditions of GC-FID with a carbowax capillary column (30 m × 0.32 mm × 0.25 μm) were as follows: both injector and detector temperatures were 250 °C; the column temperature was maintained at 60 °C for 2 min, then sequentially increased by 10 °C/min up to 200 °C, and 5 °C/min up to 240 °C; the final temperature (240 °C) was maintained for 7 min; helium was used as the carrier gas (1 mL/min); nonadecanoic (C19) acid methyl ester was used as the internal calibration standard FAME. The FAME profiles and concentrations were identified using a standard solution (37 comp. FAME mix, Supelco, Bellefonte, PA, USA). The FAME content was determined as follows:
FAME   content   ( % ) = ( ∑ A ) − A E I A E I × C E I × V E I m × 100
where ΣA is the total peak area, AEI is the peak area that corresponds to the internal standard, CEI is the concentration of the internal standard (mg/mL), VEI is the volume of internal standard (mL), and m is the injection mass of the crude biodiesel sample (mg).

2.6. Principal Component Analysis and Weighted Euclidean Distance

Principal component analysis (PCA) was performed for multivariate analysis of the FAME component data from each sewage sludge sample using the “princomp” procedure in SAS 9.4. The multivariate analysis of PCA is a statistical technique used to reduce dimensional output through the princomp procedure of complex high-dimensional data [30]. The FAME components for each sewage sludge sample could be displayed in two-dimensional PC plots obtained from the PCA analysis.

2.7. Statistical Analysis

Statistical analysis was conducted with SPSS software (SPSS V12.0 IBM Corporation, Somers, NY, USA). Every experiment, including lipid content and biodiesel conversion, was done in triplicate. Significant differences between analyses were determined by one-way analysis of variance (ANOVA). The significance for all p-values was less than 0.05 unless otherwise stated.

3. Results

3.1. Lipid Content of Sewage Sludge

Table 1 shows the basic information of the three STP candidates. Most of the information, such as process configuration and the operating conditions of each STP, was referenced from data provided by the Ministry of Environment in Korea (MOE) [5]. The SRT was obtained via discussion with the facility operator of each STP. Table 2 shows the solid and lipid contents of each sewage sludge sample. The solid content (TS and VS) between sludge samples was similar except for the “D” sludge taken from the thickener. Despite the fact that the “D” sludge sample had a two-fold higher solid content than others, the VS content (VS/TS) among all sludge samples ranged from 76.5% to 87.1%.
The lipid content of PS obtained from all STPs was within a relatively narrow range of 14.0–16.8%. That of the WAS was also within a narrow range of 10.9–11.7%, albeit slightly lower than that for PS. The lipid content in sewage sludge was reported in a much wider range; the global ranges of lipid content in the PS and WAS were 7–35% and 5–12%, respectively [25,31,32,33]. There are not many reports on the differences in PS lipid content among countries; however, it is presumed that oil consumption in kitchen wastewater affects lipid content in the incoming sewage. Based on an Food and Agriculture Organization (FAO) report [34], European countries consume twice as much fat as Korea (164 vs. 86 g/day/cap) in their diet. Conversely, the regional difference in WAS lipid content between Korea and Europe was not significant, which suggests that the lipid content of WAS is more dependent on the biological characteristics.

3.2. Crude Biodiesel Potential of Sewage Sludge

Figure 1 shows the mass fractions of the lipid extracted from the sewage sludge. We classified sewage sludge lipids into three categories in this study: FAMEs, non-FAME TLs, and non-TLs. The sum of the two former fractions (i.e., FAMEs and non-FAME TLs) was defined as the TLs extractable via in situ transesterification in this study. The fraction of TLs in the PS for the three STPs ranged from 71% to 81 %, demonstrating the relatively large potential of TLs in PS. The FAME fraction in the PS was about half that of the TLs, that is, ranging from 36% to 41% of the total lipids. Meanwhile, the FAME fraction in the WAS was only 12% to 32% of the total lipid, which was noticeably lower than PS and had a wider range. These results indicated that the natural oil (FOGs) present in the PS could be more readily transesterified and converted to FAMEs than the microbial phospholipids of the WAS.
Generally, the PS contains fat, oil, and greases (FOGs), which are indigenous lipids in sewage, and which attach to particles during settling in a primary sedimentation tank. In contrast, lipids in WAS accumulate in the microbial cell membrane in the form of phospholipids, as a result of the metabolic pathway of microorganisms. Thus, differences in the metabolic pathways of microorganisms, probably depending on the purpose of the biological treatment process, lead to differences in the lipid fraction of the WAS [12,35,36]. The configuration and operational factors of the biological treatment process are widely modified from the conventional activated sludge process depending on the target pollutants, i.e., organics or nutrients. For example, the modified Ludzack–Ettinger (MLE) process focuses on nitrogen removal through nitrification–denitrification, and, in order to ensure nitrification, it generally maintains a relatively long SRT. Conversely, the anaerobic/anoxic/oxic (A2O) process focuses on the simultaneous removal of nitrogen and phosphorus. Phosphorus accumulating organisms (PAOs) can grow well upon being exposed to repeated anaerobic and aerobic circumstances, and more phosphorus can be removed by from the sludge with PAOs under a shorter SRT. The nitrogen and phosphorus removal (NPR) process is a modified A2O process and is characterized by providing a different SRT between PAOs and nitrifiers; the latter can grow attached to biomedia under a longer SRT.
Figure 2 illustrates that there is a close correlation between the extracted TL fraction and the SRT applied to each process. The TL fraction of the “D” STP sample, i.e., from the A2O process, was slightly lower than the others. This result agrees with reports of organic carbon being synthesized into intracellular carbon, i.e., polyhydroxyalkanoates (PHAs) or polyhydroxybutyrate (PHB), in PAOs grown for many biological phosphorus removal processes, such as A2O [37,38]. These internal carbons can be included in the non-TL fraction, because they are readily extractable using chlorinated hydrocarbon solvents (e.g., chloroform), but not using the in situ transesterification process. Although the NPR process adopted by the “H” STP has an intermediate SRT compared to the MLE process adopted by the “J” STP and the A2O process adopted by the “D” STP, its TL fraction was similar to that of the “J” STP sample. This is presumably associated with the inclusion of nitrifiers grown under a higher SRT in the collected WAS sample [39].

3.3. FAME Composition of Sewage Sludge Crude Biodiesel

Fatty acid composition, as well as content, is a very important indicator to determine the quality of biodiesel as a fuel. Fatty acids with carbon chains from C12 (lauric acid) to C24 (lignoceric acid) are classified into saturated and unsaturated fatty acids, according to the presence of double bonds in the carbon chain. The composition of the individual FAME determines the fuel characteristics of the biodiesel, such as the velocity of combustion, ignition temperature, oxidation stability, and viscosity. Compositions of FAMEs with different carbon chain lengths also affect the cetane number; the cetane number increases with FAME carbon chain length [40]. The cetane number is an important measure of ignition delay for a fuel source. A lower cetane number corresponds to a longer ignition delay, which has implications in engine performance (i.e., knocking). The melting point and viscosity are closely related to unsaturation [41,42]. The major fatty acids in vegetable oils are C16:0 (palmitic acid) and C18:0 (stearic acid). In addition to these two common fatty acids, vegetable oils contain other fatty acids, such as C12:0 (lauric acid), C14:0 (myristic acid), C16:1 (palmitoleic acid), C18:1 (oleic acid), C18:2 (linoleic acid), and C18:3 (linolenic acid) [43].
Figure 3 shows that more than 15 kinds of FAMEs were identified from the biodiesel derived from sewage sludge. Of the identified FAMEs, the C16 and C18 FAMEs contributed to more than 88% and 77% of PS and WAS, respectively. They are major components of conventional vegetable oil-derived biodiesel, such as cottonseed or coconut oil, indicating that sewage sludge is a very favorable feedstock for the production of good biodiesel [44]. Similar to the lipid fractions shown in Figure 1a, the FAME compositions among the PS samples were similar to each other, while those among the WAS samples were different. Proper proportioning of the saturated and unsaturated FAMEs is important for the desired fuel quality of biodiesel, in terms of combustion rate and fluidity. Even though the optimal ratio between the saturated and unsaturated FAMEs remains to be established for a biodiesel quality standard, existing biodiesel feedstocks, such as palm oil or beef tallow, have a ratio in the range of 0.1–2 [45,46]. Of the samples, FAMEs derived from the WAS of “J” and “D” STPs were coincident with this range, in terms of saturated/unsaturated ratio. However, “H” STP samples contained very high concentrations of saturated FAMEs (ratio of approximately 9.0). The high concentration of saturated FAMEs would lead to a reduction in the fluidity of the biodiesel.
The FAME composition shown in Figure 3 was too complex for a direct comparison among the different samples, since it represented a multi-dimensional relationship with many variables. For high-dimensional datasets that are difficult to distinguish graphically, PCA is a powerful tool as it identifies components (PC1 and PC2) extracted from the whole FAME composition data for each sample, plotted as points on a two-dimensional (2D) plot, allowing clear visualization of the information between samples [47]. Figure 4 shows the PCA results and the proportional variance given by the eigenvector coefficients for PC1 and PC2 for each sludge sample. The PC1 and PC2 displayed on the x-axis and y-axis accounted for 45.7% and 21.6% of the total variance, respectively. In the PC plot, the long distance between PS and WAS samples illustrated that these two sewage sludge samples were statistically different in terms of FAME components. Among the PS samples, the difference between J1 and the other two samples (D1 and H1) was significant due to the different sewer systems and the regional characteristics. This STP receives domestic sewage mostly through a combined sewer system (Table 1). Also, the “J1” STP is in a metropolitan area (Seoul) with more than 10 million people. These two reasons might affect the lipid components of the incoming wastewater. The FAME composition of the WAS samples was influenced by the biological treatment process. The distance between WAS samples in the PC plot showed that the “H” STP had the most unique FAME composition of the STPs. Unlike “J” and “D” STPs, the “H” STP adopts the unique process of NPR, which is a biomedia process that facilitates the attached growth of microorganisms, e.g., nitrifiers.

3.4. Effect of Microbial Community Structure on FAMEs Recovered from WAS

In all WAS samples, bacterial phyla were dominated by Proteobacteria (48.81%), Bacteroidetes (26.11%), and Nitrospirae (6.59%). This is a similar composition to previously published data, where Proteobacteria (57.30%) and Bacteroidetes (26.60%) were found [48]. The microbial community distribution of the “H” STP, particularly the composition of the three phyla noted above, significantly differed from other WAS samples. Instead of a lower Proteobacteria content, the Bacteroidetes (30.97%) and Nitrospirae (11.59%) contents were comparatively high at the phylum level (Figure 5). The greater presence of Nitrospirae was most likely related to the purpose of the biological treatment process (i.e., nitrogen removal). As described above, the NPR process at the “H” STP employs suspended biomedia, in order to increase the retention time of slowly growing nitrifiers, such as Nitrospirae [49]. It was reported that the lipid content of Nitrospirae is normally two times greater than that of Proteobacteria, which was the predominant class of the other WAS samples [50,51]. The relatively larger proportion of slowly growing microorganisms, such as Nitrospirae, might contribute to the higher FAME production potential from the WAS sample of the “H” STP (Figure 1b). Simultaneously, this also differentiated the FAME composition from the other two STPs, as discussed above (Figure 4).
The overall results indicated that the crude biodiesel FAME compositions, as well as the microbial community structures, were substantially dependent on the configuration of the biological process and operational factors, particularly SRT. The population species varied depending on whether the treatment process targeted phosphorus removal (or simultaneous phosphorus and nitrogen removal) or nitrogen removal. Longer SRTs were more favorable for the growth of nitrifying bacteria (i.e., Nitrospirae), which led to an increase in microbial lipid content.

4. Conclusions

The ultimate goal of this study was to characterize the crude biodiesel production potential of sewage sludge, based on the characteristics of the sewer system and the configuration and operation of the biological processes. Both PS and WAS contained a sufficient amount of extractable lipids and a similar FAME composition to vegetable oil. In terms of crude biodiesel productivity, PS had a more consistent and higher lipid content than WAS, even though it was affected by sewer system and regional characteristics. In contrast, the FAME yield and the FAME composition derived from WAS were very diverse depending on the microbial ecology and physiological state, features which were determined by the biological configuration and operational factors, such as SRT. The overall results suggest that, in order to use the WAS as a biodiesel feedstock, it may be desirable to adopt a biological process with a long SRT and a higher proportion of nitrifiers in the microbial population.

Author Contributions

Conceptualization, J.W.L.; data curation, O.K.C., Z.H., K.Y.P., J.-K.K. and A.S.; formal analysis, O.K.C. and J.Y.P.; project administration, J.W.L.; writing—original draft, O.K.C.; writing—review and editing, Z.H., J.-K.K. and J.W.L.

Funding

This research was funded by National Research Foundation of Korea (grant number NRF-2018R1A6A3A11045472) from the Korea Ministry of Education (MOE) and the National Research Foundation of Korea (grant number NRF-2017R1A2B4012101) from the Korea Ministry of Science, ICT, and Future Planning (MSIP).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Mass fraction of lipids in (a) primary sludge (PS) and (b) waste activated sludge (WAS) extracted via in situ transesterification.
Figure 1. Mass fraction of lipids in (a) primary sludge (PS) and (b) waste activated sludge (WAS) extracted via in situ transesterification.
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Figure 2. Relationship between transesterifiable lipid (TL) fraction from WAS and solid retention time (SRT).
Figure 2. Relationship between transesterifiable lipid (TL) fraction from WAS and solid retention time (SRT).
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Figure 3. Fatty acid methyl ester (FAME) composition profile obtained from the in situ transesterification of (a) PS and (b) WAS for each STP.
Figure 3. Fatty acid methyl ester (FAME) composition profile obtained from the in situ transesterification of (a) PS and (b) WAS for each STP.
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Figure 4. Principal component (PC) plot for FAME composition obtained from sewage sludge. J1 and J2 depict the PS and WAS from the “J” STP, respectively. Abbreviations were similarly applied to the other samples.
Figure 4. Principal component (PC) plot for FAME composition obtained from sewage sludge. J1 and J2 depict the PS and WAS from the “J” STP, respectively. Abbreviations were similarly applied to the other samples.
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Figure 5. Comparison of the microbial community structures of WAS in each STP.
Figure 5. Comparison of the microbial community structures of WAS in each STP.
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Table
Table 1. Summarized information on sewage treatment plant (STP) facilities from which sewage sludge samples were taken.
Table 1. Summarized information on sewage treatment plant (STP) facilities from which sewage sludge samples were taken.
STP SiteInfluent TypeSewer System (%)Process Capacity (m3/day)Biological ProcessSolid Retention Time (days)
CombinedSeparated
JDomestic9281,590,000MLE 115–20
DDomestic4753900,000A2/O 24–10
HDomestic37637200NPR 310–15
1 Modified Ludzack–Ettinger process. 2 Anaerobic/anoxic/oxic process. 3 Nitrogen and phosphorus removal process.
Table
Table 2. Solid and lipid content of raw wastewater sludge for each STP. TS—total solids; VS—volatile solids; PS—primary sludge; WAS—waste activated sludge.
Table 2. Solid and lipid content of raw wastewater sludge for each STP. TS—total solids; VS—volatile solids; PS—primary sludge; WAS—waste activated sludge.
STP SiteTypeTS (%)VS/TS (%)Lipid Content (%)
JPS2.07 (±0.03)82.51 (±0.79)14.01 (±0.50)
WAS2.24 (±0.08)81.46 (±0.98)11.55 (±0.79)
D 1PS4.58 (±0.09)83.89 (±1.83)16.06 (±0.93)
WAS5.15 (±0.06)76.48 (±0.87)10.88 (±0.10)
HPS2.11 (±0.05)81.71 (±3.66)16.78 (±0.12)
WAS1.73 (±0.02)87.13 (±0.98)11.68 (±0.27)
1 “D” STP sludge was collected from the thickener.

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Choi, O.K.; Hendren, Z.; Park, K.Y.; Kim, J.-K.; Park, J.Y.; Son, A.; Lee, J.W. Characterization and Recovery of In Situ Transesterifiable Lipids (TLs) as Potential Biofuel Feedstock from Sewage Sludge Obtained from Various Sewage Treatment Plants (STPs). Energies 2019, 12, 3952. https://doi.org/10.3390/en12203952

AMA Style

Choi OK, Hendren Z, Park KY, Kim J-K, Park JY, Son A, Lee JW. Characterization and Recovery of In Situ Transesterifiable Lipids (TLs) as Potential Biofuel Feedstock from Sewage Sludge Obtained from Various Sewage Treatment Plants (STPs). Energies. 2019; 12(20):3952. https://doi.org/10.3390/en12203952

Chicago/Turabian Style

Choi, Oh Kyung, Zachary Hendren, Ki Young Park, Jae-Kon Kim, Jo Yong Park, Ahjeong Son, and Jae Woo Lee. 2019. "Characterization and Recovery of In Situ Transesterifiable Lipids (TLs) as Potential Biofuel Feedstock from Sewage Sludge Obtained from Various Sewage Treatment Plants (STPs)" Energies 12, no. 20: 3952. https://doi.org/10.3390/en12203952

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