Elsevier

Renewable Energy

Volume 36, Issue 6, June 2011, Pages 1802-1807
Renewable Energy

Enhancing biomethanation of municipal waste sludge with grease trap waste as a co-substrate

https://doi.org/10.1016/j.renene.2010.11.014Get rights and content

Abstract

Grease trap waste (GTW) presents a challenge to wastewater treatment processes due to its slow biodegradation kinetics, high oxygen demand, and risks of pipeline blockage. The objective of this work was to evaluate the feasibility of GTW as an organic-rich co-substrate to improve biomethane production in the anaerobic digestion of municipal waste sludge (MWS) from sewage treatment, one of the most abundant feed materials to municipal anaerobic digesters. Waste characterization confirmed the high organic content of GTW at 138 gVS/L, which was 626% higher than that of MWS (19 gVS/L). The methane potential of GTW approximated 145 LMethane/LGTW, which was more than 15 times higher than that of MWS (8.9 LMethane/LMWS). When GTW was added as a co-substrate in addition to MWS, the high methane potential and organic content of GTW resulted in significant improvement in methane production during the anaerobic co-digestion of MWS, e.g. a 65% increase at the GTW loading of 5.5 gVS/L, representing a less than 4% (vol/vol) addition of GTW. Thus, the operational feasibility of anaerobic co-digestion using GTW as the co-substrate is enhanced by the insignificant volumetric GTW loading required for significant improvements in methane production. Process inhibition and reduction in biogas production, however, occurred with higher GTW loadings, suggesting the importance of proper GTW loading rates for the implementation of anaerobic co-digestion processes effective in improving biomethanation of municipal waste sludge.

Introduction

The primary components of grease trap waste (GTW) are essentially spent fat, oil and grease with associated solids and debris from food service establishment. GTW is typically captured to prevent entry of these materials into municipal sewer collection systems to avoid the occurrence of sewer line obstruction and pump failure due to grease coating, congealing, or accumulating in pipes and pumps [1]. According to a 1998 study sponsored by the US Department of Energy’s National Renewable Energy Laboratory, the annual production of GTW per capita averages 13.4 pounds in the U.S. [2]. Given the large quantities available and the high organic content of GTW, its potential as an energy source, instead of a waste stream, has been increasingly realized [3], [4].

Current waste collection practices, however, frequently result in GTW commingled with other municipal or household waste, such as septage, rendering municipal wastewater treatment facilities as the only feasible disposal option [5]. Yet, treatment of GTW in conventional aerobic processes presents a challenge to wastewater treatment facilities primarily due to its slow biodegradation kinetics, high oxygen demand, and risks of pipeline blockage [6]. Therefore, interest in the anaerobic treatment of GTW as a high-strength organic waste has grown as an alternative treatment process with the potentials for energy recovery as biogas [7], [8].

Anaerobic digestion is a widely used technology for the treatment of organic wastes including municipal, industrial, and agricultural wastes [9], [10]. The use of GTW for anaerobic digestion, particularly as a single substrate, however, has been complicated by problems such as nutrient imbalance, rapid acidification, and accumulation of inhibitory compounds [11], [12].

Anaerobic co-digestion has emerged as an alternative concept with potentials to overcome these challenges. Primary advantages of anaerobic co-digestion may include improved nutrient ratios in mixed substrates and enhanced pH buffering capacity, which could lead to more efficient waste treatment and biogas production [13]. While anaerobic co-digestion has been studied and practiced for a broad range of organic wastes, few studies have been conducted on the co-digestion of municipal waste sludge (MWS) derived from sewage treatment with GTW as a co-substrate [14], [15]. The use of GTW as a substrate for anaerobic digestion, while desirable with the recovery of biogas as a renewable energy source, remains difficult with large variations in waste characteristics as a result of differences in collection practices and potential inhibitory effects due to the high lipids content of GTW [16], [17]. Therefore, the objective of this work was to evaluate the feasibility of GTW commingled with septage as an organic-rich co-substrate to improve the anaerobic digestion of MWS, one of the most important feed materials to municipal anaerobic digesters [18].

Section snippets

Source materials

All waste materials used for anaerobic digestion in this study were obtained from a secondary wastewater treatment plant located in Knoxville, Tennessee, USA. The GTW sample was taken from a single truck delivery collected from local restaurants and food processing facilities mixed with septage. The MWS sample was taken from the stream of thickened sludge from a dissolved air flotation unit fed with primary and secondary sludge. The thickened sludge was further stabilized by anaerobic

Waste characterization

To evaluate the potential of GTW as a co-substrate for the anaerobic digestion of MWS, the characteristics of GTW were analyzed and compared to those of MWS as well as the inoculum material (Table 2). The content of organic material in GTW (138 gVS/L) was much greater than that in MWS (19 gVS/L). Similarly, another measure of organic content, the total chemical oxygen demand (CODt), also shows that GTW contained more organic material than MWS, suggesting the higher biogas production potential

Conclusion

In this study, we evaluated the potential of organic-rich GTW mixed with septage as a co-substrate in improving the anaerobic digestion of MWS. GTW was shown as a substrate with high organic content and methane potential. The use of GTW as a co-substrate at proper loadings could significantly improve the process efficiency of the anaerobic digestion of MWS by enhancing methane production by up to 65% at volumetric GTW loading rates less than 4% (VGTW/VDigester). Thus, the operational

Acknowledgements

This work was partly supported by a U.S. Environmental Protection Agency grant (SU-83431801). Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. MKH was supported by the Pre-Collegiate Research Scholars program at the University of Tennessee,

References (34)

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    The critical factor, however, is the choice of waste. Widely reported wastes for co-digestion are sewage sludge, flower and vegetable waste, food industry sludge, agro-industry waste, organic fraction of municipal solid waste (OFMSW), and manure (Aymerich et al., 2013; Luostarinen et al., 2009; Zhu et al., 2011). On the other hand, there are reports which also suggest no significant improvement achieved with co-digestion (Silvestre et al., 2015; Xie et al., 2017).

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1

Current address: Pratt School of Engineering, Duke University, Durham, NC 27708, USA.

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