In-vessel composting system for converting food and green wastes into pathogen free soil amendment for sustainable agriculture
Graphical abstract
Introduction
The disposal of excess foodwaste produced in urban environment is a serious issue (Kim et al., 2008, Li et al., 2013, Pandey et al., 2016a). More than 40% of food produced using precious land and water resources goes as a waste (Gustavsson et al., 2011, Gunders, 2012). A major portion of this foodwaste reaches to landfills, and controlling excessive influx of foodwaste into landfills requires improved foodwaste recycling methods. Currently, the two conventional methods for treating food and green wastes are composting and anaerobic digestion (AD) (Parthan et al., 2012, Tsilemou and Panagiotakopoulos, 2006). The major advantage of AD method is that it facilitates production and capture of bio-methane, which can be utilized as a renewable energy source (Lins et al., 2014, Pandey et al., 2011, Zhu et al., 2014). Composting is a natural treatment method and requires minimal external energy input to complete the process (Goldstein, 2014, Watteau and Villemin, 2011, Zhou et al., 2014). Both of these methods result in end products which are useful for fertilizing the crops (Razali et al., 2012, Sangamithirai et al., 2015, Zhu et al., 2014). However, currently there is a growing public concern with regards to the risk of foodwaste borne pathogens reaching to crop land when digestate (i.e., compost or AD effluent) is used in the form soil amendments (Larney et al., 2003, Pandey et al., 2016a).
The inactivation of pathogens is often uncertain in both the AD (Pandey et al., 2011, Pandey et al., 2016a, Pandey et al., 2016b, Pandey et al., 2016c) and composting processes (Droffner and Brinton, 1995, Cekmecelioglu et al., 2005, Millner et al., 2014). As an example, previous studies found that the pathogen survival in composting can prolong from 1 to 2 weeks depending on the temperature of compost piles (Vinnerås, 2007, Droffner and Brinton, 1995). In anaerobic digestion, which is mainly run in mesophilic temperature, pathogen survival can extend for several weeks (Pandey et al., 2011). The application of digestate with elevated level of pathogens will likely to increase the influx of pathogens in cropland if the contaminated digestate is applied as a soil amendment (Heringa et al., 2010, Li et al., 2013). In addition to uncertainty in pathogen inactivation, a major drawback of composting and AD processes is that these are slow processes, and often require 30–90 days to complete the process (Iyengar and Bhave, 2006, Kim et al., 2008, Lins et al., 2014). Because these processes are slow, relatively a large space is needed to design AD and composting facilities, which is often cost prohibitive (Parthan et al., 2012, Tsilemou and Panagiotakopoulos, 2006). Therefore, improved treatment methods capable of converting organic wastes including food and green wastes into compost/soil amendments in shorter period and eliminating waste borne pathogens are needed.
Previous studies showed that in-vessel composting can accelerate the composting process (Antizar-Ladislao et al., 2005, Sangamithirai et al., 2015, Walker et al., 2009) and reduce the composting time. Iyengar and Bhave (2006) used a in-vessel (mixed and non-mixed) composting system for converting household wastes into humus for improving the soil nutrients. An et al. (2012) also tested an in-vessel composting system to evaluate the composting of agro-industrial and industrial wastes. In a similar research, Kim et al. (2008) used a pilot-scale in-vessel composting system for reducing the time needed for foodwaste treatment. Although these studies provided important insight in terms of using in-vessel composting system to accelerate the process, understanding of pathogen inactivation during the in-vessel composting process is still weak. The uncertainty in survival of foodborne pathogens during composting is a common concern (Larney et al., 2003; Pandey et al., 2016a, Pandey et al., 2016b). The increased emphasis on controlling foodborne pathogens and protecting the public health requires application of soil amendment with minimum pathogen risks to the health of soil and crops, animal, environment, and human (Angulo and Mølbak, 2005, Heringa et al., 2010, Park and Diez-Gonzalez, 2003). Therefore, to better understand the potential benefits of in-vessel composting system, and develop an advanced in-vessel composting system, a series of pilot-scale and lab-scale experiments was executed in this study. The objectives were to: 1) evaluate the performance of in-vessel composting on foodborne pathogen inactivation; 2) assess the quality (pH, carbon, and C: N ratio) of digestate produced during in-vessel composting system; and 3) develop the predictive models for calculating pathogen inactivation in the in-vessel composting system.
Section snippets
Pilot-scale and bench-scale systems
A pilot-scale study was conducted at Teaching and Research Animal Care Services (TRACS), and a bench-scale study was conducted at Extension Lab of the Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis (UC Davis), California, USA. The schematics of pilot-scale and bench-scale experiments are shown in Fig. 1. The bench-scale experiments were carried-out in two reactors (each 1000 mL capacity). The pilot-scale experiments were executed
Temperature, moisture, and carbon contents
Fig. 2 shows increment of temperature during pilot-scale and lab-scale experiments, and the results indicate that the desired temperature (≈55–60 °C) in pilot-scale experiment was obtained within 20–30 min, while in bench-scale the come-up time was 50–60 min. The difference in time needed for achieving composting temperature could be due to the fact that in pilot-scale system heat was applied with the help of in-built convection heating jacket, while in bench-scale system heat was applied with
Conclusions
To improve the foodwaste conversion into pathogen-free soil amendment, this study developed an innovative controlled in-vessel composting system, which accelerates the composting process. Such system is needed to address the issue of excessive foodwaste reaching to landfills and controlling foodwaste borne pathogens. Increasing emphasis on reducing the excessive influx of food waste and green waste into landfill requires identifying improved and accelerated methods capable of converting these
Acknowledgments
Authors would like to thank the Diamond Developers Co, Ltd (VBOM) (Award #201500387) and University of California, Agricultural and Natural Resources (ANR) Division for supporting this work generously. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the view of the sponsoring agencies, which suppoted the study.
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