Research article
Reducing greenhouse gas emissions and enhancing carbon and nitrogen conversion in food wastes by the black soldier fly

https://doi.org/10.1016/j.jenvman.2020.110066Get rights and content

Highlights

  • Greenhouse gas and NH3 emissions were measured for black soldier fly (BSF) treatment.

  • BSF recycled 1.95–13.41% of C and 5.40–18.93% of N from food waste into biomass.

  • BSF bio-conversion of food waste greatly decreased CH4 and N2O emissions.

  • Increasing initial pH value of the substrate promoted larval growth.

Abstract

Currently, sustainable utilisation, including recycling and valorisation, is becoming increasingly relevant in environmental management. The wastes bioconversion by the black soldier fly larva (BSFL) has two potential advantages: the larvae can convert the carbon and nitrogen in the biomass waste, and improve the properties of the substrate to reduce the loss of gaseous carbon and nitrogen. In the present study, the conversion rate of carbon, nitrogen and the emissions of greenhouse gases and NH3 during BSFL bio-treatment of food waste were investigated under different pH conditions. The results showed that the pH of the raw materials is a pivotal parameter affecting the process. The average wet weight of harvested BSFL was 13.26–95.28 mg/larva, with about 1.95–13.41% and 5.40–18.93% of recycled carbon and nitrogen from substrate at a pH from 3.0 to 11.0, respectively. Furthermore, pH is adversely correlated with CO2 emissions, but positively with NH3 emissions. Cumulative CO2, NH3, CH4 and N2O emissions at pH ranging from 3.0 to 11.0 were 88.15–161.11 g kg−1, 0.15–1.68 g kg−1, 0.19–2.62 mg kg−1 and 0.02–1.65 mg kg−1, respectively. Compared with the values in open composting, BSFL bio-treatment of food waste could lead greenhouse gas (especially CH4 and N2O) and NH3 emissions to decrease. Therefore, a higher pH value of the substrate can increase the larval output and help the mitigation of greenhouse gas emissions.

Introduction

Globally, about 1.3 billion tons of food are wasted each year, equivalent to about $1 trillion in annual economic losses (FAO, 2014). Nowadays, the treatment of FW has become a serious issue worldwide due to the high costs related to its environmental management (Fersiz and Veli, 2015). The main approaches in FW management include disposal in landfills, incineration and composting for the production of fertilisers (Avagyan, 2017). Despite this, about one third of all the food produced today goes in landfills (Stuart, 2009). In Canada, for example, in the year 2017, 12.9 million tons of FW was produced, but only 4.4 million tons were recycled (Avagyan, 2017). In addition to the enormous financial costs, FW also result in many environmental problems, such as landfill consumption, odour nuisance and generation of leachate and landfill gas (Lee et al., 2007). On the other hand, the burning of FW reduces its economic value, and the dumped product can cause health and environmental problems (release of dioxins, etc.) (Avagyan, 2018). In order to alleviate this, it is necessary to develop economic and environmentally friendly alternatives (Avagyan, 2017, Avagyan, 2013).

Recently, composting has become an important way to manage FW, as it can reduce the volume and weight of FW, and produce innoxious, stable and rich in nutrients soil amendment materials even though it shows some limit (Avagyan, 2018, Yang et al., 2015). In particular, the biodegradation of organic matter can lead to the substantial loss of C and N during long processes, which both reduces the end-product quality and causes secondary environmental pollution. For example, some studies indicated that 16–74% of initial TN and 14–59% of initial TOC are lost during composting, mostly in the form of NH3 and CO2, respectively (Chen et al., 2019). About 0.2–9.9% of initial TN and 0.08–6% of initial TOC in organic wastes are lost in the form of N2O and CH4 (Yang et al., 2015), which are GHG of high concern (Chowdhury et al., 2014).

Currently, biowastes are treated by BSFL widely, and this new approach has attracted considerable interest by researchers worldwide. Previous studies were mostly focused on the bio-treatment of diverse organic wastes, and the residues can be utilised as fertiliser; Meanwhile, larvae provide protein- and fat-rich biomass, which can be further used for biodiesel production and animal feed (Salomone et al., 2017). In fact, the conversion of waste into protein for use as feed for aquaculture and poultry in USA (AAFCO, 2016), EU (Cutrignelli et al., 2018), and other parts of the world is allowed. Hermetia. Illucens L. is one of the authorised insect species. Practical approaches have been applied to ensure the retention of nutrients and control C and N loss in composting (Wang et al., 2018), including the use of different kinds of bulking agents which can increase the C/N ratio in raw materials, the modification of the aeration rate, and the addition of chemical agents or mineral additives. Compared with these methods, BSFL biowaste treatment can lead to end-products of higher economic value at lower costs.

The sustainable use of waste, including recycling and valorisation, is the current trend in waste management (Avagyan, 2013, Sánchez et al., 2015). However, there are few studies focused on the environmental benefit related to the treatment of wastes with BSFL in terms of sustainability. Mertenat et al. (2019) and Ermolaev et al. (2019) found that the BSFL FW treatment has potential to cut down GHG emissions compared with conventional composting. These researchers also suggest that there is still need of further exploration based on various raw materials and process parameters, to study the direct GHG emissions, C and N recycling during BSFL treatment, in order to correctly evaluate its environmental sustainability. Additionally, the FW biodegradation during composting has been reported to be mostly controlled by factors such as pH, moisture, C/N ratio and temperature (Cerda et al., 2018). Especially, the quick biodegradation of readily available organic matter can intensively acidify the substrate, resulting in low pH values in the initial phase of FW treatment and eventually inhibiting the larval growth and microbial activity (Chan et al., 2016, Ma et al., 2018). For this reason, to optimise the biowaste treatment by adjusting the initial pH of the substrate is of great significance. This may not only accelerate the decomposition of FW but also increase the yield and quality of BSFL. To date, the effects of pH on the bioconversion of C and N and on the emissions of GHG and NH3 during BSFL bio-treatment of FW have not been reported, to the authors' knowledge. This study aims to investigate the GHG and NH3 emissions during BSFL bio-treatment of FW under different pH conditions of the feeding substrate, as well as to improve the recycling of C and N from FW to larvae, and to mitigate the GHG emissions by adjusting the initial pH of FW. Besides, the amount of C and N was monitored alongwith parameters such as changes in larvae weight and physico-chemical properties of the substrate. The results of this study can provide important insights for the sustainable use of wastes by BSFL biowaste treatment.

Section snippets

Raw materials

BSFL (Hermetia illucens L., Stratiomyidae: Diptera) used in this study were bred at Wuhan ChaoTuo Ecology Agricultural Ltd. Food waste was collected from the restaurants in Wuhan, China, Hubei Tianji Bioengineer Co. Ltd, China. The chopped rice straw (RS) was obtained from the experimental field of Huazhong Agricultural University, Wuhan China. The physical and chemical characteristics are shown in Table 1.

Experimental design

Before BSFL rearing, FW and RS were mixed at a fixed ratio (FW:RS, 9: 1 w/w) to adjust

Dynamics of pH value, larval growth during black soldier fly larvae bioconversion process

As one of the crucial factors affecting the BSFL activity and GHG emissions, time-changes in pH are presented in Fig. 1a, with reference to treatments at different nominal pH (from 3.0 to 11.0). BSFL were able to adjust the pH of the substrate. However, this capability was not observed when the substrate was highly acidic (pH 3.0). The final pH value (after 10 days), for treatments at nominal pH from 5.0 to 11.0, ranged from about 8 to 9, indicating that the residues after BSFL conversion could

Conclusions

This study demonstrates that increasing the pH of the initial substrate effectively accelerates the BSFL growth and decreases CO2 emissions, but simultaneously increases NH3 emissions. The BSFL bio-treatment of FW could reduce CH4, N2O and NH3 emissions, when compared with traditional composting methods. BSFL can be harvested and preserved, providing a short-term means of C and N sequestration, rather than allowing them to be directly decomposed by microbes and released in the form of gases.

Credit author statement

Wangcheng Pang: Designed experiments, Carried out experiments, Writing- Original draft preparation. Dejia Hou: Designed experiments, Carried out experiments, Writing-Reviewing and Editing. Jiangshan Chen: Carried out experiments, Analysed data. Elhosseny E. Nowar: Commentary and revision. Zongtian Li: Commentary and revision. Jeffery K. Tomberlin: Commentary and revision. Ziniu Yu: Financial support for the project leading to this study. Ronggui Hu: Financial support for the project leading to

Declaration of competing interest

The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was financially supported by the National Key Research and Development Program of China (Project No. 2018YFD0500203). Fundamental Research Funds for the Central Universities (Project No. 2662017JC045 and Project No. 2662017JC026).

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    Wancheng Pang and Dejia Hou contributed equally to this work.

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