Biodiesel production from swine manure via housefly larvae (Musca domestica L.)

Highlights

• Swine manure can be easily converted into housefly biomass rich in grease.

• Producing biodiesel with housefly larvae grease can lower the feedstock costs.

• We have optimized variables affecting the yield of esterification of methyl esters.

• We report components of larvae grease and the relevant biodiesel properties.

Abstract

Although biodiesel is a sustainable and renewable diesel fuel, the current feedstock predominantly from edible oils limits the economic feasibility of biodiesel production and thus the development of a cost-effective non-food feedstock is really essential. In this study, approximately 21.6% of crude grease was extracted from housefly (Musca domestica L.) larvae reared on swine manure, and the extracted grease was evaluated for biodiesel production concerning the variables affecting the yield of acid-catalyzed production of methyl esters and the properties of the housefly larvae-based biodiesel. The optimized process of 8:1 methanol/grease (mol/mol) with 2 vol% H₂SO₄ reacted at 70 °C for 2 h resulted in a 95.7% conversion rate from free fatty acid (FFA) into methyl esters. A 90.3% conversion rate of triglycerides (crude grease) to its esters was obtained from alkaline trans-esterification using sodium hydroxide as catalyst. The major fatty acid components of this larvae grease were palmitic (29.1%), oleic (23.3%), palmitoletic (17.4%) and linoleic (17.2%). The housefly larvae-based biodiesel has reached the ASTM D6751-10 standard in density (881 kg/m³), viscosity (5.64 mm²/s), ester content (96.8%), flash point (145 °C), and cetane number (52). These findings suggest that the grease derived from swine manure-grown housefly larvae can be a feasible non-food feedstock for biodiesel production.

Introduction

Biodiesel is a form of environment friendly fuel made from animal fats or vegetable oils, but the cost of feedstock is a major economic factor affecting biodiesel development [1].

To cope with this problem, some non-edible plant oils have been investigated, such as kusum (Schleichera triguga), mahua (Madhuca indica) and rubber seed oil [2], [3], [4]. But most of those plants require special growth conditions, a long cycle and large area of land, and more importantly, the yield of those oil seeds mainly depends on climate. Moreover, efforts have been made to obtain cheaper feedstock from solid organic waste. Lipomyces starkeyi can live on and convert sewage sludge into lipids, but a pretreatment of the sludge with alkaline or acid hydrolysis is required, which is highly energy-consuming and less environmental friendly [5]. Some potential feedstock from waste lipids such as waste cooking oil and rice bran oil have been investigated for biodiesel production, too [6], [7], but the information is scarce on using those materials to produce biodiesel on an industrial scale.

Some animal waste can be used for biodiesel production, such as chicken fat extracted from feather meal which is prepared from chicken wastes like chicken feathers, blood, offal and trims [8]. Another important candidate animal waste resource is swine manure. In developing countries, voluminous quantities of manure are produced in traditional pig industry and currently the disposal of the manure has become a major public concern due to its huge potential threat to public health and depletion of natural resources if improperly managed [9]. However, some saprophagous maggots, such as black soldier fly (Hermetia illucens Linnaeus) and particularly housefly (Musca domestica Linnaeus) can be used to degrade animal manure [10], [11]. Using a cheaper feedstock like the grease from maggots grown on organic waste has proved to be an alternative to lower the biodiesel costs, but can also mitigate the environmental pressure [12].

Housefly is a world-wide distributed fly species with a short developmental stage, high productivity, and the ability to live on virtually all organic wastes. Now the artificial breeding technology of housefly on an industrial scale has been well studied. The whole developmental period (from egg to adult) of housefly fed on swine manure at 30 °C is about 9 days, probably the shortest among the saprophagous flies. As a promising insect in organic waste management, the developmental period (from egg to adult) of black soldier fly is about 33 days at 30 °C, nearly 4 times that of housefly [13]. In addition, the adult longevity of black soldier fly is about 15 days, but that of housefly is longer than one month [13], [14]. Actually, a shorter larvae stage can curtail the production cycle of converting the manure into insect biomass and a longer adult stage is helpful for collecting the eggs needed for processing the manure on a large scale. Easy culture, high reproductive rate, well understood biology, and the ability to flourish in virtually all animal feces will be the sound reasons for the choice of housefly as the agent in manure management [15]. Moreover, the pupae from swine manure can be used to breed the next generation of housefly, thereby lowering the cost of larvae biomass production.

On the other hand, the ability of decomposing swine manure was approximately 0.2 g per individual housefly larva, nearly 10 times lower than that per individual black soldier fly larva, but the biodegration can be achieved by increasing the number of housefly larvae incubated in the manure. It is worth emphasizing that the housefly larvae grease content can reach 29.7% [16], a value nearly equivalent to that of black soldier fly larvae fed on swine manure (29.1%), and the latter is considered to be a potential feedstock for biodiesel [17]. However, few studies have been reported on the use of housefly larvae grease as a feedstock for biodiesel production.

In this study, we evaluated the ability of swine manure in raising housefly larvae used for biodiesel production and optimized several key variables affecting the yield of acid-catalyzed production of methyl esters: the molar ratio of methanol to grease, amount of catalyst, reaction time and temperature. We also analyzed the major fatty acid components of the housefly larvae grease and compared the fuel properties of the resulting biodiesel to those of rapeseed-based and black soldier fly larvae-based biodiesels based on the standard ASTM D6751-10.