From waste to energy: Microalgae production in wastewater and glycerol

Highlights

• Highest biomass and lipid productivities where achieved at mixotrophic cultivation.

• Lipids from mixotrophic biomass were more suitable for biodiesel production.

• Nutrient removal achieved below the most strict threshold limits.

• Positive energy production scenarios can be designed coupling distinct processes.

Abstract

The present work aimed to evaluate the auto/mixotrophic growth of microalgae using domestic wastewater (WW) amended with glycerol aiming biofuels production. The best results were obtained with the highest glycerol supplementation (50 mM). In such condition, Chlorella vulgaris and Botryococcus terribilis showed a biomass productivity of 118 and 282 mg l⁻¹ d⁻¹, which produced about 18 and 35 mg l⁻¹ d⁻¹ of lipids, respectively. Thus, if scaled-up (200 m³ d⁻¹ of WW, 240 working days y⁻¹) biomass and lipid yields may be about 5.6 tons y⁻¹ and 894.2 kg y⁻¹ or 13.5 tons y⁻¹ and 1.6 tons y⁻¹ for C. vulgaris and B. terribilis, respectively. The mixotrophic production of lipids can generate high quality biodiesel according to estimations using their fatty acids profiles. The whole process can be advantageously combined with the production of other biofuels (e.g. methane and bio-ethanol) in a biorefinery scenario. This combination of algal biomass production with waste treatment (WW amended with glycerol) can have a significant impact in the water treatment sector and local markets.

Introduction

It is estimated that the volume of domestic effluents generated by North America, Europe and Latin America is of approximately 70, 63 and 47 km³ y⁻¹, respectively [1]. Thus, the current main challenge of a Wastewater Treatment Plant (WWTP) is not only to produce reusable clean water, but it is also to find new technologies for supporting such an activity [2]. Conventional techniques can remove only a fraction of the total nitrogen (40%) and phosphorous (12%) present in the effluent. In order to improve the process new methods (tertiary steps) and, consequently, additional costs are required [3]. The European Directive 98/15/EC establishes a threshold of 10 and 1 mg per liter for total N and P. WWTP effluents commonly show N and P values around 20–70 and 4–12, respectively [3]. Therefore, there is still a clear need for new developments and biological systems are often considered to be the ideal means for responding to such a demand [4]. The economic costs are, however, a primary concern once improved nutrients removal would require an overall increase in energy consumption of about 60–80% [5]. Therefore, in order to reduce costs, new systems should explore the combination of wastewater treatment with the production of renewable energy in order to offset final costs [6], [7].

Microalgae based systems have shown a high potential to assist with nutrient removal [8], [9], [10]. On the other hand, such a process can be improved if treatment is associated with generation of valuable co-products [7]. Biofuels have been advocated as a suitable option to replace fossil fuels [11], [12]. However, several social and environmental issues are associated with increasing land crops based biodiesel and microalgae based systems were identified as capable of overcoming both economic and ecological problems [12], [13], [14]. In addition, microalgae systems can generate further commodities such as bio-ethanol, bio-kerosene, bio-plastics, bio-hydrogen, biogas, and other chemicals derivatives [15], [16], [17].

Although microalgae systems show high potential, the main bottleneck toward an effective application in the energy sector is the cost associated with both upstream and downstream processing [45], [12]. In complement to the commonly explored autotrophic activity, mixotrophic algae systems have also been considered as a viable alternative for supporting innovative bioprocesses [18], [19], [11], [10]. Mixotrophic based systems have been previously positively tested for Chlorella and Botryococcus [20], [21], [22], [23]. These microalgae genera have been shown to have a high potential for the production of bioenergy and they also have been used for wastewater treatment. However, since the metabolic response may be strain specific, it is still necessary to evaluate their performance and, most importantly, assess their potential lipid yields at such condition.

Finding cheap supplemental carbon sources for algae cultivation is important for minimizing the economic impact. Glycerol, for instance, is currently being produced at significant amounts as the by-product of biodiesel transesterification [24] and, in 2010; the worldwide production was of about 1.8 billion liters [25] with a commercial demand of only 0.8 million per year [24]. Thus, currently, glycerol is cheaply available and with very poor commercial perspectives. Some algae mixotrophic systems have been reported using glycerol as carbon source [11], [23], [26], but their results regarding biomass and lipid yields are in need of complementation.

Therefore, the use of glycerol as organic supplementation for algal growth is an innovative suggestion and it may significantly contribute to environmental and economic advantages for WWTP worldwide. Thus, the aim of the present work was to evaluate the potential of using glycerol as carbon supplementation to WWTP effluent and its effects on supporting the mixotrophic growth of lipid producing microalgae.