Feasibility study of SCFAs production from microalgae during hydrogen fermentation

Highlights

• Feasibility test of SCFAs production from microalgae during H₂ fermentation.

• The highest H₂ yield of 36 mL H₂/g dcw at 40 g dcw/L.

• The highest 34% SCFAs conversion efficiency at 10 g dcw/L.

• Description of H₂ and SCFAs production rate by Andrew's Eq. and Monod Eq.

Abstract

In the present work, the feasibility of short-chain fatty acids (SCFAs) production from microalgae was investigated during hydrogen (H₂) fermentation. The fermentation was conducted at various substrate concentrations ranging 2.5–100 g dry cell weight (dcw)/L by using anaerobic mixed cultures under mesophilic condition. It was found that H₂ yield increased to 36 mL H₂/g dcw as substrate concentration increased to 40 g dcw/L. However, a significant decrease in H₂ yield was observed at substrate concentration of ≥60 g dcw/L. As substrate concentration increased, SCFAs concentration gradually increased, reaching 12,410 mg COD/L at 100 g dcw/L. However, in terms of SCFAs conversion efficiency, it increased from 17 to 34% (on chemical oxygen demand basis) with substrate concentration increase up to 10 g dcw/L. Acetate was the major product at 2.5 g dcw/L, but butyrate became dominant at substrate concentration of 5–60 g dcw/L. At 80 and 100 g dcw/L, lactate became the major product. H₂ and SCFAs production rate were successfully described by kinetic models of Andrew's Eq. and Monod Eq., respectively. From the fermentation performance obtained, it can be concluded that microalgae could substitute the current attempt that acidifying sewage sludge to use as an external carbon source in denitrifier.

Introduction

The concerns on environmental pollution, climate change, and energy security have brought about a lot of research efforts on developing alternative production chains in transport and chemical sectors [1]. Biorefinery concept is considered the alternative and sustainable way to create value-added biofuels and products through a biomass-based industry [2]. Biomass is defined as the contemporary plant matter formed by photosynthetic capture of solar energy, splitting into terrestrial and aquatic one depending on its growth environment. Compared to terrestrial biomass, microalgae, one of the representing aquatic biomass, has a number of potential advantages: faster growth rate with higher biodegradability, lower need for land use, and no competition with edible agricultural crops [3]. In addition, microalgae can be converted into a wide range of metabolites and chemicals including proteins, carbohydrates and lipids, which can subsequently be transformed into biofuels and value-added products via thermochemical and fermentation processes [4].

Hydrogen (H₂) has garnered huge interest as a promising alternative energy carrier in last years since it has high energy content and its combustion only produces water as byproduct [5]. From biomass by fermentation routes, H₂ can be produced in two ways depending on the light dependency. The photo-driven process has an advantage of high H₂ yield, but there is a limitation that requires big-sized fermenter caused by low H₂ production rate (HPR) [6]. Meanwhile, dark fermentative H₂ production, in short, H₂ fermentation, proceeded in a fast manner and can directly use solid materials as feedstock [7]. In addition, short-chain fatty acids (SCFAs, C2–C5) can be obtained as byproducts, during H₂ fermentation [8]. The produced SCFAs can be used as external carbon sources in biological nutrient removal process in wastewater treatment, and are highly suitable substrates for polyhydroxyalkanoate (PHA) production [9], [10].

There have been a few studies on SCFAs production by using pure culture, but a relevant literature on fermentation using mixed culture is scarce. The use of mixed cultures can offer several advantages compared to that of pure cultures in engineering point of view, such as simpler operation, better substrate utilization, and diverse metabolic capabilities [11], [12]. There are lots of factors that affect fermentation from microalgae using mixed cultures such as substrate concentration, pH, temperature, solids retention time (SRT), etc. Among these factors, substrate concentration has a significant effect on product distribution and yield, and it is a basic parameter judging the feasibility of invented process [13], [14].

This study aimed to investigate the feasibility of SCFAs production from microalgae during H₂ fermentation. The batch tests at various substrate concentrations of microalgae (2.5–100 g dcw/L) were conducted by using anaerobic mixed cultures under mesophilic condition. The results obtained here might helpful to understand the mechanisms for SCFAs and H₂ production from microalgae and beneficial for the sustainability of microalgae-based biorefinery.