Sustainable Solutions for Environmental Pollution. Группа авторов

Sustainable Solutions for Environmental Pollution - Группа авторов


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electrode. However, with the applied voltage >1.5 V, the performance of both electrodes declined. With an increase in applied voltages, hydrogen production on the cathode linearly increased in both reactors. Moreover, the stainless-steel electrode led to higher hydrogen production as compared to the graphite, which is consistent with some of the reports on superior hydrogen evolution reaction (HER) with the metal electrode (Cheng et al., 2009). Thus, superior HER on the stainless surface could introduce instability of cathode biofilms. Overall, this study demonstrated considerable scope to optimize process parameters and design of EF systems to maximize productivity and yield of target value-added products.

      1.3.4 Microalgae Derived Lipids

      Up to date, fossil fuels have been widely adopted as energy sources across the world. However, there is a great need to significantly develop the renewable fuels (e.g., non-fossil fuel) as energy sources on a large scale to outcompete the fossil fuels, eliminating the issues of aggravating CO2 concentrations and global warming (IPCC, 2018; Liu et al., 2019; Liu et al., 2020c). Out of many renewable fuels, microalgae-derived biofuels have demonstrated to be highly promising due to their advantages, such as high biomass production per unit area, a high weight ratio of lipids and ability to accumulate lipids in oleosomes, and no competition for arable land usage (Adeniyi et al., 2018; Chisti, 2007; Hallenbeck et al., 2016; Hu et al., 2008; Rittmann, 2008). Despite these attracting features, several drawbacks for the microalgae biofuel generation have been reported, emphasizing their high costs, and environmental concerns associated with algae harvesting and lipids extraction (Markou and Nerantzis, 2013; Pierobon et al., 2018; Rittmann, 2008; Sills et al., 2013). For instance, the requirement of pre-treatment (e.g., acid/alkaline hydrolysis, pulsed electric fields, ultrasound (Sheng et al., 2011; Zbinden et al., 2013)) for microalgae processing are generally highly energy intensive (e.g., in terms of both capital and operational costs), which was one of the arguments against further development and scaling-up. Lipid extraction is a critical step prior to the microalgal biodiesel production process. However, highly toxic chloroform-methanol solvents have been widely adopted for lipid extractions due to their effective performance (e.g., the ability to penetrate cell wall and membrane) (Bligh and Dyer, 1959; Folch et al., 1957). However, they must be replaced by other non-toxic solvents (e.g., hexane-isopropanol) due to the environmental and public health concerns (Lai et al., 2016a; Lai et al., 2014). Hence, a greener approach is required to address the challenges associated with microalgae-derived lipids extraction.

      However, the SF itself can suffer from the fact that the protein fermentation is slower than the carbohydrate fermentation (Lu et al., 2010). Furthermore, accumulation of short-chain carboxylates (e.g., high COD in the system) is another challenge (Lai et al., 2016b), resulting in the following two cases: (1) wasting the electrons from the feed biomass (e.g., not all the electrons are used up or recovered as value-added products); and (2) lowering pH, which inhibits the fermentation process (Siegert and Banks, 2005). Hence, stimulating the protein and SCFAs biodegradation is of great interest to assist lipid extractions. Previous studies have reported that MECs have the potential to accelerate the fermentation of protein along with other complex organics, such as SCFAs (Lu et al., 2012; Velasquez-Orta et al., 2009). Furthermore, with the increased removal of protein, hydrogen bonds between membrane proteins and lipids can be disrupted, rupturing the cell membrane and exposing intracellular lipids for much simpler extraction (Cooney et al., 2009; Sheng et al., 2011). Hence, this was the main motivation behind coupling MEC with SF, also known as the electro-selective fermentation (ESF), to significantly enhance lipid wet-extractions (Liu et al., 2019; Liu et al., 2020b; Liu et al., 2020c).

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      Finally, Liu et al. (2020b) have compared various SRTs on the ESF performance. In their study, ESF systems with different SRTs were compared (System A: start with a 6 d SRT, then switch to 2 d SRT vs. System B: start with a 2 d SRT, then switch to 6 d SRT). The ESF system A exhibited the highest lipid extractability (25%) when operated with a 6 d SRT, and gave the highest lipid productivity (450 mg/L/d) when switched to a 2 d SRT. This was attributed to establishing the microbial community containing protein fermenters at a 6 d SRT and washing out of lipid fermenters when the system A was


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