Declining fossil energy resources and increasing global temperatures have steadily raised concerns on meeting increased future energy demands. Renewed interest in alternative energies has been kindled by environmental concerns and economic challenges. Biofuel, an excellent alternative to traditional fossil fuel-derived energy sources, can be produced from abundant supplies of renewable biomass. Volatile fatty acids (VFAs) are carboxylic acids with less than C6, which can be obtained during anaerobic digestion. VFAs are mainly com-posed of acetic, propionic, and butyric acids with various ratios of 6:1:3, 5:1:5, or 8:1:1. VFAs can be used as precursors for synthesizing mixed alcohols or biochemicals and carbon source for microbial lipid production.
In this study, the objective is to optimize VFAs production with various approaches and to enrich VFAs concentration with desalination technology. Numerous attempts have been made to enhance VFAs and $H_2$ production during acidogenesis in the anaerobic digestion processes. This paper implemented response surface methodology (RSM) as statistical tool to optimize VFAs and $H_2$ production from mixed cultures of Saccharina japonica, with respect to two independent variables: methanogenic inhibitor concentration and temperature. When testing the effects of various types of methanogenic inhibitors on acidogenic processes, doses of $beta$ -Cyclodextrin ($beta$ -CD) proved to have greater effects on VFAs concentration compared to other methanogenic inhibitors, showing the highest production of VFAs. RSM showed that VFAs production reached a peak of 12.53 g/L at $38.57^\circ C$ and 7.41 g $beta$ -CD/L, conditions under which $H_2$ production was also nearly maximal. Quantitative polymerase chain reaction showed that the concentration of $beta$ -CD correlated with shifts in the bacterial community population, indicating that $beta$ -CD effectively inhibited methanogens.
Additionally, to enhance the VFAs concentration and productivity, bioreactors in four series were developed to implement the MSC-HCDC system. A 4L multistage reactor was constructed, and broth was mixed by an overhead stirrer. In a flask scale, when experiments were conducted at a constant temperature of $45^\circ C$, dilution rate of 0.2/day, feeding ratio of 1:1:1:1, pH in feed stream of pH 10, and methanogenic inhibitor of 7g $beta$ -CD /L, VFAs concentration was measured to be 23.04g/L after 14 days, which was 24% higher than the experiment conducted with only a 1:1:1:1 feeding ratio and a 0.2/day dilution rate. Under this condition, there was a reduction in butyric and lactic acid production. In a four-stage bioreactor, VFAs concentration was 31.17g/L and its productivity was 6.23g/L/day on the 28th day. The composition of VFAs in this bioreactor is completely different than that in the flask scale. Although the concentration of valeric and caproic acids were negligible in the flask scale, VFAs with C4 to C6 were the main soluble compounds generated in this bioreactor.
However, the concentration of VFAs in the fermentation broth is generally below 35g/L, which makes the purification process expensive. The series of ordinary distillation columns are hardly applicable to the separation of VFAs. Thus, a cost-effective enrichment process is essential for developing an industrial-scale process. Additionally, several problems exist with VFAs when removing water: hydrophilicity, azeotrope in each acids, and a boiling point near $100^\circ C$. Forward osmosis (FO) has recently become increasingly popular. FO is an osmotic membrane process whereby the driving force for separation is an osmotic pressure gradient between the feed and draw solutions. In this study, to effectively enrich VFAs, permeation characteristics including water flux, reverse salt flux, and rejection of volatile fatty acids (VFAs) in the FO system were investigated. In FO, the pH value of VFAs feed solution affected rejection and water flux most significantly, while temperature, concentration of draw solution, and types of draw solute mainly influenced the water flux. The water flux and rejection of VFAs was also dependent on both membrane orientations. Different types of feed solutes showed different rejection and flux behaviors with either dependence or independence on feed pH. When hydraulic pressure was applied, the PAO system with NH4OH showed higher water flux and a lower rejection rate, while the PAO system with NaOH showed higher water flux and a higher rejection rate. Additionally, not only the chemical solution, but also the real fermentation product can be enriched using the FO system. Furthermore, we investigated the delta pi zero ($\Delta \pie$ =0) reverse osmosis (RO) system. Like reverse osmosis, delta pi zero RO applies lower hydraulic pressure for separation and enrichment since the $\Delta \pie$ =0 RO system has lower or zero osmotic pressure difference. With delta pi zero RO, enrichment of feed solution can be maximized while preserving energy and minimizing operation expenses.
In conclusion, these findings paved a new route to produce and enrich VFAs as biofuels and bio-chemicals. If VFAs from Saccharina japonica are converted to biofuels such as mixed alcohol, higher concen-tration and productivity of VFAs will be needed. Thus, these approaches may provide useful information in the extension of a VFAs platform for microbial biodiesels.