Electro-dialytic separation of volatile fatty acids from acid fermentation broth of food waste

Abstract

The yearly generation of food waste in Korea exceeds 5 million tons since. However, conventional disposal methods (ocean dumping and landfill) were banned by the international convention and domestic regulations, and it has led to the recycle of food waste into biogas, fertilizer and forages. Among them, anaerobic digestion to produce biogas has been considered cost-effective and energy-positive technologies. Organic wastes such as food wastes generally undergoes the fermentation process during anaerobic digestion, which produces volatile fatty acids from organic matters. The volatile fatty acids (VFAs) can be utilized as a wide range of applications including a raw material for polyhydroxylalkanoates (PHA) which is biodegradable polymers, synthesis of bio-alcohol or biodiesel, and biological nutrients removal. However, for utilization of VFAs, separation process is required to remove impurities in the acid fermentation broth. A number of technologies such as crystallization and precipitation, liquid-liquid extraction and adsorption have been applied to separate VFAs. Although many of above technologies could achieve high recovery of VFA separation performance, large amount of chemicals were consumed during the complicated separation processes. Comparing with above technologies, Electrodialysis (ED) is an ion separating technology, thus simply consisted with ion exchange membranes (IEM) and electrodes. This simple process can separate only ions without other materials from the feed solution. However, the driving forces is electrical energy, thus the higher energy consumption has been considered as the biggest hurdle for the commercial application of VFA by ED. Therefore, in this study, we investigated energy-efficient operating condition for separation of VFAs from the acid fermentation broth with ED process, and provided the mechanistic explanation between the transport of VFA and electrical energy consumption.

In Chapter 1, the research background, objectives and the scope of this research were described. In Chapter 2, summary of recent advances in ED and VFA separation were conducted with the fundamental concepts. In Chapter 3, in order to reduce the electrical energy consumption of electro-dialytic VFAs separation, operational parameters in ED system and changeable characteristics of acid fermentation broth were explored with model solutions. Lab-scale electrodialyzer was applied and the pH of the feed, the concentration of the feed, the volumetric ratio of the product to the feed and the applied voltage was evaluated. The pH of the feed was related to dissociation of VFAs and VFAs ion affected by electric field was increased. The concentration of the feed and the volumetric ratio of the product to the feed influenced mass transfer by diffusion. The increase of applied voltage enhanced the electric field and mass transfer by electro-migration. Therefore, the operating time was decreased and the energy consumption was increased proportionally. In conclusion, high pH, high concentration difference and lower applied voltage were the condition for the energy-efficient operation. In Chapter 4, the investigation on the impact of ion species in the feed to ED operation was examined based on the water dissociation phenomena. It is further proven by the changes of cation species with various diffusion coefficient. Three kinds of VFAs (acetic, propionic butyric acid) and three types of cation ($K^+$, $Na^+$, $Ca^{2+}$) in pH adjusting reagent were evaluated respectively. Acetic acid with higher diffusion coefficient was separated quicker consuming lower energy than others. In contrast, $Ca^{2+}$ with lower diffusion coefficient was revealed to reduce water dissociation in anion exchange membrane and therefore the energy consumption for separation of VFAs was improved. And ED operation with a pilot-scale electrodialyzer was examined for the comparative evaluation of the experimental results with lab-scale electrodialyzer. The effective membrane area of the pilot-scale electrodialyzer was 22 times larger than that of lab-scale electrodialyzer. At 0.5 V/cell, lab-scale electrodialyzer took 2 and half hours to recover 130 mmol of acetic acid. And pilot-scale electrodialyzer took an hour to recover 6 mol of VFAs. Therefore, without consideration for the 22-fold increase in the membrane area, 2.5 times of additional improvement on performance of VFAs separation was observed. The reason was the shortening of the distance between electrodes by scale-up. In Chapter 5, the conclusion of this research was presented. Therefore, this study can contribute to provide the fundamental knowledge for ED operation with the object of recovery of VFAs from the acid fermentation broth and to give guidelines on selecting pH adjusting reagent during acid fermentation for energy efficient separation of VFAs by ED.