Separation of waste-derived volatile fatty acids from fermented wastewater

The aim of this thesis is to provide a practice to enable separation of volatile fatty acids (VFAs) from fermented wastewater. Two affinity separation techniques, namely liquid-liquid extraction (LLX) and adsorption, were initially proposed as the candidates which were to be assessed and compared. A sample of an actual fermented wastewater was represented by a few model solutions containing various VFAs and salts.

Part 1. Liquid-liquid extraction
The first candidate to examine was LLX. The practice of LLX can be seen as migration of a VFA from its home solution into a solvent, followed by removing it from the solvent to obtain a high purity VFA stream and reuse the solvent. This simple concept is the core of all elaborate LLX-based processes designed to meet various demands imposed by the nature of the carboxylic acid and its home solution. This work has utilized a phosphonium-based ionic liquid as the solvent for VFAs. It has explored migration of VFAs from fermented wastewater model solutions into the ionic liquid and removal of migrated VFAs from the ionic liquid to realize a practical process. Moreover, it has provided a road map for future separations-oriented research involving an ionic liquid.

Part 2. Adsorption
Adsorption was the second candidate to study. The affinity needed to adsorb VFAs from an aqueous solution is induced by a solid, namely an adsorbent, often immobilized in a column. The following step, referred to as desorption, removes the adsorbed VFAs from the adsorbent and prepares it to be reused. This thesis has introduced a non-functionalized adsorbent for separation of VFAs from fermented wastewater with no capacity for its mineral impurity which in turn gives the adsorbent an excellent selectivity for VFAs. Further adsorption and desorption experiments in a column proved that the non-functionalized adsorbent was very stable.