## 1 Introduction  
One of the consequences of the ever-growing global population is the generation of enormous amounts of different wastes on a daily basis. In order to build modern societies that maintain the balance between social, economic, and environmental sustainability, these waste streams should be effectively and efficiently treated. Among different waste streams, handling wastewater is of critical importance as its poor treatment and discharge to water bodies imposes a substantial environmental and ecological imbalance on marine life, in addition to diverse direct and indirect effects on human health [1, 2]. In conventional wastewater treatment (WWT) systems, in order to remove carbonaceous compounds and nutrients such as nitrogen and phosphorous to obtain clean water accepted for discharge, the received influent wastewater undergoes different treatment stages of biochemical oxygen demand (BOD) reduction, nitrification, and denitrification (with different orders in different treatment plants) [3]. Figure 1 presents a general schematic of the main wastewater treatment stages. Conventionally, the excess sludge discharged at different stages of treatment is directed to biogas and fertilizer production through anaerobic digestion (AD). For instance, in Sweden, about 204,000 tons of dry matter of sewage sludge (20.74 kg per capita) was produced in 2017, of which only 34% was used for agricultural purposes [4]. In addition to the excess sewage sludge, food waste is one of the most studied wastes streams that is mainly treated using anaerobic digestion due to its richness in organic strength [5; 6]. The biochemical reactions in the AD process take place in four interdependent stages namely: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 2) [7]. Biomethane is produced in the last stage of the AD process, while other metabolites such as volatile fatty acids (VFAs) and hydrogen are formed in the intermediate stages [8]. Biogas suffers low commercial value; however, a promising but hitherto not well-developed line of research is to extract and transform the intermediate products of AD (VFAs and H\({}_{2}\)). These compounds can be further converted into several high-value products, such as bioplastics, butanol and biodiesel [9]. The VFAs, such as acetic, butyric, and propionic acids, produced through AD, can be used directly or indirectly as chemical building blocks in, for instance, textile and plastic production industries [8]. Along with VFAs, hydrogen is produced during acidogenesis, which has attracted great attention as a clean energy source. However, issues such as VFA loss due to conversion to methane, change in pH, and cell inhibition by a high concentration of VFAs hinder obtaining high VFAs yields. In a mixed microbial culture, if the methanogens are not inhibited or the products of acidogenesis are not recovered efficiently, the VFAs will be lost through methanogenesis for methane production. Therefore, it is of great importance to be able to control the AD process to produce mainly desirable VFAs of higher value and application. However, a solution with a mixture of VFAs has a low market value, therefore, it needs to either be converted to other bioproducts or to be separated into pure components. Due to the formation of an azeotropic mixture with water, VFAs separation and recovery is laborious and energy intensive [10]. Thus, the direct application of VFAs mixture can eliminate extra energy intensive recovery processes and enhance economic feasibility. Figure 1: General processing stages at a conventional wastewater treatment plant aimed at biogas production from excess sewage sludge. During the wastewater treatment process, unlike nitrification bacteria that use inorganic carbon sources such as CO\({}_{2}\) to thrive on, the bacteria responsible for denitrification require organic carbon source for maintaining growth and acquiring energy [11].