Recovery Techniques Enabling Circular Chemistry from Wastewater

Abstract

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Keywords: extracellular polymeric substances; long-chain dicarboxylic acids; medium-chain carboxylic acids; separation technology; unsaturated fatty acids; volatile fatty acids.

Figure 1

Scope and application of bio-based volatile fatty acids, figure taken from [17].

Figure 2

Possible fermentation pathways in mixed microbial culture, reused from [27].

Figure 3

Possible interactions between the aromatic ring of a non-functionalized polystyrene based adsorbent and VFAs, (a) purely hydrophobic interactions, and (b) hydrogen bond–pi interactions.

Figure 4

Schematic view of adsorption-thermal desorption process to recover VFAs from a fermentation broth, redrawn from the work of [36] with permission from the American Chemical Society.

Figure 5

Integrated recovery and esterification of the carboxylate from a fermentation broth by CO₂-expanded methanol technique and using paper mill wastewater as a feedstock, redrawn from [58].

Figure 6

Schematics of the bioreactor and product separation system comprised of membrane-assisted extraction and back-extraction and a membrane electrolysis cell as described in [10].

Figure 7

General structure of various PHAs, reproduced from [87].

Figure 8

Production process of PHA via a wastewater treatment plant (WWTP) taken from [92].

Figure 9

Thermal degradation of PHB toward crotonic acid, by H. Ariffin et al., reproduced with permission from [13]; published by Elsevier, 2008.

Figure 10

Schematic view of fluidized bed PHB pyrolysis process. The figure is reproduced with permission from [105], published by Elsevier, 2014.

Figure 11

Schematic view of aqueous two-phase system formation.