Waste to bioproduct conversion with undefined mixed cultures: the carboxylate platform

Our societies generate increasing volumes of organic wastes. Considering that we also need alternatives to oil, an opportunity exists to extract liquid fuels or even industrial solvents from these abundant wastes. Anaerobic undefined mixed cultures can handle the complexity and variability of organic wastes, which produces carboxylates that can be efficiently converted to useful bioproducts. However, to date, barriers, such as inefficient liquid product separation and persistence of methanogens, have prevented the production of bioproducts other than methane. Here, we discuss combinations of biological and chemical pathways that comprise the ‘carboxylate platform’, which is used to convert waste to bioproducts. To develop the carboxylate platform into an important system within biorefineries, we must understand the kinetic and thermodynamic possibilities of anaerobic pathways, understand the ecological principles underlying pathway alternatives, and develop superior separation technologies.

Introduction

The discrepancy between the rates of discovery of new oil reserves and consumption will undoubtedly lead to a future oil crisis. Our societies are also generating an increasing quantity of organic wastes, such as industrial and agricultural wastewater. An opportunity therefore exists to shift the view of these waste streams from pollutant to renewable resource. In the biorefinery concept, the value of each stream must be maximized (similar to oil refineries) [1], and such waste treatment creates an opportunity to generate additional fuels or chemicals (i.e. bioproducts), while simultaneously recycling nutrients and water. Processing steps within biorefineries, such as chemical/physical pretreatment, enzyme production, and fermentation and extraction steps, all create large volumes of wastewater that must be treated. The two best-known biorefinery platforms are the sugar platform, in which purified enzymes convert biomass into five- and six-carbon sugars as intermediate feedstock chemicals that are converted further by, for example, fermentation to fuels; and the syngas platform, in which thermochemical systems convert biomass into syngas (i.e. synthesis gas, such as CO, H₂, and CO₂) as feedstock chemicals that are converted further by, for example, catalysis to fuels (National Renewal Energy Laboratory: www.nrel.gov/biomass/biorefinery.html). We envision a third important platform – the carboxylate platform – to convert organic feedstocks, which are often derived from industrial and agricultural wastes, to short-chain carboxylates as intermediate feedstock chemicals, using hydrolysis and fermentation with undefined mixed cultures in engineered systems under anaerobic conditions. The differences in platforms are essentially based on the method of biomass conversion and its resultant chemicals (e.g. sugar, syngas and carboxylates), because the subsequent conversion step into bioproducts is interchangeable between platforms. The use of undefined mixed cultures in waste treatment systems is vital, because they can tolerate the complexity and variability of substrates owing to the metabolic flexibility conferred by the many members of the community 2, 3. Furthermore, they are open and anaerobic systems, which makes energetically unfavorable sterilization and aeration superfluous [3].

The terminology ‘carboxylate platform’ is not new, and has been used to describe an undefined-mixed-culture process to generate a mixture of carboxylates as intermediate platform chemicals towards generation of complex fuels [4]. Carboxylates are dissociated organic acids that are characterized by the presence of at least one carboxyl group. The short-chain carboxylates – acetate, propionate, lactate and n-butyrate – are the main organic products of undefined mixed cultures through primary fermentation reactions (Figure 1a,b). They are themselves valuable products when separated from the culture broth, but often they are substrates for further fermentation in the same undefined mixed culture through secondary fermentation reactions (Figure 1c–j) or in separate bioprocesses. The carboxylates from primary fermentation can also be further processed with separate pure-culture biochemical, electrochemical, and thermochemical steps (chemical post-processing step in Figure 2). An important carboxylate flux occurs within the undefined mixed culture; as such, anaerobic digestion is included within the carboxylate platform because short-chain carboxylates are the (pen)ultimate intermediate platform products for gaseous methane formation (Figure 1). Even though pure culture and defined mixed culture studies are performed to understand the underlying ecological principles of undefined mixed communities 5, 6, 7, the commercial process to convert biomass into carboxylates must be an undefined microbial processing step to handle the complexity of the organic waste stream.

This review is a much-needed update to previous reviews 2, 8, because several additional bioprocessing schemes have been developed in the interim, to generate energy-rich chemicals with undefined mixed cultures, rather than with pure or defined cultures. The organization of this review is based on the production and subsequent conversion of the primary fermentation end products, the carboxylate feedstock chemicals: acetate (C2), propionate and lactate (C3), n-butyrate (C4), and mixed carboxylates. In this review, we discuss how these chemicals can be further processed into high-volume fuels or industrial solvents because these bulk bioproducts would have the largest impact in an integrated biorefinery. Table 1 shows balanced chemical equations and their thermodynamic values under standard biological conditions for the reactions and processes that we discuss in this review. Other chemicals that might also be generated within a carboxylate platform concept, but are not discussed here, are iso-butyrate, long-chain fatty acids, and biopolymers, such as poly(lactic acid).