Carbon recovery from wastewater through bioconversion into biodegradable polymers

Highlights

• The fundamentals of the 3-step MMC PHA production process are well established.

• Technical-economic analysis and LCA of 3-step PHA production process are promising.

• Novel approaches are under investigation with focus on not fermented feedstock.

• Integrating PHA production in wastewater treatment facilities is feasible.

• A viable chain between PHA producers and stakeholders has to be established.

Abstract

Polyhydroxyalkanoates (PHA) are biodegradable polyesters that can be produced in bioprocesses from renewable resources in contrast to fossil-based bio-recalcitrant polymers. Research efforts have been directed towards establishing technical feasibility in the use of mixed microbial cultures (MMC) for PHA production using residuals as feedstock, mainly consisting of industrial process effluent waters and wastewaters. In this context, PHA production can be integrated with waste and wastewater biological treatment, with concurrent benefits of resource recovery and sludge minimization.

Over the past 15 years, much of the research on MMC PHA production has been performed at laboratory scale in three process elements as follows: (1) acidogenic fermentation to obtain a volatile fatty acid (VFA)-rich stream, (2) a dedicated biomass production yielding MMCs enriched with PHA-storing potential, and (3) a PHA accumulation step where (1) and (2) outputs are combined in a final biopolymer production bioprocess. This paper reviews the recent developments on MMC PHA production from synthetic and real wastewaters. The goals of the critical review are: a) to highlight the progress of the three-steps in MMC PHA production, and as well to recommend room for improvements, and b) to explore the ideas and developments of integration of PHA production within existing infrastructure of municipal and industrial wastewaters treatment. There has been much technical advancement of ideas and results in the MMC PHA rich biomass production. However, clear demonstration of production and recovery of the polymers within a context of product quality over an extended period of time, within an up-scalable commercially viable context of regional material supply, and with well-defined quality demands for specific intent of material use, is a hill that still needs to be climbed in order to truly spur on innovations for this field of research and development.

Introduction

Polyhydroxyalkanoates (PHA) are completely biodegradable polyesters that can be biologically produced from renewable resources in contrast to most oil-based production of biologically recalcitrant thermoplastics. The most common PHA are poly(3-hydroxybutyrate), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate), (P(3HB) and P(3HB-co-3HV), respectively). These materials are thermoplastic polyesters with comparable mechanical properties to those of conventional oil based polymers such as polypropylene and polyethylene [1]. The current PHA price depends on monomer composition and it is usually higher for the copolymers; overall, it ranges from 2.2-5.0 €/Kg [2], [3], [4], which is less than typical range of 10–12 €/Kg reported from the beginning of the last decade [2]. Notwithstanding the burden of costs and environmental impacts of plastic trash, the current PHA prices are not deemed to be commercially competitive with respect to conventional petroleum-based polymers, which typically cost less than 1.0 €/Kg [2].

In order to provide a commercial entry of PHA based bioplastics in substitution to the established classical plastics, it is often expressed that the production methods of PHA must become cheaper. Strategies to improve the production economy have included the development of open mixed- instead of pure-culture methods, and the utilization of perceived low or negative cost, waste or residual feedstock that do not compete with primary food crops [5]. Downstream processing costs for PHA recovery from biomass plays a pivotal role in the PHA manufacturing process with respect to material quality, application opportunity, and, in following, its market value [6]. Different types of PHAs offer distinctly different material properties and this also factors into a range in scope of engineering applications and within that spectrum, different regional and niche business models.

PHA are part of the key carbon transformation products occurring during biological wastewater treatment by activated sludge. Dynamic conditions, such as those caused by discontinuous feeding regimes and/or alternate redox conditions (anaerobic/anoxic/aerobic), applied in a variety of process configurations can be used to promote for the enriched presence PHA-storing microorganisms in activated sludge biomass [7]. Given the wide spread utilization of activated sludge in water quality management services and the fact that PHA storing organisms are naturally present in such bioprocesses, much of the MMC PHA research and development has gravitated around biological wastewater treatment with the use of process residuals, wastes and wastewaters. This synergy of service and by-product generation is anticipated to be a logical means to decrease both CAPEX and OPEX costs. For example, in comparison to pure culture PHA production, MMC PHA methods are simpler and less costly since sterile conditions and infrastructure for an axenic bioprocess are not required. Also, PHA are recognised as key platform chemical raw materials within bio-refinery frameworks of bio-based schemes [6], [8]. In particular, in wastewater and solid waste management, PHA production via MMC represents an opportunity for recovering raw wastewater organic carbon by means of biological treatment [9], converting traditional end-of pipe waste treatment into an opportunity for residuals bio-refineries [10].

Over the past 15 years fundamental insights on MMC PHA production have come from laboratory scale research, and this research has been typically focused on three different biological process elements of a so-called three-step process [11], [12], [13] (Fig. 1). More recently, efforts have been devoted to the implementation of such a 3-step process at pilot scale, by the integration of MMC PHA production to wastewater treatment in different process configurations, and with the potential concurrent benefits of sludge minimization [14], and/or process water quality management [15], [16]. The results of this process integration are a benefit of joining essential regional services of waste management with outcomes of bio-based resource generation [17].

This review aims to a) outline strengths and weaknesses of the three-step MMC process, and b) examine methods and opportunities for integrating PHA production with municipal and industrial wastewater treatment infrastructures. A section dedicated in focus of review to the fourth process element of PHA recovery techniques from MMC is also provided. The properties of PHAs and their potential applications are beyond the scope of the present work, but the interested reader may refer to recent publications [1], [18]. Downstream processing and PHA recovery are principal factors influencing polymer properties (physical-chemical, thermal, rheological and mechanical). These properties can strongly affect processing and application challenges and opportunities, wherein production costs versus material value are ultimately judged in commercial business models. The specific nature of the application of the PHA as platform chemicals or bioplastic ingredients will influence the business case to justify the needed capital investments for the material commercial supply. A range of specialized niche and/or high-end product applications may well support a higher cost for these bio-based polymers within specific, and hoped for, regionally driven circular economies supporting renewable resource value chains.