Sewage sludge as carbon source for polyhydroxyalkanoates: a holistic approach at pilot scale level

Highlights

• Polyhydroxyalkanoates production from mixed microbial cultures and sewage sludge.

• Sewage sludge fermentation in thermophilic conditions after thermal pre-treatment.

• Characterization of polyhydroxyalkanoates derived from sewage sludge at pilot scale.

• Overall yield of polyhydroxyalkanoates produced from sewage sludge at pilot scale.

Abstract

In the present study, polyhydroxyalkanoates (PHA) production by mixed microbial cultures (MMC) has been carried out using thermally pre-treated excess thickened waste activated sludge (WAS), applying the feast-famine approach at pilot scale. The preliminary results of WAS fermentation conducted both in mesophilic and thermophilic conditions, in combination with a thermal pre-treatment at 70 °C for 48 h, highlighted how the thermal hydrolysis has a crucial role for the solubilization of the chemical oxygen demand (COD), which allows a significant increase of the final volatile fatty acid (VFA) concentration (roughly 8.5 g COD_VFA/L). Since thermophilic fermentation after thermal pre-treatment was the best performing condition, it has been applied at pilot scale in order to routinely produce VFA as precursors for the following PHA synthesis. The selection and enrichment of PHA-producing biomass was successfully established and maintained in a Sequencing Batch Reactor (SBR) during the whole experimentation period, under short hydraulic retention time (HRT; 2 days) and medium-low organic loading rate (OLR; 2.0–2.2 g COD/L d). At the end of the production process an average PHA content of 53 ± 3 %w/w was achieved. The polymer was finally extracted and recovered from the biomass using traditional chloroform and sodium hypochlorite extraction and then characterized for the quantification of thermal properties (Tm = 156.8–160.9 °C) and molecular weight (Mv = 396–405 kDa). A final overall mass balance, usually poorly reported in the literature, has been also assessed, resulting in an overall yield of 56 g PHA per kg of volatile solids (VS).

Introduction

Large quantities of sewage sludge, produced inside wastewater treatment plants (WWTPs), require adequate and environmentally safe management and disposal. Indeed, sludge management is one of the most critical issues in WWTPs operation, and it can represent from 20% to 60% of the overall operating costs (Andreoli et al., 2007). During the last few decades, sewage sludge conversion into value-added products, such as bioenergy or biobased materials, became significantly attractive. This alternative has the advantage of limiting the disposed sludge quantity and can contribute to reduce the overall operating costs (Alloul et al., 2018). In this context, anaerobic digestion is a very useful and consolidated technology for sludge management, since it can convert the biodegradable organic carbon into biogas (Alloul et al., 2018). On the other hand, WWTPs and anaerobic digesters are often designed with spare capacity to cater for variation in the wastewater flow and future population growth (Morgan-Sagastume et al., 2016). However, recent progress in water conservation and slower population growth in many developed countries have left many wastewater treatment facilities with oversized digesters not utilized. In addition, volatile fatty acids (VFAs), as an important intermediate product of anaerobic digestion, are a promising substrate for many different bioprocesses (Luo et al., 2019), e.g., biopolymer production (Valentino et al., 2018) bioenergy generation (Lee et al., 2014), as well as biological nutrient removal (BNR) (Liu et al., 2018). Indeed, VFA-rich liquid has great potential to be used directly as an external carbon source in biological nutrient removal which can make WWTPs self-sufficient plants in terms of chemical use (Zhang et al., 2013). Therefore, in recent studies, the use of VFAs, produced from co-digestion of sewage sludge and external organic waste (Owusu-Agyeman et al., 2020) or from acidogenic fermentation of cellulosic primary sludge (CPS) (Da Ros et al., 2020), as a carbon source for BNR has been evaluated. In this view, Da Ros et al. (2020) developed an economical assessment of two scenarios for the implementation of cellulosic primary sludge recovery considering the anaerobic digestion as reference scenario. The results showed that the production of bio-based VFAs from CPS as carbon source in BNR and/or as chemical precursors for biopolymers synthesis give higher net benefits instead of the only biogas production. Moreover, even though anaerobic digestion is a mature and sustainable technology for sewage sludge valorization, other recent studies indicated the viability of applying alternative technologies with the aim of producing or recovering higher-value products, such as polyhydroxyalkanoates (PHAs) (Alloul et al., 2018; Kleerebezem et al., 2015; Morgan-Sagastume et al., 2016). PHAs are biodegradable polyesters synthesized by several types of bacteria able to accumulate PHA as intracellular carbon and energy source, under nutrient-limited growth conditions (Lorini et al., 2020). Since they are completely biodegradable and they have thermoplastic properties comparable to traditional plastics, the interest on these biopolymers is progressively increasing, also considering the possibility to exploit organic fermentable wastes as substrates for their synthesis and to produce them from mixed microbial cultures (MMC) (Rodriguez-Perez et al., 2018). Sewage sludge is currently one of the most available waste within urban scenarios, however disposal problems are encountered and several legislations within European Union regulate sludge management (Li et al., 2018). More recently, Italian legislation included the production of biopolymers as one of the alternatives for energy and materials recovery from sludge (“Discipline for the management of waste consisting of sewage sludge and implementation of Directive 86/278/CEE concerning the protection of the environment, especially the soil, in the use of sewage sludge, 2019). Therefore, integration of PHAs production and wastewater treatment may help to both solve waste disposal issues and increase PHA competitiveness with the traditional fossil-based polymers for the plastics industry.

During the last few years, some studies described different methods for PHA production at pilot scale, to be integrated in existing WWTPs by using available resources: the organic fraction of municipal solid waste (OFMSW) (Valentino et al., 2018), mixture of OFMSW and waste activated sludge (WAS) (Moretto et al., 2020a), WAS and municipal wastewater (Morgan-Sagastume et al., 2015); primary sludge and municipal wastewater (Bengtsson et al., 2017); WAS and anaerobic digestion rejected water (Frison et al., 2015). In the view of PHA production implementation, the quality of the polymer needs to be also considered. Previous studies demonstrated the possibility of producing biobased materials from waste-derived PHA at high viscosimetric molecular weight (M_v), obtained by electrospinning for packaging applications (Melendez-Rodriguez et al., 2020). On the other hand, PHA characterized by low Mv or even stabilized PHA-rich biomass stabilized may be exploited in different environmental application: both extracted PHA and raw PHA-rich biomass are an efficient slow-release carbon source in anaerobic conditions for enhancing the biological reductive dichlorination in the frame of groundwater remediation from chlorinated hydrocarbons (Amanat et al., 2021a, 2021b). These results clearly suggested the possibility to directly use the PHA-rich biomass derived from waste, with no need of extraction procedures, bringing remarkable advantages from an economic and environmental point of view.

The present study focuses on PHA production by MMC using pre-treated excess thickened WAS, applying the feast-famine approach in a traditional three-step process, developed at pilot scale in Treviso municipality (northeast of Italy). Furthermore, the paper shows the preliminary results of the fermentation of WAS conducted both in mesophilic and thermophilic conditions, also evaluating the effects of a thermal pre-treatment. At the end of the production process, PHA synthesized from WAS has been extracted from the biomass and then characterized for the quantification of thermal properties and molecular weight. A final mass balance, usually poorly reported in the literature, has been also assessed.