Elsevier

Waste Management

Volume 92, 1 June 2019, Pages 21-29
Waste Management

Optimization of urban waste fermentation for volatile fatty acids production

https://doi.org/10.1016/j.wasman.2019.05.010Get rights and content

Highlights

  • Acidogenic fermentation process was optimized for urban bio-waste valorisation.

  • Thermal pre-treatment favoured the urban bio-waste organic matter solubilisation.

  • Mesophilic alkaline fermentation enhanced VFA production yields.

  • Higher HRTs and lower OLRs gave the best performance in a CSTR with 39 g CODVFA/L.

  • Alkaline urban bio-waste fermentation increased the HV precursors concentrations.

Abstract

The problem of waste disposal has recently focused on practices for waste recycling and bio-resources valorization. Organic waste produced in urban context together with biological sludge produced in wastewater treatment plants (WWTPs) can be used as renewable feedstock for the production of building blocks of different products, from biopolymers to methyl esters. This paper deals with the optimization of the fermentation process in order to transform urban organic waste (a mixture of pre-treated food waste and biological sludge) into added-value volatile fatty acid (VFA) rich stream, useful for biological processes within a biorefinery technology chain. Different temperatures, pH, hydraulic retention times (HRTs) and organic loading rates (OLRs) were tested both in batch and continuous trials. Batch tests showed the best working conditions at 37 °C and pH 9, using the bio-waste feedstock thermally pre-treated (76 h at 72 °C). These conditions were applied in continuous process, where higher HRT (6.0 d) and lower OLR [7.7 kg VS/(m3 d)] gave the best performances in terms of process yield and maximum VFA level achieved: 0.77 CODVFA/VS(0) and 39 g CODVFA/L. An optimized fermentation process is crucial in a biorefinery perspective since it has to give a final stream of constant composition or tailored products suitable for further applications.

Introduction

Waste management and disposal is one of the most pressing issues mainly due to the exponential population increase in the last decades, especially in urban areas where the majority of the population is located (Pfaltzgraff et al., 2013). The main organic refuses produced in this context are food waste, also known as the organic fraction of municipal solid waste (OFMSW), and biological sludge coming from the wastewater treatment. In Europe, the average organic matter production expressed as total solid (TS) per day (d) pro capita (person equivalent, PE) is approximately 55 g TS/(PE) d of OFMSW and 39 g TS/(PE) d of biological sludge (Colombo et al., 2017, IPCC, 2006). It is quite clear that these kinds of organic waste are highly available especially in urban areas and at present time they are handled and treated separately. The sludge is typically digested, dewatered and then sent to final disposal, and the OFMSW is sent to composting. The separate treatment of biological sludge and OFMSW is generally not convenient from both energetic and environmental point of view. As a matter of fact, anaerobic digesters treating sludge in wastewater treatment plants (WWTPs), especially waste activated sludge (WAS), are often low loaded and underperforming (Bolzonella et al., 2005), while aerobic composting is a highly energy consuming process. The advantages of a combined treatment of OFMSW and WAS have already been exploited by the anaerobic co-digestion approach, with the experiences reported in previous studies (Mata-Alvarez et al., 2014). This technology directs all the organic matter into a single step, in which the conversion into biogas and energy is realized (Scarlat et al., 2018). Digestate is also produced together with biogas and energy, with environmental concerns regarding its further stabilization and disposal and the possible presence of heavy metals, pharmaceuticals and/or other pathogens (Mata-Alvarez et al., 2014). OFMSW mixing with WAS generates several benefits, such as diluition of potential toxic compounds and improved nutrient balance (Zahedi et al., 2016). A more effective application of this approach can be found in urban contexts where the OFMSW is coming from a source separate collection or a door-to-door collection, since the biodegradability of the collected waste improves substantially due to the increase of organic matter content and the decrease of inert materials (Bernstad et al., 2013). Novarino and Zanetti (2012) reported the application of a mechanical pretreatment method to further separate the inert material and homogenize the organic matter, enhancing the anaerobic co-digestion process. The integration of OFMSW and WAS treatment can be easily realized in existing WWTPs, where anaerobic digesters are in most cases already present, in order to improve the WWTP energy balance. Authors reported some co-digestion experiences in full-scale plants, such as the Rovereto WWTP, located in the Trento province in northern Italy, in which the OFMSW and mixed sludge co-digestion was implemented in 2014 (Mattioli et al., 2017). In the Treviso province (northeast Italy) the co-digestion approach of OFMSW and WAS in the full-scale WWTP was proposed and implemented since 1999 (Bolzonella et al., 2006, Pavan et al., 2000). In this specific urban scenario, the waste separate collection is very efficient and reaches 87.9% on the total wastes (ISPRA, 2017) making the organic waste treatment integration an eligible way for valorization and recovery of the organic matter. A possible and innovative approach for the realization of this treatment integration can be found in an urban biorefinery. The urban biorefinery represent a technology chain in which the organic material of urban waste can be converted into new added-value bio-based products (Valentino et al., 2018). Bio-based products obtainable in an urban biorefinery through the exploitation of organic waste are biofuels (Stephen and Periyasamy, 2018), platform chemicals (Kiran et al., 2014), and bioplastics (Valentino et al., 2017). Some of the most important intermediates that allow the conversion of organic waste into these valuable bio-based products are volatile fatty acids (VFA), which in most cases are the direct precursors for biopolymer synthesis, such as polyhydroxyalkanoates (PHA) obtained from pure and mixed microbial culture (MMC) (Valentino et al., 2014). VFA are produced during the anaerobic fermentation process of almost all kinds of biodegradable organic waste (Strazzera et al., 2018). Previous studies reported experiences of anaerobic fermentation on dewatered sludge (Hao and Wang, 2015), cellulosic substrates (Keating et al., 2013), cheese whey (Colombo et al., 2017, Valentino et al., 2015) and OFMSW (Chen et al., 2017, Girotto et al., 2017, Korkakaki et al., 2016). In a platform where anaerobic and aerobic processes are combined, VFA obtained from waste fermentation are intermediate chemicals for the conversion of organic matter into the aforementioned biodegradable added-value products (Koller et al., 2017). Indeed, in a scenario where the OFMSW is coming from a highly efficient source separate collection and the anaerobic co-digestion of OFMSW and WAS is already implemented, the urban biorefinery concept finds its perfect integration. The municipality of Treviso (northeast Italy) is a representative example where the OFMSW-WAS mixture is currently sent to anaerobic co-digestion. Indeed, the hypothesis of driving part of this organic source into an anaerobic fermentation step for the VFA production could be an eligible way for an efficient urban organic waste management. Within this route, the combined VFA and methane production from OFMSW-WAS mixture has been recently demonstrated (Valentino et al., 2019) in the same urban context. However, in order to enhance the PHA synthesis and productivity, the fermentation of OFMSW-WAS mixture needs to be optimized, maximizing the VFA production and, as a consequence, the PHA potentially obtainable. In this study, different fermentation conditions were tested by means of batch tests on a mixture of OFMSW and WAS. These batch trials were conducted in order to find the best working conditions for the fermentation process, namely optimum pH value and temperature. Once the best condition was found, a continuous lab scale trial in a continuous stirred tank reactor (CSTR) was set-up to better represent an acidogenic fermentation process under different applied HRT and OLR.

Section snippets

Substrate characterization

The substrates used in this study were thickened WAS and pre-treated OFMSW both available inside the Treviso WWTP. The WAS has been collected from the static thickener of the full-scale plant; the OFMSW came from the source sorted collection in 50 districts of the Treviso Province and was transferred to the full-scale WWTP after its pre-treatment (squeezing and homogenization) in a dedicated plant. The mixture used for all trials was composed by volumetric fractions of 65–70% thickened WAS and

Batch fermentation tests

All batch tests were started with the same urban waste mixture; indeed, the initial solids as well as the macronutrients (nitrogen and phosphorus) content were similar in all trials. The urban waste mixture was extremely homogeneous since the OFMSW was squeezed in the pre-treatment and resulted as liquid slurry after mixing with thickened WAS. VFA concentrations were monitored daily for each batch test. The main results are summarized in Table 3.

Conclusions

This study assessed how the fermentation process of a specific waste mixture of urban origin could be optimized in terms of organic matter solubilisation and VFA production. For this work, a mixture composed by thickened WAS (65–70% v/v) and squeezed OFMSW (30–35% v/v) was used as renewable feedstock. A first screening of batch trials revealed that alkaline pH (9.0) and mesophilic temperature (37 °C), coupled with thermal pre-treatment (72 °C, 76 h) gave the best performances in terms of

Acknowledgements

This work was supported by the ‘‘REsources from URban BIo-waSte” - RES URBIS (GA 7303499) project in the European Horizon2020 (Call CIRC-05-2016) program. Alto Trevigiano Servizi S.r.l. is also gratefully acknowledged for the hospitality.

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