An urban biorefinery for food waste and biological sludge conversion into polyhydroxyalkanoates and biogas

This study focuses on the application of the concept of circular economy, with the creation of added-value marketable products and energy from organic waste while minimizing environmental impacts. Within this purpose, an urban biorefinery technology chain has been developed at pilot scale in the territorial context of the Treviso municipality (northeast Italy) for the production of biopolymers (polyhydroxyalkanoates, PHAs) and biogas from waste of urban origin. The piloting system (100\u2013380 L) comprised the following units: a) acidogenic fermentation of the organic fraction of municipal solid waste (OFMSW) and biological sludge; b) two solid/liquid separation steps consisting of a coaxial centrifuge and a tubular membrane (0.2 \u3bcm porosity); c) a Sequencing Batch Reactor (SBR) for aerobic PHA-storing biomass production; d) aerobic fed-batch PHA accumulation reactor and e) Anaerobic co-digestion (ACoD). The thermal pre-treatment (72 \ub0C, 48 h) of the feedstock enhanced the solubilization of the organic matter, which was converted into volatile fatty acids (VFAs) in batch mode under mesophilic fermentation conditions (37 \ub0C). The VFA content increased up to 30 \ub1 3 g COD/L (overall yield 0.65 \ub1 0.04 g CODVFA/g VS(0)), with high CODVFA/CODSOL (0.86 \ub1 0.05). The high CODVFA/CODSOL ratio enhanced the PHA-storing biomass selection in the SBR by limiting the growth of the non-storing microbial population. Under fully aerobic feast-famine regime, the selection reactor was continuously operated for 6 months at an average organic loading rate (OLR) of 4.4 \ub1 0.6 g COD/L d and hydraulic retention time (HRT) of 1 day (equal to SRT). The ACoD process (HRT 15 days, OLR 3.0\u20133.5 kg VS/m3 d) allowed to recover the residual solid-rich overflows generated by the two solid/liquid separation units with the production of biogas (SGP 0.44\u20130.51 m3/kg VS) and digestate. An overall yield of 7.6% wt PHA/VS(0) has been estimated from the mass balance. In addition, a preliminary insight into potential social acceptance and barriers regarding organic waste-derived products was obtained

Optimizing a waste collection system with solid waste transfer stations

In recent years, stricter environmental regulations, as well as an increased public concern, have progressively forced new landfills to be located more and more away from urban centers. This has stimulated the use of solid waste transfer stations, where the solid waste is transferred from small collection vehicles to large transportation vehicles. In this paper, we tackle the problem of determining the routes for both collection and transportation vehicles, as well as their synchronization at the transfer stations. We divide the problem into two phases and propose an exact approach utilizing a mathematical formulation, as well as a constructive heuristic and a matheuristic for the first phase, and a heuristic approach for the second phase. Computational results show that the approach combining the matheuristic for the collection phase with the heuristic for the transportation phase is able to achieve consistent reductions in terms of number of collection vehicles needed

Optimization of urban waste fermentation for volatile fatty acids production

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

Environmental assessment of the production of bio-plastics from urban bio-waste

The organic solid waste and sewage sludge management has important consequences on the overall environmental and economic performance of urban waste management systems. The RESURBIS is a Horizon 2020 project that proposes a biorefinery concept for the combined treatment of all the bio-waste produced in an urban area, mainly focusing on the production of polyhydroxyalkanoate (PHA). The objective of this study is to compare the global warming potential of the the business as usual (where organic solid waste is incinerated and sewage sludge is treated in an anaerobic digester and then incinerated) and the RESURBIS bio-refinery, using a consequential Life Cycle Assessment approach. The preliminary results show that the actual treatment of 1,000 kg of the considered bio-waste in the Great Copenhagen area causes 18 kg CO2-eq, while in the RESURBIS bio-refinery reduces this environmental burden to 8 kg CO2-eq. These preliminary results need to be confirmed by a more detailed environmental modelling

An urban biorefinery for food waste and biological sludge conversion into polyhydroxyalkanoates and biogas

This study focuses on the application of the concept of circular economy, with the creation of added-value marketable products and energy from organic waste while minimizing environmental impacts. Within this purpose, an urban biorefinery technology chain has been developed at pilot scale in the territorial context of the Treviso municipality (northeast Italy) for the production of biopolymers (polyhydroxyalkanoates, PHAs) and biogas from waste of urban origin. The piloting system (100–380 L) comprised the following units: a) acidogenic fermentation of the organic fraction of municipal solid waste (OFMSW) and biological sludge; b) two solid/liquid separation steps consisting of a coaxial centrifuge and a tubular membrane (0.2 μm porosity); c) a Sequencing Batch Reactor (SBR) for aerobic PHA-storing biomass production; d) aerobic fed-batch PHA accumulation reactor and e) Anaerobic co-digestion (ACoD). The thermal pre-treatment (72 °C, 48 h) of the feedstock enhanced the solubilization of the organic matter, which was converted into volatile fatty acids (VFAs) in batch mode under mesophilic fermentation conditions (37 °C). The VFA content increased up to 30 ± 3 g COD/L (overall yield 0.65 ± 0.04 g CODVFA/g VS(0)), with high CODVFA/CODSOL (0.86 ± 0.05). The high CODVFA/CODSOL ratio enhanced the PHA-storing biomass selection in the SBR by limiting the growth of the non-storing microbial population. Under fully aerobic feast-famine regime, the selection reactor was continuously operated for 6 months at an average organic loading rate (OLR) of 4.4 ± 0.6 g COD/L d and hydraulic retention time (HRT) of 1 day (equal to SRT). The ACoD process (HRT 15 days, OLR 3.0–3.5 kg VS/m3 d) allowed to recover the residual solid-rich overflows generated by the two solid/liquid separation units with the production of biogas (SGP 0.44–0.51 m3/kg VS) and digestate. An overall yield of 7.6% wt PHA/VS(0) has been estimated from the mass balance. In addition, a preliminary insight into potential social acceptance and barriers regarding organic waste-derived products was obtained

Pilot-Scale Polyhydroxyalkanoate Production from Combined Treatment of Organic Fraction of Municipal Solid Waste and Sewage Sludge

Although the organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) originate from the same urban area and contain similar organic matter, they are collected separately and handled with different technologies. In this work, a combined treatment of OFMSW-SS mixture was investigated at pilot scale, by using a three-step mixed microbial culture (MMC) process in order to produce polyhydroxyalkanoate (PHA) as final high value biobased product. Biomass selection efficiency was quantified by PHA-specific storage rate that was 258 mg CODPHA/g CODXa/h under the optimized process condition. In fed-batch tests, PHA-storing MMC was able to accumulate up to 46 wt % PHA. In the perspective of a full-scale application and taking into account the mass flows in each process step, an overall yield of 65 g PHA/kg TVS was estimated

Una bioraffineria urbana per la conversione della frazione organica dei rifiuti solidi urbani (FORSU) e fanghi municipali in biopolimeri e biogas

Una bioraffineria urbana in scala pilota è stata sviluppata nel contesto territoriale del comune di Treviso (TV) per la produzione di biopolimeri (poliidrossialcanoati, PHA) e biogas da rifiuti organici urbani. In tale contesto, la raccolta differenziata altamente efficiente (87,9% sui rifiuti totali) garantisce la produzione di Frazione Organica di Rifiuti Solidi Urbani (FORSU) di alta qualità (alto contenuto organico biologicamente valorizzabile). Attualmente, la frazione liquida proveniente dal pre-trattamento meccanico della FORSU viene miscelata con fango biologico secondario (WAS) ed inviata al co-digestore anaerobico (ACoD) in piena scala all’interno dell’impianto di trattamento acque (WWTP) di Treviso (70.000 PE) per la produzione di solo biogas. Il sistema sviluppato in scala pilota (100-380 L) prevede l’upgrade dello schema attualmente operativo in piena scala tramite l’installazione di una linea aerobica per la produzione di PHA. Le unità pilota sono le seguenti: a) fermentatore acidogenico per la produzione di acidi grassi volatili (VFA); b) separazione solido/liquido (membrana tubolare di ultrafiltrazione); c) un Sequencing Batch Reactor (SBR) per la produzione aerobica di biomassa mista (MMC) PHAproduttrice; d) reattore aerobico fed-batch per la produzione intracellulare di PHA; e) ACoD per la produzione di biogas dai residui solidi miscelati con WAS prodotto dall’impianto in piena scala. FORSU pressata e WAS sono stati miscelati in una frazione volumetrica del 30% e 70% rispettivamente. La fermentazione acidogenica è stata condotta dopo un prepretrattamento termico (72°C, 48 ore), in modalità batch (5 d) e in mesofilia (37°C). Il processo fermentativo avveniva in un intervallo di pH compreso tra 5.0-5.5 ed era caratterizzato da un rendimento (YVFA) medio pari a 0.43 g CODVFA/g VS(0). La concentrazione di VFA in uscita era pari a 30 g CODVFA/L, con un rapporto CODVFA/CODSOL di 0.84. Questa caratteristica fondamentale della matrice fermentata permetteva di condurre la selezione aerobica della biomassa PHA-accumulante, la cui crescita è garantita dalla presenza di VFA, in modo efficace. Il reattore SBR è stato mantenuto operativo per più di 8 mesi, applicando un OLR di 3.2-4.0 kg COD/m3 .d e HRT pari a 1.0 d (uguale a SRT). La biomassa selezionata era in grado di accumulare fino al 60% di PHA rispetto al suo peso secco (w/w). In relazione al rifiuto organico di partenza, è stato stimato un rendimento complessivo di 107 g PHA/kg VS (approssimativamente del 10%). Il processo mesofilo (37°C) di codigestione (HRT 15-18 d, OLR 2-2.5 kg VS/m3 .d) ha permesso di produrre biogas dai flussi secondari in eccesso ricchi in solidi (30% w/w TS) dopo diluizione con WAS. La produzione specifica di gas (SGP) era pari a 0.4 m3 /kg VS. Applicando tale bioraffineria all’attuale impianto in piena scala è stata stimata una produzione di 114 ton PHA/y, 2314 m3 biogas/d e 5.9 MWh/d di energia elettrica. Rispetto allo scenario ACoD attuale, questo schema può essere ugualmente redditizio se il PHA prodotto è commercializzato ad una soglia minima di 0.82 €/kg. Quindi, un prezzo di mercato più alto e ancora ragionevole potrebbe rendere la bioraffineria un modello di valorizzazione dei rifiuti urbani preferibile rispetto al tradizionale approccio di digestione anaerobica

Novel routes for urban bio-waste management: A combined acidic fermentation and anaerobic digestion process for platform chemicals and biogas production

A combined acidic fermentation and anaerobic digestion (AD) treatment has been developed on pilot scale for urban bio-waste conversion into volatile fatty acid (VFA) and biogas. The specific waste mixture was composed by the pre-treated organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS), both produced inside the Treviso (northeast Italy) municipality. The effect of temperature (37 °C and 55 °C) was investigated in both steps. Only the mesophilic fermentation process provided a VFA-rich stream (19.5 g COD VFA /L) with stable physical-chemical features, with no need of chemicals addition for pH control. The sludge buffering capacity made this step technically feasible. The AD step was performed on the solid-rich fraction of fermented bio-waste, after dilution with excess WAS. No relevant differences were observed under the two investigated temperature: in the steady state (organic loading rate of 2.5 kg VS/m 3 d), the specific biogas production was 0.40 and 0.45 m 3 /kg VS at 37 °C and 55 °C respectively, with similar CH 4 content (63–64% v/v). The scaled-up version of the system (in an average urban municipality of 170,000 Person Equivalent) revealed that the whole process is thermally sustainable if both reactors are operated at mesophilic temperature: 36% of surplus thermal energy and 13,03 MWh/d of produced electricity, which corresponds to a revenue of 609,605 €/year. In addition, 2,262 kg COD VFA /d are available for parallel purposes, such as the synthesis of bio-products with higher added value than bio-methane (e.g. biopolymers)

Biopolymers from Urban Organic Waste: Influence of the Solid Retention Time to Cycle Length Ratio in the Enrichment of a Mixed Microbial Culture (MMC)

In this study, the performance of the selection process for polyhydroxyalkanoate (PHA) production from mixed microbial cultures (MMCs) at pilot scale was deeply investigated with the solid retention time (SRT) to cycle length (CL) ratio as main affecting parameter. Four different runs were tested by varying the SRT/CL ratio maintaining the same organic loading rate (OLR). The pilot-scale selection and accumulation reactors were fed with a fermented mixture of source-selected organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS), refined with a centrifuge and membrane unit for the coarse solid removal. The selected biomass obtained in the most performing run was characterized by a specific storage rate of 375 mg CODP/g CODXa h and a storage yield of 0.46 CODP/CODSOL. Accumulations performed with the same biomass were characterized by a storage yield of 0.62 CODP/CODVFA. The microbiome composition was assessed. In the most performing run, putative PHA-storing bacteria affiliated with Paracoccus genus were found at high abundance (36.8%), in contrast to all other runs. An overall PHA yield of 110 g PHA/kg VS was estimated for the best scenario, revealing an interesting perspective for biorefinery technology chains based on the three-stage process for PHA production