Reuse of process water in a waste-to-energy plant: An Italian case of study

The minimisation of water consumption in waste-to-energy (WtE) plants is an outstanding issue, especially in those regions where water supply is critical and withdrawals come from municipal waterworks. Among the various possible solutions, the most general, simple and effective one is the reuse of process water. This paper discusses the effectiveness of two different reuse options in an Italian WtE plant, starting from the analytical characterisation and the flow-rate measurement of fresh water and process water flows derived from each utility internal to the WtE plant (e.g. cooling, bottom ash quenching, flue gas wet scrubbing). This census allowed identifying the possible direct connections that optimise the reuse scheme, avoiding additional water treatments. The effluent of the physical–chemical wastewater treatment plant (WWTP), located in the WtE plant, was considered not adequate to be directly reused because of the possible deposition of mineral salts and clogging potential associated to residual suspended solids. Nevertheless, to obtain high reduction in water consumption, reverse osmosis should be installed to remove non-metallic ions (Cl , SO4 2 ) and residual organic and inorganic pollutants. Two efficient solutions were identified. The first, a simple reuse scheme based on a cascade configuration, allowed 45% reduction in water consumption (from 1.81 to 0.99 m3 t M1SW, MSW: Municipal Solid Waste) without specific water treatments. The second solution, a cascade configuration with a recycle based on a reverse osmosis process, allowed 74% reduction in water consumption (from 1.81 to 0.46 m3 t M1SW). The results of the present work show that it is possible to reduce the water consumption, and in turn the wastewater production, reducing at the same time the operating cost of the WtE plant

Early prediction of BMP tests: A step response method for estimating first-order model parameters

The Biochemical Methane Potential (BMP) test is an essential tool for supporting real-scale facilities, for instance to derive practical knowledge about a digester performance. However, its broader application is limited by long test duration and high cost. This work proposes a new method for early prediction of BMP first-order kinetic parameters (the maximum methane yield, B0, and the kinetic constant rate k), based on the analysis of a part of data collected from the experiment. Akaike and Bayesian information criteria were used to verify that the prevailing degradation kinetics is that of first-order, for many substrates. An algorithm was developed, providing good early estimates within a short time (4e10 days): in 92.5% of cases, the relative error of the final BMP estimate was found to be in the 1e13% range, with a relative Root Mean Squared Errors (rRMSE) of below 10%. Results suggest that it’s possible to shorten BMP test duration by leveraging data collected in the first part of the experiment

Evaluation of the anaerobic degradability of food waste collection bags made of paper and bioplastic

In Italy, the organic fraction is the most relevant among all the separately collected materials in the municipal solid waste. This project aims to analyse the environmental performances of the organic waste treatment chain, with a focus on the type of bags used for the collection. Currently, bioplastic bags are largely predominant in Italy, while paper bags account for a very small amount. In this research, the degradation potential and the corresponding biomethane production of bioplastic and paper bags were evaluated at the laboratory level, by means of BMP (Biochemical Methane Potential) tests. Then, to better simulate the operating conditions of full-scale digesters, semi-continuous co-digestion tests with the food waste were performed. The results of BMP tests under thermophilic conditions indicated a good degradability of both bioplastic and paper bags. On the contrary, the semi-continuous co-digestion tests showed significant differences between the two materials, with a limited degradation of bioplastic bags and a high compatibility of paper bags with the anaerobic digestion

ANALISI SPERIMENTALE SULLA DEGRADAZIONE ANAEROBICA DI SACCHETTI IN CARTA O IN BIOPLASTICA PER LA RACCOLTA DEL RIFIUTO ALIMENTARE

In Italia, la quantità di bioplastiche compostabili (conformi allo standard tecnico UNI EN 13432:2002) raccolte con il rifiuto organico è in costante crescita (+210% negli ultimi tre anni). La legislazione italiana prevede che questi manufatti debbano essere accettati da tutti gli impianti di trattamento biologico, siano essi aerobici o anaerobici. Tuttavia, lo standard richiede la sola valutazione della degradabilità aerobica, mentre non è generalmente necessario testare il comportamento in condizioni anaerobiche. Questo aspetto è analizzato nel presente articolo, con la valutazione della degradabilità anaerobica di uno specifico manufatto in bioplastica, ovvero i sacchetti utilizzati per la raccolta del rifiuto alimentare, il cui utilizzo è ampiamente diffuso in Italia. In dettaglio, sono state eseguite prove di biometanazione (BMP – Biochemical Methane Potential) in condizioni termofile su quattro tipologie di sacchetti in bioplastica, includendo sia quelli specificamente progettati per la raccolta del rifiuto alimentare sia gli shopper, che possono essere riutilizzati allo stesso scopo dopo il primo utilizzo per il trasporto della spesa. Le prove sono state effettuate confrontando il comportamento dei sacchetti in bioplastica con quello di un sacchetto in carta specificamente realizzato per la raccolta del rifiuto alimentare. I risultati delle prove indicano una buona degradabilità (>71%) dei sacchetti in bioplastica. Tuttavia, essi sono caratterizzati da particolari cinetiche di degradazione, con un andamento a gradini o una prolungata fase di latenza iniziale, che ne limitano la conversione in metano nel caso di alimentazione continua. Al contrario, prospettive molto interessanti sono offerte dal sacchetto in carta, che mostra, in aggiunta a una buona degradabilità anaerobica (74%), una cinetica di degradazione molto rapida.In Italy, the amount of compostable bioplastics (fulfilling the technical standard UNI EN 13432:2002) collected with the organic waste is constantly growing (+210% in the last three years). The Italian market of biodegradable and compostable bioplastics was originally driven by bags used for food waste collection, for shopping at the groceries and for containing loose foods like fruit and vegetables at the supermarket. In recent years there was a robust growth of the sector of other packages as well as rigid items such as disposable tableware. According to the Italian legislation, these items must be accepted by all biological treatment plants (both aerobic and anaerobic). Anyway, the standard UNI EN 13432:2002 requires only the assessment of the aerobic degradability, while it is generally not necessary to test the behaviour under anaerobic conditions. This aspect is in contrast with the increasing diffusion of anaerobic digestion plants observed in recent years. In particular, in Italy, about 2.9 million tonnes of food waste were sent to anaerobic digestion or integrated (anaerobic/aerobic) plants in the year 2019, compared to only 1.7 million tonnes sent to composting. For this reason, the present research was performed with the aim to assess the anaerobic degradability of bioplastic bags for the food waste collection (typically employed in Italy for this purpose) at the laboratory scale. In detail, BMP (Biochemical Methane Potential) tests were performed under thermophilic conditions on four commercial types of bioplastic bags, including both bags specifically designed for the food waste collection and shoppers, that can be reused for the same purpose after being used for the overall shop at the supermarkets. Both types are made with the Mater-Bi® polymer, a compostable bioplastic according to the UNI EN 13432:2002 standard, whose composition is 70% polybutylene adipate terephthalate, 20% starch, and 10% additives. The tests were performed comparing the behaviour of bioplastic bags to that of a paper bag specifically designed for the food waste collection. Results of BMP tests indicated a maximum anaerobic degradability for bioplastic bags in the range 71% – 93%, resulting for three out of the four tested bags higher than that of the paper bag, equal to 74%. As regards the kinetic behaviour, three out of the four bioplastic bags showed stepped degradation trends, similarly to what typically observed for the anaerobic degradation of starch-based biopolymers. The stepped trend was modelled by means of the combination of two Gompertz models. The degradation kinetic of the fourth bioplastic bag, described by means of a Gompertz model, was also peculiar, since it showed a lag phase lasting up to more than 10 days. As regards the examined paper bag, the kinetic described by a Gompertz model was very fast and characterised by a very short lag phase (about 1 day). The degradation kinetic modelling was useful to estimate the anaerobic degradability of bags when digested under continuous feeding conditions (i.e. conditions similar to the real management at full-scale anaerobic digestion plants). The particular degradation kinetics of bioplastic bags limit their conversion into methane in these conditions. In particular, assuming an anaerobic digestion process characterised by a hydraulic retention time of 21 days (a typical value in full-scale digesters treating the food waste under thermophilic conditions), the degradability (in the range 44% – 69%) resulted sensibly lower than that of the examined paper bag, equal to 70%, in contrast to what observed in terms of maximum degradabilities

Integration of sludge ozonation with anaerobic digestion: From batch testing to scenario analysis with energy, economic and environmental assessment

A methodological procedure, based on results from batch experiments, is proposed and applied to a selected wastewater treatment plant generating a poorly degradable sludge, to identify the best configuration and ozone dosage for full-scale application of sludge ozonation. Samples of pre-thickened and digested sludge were collected, tested at different ozone dosages and characterized to gather useful data for energy, economic and carbon footprint balances. The most viable scenario was found to be sludge pre-treatment at the lowest tested dosage (20 mg O3/g VS), yielding energy, cost and GHG emission net savings of 177 MWh/y, 57.8 k€/y and 6.38 Mg CO2-eq./y, respectively. Sensitivity analyses, conducted by varying the specific energy required for ozone generation and the unit costs for sludge disposal and resource supply, confirmed the stability of this scenario, whereas a field pilot-scale testing is advisable to verify modified process conditions for a safe and efficient application of sludge ozonation. The proposed methodology, including laboratory batch anaerobic digestion tests, scenario definition and energy/economic/environmental balances, could be preliminary applicable to all situations to broadly analyze all involved aspects and give a useful overview about the effective applicability of sludge ozonation

Sustainable Alternatives for Tertiary Treatment of Pulp and Paper Wastewater

In this work, different alternatives to conventional tertiary treatment of pulp and paper (P&P) wastewater (WW), i.e., physicochemical coagulation-flocculation, were investigated to enhance the environmental and economic sustainability of industrial wastewater treatment. In particular, following a preliminary characterization of secondary effluents, cloth filtration and adsorption were studied, the former by pilot-scale tests, while the latter at laboratory scale. An economic analysis was finally accomplished to verify the full-scale applicability of the most promising technologies. Cloth filtration showed excellent total suspended solids (TSS) removal efficiency (mean 81% removal) but a very limited influence on chemical oxygen demand (COD) (mean 10% removal) due to the prevalence of soluble COD on particulate COD. Adsorption, instead, led to a good COD removal efficiency (50% abatement at powdered activated carbon—PAC—dosage of 400 mg/L). The economic analysis proved that adsorption would be convenient only if a local low-cost (100  /ton) adsorbent supply chain was established. Ultrafiltration was considered as well as a potential alternative: its huge capital cost (19 M ) could be recovered in a relatively short timeframe (pay-back time of 4.7 years) if the ultrafiltrated effluent could be sold to local industries

Performance Analysis and Microbial Community Evolution of In Situ Biological Biogas Upgrading with Increasing H2/CO2 Ratio

The effect of the amount of hydrogen supplied for the in situ biological biogas upgrading was investigated by monitoring the process and evolution of the microbial community. Two parallel reactors, operated at 37°C for 211 days, were continuously fed with sewage sludge at a constant organic loading rate of 1.5 gCOD∙(L∙d)-1 and hydrogen (H2). The molar ratio of H2/CO2 was progressively increased from 0.5 : 1 to 7 : 1 to convert carbon dioxide (CO2) into biomethane via hydrogenotrophic methanogenesis. Changes in the biogas composition become statistically different above the stoichiometric H2/CO2 ratio (4 : 1). At a H2/CO2 ratio of 7 : 1, the methane content in the biogas reached 90%, without adversely affecting degradation of the organic matter. The possibility of selecting, adapting, and enriching the original biomass with target-oriented microorganisms able to biologically convert CO2 into methane was verified: high throughput sequencing of 16S rRNA gene revealed that hydrogenotrophic methanogens, belonging to Methanolinea and Methanobacterium genera, were dominant. Based on the outcomes of this study, further optimization and engineering of this process is feasible and needed as a means to boost energy recovery from sludge treatment

Selective microbial resolution of lupanine racemate: Bioprocess development and the impact of carbon catabolite repression on industrial wastewater valorisation

An environmentally friendly bioprocess for the valorisation of the lupanine enantiomeric mixture existing in lupin bean processing wastewater was developed. Pseudomonas putida LPK411, which is capable of enantioselectively biodegrading lupanine enantiomers, was employed for the resolution of the lupanine racemate content of unrefined and pretreated industrial effluents. The optimal culture conditions for racemic lupanine biodegradation by LPK411 were determined as 31 °C, pH 6–7, and 1.5 g L−1 initial lupanine concentration. The results obtained for enantioselective resolution of the effluents by P. putida LPK411, grown in shake-flasks, and a lab-scale bioreactor under batch operation, demonstrated that lupanine resolution was substantially improved in the bioreactor, exhibiting L-(–)-lupanine enantiomeric excess > 93% for all feedstocks used. Moreover, a fed-batch bioprocess was conducted using racemic lupanine resulting in 53% and 49% enhanced D-(+)-lupanine biodegradation and biomass production compared to the corresponding batch experiment respectively, while L-(–)-lupanine concentration increased by 49%. Monitoring the transcriptional kinetics of luh and crc genes employing industrial wastewater and the alkaloid racemate in synthetic media demonstrated that although expression from the lupanine catabolic route was rapidly induced upon supply of lupanine as a single substrate, LPK411 preferably utilised other carbon molecules of the real effluent over lupanine, suppressing the alkaloid’s catabolic pathway via the carbon catabolite repression regulatory system. The study exemplified the impact of a preferred compound on the main metabolic route of a bioprocess, demonstrating the importance of molecular interactions in biorefineries developed based on the mixture of substrates contained in renewable bioresources. Future research should aim at optimising and up-scaling the enantioselective biodegradation process proposed

Processes, applications and legislative framework for carbonized anaerobic digestate: Opportunities and bottlenecks. A critical review

Char is a valuable product obtained from thermochemical conversion processes of different biomass feedstocks, mainly pyrolysis and hydrothermal carbonization (HTC). In this work, anaerobic digestion (AD) integration with pyrolysis/HTC is critically reviewed, considering anaerobic digestates as feedstocks for char production. This virtuous interconnection can boost sustainable digestate valorization in the circular economy framework. Different substrates for AD are investigated, including sewage sludge, food waste, agricultural residues, and animal manure. The available thermochemical technologies, including pyrolysis, HTC and other processes are considered, analyzing the effects of substrate characteristics and process parameters on char quality. The possible fields of char application are successively presented, including agricultural application, energy recovery, pollutants adsorption, catalysts production, and electrochemical technologies; the advantages and drawbacks of each application are highlighted. Limitations still preventing the full-scale application of digestate-derived char production and utilization include the variability in substrate characteristics and the presence of undesired pollutants (especially in sewage sludge digestate), full-scale development of thermochemical plants, lacking legislative frameworks, uncertain economic sustainability, limited eco-toxicological studies, and stakeholders’ acceptance. Future research needed on the topic is finally depicted, with the aim of widening digestate reuse applications, as thermochemical processes may prevent safety concerns linked to direct agricultural reuse, leading to sustainable biorefinery platforms