Upgrading of a mechanical biological treatment plant with a solid anaerobic digestion batch: a real case study

Effect of green waste and lime amendments on biostabilisation, physical-chemical and microbial properties of the composted fine fraction of residual municipal solid waste

Implementation of guidelines to reduce the amount of biodegradable municipal waste (BMW) sent to landfill has created a need in the waste-management industry to investigate possible methods of accelerating biostabilisation of residual BMW. The effect of commercially feasible manipulations (lime and green waste (GW)) on the rate of biostabilisation of the fine (<20 mm) fraction of residual BMW was investigated. The physical and chemical attributes of the composted wastes were measured, and their bacterial communities profiled using traditional culture-based methods. In addition, ammonia-oxidising microbes were monitored during the biostabilisation process using molecular profiling methods. Addition of GW accelerated biostabilisation, reduced conductivity and increased the levels of ammonia-oxidising bacterial (AOB) and archaeal (AOA) genes. The best stability was noted in the dual (Lime + GW) treatment, which was under the limit of 13 mmol O₂ kg DM^-1 h^-1 recommended by the Irish compost standard. Biostabilised wastes met recommendations for source-segregated compost for pH (6-8) and pathogens (E. coli and Salmonella), but not heavy metals, indicating their unsuitability for uses other than landfill cover. Levels of AOA genes (log 3-6 g^-1 DM) were higher than AOB (log 1-6 g^-1 DM, indicating AOA may contribute more to potential ammonia oxidation in residual BMW composting.

Minimization of spreading of SARS-CoV-2 via household waste produced by subjects affected by COVID-19 or in quarantine

Currently available evidence supports that the predominant route of human-to-human transmission of the SARS-CoV-2 is through respiratory droplets. Indirect hands contact with surfaces contaminated by infectious droplets subsequently touching the mouth, nose or eyes seems to be another route of an indirect contact transmission. Persistence of the virus on different surfaces and other materials has been reported in recent studies: SARS-CoV-2 was more stable on plastic and stainless steel than on copper and cardboard. Viable virus was detected up to 72 h after application to different surfaces, although infectivity decay was also observed. This evidence suggests the likelihood that waste generated from patients affected by COVID-19 or subjects in quarantine treated in private houses or in areas different from hospitals and medical centres could be contaminated by SARS-CoV-2. Consequently, waste streams may represent a route for viral spreading being a potential risk also for the operators directly involved in the different phases of waste management. To address this concern, a specific multidisciplinary working group was settled by the Italian National Institute of Health (ISS) during the COVID-19 emergency, in order to establish guidelines related to solid waste collection, delivering, withdrawal, transport, treatment and disposal. Temporary stop of waste sorting, instructions for the population on how to package waste, instructions for Companies and operators for the adoption of adequate personal protection equipment (PPE), the use and sanitation of proper vehicles were among the main recommendations provided to the community by publications of freely downloadable reports and infographics in layman language. Incineration, sterilization and properly managed landfills were identified as the facilities to be preferentially adopted for the treatment of this kind of waste, considering the main inactivation strategies of SARS-CoV-2 (e.g. treatment length > 9 days and temperature > 70 °C for more than 5 min).

Management of the biodegradable fraction of residual waste by bioreactor landfill

The performances of an integrated system based on mechanical biological treatment and bioreactor landfill with leachate recirculation for managing the mixed municipal solid waste generated in a given Italian district were investigated. In the mechanical biological treatment the municipal solid waste was mechanically sorted into two main streams: a dry and a mechanically sorted organic fraction consisting of 45,000 tonnes year^-1. After being sorted the mechanically sorted organic fraction was aerobically pretreated before being disposed of in the 450,000 m³ bioreactor landfill. Experimental runs showed that an aerobic pretreatment period ranging from 15 to 30 days was able to maximize the methane generated by the mechanically sorted organic fraction once landfilled up to 10 Nm³ tonne^-1. The aerobic pretreatment leads to a significant volatile solids reduction in the first 30 days, after which the volatile solids concentration remained quite constant. Similarly the potential dynamic respirometer index was significantly reduced in the first 15 days of the aerobic pretreatment decreasing from about 5,000 to about 3,500 mgO₂kgVS^-1h^-1. The whole amount of electrical energy producible by the landfill ranged from 18.5 kWh tonne^-1 to 21 kWh tonne^-1, depending on the strategies adopted for the activation in bioreactor mode of each landfill cell by the leachate recirculation.

Effectiveness of municipal solid waste incinerators in replacing other fuels. A primary energy balance approach for the EU28

The latest European Union legislation introduced the possibility of considering high efficiency incineration of waste as a recovery operation, that is, to use waste as a means of producing energy and hence able to replace other fuels. This possibility has been further investigated by expanding the boundaries for the mass and energy balance of municipal solid waste incinerators operating in the EU28. An energetic analysis based on a hybrid primary energy (MJ Mg^-1) approach was performed also using the cumulative energy demand index. Average results showed a net hybrid primary energy >0 for those municipal solid waste incinerators recovering only electricity, indicating that no primary energies can be replaced. For those operating in combined heat and power mode, an average hybrid primary energy ranging from about -200 MJ Mg^-1 to about -4800 MJ Mg^-1 was detected for large-size municipal solid waste incineration facilities (>200,000 Mg y^-1). The value of hybrid primary energy for medium and small facilities ranged from about +3000 MJ Mg^-1 to -4000 MJ Mg^-1. Furthermore, in some operating conditions landfill had a lower hybrid primary energy than those of small-size municipal solid waste incinerators. To some degree, these results are not in agreement with the classification of municipal solid waste incinerators based on the energy efficiency formula, particularly for those recovering only electrical energy.

Treatment of mechanically sorted organic waste by bioreactor landfill: Experimental results and preliminary comparative impact assessment with biostabilization and conventional landfill

Treatment and disposal of the mechanically sorted organic fraction (MSOF) of municipal solid waste using a full-scale hybrid bioreactor landfill was experimentally analyzed. A preliminary life cycle assessment was used to compare the hybrid bioreactor landfill with the conventional scheme based on aerobic biostabilization plus landfill. The main findings showed that hybrid bioreactor landfill was able to achieve a dynamic respiration index (DRI)<1000 mgO2/(kgVSh) in 20weeks, on average. Landfill gas (LFG) generation with CH4 concentration >55% v/v started within 140days from MSOF disposal, allowing prompt energy recovery and higher collection efficiency. With the exception of fresh water eutrophication with the bioreactor scenario there was a reduction of the impact categories by about 30% compared to the conventional scheme. Such environmental improvement was mainly a consequence of the reduction of direct and indirect emissions from conventional aerobic biostabilization and of the lower amount of gaseous loses from the bioreactor landfill.

Impact of biological treatments of bio-waste for nutrients, energy and bio-methane recovery in a life cycle perspective

Composting of the source-segregated organic fraction of municipal solid waste was compared in a life cycle perspective with conventional anaerobic digestion (AD), aimed at electricity substitution, and with AD aimed at biogas upgrading into bio-methane. Three different uses of the bio-methane were considered: injection in the natural gas grid for civil heating needs; use as fuel for high efficiency co-generation; use as fuel for vehicles. Scenarios with biogas upgrading showed quite similar impact values, generally higher than those of composting and conventional AD, for which there was a lower impact. A decisive contribution to the higher impact of the scenarios with bio-methane production was by the process for biogas upgrading. In any case the substitution of natural gas with bio-methane resulted in higher avoided impacts compared to electricity substitution by conventional AD. The uncertainty analysis confirmed the positive values for eutrophication, acidification and particulate matter. Large uncertainty was determined for global warming and photochemical ozone formation.

Improvement of the management of residual waste in areas without thermal treatment facilities: A life cycle analysis of an Italian management district

Starting from an existing waste management district without thermal treatment facilities, two different management scenarios for residual waste were compared by life cycle assessment (LCA). The adoption of a bioreactor landfill for managing the mechanically sorted organic fraction instead of bio-stabilization led to reduction of global warming and fresh water eutrophication by 50% and 10%, respectively. Extraction of recyclables from residual waste led to avoided emissions for particulate matter, acidification and resource depletion impact categories. Marginal energy and the amount of energy recovered from landfill gas marginally affected the LCA results. On the contrary the quality of the recyclables extracted can significantly modify the eco profile of the management schemes.

Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste

The paper investigates, at a laboratory scale, the applicability of anaerobic digestion for the treatment of pressed-off leachate produced in a biomechanical treatment plant for municipal solid waste. Batch tests show that the anaerobic process proceeds smoothly and produces about 10,000 mL of methane per litre of treated leachate. The process is characterized by a lag phase lasting about 30 days, and is completed in about 2 months. Chemical oxygen demand (COD) and volatile fatty acids monitoring allows studying process kinetics that are modelled through a triple linear expression. Physical and biological treatments are also investigated to reduce the residual organic charge of the produced digestate. The best performances are obtained via aerobic degradation followed by assisted sedimentation. This cycle reduces the residual COD of about 85%, and allows the correct disposal of the final waste stream.

Life cycle analysis of management options for organic waste collected in an urban area

Different options for managing the organic fraction (OF) of municipal solid waste generated in a given urban area were analyzed by life cycle assessment (LCA) for different source segregation (SS) intensities ranging from 0 to 52%. The best management option for processing the OF remaining in the residual organic fraction (ROF) for the different SS intensities was by incineration. Landfilling and mechanical biological treatment (MBT) of ROF gave higher impacts. Aerobic treatment alone or combined with anaerobic digestion (AD) for processing the source-segregated organic fraction (SSOF) led to relevant environmental impact reduction even if the difference between the two options was quite negligible. The weighted impact showed that scenarios using incineration always gave environmental gains, whereas there was a higher environmental burden with the scenarios using MBT.

A holistic life cycle analysis of waste management scenarios at increasing source segregation intensity: the case of an Italian urban area

Life cycle analysis of several waste management scenarios for an Italian urban area was performed on the basis of different source segregation collection (SS) intensities from 0% up to 52%. Source segregated waste was recycled and or/recovered by composting. Residual waste management options were by landfilling, incineration with energy recovery or solid recovered fuel (SRF) production to substitute for coal. The increase in fuel and materials consumption due to increase in SS had negligible influence on the environmental impact of the system. Recycling operations such as incineration and SRF were always advantageous for impact reduction. There was lower impact for an SS of 52% even though the difference with the SS intensity of 35% was quite limited, about 15%. In all the configurations analyzed, the best environmental performance was achieved for the management system producing SRF by the biodrying process.

What is the acceptable margin of error for the oxygen uptake method in evaluating the reactivity of organic waste?

The acceptable margin of error for the organic waste reactivity measured by the oxygen uptake method was assessed. Oxygen uptake was determined by the Dynamic Respiration Index (DRI) (mgO2/kgVS h). The composed uncertainty (uC) of the experimental set up used for the DRI test was evaluated and the uncertainty (u) of all the components of the apparatus was evaluated. A procedure for calculating the uC of the apparatus is proposed. The components affecting the uC of the DRI to a more significant extent were the one of the oxygen mass rate and the u of the amount of VS in the sample analyzed. For a confidence level of 99.73%, the extended uC (UC) interval for a DRI = 1024 mgO2/kgVS h was ± 440 mgO2/kgVS h, whereas for a DRI = 3,489 mgO2/kgVS h, the UC interval was ± 1288 mgO2/kgVS h. When oxygen consumption and VS content become lower than 600 mgO2/h and 0.9 kg, respectively, the UC interval is similar to the measured DRI.

Urban mining: quality and quantity of recyclable and recoverable material mechanically and physically extractable from residual waste

The mechanically sorted dry fraction (MSDF) and Fines (<20mm) arising from the mechanical biological treatment of residual municipal solid waste (RMSW) contains respectively about 11% w/w each of recyclable and recoverable materials. Processing a large sample of MSDF in an existing full-scale mechanical sorting facility equipped with near infrared and 2-3 dimensional selectors led to the extraction of about 6% w/w of recyclables with respect to the RMSW weight. Maximum selection efficiency was achieved for metals, about 98% w/w, whereas it was lower for Waste Electrical and Electronic Equipment (WEEE), about 2% w/w. After a simulated lab scale soil washing treatment it was possible to extract about 2% w/w of inert exploitable substances recoverable as construction materials, with respect to the amount of RMSW. The passing curve showed that inert materials were mainly sand with a particle size ranging from 0.063 to 2mm. Leaching tests showed quite low heavy metal concentrations with the exception of the particles retained by the 0.5mm sieve. A minimum pollutant concentration was in the leachate from the 10 and 20mm particle size fractions.

Experimental and life cycle assessment analysis of gas emission from mechanically-biologically pretreated waste in a landfill with energy recovery

The global gaseous emissions produced by landfilling the Mechanically Sorted Organic Fraction (MSOF) with different weeks of Mechanical Biological Treatment (MBT) was evaluated for an existing waste management system. One MBT facility and a landfill with internal combustion engines fuelled by the landfill gas for electrical energy production operate in the waste management system considered. An experimental apparatus was used to simulate 0, 4, 8 and 16weeks of aerobic stabilization and the consequent biogas potential (Nl/kg) of a large sample of MSOF withdrawn from the full-scale MBT. Stabilization achieved by the waste was evaluated by dynamic oxygen uptake and fermentation tests. Good correlation coefficients (R(2)), ranging from 0.7668 to 0.9772, were found between oxygen uptake, fermentation and anaerobic test values. On the basis of the results of several anaerobic tests, the methane production rate k (year(-1)) was evaluated. k ranged from 0.436 to 0.308year(-1) and the bio-methane potential from 37 to 12Nm(3)/tonne, respectively, for the MSOF with 0 and 16weeks of treatment. Energy recovery from landfill gas ranged from about 11 to 90kWh per tonne of disposed MSOF depending on the different scenario investigated. Life cycle analysis showed that the scenario with 0weeks of pre-treatment has the highest weighted global impact even if opposite results were obtained with respect to the single impact criteria. MSOF pre-treatment periods longer than 4weeks showed rather negligible variation in the global impact of system emissions.

Leachate purification of mechanically sorted organic waste in a simulated bioreactor landfill

The bioreactor environment for the mechanically sorted organic fraction (MSOF) of residual municipal solid waste was simulated for a period of 300 days. A closed-loop system was implemented for analysing the leachate purification process due to its recirculation through MSOF. Maximum concentrations of Cu, Zn and Ni in the leachate were about 60, 20 and 15 mg L(-1), whereas Pb and Cr were about 5.5 and 7 mg L(-1). About 60 days from the start of recirculation these concentrations dropped to < 1 mg L(-1). Chemical (COD) and biological oxygen demand (BOD5) dropped, respectively, by about 50 and 80%, achieving a COD-to-BOD5 ratio > 7. Volatile fatty acids, which were about 10 g L(-1), fell to about 3.5 g L(-1) whereas biomethane production was about 34 NL kg TS(-1). As expected, recirculation under strictly anaerobic conditions gave minor benefits in reducing nitrogen and ammonium which reached final concentrations of about 4 and 3.8 g L(-1), respectively.