Energy, environmental and operation aspects of a SRF-fired fluidized bed waste-to-energy plant

Life cycle sustainability assessment of advanced treatment techniques for urban wastewater reuse and sewage sludge resource recovery

Wastewater treatment plants can become a source of valuable resources, such as clean water, energy, fuels and nutrients and thus contribute to the sustainable development goals and a transition to a circular economy. This can be achieved by adopting advanced wastewater and sludge treatment techniques. However, these have to be evaluated on their sustainability to avoid any unintentional consequences. Therefore, this paper presents a life cycle sustainability assessment of advanced wastewater and sludge treatment techniques by integrating the environmental, economic and social aspects. The options considered for advanced wastewater treatment are: i) granular activated carbon; ii) nanofiltration; iii) solar photo-Fenton; and iv) ozonation. The technologies for advanced sludge treatment are: i) agricultural application of anaerobically digested sludge; ii) agricultural application of composted sludge; iii) incineration; iv) pyrolysis; and v) wet air oxidation. The results for the advanced wastewater treatment techniques demonstrate that nanofiltration is the most sustainable option if all the sustainability aspects are considered equally important. If, however, a higher preference is given to the economic aspect, ozonation and granular activated carbon would both be comparable to nanofiltration; if the social aspect is considered more important, only activated carbon would be comparable to nanofiltration. Among the sludge treatment methods, agricultural application of sludge is the most sustainable technique for mean-to-high resource recovery. If the recovery rate is lower, this option is comparable with incineration and pyrolysis with high recovery of their respective products. This work helps to identify the most sustainable techniques that could be combined with conventional wastewater treatments for promoting wastewater reuse and resource recovery across a wide range of operating parameters and products outputs. The findings also support the notion that more sustainable wastewater treatment could be achieved by a circular use of water, energy and nutrients contained in urban wastewaters.

Comparing the quantity and quality of glass, metals, and minerals present in waste incineration bottom ashes from a fluidized bed and a grate incinerator

Bottom ash is the primary solid residue arising from municipal solid waste incineration. It consists of valuable materials such as minerals, metals and glass. Recovering these materials from bottom ash becomes evident when integrating Waste-to-Energy within the circular economy strategy. To assess the recycling potential from bottom ash, detailed knowledge of its characteristics and composition is required. The study at hand aims to compare the quantity and quality of recyclable materials present in bottom ash from a fluidized bed combustion plant and a grate incinerator, both located in the same city in Austria and receiving mainly municipal solid waste. The investigated properties of the bottom ash are grain-size distribution, contents of recyclable metals, glass, and minerals in different grain size fractions, and the total and leaching contents of substances in minerals. The study results reveal that most recyclables present are of better quality for the bottom ash arising at the fluidized bed combustion plant. Metals are less corroded, glass contains fewer impurities, minerals contain fewer heavy metals, and their leaching behavior is also favorable. Furthermore, recoverable materials, such as metals and glass are more isolated and not incorporated into agglomerates as observed in grate incineration bottom ash. Based on the input to the incinerators more aluminum and significantly more glass can potentially be recovered from bottom ash from fluidized bed combustion. On the downside, fluidized bed combustion produces about five times more fly ash per unit of waste incinerated, which is currently disposed of in landfills.

Industrial tests of co-combustion of alternative fuel with hard coal in a stoker boiler

This paper presents the results of industrial research on co-combustion of solid recovered fuel (SRF) with hard coal in a stoker boiler type WR-25. The share of SRF in the fuel mixture was 10%. During the co-combustion of SRF, no technological disturbances of the boiler were noted. The obtained SO₂ and NO_X emissions were comparable with coal combustion, but dust emissions increased. During the co-combustion of the coal mixture with 10% of alternative fuel, acceptable standards for co-incineration of waste were exceeded for NOx, dust, CO, HCl, HF, heavy metals, dioxins, and furans. The by-products of waste co-combustion with coal were non-hazardous waste. The obtained results constitute a very important contribution to the process of boiler retrofitting toward a waste co-incineration unit, and to meeting the legislative and environmental requirements.Implications: Due to some challenges related to waste storage and transportation, combustion in incineration plants and Waste-to-Energy plants is not possible. The adaptation (formal and technical) of medium scale boilers as co-incineration plants reveals high potential. Nevertheless, the lack of experience and investigations of waste co-combustion in real industrial scale grate boilers is observed. Thus, the implication of this article results consists of the investigations using industrial scale mechanical grate boiler (different from incineration type). Moreover, the investigations were carried out in a low-capacity boiler (~50% of nominal capacity). This novel experience is very important because reduced heat dissipation into the grid caused by high ambient temperatures occurs very frequently. These tests are valuable from the point of view of retrofitting the unit to obtain technological and emission parameters that would allow obtaining the status of a waste co-incinerating unit. The results of these investigations are addressed to power plant management board and engineering staff.

DPSIR Model Applied to the Remediation of Contaminated Sites. A Case Study: Mar Piccolo of Taranto

The study critically analyses the complex situation of the Mar Piccolo of Taranto (South of Italy), considered one of the most polluted marine ecosystems in Europe. In order to investigate possible cause–effect relationships, useful to plan appropriate planning responses or remediation technologies to be adopted, the Driver–Pressure–State–Impact–Response (DPSIR) model was applied. Methodologically, about 100 references have been considered, whose information was organized according to the logical scheme of the DPSIR. The results showed how the Mar Piccolo is the final receptor of pollutants coming from all industrial and agricultural activities, especially due to its natural hydrogeological network conformation. The anthropic activity represents a critical impact on the ecosystem due to the subsequent marine litter. The mobility of contaminants from sediments to the water column showed the potential risk related to the bioaccumulation of organisms from different trophic levels, posing a threat of unacceptable magnitude to human safety. The paper concludes by discussing the actions currently implemented by the authorities in response to the anthropogenic impacts as well as the need for new ones concerning both plans, programs, and remediation interventions. The case study shows how the DPSIR is a useful framework to organize extensive and heterogeneous information about a complex environmental system, such as the one investigated. This preliminary organization of the available data can represent the starting point for the development of a DPSIR-based Environmental Decision Support System (EDSS) with robust cause–effect relationships.

Experimental Study of Entrainment and Mixing of Renewable Active Particles in Fluidized Beds

Fluidized bed combustors were initially designed and built basically for the utilization of fossil fuels, mostly coal. The actual worldwide trend of transitioning from fossil fuels to renewables requires sufficient knowledge on the fluid mechanics of these new particle types because of the significant differences in their shapes, sizes, densities, and homogeneities. This article presents experimental results on the particle entrainment and mixing of some industrially relevant fuels such as solid refused fuel/refuse derived fuel (SRF/RDF), bark, sunflower shell, and wheat shell. The measurements were performed on a lab-scale fluidized bed experimental facility. The results show that sunflower shell is entrained in the highest degree; however, at very low velocity, the entrainment of wheat shell is the most intensive. The entrainment behaviors of the investigated SRF and bark samples are similar. On the other hand, the mixing results showed that the SRF has relatively high mass fractions in the bottom and centeral regions of the fluidized bed at low superficial velocities, while at elevated velocities, the entire mass of this fuel is shifted upwards. Interestingly, just the opposite tendency can be observed in cases of all other investigated biomass fuels. Finally, the nonspherical renewable active particles have markedly higher concentrations in the bottom region of the bed compared to spherical ones.

Sustainable design of large wastewater treatment plants considering multi-criteria decision analysis and stakeholders' involvement

The typical treatment scheme of a large municipal wastewater treatment plant (LWWTP) is almost always the result of design based on technical and economic criteria. Once a threshold in terms of population equivalent (PE) is reached, it is possible that additional sludge thermal treatment might be required. Aspects such as greenhouse gas (GHG) emissions and land use for the construction of the WWTP or the service landfill are considered marginal in current design practice; in a world that requires increasingly attention to the environment, these criteria cannot be ignored when defining the treatment scheme of a LWWTP. With the intent of providing a sustainable approach to design, this study aims to identify the best treatment scheme for a LWWTP with a 720,000 PE size. Methodologically, the study involves the development of an approach based on multi-criteria decision analysis (MCDA). Six alternative treatment schemes were considered; two simplified schemes, without primary sedimentation, with extended aeration activated sludge processes and aerobic sludge stabilization; four full schemes, with primary sedimentation and anaerobic sludge digestion. Some schemes differ for the organic loading rate applied; others for the use of sludge incineration. Subsequently, six evaluation criteria (ECs) such as GHG emissions, electricity consumption, running costs, WWTP planimetric area, surface for the service landfill, as well as WWTP as biorefinery have been considered. The weighting of the ECs involved the participation of the main stakeholders in such a decision-making process, following a bottom-up approach. The resolution of the MCDA problem allowed the identification of the full scheme based on primary sedimentation, biological activated sludge at low organic load (0.210 kgBOD₅/kgVSS/d) and anaerobic sludge digestion as the best solution. The sensitivity analysis, able to indirectly take into account the multitude of decision makers involved, allowed corroborating the results. The obtained treatment scheme was different from that generally adopted in current design practice for LWWTPs.

Alternating pure oxygen and air cycles for the biostabilization of unsorted fraction of municipal solid waste

Biostabilisation is a process of treating the unsorted fraction of municipal solid waste (UFMSW) mechanically pre-treated. Although concepts such as circular economy would seem to limit biostabilization, several authors have recently described the advantages of biostabilization in regions where recycling systems are inadequate. In this perspective, the development of new MBT technologies is of considerable importance. The objective of the study was to evaluate the effects of the use of alternating air and oxygen cycles on the treated waste stability as well as on the quality of leachate and process gaseous emissions. Two Herhof biocells were prepared for this purpose. One implemented the conventional process and the other the "Air + O₂" process. The biostabilization of the inlet UFMSW (3965 ± 1965 mgO₂/kgVS/h) resulted in a final product with a dynamic respirometric index almost equal in both processes. The mass balance indicated that of the 400 tons representing the input waste, 37.57% were biostabilized waste, 0.29% leachate and 62.14% CO₂ and odours. However, the biostabilized waste was lower than that of the conventional process (equal to 40.18%). The Air + O₂ system resulted in a shorter duration, increased production of leachate (although characterized by higher quality) and process gaseous emissions quality. The energy balance (20.3 kJ/kg per input waste) and cost analysis (80.0 €/ton per input waste) showed values equal or better to those of the conventional system. By contrast, weakness was in the O₂ diffusion system. Although a life cycle analysis is necessary, the results highlighted the feasibility of the proposal especially for emergency situations.