Economic, energetic and environmental analysis of the waste management system of Reggio Calabria

The waste management and treatment process is illustrated with reference to a case study, concerning USW disposal system of Reggio Calabria. Particularly an economic, energetic and environmental analysis of the different process phases has been effected.

The society which manages the service handles waste collection, transport and sending to recovery, effecting a material selection treatment before their final disposal. 

2.1 Case study: USW disposal system OFC

Monthly urban solid waste production in the city and its amount of selected collection in 2015 (year in which the society started its collection service) are reported in Figure 1: it can be observed that the differentiated amount in 2015 was still low (equal to 17,5%).

Referring to EWC (Europe Waste Code) the collected quantities have been reported in Figure 2, pointing out the analysed components.

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Figure 1. USW collected mass and differentiated percentage

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Figure 2. Waste quantity with reference to their classification according to EWC

Figure 3 reports the waste journey from the collection site to the disposal (for undifferentiated and organic components) or selection (for multi-material and paper) ones and the subsequent journey of selected materials to recycling plants.

Non-all the waste typologies are sent to treatment: in the analysed year only 75% of the selected component (13,1% of total collected waste), formed by paper and cardboard, multi-material and organic has been treated.

In undifferentiated waste, a part has been sent to stabilization and the remaining part to landfill.

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Figure 3. Waste components travel from the collection site to the selection, disposal and recycling sites

2.2 Paper and cardboard

After being collected, the paper and cardboard fraction is sent to a selection plant in order to remove impurities through manual selection. A subsequent pressing gives the final cubic form of bales, allowing easy loading and transport.

2.3 Multi-material

Multi-material includes different fractions simultaneously collected: plastic, glass and metals (aluminium) that are separated in a selection plant; glass and plastic are prevailing in volume and weight.

After a first manual selection, a dimensional reduction is carried out through an oscillating sieve, removing materials with large or very small dimensions.

Undersize materials are directed on a tape where, after a second manual selection of residual voluminous fractions, light materials (cans and plastic) are aeraulically separated through an extractor hood and metals are captured by an over belt magnet. The hood directs materials to a cyclone, where they fall into a hopper and are sent to a press in bales, distinguished by material (containers, white bottles, colored bottles, cans, and so on).

After a further manual selection, the material is sent to recycling, plastics after a subdivision by polymers (PET, PE, etc.) and colour, carried out by optical readers, that allow to obtain highly pure materials.

2.4 Organic

The fraction of organic waste is transformed in high quality fertilizing through a composting process, a biological stabilization in solid phase taking place in aerobic conditions.

Organic waste follows a precise treatment sequence. After a compliance test, waste is pre-treated through crumbling and mixturing and undergoes a primary biologic treatment aimed to remove through sieving metals and plastic.

The mixture of wet and wood-cellulose wastes (these latter, with structure function, are used for particularly wet waste) goes to composting in reinforced concrete basins covered by holed tiles, where it remains for 21-23 days; a periodic blowing of purified air by a bio-filter guarantees idoneous growth of bacteria in bio-oxidation phase.

2.5 Undifferentiated waste

Undifferentiated waste can be sent to landfill or to incinerators/waste to energy plants. In the analysed case, such component is sent to a treatment/transference plant, where materials are selected before their disposal in landfill.

Energetic assessments have been carried out evaluating fuel consumption necessary for transport and electric energy consumption required to select and recycle multi-material components.

Every waste during transport and treatment generates CO₂ and other pollutant emissions, that have been determined using the related emission factor; as concerns CO₂, the total amount emitted through the whole process has also been evaluated.

4.1 Step 1. From the collection to the selection plants

4.1.1 Transport

Fuel consumption $F C_{i}$ (diesel, kg) necessary to transfer the i-th waste to the plant has been determined using the expression:

$E C_{i}=N_{i} \times d_{i} \times f c$        (3)

where:

– is the number of required trips

– is the plant distance (km)

– is the unitary fuel consumption (0,42 kg/km).

The corresponding energy consumption $E_{i}$ (kWh) during the transport (Table 6) is then given by:

$E_{i}=F C_{i} \times L C V$       (4)

where LCV diesel Low Calorific Value (11,86 kWh/kg).

Table 6. Transport consumption

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Moreover, j-th pollutant emissions generated during the transport of the i-th waste have been determined using the expression:

$e_{i j}=F C_{i} \times E F_{j}$      (5)

in which EF_j (g/kg) is the emission factor of the j-th pollutant [20], [21] (Table 7). The results are reported in Table 8.

Table 7. Emission factors of the main combustion products [20]

Table 8. Step 1 Transport emissions

4.1.2 Selection/disposal

Energy consumed by the selection of i-th waste $E_{i}$ has been evaluated through the expression:

$E_{i}=P_{i} \times t_{i} \times N_{d i}$     (6)

where

–  plant power (kW)

–  working time (h)

–  number of yearly working days.

Consequently, the j-th pollutant emissions generated by the selection of i-th waste are computed through the expression:

$e_{i j}=E_{i} \times E F_{j}$     (7)

with EF_j emission factor of the j-th pollutant (kg/kWh).

Using average values, we have made the hypothesis that plants are able to select 75 t/day.

In Table 9 energy consumption, emission factors and CO₂ emissions for multi material and paper selection are reported. Table 10 reports emission factors and Table 11 the related emissions of the main gases (CH₄ and CO₂) originating from undifferentiated and organic wastes, which do not undergo energetic treatments.

Table 9. Energy consumption and CO₂ emissions originating from multi material and paper selection

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It can be seen that very high emissions originate from undifferentiated waste, due to its quantity; the most relevant ones are ascribable to CH₄, but also CO₂ amount is noteworthy.

Table 10. Emission factors of the main landfill and organic gas emitted [22], [23] 

Table 11. Undifferentiated and organic components emissions

4.2 Step 2. From selection plants to recycling ones

4.2.1Transport

Fuel and energy consumption and the relative pollutant emissions have been calculated using the expressions described in 4.1.1. Table 12 reports the consumption values, Table 13 shows gas emissions, evaluated using emission factors reported in Table 7.

4.2.2 Recycling

The selected wastes undergo a recycling process. The plants unitary consumptions used in the analysis and the total energy consumption are reported in Table 14, CO₂ emissions are reported in Table 15.

Table 12. Transport consumption

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Table 13. Step 2 Transport emissions

Table 14. Energy consumption by recycled material

Table 15. Recycling CO₂ emissions

4.2.3 Saved energy

Saved energy by waste recycling has been evaluated as difference between energy consumed during production by raw material and from recycling.

Table 16. Production specific consumption

From Table 16, reporting specific consumption referring to production by raw material and recycling, it can be seen that the greatest energy consumption is associated to plastic production (45 kWh/t), with reference to which the greatest energy saving is obtained (30 kWh/t), followed by that of aluminium (ca. 15 kWh/t). In Table 17 and in Figure 4 the respective total energy consumptions are reported: due to the collected quantities, the greatest ones are observed for glass, the recycling of which allows saving only 2 kWh/t. Moreover, Table 17 reports total emissions avoidable through recycling.

Table 17. Total consumption referred to production by raw material and recycling and relative avoidable emission

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Figure 4. Energy consumption for material production

4.2.4 Total consumption and emissions

Table 18. Total energy consumption

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Table 19. Total CO₂ emissions

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Total consumption and CO₂ emissions referring to transport and treatment (Step 1 and 2) are reported in Tables 18 and 19. Concerning multi-material components in Step 1, they have been determined using the respective weight percentages.

Due to the involved quantities, the most relevant emissions are originated from undifferentiated waste.

4.2.5 Scenarios

The analysed case has been compared with different scenarios (Table 20) showing increasing percentage of differentiated collection (35% and 70%), relating them to the least advanced disposal modality, landfill. In Table 21 the total cost of waste management in the scenarios is reported. Moreover, Figures 6 and 7 respectively show the scenarios cost referring to waste components and phases of disposal Steps.

Table 20. Analysed scenarios

Table 21. Total cost of waste management for the scenario

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Figure 6. Cost of waste management scenarios referred to waste components

Particularly in Figure 7, referring to each phase of the two steps, also benefits can be represented, differently from Figure 6, which reports the final cost (or benefit) associated to each material disposal. It can be observed that, due to the recyclable material sales, increasing the percentages of differentiated collection can markedly reduce scenarios costs.

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Figure 7. Cost and benefit of waste management scenarios referred to the Steps phases

Table 22. Scenarios total energy consumption

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Figure 8. Scenarios energy consumption referred to waste components.

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Figure 9. Scenarios energy consumption referred to Steps.

In Table 22 and Figures 8-9 the scenarios total consumptions are reported: the figures respectively refer to waste components and disposal steps. As it is possible to observe, consumption markedly increase as the percentage of differentiated selection increases, but this is pre-eminently due to the distance of the glass recycling plant.

Finally, Table 23 and Figures 10-11 show the corresponding scenarios CO₂ emissions; the figures respectively refer to waste components and Steps phases

Table 23. Scenarios CO₂ emissions

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Figure 10. Scenarios CO₂ emissions referred to components.

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Figure 11. Scenarios CO₂ emissions referred to Steps phases.

It can be observed that, like cost, also CO₂ emissions reduce in advanced scenarios, thanks to the reduced quantities sent to landfill: for 70% differentiated selection, CO₂ amount becomes half the value observed in the analysed case (17,5%).