Comparative study of different sewage sludge incineration treatments based on environmental and economic life cycle assessment

Abstract

Incineration is one of the most widely used treatments in the field of sewage sludge disposal. However, the choice of sewage sludge incineration process is still controversial. In this study, the comparative life cycle assessment of sewage sludge incineration processes, including the mono-incineration, co-incineration in coal-fired power plants and co-incineration in municipal solid waste (MSW) incineration plants, was carried out from the perspective of environment, carbon footprint and economy. The environmental assessment results show that terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity and human non-carcinogenic toxicity are the most significant environmental impacts. And the environmental performance of co-incineration in coal-fired power plants is the best. Moreover, the environmental impact is most sensitive to the dehydrant, electricity and fly ash chelating agent. Co-incineration in MSW incineration plants has the lowest carbon emissions, with only 70.50% and 82% of the carbon emissions from mono-incineration and co-incineration in coal-fired power plants, respectively. Furthermore, sewage sludge mono-incineration has the highest disposal costs because of the higher depreciation and solid waste disposal costs. The comprehensive evaluation results reveal that the optimization should focus on the selection of dehydrant and fly ash chelating agent, as well as the improvement of the equipment efficiency.

Keywords

Sewage sludge incineration co-incineration life cycle assessment carbon footprint comparative study

Introduction

Sewage sludge is the coproduct of the sewage treatment process in wastewater treatment plants (WWTPs), containing a lot of heavy metals, organic pollutants and pathogens (Xu et al., 2014). Improper disposal of sewage sludge will cause serious air, soil and water pollution (Rostami et al., 2020). However, the disposal problem of sewage sludge is more aggravated with the rapid development of economy, urbanization and industrialization (Ding et al., 2021). Thus, sewage sludge has become a growing environmental problem worldwide.

The goal of sewage sludge disposal is reduction, stabilization, harmlessness and resource (Yang et al., 2015). Conventional methods for sewage sludge disposal include landfilling, composting, anaerobic digestion, incineration and some new processes (e.g. wet-oxidation, pyrolysis and earthworm breeding) (Zhang et al., 2014; Zhou et al., 2022). Landfilling has been gradually prohibited due to serious pollution problems among the sewage sludge disposal methods (Hossain et al., 2020). Composting and anaerobic digestion are the most commonly used methods, with the former adopted in 20 countries and the latter in 24 countries in the European Union (EU; Havukainen et al., 2022). In the EU, the extent of landfilling as a sewage sludge disposal method declined from 15% in 2005 to 7% in 2015 and that of agricultural application decreased from 43% in 2005 to 28% in 2015, while the proportion of sewage sludge incineration increased from 21% to 38% during the corresponding period (Teoh and Li, 2020). Incineration can significantly reduce the volume/mass of sewage sludge, which is important in densely populated areas with scarce land resources (Schnell et al., 2020). Moreover, incineration can also decompose toxic organic compounds and realize energy recovery (Hu et al., 2015).

Sewage sludge incineration can be carried out independently in a newly built incinerator. Meanwhile, sewage sludge can also be incinerated with other existent energy equipment (coal-fired power plants, municipal solid wastes (MSWs) incineration plants, etc.), but the impact on the stability, economy and pollutant emission of the original equipment must be considered (Cieslik et al., 2015). The sewage sludge co-incineration process has been encouraged in some countries (Chen et al., 2020). MSW incineration plants have the complete flue gas treatment devices, which can achieve co-incineration with sewage sludge directly after proper technical transformation (Sun et al., 2020). Co-incineration in coal-fired powers plants can be done with simple modifications. However, the flue gas treatments of coal-fired power plants are generally not considered for dioxin emissions due to the absence of activated carbon injection and the possibility of high-temperature heating surface corrosion problems because of chlorine in the sewage sludge. The efficiency of the boiler can also be adversely affected when the sewage sludge is blended in larger quantities. Due to the limitation of processing capacity, the co-incineration process cannot meet the demand of sewage sludge disposal.

Life cycle assessment (LCA) is a method to compile and evaluate the potential environmental impact of the product system or process based on the input and output during the whole life cycle (Li et al., 2013). Standardized methodologies, such as ISO 14044, have made LCA an important reference tool for achieving sustainable development goals (Blanco et al., 2022). The method was usually used to evaluate and compare different treatment processes of wastewater in the wastewater treatments plants (Gourdet et al., 2017). Moreover, some evaluations have been performed in the field of sewage sludge incineration. The LCA results between sewage sludge co-incineration in coal-based power plants and separate coal combustion in power plants indicated that co-incineration process caused greater environmental load than that of coal-based energy production, but co-incineration process presented higher economic benefits (Hong et al., 2013). The comparative LCA of mono-incineration of sewage sludge and MSW, co-incineration of sewage sludge and MSW indicated that the most significant environmental impacts were non-carcinogens, ozone layer depletion, terrestrial ecotoxicity and global warming potential. Moreover, the co-incineration of sewage sludge and MSW has the lowest environmental load and the best economic benefit (Chen et al., 2019). The evaluation of sewage sludge incineration in the fluidized bed and cement kiln revealed that the incineration in fluidized bed has the better environmental performance in most environmental impact categories (Abusoglu et al., 2017). The sewage sludge mono-incineration process appeared to the worst environmental, energy and economic performance among the four processes including mono-incineration, co-incineration in MSW incineration plants, coal-fired power plants and cement kiln (Xiao et al., 2022). However, the life cycle inventory (LCI), including heavy metal leaching toxicity of fly ash and residue, flue gas treatment materials, fly ash chelating agents and assignment problems of materials and pollutants during the co-incineration process, was not considered comprehensively in the above researches. And the environmental impact of drying stage is not involved. In addition, many countries are committed to carbon neutrality with the growing concern about global warming (Zhang et al., 2022). Carbon footprint research is an effective method to measure the greenhouse effect potential of the process. Carbon footprint research of WWTPs was mainly concerned currently. However, carbon emissions of sewage sludge disposal have been regrettably neglected (Chai et al., 2015; Wu et al., 2022). Furthermore, while previous studies used evaluation methods such as CML 2001 and Impact 2002+, this study adopted the ReCiPe 2016 evaluation method with richer midpoint environmental impact categories, and conducted a more comprehensive sensitivity analysis.

The aim of the study was to choose a reasonable, efficient and environment-friendly sewage sludge incineration treatment method, and provide the reference for the optimization direction of sewage sludge incineration process. Therefore, a comparative study between sewage sludge mono-incineration and co-incineration was conducted from the perspectives of environment, carbon footprint and economy. In consideration of the large fluctuation of sewage sludge composition, the sensitivity analysis was conducted to provide reference for the choice and optimization of sewage sludge incineration processes.

Materials and methods

The environmental, carbon footprint and economic assessments of different sewage sludge incineration processes were conducted based on LCA, a method for systematic assessment of processes or products according to ISO14040 and ISO14044 standards (Martin et al., 2020). The assessment procedure comprises four steps, including goal and scope definition, inventory analysis, life cycle impact assessment and life cycle interpretation (Cankaya and Pekey, 2019).

Goal and scope definition

This study mainly evaluated the environmental, carbon footprint and economic performance of three sewage sludge incineration treatments, including mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants. The system boundary of sewage sludge incineration processes is shown in Figure 1, from the primary sewage sludge produced in WWTPs to the ash residue disposal after incineration, including all materials inputs and pollutants outputs. Furthermore, the environmental impact of infrastructure was not considered in this article due to the large processing capacity and long service life (Hu et al., 2020). The functional unit (FU) is 1 tonne sewage sludge (80% moisture content). All input and output data collections were based on the defined system boundary and FU.

Case 1: The sewage sludge mono-incineration process mainly comprises four stages, including dewatering, drying, incineration and ash residue treatment stages. Dewatering process is generally carried out in the WWTPs. First, the primary sewage sludge is mixed with dehydrants which consist of ferric chloride and calcium oxide. Then, the sewage sludge will be sent to the filter press for mechanical squeezing dewatering. Meanwhile, the filter pressing wastewater is directly sent back to the WWTPs for further treatment. Thus, the pollution emission of wastewater is not considered. The dewatered sewage sludge will be transported to the incineration plant, and delivered to the dryer for heat exchange with the high-temperature working fluid generated by sewage sludge incineration. The evaporated water from the sewage sludge will be de-dusted and condensed into wastewater, which is discharged after meeting the discharge standard, while the non-condensable gas will enter the furnace with the secondary air for incineration. Finally, the dried sewage sludge enters the incinerator through the feeding device to produce the high-temperature working fluid. Low-temperature flue gas after heat exchange shall be discharged after reaching the emission standard by desulfurization, denitration, activated carbon adsorption and electric precipitation.

Case 2: The sewage sludge co-incineration in coal-fired power plants can be performed on the existing device after equipping the pre-treatment facilities and modifying the boiler fuel system. Meanwhile, deodorization and other environmental protection facilities need to be built. The co-incineration in coal-fired power plants also consists of four stages, including dewatering, drying, co-incineration and ash residue treatment stages. The mechanical dewatering process is consistent with that of the mono-incineration process. Dewatered sludge is transported to coal-fired power plants and then thermally dried. The incineration and pollutant emissions shall comply with the relevant standards of coal-fired power plants. The co-incineration with a small amount of sewage sludge has no significant impact on the normal operation of coal-fired power plants.

Case 3: The sewage sludge co-incineration in MSW incineration plants utilizes the incinerator and flue gas treatment equipment of MSW incineration plant to dispose the sewage sludge by direct feeding. Compared with the conventional MSW incineration plants, the sewage sludge co-incineration process mainly increases the pre-treatment equipment (e.g. transportation, storage and drying). The sewage sludge co-incineration in MSW incineration plants also can be divided into four stages, including dewatering, drying, co-incineration and ash residue treatment stages. In addition, the element analysis of sewage sludge is relatively similar to that of MSW, and the impact on the flue gas purification system is controllable. Thus, the co-incineration has little impact on original incineration conditions.

Life cycle inventory

Based on the system boundary and FU, the inventory data of materials, power, transportation and pollutant emissions of three incineration processes were collected. The data of mono-incineration and co-incineration in coal-fired power plants were mainly obtained from field survey. The specific properties of those plants are provided in Supplemental Table S1. The relevant data of co-incineration in MSW incineration plants were mainly collected from literature. As the data were collected from different places in China, the composition and calorific value of the sewage sludge were different. The industrial analysis and element analysis of sewage sludge for mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants are shown in Table 1. The input and output data were gathered and organized based on this composition of sewage sludge, including chemicals, power, water, transportation, flue gas pollutants and heavy metals, which were described as follows. Based on the inventory data of separate coal/MSW incineration and co-incineration with sewage sludge, the linear calculation was used to obtain the material consumption and pollutant emission generated by the sewage sludge alone in the co-incineration process. The complete inventory data are provided in Supplemental Tables S2–S9.

Table 1.

Industrial analysis and elemental analysis of sewage sludge.

Sewage sludge	Industrial analysis (%)				Elemental analysis (%)					LHV Q_net,d (MJ kg⁻¹)
Sewage sludge	M _t	A _d	V _d	FC_d	C _d	H _d	O _d	N _d	S _d	LHV Q_net,d (MJ kg⁻¹)
Case 1	34.9	43.9	40.9	15.2	20.37	4.04	27.73	3.27	0.70	6.75
Case 2	61.30	58.48	37.11	4.41	18.28	3.00	15.39	4.20	0.65	7.24
Case 3	40	21.92	N.A.	N.A.	49.36	5.40	16.97	4.88	1.48	N.A.

LHV: low calorific value; N.A.: Not Available.

The inventory data of the sewage sludge mono-incineration process were obtained from a 450 tonne day⁻¹ (80% moisture content) bubbling fluidized bed incineration plant in eastern China. The sewage sludge dewatering process was carried out in the WWTPs. And the distance from the WWTPs to mono-incineration plant was 30 km. The plate and frame filter press were adopted as the dewatering device, and the dryer was disc type drying machine. The dried sewage sludge will be delivered to a bubbling fluidized bed for incineration. Selective non-catalytic reduction (SNCR) denitration, semi-dry desulfurization, activated carbon adsorption and bag dust removal processes were chose for flue gas purification to ensure the flue gas meet the strict emission standard. Fly ash should be landfilled after chelated and stabilized according to hazardous waste disposal standards.

The co-incineration in coal-fired power plants data of sewage sludge come from a 300 MW pulverized coal furnace power in southern China and the proportion of sewage sludge mixed was 10%. Moreover, the boiler is a single chamber, balanced ventilation, solid slag removal, steel suspension structure, low nitrogen combustion and subcritical boiler. The flue gas adopts selective catalytic reduction (SCR) denitration, seawater desulfurization and electrostatic dust removal process.

The co-incineration in MSW incineration plants data were provided from the grate furnace with the capacity of 800 tonne·day⁻¹. The proportion of sewage sludge mixed was 30%. Due to the lack of complete data on the materials consumption for flue gas treatment, the data of MSW incineration were used to replace and the power consumption was calculated based on 20% auxiliary power consumption rate (Han, 2013). The sensitivity analysis indicated that this alternative is feasible. The co-incineration data of flue gas emission and ash residue heavy metals were obtained from literatures (Chen et al., 2019; Yan et al., 2018).

In addition, the background data were from the database Ecoinvent 3.0, which is one of the most comprehensive international LCI databases. Ecoinvent database contains relevant, reliable, transparent and accessible information of several thousands of LCI datasets in the areas of energy, transport, agriculture and electronics as well as waste treatments (Pascual-Gonzalez et al., 2016). The transport distance of sewage sludge from the WWTPs to incineration plant and ash residue from incineration plant to landfill site was set as 30 km. Due to the late development of sewage sludge incineration process in China, many sludge mono-incineration plants cannot be built near WWTPs because of space, opposition from residents and so on. Meanwhile, to enhance the generalizability of the comparison results between different incineration processes, the same distance was adopted. Moreover, the transportation distance of chemicals was set as 300 km. The fly ash chelating agent is dithiocarbamate, with the addition ratio of 2.5%.

Life cycle environmental impact assessment

Life cycle impact assessment is a stage in which the inventory data are modelled and converted into specific environmental impact categories to help explain environmental damage (Ioannou-Ttofa et al., 2021). SimaPro software is used for analysing LCI which is widely applied in the field of LCA (Lopes Silva et al., 2019) and the impact assessment is carried out using the ReCiPe 2016 method (Morero et al., 2017). The ReCiPe 2016 method is one of the most widely used and recognized method, providing a harmonized implementation of cause–effect pathways for the calculation of both midpoint and endpoint characterization factors (Dekker et al., 2020). The various environmental impact loads and the contribution of each stage to the environmental impact are studied based on the ReCiPe 2016 midpoint which includes eighteen categories, including global warming, stratospheric ozone depletion, ionizing radiation, ozone formation/human health, fine particulate matter formation, ozone formation, terrestrial ecosystems, terrestrial acidification, freshwater eutrophication, marine eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity, human non-carcinogenic toxicity, land use, mineral resource scarcity, fossil resource scarcity and water consumption.

Sensitivity analysis

Sensitivity analysis is a method to confirm the robustness of LCA results, and ISO standard recommends sensitivity analysis to determine the impact of different systems, process or data sources on LCA results (Gourdet et al., 2017). The composition of sewage sludge varies greatly due to different climate and human conditions. In addition, the material consumption and pollutant emission from various devices and processes are also different. Therefore, sensitivity analysis is conducted to study the fluctuation of results based on the field investigation data. The key parameters with greater environmental impact were selected through contribution analysis. The sensitivity analysis of evaluation results was carried out by adding a disturbance with a change of ±25% to the parameters, so as to obtain the fluctuation range of results and optimization suggestions (Li et al., 2017).

Results and discussion

Environmental impact analysis

The environmental impact evaluation results of three incineration processes based on LCA are shown in Figure 2. Figure 2(a) shows that the sewage sludge mono-incineration process has the most significant impact on the five types of environmental impact, namely terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET), human carcinogenicity (HCT) and human non-carcinogenicity (HNCT), with MET having the highest environmental impact load. The environmental impact normalized values of TET, FET, MET, HCT and HNCT are 0.40, 4.39, 7.21, 0.97 and 1.04, respectively. The stage distribution of environmental impact reveals that dewatering, drying, incineration and ash residue landfilling account for 34.7%, 6.96%, 18.96% and 39.4% of the main environmental impact category MET, respectively. The contribution analysis shows that among the 18 types of environmental impacts, the dewatering stage has the most significant environmental impact. Because the dewatering stage accounts for more than 50% of the total environmental load among eight types of environmental impact, due to the indirect environmental impact caused by dehydrants and power consumption. The environmental impact of the drying stage is minimal due to the relatively lower power consumption and pollutant emissions. Even if the gaseous pollutants and heavy metals in the flue gas are controlled at low concentrations, sewage sludge mono-incineration stage still accounts for a certain percentage of the environmental impact load due to the use of flue gas cleaning chemicals and pollutants. The environmental impact of the ash residue landfilling stage is mainly caused by fly ash chelating agents used to stabilize heavy metals in fly ash.

Figure 2(b) and (c) reveals that the co-incineration in coal-fired power plants and co-incineration in MSW incineration plants also have the most significant impact on the five types of environmental impact, including TET, FET, MET, HCT and HNCT, with MET having the highest environmental impact load. The environmental impact normalized values of TET, FET, MET, HCT and HNCT for co-incineration in coal-fired power plants are 0.94, 2.88, 4.64, 0.94 and 0.64, respectively. Dewatering, drying, co-incineration and ash residue landfilling stages account for 53.90%, 10.81%, 32.27% and 3.02% of the MET, respectively. Besides, among the five main environmental impact types, dewatering stage accounts for more than 50% of the four types of environmental impacts. The environmental impact of the drying and co-incineration stages cannot be neglected either. Ash residue landfilling stage accounts for a small proportion in all environmental impact categories, because fly ash from coal-fired power plants does not need chelation treatment due to the low heavy metal concentration.

For co-incineration in MSW incineration plants, the environmental impact normalized values of TET, FET, MET, HCT and HNCT are 0.30, 3.21, 5.24, 0.94 and 0.62, respectively. Dewatering, drying, co-incineration and ash residue landfilling stages account for 47.79%, 9.59%, 28.23% and 14.39% of the MET, respectively. The dewatering stage also brings the greatest environmental load. As the MSW incineration plant uses the grate incinerator, the amount of fly ash chelating agent is small due to the less fly ash. Compared with the mono-incineration of sewage sludge, the environmental impact of the ash residue landfilling stage in this process is relatively lower.

Comparative analysis indicates that the most significant environmental impact types of the three incineration processes are TET, FET, MET, HCT and HNCT with the largest being MET and FET. And the dewatering stage accounts for a large proportion in the most environmental impact categories of three incineration processes. The environmental impact of the ash residue landfilling stage accounts for a high proportion in mono-incineration and co-incineration in MSW incineration plants. The environmental impact load of the drying stage is relatively low for the three processes.

The comparison of the characterization values of the five major environmental impact for the sewage sludge incineration processes is shown in Figure 3. Among the five major environmental impact categories, compared with co-incineration, the mono-incineration of sewage sludge has the largest environmental impact load in FET, MET, HCT and HNCT, due to the high pollutant emission of mono-incineration stage and fly ash chelating agents. The load of MET for mono-incineration, which is the most significant environmental impact, is 1.62 and 1.38 times higher than those of the co-incineration in coal-fired power plants and co-incineration in MSW incineration plants, respectively. Specially, the co-incineration in coal-fired power plants has the highest environmental load of TET, because of the higher content of mercury, cadmium and other heavy metals in the flue gas. But the co-incineration in coal-fired power plants shows the lowest environmental impact load in MET and FET. The environmental loads of the two co-incineration processes are similar to the two environmental impact types, namely HCT and HNCT. The comparison of co-incineration processes between in coal-fired power plants and MSW incineration plants indicates that co-incineration in coal-fired power plants has the better environmental performance except for TET in the five main environmental impact categories. The normalized value of environmental impact loads indicates that MET and FET are the most significant environmental effects compared to TET, HNCT and HCT. Thus, the co-incineration in coal-fired power plants has the better comprehensive environmental performance, although TET has the highest environmental load and can be better if the heavy metals in the flue gas of coal-fired power plants are more strictly controlled.

Based on the above analysis results, even if heavy metals leaching toxicity and more complete material consumption are considered, the environmental impact of co-incineration process is generally lower than that of mono-incineration process, which is consistent with the conclusions of relevant studies (Chen et al., 2019; Xiao et al., 2022). But the most significant types of environmental impacts differ from previous studies (Chen et al., 2019; Xiao et al., 2022). Due to the different evaluation methods, the most significant types of environmental impacts were evaluated differently in each study.

Carbon footprint analysis

The carbon footprint analysis of sewage sludge incineration processes in this study is mainly carried out based on the LCA method, which is an important assessment indicator of environmental impact. The carbon emissions of sewage sludge incineration include direct and indirect emissions. The direct emissions are from the carbon contained in the sewage sludge, while the indirect emissions are mainly caused by electricity, transportation, chemicals and so on, excluding facility construction. However, Intergovernmental Panel on Climate Change (IPCC, 2006) stipulates that the direct emissions from sewage sludge incineration are not included in the carbon emission accounting because of biological origin. The carbon emission factors are mainly from Ecoinvent 3.0 database. And the carbon emission factor of electric power is corrected by 0.7921 kg CO₂eq kWh⁻¹ based on the Eastern China regional grid baseline in 2019 (Ministry of Ecology and Environment of the People’s Republic of China, 2020).

The carbon footprint analysis in this study focused on indirect carbon emissions during sewage sludge disposal based on IPCC 2006 related standards (IPCC, 2006). Meanwhile, the direct carbon emission of sewage sludge incineration changes greatly due to the large fluctuation of sewage sludge composition. The investigated mono-incineration device of sewage sludge cannot generate power because the working fluid of the device is heating conduction oil. Meanwhile, the accurately corresponding power generation capacity of sewage sludge in co-incineration is obtained difficultly. Therefore, the carbon sink is not considered. As Figure 4 shown, the carbon footprint analysis reveals that the carbon emissions of sewage sludge mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants are 108.02, 100.47 and 76.15 kg CO₂eq tonne⁻¹, respectively. The contribution analysis shows that the dewatering stage has the highest carbon emissions, while the carbon emissions of flue gas treatment material, chelating agent and transportation are relatively low. Among the three processes, the carbon emission contribution of dehydrating agents is above 50%. And carbon emissions caused by electricity consumption rank second. The contribution of transportation is also higher than that of flue gas treatment materials and fly ash chelating agent.

Compared with the mono-incineration processes, the carbon emission of sewage sludge co-incineration is the lowest. Fly ash without chelation and seawater desulfurization without desulfurizer lead to lower carbon emissions for co-incineration in coal-fired power plants. Considering conventional wet desulfurization, carbon footprint analysis shows that the carbon emission of this process is 102.90 kg CO₂eq tonne⁻¹, which is 2.43 kg CO₂eq tonne⁻¹ more than that of seawater desulfurization. The minimum emissions of co-incineration in MSW incineration plants are mainly due to the biomass electricity used in the process, which are not calculated in the carbon emissions. Based on the above analysis, the life cycle carbon emission reduction strategy of sewage sludge incineration mainly includes the development of green dehydrant and fly ash chelating agent, more efficient dewatering technology and the improvement of energy efficiency of incineration equipment.

Economic analysis

The economic analysis of sewage sludge incineration was mainly performed by calculating the cost of sewage sludge disposal, including depreciation, material, water, electricity, maintenance, personnel, transportation cost and so on. And the annual operation time of incineration equipment was 8000 hours. The depreciation period of equipment was 10 years, and that of building was 15 years. The material price was determined according to the average price in the Chinese market, as shown in Supplemental Table S10. The annual equipment maintenance cost was calculated as 2% of the equipment cost. In addition, the loan fund was calculated as 60% of the total investment with the 4% of annual loan interest rate for 5 years. The other costs of case 1 were loan interest. And the other costs of case 2 were loan interest and the increase in coal consumption due to co-incineration. Moreover, the other costs of case 3 are mainly loan interest, loss of power generation income and the MSW disposal fee.

As Table 2 shown, the economic analysis reveals that the costs of mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants are USD 41.06, USD 23.95 and USD 27.15 tonne⁻¹, respectively. Compared with mono-incineration, the sewage sludge co-incineration in coal-fired power plants and co-incineration in MSW incineration plants can save USD 17.11 and USD 13.91 tonne⁻¹, respectively. The cost analysis of the sewage sludge mono-incineration process indicates that the depreciation and material account for 53.47% of the total cost due to the higher investment and the large amount of agent consumption in the dewatering, flue gas disposal and fly ash chelation stages. The labour cost of the mono-incineration process is also higher compared to the co-incineration process, which can utilize the available labour. Moreover, the mono-incineration in the fluidized bed will produce a large amount of fly ash, which will lead to high hazardous solid waste disposal costs.

Table 2.

Economic analysis of different sewage sludge incineration processes.

Categories	Unit	Case 1	Case 2	Case 3
Investment	Million USD	18.84	3.69	4.36
Material	USD/FU	11.13	10.87	10.90
Water and electricity	USD/FU	4.12	1.43	0.62
Labour cost	USD/FU	4.83	0.71	0.60
Maintenance	USD/FU	1.82	0.36	0.36
Solid waste disposal	USD/FU	4.25	1.11	1.74
Transportation	USD/FU	2.58	2.37	2.81
Depreciation	USD/FU	10.82	1.82	1.82
Others	USD/FU	1.51	5.28	8.29
Total	USD/FU	41.06	23.95	27.15

FU: functional unit.

Compared with the sewage sludge mono-incineration process, the cost of co-incineration process is lower, especially co-incineration in coal-fired power plants. The cost analysis of the co-incineration process reveals that other costs of the co-incineration in coal-fired power plants and co-incineration in MSW incineration plants account for a large proportion. Therefore, for the co-incineration in coal-fired power plants, the increase in coal consumption will significantly affect the cost of sewage sludge disposal. For the co-incineration in MSW incineration plants, the loss of power generation revenue and MSW disposal fees will have a significant impact on the cost of sewage sludge disposal. The blending ratio and the overall efficiency of the device after co-incineration should be seriously considered. Although the dehydrated sludge transportation distance for the three incineration processes was the same, the transportation cost of co-incineration in coal-fired power plants was the lowest. The reason was that the transportation cost not only includes the transportation of dehydrated sludge, but also includes the transportation of flue gas purification materials, fly ash chelating agents and so on.

Moreover, the transportation costs for all three processes were calculated based on the same transportation distance, so the true transportation costs may vary to some extent depending on the situation. However, the transportation cost accounted for a small proportion of the total cost. And the transportation costs for the sludge mono-incineration process were USD 0.66 and USD 7.75 for sludge transportation distances of 0 and 100 km, respectively. Therefore, the effect of transportation distances on the comparison results can be ignored. The sewage sludge mono-incineration process still exhibits the worst economic performance, which is consistent with the conclusions of relevant studies (Chen et al., 2019, Xiao et al., 2022).

Sensitivity analysis

The sensitivity analysis of MET was conducted to verify the reliability of the evaluation results, because MET is the most significant environmental impact of the three incineration processes. Meanwhile, as the investigated coal-fired power plants adopted seawater desulfurization, LCA of conventional wet desulfurization was conducted to explore the influence of desulfurization methods on environmental assessment results.

Figure 5 shows the changes in environmental impact load for MET based on the ±25% disturbance of dehydrant, electricity, flue gas treatment material, transportation and fly ash chelating agent. As Figure 5(a) shown, the environmental impact of the sewage sludge mono-incineration process is most sensitive to fly ash chelating agent, dehydrants and electricity in order. However, the sensitivity of transportation and flue gas treatment materials is low. Figure 5(b) indicates that the environmental impact of co-incineration in coal-fired power plants is most sensitive to dehydrants and electricity. In addition, the environmental impact load is less sensitive to desulfurization methods. Evaluation result reveals that the environmental load of wet desulfurization is only 0.21% changed compared with seawater desulfurization. Figure 5(c) reveals that the environmental impact of co-incineration in MSW incineration powers is most sensitive to the electricity, dehydrants and fly ash chelating agent. The environmental load of MET for three processes (mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants) varies by 7.28%, 11.31% and 10.02% due to a 25% change of dehydrants, respectively. The environmental load of MET for three processes varies by 7.14%, 11.46% and 10.53% due to a 25% change in electricity, respectively. The environmental load of MET for mono-incineration and co-incineration in MSW plants varies by 9.69% and 3.72% due to a 25% change of fly ash chelating agent, respectively.

Comprehensive analysis revealed that the environmental impact load of three incineration processes is most sensitive to dehydrants, fly ash chelating agent and electricity, while the impact of transportation and flue gas treatment material on environmental load is relatively low. The optimization of process environmental performance should focus on the selection and dosage of dehydrants and fly ash chelating agent and the improvement of equipment efficiency.

Conclusion

In this study, the comparative LCA of three sewage sludge incineration processes, including mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants, was carried out from the perspective of environment, carbon footprint and economy. And the sensitivity analysis of environmental impact assessment was conducted to provide reference for process optimization.

The sewage sludge mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants have the most significant impact on the five types of environmental impact, including TET, FET, MET, HCT and HNCT. Furthermore, the sewage sludge co-incineration in coal-fired power plants has the best comprehensive environmental performance. However, the potential environmental impact caused by dioxin emissions should be taken into account due to the lack of activated carbon adsorption equipment for co-incineration in coal-fired power plants. In addition, the corrosion of boiler high-temperature heating surfaces by chlorine in sewage sludge cannot be ignored. The results of environmental assessment are most sensitive to dehydrants, fly ash chelating agent and electricity. Desulfurization method has little impact on the environmental load of incineration.

The carbon footprint analysis reveals that the indirect carbon emissions from the sewage sludge mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants are 108.02, 100.47 and 76.15 kg CO₂eq tonne⁻¹, respectively. Sewage sludge co-incineration in MSW incineration plant has the lowest carbon emissions, only 70.50% and 75.79% of mono-incineration and co-incineration in coal-fired power plants, respectively. The emission contribution of the dewatering stage is the highest while that of the ash residue treatment stage is the lowest.

Economic analysis shows that the sewage sludge mono-incineration, co-incineration in coal-fired power plants and co-incineration in MSW incineration plants are USD 41.06, USD 23.95 and USD 27.15 tonne⁻¹, respectively. The higher investment and more hazardous solid waste of mono-incineration led to higher disposal cost. However, as the co-incineration process can make full use of existing equipment with less investment, the disposal cost is lower.

The above evaluation analysis provides references for the selection of a reasonable and environment-friendly sewage sludge incineration process and offers suggestions for the optimization direction, which should focus on improving the dewatering and fly ash heavy metal chelation process, as well as the energy utilization efficiency of the equipment.

Supplemental Material

sj-docx-1-wmr-10.1177_0734242X231187560 – Supplemental material for Comparative study of different sewage sludge incineration treatments based on environmental and economic life cycle assessment

Supplemental material, sj-docx-1-wmr-10.1177_0734242X231187560 for Comparative study of different sewage sludge incineration treatments based on environmental and economic life cycle assessment by Xiaoyong Zhang, Xiaoping Chen, Jun Xiao, Xiaowei Peng, Jianguo Wang, Jiliang Ma, Daoyin Liu and Cai Liang in Waste Management & Research

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by National Key Research and Development Program of China (grant number: 2020YFC1908703).

ORCID iD

Xiaoyong Zhang

Supplemental material

Supplemental material for this article is available online.

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