The significant benefits of using LCA in geo-environmental engineering include: (i) pinpoint specific phases or activities with the highest CFP, allowing for targeted reduction efforts; (ii) evaluate the environmental impact of different material choices, construction methods, or remediation techniques to inform sustainable decision-making; (iii) transparent communication of LCA results can demonstrate commitment to sustainability and build trust with stakeholders; and (iv) LCA can help in meeting environmental regulations and standards related to greenhouse gas emissions and resource use.
3.1.1. Solid Waste Management (SWM)
In a study by Hong et al. [
89], the environmental and economic impacts of the three medical waste disposal scenarios (i.e., pyrolysis, chemical disinfection, and steam sterilization) were quantified via an LCA coupled with life cycle costing (LCC). The results (
Figure 3; data from Hong et al. [
89]) showed that steam sterilization exhibit the highest CFP (3.73×10
3 kg CO
2e) followed by pyrolysis (1.26×10
3 kg CO
2e) and chemical disinfection (8.00×10
2 kg CO
2e) because of the differences in energy consumption. The CFP main contributors are: (
i) for steam sterilization, electricity (26.1%) and diesel (66.7%); (
ii) for pyrolysis, electricity (23.8%), diesel (36.1%) and sodium hydroxide (29.9%); and (
iii) for chemical disinfection, electricity (66%) and lime (11%).
In addition, from an economic viewpoint, the economic burden contributing factors are: (i) for steam sterilization, electricity and diesel (39%), and labor, and human health protection (69.1 %); (ii) for pyrolysis, sodium hydroxide, electricity and active carbon (43.3%), and labor, human health protection, and investment (18.4%); and (iii) for chemical disinfection, electricity, chlorine and landfill (27.5%), and investment, labor, and human health protection (66%). However, the chemical disinfection showed the highest net profit (220.13 USD/t) followed by pyrolysis (189.96 USD/t) and steam sterilization (28.66 USD/t). Therefore, a balance between the achieving sustainability goals and economic benefit must be realized in choosing the best technology for the treatment of this medical waste.
3.1.2. Wastewater Treatment (WWT)
Recently, several LCA methodologies such as IMPACT World
+ [
90], ReCiPe [
91], and LIME 2 [
92] have been applied to WWT plants [
93,
94,
95]. For instance, the LCA studies were conducted to: (
i) evaluate all the elements of a wastewater treatment system [
96,
97]; (
ii) evaluate the negative influence of septic tanks on the soil and water environment [
98,
99,
100,
101]; (
iii) assess alternatives for a city’s water-wastewater service system [
102]; (
iv) select a sustainable sewerage servicing systems and technologies [
103]; (
v) assess the vertical and horizontal flow in constructed wetlands [
104]; and (
vi) evaluate the system of wastewater collection, transport and treatment [
9].
Studies suggest that operation related stages, including WWT, sludge treatment, and sludge disposal, are the main contributors to the environmental impact of conventional WWT plants ( >90%) [
105,
106]. Of the four processes related to a conventional WWT plants (i.e., construction, operation, sludge treatment and disposal, and emissions to air), the environmental impact of the operational process was most significant, being 85–97% of the total impact among all seven impact categories considered. Also, in a study by Limphitakphong et al. [
107], the LCA of WWT plants in three impact categories (climate change, eutrophication, and acidification) showed that eutrophication accounted for over 90% of the total environmental impact. Therefore, it is important to assess effluent water quality thoroughly when comparing WWT options with different effluent qualities.
In addition, in a study by Lorenzo-Toja et al. [
97], the environmental efficiency of multiple units in 47 WWT plants located in different regions of Spain and operated for 4 years was analyzed using LCA and data envelop analysis (DEA). The results suggested that
(i) for most of the facilities, the efficiency standards tended to remain constant through time; (
ii) significant differences were detected among plants with different legislation thresholds for their effluent withdrawal; (
iii) WWT plants discharging to non-sensitive water bodies appeared to be able to attain efficiency values near the benchmark, whereas facilities with stricter thresholds (i.e. sensitive water bodies) resisted to achieve those values, especially in the case of the smaller plants; and (
iv) the use of the slacks-based measure of efficiency (SBM) model for one single year of operation is a good proxy for the evaluation of the environmental efficiency of these systems.
Also, in a study by Mishima et al. [
108], the environmental load from the septic systems in Japan was evaluated using LCA and the results have shown that the mean environmental load of the effluent during the operation stage was 37.6%, and the operation stage accounts for over 99% of the involved eutrophication, biological toxicity, and toxic chemicals, which are strongly related to the quality of the effluent. The septic systems used in the LCA consisted of the sedimentation tank, anaerobic tank, aerobic tank, clarification tank, and disinfection tank. These systems were further classified into: (a) standard-structure type (S) as per the Japanese specification and effluent water quality of BOD = 20 mg/l; and (b) certified structure types (C1-C3) which are freely designed and manufactured by septic system manufacturers and certified with the authorized effluent water quality based on the treatment performance test. The effluent water quality standards are BOD=20 mg/l and T-N=20 mg/l for C1, BOD=10 mg/l, and T-N 20 mg/l for C2, and BOD=10 mg/l, T-N= 10 mg/l, and T-P = 1 mg/l for C3. The average total volumes of these septic tanks are 2.95, 2.10, 2.74, 2.89 m
3 for S, C1, C2, C3, respectively.
For the LCA, the following four life-cycle stages of the septic systems were considered: (i) manufacturing, including the fabrication of the septic systems’ components; (ii) installation which involves transportation from the factory to the installation site; (iii) operation which includes operation, maintenance, sludge treatment, and effluent discharge. Items such as material consumption, electricity consumption, direct GHG emissions, and indirect GHG emissions were considered in the operational process. The COD, T-N, total phosphorus (T-P), and ammonium nitrogen (NH4–N) in the effluent were calculated for the effluent discharge, and the quality of the gray water influent was assumed to be: BOD, COD, T-N (100% as NH4–N), and T-P of 145, 65, 10, and 1.5 mg/L, respectively. The average amount of sludge generated was assumed to be 1.2 L/p/d; and (iv) disposal whereby at the end of the life cycle the septic tank components would be incinerated, and the residues would be landfilled.
The functional unit was assumed to be 1-year use of a five population equivalents (PE) septic system by one household, and the lifespan of a septic system was assumed to be 40 years. Throughout the life cycle of the septic system, 10 impact categories were considered: climate change, eutrophication, biological toxicity, toxic chemicals, urban area air pollution, acidification, ozone layer destruction, photochemical oxidant, resources depletion, and waste.
The unit GHG emissions from the septic systems were assumed to be: (i) 2,477 (for S), 1,984 (for C1), and 1,044 (for C2 and C3) g-CH4/person/year; and (ii) 71.7 (for S), 54.5 (for C1), 123.2 (for C2 and C3) g-N2O/person/year.
Also, the LCA was estimated using the impact assessment model LIME2 [
92] and the ecotoxicity estimation model [
108]. The results (
Figure 4) indicated that the total GHG emissions during operation of the standard structure is the highest (744 Kg-CO
2e/year/household) followed by certified structures C3 (722), C2 (667) and C1 (655), respectively. For the standard structure, the GHG emissions are distributed (
Figure 4B), from the highest to the lowest, as electricity (40.1%), direct emissions (CH
4 and N
2O) (34%), sludge treatment (21%), maintenance (2.9%), and indirect (2%). The GHG contribution due to sludge treatment was in deceasing order C1 (24%), C2 (23.3%), C3 (21.5) and Standard (21%). However, for the direct emissions, the GHG emissions were 34, 29, 28, 26.4% for standard, C1, C2, and C3, respectively. The GHG emissions due to electricity were 48, 44, 41and 40.1% for C3, C2, C1, and standard, respectively. Therefore, the contribution of electricity consumption was the highest among all types of the septic systems, followed by the direct emissions of the systems (CH
4 and N
2O) and the sludge treatment process; hence optimal control during operation is needed to reduce GHG emissions.
For the LCA, of the analyzed 10 impact categories, the major contributors to the environmental impact were GHG, eutrophication, biological toxicity, and air pollution. For the standard structure, the LCA impact categories were in decreasing order GHG (35%), eutrophication (23%), biological toxicity (17%), and air pollution (12%); whilst, for the certified structure C3, GHG (41%), eutrophication (29%), biological toxicity (8%), and air pollution (16%). The LCA have also indicated that both operation (about 30%) and disposal, due to landfill space requirement, (60-65%) stages have the major impacts. To reduce the impact of landfill disposal, implementation of nature-based solutions (NbS) (biodegradation, phytoextraction, phytovolatilization, and revegetation) is highly recommended [
109,
110,
111,
112,
113].
In another study by Khan et al. [
80], an LCA was employed to compare the CFP of three treatment processes, incineration, mechanical recycling and chemical recycling of a functional unit of 1 ton of re-pulping waste reject. Using biomass-based heat sources, the generated CFPs were 560, 740, and 1900 kg CO
2e/t of waste for chemical recycling, mechanical recycling, and incineration scenario, respectively. However, using natural gas-based heat sources, the generated CFPs were 290, 430, and 960 kg CO
2e/t of waste for chemical recycling, mechanical recycling, and incineration, respectively. Therefore, the chemical recycling would be the best option with using natural gas as the energy source. It is to be noted that it is useful to conduct uncertainty analysis to evaluate the impact of the assumptions used in the LCA.
In addition to identifying the environmental impacts from WWT plants via LCA, the trade-off of the new integration of existing technologies in terms of cost and environmental impact can be assessed [
114]. In a study by Meneses et al. [
115], the technologies adopted for more stringent effluent standards from WWT plants (such as, 10-15 mg N/l and 1-2 mg P/l by EU Urban Waste Water Directive) could improve effluent quality; however, it may require additional energy consumption, utilization of more chemical reagents, and production of more sludge. In another study by Hauck et al. [
116], a 16% reduction in marine eutrophication was obtained; however, the GHG impacts increased by 9% from the traditional operation of the Dokhaven WWT plant, The Netherlands, because of the increasing use of electricity. The results also indicated that the trade-off between effluent quality and other environmental impacts should not be neglected when applying advanced technologies.
3.1.3. Contaminated Soil Remediation (CSR)
In a study by Cappuyns [
117], the environmental impacts of two contaminated sites by mineral oil, and one site by mineral oil and BTEX (Benzene, Toluene, Ethylbenzene, and Xylene), were quantified by means of the BATNEEC (Best Available Technology Not Entailing Excessive Costs) methodology, which uses a multicriteria analysis considering environmental, technical and financial aspects. The CFPs were calculated by a methodology based on the principles of LCA (REC; Risk reduction, Environmental merit and Costs), and an environmental merit index (E) was determined based on interviewing a panel of environmental experts. The CFPs were calculated by using three methods:
a) the Soil Remediation Tool (SRT) [
118], which allows users to estimate sustainability metrics for various CSR technologies such as excavation, soil vapor extraction, pump and treat, enhanced in situ biodegradation, thermal treatment, in situ chemical oxidation, permeable reactive barrier, long-term monitoring, and monitored natural attenuation;
b) the SiteWise tool [
119], which considers variety of remedial actions such as site investigation, construction, operation and monitoring; and
c) Tauw CO
2 calculator [
120,
121], which considers different CSR options (in-situ and ex-situ), and the possibility of using renewable energy sources to decrease the overall environmental impacts of a CSR operation.
The results showed that the CFP of soil excavation, as a CSR option, was mainly attributed to the transport of soil (30.1-55.4%), while the excavation activities themselves (6.1-10.6%) and the in-situ treatment of the groundwater (1.6-3.7%) were of secondary importance, and the pumping of the groundwater had a negligible (0.2-0.3%) contribution. Transport of the equipment and personnel to the site only had a marginal contribution to the total CO2 emissions.
Therefore, optimization of these tools, taking into account new developments in the field of CSR, such as the use of more accurate screening tools to delineate the extent of the contamination, the use of innovative (sustainable) CSR technologies or the use renewable energy sources, will further improve the environmental impacts assessment of CSR activities [
10,
117].
In another study by Amponsah et al. [
122], the GHG emissions (CO
2e) from six ex-situ CSR technologies, including excavation and land disposal, excavation and incineration, thermal desorption, soil vapor extraction, bioremediation, and soil washing were evaluated. The results showed (
i) a large variation in the GHG emissions of treated soil varying from 3.1 × 10
−7 t to 8.2 t CO
2e/m
3; (
ii) incineration had the highest mean GHG emissions (0.7 t CO
2e/m
3) and thermal desorption the lowest (0.07 t CO
2e/m
3); (
iii) a large variation range of GHG emissions from the excavation and disposal, and excavation and incineration technologies, varying from 3.1 × 10
−7 t to 8.2 t CO
2e/m
3 of treated soil. This information provides the decision makers with new prospects for choosing the most sustainable CSR technology via green practices throughout the remedial stages (investigation, design, construction, operation, and monitoring).
In another study by Huysegoms et al. [
76], LCA, to assess the impacts linked to the remediation works, and Cost-Benefit Analysis (CBA) were used to assess the CSR technologies (excavation of contaminated soils and off-site thermal treatment). Both the soil and the groundwater at the site were contaminated by poly-aromatic hydrocarbons (PAH) and cyanide. The results from the LCA were monetized using a Stepwise 2006 technique, that can be fully implemented in the LCA-SimaPro software to provide users with values on three overall safeguard subjects (human well-being, biodiversity and resource productivity), which are linked to the sustainability three pillars (people, planet, profit), and Ecovalue 08 technique, that used to monetized endpoint impact based on market valuations of resource depletion and individual Willingness To Pay (WTP) estimates for environmental quality. The results from the Stepwise 2006 are expressed in terms of Quality Adjusted Life Years (QALY) for human well-being, Biodiversity Adjusted Hectare Years (BAHY) for biodiversity and euros for resource productivity. The use of such monetization analysis is not a routine process for site remediation evaluation [
86].
The LCA analysis reported by Huysegoms et al. [
76] showed that the impact categories that mostly contribute to the environmental impact of the adopted CSR technologies are energy consumption, climate change, and formation of particulate matter, where the off-site treatment, and transportation contributed about 70%, and 10%, respectively, of the total impact. The minimum and maximum monetized values of the environmental impact parameters for the proposed remedial method were 17,758 and 259,996 EU2015, respectively, with impact category contributions in a decreasing order as abiotic depletion (44.6%), global warming (26.64%), human toxicity (19.91%), eutrophication (5.44%), acidification (3.26%), and photochemical oxidation (0.13%). Based on the CBA, the NPV contribution of the remedial work, externality, and site restoration to the total NPV were estimated at 63.54%, 29,72%, and 6.73%, respectively. However, since the pollution source has been removed, the NPV of the positive health impact was estimated at 48.28% of the total NPV, indicating that the project is not socially profitable in the short term (30 years) because of the limited health benefits of the remediation and the large cost of transport of soil to off-site treatment facilities (39.74% of the total NPV).
From the preceding analysis, it can be seen that the social CBA is a method that can be used to address all negative and positive impacts of a remediation; whilst, the LCA provides an in-depth, extensive and substantiated view on the environmental impacts, negative and positive, of the remediation project. Therefore, combining both methods would be an important step; however, the LCA needs to be monetized. The analysis showed that the resultant costs of CO2/t are 44-euro for the CBA, 83-euro for LCA with Stepwise 2006 technique, and 11 to 222-euro for LCA with Ecovalue 08 technique. It is worth noting such differences are attributed to the type of impacts covered in each method. The LCA with monetization gives a more extensive view on the environmental effects, including eutrophication and acidification, whilst, social CBA addresses air pollutants from the remediation works, monitoring and transportation, meaning that the use of monetized LCA is a more appropriate and comprehensive method due to inclusion of variety of environmental impacts.
Recently, in a study by Ashkanani et al. [
123], the LCA of thirteen (13) distinct CSR technologies for heavy polyaromatic hydrocarbons (PAH)-contaminated soils was evaluated based on nine well-defined LCA criteria, each assigned an LCA score ranging from 1 (indicating the lowest environmental impact, very low category, VL) to 5 (the highest impact, very high category, VH). And to rank these technologies, a sub-categories were assigned as low (L), medium (M), and high (H) for scores of 2, 3, and 4, respectively. The thirteen CSR technologies were grouped into: (
i) physico-chemical (sheet pile wall, pumping (P), dual phase extraction (DPE), soil vapor extraction (SVE), chemical oxidation (S-ISCO), passive soil vapor extraction (PSVE)); (
ii) thermal (thermal desorption (ISTD), steam enhanced extraction, thermal resistivity - ERH); and (
iii) biological (stimulation reductive dechlorination process (SRD), excavation plus off site treatment, soil mixing with micro-scale (ZVI), natural attenuation (NA)). The nine environmental criteria considered in the LCA were resource consumption, CO
2 emission, toxicity, waste, energy consumption, carbon footprint (CFP), cost planning and construction, initial capital cost, and ongoing operating cost.
The results shown in
Figure 5 (data from Ashkanani et al. [
123]) indicate that the thermal technologies have the highest CFPs, total LCA and LCCA impacts. The biological treatments have low to moderate LCA impact (
Figure 5B) but have higher economic costs in comparison with the physico-chemical treatments (
Figure 5C). The rankings of these technologies are shown in
Table 1 with specific rankings to each criterion considered in the LCA. Therefore, for choosing the appropriate technology, a balance must be made between the economic cost and achieving sustainability goals.
The vital challenges of using LCA in geo-environmental engineering include: (i) consistent and reliable data on material production, energy consumption, and emissions across different regions and sectors can be challenging to find; (ii) modeling complex systems and accurately assessing indirect impacts can require specialized expertise and software; and (iii) life cycle inventory data and impact assessment methods may carry uncertainties, requiring careful interpretation and sensitivity analysis.
The crucial considerations of using LCA in assessing geo-environmental engineering projects include: (i) clearly define the specific processes and activities included in the assessment, whether it's a single construction project, a specific remediation technique, or the entire life cycle of a piece of infrastructure; (ii) quantify the energy, materials, water, and other resources consumed at each stage of the life cycle, as well as the emissions and waste generated; (iii) assess the environmental impacts of these inputs and outputs using various categories like climate change, resource depletion, human health, and ecosystem quality; and (iv) analyze the results to identify hotspots with the highest environmental impact and explore alternative materials, design strategies, or management practices to reduce those impacts.
In addition, it is recommended to [
117]: (i) first step, an initial ranking of soil remediation techniques based on more qualitative site-specific criteria is required. The ranking can be done using a BATNEEC analysis [
75], which is based on the principles of a multicriteria analysis; and (
ii) second step, a quantitative assessments using LCA and related techniques may be of value for an in-depth analysis.