This study benchmarks vermifiltration (VF) as secondary wastewater treatment in three nature-based decentralized treatment plants using life-cycle assessment. The comparison is justified by the comparatively easier and cheaper operation of VF when compared to more traditional technologies, including small rate infiltration (SRI), constructed wetlands (CW), and activated sludge (AS). Standard life cycle assessment was used and applied to three case studies located in southern Europe. Material intensity during construction was highest for VF, but impacts during operation were lower, compensating those of the other phases. Impacts during the construction phase far outweigh those of operation and dismantling for facilities using constructed wetlands and activated sludge, when the number of served inhabitants is small, and due to lack of economies of scale. VF used as secondary treatment was shown to contribute to reducing the environmental impacts, mainly in constructed wetlands and activated sludge. The replacement of CW by VF seems to bring important environmental benefits in most impact categories, in particular in the construction phase. The replacement by VF in facilities with SRI seems to result in the improvement of some of the impact categories, in particular in the operation phase. As for dismantling, no conclusive results were obtained.

  • Vermifiltration is a promising technology.

  • An LCIA is needed to better understand vermifiltration.

  • The quantification of the environmental impacts will help researchers in future works.

  • Constrution, operation and dismantling phases are crucial to understand the life cycle in Wastewater Treatment Plants.

  • LCIA is an important tool to study the environmental impacts of wastewater treatment.

Wastewater treatment plants (WWTPs) are designed to reduce wastewater pollution and minimize negative impacts on human health and the environment (Zang et al. 2015). However, due to material, energy consumption, and emissions, WWTPs can cause environmental impacts (Capodaglio & Olsson 2019). Thus, the selection of a specific wastewater treatment technology should not be focused solely on technological insight, but should also integrate the human and environmental activities surrounding it; that is, optimize the balance between technology and environment. A plethora of methods have been suggested to calculate the achievement of such a balance. The most frequent in wastewater technologies include decision support systems for designing sustainable technologies by (multi)objective optimization (Padrón-Páez et al. 2020); and aggregating indices, including (i) the use of sets of (mainly local) indicators that incorporate environmental, societal, and economic sustainability (Cossio et al. 2020); (ii) life cycle impact assessment (LCA) (Myllyviita et al. 2016); (iii) energy analysis (Meneses-Jácome & Ruiz-Colorado 2020); (iv) socio-eco-efficiency analysis (Anwar et al. 2021); (v) a variety of footprints (Chen et al. 2020); and (vi) eco-efficiency (Dong et al. 2017).

Urban wastewater treatment for small rural communities, where technical and financial resources are usually limited, can pose a problem as solutions adopted in larger urban areas are not applicable (Capodaglio et al. 2017). Besides, the problem associated with the current treatment technologies is the difficulty in reaching sustainability (Bdour et al. 2009), assuming there is agreement about the measuring method. As for the latter, the largest consensus has been towards life cycle assessment, as shown by the more than two hundred LCAs made for wastewater treatment facilities over the last twenty years. Despite this wealth of experience, there is not yet agreement about many specificities of the method in the wastewater industry (Corominas et al. 2020), namely because the environmental impacts are strongly dependent on the geographical location, size, and installed unit operations, but also on socio-economic factors of the served population. The reproduction of methods and results from one place to another is therefore more difficult than in other industries. Given the diversity of possible combinations of natural and technological possibilities, the set of available LCA studies is still limited. It is, for instance, at the level of an emergent technology such as carbon capture, and inferior to the over 300 made for a sub-category of the waste industry, such as waste-to-energy technologies (Mayer et al. 2019).

A number of small-scale treatment systems have been developed for small rural communities, which are adapted to the needs of these areas. These WWTPs have also good applicability in developing countries as they require less investment and have less technically demanding maintenance operations common in large centralized facilities (Muga & Mihelcic 2008). Given the need for alternative solutions, priority has been given to technologies that have minimum energy requirements, simple operational procedures, and sufficient level of inertia when facing large fluctuations in the flow and water quality (Salas 2004). Several types of decentralized wastewater treatment technologies for rural areas include: (i) active sludge systems (AS) (Ahansazan et al. 2014), anaerobic sludge reactors (e.g. septic tank) (Tilley et al. 2014), stabilization ponds (Mara 2006), constructed wetlands (Vymazal 2011), infiltration systems (Li et al. 2015), and hybrid nature-based systems (Capodaglio et al. 2021).

Vermifiltration (VF) is an example of the latter, recognized as a low-cost and sustainable technology to treat wastewater (Jiang et al. 2016), sewage sludge (Zhao et al. 2010) and fecal matter (Furlong et al. 2016). On top of the above-mentioned advantages over conventional wastewater treatment solutions, VF has been shown to have higher efficiencies when flows are small (Li et al. 2008). It is a bio-oxidative process combining filtration with vermicomposting processes in vermifilters (VFs) (Tompkins et al. 2019). Filter packing in VFs is constituted by an organic packing and an inert packing (Lourenço & Nunes 2017a). It usually includes sawdust and vermicompost (Lourenço & Nunes 2017b), peat and wood flour (Li et al. 2009), wood chips, (Li et al. 2008) for organic packing; and gravel, quartz sand (Adugna et al. 2019), river bed gravel, mud balls, glass balls (Kumar et al. 2014), ceramsite and coal, for inert packing (Wang et al. 2010). Many of these packing materials are wastes from other activities, which are recycled into secondary raw materials, in line with circular economy principles. VF has found application in households, small communities, and to treat mixtures of urban and industrial wastewater (Sinha et al. 2014). It is also accepted that VF can be more cost-effective when compared with conventional WWTP (Liu et al. 2012) since it does not require mechanical equipment other than pumps (Sinha et al. 2008). Several VFs are already working in France (1,000 PE) (RECYCLAGUA 2019), Spain, Chile (Fundación Chile 2019), Brazil (Madrid 2016), and P. R. China (Nie et al. 2013).

Like other WWTPs, small-scale treatment systems should be assessed carefully considering the inputs and outputs fluxes of the life cycle phases, evaluating its environmental performance as a whole system. The method begins with the life cycle inventory (LCI), which includes the study boundaries, resources (raw materials and energy), emissions (atmospheric, waterborne, and solid waste), and disposal practices. It consists of detailed tracking of all the flows in and out of the product system, including raw resources or materials, energy, water, and greenhouse gases (GHG), and substances. Conversion of inventory into impacts is made in the characterization phase (LCIA). Characterization factors are used to convert each LCI information into impact categories (Van den Bossche et al. 2006).

LCIA has been used to explore the sustainability of WWTPs (Ortiz et al. 2007) and to compare between alternative unit treatment processes, and alternative integrated wastewater management solutions (Palme et al. 2005), but mostly focusing on traditional technologies for large population sizes in an urban environment. Small-scale WWTPs in small rural communities, which commonly serve population sizes below 2,000 person-equivalent (PE) or process influent flow rates below 200 m3 day−1 (Lens et al. 2001), have received little attention.

This study benchmarks vermifiltration (VF) as secondary wastewater treatment in three natured-based decentralized treatment plants using life-cycle assessment. The comparison is justified by the comparatively easier and cheaper operation of VF when compared to more traditional technologies, including small rate infiltration (SRI), constructed wetlands (CW), and activated sludge (AS). VF is also cheaper and less technical, engineering-intensive to install and operate (Singh et al. 2019). The study uses the international standards for LCA (ISO 14040 2006) and focuses also on obtaining important data related to the different life cycle phases for the vermifiltration technology in an environmental sustainability perspective.

The present study focuses on WWTPs used for wastewater treatment in rural areas with communities up to 120 PE. Three real case-studies were chosen: (i) a slow rate filtration plant (SRI); (ii) a constructed wetland (CW); and (iii) a self-supporting activated sludge prefabricated steel unit (AS). The data from each WWTP necessary for the inventory and analysis of the current WWTPs were retrieved from published studies (Machado et al. 2007; Nogueira et al. 2009). To ease interpretation, the case studies were named according to their original treatment systems. The systems were evaluated first by considering their present process diagram (base solution), and after a hypothetical replacement of the secondary treatment by vermifiltration (VF) (alternative solution).

SRI includes a pre-treatment followed by the slow infiltration system with Populus euroamericana and Eucaliptus colmadulensis as the used biomass, which occupies an area of 2,000 m2. It has a design capacity of 40 PE in winter and 120 in summer, with a flow rate of 5.0 m3 day−1 in winter and 15.0 m3 day−1 in summer. The biomass produced is harvested each 5 years, shredded, and sold to the paper pulp industry. The irrigation system is formed by a polyethylene piping network. CW is an experimental infrastructure located at Carrión de los Céspedes (Spain). It includes an Imhoff tank, which is used as primary treatment, and two vertical-flow wetlands in series with 317.0 m2 each, followed by a horizontal-flow wetland with 277.0 m2. Grown biomass is cut yearly and transported to a landfill. It has a design capacity of 120 PE and processes a flow rate of 15.0 m3 day−1. AS is a full-scale municipal infrastructure located in Vila Verde (Braga, Portugal). It has a design capacity of 500 PE and treats a flow rate of 60.0 m3 day−1. It is constituted by an activated sludge tank with two surface aerators, each working 11 h day−1, a primary clarifier, and a secondary clarifier. It is assumed that all sludge produced in CW and AS is deposited in landfill for biogas production.

VFs are packed media filters using earthworms where wastewater is loaded intermittently into the upper surface and allowed to percolate through the system (USEPA 2002). Sizing is made according to the influent flow rate, hydraulic retention time (HRT), hydraulic loading rate (HLR), organic loading rate (OLR), packing depth, and abundance of earthworms (Singh et al. 2017).

The data necessary to compute the materials and resources necessary to build the VF were retrieved from literature, based on sizing optimization parameters (Lourenço & Nunes 2017a). The VF is made of pre-cast concrete and is constituted by an organic packing (sawdust) (0.3 m depth) and an inert packing (sand with 0.3 m depth and aggregate with 0.3 depth). The bottom of the VF includes a clarifier for wastewater recirculation.

Life cycle inventory (LCI)

A full comparative life cycle inventory was carried out for the three WWTPs for the base solution and the alternative. A material balance was made for each individual process. In the analysis, all the inputs required and all the outputs generated were identified and quantified. The computations were made in OpenLCA® Nexus (version 1.7) (openLCA.org 2019), using Ecoinvent database, v. 3.5 (ECOINVENT 2019). CML Baseline v.4.4 (January 2015) was used to perform the quantification of the impacts, and EU25+3 (2000) for the normalization.

Though VFs are suited for primary or secondary treatment (Li et al. 2009), in order to keep the original primary treatments of the case studies, VF was sized for secondary treatment only. SRI and AS included a septic tank, and CW an Imhoff tank (Figure 1) as primary treatments, which were maintained when studying the replacement of the original secondary treatment by VF. So, the LCIA is used to assess the impacts of using vermifiltration as secondary treatment, replacing wetlands and the activated sludge tank (Figure 1).

Figure 1

Flowchart for SRI, CW and AS original WWTPs with VF alternative.

Figure 1

Flowchart for SRI, CW and AS original WWTPs with VF alternative.

The sizing of the VF followed optimized operational parameters obtained before (Lourenço & Nunes 2017b), namely: hydraulic retention time (HRT) (6 h), hydraulic loading rate (HLR) (0.89 m3 m−2 day−1), organic loading rate (OLR) (0.8 kg BOD m−3 day−1), abundance of earthworms (20 g L−1), recirculation flow rate (Qr/Qmix) (0.7) and the packing materials, which includes sawdust (0.3 m), quartz sand (0.3 m) and gravel (0.3 m), and wet to dry time ratio (W:D) (1:3) (Wang et al. 2014). The growth of earthworm's biomass was 0.12 g L−1 packing.day (Zhao et al. 2010). The size of the population of earthworms needed to be maintained constant throughout the exploration period, leading to the need to subtract some individuals. These are assumed to be released back in nature. Since both SRI-VF and CW-VF receive the same wastewater inflow (15 m3 day−1) (Table 1), and in order to keep the same W:D ratio in the VF (Wang et al. 2014) we assumed the same pumping work time and thus, the same electricity consumption.

Table 1

Detailed construction data and operation for the WWTPs

ParameterReference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Construction 
Unit operation area (m2)a 2,000.0 594.0 95.0 12.5 12.5 50.0 
Total plant area (m2)b 2,014.8 620.7 139.4 27.3 39.2 94.4 
Flow rate (m3 day−115.0 15.0 60.0 15.0 15.0 60.0 
Population served (No.) 120.0 120.0 500.0 120.0 120.0 500.0 
Wet to dry time ratio (W:D)b – – – 1:3 1:3 1:3 
Organic load (kg BOD PE−1 year−125.0 25.0 15.0 25.0 25.0 15.0 
Earthwork volume (m3)c – – – 0.270 0.270 0.110 
Total distance travelled (km PE−1)d 137.7 137.7 26.0 137.7 137.7 26.0 
Operation* 
BOD load (kg)e 740.0 740.0 453.0 740.0 740.0 453.0 
COD load (kg) 1,272 1,272 1,005 1,272 1,272 1,005 
Pollutants removal (kg BOD) 666.4 629.4 397.2 654.6 654.6 400.5 
Pollutants removal (kg COD) 1,081.1 1,017.5 828.5 960.6 960.6 758.8 
Primary sludge production (kg)f – 1,080.0 1,080.0 1,080.0 1,080.0 1,080.0 
Secondary sludge production (kg)g – – – 67–413 67–413 53–326 
Total distance travelled (km)h 0.483 0.483 0.115 0.525 0.525 0.151 
ParameterReference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Construction 
Unit operation area (m2)a 2,000.0 594.0 95.0 12.5 12.5 50.0 
Total plant area (m2)b 2,014.8 620.7 139.4 27.3 39.2 94.4 
Flow rate (m3 day−115.0 15.0 60.0 15.0 15.0 60.0 
Population served (No.) 120.0 120.0 500.0 120.0 120.0 500.0 
Wet to dry time ratio (W:D)b – – – 1:3 1:3 1:3 
Organic load (kg BOD PE−1 year−125.0 25.0 15.0 25.0 25.0 15.0 
Earthwork volume (m3)c – – – 0.270 0.270 0.110 
Total distance travelled (km PE−1)d 137.7 137.7 26.0 137.7 137.7 26.0 
Operation* 
BOD load (kg)e 740.0 740.0 453.0 740.0 740.0 453.0 
COD load (kg) 1,272 1,272 1,005 1,272 1,272 1,005 
Pollutants removal (kg BOD) 666.4 629.4 397.2 654.6 654.6 400.5 
Pollutants removal (kg COD) 1,081.1 1,017.5 828.5 960.6 960.6 758.8 
Primary sludge production (kg)f – 1,080.0 1,080.0 1,080.0 1,080.0 1,080.0 
Secondary sludge production (kg)g – – – 67–413 67–413 53–326 
Total distance travelled (km)h 0.483 0.483 0.115 0.525 0.525 0.151 

*Data for the 30 years of project.

aExclusively for secondary treatment.

bSum of primary and secondary treatment areas. 6 h wet time/18 hour dry time (Wang et al. 2014).

cData not available for the reference situation.

dTransportation includes the way back, excluding the inputs which were not used for each particular WWTP.

eThe organic load discharged to the receiver.

fBased on a daily production of 0.11 L PE−1day−1 (Paixão 2004) and a bulk density of 900 kg m−3 (Lourenço 2014). According to Machado et al. (2007) SRI did not produce primary sludge.

gWe opted for recirculation of this material in VFs. Based on a production of 0.07 to 0.43 kg suspended solids kg−1 COD removed (Xing et al. 2011). Production in the reference solution was not reported.

hDistance to landfill, including the way back, plus the distance travelled for sawdust transport. The reference solution does not include the sawdust transport, which was 0.042 km PE−1 in SRI-VF and in CW-VF, and 0.036 km PE−1 in AS-VF.

Since the lifetime of a typical WWTP is between 25 and 50 years (Piao et al. 2016), the expected lifetime for the mechanical equipment was set at 15 years, and at 30 years for construction works.

The principles and requirements for LCI follow those established in the ISO 14000 series standards, namely for goal and scope (ISO 14040 2006), and inventory analysis (ISO 14044 2006). According to the standards, the functional unit (FU) may be one cubic meter (Piao et al. 2016), or one person equivalent (PE) Kärrman & Jönsson (2001)). In the present study, the functional unit is one PE.

The system boundary was established at the WWTP fence, with a gate to gate approach, which includes the analysis from reception of the ready to use raw materials and influent watewater to the outflow of treated wastewater and wastes. It is a common procedure when unit process are specifically studied. It was assumed that the sludges from the primary treatment are deposited in landfill. This assumption is intended to close the boundary for simulation purposes.

Construction, operation, and dismantling phases were included, following common procedures (Buyukkamaci 2013; Li et al. 2013). The LCI included detailed tracking of all inputs and outputs throughout the facilities' life cycle (construction, operation, dismantling), including assessment of raw resources or materials, water, energy and fuel, solid wastes, air and water emissions. The analysis involved several individual unit processes, being the inputs and outputs assigned to each of them. All the energy requirements for the processes identified in the LCI were first quantified in terms of fuel and electricity units. The fuel used to transport raw materials to each process is included as a part of the LCI energy requirements. Emissions are categorized as atmospheric emissions, water pollutants, and solid wastes. Atmospheric emissions (as greenhouse emissions) include carbon dioxide, methane and nitrogen oxide, and carbon dioxide equivalent, all reported as kg PE−1. Water pollutants are reported as kg of pollutant per volume of wastewater per PE, which includes biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total suspended solids (TSS). The volume of wastewater produced per PE was considered equal to 90 L (Paixão 2004).

Construction

The input flows considered in the construction phase included (i) fossil fuel (kg); (ii) construction materials (kg) and (iii) transport of materials (km). All values are reported as per functional unit. Design data for the different treatment schemes are shown in Table 1.

Fossil fuel consumption was associated to (i) excavator (150–153 HP), wheel loader (260–285 HP), truck (413–496 HP), bulldozer (354 HP), a diesel powered electricity generator (80 HP) (DSS 2019); and (ii) estimated travel distances.

The time of use of on-site and electric machinery was determined considering the unit operation area required to build the infrastructures, considering they were made from precast concrete. The latter was supplied by a local company 25 km away from the WWTPs. The sewerage system was built using PVC pipes.

Fuel consumption included diesel from fuel-powered electricity generators and transport vehicles. Fuel consumptions for the generator were based on the average values indicated by Klanfar et al. (2016). For road vehicles, heavy-duty transport vehicles (16–32 ton) in Portugal, Spain, Germany, and Luxembourg were assumed to be compliant with EURO 0–4 mix, 22 t total weight, 17.3 t max payload. Travelled distances are shown in Table 1.

Operation

During operation, four main activities were considered (i) wastewater treatment; (ii) sludge management; (iii) packing renewal, in the case of vermifiltration; and iv) substitution of equipment and parts. The wastewater treatment included the total volume of influent and effluent wastewater (m3), therefore, assuming no losses in the process. The following flows were included: (i) electricity for pumping (MJ), (ii) fossil fuel consumption (kg).

Sludge treatment included the physico-chemical primary treatment and the biological secondary treatment. The following production values were assumed: (i) 0.11 L PE−1 day−1 for primary sludge (Paixão 2004) with 80% moisture; and (ii) 0.070 kg suspended solids, from secondary sludge produced from vermifiltration (Xing et al. 2011). The percentage of nitrogen and phosphorous in the sludges were 0.45% and 0.30%. wet weight, respectively (Lourenço 2014). In larger WWTPs, polyaluminum chloride and aluminum sulfate are usually used as the inorganic flocculants in sludge thickening and dewatering (Piao et al. 2016), but due to the small size of the studied WWTP, they are not used. For VF, the secondary sludge is a mixture of earthworm castings and excess sludge (Xing et al. 2011). Due to the low production of secondary sludge in VF, it was assumed that all secondary sludge is recirculated.

It was assumed that the size of the population of earthworms in the VF increased for the first 200 days of operation, stabilizing after that at a relatively constant size for the remaining 30 years of project (Domínguez 2018). The production of earthworm castings, the precursor of vermicompost in the VF, followed a kinetic production of 11.9 mg g−1 earthworm biomass day−1 (Zhao et al. 2010).

The electricity for the operation phase was obtained from the grid, using the electricity mix of the country, AC, 1–60 kV. During this phase, the only relevant electrical equipment were a wastewater pump with 1.3 HP, which worked for 6 h day−1, and a recirculation pump with 0.07 HP, which worked for 2 h day−1 in order to keep the adequate HRT (Bhunia et al. 2019) and an adequate wet to dry ratio in the VF (Wang et al. 2014) as described previously.

The removal of the vermicompost from the uppermost layer of the VF was made every 3 months and assumed to be made using appropriate manual tools and was assumed to be used onsite to fertilize the green areas.

For sawdust, one single trip was assumed every year for each WWTP. The transport of primary sludge, secondary sludge and greases was assumed to be made at the same time, on a yearly basis in SRI-VF and CW-VF and, due to the amounts of primary sludge produced, twice a year on AS-VF. Therefore, the two flows were considered together. Transport of materials and wastes in and out of the facilities was assumed to be made by a 7.5 ton lorry, 3.3 t max payload, EURO 0–4 mix. The distance from the WWTPs to the landfill was 29.0 km (from Carrión de los Céspedes to Seville, Spain) (SRI-VF and CW-VF), and 14.4 km (from Vila Verde to Ferreiros – Braga, Portugal) (AS-VF). Other detailed information is shown in Table 1 and Figure 2.

Figure 2

Total COD, TN and TP emissions for operation for the WWTPs.

Figure 2

Total COD, TN and TP emissions for operation for the WWTPs.

Figure 3

Concrete, gravel and sand consumptions for the WWTPs studied.

Figure 3

Concrete, gravel and sand consumptions for the WWTPs studied.

Dismantling

The flows in the dismantling phase included (i) fossil fuels consumption for the on-site machinery, electricity generator, concrete crushing machine and transportation (kg), and (ii) construction and general wastes (kg).

Fossil fuel consumption was related to the excavator (150–153 HP), wheel loader (260–285 HP), truck (413–496 HP), and bulldozer (354 HP) and stone crushing machine (50 HP) with a mean fuel consumption of 32 L h−1 (Klanfar et al. 2016). A pneumatic hammer with 3.35 HP was fed by a diesel-powered electricity generator with 80 HP and a fuel consumption of 18 L hour−1 at full load (DSS 2019). All the fuel consumptions were based on the same average values described above.

About 50% of the construction and demolition wastes were assumed to be crushed and recycled, following international experience (Trevor & Vallero 2019). Thus, detailed dismantling data does not include as wastes the materials that are recovered for reuse or recycling, which includes 50% of the total of concrete, gravel, sand, steel, and all the metal parts of the pumps. The remaining general wastes, which include steel mesh, PVC and HDPE pipes and PP, were considered to be 50% recycled. Transport of wastes out of the facilities was assumed to be made in the same conditions described in construction phase.

Life cycle impact assessment (LCIA)

The impact assessment step was made using the CML Baseline (v. 4.4, January 2005) characterization method, since it is one of the few that considers organic matter and nutrients as emissions. Normalization was made using EU25+3 (2000). No allocation was made. The selection of impacts was based on the works made by Corominas et al. (2013) and Jeppsson & Hellström (2002): Abiotic deplection (AD) (kg Sb eq.), Acidification (AC) (kg SO2 eq.), Eutrophication (EUT) (kg PO43− eq.); Fresh water aquatic ecotoxicity (FWT) (kg 1,4-DB eq.); Global warming potential (GWP) (kg CO2 eq.); MAEC (Marine aquatic ecotoxicity) (kg 1,4-DB eq.); Ozone layer depletion (OLD) (kg CFC-11 eq.); Human toxicity (HT) (g 1,4-DB eq.); Photochemical oxidation (PO) (kg C2H4 eq.); and Terrestrial ecotoxicity (TE) (1,4-DB eq.).

LCI

Table 4 shows the most important inputs for the three WWTP scenarios under study. The area of land occupied by the alternative VF solution is much lower than in the base solution, decreasing from 16.7 to 0.230 m2 PE−1 in SRI-VF; and from 4.95 to 0.330 m2 PE−1 in CW-VF. Given that the size of the activated sludge tank is similar to the vermifilter, the occupied land is equal, 0.190 m2 PE−1. Despite having the same served population (120 PE), SRI-VF occupies less land area than CW-VF due to different primary treatment systems, septic tank in SRI-VF and Imhoff tank in CW-VF. Imhoff tanks are used by small communities and their underground construction minimizes land use (Tilley et al. 2014). The estimated values are close to those obtained elsewhere for VF-based solutions. In France (Combaillaux), the typical area of land for a VF was 0.25 m2 PE−1 (with no pre-treatment included) (RECYCLAGUA 2019). In rural areas of China, Nie et al. (2013) found that the typical area of land for VF varied from 0.060 to 0.21 m2 PE−1 (with no pre-treatment included). In India, typical land area was reported to vary between 0.5 and 0.6 m2 PE−1 (Sinha et al. 2014).

The amount of materials used during the construction phase of the alternative were, by decreasing order, concrete (515.2 kg PE−1 in CW-VF), sand (46.9 kg PE−1 both in SRI-VF and CW-VF), gravel (44.7 kg PE−1 both in SRI-VF and CW-VF), and steel (18.2 kg PE−1 in CW-VF). These are somewhat higher than those found elsewhere for similar infrastructures (Sapkota 2016), namely for cement (8.98–21.1 kg PE−1), sand (18.27–27.46 kg PE−1), aggregate (28.58–73.71 kg PE−1), steel (1.23–3.13 kg PE−1), and PVC (0.006–0.31 kg PE−1). Concrete use was always higher in the alternative solution due to the need to build the vermifilter (Table 4). Gravel represents one of the most significant inputs during construction of CW, resulting in being one the materials with the highest variations between the base and alternative solutions. In the opposite direction go sand and iron, both used in the construction of the vermifilter (Table 4 and Figure 3).

Steel, stainless steel, and pumps came from suppliers far away from the construction sites, justifying the large transport distances (stainless steel, 5.50 km PE−1 for both SRI/SRI-VF and CW/CW-VF, and 1.20 km PE−1 for AS/AS-VF; pumps, 6.33 km PE−1 for both SRI/SRI-VF and CW/CW-VF, and 1.40 km PE−1 for AS/AS-VF).

Fossil fuel consumption is proportional to travelled distances, and the number of hours of electricity generator use in the construction and demolition phases. Steel and stainless steel transportation showed the highest fossil fuel consumption since these materials have the longest distance travelled between each manufacturer and the construction site (Table 2). In terms of fuel consumption, no significant alteration between the reference solution and the alternative was found due to the negligible contribution of the transport of sawdust for filling of the vermifilters.

Table 2

Estimated travel distances for transportation of inputs used in the construction of the WWTPs (per PE)

InputaDistance to WWTP (km)
SRI-VFCW-VFAS-VF
Pre-cast concrete 0.42 0.42 0.10 
Gravel 9.7 9.7 0.01 
Quartz sand 10.5 10.5 1.20 
Steel (inc. stainless steel) 36.3 36.3 8.00 
Geotextile fiber (PP) 10.6 10.6 4.20 
Pipes (PVC) 19.2 19.2 0.500 
Pipes (HDPE) 11.5 11.5 0.400 
Sawdust 0.040 0.040 0.020 
Earthwormsb – – – 
Pumps 39.2 39.2 8.70 
Total 137.9 137.9 23.8 
InputaDistance to WWTP (km)
SRI-VFCW-VFAS-VF
Pre-cast concrete 0.42 0.42 0.10 
Gravel 9.7 9.7 0.01 
Quartz sand 10.5 10.5 1.20 
Steel (inc. stainless steel) 36.3 36.3 8.00 
Geotextile fiber (PP) 10.6 10.6 4.20 
Pipes (PVC) 19.2 19.2 0.500 
Pipes (HDPE) 11.5 11.5 0.400 
Sawdust 0.040 0.040 0.020 
Earthwormsb – – – 
Pumps 39.2 39.2 8.70 
Total 137.9 137.9 23.8 

aFrom each manufacturer/supplier, including the way back.

bAssumed to be picked in nature near the WWTPs.

The inventory during operation for the three WWTPs is reported in Table 3. Discussion is henceforward referred to a period of 30 years. Both SRI and CW treat the same total inflow of 1,369 m3 PE−1 while AS treats 1,314 m3 PE−1. Water consumption in the reference solution was 142.7 kg PE−1 in SRI and CW to 1,629 kg PE−1 in AS-VF and, in the alternative of 142.8 kg PE−1 both in SRI-VF and CW-VF to 965.5 kg PE−1 in AS-VF. The amount of sawdust used during operation was 74.3 kg PE−1 for both SRI-VF and CW-VF, and 71.3 kg PE−1 for AS-VF. No sawdust or earthworms were used or vermicompost produced in the reference solution since they are specific to the filter packing of VF.

Table 3

LCI results for the construction, operation and dismantling phases of the three WWTPs (per PE)

FlowUnitReference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Construction phase 
Concretea kg 175.2 110.8 – 487.1 515.2 354.4 
Gravel kg – 1.08×104 – 44.7 44.7 42.9 
Sandb kg 9.54 0.170 0.270 46.9 46.9 45.0 
Steel (inc. stainless steel) kg – – 24.28 4.1 18.2 3.0 
Iron kg 169.5 0.3400 7.100 206.0 661.7 625.2 
Polypropylene kg 0.121 0.643 – 8.07×10−3 8.07×10−3 7.75×10−3 
PVC kg 0.034 3.17×10−3 – 0.137 0.137 0.033 
HDPE kg 4.57 1.47×10−2 – 7.92×10−3 7.92×10−3 1.90×10−3 
Sawdust kg – – – 7.46 7.46 7.16 
Earthworms kg – – – 7.5 7.5 7.2 
Fossil fuel (on-site machinery) kg 0.386 0.698 0.278 0.386 0.698 0.278 
Fossil fuel (electricity generator) kg 0.498 0.902 0.360 0.498 0.902 0.360 
Total fossil fuel kg 0.884 1.60 0.638 0.884 1.60 0.638 
Operation 
Raw wastewater m3 1,369.0 1,369.0 1,314.0 1,369.0 1,369.0 1,314.0 
Emissions to water (COD) kg 190.8 254.4 176.2 311.4 311.4 245.9 
Emissions to water (TN)c kg 39.8 76.3 63.2 77.4 77.4 55.0 
Emissions to water (TP)c kg 29.3 60.7 10.5 36.6 36.6 7.50 
TN (in primary sludge)d kg – 4.88 4.88 4.88 4.88 4.88 
TP (in primary sludge)d kg – 3.25 3.25 3.25 3.25 3.25 
Sawdust consumptione kg – – – 74.3 74.3 71.3 
Production of earthwormsf kg – – – 0.75 0.75 0.72 
Production of vermicompost kg – – – 16.3 16.3 15.6 
Electricity consumptiong MJ 3,940.0 3,940.0 1.16×106 6,340.0 6,340.0 4,520.0 
Dismantling 
Construction wasteh kg 2.43×10−5 1.77×10−4 – 291.0 312.0 222.0 
General wastei kg 0.179 1.309 2.517 0.119 0.119 0.114 
Fossil fuel (on-site machinery) kg 0.386 0.698 0.278 0.386 0.698 0.278 
Fossil fuel (electricity generator) kg 0.125 0.225 0.090 0.125 0.225 0.090 
Fossil fuel (crushing machinery) kg – – – 0.664 1.200 0.479 
Total fossil fuel kg 0.510 0.924 0.368 1.170 2.130 0.848 
FlowUnitReference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Construction phase 
Concretea kg 175.2 110.8 – 487.1 515.2 354.4 
Gravel kg – 1.08×104 – 44.7 44.7 42.9 
Sandb kg 9.54 0.170 0.270 46.9 46.9 45.0 
Steel (inc. stainless steel) kg – – 24.28 4.1 18.2 3.0 
Iron kg 169.5 0.3400 7.100 206.0 661.7 625.2 
Polypropylene kg 0.121 0.643 – 8.07×10−3 8.07×10−3 7.75×10−3 
PVC kg 0.034 3.17×10−3 – 0.137 0.137 0.033 
HDPE kg 4.57 1.47×10−2 – 7.92×10−3 7.92×10−3 1.90×10−3 
Sawdust kg – – – 7.46 7.46 7.16 
Earthworms kg – – – 7.5 7.5 7.2 
Fossil fuel (on-site machinery) kg 0.386 0.698 0.278 0.386 0.698 0.278 
Fossil fuel (electricity generator) kg 0.498 0.902 0.360 0.498 0.902 0.360 
Total fossil fuel kg 0.884 1.60 0.638 0.884 1.60 0.638 
Operation 
Raw wastewater m3 1,369.0 1,369.0 1,314.0 1,369.0 1,369.0 1,314.0 
Emissions to water (COD) kg 190.8 254.4 176.2 311.4 311.4 245.9 
Emissions to water (TN)c kg 39.8 76.3 63.2 77.4 77.4 55.0 
Emissions to water (TP)c kg 29.3 60.7 10.5 36.6 36.6 7.50 
TN (in primary sludge)d kg – 4.88 4.88 4.88 4.88 4.88 
TP (in primary sludge)d kg – 3.25 3.25 3.25 3.25 3.25 
Sawdust consumptione kg – – – 74.3 74.3 71.3 
Production of earthwormsf kg – – – 0.75 0.75 0.72 
Production of vermicompost kg – – – 16.3 16.3 15.6 
Electricity consumptiong MJ 3,940.0 3,940.0 1.16×106 6,340.0 6,340.0 4,520.0 
Dismantling 
Construction wasteh kg 2.43×10−5 1.77×10−4 – 291.0 312.0 222.0 
General wastei kg 0.179 1.309 2.517 0.119 0.119 0.114 
Fossil fuel (on-site machinery) kg 0.386 0.698 0.278 0.386 0.698 0.278 
Fossil fuel (electricity generator) kg 0.125 0.225 0.090 0.125 0.225 0.090 
Fossil fuel (crushing machinery) kg – – – 0.664 1.200 0.479 
Total fossil fuel kg 0.510 0.924 0.368 1.170 2.130 0.848 

aAs pre-cast concrete.

bAs quartz sand.

cIn the reference solution, the distance suppression of the sawdust transportation was negligible. Total nitrogen in raw wastewater was established as 1.75 of the NH4+ emissions and total phosphorus as 1.67 of the PO43− emissions (Henze & Comeau 2008).

dAccording with the values from Lourenço (2014).

eThe application of sawdust during the packing renewal.

fBiomass growth.

gIncludes the electricity consumption for lights and, in the alternative solution, the electricity required to pump the wastewater from primary treatment and for effluent recirculation. The value for AS was adjusted from MJ to GJ due to inconsistency of original units.

h50% of the construction waste was assumed to be recycled by private companies (50% of concrete+gravel+steel+sand)).

iStainless steel+PP+PVC+HDPE. In the reference situation, values from construction and general waste were the ones reported by Machado et al. (2007).

Emissions to water courses were computed for the chemical oxygen demand (COD), total nitrogen and total phosphorus. The accumulated COD emissions during operation were of 311.4 kg PE−1 in SRI-VF and in CW-VF, and of 245.9 kg PE−1 in AS-VF (Table 3), therefore between 22 and 63% higher than in the reference solution (190.8 kg PE−1 in SRI, 254.4 kg PE−1 in CW, and 176.2 kg PE−1 in AS). They are nonetheless about ten times lower than those reported for treatment plants of similar size (Hospido et al. 2008), so the emission of nutrients is already well optimized in the studied base and alternative solutions.

Electricity consumption in the reference solution varied from 3,940 MJ PE−1 in SRI and CW to 1.16×106 MJ PE−1 and, in the alternative solution, from 4,520 MJ PE−1 in AS-VF to 6,340 MJ PE-1 in SRI-VF and CW-VF. The values of electricity were higher for SRI-VF and CW-VF compared with SRI and CW due to the introduction of a new pump. It was lower in the alternative solution for AS due to the elimination of the air injection system. Electricity consumption is a key element in the overall environmental performance of a WWTP. In fact, energy use has been already identified as one of the major sustainable development indicators for wastewater treatment systems (Palme et al. 2005). Energy consumption in the reference solution and in the alternative are in line with values reported by other authors. Nie et al. (2013), in a study with a tower VF to attend a served population of 230 PE, reported electricity consumptions were in the range of 200 MJ PE−1 to 750 MJ PE−1. Tillman et al. (1998) found, for a period of 30 years, for two small WWTPs in Sweden, electricity consumptions in the range of 228 MJ PE−1 to 576 MJ PE−1. Hospido et al. (2008), studying thirteen small WWTPs in Galicia (Spain), found mean electricity consumptions in the range between 102 and 427.8 MJ PE−1. Magar (2016) refers electricity consumptions during operation of 234 MJ PE−1 and 1,944 MJ PE−1 when managing three small WWTPs in Norway and De Feo et al. (2016) refer electricity consumptions of 4,320 MJ PE−1, and Weiss et al. (2008) of 4.5×105 MJ PE−1. Treating wastewater using VF, Laws (2003) refer electricity consumption in the range of 5.0×105 MJ PE−1 to 3.7×106 MJ PE−1, due to the use of UV for disinfection. In fact, an increase of 30% in the total use of electricity can be expected when UV disinfection is used (Nie et al. 2013). Electricity demand in WWTPs is also influenced by the inflow since higher inflows show generally lower consumption per PE (Trapote et al. 2014), due to economies of scale in larger infrastructures, leading to larger but more efficient equipment, better performing automation, and better-trained staff operators.

The total production of sludges during the operation period was the same on all WWTPs, equal to 1,100 kg PE−1, which is similar to values referred by Laws (2003) for his VF study in Chile, of near 1,056 kg PE−1 (after correcting his moisture content to 80%). Magar (2016) refers sludge productions between 5.98 and 50 kg PE−1, Hospido et al. (2008) indicates productions in the range of 84.6 kg PE−1 to 717.3 kg PE−1; and Tillman et al. (1998) in the range of 96.3 kg PE−1 to 768 kg PE−1. The prediction of vermicompost production was 16.3 kg PE−1 in SRI-VF and in CW-VF, and 15.6 kg PE−1 in AS-VF (Table 3).

During the dismantling phase, the main difference between the base and alternative solutions resides in the need to dismantle the vermifilter, which resulted in an added amount of construction waste of around 300 kg PE−1. Other unspecified waste was also created, but in small amounts (Table 3). During this phase, the heavy duty equipment and generator were responsible for the consumption of a substantially larger amount of fuel in the alternative solution than in the reference (about 900 times for SRI-VF; 540 times for AS-VF; and 90 times for CW).

LCIA

The construction, operation and dismantling of the WWTPs lead to positive net emissions of most selected LCIA indicators (Table 4). The exceptions were for a minor reincorporation of abiotic resources in AS and in all the alternative solutions during construction, which is due to the way material recycling is accounted in CML's characterization factors. Due to the way LCIA results are presented, the positive indicator values may correspond to negative environmental impacts, and the opposite to the negatives. We will refer throughout the text to the signal of impacts having these relationships in mind.

Table 4

Results per impact category by each one of the three phases for the two scenarios (per PE)

PhaseImpact categoryReference unitImpact result
Reference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Const AD kg Sb eq. 1.47×10−5 2.20×10−4 −8.26×10−4 −9.93×10−5 −5.76×10−4 −7.22×10−5 
O&M 1.80 1.80 530.1 2.898 2.898 2.065 
Dism 1.49×10−7 2.20×10−7 1.98×10−7 1.14×10−7 1.83×10−7 2.98×10−7 
Sum 1.800 1.800 530.099 2.989 2.897 2.065 
Const AC kg SO2 eq. 0.0451 8.21 0.165 0.196 0.280 0.113 
O&M 0.790 0.321 6.190 0.190 0.190 0.953 
Dism 1.75×10−3 3.17×10−3 1.26×10−3 2.23×10−3 3.10×10−3 4.90×10−3 
Sum 0.837 8.53 6.36 0.388 0.473 1.07 
Const EUT kg PO43− eq. 7.7×10−3 1.83 0.0248 0.037 0.048 0.0197 
O&M 13.1 24.3 20.7 8.92 8.92 7.49 
Dism 1.84×10−4 3.33×10−4 1.33×10−4 3.90×10−4 4.93×10−4 7.62×10−4 
Sum 13.1 26.1 20.7 8.96 8.97 7.51 
Const FWT kg 1,4-DB eq. 0.433 40.4 0.9977 1.75 1.81 0.459 
O&M 0.270 3.19 2.15 0.106 0.106 0.964 
Dism 3.80×10−3 6.88×10−3 2.74×10−3 2.42×10−3 4.14×10−3 6.82×10−3 
Sum 0.707 43.6 3.15 1.86 1.92 1.43 
Const GWP kg CO2 eq. 23.0 1.78×103 46.3 81.7 108 52.7 
O&M 39.5 150 218 26.4 26.4 130 
Dism 0.234 0.425 0.169 0.396 0.520 0.811 
Sum 62.7 1.93×103 264 108 135 183 
Const HT kg 1,4-DB eq. 6.63 349 3.47 22.5 24.5 11.4 
O&M 3.43 2.88 33.1 2.57 2.57 16.8 
Dism 0.038 0.068 0.027 0.041 0.059 0.094 
Sum 10.1 352 36.6 25.1 27.1 28.3 
Const MAEC kg 1,4-DB eq. 213 2.47×104 463 770 102 469 
O&M 411 1.11×103 4.67×103 495 495 5.93×103 
Dism 11.0 20.0 7.95 9.78 15.0 24.1 
Sum 635 2.58×104 5.14×103 1.28×103 612 6.42×103 
Const OLD kg CFC-11 eq. 4.32×10−7 9.96×10−6 1.11×10−6 1.38×10−6 2.08×10−6 9.96×10−7 
O&M 2.93×10−6 2.4×10−6 3.39×10−5 2.4×10−6 2.94×10−6 7.33×10−7 
Dism 6.04×10−10 1.10×10−9 4.36×10−10 8.40×10−10 1.15×10−9 1.80×10−9 
Sum 3.36×10−6 1.24×10−5 3.50×10−5 3.78×10−6 5.02×10−6 1.73×10−6 
Const PO kg C2H4 eq. 3.89×10−3 0.594 0.019 0.017 0.027 0.010 
O&M 0.013 0.047 0.084 9.79×10−3 9.97×10−3 0.051 
Dism 1.42×10−4 2.58×10−4 1.03×10−4 1.69×10−4 2.40×10−4 3.80×10−4 
Sum 0.017 0.641 0.103 0.027 0.037 0.061 
Const TE kg 1,4-DB eq. 0.020 0.527 0.060 0.091 0.125 0.064 
O&M 0.019 0.021 0.220 0.021 0.021 0.235 
Dism 0.011 0.019 7.64×10−3 5.93×10−3 0.011 0.018 
Sum 0.050 0.567 0.288 0.118 0.157 0.317 
PhaseImpact categoryReference unitImpact result
Reference solution
Alternative solution
SRICWASSRI-VFCW-VFAS-VF
Const AD kg Sb eq. 1.47×10−5 2.20×10−4 −8.26×10−4 −9.93×10−5 −5.76×10−4 −7.22×10−5 
O&M 1.80 1.80 530.1 2.898 2.898 2.065 
Dism 1.49×10−7 2.20×10−7 1.98×10−7 1.14×10−7 1.83×10−7 2.98×10−7 
Sum 1.800 1.800 530.099 2.989 2.897 2.065 
Const AC kg SO2 eq. 0.0451 8.21 0.165 0.196 0.280 0.113 
O&M 0.790 0.321 6.190 0.190 0.190 0.953 
Dism 1.75×10−3 3.17×10−3 1.26×10−3 2.23×10−3 3.10×10−3 4.90×10−3 
Sum 0.837 8.53 6.36 0.388 0.473 1.07 
Const EUT kg PO43− eq. 7.7×10−3 1.83 0.0248 0.037 0.048 0.0197 
O&M 13.1 24.3 20.7 8.92 8.92 7.49 
Dism 1.84×10−4 3.33×10−4 1.33×10−4 3.90×10−4 4.93×10−4 7.62×10−4 
Sum 13.1 26.1 20.7 8.96 8.97 7.51 
Const FWT kg 1,4-DB eq. 0.433 40.4 0.9977 1.75 1.81 0.459 
O&M 0.270 3.19 2.15 0.106 0.106 0.964 
Dism 3.80×10−3 6.88×10−3 2.74×10−3 2.42×10−3 4.14×10−3 6.82×10−3 
Sum 0.707 43.6 3.15 1.86 1.92 1.43 
Const GWP kg CO2 eq. 23.0 1.78×103 46.3 81.7 108 52.7 
O&M 39.5 150 218 26.4 26.4 130 
Dism 0.234 0.425 0.169 0.396 0.520 0.811 
Sum 62.7 1.93×103 264 108 135 183 
Const HT kg 1,4-DB eq. 6.63 349 3.47 22.5 24.5 11.4 
O&M 3.43 2.88 33.1 2.57 2.57 16.8 
Dism 0.038 0.068 0.027 0.041 0.059 0.094 
Sum 10.1 352 36.6 25.1 27.1 28.3 
Const MAEC kg 1,4-DB eq. 213 2.47×104 463 770 102 469 
O&M 411 1.11×103 4.67×103 495 495 5.93×103 
Dism 11.0 20.0 7.95 9.78 15.0 24.1 
Sum 635 2.58×104 5.14×103 1.28×103 612 6.42×103 
Const OLD kg CFC-11 eq. 4.32×10−7 9.96×10−6 1.11×10−6 1.38×10−6 2.08×10−6 9.96×10−7 
O&M 2.93×10−6 2.4×10−6 3.39×10−5 2.4×10−6 2.94×10−6 7.33×10−7 
Dism 6.04×10−10 1.10×10−9 4.36×10−10 8.40×10−10 1.15×10−9 1.80×10−9 
Sum 3.36×10−6 1.24×10−5 3.50×10−5 3.78×10−6 5.02×10−6 1.73×10−6 
Const PO kg C2H4 eq. 3.89×10−3 0.594 0.019 0.017 0.027 0.010 
O&M 0.013 0.047 0.084 9.79×10−3 9.97×10−3 0.051 
Dism 1.42×10−4 2.58×10−4 1.03×10−4 1.69×10−4 2.40×10−4 3.80×10−4 
Sum 0.017 0.641 0.103 0.027 0.037 0.061 
Const TE kg 1,4-DB eq. 0.020 0.527 0.060 0.091 0.125 0.064 
O&M 0.019 0.021 0.220 0.021 0.021 0.235 
Dism 0.011 0.019 7.64×10−3 5.93×10−3 0.011 0.018 
Sum 0.050 0.567 0.288 0.118 0.157 0.317 

Negative values indicate removal of the indicator substance, therefore a positive impact.

Const., Construction phase; O&M, Operation phase (values for 30 years); Dism., Dismantling phase; Sum, Sum of all three phases.

Considering the reference solution, the treatment showing worst impacts in most categories throughout the life cycle is CW due to the impacts during the construction phase, followed by AS and SRI (sums of each phase are described in Table 4; see more detailed analysis in Supplementary Material). The only exception was OLD, where AS has marginally higher emissions of CFC-11 eq. made during the production of the stainless steel activated sludge prefabricated unit. In CW the construction phase contributes most to its life cycle impacts due to the earthworks involved, namely in building access roads, clearing, constructing basins and dikes, installing piping and valving, planting, seeding, liming, fertilizing, and mulching dikes and disturbed areas. In the remaining systems, operation is the phase where impacts are highest. Dismantling has the lowest impact in our simulations due to the assumption of recycling 50% of demolition wastes.

In the alternative solution, the replacement of the constructed wetland or of the activated sludge by the vermifilter seems to bring important environmental benefits, as reflected in an improvement in most impact categories, in particular in the construction phase. This is due to VF being less energy-intensive for inputs such as steel and plastic materials. The substitution of slow rate filtration by the vermifilter results in the improvement of indicators AC and EUT, but in the deterioration of the remaining indicators. Thus, the vermifilter would be a better environmental solution than CW and AS in the studied WWTPs and, according to the performances obtained by the impact categories, a more sustainable solution (Hannouf & Assefa 2018).

A more detailed analysis shows that during the operation phase, the vermifilter has lower impacts than the remaining solutions in most impact categories. The exceptions are in AD, with the indicator worsening between one and two orders of magnitude (2.898 kg Sb eq. PE−1 in SRI-VF and CW-VF, and 2.065 kg Sb eq. PE−1 in AS-VF); and MAEC and TE, with only marginal deterioration of the indicators' values (for MAEC 495 kg 1,4 DB eq. in SRI-VF and CW-VF, and 5.93×103 kg 1,4 DB eq. in AS-VF; and for TE 0.021 kg 1,4 DB eq. in SRI-VF and CW-VF, and 0.235 kg 1,4 DB eq. in AS-VF). The higher emissions of COD (311.4 kg PE−1 in SRI and CW, and 249.9 kg PE−1 in AS-VF), but lower emissions of nutrients (77.4 kg TN PE−1 in SRI-VF and in CW-VF, and 55.0 kg PE−1 in AS-VF; and 36.6 kg TP PE−1 in SRI-VF and in CW-VF, and 7.50 kg PE−1 in AS-VF) in the alternative may help justify these differences.

Studies report the high relative weight of operation phase impacts of WWTPs with electricity and sludge managment usually being the most important flows during operation (De Feo et al. 2016). Several authors even assume that the contribution of construction and dismantling is negligible if compared with operation, especially during a relatively long lifetime (Corominas et al. 2013). Our results, on the contrary, show that for very small WWTPs the impacts of the construction phase do not dilute enough throughout the lifecycle and number of served inhabitants, unlike the economies of scale of larger facilities.

Dismantling was the phase that contributed less for the impact categories on both solutions, being in some categories negligible or even zero.

This research aimed to find the usefulness of that the results found, especially the fact that the replacement of constructed wetland or activated sludge by vermifilter, in construction phase, seems to reflect in an improvement in most of the environmental impact categories. In the operation phase, the VF has lower environmental impacts than the remaining solutions for most impact categories. Future studies should include social and economic analysis and improve the LCA analysis with a full perspective of circular economy.

This study made a full comparative life cycle impact assessment for nature-based solutions for wastewater treatment in rural areas where a slow rate filtration plant, a constructed wetland, and an activated sludge system (designated reference solution), were all evaluated as were, and after substitution of the secondary treatment by vermifiltration (alternative solution). Detailed lifecycle inventory was obtained for each solution, which showed that more material resources are used during construction than in any other phase. Given the small served population, the material intensity (per PE) was higher than that found in other larger facilities. On the contrary, electricity was the resource more used during operation, which was an expected result. They are more aligned with consumption rates found elsewhere, indicating that the operation of the facilities follows standardized procedures, therefore with little optimization freedom.

The replacement of constructed wetland by vermifiltration as secondary treatment seems to bring important environmental benefits in most impact categories, in particular in the construction phase. The replacement by VF in facilities with SRI seems to result in the improvement of some of the impact categories, in particular in the operation phase. As for dismantling, no conclusive results were obtained for the replacement by VF.

Our results show that the impacts during the construction phase outweigh those of the other phases when the number of served inhabitants is small, due to lack of economies of scale for all the scenarios. The use of vermifiltration as alternative secondary treatment, in particular to constructed wetlands and activated sludge, would help reduce the impacts of several environmental categories. VF would be a better environmental solution than CW and AS in the studied WWTPs. We should note here that, as in all LCA studies, the results can be affected by the specificity of local conditions. These were minimized here as far as possible by setting the appropriate limits for the boundaries of the systems.

Life-cycle impact assessment provides but another measure of environmental sustainability and efficiency of alternative technologies, which should complement socio-economic and technological constraints (Lourenço & Nunes 2020).

The first author benefited from the financial support of FUTURAMB. The second author was supported by project GROUNDWATER – Citizen science and ICT-based enhanced information systems for groundwater. 2020–2024. European Union Horizon 2020.

All relevant data are included in the paper or its Supplementary Information.

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