Climate change

The climate change category is one of the best known and commonly used as an environmental impact indicator, although the discharge of effluent into the environment does not have an important effect on this category. The energy intensity in the treatment process makes it important to include it for evaluation (^{Lorenzo-Toja et al., 2016b}). In this context, electric power proved to be the main contributor in all technologies evaluated as shown in Figure 3.

Figure 3 Contribution of the kind of technology for the removal of 1 kg BOD₅ for the climate change category 

Ranges between 1.24-3.49 kg CO_2eq /kg BOD₅ were obtained for aerated lagoon plants, with an average of 2.38 kg CO_2eq /kg BOD₅, where the consumption of electric energy contributed 59% of the impacts. A 27% of the contribution to the environments impacts were due to the use of chemicals inputs, where the disinfection stage had the higher contribution (15.4%) due to the use of sodium hypochlorite (15%) and chlorine gas (0.4%), followed by inputs chemicals such as ferric chloride (9.8%), sodium metabisulfite (1.1%) and polymer (0.7%). Finally, the disposal of sludge contributed 11% of the impacts, mainly due to methane emissions in the landfill and monorefilled landfill disposal.

It was found that DWWTP 2 and 4 obtained the worst performances in energy terms (2.7 and 5 kWh / kg of BOD₅ respectively), with both having in common the lack of use of supplies and electrical energy for sludge stabilization, so the energy demand is attributed exclusively to the treatment of the waterline showing energy inefficiencies in its process, compared to the other aerated lagoon plants, such as DWWTP 5, which has a consumption of 1.1 kWh/kg of BOD₅ without sludge purge.

Ranges of 0.67-3.98 kg CO_2eq / kg BOD₅ with an average of 1.96 kg CO_{2 eq}/kg BOD₅ were found by active sludge technology. Being the electricity the main contributor with consumptions that vary from 0.8 - 5.5 kWh / kg BOD₅ and thus causing 71% of the impacts. Followed by diesel fuel used in the generation of electrical energy, and the use of chemical inputs, with a contribution of 16% and 10%, respectively. In the case of the use of inputs products, the disinfection stage contributed 6%, where the sodium hypochlorite was responsible for 99% of these impacts, having an average consumption of 0.2 kg/kg BOD₅. The atmospheric emissions related to the burning of diesel fuel and biogas generated in anaerobic digestion plants contributed only 2.3%, proving to be negligible compared to the other contributions.

The plants with the most impact on removing 1 kg BOD₅ in the climate change category were DWWTPs 8, 9 and 10, in agreement with the higher energy and fuel consumption required for their process. These DWWTPs having in common that they do not use anaerobic digestion with biogas recovery and requiring other power supply for their energetic demands.

In the case of the biofilter and vermi-biofilter DWWTPs electrical power also proved to be primarily responsible for impacts with 80% and 74% respectively, followed by biofilter fuel (11%), and the use of chemicals for vermi-biofilter (25%). In the last one, the use of sodium hypochlorite contributed about 20% of these impacts.

Freshwater eutrophication

The greatest impacts for the freshwater eutrophication category were obtained in aerated lagoons, varying from 8.14x 10^-4 to 2.15x10^-3 kg P eq/kg BOD₅, with an average of 1.30x10^-3 kg P eq/kg BOD₅. For active sludge wastewater treatment plants, a range between 8.20x10^-5 and 2.36x10^-3 kg P eq/kg BOD₅ was obtained, with an average of 1.15x10^-3 kg P eq/kg BOD₅. Impacts of 5.56x10^-4 kg P eq/kg BOD₅ and 7.18x10^-4 kg P _eq/kg BOD₅ were obtained in the biofilter and vermi-biofilter technologies, respectively, suggesting that biofilter technology has the best environmental performance. Electrical consumption turned out to be the process with the greatest contribution to this category, this being between 70-80% (Figure 4); except for DWWTP 7, mainly due to the change in the energy matrix used in the plant (Aysén matrix). In the case of DWWTP 7, sodium hypochlorite was the main contributor to the freshwater eutrophication category.

Figure 4 Contribution by kind of technology for the removal of 1 kg BOD₅ for the category of freshwater eutrophication. 

The second contributor to the freshwater eutrophication category was the use of chemicals inputs, with a contribution of 24% and 20% for aerated lagoons and vermi-biofilter technologies, respectively. Sodium hypochlorite was one that contributed the most to these impacts with 13% for aerated lagoons and 16% for vermi-biofilter, followed by ferric chloride (10%) and sodium metabisulfite (4%). On the other hand, for active sludge and biofilter, the second contributor was diesel fuel.

Freshwater Ecotoxicity

In the freshwater ecotoxicity category the impacts varied from 0.03 to 0.12 kg 1.4-DCB _eq/kg BOD₅, with an average of 0.07 kg 1.4-DCB _eq/kg BOD₅ for aerated lagoons, having this technology the greatest impacts. For active sludge values between 0.01 - 0.12 kg 1.4-DCB _eq / kg BOD₅ were obtained, with an average of 0.06 kg 1.4-DB eq / kg BOD5. Impact of 0.01 kg 1.4-DCB _eq/kg BOD₅ and 0.02 kg 11.4-DCB _eq/kg BOD₅ were obtained by biofilter and vermi-biofilter technologies, respectively, suggesting that biofilter technology had the best environmental performance. The electricity was the main contributor to the freshwater ecotoxicology category for the case of vermi-biofilter and biofilter technologies, with 62.4% and 77.6% of the contribution, respectively (Figure 5), being the emission of copper the main aspect within this category (30 - 36%). On the other side, water emissions (discharge) from the dumping of treated waters contributing 48.8% and 59.2% for the technologies of aerated lagoons and active sludge, respectively, thus being the residual chlorine the main contributed (between 48 - 55%).

Figure 5 Contribution of kind of technology for the removal of 1 kg BOD₅ in the freshwater ecotoxicity category. 

For vermi-biofilter technology the second contributor to the freshwater ecotoxicity category corresponds to the use of chemicals inputs represent a 32%, while for aerated lagoons the contribution of chemical products was a 30%. For biofilter technology, chemists accounted for 10.2%, mainly due to the use of ferric chloride (8.2%), and fuel use (10.6%). On the other hand, the use of electrical energy contributed 26.4% and 28.1% for the technologies of aerated lagoons and active sludge respectively.

DWWTP 11 shows the lowest values for this category with 0.01 kg 1.4-DCB _eq/kg BOD₅, attributed to the use of ozone for the disinfection stage, where, residual chlorine emissions were not generated. Finally, the technologies that presented the highest consumption of electrical energy (lagoons and sludge with 2.6 and 2.3 kWh / kg of BOD₅ respectively), did not show a significant impact in this category, mainly due to the greater contribution of emissions to water, specifically for residual chlorine, since this substance has a high characterization factor equivalent to 19 kg 1.4-DCB _eq/kg Cl.

Human toxicity

For aerated lagoons, the impacts averaged 1.40 kg 1.4-DCB _eq/kg BOD₅, while for active sludge, 1.15 kg 1.4-DCB _eq/kg BOD₅ was obtained. In the case of biofilter and vermi-biofilter, these technologies show the best environmental performance impact results with 0.42 kg 11.4-DCB _eq/kg BOD₅ and 0.57 kg 1.4-DCB _eq/kg BOD₅ respectively. In the same way, electrical energy was the process with the greatest contribution for the human toxicity category (Figure 6), with a contribution of 44% and 74% for aerated lagoons and biofilter, respectively. This result is mainly due to the use of manganese, which comes from electrical power generation by the use of coal within the composition of the energy matrix. Followed then by use of chemicals inputs (about 30%), mainly for sodium hypochlorite, providing 15.5% for air lagoons and 27% for biofilter in the disinfection stage, followed by ferric chloride with 13% in aerated lagoons and biofilter fuel (10 %). For active sludge, water emissions (discharge) had a contribution of 29%, mainly due to the chlorine emitted by the effluent, where the latter has a characterization factor equivalent to 170 kg 1.4-DCB _eq/kg Cl for this category.

Figure 6 Contribution by type of technology for the removal of 1 kg BOD₅ for the category of human toxicity. 

Eco-efficiency assessment

Vermi-biofilter proved to be the most eco-efficient technology with a higher indicator corresponding to 6.7 m³ / kg CO_{2 eq}, followed by biofilter with 5.1 m³ / kg CO_{2 eq}. On the other hand, the vermi-biofilter technology was the most eco-efficient treatment technology with indicators of 10 984.1 m³/kg P _eq, followed by active sludge with 10 518.5 m³/kg P _eq, in the freshwater eutrophication category, as shown in Table 3.

When the eco-efficiency of the plants is analyzed in the climate change category, the DWWTP 3, 5, 7, 11, 12 and 13 have the best indicators for lagoons and active sludge technologies, in agreement with the size of the plants, as was pointed by (^{Lorenzo-Toja et al., 2014}), with exception of the DWWTP 7 which corresponds to a small plant (662.8 inhabitants). The change in the electrical matrix is an important point for the determination of its impacts, and therefore its eco-efficiency. In the same way in the category of freshwater eutrophication, the DWWTP 7 obtains the best eco-efficiency indicator attributable to the energy matrix type mentioned above.

According to factor X (Table 3) in the climate change category, the vermi-biofilter is almost twice the time eco-efficient than aerated lagoons and active sludge plants. Finally, for the freshwater eutrophication category, vermi-biofilter plants turned out to be twice eco-efficient than for aerated lagoons, followed by biofilter, and to a lesser measure with active sludge.

Sensitivity analysis

The results of the environmental impact for the sensitivity analysis (Figure 7), show a decrease in environmental impacts by increasing the unconventional renewable electrical energy in the energy matrix. In the climate change and human toxicity categories a decrease close to 20% and 35% were obtained for vermi-biofilter with 60%-NCRE and vermi-biofilter with 70%-NCRE, respectively. On the other hand, the opposite is true for the category of freshwater ecotoxicity, however, the differences in environmental impact did not exceed 9%, mainly attributed to the 11% increase in photovoltaic and wind energy generation, which 43.2% of the impact contributed to 60%-NCRE, and 60.5% to 70 -NCRE.

Figure 7 Environmental impacts for vermi-biofilter for different compositions in the energy matrix. 

Finally, for the eco-efficiency indicator, the vermi-biofilter plant with the highest percentage of NCRE (70%-NCRE) for both categories studied was more eco-efficient. In the category of climate change, the conventional matrix plant decreased its eco-efficiency by 35% and 19% for the matrix with 60%-NCRE, for the freshwater eutrophication category, the decrease in eco-efficiency for the plant with Conventional energy matrix was 54% and for the plant with 60%-NCRE it was 34% with respect to the plant with the highest NCRE (Figure 8).

Figure 8 Eco-efficiency indicators with energy matrix projection. 

Finally, with this assessment, it can be demonstrated that eco-efficiency can vary without modifying the flows (required inputs), in terms of efficiency, if not, that the indicator can be altered by simply changing the composition of the electrical matrix, that is, without a change in its design, because the electricity corresponds to an environmental hotspot for the treatment systems.