Wastewater reclamation will be a significant part of future water management and the environmental assessment of various treatment systems to reuse wastewater has become an important research field. The secondary treatment process and sludge handling on-site are, especially, electricity demanding processes due to aeration, pumping, mixing, dewatering, etc. used for operation and are being identified as the main contributor for many environmental impacts. This study discusses how the environmental performance of reuse treatment systems may be influenced by surrounding conditions. This article illustrates and discusses the importance of factors commonly treated as externalities and as such not being included in optimization strategies of reuse systems, but that are necessary to environmentally assess wastewater reclamation systems. This is illustrated by two up-stream and downstream processes; electricity supply and the use of sludge as fertilizer commonly practiced in regions considered for wastewater reclamation. The study shows that external conditions can have a larger impact on the overall environmental performance of reuse treatment systems than internal optimizations could compensate for. These results imply that a more holistic environmental assessment of reuse schemes could provide less environmental impacts as externalities could be included in measures to reduce the overall impacts.

INTRODUCTION

The need for wastewater reuse has been identified by several global and internationally recognized organizations (World Health Organization 2006; ACWUA 2010; National Research Council 2012; US Environmental Protection Agency 2012). The reuse of water and especially the reuse of municipal wastewater will become a significant part of future water management and become an important part of the water supply for various water sectors. Technologies for treating wastewater to various quality standards for reuse are already available (Best Available Technology (BAT)), however the selection of the right reuse treatment-system is not straightforward and there is not a single solution which fits all. Further, the total environmental impact of reusing wastewater will be different compared with common sewage treatment due to additional use of, for example, chemicals and electricity, but at the same time reuse of water that otherwise would be discharged. An increased impact is likely if various treatment technologies for secondary, tertiary and quarterly treatment are not optimized to an overall treatment system. It is thus crucial to assess different treatment systems based on BAT in order to offer the most optimal solution, that is matching the local regulatory, but also environmental and economic requirements in the form of minimum total environmental and cost impact.

One of the most recent and complete wastewater-reuse assessment studies reported is provided by Baresel et al. (2015a, 2015b), comprising eight different treatment trains for targeted reuse effluent qualities for agriculture (AG), industrial (I) and groundwater (GW) recharge applications. The study showed that negative environmental impacts of more advanced treatment technologies to meet higher effluent qualities are reduced or even insignificant for some impact categories (key performance indicator (KPI)) with increasing plant size. The study further indicated that sludge on-site handling and the secondary treatment process (commercial active sludge) were the dominating processes when evaluating the total environmental performance of the different wastewater treatment systems for wastewater reclamation. Electricity, especially, for various water and sludge handling processes such as aeration, pumping, mixing, dewatering, etc. was identified as the main contributor for many KPIs. Considering the current focus on global warming potential (GWP), the use of electricity was the main contributor to GWP for all investigated treatment systems for industrial and groundwater recharge reuse and the second dominating contributor for treatment systems facilitating water reuse for agriculture. For the electricity mix of Spain, which served as a model region, it was also the main contribution (>90%) to the category Ecotoxicity origins from the use of electricity.

Environmental assessments of reuse systems such as provided by Baresel et al. (2015a, 2015b) aim at optimizing and reducing environmental impacts in order to push forward the implementation of wastewater reclamation to meet societies' demands in the future and fight global water stress. The environmental assessment, however, has always been limited as systems boundaries needs to be defined clearly in order to carry out such assessment. Assessing the technical treatment systems may, however, be influenced by the choice of boundary and selected surrounding conditions. Such external conditions may have a larger impact than internal optimizations (Pasqualino et al. 2009; Chen et al. 2012; Dahlgren et al. 2014). Dahlgren et al. (2014) have shown that the sludge handling on-site can have a significant impact on the overall environmental performance of reuse treatment systems if for example in focus regions commonly applied aerobic stabilization is replaced with anaerobic stabilization. The produced bioenergy can be used for the secondary water treatment. Despite the emission of methane in the anaerobic step, better dewaterability and a decrease of the needed external energy for the water treatment may reduce the overall impact of several impact categories such as GWP and eutrophication potential (EP).

A dominating impact of external conditions on the overall environmental performance on reclamation schemes would imply that sub-optimizations of single treatment processes could be avoided and that reuse schemes could be less environmentally impacting if the wider context or externalities outside the system boundaries would be included in the assessment. It would further imply that a more holistic approach would be required when implementing wastewater reclamation schemes, which at the same time would offer the potential of significantly reducing the total environmental impact. The decision on whether utilization of wastewater reuse would be sustainable or not would then also be defined by such externalities in some locations.

This article illustrates and discusses the importance of factors commonly treated as externalities and as such not included in the assessment and optimization strategies but that are necessary to environmentally assess wastewater reclamation systems. This includes the influence on the environmental impact of wastewater reclamation systems by the type of electricity supply and the use of wastewater sludge as a fertilizer, as commonly utilized in regions considered for wastewater reclamation.

MATERIALS AND METHODS

For this study, two of the systems investigated by Baresel et al. (2015a, 2015b) were selected. First, a reclamation treatment line for agricultural reuse of treated wastewater and a system to reclaim water for industrial reuse. Both treatment lines consisted of secondary treatment a modified sequencing batch reactor (SBR) called ICEAS (SBR with continuous inflow and a pre- and main reaction zone) operated in an incomplete nitrification mode for AG water quality and a complete nitrification/denitrification mode for I water quality respectively. The AG-line was completed with a rapid gravity sand filter and treatment with UV irradiation (UV) as tertiary and disinfection treatment steps. The industrial reclamation line consisted of a submerged ultra-filter (sUF), ozone treatment followed by sodium hypochlorite (Cl) as tertiary and disinfection treatment steps. The systems were physically studied at the R&D-facility at Hammarby Sjöstadsverk in Stockholm and life-cycle assessment (LCA) models were built in the LCA-tool GaBi 6.3, supplemented with modules describing the surrounding infrastructure (Baresel et al. 2015a, 2015b). Spain was chosen as a model region. Even though different plant sizes were included in the original study, only plant sizes of 100,000 pe are used for exemplification in this article. The cases described here include the following:

  • Extension of downstream boundary to include sludge fertilizing.

  • Various electricity mixes compared with internal optimization strategies and process-internal greenhouse gas (GHG) emissions.

Each treatment train was assessed by an attributional life-cycle assessment according to the ISO standard (ISO14044:2006), which comprised the treatment from the influent water to the reclaimed water. The functional unit was defined as 1m3 of reclaimed water. The system boundary comprises the complete system of all sewage treatment processes including related on-site sludge handling. Inputs across that boundary include the untreated sewage influent and the natural resources, which are necessary to generate energy, produce material commodities and construction materials and services, and to transport materials to the site of the plant. Outputs are the reclaimed water at the outlet from the plant and the sludge after on-site stabilization ready for transport to disposal. As environmental KPIs, maximum potential impacts (midpoint indicators) were used. Operational data from experiments and the mass-balance modelling in Matlab were used. Data on materials and construction of the equipment was from existing or designed installations. Upstream data to describe the peripheral processes (chemicals, energy and construction) were collected from relevant literature or from life-cycle inventory databases as modules from suitable databases; Ecoinvent 2.2 and ProfDB. A detailed description of the systems setup and information about the LCA methodology is provided by Baresel et al. (2015a, 2015b).

Fertilizing with sludge

The disposal of biosludge by using it as an agricultural fertilizer is the most common use in regions of interest for wastewater reclamation (Baresel et al. 2015b) and has therefore been included in the system. The approach is to calculate the acreage of land that can be fertilized with the quantity of sludge generated from the production of 1 m3 of reclaimed water. The environmental impacts of fertilizing the same acreage with mineral fertilizers are then deducted. The deducted impacts include production and delivery of the NP-fertilizer diammonium hydrophosphate (DAP). This fertilizer was chosen since DAP is the NP-fertilizer, which has a composition comparable to the plant-available N:P ratio in the sludge and for which data are available. In GaBi it is modeled by two modules from the database Ecoinvent 2.2, ‘diammonium phosphate as nitrogen' and ‘diammonium phosphate as P2O5' respectively. Both these modules refer to 1 kg N respectively 1 kg P2O5 in diammonium phosphate with a nitrogen content of 18% and a P2O5-content of 46%. Allocation factors between the products are based on energy requirements of the respective nutrients for the production process; 60% for diammonium phosphate as nitrogen and 40% for diammonium phosphate as P2O5. Since the N content of DAP is given as 18% in the module, it is somewhat below the theoretical nitrogen content, which is 21%. The only emissions considered are greenhouse gases and compounds causing acidification and eutrophication. Being an important aspect, toxicity is also included in this study but due to the lack of data on the contents of metals in mineral fertilizers (except for cadmium) no comparison can provided.

The location of fertilized land is selected to be in Spain in order to comply with the earlier assumption of this location within the study. The required dosage of plant-available nitrogen is defined as 102 kg/(ha·yr) (Tidåker et al. 2005) and the maximum permitted dosage of nitrogen in Spain is 170 kg/(ha·yr) (Real Decreto 261/1996). There is no maximum permitted dosage of phosphorus in Spain except for some specific regions (Real Decreto 1310/1990; Amery & Schoumans 2014), which implies that the maximum allowed dosage of nitrogen of 170 kg/(ha·yr) is ruling. The sludge from the ReUse-project (Baresel et al. 2015a, 2015b) contains about 0.26 kg P/(kg total N) and 0.86 kg P/(kg plant-available N). More information on used data to model fertilizing with sludge and with a mineral NP-fertilizer is provide in the Supporting Information (available with the online version of this paper). For comparison, Spanish (Real Decreto 261/1996) and limits Swedish (SEPA 1998) are presented in Table 1.

Table 1

Nutrient concentrations in pilot-sludge and Spanish and Swedish limits for application in agriculture

  Sludge AG (plant size 100 kpe) kg/d Limit 
   Spain kg/(ha·yr) Sweden kg/(ha·yr) 
Total P 77.8 – 22–35* 
Total N 297 170 150§ 
Plant available N 90.6 102 102 
  Sludge AG (plant size 100 kpe) kg/d Limit 
   Spain kg/(ha·yr) Sweden kg/(ha·yr) 
Total P 77.8 – 22–35* 
Total N 297 170 150§ 
Plant available N 90.6 102 102 

*22 kg/(ha·yr) as an average for the complete farm, applies for soils with high content.

§Only given as NH4-N.

Based on pilot-plant measurements (provided in the Supporting Information) and recalculated to consider an increase in concentration due to the volatile suspended solids (VSS) digested in the aerobic digester (e.g. assuming a decrease in amount of digested sludge (DS) without any loss of metals), an investigation of potential limitations of sludge application in agriculture was performed with the treatment train AG as a basis.

Electricity use and GHG emissions

The influence of the type of electricity supply has been studied by replacing the Spanish average electricity mix by two other electricity sources, namely Swedish average electricity and US average electricity mix (Table 2). The Spanish electricity mix has been adjusted to 2012 with statistics (IEA 2013). The composition of the Swedish electricity mix was collected from statistics from Swedish power network for the year 2011 (Svenska Kraftnät 2011). Data for the US electricity mix were database data from ProfDB (GaBi 2014) for the year 2012. Supply mixes that consist of both production and electricity gross import were used. The electricity from thermal sources was split into separate energy wares according to IEA statistics from 2009 (IEA 2013). Both the Spanish and the Swedish electricity models were supplemented with data from the database ProfDB (GaBi 2014) in order to describe the individual modes of electricity generation (hydropower, nuclear power, etc.). The electricity mixes used were the most recent updates of the LCA-databases published in 2014 but indicate a certain delay in the updating procedure of LCA-databases. As these date sets are widely used, a comparison with other studies becomes possible even so, the electricity mixes may have changed during the last 2–3 years.

Table 2

Composition of electricity mixes for Spain, USA and Sweden

  Electricity mix %
 
Origin Spain (2012) USA (2012) Sweden (2011) 
Biogas 0.18 0.23 0.02 
Biomass (solid) 0.73 0.97 4.41 
Coal gases 0.34 0.07 – 
Hard coal 11.10 43.48 3.14 
Heavy fuel oil 5.75 1.10 0.39 
Hydro 7.97 6.54 47.19 
Lignite 0.23 2.00 0.73 
Natural gas 32.56 23.25 1.43 
Nuclear 20.39 19.16 37.38 
Photovoltaics 2.83 0.02 0.01 
Wind 17.79 2.17 4.19 
Waste 0.46 0.53 0.78 
Geothermal – 0.08 – 
Solar thermal – 0.40 – 
Industrial gas – – 0.14 
Peat – – 0.20 
  Electricity mix %
 
Origin Spain (2012) USA (2012) Sweden (2011) 
Biogas 0.18 0.23 0.02 
Biomass (solid) 0.73 0.97 4.41 
Coal gases 0.34 0.07 – 
Hard coal 11.10 43.48 3.14 
Heavy fuel oil 5.75 1.10 0.39 
Hydro 7.97 6.54 47.19 
Lignite 0.23 2.00 0.73 
Natural gas 32.56 23.25 1.43 
Nuclear 20.39 19.16 37.38 
Photovoltaics 2.83 0.02 0.01 
Wind 17.79 2.17 4.19 
Waste 0.46 0.53 0.78 
Geothermal – 0.08 – 
Solar thermal – 0.40 – 
Industrial gas – – 0.14 
Peat – – 0.20 

Emissions of nitrous oxide (N2O) used in the calculations are mainly based on actual measurements during a period of 6 months within the ReUse-project accounting for 2.09% of N2O per TN being removed for AG-line and 0.2% of N2O per removed TN for I-line. However, higher emissions have been reported in literature and are accounted for in one scenario (Global Water Research Coalition report 2011; Rodriguez-Caballero & Pijuan 2013; Sun et al. 2013). Details on specific emissions and measurement setup are provided by Baresel et al. (2015a, 2015b).

RESULTS

Fertilizing with sludge

Considering the measured nutrient concentrations in the ReUse-sludge from the AG-system, and the presence of restrictions only for nitrogen, the total amount of sludge from the reclamation trains that could be used on agriculture land is limited to 8,036.5 kg dry solids/(ha·yr). In other words, supporting phosphorus fertilizing may be required if more phosphorus fertilizing is desired than the maximum amount of sludge that can be spread per hectare provides. The N:P-ratio can be affected in the sludge before fertilizing or by additional phosphorus fertilizing after sludge application. The total agricultural area that could be fertilized with sludge produced from the ReUse-train amounts to 184.4 ha when meeting nitrogen limits. With the current composition of the biosludge as produced in the study and with a dose that meets the required amount of plant-available nitrogen; about 0.26 kg phosphorus per kg N is broad on the fields. This implies that enough phosphorus is supplied if considering recommended N:P ratios of commercial NP-fertilizer for wheat (NPK (ratio of elemental N, P and K) 8-24-8 plus NAC 27; see Bellido 2010).

The result of the environmental impacts evaluation of the wastewater reclamation system with or without the use of the produced sludge as fertilizer is shown for the impact category GWP, EP and acidification potential (AP) in Figure 1. The major difference in impacts of the wastewater treatment and sludge application on GWP is due to N2O emissions from the sludge during and after the distribution on the field. The major avoided impact (negative bar) is due to avoided emissions that would occur during manufacturing (European average) of exactly the same amount of mineral fertilizer. This avoided impact presents the benefit of reusing the sludge as a fertilizer. The largest benefit can be observed for the AP due to avoided consumption of energy when producing mineral fertilizer. When fossil fuels are combusted, sulfur dioxide (SO2) and nitrogen oxides (NOx) are emitted which increases AP. This is also true for GWP but the increased emissions of nitrous oxide when fertilizing with sludge compared with mineral fertilizers imply a larger impact than the avoided GHG emissions. The avoided EP from fertilizing with sludge is due to the avoided emissions from mineral fertilizing to the aquatic environment. Higher emissions of nitrate if applying sludge fertilizing than for mineral fertilizers, however, cause an increased EP.
Figure 1

Environmental impacts of utilizing sludge from wastewater reclamation treatment as fertilizer on agriculture land for 1 m3 of reclaimed wastewater (for plant size 100,000 pe and CML 2001 – April 2013) for environmental impact category (KPI): (a) GWP; (b) EP; and (c) AP.

Figure 1

Environmental impacts of utilizing sludge from wastewater reclamation treatment as fertilizer on agriculture land for 1 m3 of reclaimed wastewater (for plant size 100,000 pe and CML 2001 – April 2013) for environmental impact category (KPI): (a) GWP; (b) EP; and (c) AP.

The largest negative impact of reusing sewage sludge as a fertilizer is for the terrestric ecotoxicity and that is mostly due to the emission of Cr to agriculture soil (62% of contribution). Because the quantification of the impact of fertilization with sludge on terrestric ecotoxicity is not complete due to the lack of data on the content of metals in mineral fertilizers (except for cadmium), it is not included in Figure 1. It was not possible to calculate avoided impacts properly and therefore only sewage sludge specific data are presented in Table 3.

Table 3

Addition of metals on agricultural land based on nitrogen limits (Supporting Information, available with the online version of this paper)

  Content in sludge from AG Load limit kg/(ha·yr)
 
 4572.6 kg DS/(ha·yr) Spain Sweden 
Cadmium 0.00101 0.15 0.00075 
Copper 0.500482 12 0.3 
Lead 0.021991 15 0.025 
Mercury 0.000851 0.1 0.0015 
Nickel 0.050352 0.025 
Zinc 0.561147 30 0.6 
Chromium 0.08675 0.04 
Total P 87.6 – 22–35 
Plant available N 102 102* 102 
  Content in sludge from AG Load limit kg/(ha·yr)
 
 4572.6 kg DS/(ha·yr) Spain Sweden 
Cadmium 0.00101 0.15 0.00075 
Copper 0.500482 12 0.3 
Lead 0.021991 15 0.025 
Mercury 0.000851 0.1 0.0015 
Nickel 0.050352 0.025 
Zinc 0.561147 30 0.6 
Chromium 0.08675 0.04 
Total P 87.6 – 22–35 
Plant available N 102 102* 102 

*Limiting.

The analysis of average concentrations of heavy metals in the sludge recalculated to relevant units showed that these were well below the Spanish limits (RD 1310/1990) but also Swedish limits (Table 3). Assuming that nitrogen limits the amount of sludge to be used per hectare (Table 3), the applied quantities of cadmium, copper, lead, mercury, and nickel do not exceed the Spanish limits. However, quantities of cadmium and copper would exceed Swedish limits and would thus require a reduced sludge quantity to be distributed on agriculture land per hectare. These emissions of heavy metals imply a significant environmental impact in the form of terrestic ecotoxicity in the applied assessment model (not shown here). Even if load limits for these substances are not exceeded as in the case of Spain, their toxic impact dominated the complete environmental impact assessment, which was one of the reasons not to include sludge disposal in the original study, as technical aspects of water and sludge treatment would not have been able to be investigated. As explained in Baresel et al. (2015a, 2015b) this was possible only because produced sludge quality and quantity was identical for all investigated reuse systems.

Electricity mixes compared with internal optimization strategies and process-internal GHG emissions

In order to investigate the impact of changing of electricity mix compared with different optimization strategies that will result in a decrease of electricity consumption of a biological treatment step (a SBR) by 10 and 20%, sensitivity analysis was performed where Spanish, Swedish and US electricity grids were compared for the same electricity consumed.

Figure 2(a) shows that a reduction of the electricity use within the reuse treatment system has only minor impact on the GWP for the complete system, whereas external factors such as the site location and therefore the used electricity mix composition may have significant impact on the overall GWP of the wastewater reclamation process. As the percentage of green electricity increases, going from Spanish to Swedish electricity mix (see also Table 2), GWP decreases by 60% for the same electricity consumption. Higher percentage of fossil fuels like hard coal as it is used in US mix increased the GWP by 50%. Nuclear power is commonly considered as green electricity at a time horizon of 100 years. As the Swedish electricity mix consists of almost 40% nuclear power, different assessment of this electricity source would of course significantly alter the outcome of the performed evaluation. The impact of various electricity mixes is most significant for GWP but also changes all other KPIs in a similar way.
Figure 2

Impact of available electricity mix (a) used for the wastewater reclamation system (I, 100,000 pe, 0.2% N2O of Ntot), and (b) variable internal GHG emissions (AG 100,000 pe) on GWP.

Figure 2

Impact of available electricity mix (a) used for the wastewater reclamation system (I, 100,000 pe, 0.2% N2O of Ntot), and (b) variable internal GHG emissions (AG 100,000 pe) on GWP.

The assessment of various emissions of nitrous oxide (N2O) from the biological nitrogen removal process at constant electricity consumption (based on the default Spanish electricity -mix; Figure 2(b)) showed that the overall environmental impact caused by high emissions of greenhouse gases from the biological processes may even be more significant than the impact different electricity mixes may have. This implies that an optimization of the secondary biological treatment to minimize nitrous oxide emission can reduce the overall environmental impact of the whole treatment system considerably. The effect of an environmentally impactful electricity-mix used for the treatment process and at the same time high emissions of greenhouse gases from the biological treatment may then dominate the total environmental impact of a treatment system in a similar way as terrestic ecotoxicity from metals in sludge used as fertilizer.

DISCUSSION AND CONCLUSIONS

The choice of external conditions for wastewater reclamation systems may play a more significant role than the actual technical optimization of treatment trains. This suggests that such external factors have to be considered in a more holistic system perspective when planning for wastewater reuse systems. The traditional focus on technical systems as independent solutions that can be fitted into any environment (region) and optimized towards lowest possible environmental impact cannot comprise the complexity externalities may put on the system. With this said, the main driving factors to implement wastewater reclamation system will always be the actual need for reclaimed water and location of such systems is thus very well defined. However, wastewater reclamation may get a better regional understanding and support if considering other aspects than solely the actual water treatment technology used. This may for example include supporting green electricity that can decrease the overall environmental impact of such reuse schemes. Even if such externalities are not a direct part of the reuse system, they can significantly influence the overall environmental impact of such systems.

The present study shows that the design of wastewater reclamation projects should specifically consider external factors such as the electricity supply as an important part of the planning. Improvements in this area may lead to significant lower overall environmental impacts than the attempt to decrease electricity consumption. On the other hand, it has been also shown that the optimization of the secondary treatment processes to decrease emissions of nitrous oxide (N2O) may also have a significant impact on the GWP and therefore the overall environmental impact of the reuse treatment system.

Results also indicate that including the reuse of sludge into the system boundaries has a significant environmental impact of certain KPIs and can hide smaller impacts from the treatment processes and can therefore influence the evaluation of two different treatment lines.

These findings are of course not limited to wastewater reclamation systems but generally applicable for water treatment. A holistic environmental impact assessment allows optimization of efforts to reduce the total impact. This may also gain increased public acceptance of reuse systems and avoid sub-optimization within the core system in the case where that resource is allocated outside the system, e.g., in proper sludge handling and regenerative energy sources, would imply a more significant improvement of the environmental performance.

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Supplementary data