Article pubs.acs.org/est
Environmental Benefits and Burdens of Phosphorus Recovery from Municipal Wastewater Zenah Bradford-Hartke,† Joe Lane,‡ Paul Lant,‡ and Gregory Leslie*,† †
School of Chemical Engineering, The University of New South Wales, Kensington 2052, Sydney, New South Wales, Australia School of Chemical Engineering, The University of Queensland, St. Lucia 4072, Brisbane, Queensland, Australia
‡
S Supporting Information *
ABSTRACT: The environmental benefits and burdens of phosphorus recovery in four centralized and two decentralized municipal wastewater systems were compared using life cycle assessment (LCA). In centralized systems, phosphorus recovered as struvite from the solids dewatering liquid resulted in an environmental benefit except for the terrestrial ecotoxicity and freshwater eutrophication impact categories, with power and chemical use offset by operational savings and avoided fertilizer production. Chemical-based phosphorus recovery, however, generally required more resources than were offset by avoided fertilizers, resulting in a net environmental burden. In decentralized systems, phosphorus recovery via urine source separation reduced the global warming and ozone depletion potentials but increased terrestrial ecotoxicity and salinization potentials due to application of untreated urine to land. Overall, mineral depletion and eutrophication are well-documented arguments for phosphorus recovery; however, phosphorus recovery does not necessarily present a net environmental benefit. While avoided fertilizer production does reduce potential impacts, phosphorus recovery does not necessarily offset the resources consumed in the process. LCA results indicate that selection of an appropriate phosphorus recovery method should consider both local conditions and other environmental impacts, including global warming, ozone depletion, toxicity, and salinization, in addition to eutrophication and mineral depletion impacts.
1. INTRODUCTION Phosphorus removal from municipal wastewater, traditionally to prevent eutrophication of receiving waters,1,2 now presents an opportunity to generate local supplies of phosphorus fertilizers. The economic case for phosphorus recovery at municipal Wastewater Treatment Plants (WWTPs) has improved through decreasing processing costs3,4 and increasing prices and demand for phosphorus commodities. From 1983 to 2013, the phosphate rock price increased by 133%,5 while global phosphate consumption increased by 25%.6,7 As the quality and availability of reserves decline8 and jurisdictions promulgate legislative targets,9−11 utilities will be required to upgrade the WWTP’s phosphorus recovery capacity,12−15 despite uncertainty regarding the size16−18 and projected exhaustion of global reserves.18−20 Notwithstanding the economic and resource recovery case,21 the literature contains conflicting data on overall environmental impacts of alternative recovery processes. For example, one study found that phosphorus recovered as struvite from solids dewatering streams was less energy intensive than chemical (FeSO4) removal by a factor of 2.3 and a factor of 1.4−1.7 lower than fertilizer production.22 Conversely, another study found struvite precipitation was more energy intensive than mineral fertilizer production by a factor of 2 and a factor of 10 greater than applying sludge to land.23 Including the impacts of © 2015 American Chemical Society
soil metal toxicity further highlights the difficulties when comparing processes. One study found that while recovery as struvite resulted in 5 times less cadmium applied to land than reuse of biosolids, there was only a marginal reduction compared to mineral fertilizer use.23 Conversely, another study found that concentrations of cadmium (72×), chromium (161×), and arsenic (4.5×) were higher in mineral fertilizer than struvite.24 Comparison of different phosphorus recovery options is possible with Life Cycle Assessment (LCA) methodology25 provided the studies have a common functional unit and consistent system boundaries. Life cycle analyses of WWTPs26,27 and biosolids processing options28,29 have included the environmental impacts of phosphorus removal26 and recovery.27−29 However, differences in the process designs, functional units, system boundaries, and impact categories prevent direct comparison of the results. This paper uses LCA methodology to assess the environmental benefits and burdens of implementing phosphorus recovery into wastewater treatment systems on Australia’s east Received: Revised: Accepted: Published: 8611
March 6, 2015 May 20, 2015 June 29, 2015 June 29, 2015 DOI: 10.1021/es505102v Environ. Sci. Technol. 2015, 49, 8611−8622
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Environmental Science & Technology coast. Australia is highly urbanized with almost 90% of residents living in coastal urban areas,30,31 and 87% of the population is connected to a sewerage system.32 However, many agricultural crops including grains are grown inland,33 which indicates that transportation of recovered phosphorus fertilizers from population center to agriculture may be an important contribution to the impacts of phosphorus recovery. Currently 60% of WWTP biosolids are reused in agriculture,34 which is a form of phosphorus recovery. However, the benefits of avoided synthetic fertilizer production and preservation of mineral reserves is only achieved if biosolids application is coupled with reduced application of synthetic fertilizer. Less common recovery processes include urine source separation35−37 and struvite precipitation from biosolids dewatering streams.38,39 Additionally, recent water shortages have seen growth in membrane based water recycling plants40 which generate another potential feedstock (reverse osmosis brine) for mineral recovery processes, including phosphorus recovery.41−44 Consequently, the analysis considers two decentralized and four centralized systems, including one centralized plant with advanced treatment for water reclamation. Recovered phosphorus products include urine, biosolids, and struvite precipitation from solids dewatering and brine streams.
2. MATERIALS AND METHODS LCA methodology involves four steps: goal and scope definition; life cycle inventory; impact assessment; and interpretation.45 2.1. Goal and Scope. The goal was to assess the environmental impact of implementing phosphorus recovery into wastewater treatment systems on Australia’s east coast (Figure 1, Table S1). The system boundary contained foreground processes, including effluent discharges and direct gaseous emissions (Tables S1 and S7). Background processes, including production of energy, chemicals, and materials required during the construction and operation of new infrastructure to achieve, or increase capacity for, phosphorus recovery were included (Tables S2 and S3). Infrastructure required to produce electricity and chemicals was excluded. The system boundary included credits for avoided fertilizer production, as all recovered phosphorus was assumed to replace commercial fertilizers.46 The analysis considered 50 years of plant operation, including transport of recovered phosphorus to agriculture (Table S5). 2.1.1. Functional Unit. The functional unit was defined as the recovery of 1 kg of plant available phosphorus able to offset synthetic fertilizer. 2.2. Case Studies. Decentralized WWTPs are typically simple, package plants based on septic tanks, extended aeration or packed beds, rather than advanced nutrient removal processes used in centralized plants.47−49 Phosphorus recovery in decentralized systems (Cases 1 and 2) was modeled on the Currumbin Ecovillage WWTP serving 109 domiciles in southeast Queensland.36,50,51 The cluster-scale package plant treats comingled wastewater in anaerobic septic tanks and aerobic textile filters, followed by membrane filtration and disinfection. Treated water is reused on-site, and septic tank solids are periodically transported to a centralized WWTP.36,50 Case 1 (USS + CT) considered installation of urine source separation (USS) at single domiciles and phosphorus recovery by use of stored urine in agriculture (Figure 1, Table S1).
Figure 1. Process flow diagrams for case studies 1−6. Implementing phosphorus recovery requires modifications to the existing infrastructure (indicated in black) but may also avoid processes (indicated in red). Existing infrastructure that remains unchanged by the implementation of phosphorus recovery was excluded from the system boundary (indicated in gray).
Installation of USS is coupled with greywater diversion to land and faeces processing in a composting toilet (CT). Case 2 (Ecovillage + USS) considered installation of USS at single domiciles and use of stored urine in agriculture. The remaining wastewater fractions (greywater and brownwater) are treated in the onsite cluster scale WWTP, with changes in performance of the anaerobic septic tanks and aerobic trickling filters due to urine separation included in the model (Figure 1, Table S1). Phosphorus recovery in conventional centralized systems (Cases 3, 4, and 5) was modeled on a 10 MLD (50,000 EP) five stage Bardenpho biological nutrient removal (BNR) plant discharging to an ocean outfall, representative of facilities on Australia’s east coast.49 The plant includes anaerobic lagoons for stabilization of biosolids prior to dewatering and transport to landfill. 8612
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Environmental Science & Technology Table 1. Phosphorus Mass Balance for Cases 1−6f
EP influent load
1 USS + CT
2 Ecovillage + USS
kg/d g/EP/d
273a 0.5 (100%) 1.7 (100%) stored urine
273a 0.5 (100%) 1.7 (100%) stored urine
50,000 120 (100%) 2.4 (100%) stabilized biosolids
%
100
100
kg/d g/EP/d kg/d g/EP/d kg/d g/EP/d kg/d g/EP/d
0.1 0.4 0.0 0.0 0.2 0.7 0.2 0.6
phosphorus product phosphorus availability in recovered productb phosphorus recovered that offsets fertilizerb phosphorus recovered but unavailablec phosphorus retained in the effluent phosphorus lost to system constraints
(22%) (22%) (0%) (0%) (41%) (41%) (36%)d (36%)d
0.1 0.4 0.0 0.0 0.2 0.7 0.2 0.7
(22%) (22%) (0%) (0%) (39%) (39%) (39%)d (39%)d
3 biosolids
4 Chem-P
5 dewatering struvite
6 brine struvite
50,000 120 (100%) 2.4 (100%) struvite
50,000 120 (100%) 2.4 (100%) struvite
70, sensitivity 25−100
50,000 120 (100%) 2.4 (100%) stabilized biosolids with chemical addition 50, sensitivity 25−85
100
100
26.0 (22%) 0.5 (22%) 11.1 (9%) 0.2 (9%) 82.9 (69%) 1.7 (69%) 0.0 (0%) 0.0 (0%)
54.7 (46%) 1.1 (46%) 54.7 (46%) 1.1 (46%) 10.2 (9%) 0.2 (9%) 0.0 (0%) 0.0 (0%)
35.5 (30%) 0.7 (30%) 0.0 (0%) 0.0 (0%) 50.3 (42%) 1.0 (42%) 34.2 (29%)e 0.7 (29%)e
32.3 (27%) 0.6 (27%) 0.0 (0%) 0.0 (0%) 10.8 (9%) 0.2 (9%) 76.9 (64%)e 1.5 (64%)e
a
Rounded to integer from 2.5 EP/domicile. bAvailable for uptake by plants. References for urine,122−125 biosolids,29,123,126−128chemically enriched solids,127,129 and struvite.124,130,131 Refer to the SI for further comments. cUnavailable for uptake by plants. dLosses due to time spent away from home and precipitation in the storage system. eLosses due to phosphorus retained in the solids which are diverted to landfill. fThe influent phosphorus load is shown (on a mass basis and % of influent load) as the phosphorus recovered that can offset fertilizer, the phosphorus that is recovered but unavailable for uptake by plants, the phosphorus that is retained in the effluent, and the phosphorus that is lost due to system constraints. Refer to the SI for further details on the process models for case studies.
concentration (12 mg/L TP), wastewater flow (10 ML/d), and an EP of 50,000. Differences between phosphorus loads in decentralized and centralized treatment plants is attributed to the assumed use of low phosphorus detergents at the Ecovillage and the calculation of loads per person from an influent concentration and equivalent population in centralized settings. 2.4. Inventory Data. 2.4.1. Infrastructure Used To Recover Phosphorus from Wastewater. Inventory data for foreground processes including operation of phosphorus recovery infrastructure was based on literature values (Tables S2 and S3). Data for background processes including chemical, energy, and materials production were taken from the inbuilt Australasian57 and Ecoinvent58 libraries in SimaPro.59 2.4.2. Fertilizer Offsets from Recovered Phosphorus. Recovered phosphorus was assumed to offset the use of diammonium phosphate (DAP) and urea. The availability of recovered phosphorus was expressed as a percentage of the plant available phosphorus in synthetic fertilizer (Table 1). Urea offsets for recovered nitrogen were calculated net any reduced application associated with DAP offsets60 (Table S1). Potassium in urine was assumed to be readily available61 and offsets potassium chloride (Table S1). 2.5. Impact Assessment. Impacts were assessed using the Hierarchist ReCiPe(H) midpoint method (v1.08) which is based on common policy principles including time frame.62 Midpoint impact methods are also referred to as a problemorientated approach and directly relate the inventory results into environmental impacts such as global warming potential and ozone depletion potential. While the ILCD recommends USEtox as the impact method for human toxicity, it does not include terrestrial ecotoxicity.63 However, since all recovered phosphorus products are applied to land; terrestrial ecotoxicity was identified as an interesting impact result to consider. The ReCiPe method was selected as it complies with all essential aspects for human toxicity63 and includes terrestrial ecotoxicity.62 Modifications included the following:
Case 3 (Biosolids) considered the diversion of dewatered BNR biosolids from landfill to agriculture (Figure 1, Table S1). Case 4 (Chem-P) considered chemical dosing (FeCl3) to enrich biosolids phosphorus levels and use of dewatered chemically enriched solids in agriculture (Figure 1, Table S1). Case 5 (Dewatering struvite) considered installation of a struvite reactor on the biosolids dewatering stream and use of the struvite in agriculture. Phosphorus recovery in an advanced centralized water reclamation facility was modeled on a 5 stage Bardenpho BNR followed by a microfiltration and reverse osmosis (RO) plant to produce high-grade water for industry. RO brine is discharged via an ocean outfall. Plants with similar configurations exist in Wollongong,52 Melbourne,53 and other coastal locations in the eastern states of Australia. Case 6 (Brine struvite) considered the installation of a struvite reactor on the RO brine stream and use of struvite in agriculture,41 followed by ocean discharge of the reactor effluent (Figure 1, Table S1). Further details on the process models for all case studies are provided in the Supporting Information (SI). All cases assumed discharge of effluents to marine environments and application of recovered products nearby inland freshwater catchments. 2.3. Phosphorus Loads. Decentralized cases assumed a phosphorus load of 1.7 g/EP/d, based on data from Currumbin Ecovillage36,50,51 and literature values41 for combined wastewater (urine, faeces, and greywater) (Table S4). Each person generates 1.5 L of urine daily.54 However, due to the assumed daily movements of residents only 55% of urine is collected at home (Table S2), compared to 100% of greywater and faeces. While greywater in Australia may contain high phosphorus loads due to voluntary phosphate removal from detergents,55,56 Ecovillage residents were assumed to use low phosphorus detergents. Centralized cases assumed a phosphorus load of 2.4 g/EP/d, based on literature values for combined influent wastewater.49 The load per equivalent population was based on the 8613
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Environmental Science & Technology • Mineral resource depletion was considered using the Centre of Environmental Science (Centrum Milieukunde Leiden (CML)) method64 as it is the only recent model which includes phosphorus. The phosphorus recovered and other minerals used were compared: ◦ By the mass of phosphorus recovered and other minerals used (mass basis) ◦ By the significance of phosphorus recovered and other minerals used to mineral depletion, expressed as equivalent depletion of antimony based on the relative abundance of minerals (CML weighted) (see S2.1). • Marine and freshwater eutrophication were considered separately following the approach in ReCiPe 1.08.65 Australian WWTPs typically have discharge limits for nitrogen and phosphorus, implying that the receiving waters are sensitive to both. The eutrophication metric was adapted to these circumstances66 by considering both nitrogen and phosphorus discharges to be limiting in marine environments and phosphorus assumed to be the limiting nutrient in freshwater catchments (see S2.2). • Ozone depletion included a characterization factor for nitrous oxide (N2O), as N2O has been demonstrated to affect options analyses in the wastewater industry.67 • The salinization potential was included,68 as this is a challenge for Australian agriculture.69 • Land occupation, transformation, and ionizing radiation were excluded. 2.6. Interpretation. Impact results are presented using default inventory assumptions. The sensitivity of results to phosphorus availability in biosolids (Table 1) and transport distance is presented in the Supporting Information (S2.2).
Precipitation of struvite has the potential to recover 30% of the influent phosphorus when the reactor is located on the solids dewatering stream (Case 5), compared with 27% of the influent phosphorus when the reactor is located on the RO brine (Case 6). After struvite precipitation from the solids dewatering, 42% of phosphorus is retained in the effluent compared to 9% when struvite is precipitated from the brine. The remaining 29% in the dewatering struvite case and 64% in the brine struvite case is unavailable because the load entering the struvite reactor is only a fraction of the load entering the WWTP (Table 1). In the struvite cases, the biosolids could also be applied to land to increase the phosphorus recovered (49%) but is beyond the scope of this analysis. 3.2. Impacts of Phosphorus Recovery. The results are grouped into the following: key drivers for phosphorus recovery, mineral depletion, and eutrophication (Figure 2 and
3. RESULTS 3.1. Phosphorus Recovered. Implementing recovery has the potential to recover 22% of the 1.7 g/EP/day phosphorus entering decentralized systems and up to 46% of the 2.4 g/EP/ day entering centralized systems (Table 1). Phosphorus recovered as urine, struvite from solids dewatering, and struvite from brine was available to plants (0% recovered but unavailable) (Table 1). The dewatering and brine struvite cases recover comparable phosphorus quantities, as the lower phosphorus concentration and removal efficiency of the brine reactor (Table S1) are offset by higher flows. In decentralized systems, the influent phosphorus in single domiciles is distributed between greywater and faeces (41%) and urine (22%) (Case 1). In decentralized clustered domiciles, the influent phosphorus is distributed between the effluent (39%) and urine (22%) (Case 2). The remaining phosphorus in single and clustered domiciles is unavailable due to time spent away from home and precipitation in the urine storage tanks (36% and 39% unavailable system constraints). Urine separation at both the home and workplace could reduce these losses but is beyond the scope of this analysis. In centralized systems, biosolids contain 31% of the influent phosphorus, with the balance (69%) discharged as effluent (Case 3). Chemical addition in centralized systems increases the proportion of influent phosphorus contained in the solids to 91%, with the balance (9%) discharged as effluent (Case 4). However, 9% of the influent phosphorus in biosolids and 46% in chemical solids is recovered but not available to plants, due to the assumed availability of biological and chemical solids (Table 1).
Figure 2. Mineral use is expressed per kilogram of plant available phosphorus recovered over 50 years of operation (kg/kgP). The mass of mineral used (mass basis) is expressed as kg mineral used/kg P recovered. The mass of minerals after accounting for the relative abundance of minerals (CML weighted) is expressed as kg Sb eq/kg P recovered. A value of greater than 1 kg/kgP indicates mineral consumption exceeds phosphorus recovered; 1 indicates the mass of minerals used is equal to the phosphorus recovered; 0 to 1 indicates mineral use is less than phosphorus recovered; and a negative value indicates the sum of mineral use and avoided mineral consumption is less than the phosphorus recovered.
Figure 3); and potential environmental impacts, global warming, ozone depletion potential, human toxicity, terrestrial ecotoxicity, particulate matter formation, photochemical oxidant formation, fossil fuel depletion, and salinization (Figure 4 and Figure 5). The net impact of introducing phosphorus recovery in each case study is expressed per kilogram of plant available phosphorus recovered over 50 years of operation. A negative value indicates an environmental benefit, while a positive value indicates an environmental burden. The net impact of phosphorus recovery includes contributions from construction, chemical manufacture, transport of materials (inputs and products), power use, avoided fertilizers (manufacture, transport, and losses from application), phosphorus product (including losses to waterways), emissions of organic and metallic contaminants, fugitive emissions of ammonia, methane, and N2O, and effluent discharges. 8614
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Figure 3. Net change in and processes contributing to a) marine eutrophication potential and b) freshwater eutrophication potential. The net effect of phosphorus recovery in each case study is expressed per kilogram of plant available phosphorus recovered over 50 years of operation. A negative value indicates an environmental benefit while a positive value indicates an environmental burden. The relative size, or apparent absence, of each color reflects the contribution of the process to each impact.
Figure 4. Net change in and processes contributing to a) global warming potential, b) ozone depletion potential, c) human toxicity potential, and d) terrestrial ecotoxicity potential. The net effect of phosphorus recovery in each case study is expressed per kilogram of plant available phosphorus recovered over 50 years of operation. A negative value indicates an environmental benefit, while a positive value indicates an environmental burden. The relative size, or apparent absence, of each color reflects the contribution of the process to each impact.
3.2.1. Mineral Depletion. The minerals used to construct and operate a struvite reactor on the biosolids dewatering or brine stream over 50 years are less than the available
phosphorus recovered (Figure 2). Diversion of biosolids from landfill to agriculture results in a net decrease in mineral depletion though avoided mineral use in fertilizer production. 8615
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Figure 5. Net change in and processes contributing to a) photochemical oxidant formation potential, b) particulate matter formation potential, and c) fossil fuel depletion potential. The net effect of phosphorus recovery in each case study is expressed per kilogram of plant available phosphorus recovered over 50 years of operation. A negative value indicates an environmental benefit, while a positive value indicates an environmental burden. The relative size, or apparent absence, of each color reflects the contribution of the process to each impact.
ing struvite cases have the added benefit of reducing the effluent nitrogen concentration. Chemical enrichment of biosolids reduces the marine eutrophication potential by 18 kgNeq/kgP compared with 6.8 and 6.3 kgNeq/kgP for struvite precipitation from the dewatering and brine streams, respectively (Figure 3a). Implementing urine separation in decentralized systems reduces the effluent concentration of both nitrogen and phosphorus, however; the net reduction (0.2 and 0.3 kgNeq/ kgP) is negligible due to land application of treated effluent. Applying recovered products to land shifts the risk of nutrient discharge to the environment at the site of application. For the urine separation and struvite cases, the losses to waterways are similar to those of synthetic fertilizers, so the freshwater eutrophication potential remains unchanged (Figure 3b). For the biosolids cases, 30% and 50% of phosphorus in biosolids and chemical solids is considered unavailable to plants and will eventually runoff. Production of chemicals leads to additional losses, with a net increase in the freshwater eutrophication potential of 18 and 98 gPeq/kgP for biosolids and chemical solids (Figure 3b). 3.2.3. Global Warming. Recovering phosphorus as urine in decentralized systems reduces the global warming potential by 980 and 220 kgCO2eq/kgP in single and clustered domiciles (Figure 4a). The greenhouse emissions attendant with construction and transport are offset by reduced methane, power, and N2O.
Similarly, phosphorus recovered as urine upstream of a decentralized plant avoids lime use at the treatment plant and mineral use in fertilizer production, offsetting the minerals depleted during installation of urine separating toilets at clustered domiciles (Figure 2). In each case, the minerals used to recover phosphorus estimated using the mass basis and CML method were aligned. On a mass basis, installation of urine separating toilets at single domiciles, coupled with onsite greywater management and composting toilets, negates the need for the construction of a cluster scale treatment plant with lime doing and avoids the depletion of 9.5 kg minerals/kgP. However, considering the relative abundance of minerals (CML method), installation of 109 urine separating toilets with attendant brass fittings and piping exceeds the avoided mineral use in fertilizer production and lime dosing at the treatment plant, resulting in the net depletion of 91.3 kgSbeq/kgP. Finally, phosphorus recovery via chemical enrichment of biosolids consumes 1 to 1.3 kg minerals/kgP recovered. 3.2.2. Eutrophication. The impact of phosphorus recovery on eutrophication is dominated by changes in the effluent concentration in marine environments (Figure 3a) and the difference in loss rates between phosphorus products and fertilizers in freshwater environments (Figure 3b). In the biosolids case, the WWTP effluent remains unchanged, as solids are diverted from landfill to land. In other cases, phosphorus recovery has the benefit of reducing the effluent phosphorus concentration. The urine separation and dewater8616
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struvite cases also have the advantage of low contaminant levels compared to commercial fertilizers, resulting in a net reduction in the human toxicity potential of −11 and −10 kg1,4-DBeq/ kgP for struvite precipitated from the dewatering and brine streams. The organic contaminants in recovered products had little impact on the human toxicity potential. However, the terrestrial ecotoxicity potential increases by 1598 and 1593 g1,4-DBeq/ kgP when urine separation is implemented in single and clustered domiciles (Figure 4d), due to organic contaminants in urine. Diverting biosolids to agriculture increases the terrestrial ecotoxicity potential by 162 g1,4-DBeq/kgP, primarily due to the metals in biosolids rather than organic contaminants. Chemical enrichment of biosolids and struvite precipitation increases the terrestrial ecotoxicity potential by 2 and 7 g1,4DBeq/kgP (Figure 4d). The contaminant load in the chemical solids does not change (except iron), while struvite is a comparatively pure product and the contaminants are almost entirely offset by those in avoided fertilizers. 3.2.6. Photochemical Oxidant and Particulate Matter Formation. The photochemical oxidant and particulate matter formation potentials reflect the difference in direct and embodied power used in the recovery process and avoided by phosphorus recovery. The nonchemical processes also avoid methane emissions, which reduces the photochemical oxidant formation potential. Urine separation at single and clustered domiciles, biosolids diversion from landfill, and struvite precipitation from the dewatering liquid reduce the photochemical oxidant formation potential by 1113, 163, 52, and 2 gNMVOC/kgP, respectively (Figure 5a). Phosphorus recovered as chemical solids and struvite precipitated from the brine stream increase the photochemical oxidant formation potential by 144 and 106 gNMVOC/kgP (Figure 5a). Except for the biosolids case, the particulate matter formation potentials follow the trend of the photochemical oxidant formation potentials. In the biosolids case, ammonia emissions from land application increase the particulate matter formation potential by 139 gPM10eq/kgP (Figure 5b). In the urine separation cases, ammonia emissions may be particularly problematic if temperatures are high and the urine storage is not adequately sealed. However, these are offset by the avoided power use. Transport of recovered products is significant to the photochemical oxidant formation and particulate matter formation potentials in all but the struvite cases (Figure 5a and b). 3.2.7. Fossil Fuel Depletion. Urine separation at clustered domiciles, biosolids diversion from landfill, and struvite precipitation from the dewatering stream reduce the fossil fuel depletion potential by 7, 1, and 2 kg oileq/kgP, respectively (Figure 5c). For urine separation at clustered domiciles, the additional infrastructure and transport required is offset by the embodied power use in avoided fertilizers. Similarly, the fuel used to transport biosolids to agriculture and the chemicals and power used in struvite precipitation are offset by avoided fertilizer production. Urine separation at single domiciles, chemical phosphorus removal and struvite precipitation from the brine stream increase the fossil fuel depletion potential by 20, 9, and 2 kg oileq/kgP, respectively (Figure 5c). At single domiciles, urine separation does not benefit from economies-of-scale and requires additional tanks and piping to recover the same
In centralized systems, diverting biosolids to agriculture avoids 79 kgCO2eq/kgP, primarily due to avoided methane emissions from landfill (refer to S2.2.4 for electricity generation at landfill). Phosphorus recovered as chemical solids and struvite precipitated from the brine stream result in an increase of 50 and 20 kgCO2eq/kgP (Figure 4a), due to additional power and chemical use. Although struvite production from the dewatering liquid requires additional power and chemicals, there is a net reduction of 5 kgCO2eq/kgP due to avoided N2O emissions, lower power consumption, and reduced chemical dosing for pH control due to reduced nitrification. The benefits are not dependent on avoided fertilizer, with the changes in operation more significant to the global warming potential of all case studies. While transporting products does increase greenhouse gas emissions, it does not outweigh the operational benefits achieved in the nonchemical processes. Transport contributes only minimally to the struvite cases, as the phosphorus-density of struvite is higher than urine and biosolids. 3.2.4. Ozone Depletion. Reducing ozone depletion is contingent on reducing N2O emissions associated with nitrogen removal at WWTPs. Urine separation reduces the influent nitrogen load in decentralized systems by 70% (see S1.2.5). Consequently, implementing urine separation reduces the ozone depletion potential by 5351 and 6617 mgCFC-11eq/ kgP in single and clustered domiciles (Figure 4b). Struvite precipitation from the dewatering liquid reduces the ozone depletion potential by 52 mgCFC-11eq/kgP, again due to the decreased nitrogen load and associated N2O emissions at the WWTP. Diverting biosolids or chemical solids to agriculture and struvite production from the brine stream results in a net increase of 38, 73, and 50 mgCFC-11eq/kgP, respectively (Figure 4b). In the biosolids case, N2O emissions from land application are not completely offset by the avoided use of fertilizers. Similarly, for phosphorus recovered as chemical solids and struvite from the brine stream, the increase in ozone depletion potential due to power and chemical use is not offset by the avoided fertilizers. 3.2.5. Human Toxicity and Terrestrial Ecotoxicity. The human toxicity and terrestrial ecotoxicity potentials are determined by the difference in contaminants in recovered phosphorus products versus synthetic fertilizers, plus any emissions from transport or chemical manufacture. Urine separation reduces the human toxicity potential by 4 and 11 kg1,4-DB eq/kgP in the single domicile and clustered domiciles (Figure 4c). Emissions from transport increase the human toxicity potential in the urine separation cases; however, these are offset by the avoided contaminants in, and associated with production of, fertilizers. Diverting biosolids to agriculture applies metals which would have been contained in landfill, resulting in a net increase in the human toxicity potential of 16 kg1,4-DBeq/kgP (Figure 4c). Dosing ferric to recover more phosphorus does not change the metal load, except iron, which is nontoxic in the ReCiPe models. However, emissions associated with chemical manufacturing increase the human toxicity potential by 25 kg1,4-DBeq/kP. Emissions from transport also increase the human toxicity potential of the biosolids cases but are less significant than the metals in biosolids or the emissions associated with chemical manufacturing. Due to the high phosphorus-density of struvite, the contribution of transport in these cases is negligible. The 8617
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phosphorus recovery is to achieve an environmental benefit, it is important to consider other potential impacts including global warming, ozone depletion, fossil fuel depletion, toxicity, and salinity. 4.1. Study Limitations and Uncertainty. The biological only and combined biological and chemical solids cases were based on published WWTP designs.1 It is acknowledged that the biological phosphorus removal is lower than expected,77 resulting in a high effluent phosphorus load (Table 1). Australian WWTPs with low phosphorus limits (
