Assessing the Scale of Resource Recovery for Centralized and

Aug 9, 2013 - The outcome of any analysis of resource recovery scale will depend upon the nature of the technology used for resource recovery. For thi...
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Assessing the Scale of Resource Recovery for Centralized and Satellite Wastewater Treatment Eun Jung Lee,† Craig S. Criddle,*,† Phil Bobel,‡ and David L. Freyberg† †

Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States Department of Public Works, City of Palo Alto, Palo Alto, California 94301, United States

‡

S Supporting Information *

ABSTRACT: Wastewater treatment to recover water, energy, and other resources is largely carried out at centralized treatment facilities. An alternative is local treatment at satellite facilities where wastewater is removed from a collection system, resources are recovered locally, and the residuals are returned to the collection system. Satellite systems decrease the pipe and energy required for delivery of treated water and may decrease cost. But decisions regarding the geographic scale of resource recovery require consideration of many criteria. In this study, we rank water and energy recovery options for a simplified test case at three scale configurations: a centralized configuration and two hybrid configurations. We first choose criteria for decision-making. Quantitative performance metrics are defined for each criterion, weighted, and computed for each configuration. We then rank configurations. Rankings depend upon the decision-making strategy. For our test case, though, several strategies yield the same top-ranked configuration: a hybrid where communities close to the centralized facility use centralized resource recovery; communities far from the centralized facility use satellite resource recovery. Our ranking is sensitive to initial investment cost for satellite treatment. The results underscore the importance of cost-effective treatment systems and of an accurate and comprehensive analysis of design components.

1. INTRODUCTION Wastewater is increasingly recognized for its value as a resource, providing opportunities for recovery of water, energy, nutrients, and valuable materials, such as compost.1 In domestic wastewater, the highest-value resource is the water itself.1 Use of recycled water can attenuate the effects of drought and increased demand, decrease dependence upon imported water, supply freshwater to freshwater habitat, and prevent excessive freshwater discharge to salt-adapted habitat.2,3 At present, recovery of water occurs largely at the scale of the service area of a centralized wastewater treatment plant, here called a catchment. Wastewater generally flows by gravity from its sources to the centralized treatment plant, often the lowest point in the catchment. Recycling of treated water to upstream points can require significant capital (pipes, pumps) and operational expense (energy for pumping). An alternative is satellite treatment, an option that requires less pipe and energy for water delivery.4,5 But despite these benefits and government advocacy,6 utilities often assume that financing, regulatory and legal constraints, and operational complexity outweigh potential benefits. An additional impediment may be “siloed” planning of water supply and wastewater collection and treatment systems.7,8 The second most valuable resource in wastewater is renewable energy from the biodegradable organic matter.1 This energy is © 2013 American Chemical Society

typically recovered at the catchment scale. Conventional energy recovery makes use of anaerobic digestion to convert organic matter into biogas, a mixture of methane and carbon dioxide. The methane is burned, generating heat and electricity. Digestion also decreases the solids for disposal and increases their stability, decreasing the need for energy-intensive solids management. Large anaerobic digesters offer economies of scale for production of biogas and solids stabilization.9 If sufficient high quality biogas is collected, significant electricity and heat can be generated. While small-scale systems lack economies of scale, their use is increasing nonetheless. In 2005, there were more than 25 million small-scale biogas plants in operation worldwide, and the installation rate since then has exceeded 1 million per year.10 In some such systems, monetary benefits may result more from the decreased mass of solids handled and increased solids stability than from energy production.11 Further advances in anaerobic wastewater treatment11 coupled Special Issue: Design Options for More Sustainable Urban Water Environment Received: Revised: Accepted: Published: 10762

March 6, 2013 August 8, 2013 August 9, 2013 August 9, 2013 dx.doi.org/10.1021/es401011k | Environ. Sci. Technol. 2013, 47, 10762−10770

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Figure 1. Overview of key steps in the configuration and design of wastewater resource recovery systems at different geographic scales. Not shown are feedback loops needed for a complete analysis of system design options.

with improved capabilities for remote monitoring, sensing,12 and control are likely to enable cost-effective removal of organics at increasingly smaller scale. Such systems could increase production of water for local reuse and decrease risk that residual solids returned to the collection systems will settle and create nuisance conditions. But methodologies are needed to assess and rank different resource recovery configurations. In this study, we evaluate and rank water and energy recovery options for a simplified test case at three scale configurations: a centralized configuration and two hybrid configurations. These configurations are compared with an outranking methodology that incorporates input from decision makers.

audit of water is available for that time period. Most of the information remains relevant as of 2013. The PARWQCP treats ∼87 000 m3 per day (∼23 MGD) of wastewater generated by 220 000 residents. The plant is equipped with bar screens, primary sedimentation, roughing filters, nitrifying activated sludge, dual media filters, disinfection (chlorination in 2007− 2008; since replaced by ultravolet disinfection), a gravity thickener, belt filter press for dewatering, and incineration for solids stabilization.3,16 Energy sources for plant operation are imported electricity, natural gas, and landfill biogas. Incineration of primary solids and biosolids requires 4,400 kWh/ton dry solid, 64% of the total energy demand for the plant. Ash is transported by truck to hazardous waste facilities in California (cost of ∼$200 000/year). Most treated effluent is discharged to San Francisco Bay, but some is used to replace about 1.3% of the water used to irrigate a golf course and park. Stanford University has a system for recycling of water from cooling tower blow down for toilet flushing, but the amount reused is small relative to campus water use. 2.2.1. Identification of the Goal and Decision Makers. Our goal is to identify and rank resource recovery configurations for different geographic scales. Decision-makers might include municipal water and wastewater planners, consultants, and managers, nongovernmental organizations, government policy makers, or possibly the electorate (for example, in cases where a bond measure requires approval). 2.2.2. Identification of Criteria and Metrics. The second step is identification of performance criteria with quantitative metrics. A large body of literature exists to assist in the identification of criteria and metrics.17,18 For this simplified case study, we select the criteria and metrics summarized in Table 1. 2.2.3. Weighting of Criteria. For the PARWQCP test case, we drew upon our experience with the Palo Alto community to assign preference scores for each criterion in Table 1. We used a scale of 0 to 10, where 10 indicates highest preference and 0 indicates that the criterion is insignificant relative to other criteria. We used these scores to calculate criteria weights for different decision strategies based upon (1) initial investment

2. METHODOLOGY 2.1. Overview. Our aim is to identify and rank configurations for resource recovery at different geographic scales using weighted criteria. Assessment of water and wastewater systems for different geographic scale configurations requires analysis of pipeline routes, treatment options with different methods of resource recovery, iterative computation of water and solids balances, and comparison of whole systems at different scales using multiple criteria, some of which may conflict. To understand and address these issues, we perform a case study for water and energy recovery at different geographic scales in which design components are integrated for quantitative assessment of scale configurations. We define and analyze different configurations (Figure 1) and rank them using a multicriteria decision analysis (MCDA) tool13 that is widely used for environment and energy research.14,15 2.2. Case Study. For our case study, we choose the service area of the City of Palo Alto Regional Water Quality Control Plant (PARWQCP), located in Santa Clara County, California, U.S.A. The City of Palo Alto owns and operates the PARWQCP, a centralized treatment plant that serves the communities of Mountain View, Palo Alto, Los Altos, Los Altos Hills, Stanford University, and East Palo Alto Sanitary District (Supporting Information Figure S1). For the purposes of this case study, we used data for FY 2007−2008 because a detailed 10763

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Table 1. Criteria and Metrics Adopted for the Case Study criteria (1) initial investment cost ($M) (2) O&M costa ($M) (3) revenue and avoided cost resulting from resource recoverya ($M) (4) net life cycle costa ($M) (5) percent renewable energy (%) (6) resilience to water stress (%)

a

definitions and metrics Estimated initial expenses, including construction cost, engineering and construction management costs, right-of-way costs, and contingency. Present value of operation and maintenance costs Revenue from recycled water and avoided cost of imported energy. This revenue is income to the PARWQCP from the sales of recycled water. The recovered energy is used to operate the PARWQCP. Discounted costs (initial investment cost minus residual capital value, O&M and replacement costs) minus discounted benefits (revenue and avoided cost resulting from resource recovery) Fraction of energy demand met by biogas contributing to the California Renewable Portfolio Standards.19 This was computed as the percentage of the energy needed for each configuration supplied as renewable energy. Fraction of water use in a baseline year met by recycled water. This was computed as the ratio of recycled water supplied to total water use in the catchment for a baseline year (2007−2008 for the Palo Alto case study). We assume that increased water reuse enhances resilience to water stress.

Costs assume a 20-year time period and 6% discount rate (Details in SI).

Table 2. Weights Used for Ranking of Alternatives decision-making criteria initial O&M investment cost cost preference scores decision-making strategies and weights (1) normalized weights if initial investment cost is sole criterion (2) normalized weights for monetary criteria only (3) normalized weights for all criteria (4) normalized weights for net life cycle cost only (5) normalized weights for net life cycle cost and nonmonetary criteria (6) normalized weights for nonmonetary criteria

revenue and avoided cost resulting from resource recovery

net life percent cycle cost renewable energy

resilience to water stress

10

6

9

9

6

8

1

0

0

0

0

0

0.294 0.208 0 0

0.176 0.125 0 0

0.265 0.188 0 0

0.265 0.188 1 0.391

0 0.125 0 0.261

0 0.167 0 0.348

0

0

0

0

0.429

0.571

cost only; (2) monetary criteria (i.e., the first four criteria of Table 1), (3) both monetary and nonmonetary criteria (i.e., all criteria in Table 1), (4) net life cycle cost alone, (5) net life cycle cost plus the nonmonetary criteria, and (6) nonmonetary criteria only. Weights (Table 2) are calculated by dividing preference scores for each criterion by the sum of the preference scores for the applicable decision criteria. 2.2.4. Selection of a MCDA Methodology. A decisionmaking methodology is needed to balance criteria and to rank geographic scale configurations. We choose an “outranking” method20 that identifies alternatives with “better-than-others” performance on several criteria (rather than maximum performance for a single criterion) and allows comparison of criteria that have incomparable or incommensurate metrics, such as monetary and nonmonetary criteria.21 Specifically, we use PROMETHEE I and II (preference ranking organization method for enrichment evaluations I and II).22,23 We first obtain date for each of performance metrics in Table 1 for each scale configuration. We then compare configurations for each criterion in a pairwise manner. For each comparison, a preference function score is calculated using the “usual” preference function,22 that is, a configuration alternative is scored as 1 if its performance is superior to the paired alternative configuration and as 0 if its performance is equivalent to or inferior to the paired alternative. Each paired comparison receives a preference function score for each criterion. These scores are multiplied by normalized weights assigned to each criterion (Table 2) and summed to give a single number for each comparison. The weighted sums for each configuration are summed and divided by the number of alternative configurations to compute “positive outranking flows” and “negative outranking flows”. The final ranking is based on the net

outranking flows, computed as the positive outranking flow minus the negative outranking flow. These flows are dimensionless and range from −1 to +1. 23 2.2.5. Geographic Scale. Decisions regarding the geographic scale for resource recovery can be made at the scale of a single residence or building, a cluster, a catchment or “sewershed”, or several catchments within a watershed. In any such analysis, three enabling decisions are required: (1) identification of treatment plant locations, (2) definition of boundaries at each scale of interest, and (3) selection of routes for delivery of treated water. We focus on two scales: a catchment scale and a cluster scale.24 A catchment is defined as the service area of a centralized wastewater treatment plant (CTP). Resource recovery at this scale requires a wastewater collection system, a CTP, a system for storage and distribution of treated water, and a system for energy recovery from organic matter. We subdivide the catchment into “clusters”, collections of buildings that share a sewer line (e.g., a homeowner’s association, a small city, campus). Resource recovery at this scale requires removal of wastewater from the catchment collection system (a practice referred to as “scalping” and requiring a scalping system including a pump station); treatment at a satellite treatment plant (STP); return of residuals to the collection system; a system for storage and distribution of treated water; and the possibility of a system for energy recovery from organic matter. For this study, we restrict delivery of reclaimed water to the clusters from which the wastewater originated. For our case study, the catchment is that of the PARWQCP. We define five clusters using existing political boundaries: East Palo Alto Sanitary District, City of Mountain View, Cities of Los Altos and Los Altos Hills, Stanford University, and City of 10764

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3.7%, as observed at the CTP. For hybrid configurations with satellite energy recovery, we estimated the primary solids loading of the anaerobic digester at the CTP by subtracting the mass flow of thickened primary solids digested at the STPs from the mass flows of thickened primary solids digested at the CTP in 2007−2008. 2.2.7. System Configurations and Design of Resource Recovery Systems. Knowledge of water and organic balances enables comparison of the existing treatment system configuration with alternatives that include resource recovery. We analyze four resource recovery options for the CTP: (1) no recovery of energy and negligible water recovery (i.e., the status quo), (2) recovery of water but not energy, (3) recovery of energy but not water, and (4) recovery of both water and energy. For hybrid configurations, we assume that all STPs recover water, and that energy recovery is an option. In all cases, we use water and organics balances and local water quality criteria to size treatment systems, estimate costs (capital and O&M), and assess the economic benefits to the utility of water and energy recovery. The outcome of any analysis of resource recovery scale will depend upon the nature of the technology used for resource recovery. For this case study, we simplify the analysis by choosing treatment technologies a priori. Practical implementations would need to consider a broader suite of technologies. For the CTP, we assume that the PARWQCP provides sufficient removal of organic matter from the water for outdoor water reuse. The CTP also provides disinfection. In the baseline year of 2007−08, this was accomplished by chlorination/ dechlorination. For all scale configurations, we replace chlorination/dechlorination with ultraviolet irradiation at the CTP. Removal of total dissolved solids (TDS) is also required at the CTP for water that is recycled. The City of Palo Alto has set 600 ppm as a TDS goal for reclaimed water.25 TDS in the Mountain View sewer trunk line exceeds 1,130 ppm because of saltwater intrusion. We therefore assume that desalination is necessary at the CTP and at STPs serving the Mountain View cluster. Desalination options include microfiltration/reverse osmosis, microfiltration/electrodialysis reversal (EDR), and EDR. We select three-stage EDR with cartridge filtration because it reportedly meets the TDS goal (average 78% TDS removal26) and enables a high percentage of feedwater recovery (85%) at a lower present cost than other options.27 Primary and secondary organic solids are currently incinerated at the PARWQCP. About two-thirds of the energy used at the plant is required for this purpose. For energy recovery, we assume replacement of the existing incinerator by a thermophilic anaerobic digester, and we choose an energy recovery system consisting of a gas pretreatment system and: (1) a boiler alone if heating requirements required for the digester can be met with a boiler; (2) a microturbine if heating requirements for the digester can be met by waste heat from the microturbine; or (3) both a boiler and a microturbine if heating requirements of the digester can be met with increased power production by combining waste heat from a microturbine with heat from a boiler. We choose a thermophilic anaerobic digester because of its small footprint and ability to increase biogas production at low capital costs compared to conventional digesters.28 We select a boiler because it is a cost-effective and thermally efficient option for digester heating.29 For power production from the gas that remains after heating, we select microturbines because of their low air emissions.29 We assume that biogas injected into the boiler and

Palo Alto (SI Figure S1). We define three configurations: a centralized configuration where the CTP delivers recycled water to the entire catchment and two hybrid configurations in which reclaimed water is delivered from both the CTP and STPs. At present, the primary use of recycled water within the catchment is landscape irrigation, and we assume that this use will continue to be the dominant application for all configurations. For hybrid configuration 1, the CTP delivers treated water to Palo Alto, and four STPs deliver treated water to Mountain View, Los Altos/Los Altos Hills, Stanford, and East Palo Alto; for hybrid configuration 2, the CTP delivers treated water to Mountain View, East Palo Alto, and Palo Alto, and two STPs deliver treated water to Los Altos/Los Altos Hills, and Stanford. Further discussion of the rationale used to define hybrid configurations is provided in the SI. 2.2.6. Mass Balances. To determine flow rates for recycled water, a water balance is required for each scale of interest. The water balance and wastewater quality data are used to determine the mass flows of organic matter and to estimate the quantities of chemical energy available for recovery. Using average flow rates of water and wastewater for FY 2007−2008 for the City of Palo Alto, we construct an annual water balance for the catchment of the PARWQCP, including total water supply, average daily recycled water flow, average daily wastewater flow, and average daily outdoor water use for landscaping. We also construct similar mass balances for the STPs. Below we summarize key assumptions in the water and organics balances at each scale. Details are provided in the SI. For CTP within the Palo Alto catchment, we assume that the water delivered for reuse is 2.1 times the average daily outdoor water use. For STPs at the East Palo Alto, Mountain View, and Palo Alto clusters, where average daily wastewater flows are larger than the expected average daily outdoor water use, we assume that the design flow for wastewater treatment and delivery is 2.1 times the average daily outdoor water use. For STPs at the Los Altos and Los Altos Hills and Stanford clusters, where the average daily wastewater flows are less than the expected average daily outdoor water use, we assume that the ratio of the design flow to the average daily wastewater flow depends upon energy recovery at the STP. For STP systems that recover energy, we assume the quantity of digestate returned to the sewer is negligible, and that solids discharges can be timed so as to prevent nuisance conditions. We also assume that the ratio of the design flow to the average daily wastewater flow is 2.0. If energy is not recovered by digestion of solids, there may be insufficient flow for flushing of solids discharged to the sewer. We therefore assume that, for this case, only half of the average daily wastewater flow is treated and that the design flow is equal to the average daily wastewater flow. For satellite treatment plants, we also assume local storage of treated water. For Los Altos and Los Altos Hills and Stanford clusters, where average daily wastewater flow is less than average outdoor water use, we assume a storage capacity of 3 days at the average daily recycled water flow. For other clusters, where average outdoor water use is less than average daily wastewater flow, we assume a capacity of 2 days at the average daily recycled water flow. We use measured flows of thickened primary and biological solids from the PARWQCP for 2007−2008 as the mass flows of volatile solids for anaerobic digestion at the CTP. For STPs with energy recovery, we assume that the volumetric flow of thickened primary solids for anaerobic digestion is 0.6% of the influent wastewater flow and that the solids are thickened to 10765

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Figure 2. Three scale configurations evaluated for water and energy recovery. (a) CTP produces recycled water and delivers it to five clusters (yellow lines). (b) Hybrid 1: CTP produces recycled water and delivers it to one cluster (yellow line) and 4 STPs produce recycled water and deliver it to four clusters (red lines). (c) Hybrid 2: CTP produces recycled water and delivers it to three clusters (yellow lines) and two STPs produce recycled water and deliver it to two clusters (red lines). Energy recovery can occur at all scales. Table 3 summarizes all of the configurations evaluated.

followed by thermophilic anaerobic digestion to minimize footprint and maximize rate of biogas production. More detail is provided in the SI. 2.2.8. Pipeline Design. Knowledge of geographic factors and water balances enables sizing of pipes and pumps and projection of energy demands for delivery of recycled water to storage tanks. We choose the length and diameter of either PVC Schedule 40 pipe (15−36 cm), or ductile iron pipe (51− 61 cm), and turbine pumps to deliver peak treated flows to storage. We use the Hazen−Williams equation to estimate pipeline head losses. We estimate transport energy from elevation differences, frictional head losses, assumed recycled water service pressure (414 kPa),35 and pump efficiency. Assumptions are provided in the SI. 2.2.9. Scale Configurations and Performance Metrics. Figure 2 illustrates the scale configurations evaluated in this study. These configurations result in the ten options in Table 3, where each option is classified according to scale, that is, centralized (C), hybrid 1 (H1), or hybrid 2 (H2), and the resource recovered, that is, water (W), energy (E), or both. Table 3 also summarizes performance metrics for each configuration. 2.2.10. Rankings and Sensitivity Analysis. To arrive at multicriteria rankings for each decision strategy, the computed performance metrics listed in Table 3 are aggregated by application of the weights in Table 2. We used SANNA 2009,36 an MS Excel-based add-in application that supports several methods (including PROMETHEE I and II) for MCDA. We also perform a univariate sensitivity analysis of rankings (SI Figure S5) showing changes in the weighting factors

microturbine is pretreated with activated carbon to remove hydrogen sulfide and siloxane.29 We assume that stabilized biosolids are dewatered (∼30% solids)30 then transported to a site for composting. The operating costs for dewatering, transport, and composting are $30−50/ton. For STPs, there are many possible options for recovery of water (e.g., membrane bioreactors, sequencing batch reactors). We choose the Huber Vacuum Rotation Membrane Bioreactor31 with a conventional primary clarifier, a system currently used in satellite applications.31 It has the advantages of a membrane bioreactor (small aeration tank, no secondary clarifier, no sand filter, no need for ultraviolet disinfection31). It also reportedly has the lowest present cost among a set of common alternatives32,33 and is certified to meet California Title 22 standards for water reuse.31 We assume a hydraulic residence time of 9.3 h and an energy usage of 0.5 kWh/m3.34 For energy recovery at STPs, we assume use of scaled-down systems like the CTP system described previously, including a thermophilic anaerobic digester, boiler, and microturbine with an activated carbon filter. We assume that digested solids are returned to the catchment sewers. We assume that advances in the automation, operation, and maintenance of smaller-scale systems enable the same O&M costs per unit of solids digested at both the CTP and STP scale.12 For all energy recovery systems, we assume that the ratio of the flow of settled primary sludge to influent wastewater flow is equal to the ratio of settled primary sludge flow to influent flow at the existing CTP. Residual liquid is returned to the head works. For all of the high rate thermophilic anaerobic digesters, we choose a SRT of 10 days.29 We select a gravity thickener 10766

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Table 3. Performance Data Used for Preference Assessment of Ten Different Configurations geographic scale resource recovery configurations (1) centralized water reuse only (CW) (2) centralized energy recovery (CE) (3) centralized water and energy recovery (CW_CE) (4) hybrid configuration 1: water recovery only (HW1) (5) hybrid configuration 1: STP water recovery only; CTP water and energy recovery (HW_CE1) (6) hybrid configuration 1: STP water and energy recovery; CTP water and energy recovery (HW_HE1) (7) hybrid configuration 2: water recovery only (HW2) (8) hybrid configuration 2: STP water recovery only; CTP water and energy recovery (HW_CE2) (9) hybrid configuration 2: STP water and energy recovery; CTP water and energy recovery (HW_HE2) (10) no recovery

initial investment cost ($M)

O&M cost ($M)

revenue and avoided cost resulting from resource recovery ($M)

net life cycle cost ($M)

percent renewable energy (%)

resilience to water stress (%)

90 15 105 89 104

127 75 127 100 101

196 10 206 147 157

7 76 9 31 33

11 56 34 12 41

35 0 35 26 26

121

102

188

19

39

32

74 89

105 105

147 158

21 24

12 40

26 26

103

106

188

6

38

32

0

74

0

74

15

0

of the existing energy-intensive infrastructure with an energy recovery system. Strategy 6 considers nonmonetary criteria only (percent renewable energy; resilience to water stress). Centralized water and energy recovery (CW_CE) is top ranked because it offers the greatest resilience to water stress by meeting the water demands of clusters where recycled water demand exceeds wastewater produced, and it provides intermediate levels of energy recovery. The trade-off is high initial investment costs and high O&M costs for energy recovery. 3.2. Sensitivity Analysis of Criteria Preference Scores. Rankings of configurations can be sensitive to variations in input values. We use preference scores to represent the true importance of different criteria.37 Scoring by a decision-maker is likely to improve when the decision maker understands the sensitivity of configuration rankings to these scores. We performed a simple univariate sensitivity analysis to determine how changes in preference scores affect the first rank alternative and the robustness of the resulting decision. This was done by systematically varying the preference scores of each criterion in unit steps from 0 to 10, keeping constant the weights of the remaining criteria for Strategies that are based upon two or more decision-making criteria (strategies 2, 3, 5, and 6). The top ranked alternative in each strategy dominated rankings for wide variations in preference scores, indicating that this configuration (HW_HE2) is robust (SI Figure S5). 3.3. Resource Recovery Across Scale. Reclaimed water is the highest-value resource in wastewater.1 Its sale generates revenue and its use decreases dependence upon imported water. This explains the high ranking of the CW configuration. For most decision-making strategies, however, HW_HE2, a configuration that also includes satellite water and energy recovery, marginally outperforms centralized-only resource recovery. For this configuration, water that requires desalination is recovered at the centralized facility and used to meet the demand of clusters close to the CTP; water that does not require desalination is recovered at two STPs and used to meet the demand of communities far from the CTP. This configuration also takes advantage of an incidental benefit of energy recovery at STPs: their increased capacity for water extraction because fewer solids are returned to the sewers. It should be noted, however, that for this simple analysis, we assume a fixed unit price for water, ignoring the price elasticity of demand. Finally, we note that the criteria used in this study

for all strategies that consider multiple criteria (strategies 2, 3, 5, and 6).

3. RESULTS AND DISCUSSION 3.1. Net Outranking Flows. Figure 3 illustrates the net outranking flows (section 2.2.4) for each decision-making strategy and the ten configurations under consideration (Table 3). Strategy 1 minimizes initial investment cost. Decision-makers often focus on this criterion because of financing concerns, but this strategy results in poor performance with respect to other monetary criteria and does not address resource recovery criteria. Strategy 2 considers additional monetary criteria (O&M costs, net life cycle cost, revenue and avoided cost resulting from resource recovery). Centralized water reuse without energy recovery (CW) is the top-ranked configuration. Initial investment cost is intermediate and results in a low net life-cycle cost because of revenue from sale of recycled water. Of all the configurations evaluated, CW enables sale of the largest amount of recycled water. This is because it produces sufficient recycled water to meet the water demands of Los Altos and Los Altos Hills and Stanford clusters, where recycled water demand exceeds the wastewater produced. On the other hand, this strategy is poor in terms of O&M costs. Centralized water reuse with energy recovery (CW_CE) also produces enough recycled water for the entire catchment, but the advantages of energy recovery are outweighed by initial investment cost. For strategies 3−5 (Table 2), the preferred configuration is hybrid configuration 2 with both water and energy recovery (HW_HE2). Initial investment and O&M costs are intermediate but are outweighed by revenue and avoided costs from resource recovery (reflected in the lowest net life cycle cost) and by the nonmonetary benefits of resource recovery. Energy recovery at the STPs decreases discharge of biosolids to the sewer and potential for settling of solids, enabling increased treatment flow rates and production of recycled water. The result is a high percentage of renewable energy use and high resilience to water stress, with 32% of catchment water demand met by recycled water. Configuration HW_HE2 balances the advantages and disadvantages of centralized and satellite systems: clusters located far from the CTP do not require desalination because the treated wastewater has low TDS and clusters close to the CTP benefit from short pipelines and low transport energy. A significant energy recovery benefit results from replacement 10767

dx.doi.org/10.1021/es401011k | Environ. Sci. Technol. 2013, 47, 10762−10770

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Figure 3. Net outranking flows (flows are dimensionless, and range from −1 to +1) of the 10 configurations for the six strategies in Table 2. As discussed in section 2.2.4, the configuration with the highest net outranking flow is preferred.

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are illustrative and by no means exhaustive. Other criteria and metrics could be justifiably added. For different locations and conditions, other criteria will likely be significant, depending upon existing infrastructure and projected environmental and economic conditions.

ASSOCIATED CONTENT

S Supporting Information *

Map of the catchment and clusters, assumptions for balances on water and organic solids, including average daily recycled water 10768

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flow, average daily wastewater flow, average daily outdoor water use (average recycled water demand for landscaping), and total water supply during FY 2007−2008, criteria for selection of geographic scale, including assumptions regarding the definition of clusters, location of satellite facilities, location of storage tanks, and methods for delivery of water to the storage tanks, treatment train design for satellite treatment plants, cost assumptions for economic evaluations of recycled water distribution system, wastewater treatment system, energy recovery system, and storage tank, and a univariate sensitivity analysis of criteria weights. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support for this work from the Woods Institute for the Environment at Stanford University, the City of Palo Alto, the Stanford Department of Sustainability and Energy Management, and the NSF Center for Renewal of the Nation’s Water Infrastructure (RENUWIt) under award number 1028968. The authors also thank Margaret Laporte for helpful discussions.

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ABBREVIATIONS PROMETHEE preference ranking organization method for enrichment evaluations CTP centralized treatment plant STP satellite treatment plant PARWQCP Palo Alto Regional Water Quality Control Plant EDR electrodialysis reversal TDS total dissolved solids CE centralized energy recovery CW centralized water reuse HE hybrid energy recovery HW hybrid water reuse

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