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Cite This: Environ. Sci. Technol. 2018, 52, 8965−8976

The Economics of Wastewater Treatment Decentralization: A Techno-economic Evaluation Manel Garrido-Baserba,*,†,‡ Sergi Vinardell,§ María Molinos-Senante,∥,⊥ Diego Rosso,†,‡ and Manel Poch§ †

Department of Civil & Environmental Engineering, University of California, Irvine, California 92697-2175, United States Water-Energy Nexus Center, University of California, Irvine, California 92697-2175, United States § LEQUiA, Institute of the Environment, University of Girona, Girona, E-17071, Spain ∥ Department of Hydraulic and Environmental Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile ⊥ Center for Sustainable Urban Development, CONICYT/FONDAP/15110020, Av. Vicuña Mackenna 4860, Santiago, Chile

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S Supporting Information *

ABSTRACT: The existing wastewater treatment infrastructure has not adequately established an efficient and sustainable use of energy, water, and nutrients. A proposed scheme based on source separation and water-efficient use is compared to the current wastewater management paradigm (one largely based on activated sludge) using techno-economic terms. This paper explores the economic viability of adopting more sustainable management alternatives and expands the understanding of the economics of decentralization and source-separation. The feasibility of three different potential types of source-separation (with different levels of decentralization) are compared to the conventional centralized activated sludge process by using recognized economic assessment methodologies together with widely accepted modeling tools. The alternatives were evaluated for two common scenarios: new developments and retrofit due to the aging of existing infrastructures. The results prove that source-separated alternatives can be competitive options despite existing drawbacks (only when countable incomes are included), while the hybrid approach resulted in the least cost-effective solution. A detailed techno-economic evaluation of the costs of decentralization provides insight into the current constraints concerning the paradigm shift and the cost of existing technologic inertia. phenomenon in the coming decades.9 Similarly, existing and aging infrastructures in wastewater treatment facilities will soon need to be replaced. In the pursuit of sustainable wastewater management options, source separation and decentralization are slowly becoming realistic alternatives for these new and expected developments.10,11 Both wastewater treatment alternatives lead to several advantages:

1. INTRODUCTION Both economic and environmental sustainability are shifting the current paradigm in urban wastewater management.1−4 The century-old activated sludge (AS) process improved worldwide water sanitation significantly after increased urbanization and industrialization by providing safe effluent from wastewater. But this process is now being recognized as lacking economic and environmental sustainability, especially with respect to the inefficient use of energy, (recycled) water, and nutrients.5−7 Current wastewater infrastructures were designed and constructed on the basis of outdated views, requirements, conditions, and technologies of decades ago.8 Greenhouse gas emissions from the AS process itself (e.g., N2O) and from sources of energy production, lack of recovery of finite nutrients such as phosphorus (even given current depletion rates), continually rising energy costs, resilience limitations, and the need for cost-efficient technologies are among the forces that will drive cities to start building the next generation of urban wastewater solutions.3 Furthermore, the construction of new developments in cities will be a worldwide © 2018 American Chemical Society

• The organic carbon in a typical combined municipal wastewater represents a chemical energy content of approximately 1.9 kWh/m3.12−14 While anaerobic digestion has the potential to harvest this energy within the order of 0.6 to 0.9 kWh/m3 for the most concentrated black wastewater (BW) stream in houseReceived: Revised: Accepted: Published: 8965

April 14, 2018 June 25, 2018 July 2, 2018 July 2, 2018 DOI: 10.1021/acs.est.8b01623 Environ. Sci. Technol. 2018, 52, 8965−8976

Article

Environmental Science & Technology

One of the first comparison studies to date stated that depending on the scenario, source separated systems are more cost-effective than conventional systems.56 Other authors have instead estimated that the overall costs of source-separated approaches could be about twice the conventional system.57 However, none of these studies used standardized and recognized methods of cost assessment (i.e., licensed software), and their calculations were mainly based on their own experience and references. Including the potential incomes from source separation is also essential to evaluate decentralized alternatives.58 The aim of this paper is to expand the understanding of the economics of decentralization and source-separation by using standardized approaches for economic projections and evaluations in wastewater systems. The feasibility of three different potential types of source-separated systems were compared with the AS process using commercially available modeling software (i.e., CapdetWorks). The main novelty of this study is to provide a comprehensive comparison and assessment of wastewater treatment alternatives, including the following: (1) a methodology based on reliable cost-estimation software (i.e., CapdetWorks; Hydromantis, Inc.) and state-ofthe-art literature for estimating construction and operation costs; (2) consideration of only existing technologies (not requiring further innovation before their deployment) that have been accepted as feasible alternatives among wastewater experts; (3) inclusion of the potential income produced by source-separated alternatives; (4) implementation of the aforementioned analytical analysis in two of the most common scenarios in developing and developed societies: new wastewater treatment developments and the aging of existing infrastructures (retrofit), respectively. Therefore, a detailed and integrated economic analysis including the sewer system, existing and realistic alternative wastewater treatments, and resource efficiency is presented.

holds,13 the current AS process on the other hand consumes about 0.3−0.7 kWh/m3 of wastewater.15−17 • The current practice is to merge, dilute, and treat both gray wastewater (GW) and BW streams, which hinders the feasibility of nutrient recovery. Highly concentrated BW streams can be treated separately to facilitate nutrient recovery.18,19 Similarly, using a minimal amount of water yields concentrated wastewater flows which are more cost-effective for removing harmful micropollutants20,21 or recovering valuable constituents.22 • Recovering nitrogen could reduce the production of artificial fertilizers via the Haber−Bosch process, which fixes nitrogen from the air but uses up to 2% of the world’s energy23 and represents 50% of the energy in European agriculture.8 • The energy demand to run aeration blowers in the aeration-based AS process accounts for more than 50− 75% of the net power demand in wastewater treatment plants (WWTPs) needed to meet the mandated amount of dissolved oxygen.24−26 • Source separation and decentralization could reduce the current increase in energy demand (and concurrent carbon footprint) caused by the implementation of new technologies that achieve higher effluent quality at the expense of higher energy demand27 by producing renewable energy in useful forms (heat, methane) and by avoiding energy-demanding AS processes and transport28−30 • The current trend in clean decentralized energy (i.e., biogas, solar, wind) offers new possibilities of decentralized wastewater treatment, making new water reuse systems scalable, off-grid, and without the need for the transport of fossil fuels.31−36 • Vacuum toilets, as a way of source separation, can reduce BW water consumption by 90% to 35 L per person/day.10,37−40, and the overall consumption by about 25%.41 • The treatment alternatives can increase the ability of urban wastewater systems to adapt as a response to change42 and enhance climate-resilient infrastructures.6,43−46 Despite new available knowledge, expertise, and technologies to develop more economically and environmentally sustainable water resource management alternatives, practical implementation remains slow.47−50 Source separating technologies are considered “low-tech,” and hardly any resources were allocated to their development because they are still considered immature and risky by most wastewater professionals.10,50,51 Similarly, some authors suggest that developing alternative cost-efficient wastewater management systems is an issue of governance rather than technology.50,52,53 Furthermore, recent studies have highlighted that consumers (>64%) express highly favorable views of new systems combining elements of source separation, local treatment, and reduced water use.54,55 A lack of evidence pertaining to the economic viability of these alternatives hinders their consideration as feasible and credible options. Therefore, we aimed to present a clear and simple approach to the economics of source separation and decentralization to provide sound information that can support the decision-making of (waste) water authorities.

2. METHODOLOGY 2.1. Wastewater Treatment Alternatives. Two main scenarios were considered for the economic assessment of the selected wastewater management alternatives: new developments, and the retrofit of an existing WWTP. Both scenarios are evaluated for a medium-sized population of 30 000 population equivalent (PE). The population size was selected to represent an average, intermediate city scenario in which activated sludge-related configurations would typically be the preferred option to implement. Moreover, the installation of anaerobic digesters (AD) is often discouraged in centralized communities smaller than 40 000 PE due to economic and technical reasons. Phosphorus resolubilization during the hydrolysis step in AD drives highly concentrated phosphorus flows back to the main stream, leading to recirculation instead of actual P-removal, plus increased piping blockage by spontaneous struvite precipitation.59−62 The consideration of an AS without anaerobic digestion may facilitate the comparison with source-separated alternatives (anaerobicbased). Note that larger treatment plants would benefit from the use of side-stream AD (specifically by using codigestion strategies), and operational savings should be included in the case of a techno-economic analysis. For each scenario, three wastewater management alternatives or flow diagrams are applied: (i) centralized (alternative A); (ii) hybrid (alternatives B1 and B2) and; (iii) decentralized (alternative C) (Figures 1 and 2; see Table 8966

DOI: 10.1021/acs.est.8b01623 Environ. Sci. Technol. 2018, 52, 8965−8976

Article

Environmental Science & Technology

Figure 1. Flow diagrams for three wastewater treatment alternatives. (A) fully centralized alternative using activated sludge; (B) hybrid alternative following a centralized approach for gray water and a decentralized approach for black water; (C) fully decentralized approach for both black and gray water streams. Alternatives B and C can have two differentiated treatments for the liquid effluent from the anaerobic processes unit (Figure 2).

S12 in the Supporting Information for a detailed description). The centralized alternative consisted of a typical AS process. Alternative B (hybrid) is characterized by a partially centralized scheme, while alternative C represents a fully decentralized scheme. For both source-separated alternatives (B and C), the treatment of nitrogen was evaluated considering two different technologies. One approach was based on the physical− chemical recovery of the nitrogen (i.e., stripping-absorption process, alternative B1), while the other was based on the biological removal of nitrogen (i.e., Oland/anammox process, alternative B2 and C). For the sake of simplicity, the results of the two nitrogen treatment alternatives were shown only for alternative B. For the decentralized alternative (C), only the output of the most cost-effective alternative, that is, nitrogen removal by the Oland process, is presented in this paper. A detailed description of each alternative is provided as Supporting Information. 2.2. Influent Composition. Table 1 shows the typical values for an urban influent in a centralized WWTP, which traditionally combines BW and GW.10 The wastewater composition shown in Table 1 was selected as the influent for all the wastewater alternatives evaluated. In the sourceseparation cases, the two different streams were split accordingly, as shown in Table 1. Following current practices, BW is expected to be collected with vacuum toilets, which means a consumption of water per people equivalent (PE) of 5 L/PE/day.63 As for GW, its water consumption is about 108 L/PE/day.10,64

Conventional toilets were used in the sewer combined alternative (alternative A, combining and not discerning between BW and GW), which means a consumption of about 40 L/PE/day.10 2.3. Sewer Infrastructure. For comparative and standardization purposes the sewer distribution of the new development (Scenario 1) and retrofit (Scenario 2) scenarios were adapted from Roefs et al. (2016). Each development consisted of a series of districts (see Figure S4, Supporting Information) servicing 1200 PE. Each district was distributed in neighborhoods of 50 households representing a total of 120 PE. Each neighborhood had a surface area of 2.5 ha. In both the conventional and hybrid alternatives, districts were connected to a collection system that connected district to district and to the central WWTP. A backbone pipe was used as connection between neighborhoods within the district (Figure S4). At the neighborhood level, both private and public sewers were taken into account. Private sewers were defined as the sewers from the house to the first Y-joint that makes connection with the water main.56,70 A detailed description of sewer infrastructure is shown in the Supporting Information. 2.4. Model Domain. Influent Variability. This study assumes low variability in the influent concentrations for the source-separated options as the main uncertainty contributors are avoided: Industrial effluents (traditionally representing 15− 30% of flow composition), stormwater episodes, sewage characteristics (i.e., combined and separated), uncontrolled 8967

DOI: 10.1021/acs.est.8b01623 Environ. Sci. Technol. 2018, 52, 8965−8976

Article

Environmental Science & Technology

Figure 2. Flow diagram alternatives for the liquid effluent from the anaerobic processes unit: (Option 1) nitrogen recovery by a strippingabsorption system; (Option 2) nitrogen removal by the Oland process.

uncertainty could be reduced by studies incorporating wastewater dynamics and site-specific fluctuation. Sewer System. This study was based on a density of about 20 household/hectare, which is a common value for European residential areas with free-standing or double houses.73 Further research exploring the variability depending on other common population densities would be required, as there can be lower densities (i.e., US and Canada) or higher densities (i.e., highly populated cities). The model used in this study was developed by Maurer et al. (2013), and may yield significantly different results, especially if some pipe layouts needed to be arranged differently. Furthermore, the impact of assuming different distances to the central plant (currently 3 km) may result in close to negligible results for distances shorter than 30−40 km (following the aforementioned methodology). The methodology considers straight long-distance pipes (main column or artery, HDPE) with significantly lower costs (