Decentralized Treatment 21 Sewer mining toolbox helps evaluate reuse opportunities As water becomes more precious than gold in much of the world, decentralized extraction and treatment of water for reuse is drawing increased interest. Bikram Sabherwal, Jesse Wallin, and Sandeep Sathyamoorthy of Black & Veatch describe how a sewer mining toolbox can help utility managers and developers determine whether decentralized water reuse is right for them.

Sustainably increasing the avail- systems are typically smaller than Figure 1 ability, reliability, and affordability 1.9 million liters per day (mld) or of water supplies is one of the great 500,000 gallons per day (gpd), and Reuse challenges facing the global commu- potential users for small- to mid-size Additional Opportunity nity of water professionals, and re- WRRFs include housing and con- Treatment Type 2 use is high on the list of alternative dominium developments, athletic Process water supply solutions. fields, universities, and distributed Although many utilities and business parks. It is important to Basic Reuse regions gravitate toward large or note that sewer mining is not a new Treatment Opportunity megascale centralized reclamation concept and has been widely prac- Process Type 1 and reuse programs, smaller-scale ticed in many countries for more decentralized treatment systems than a decade. may be more beneficial where Until now, the vast majority of specific drivers motivate them, such sewer mining projects have been Residuals as limited budgets, smaller and designed for unrestricted spread-out demands, or lack of and other decentralized non-potable To Centralized Approved reuse (dNPR) applications. But WRRF technical expertise. Decentralized Sewer treatment systems may be highly change is at hand. relevant to promote more water There is a significant increase Figure 1. Sewer mining can be used to produce fit-for-purpose waters for a range reuse at the micro-local scale, such in the global demand for potable of reuse opportunities. Return of residuals to the sewer limits onsite residuals as in residential communities or water, particularly in urban areas. handling and disposal. Graphic by Black & Veatch multi-neighborhood developments, And there is growing confidence and to help restore ecosystems. in the feasibility and reliability of Table 1. Inputs Required for the SM.Toolbox Sewer mining can be used effec- direct potable reuse (DPR) in tively to achieve these objectives developed nations. Together, these Input Comments in urban settings. A primary ben- trends suggest that strategically efit of decentralized water reuse located decentralized DPR (dDPR) Location Reference information and where applicable and available. This allows some degree of scaling to through sewer mining, for example, systems are poised to play an local conditions and costs. is that reuse water is collected and important role in efforts to close the produced at or close to the end engineered water cycle. Although System Capacity Used in calculations. user, rather than being transported sewer mining is a valuable element Sewer System Information Used to evaluate impact on the sewer. through distribution piping from in the development of a broader Includes proximity of the sewer mining facility to a centralized treatment facility. water reuse portfolio that includes the WRRF and some limited details related to the Another benefit is the potential to dDPR, important questions remain sewer system. integrate an educational component related to a range of issues, Approximate Distance to Different Enables estimation of water distribution/ for local students of all ages. including: Users transmission costs. Sewer mining, which can be a • Appropriate technology selection Central WRRF Information Used for limited evaluation on impacts on sustainable water reuse strategy, • Costs of treatment, particularly resource recovery potential at WRRF. entails mining, or reclaiming, water considering unit cost per Process Selection Currently limited to a suite of flowsheets. from a wastewater collection system treatment volume For biological treatment these include: membrane for reuse (see Figure 1). Sewer • Risks related to public acceptance, bioreactor-based and package-plant approaches mining involves three critical given the proximity to For dDPR: advanced treatment consisting of aspects: (1) decentralized treatment neighborhoods, which is a microfiltration (MF), (RO) and is demand driven, and only the particular concern for dDPR UV mediated advanced oxidation (UV AOP). requisite volume or flow of water applications Equipment Replacement Periods Default values are based on industry experience, is extracted from the sewer; (2) the • Impact on the sewer system but user may modify. treatment process is designed to and cost implications for sewer achieve fit-for-purpose water; and system maintenance water resource recovery facilities decentralized treatment process (3) residuals from the associated • Impact on the centralized downstream merit additional are often returned to the sewer and decentralized water resource WRRFs downstream. discussion. When water is mined sent to the centralized WRRF. With recovery facility (WRRF) are from the collection system and dNPR facilities, these residuals typically returned to the sewer. The potential impacts of sewer treated for dNPR or dDPR include screenings and grit, waste These decentralized treatment mining operations on centralized applications, residuals from the from a biological treatment

World Water January / February 2017 22 Decentralized Treatment

Figure 2

14 14 dDPR systems are poised to play an 12 12 important role in 10 10 efforts to close the

8 8 engineered water

6 6 cycle.

4 4 to potential water rates in order to Capital Expenses, CAPEX ($millions) determine the economic feasibility 2 2 of such an approach. The CAPEX associated with the 100,000 200,000 300,000 400,000 Non Potable Potable dNPR scenarios considered in this model system are shown in Figure Elec,. I&C Process Systems & Equip Civil Works, Bldg 2 (left panel). As expected, the pro- cess systems and equipment are Figure 3 Figure 2. Typical outputs related to the biggest proportion of the cost, capital costs from the SM.Toolbox. constituting approximately 50 per- Capital costs associated with Non Potable Reuse for the model system cent to 60 percent of the CAPEX. Civil Site Work evaluated here. The SM.Toolbox Interestingly, for this particular sys- outputs include component level tem, upgrading the smallest of these costs with uncertainty in three broad facilities (i.e., 378,500 lpd) to a po- WRRF Exterior Building categories as illustrated here (left panel). Comparison of the CAPEX table reuse system to achieve dDPR generated using the SM.Toolbox for increases the capital cost by only WRRF Interior a 100,000 gpd NPR and dDPR system approximately 35 percent (Figure (right panel). Graphics by Black 2, right panel), while potentially & Veatch significantly increasing the value Treatment Process Systems Figure 3. The SM.Toolbox outputs of the recovered resource; this is enable evaluation of the influence of an important consideration, given significant project components. Shown the opportunity to provide potable Electrical, I & C here is an example of the impact of changes in four broad cost groupings water locally. Uncertainties remain 85 90 95 100 105 110 115 on the capital cost of the modeled related to public perceptions and Percentage Change in Baseline Cost of Facility 100,000 gpd dDPR system. Graphics acceptance of potable reuse. But by Black & Veatch the SM.Toolbox provides valuable % Change in Component / System Cost 20% 10% information for planning purposes while requiring relatively limited in- process, and backwash waste financial viability of small scale ing of the synergy between dNPR/ formation and inputs. from a process (e.g., disc sewer mining opportunities for dDPR systems and existing waste- In addition to providing the or membrane). These solids have urban dNPR and dDPR, Black water infrastructure. CAPEX, the SM.Toolbox can be potential to cause sewer blockages &Veatch has developed a Sewer used to evaluate the impact of and odor issues during low-flow Mining Toolbox (SM.Toolbox). Exploring options and varying design components or periods in sewer system. The vast When it is fully loaded with tools developing an approach elements on the CAPEX (Figure 3). majority of the energy embedded early in 2017, utilities can use this Black & Veatch recently tested the In this particular example, changes in the wastewater has been oxi- application in conjunction with toolbox using a model system for in the building exterior (e.g., façade dized in the dNPR facility, but the the company’s consulting services dNPR and dDPR within a relatively texture) have a very small impact on solids with more inert material to develop planning-level capital simple sewershed in the US state of the overall cost; a 20 percent change are returned to the central facility. and operating costs and assess California. dNPR was evaluated in in the building costs has less than a These inert solids reduce the the impact of component-level the range of 378,500 lpd (100,000 3-percent impact on the overall biodegradability of the treatment modifications on these estimates. gpd) to 1.9 mld (500,000 gpd) and costs. Therefore, if a significant facility influent, which is essential Additionally, the toolbox enables dNPR was compared with a dDPR enhancement of the building may for denitrification or biological evaluation of the impact of adding option at the 378,500 lpd scale. engender positive public acceptance, phosphorus removal, and take sewer-mining facilities on the For both dNPR and dDPR, a results from the SM.Toolbox up space in the basins, thereby collection system and the influent membrane bioreactor was used suggest that it may be beneficial to reducing process capacity. These to the centralized water resource in the model to achieve biological invest capital in this area. residuals also have to be removed recovery facility. treatment. Ultraviolet (UV) disin- Outputs from the SM.Toolbox in the central facility but are of A summary of the limited inputs fection was used for dNPR. For also include an evaluation of the limited value for energy recovery, required for the SM.Toolbox are dDPR, advanced water treatment estimated project OPEX as shown for example, through digestion shown in Table 1. Toolbox outputs with ozone, reverse osmosis and in Figure 4. This figure illustrates at the central treatment facility. include planning-level system capi- UV-mediated advanced oxidation the comparison of the OPEX for Furthermore, if the dDPR tal expenses (CAPEX) and opera- (UV-AOP) was modeled. Residuals the dNPR and dDPR options at the process implements full advanced ting costs (OPEX) with a sensitivity from the sewer mining facilities 378,500 lpd scale (left panel) along treatment, the reverse osmosis (RO) analysis and impacts on the were returned to a central Biological with an evaluation of the impacts of reject could conceivably be sent to central WRRF. Nutrient Removal WRRF, variability in unit costs for power, the central water resource recovery Project developers and utilities processing approximately 18.9 chemicals, and repair/replacement facility where it would adversely alike can use the tool to assess mld (5 million gallons per day) and (R&R) of equipment for the dDPR impact the plant’s biological financial opportunities and costs operating as a Modified Ludzack- option (right panel). For this model treatment process due to potential as well as process pros and cons. Ettinger, or MLE, process with case, energy costs not only make up toxicity issues associated with the To the authors’ knowledge, this primary . The SM.Toolbox the vast majority of the cost but also increased . is the first integrated approach to use was extended to compare the exert the most influence. A 10-per- To enable utilities to assess the developing a better understand- CAPEX and OPEX of the dDPR cent change in the power costs has

January / February 2017 World Water Decentralized Treatment 23

Figure 4 Utility managers and developers see value in sewer mining and de- centralized reuse systems. After all, 200 modular systems can be relatively Power easy to construct and implement, and the smaller size of these proj-

150 ects requires less capital outlay. Such projects may also be appealing for public-private partnerships or other Chemicals delivery approaches. But such ad- 100 vantages should be weighed against

US$ thousands potential disadvantages associated with decentralization on a case-by- 50 Equip. R&R case basis. Think of the Sewer Mining Toolbox as a cross between a scale 80% 90% 100% 110% 120% and a crystal ball; it enables users to Non Potable Potable Percentage Change in Baseline Cost of Facility weigh pros and cons and conduct

Equipment R&R Chemicals Power % Change in Component / System Cost 20% 10% planning-level evaluations and scenario analyses to assess options and opportunities, in advance and Figure 4. Typical OPEX related outputs a 378,500 lpd dDPR system. Any metrics relevant to project evalua- with limited upfront investment. from the SM.Toolbox. Breakdown of opportunities to reduce the power tion such as internal rate of return OPEX for the modeled 100,000 gpd costs either through selection of a (IRR) or net present value (NPV). Authors’ Note dNPR and dDPR options (left panel). Influence of Variability in System/ different process configuration or Bikram Sabherwal is process Component O&M costs on the overall incorporation of energy recovery fa- Planning ahead engineer, Jesse Wallin is an OPEX costs. Note that labour costs cilities, such as solar, would be high- As the global demand for water engineering manager, and Sandeep have been specifically removed from ly beneficial. Again, with relatively continues to rise, it falls on utility Sathyamoorthy, PhD, is principal this output for clarity (right panel). Graphics by Black & Veatch limited inputs into the SM.Tool- managers and their consultants to process and innovation leader box, the user is able to develop a develop alternative water supplies. in the water business of Black planning-level evaluation of the Among alternatives, reuse is king, & Veatch, a global leader in influence and impact of various and decentralized systems could engineering, procurement, and an impact of approximately 10 modifications. The CAPEX and play an increasingly important role construction services for water, percent on the baseline operations OPEX outputs can be combined to in the engineered water landscape energy, and telecommunications and maintenance (O&M) cost of output lifecycle costs and financial of the future. (www.bv.com).

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