Environ. Sci. Technol. 2009, 43, 6121–6125
Source Separation: Will We See a Paradigm Shift in Wastewater Handling?1 T O V E A . L A R S E N* ALFREDO C. ALDER RIK I. L. EGGEN MAX MAURER JUDIT LIENERT Eawag, Swiss Federal Institute of Aquatic Science and Technology, Du ¨ bendorf
LAYOUT YVONNE LEHNHARD/EAWAG
RHONDA SAUNDERS
The example of urine reveals the large potential of source separation technologies for sustainable and resource efficient future urban wastewater management.
waste; (3)), and gray water (domestic wastewater excluding toilet waste; (4)). Source separation was not new, but had long been propagated as an inexpensive and environmentally friendly technology for the poor. However, it was only considered suitable for rural areas, whereas simplified sewage is preferred for more densely populated areas (5). The novelty was that source separation could be a sustainable alternative to existing end-of-pipe systems, even in urban areas and industrialized countries. From 2000 to 2006, the Swiss Federal Institute of Aquatic Science and Technology (Eawag) completed the interdisciplinary project Novaquatis (Figure 1, (6)) that concentrated on urine source separation technology, and related topics such as consumer acceptance. When Novaquatis started, several pilot projects on urine source separation were already running in Sweden (7). We thus decided to take a more comprehensive look at the technology along a hypothetical nutrient cycle (Figure 1). In this article, we refer to many results from Novaquatis and place them in the context of the broader international experience. We are aware that much more literature is available and regret not being able to refer to all of it. However, we cite the Novaquatis review articles for specific topics, which will give guidance for more-detailed reading. Additionally, since the area is large, we concentrate on industrialized countries with only a brief outlook toward fast industrializing and developing countries. Urine source separation was practiced in many cultures in the past, mainly for efficient nutrient recycling (8). Modern urine separating (NoMix) flush toilets (Figure 2) were invented in Sweden in the 1990s, and thousands were installed in pilot projects (7), mainly motivated by the expected depletion of phosphorus (P) in the 21st century. However, modern versions of the dry urine separating toilets had been available since at least the 1970s (9). The latter drastically improve
Introduction During the 1990s, several groups started working on source separation technologies for urine (1, 2), black water (toilet 1 Editor’s Note: To our delight at ES&T, we have started to receive Features and Viewpoints by independent author(s) coincidentally overlapping both in topic and review schedule. Just as the present manuscript was being ushered into production, a second Feature concerning the needed “paradigm shift” to realize more sustainable water infrastructure was accepted. The choice was thus made to present both manuscripts in the same issue (August 15, 2009; 43, 16). Readers of this piece by Larsen et al. are therefore encouraged to read that by Guest et al. (DOI 10.1021/es9010515), which also appears herein as reference (15).
10.1021/es803001r
2009 American Chemical Society
Published on Web 07/14/2009
FIGURE 1. Novaquatis (2000-2006) was a broad interdisciplinary project on urine source separation with 26 different projects, organized in 9 work packages. In 2008, Novaquatis received the Swiss-academies award for transdisciplinary research. VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Box 1: Nutrients in urine and their consequences for water pollution control Humans produce about 1.4 L of urine and 140 g of feces (wet weight) per person per day. Urine contributes 81% of the nitrogen (N) and 50% of the phosphorus (P) in purely domestic wastewater (11). N and P can both cause eutrophication and may be removed in biological treatment plants (eqs 1-4; for the biological processes, only catabolic reactions): autotrophic ammonia oxidationa
2NH4+ + 3O2 f 2NO2- + 2H2O + 4H+
(1)
autotrophic nitrite oxidation
2NO2- + O2 f 2NO3-
(2)
heterotrophic denitrification
4NO3- + 5CH2O + 4H+ f 2N2 + 5CO2 + 7H2O (3) chemical phosphorus removal
PO43- + Fe3+ f FePO4(s)
FIGURE 2. Roediger NoMix toilet. Roediger Vacuum (www. roevac.com). hygienic handling of dry feces and are installed in large numbers in areas where flush toilets are not available. For instance, in China nearly 700,000 such toilets were in use by 2003 (10). NoMix flush toilets are still mainly used in pilot projects although some Swedish municipalities subsidize broader installation for environmental reasons (10). The rationale for urine source separation for water pollution control is summarized in Box 1.
Source Separation is Resource Efficient During the past decade, resource efficiency has gained momentum as a guiding principle for sustainable urban water management (e.g., 3, 14), recently also resulting in the formulation of a new planning and design paradigm for sustainable resource recovery from wastewater (15). Many studies showed that source separation can be resource efficient, but also that these results are sensitive toward the specific choice of technology (e.g., 16-18). Moreover, they are also very sensitive toward changes of stakeholder preferences, as shown with multicriteria decision analysis (19), a notion equally emphasized in ref 15. A principal concept leading to Novaquatis was that treating concentrated, unmixed solutions is more resource efficient than treating highly diluted, combined solutions (14). The references above support this assumption. Our own results further support the assertion that technology choice and development are extremely important. For example, whereas removing nitrogen (N) from urine is clearly much more energy efficient than at treatment plants (Box 1), recovering N from urine must compete with very energy-efficient industrial processes. Calculations show that this is possible, but challenging (20). As discussed below, good processes for P recycling exist.
Source Separation Favors On-site Technologies Upon separation of urine at the source, two nutrient management approaches exist: treatment of urine on-site or 6122
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(4)
Due to the slow growth of autotrophic microorganisms (eqs 1 and 2), the “morning peak” of N at treatment plants stemming from human urine (which is more concentrated in the morning), and the anaerobic conditions required for denitrification, treatment plants are built several times larger than if N elimination were not necessary. Without the excess P and N from urine, the P:N:C ratio is fairly balanced for biological growth, and the available P and N can thus be incorporated by the produced microorganisms. A modeling study based on a real catchment showed that this would occur at ∼60% urine separation (12). Assuming N removal from urine via 50 % NH4+ oxidation (eq 1) followed by autotrophic denitrification (also called anaerobic ammonia oxidation, shortened anammox; eq 5), the combined processes could render wastewater treatment more energy efficient, even resulting in an overall energy production (less energy for aeration, more energy from anaerobic treatment of the larger sludge production (12)): autotrophic denitrification
NH4+ + NO2- f N2 + 2H2O
(5)
In many areas worldwide nutrient removal would be required for protection of sensitive coastal areas but is not implemented (13). Urine source separation is an alternative to treatment plantssespecially in regions without sewers. a Editor’s note: in water treatment contexts, the solvated ammonium ion is synonymous with ammonia.
transport to a plant for centralized treatment. The same holds true for any other separated waste stream. We find an onsite approach for source separation more practicable, mainly because separate wastewater streams require different pipes, which is expensive. Only in small and steep catchments with short residence times of wastewater in sewers, can urine be released for recovery either in a concentrated pulse during the night into the practically empty sewers or at convenient times for peak shaving, i.e., leveling out the expensive “morning peak” of ammonia (NH4+) at treatment plants (Box 1). Both options are technically feasible (21, 22), and in Novaquatis we hoped that this could be a pioneering
approach allowing for technological learning between the present centralized- and a future on-site-treatment-system (23). However, for a perceived lack of sufficient business potential and uncertain risks, private companies interested in the Novaquatis project opted not to invest in the development of better NoMix toilets and flushing devices (based on unpublished results from Novaquatis round table discussions with representatives from the sanitary industry [producers of toilets, flushing devices, and household wastewater pipes] and the wastewater industry). Thus, only decentralized or even on-site technologies remain as an alternative to separate pipes, as transport of discarded urine by trucks seems an unattractive option for cities. It is of course possible to install different pipes for urine, feces, and gray water (24), at the price of higher investment costs (25). Nevertheless, we think that on-site technology will be more competitive in the long run, at least for urine. First, some sort of on-site technology is necessary to deal with scaling and blockage of urine-conducting pipes. Second, these years we observe an exceptional amount of technical progress that can help develop on-site technologies for wastewater treatment (see below). We discuss both topics because they well illustrate the trade-offs between transport of concentrated waste streams and on-site technologies and the opportunities supplied by new technologies. Blockage of Urine-Conducting Pipes. Biological hydrolysis of urea ([NH2]2CO) rapidly leads to a pH rise with subsequent precipitation of P compounds which can then block pipe flows (26). Two promising options to solve this problem are inhibition of biological activity, for instance with self-cleaning surfaces (overview in ref 27), or the opposite of forced precipitation in an exchangeable unit integrated into the NoMix toilet. Precipitation of magnesium ammonium phosphate (MgNH4PO4 · 6H2O, MAP, struvite) is basically a simple process with a small production of residuals (28). The main challenges are the development of a device small enough to be integrated into the toilet and good organization of the interface between user and technology. Our main point is that the technological challenge of both approaches is similar. It is thus reasonable to opt for the on-site solution, which in our opinion has better chances of becoming economically competitive. Technical Progress. Besides dealing with P, urine treatment technology should efficiently eliminate or recover N (Box 1). Many processes are possible and were tested with urine (reviewed by ref 29). Oxidation of NH4+ to nitrite (NO2-) combined with autotrophic denitrification (Box 1, eqs 1 and 5) is one good option (30, 31), but the main problem of on-site biological waste treatment is robustness. Newly developed genetic tools for the identification of microorganisms may prove useful for investigating the stability of ecosystems (32) and may allow us to break with the general opinion that complex biological processes can only be run in a central setting.
Source Separation Could Lead to Sustainable Solutions More Directly Than Traditional Approaches Obviously, source separation and on-site technologies are most attractive in areas without sewers. Whereas traditionally, source separation is only considered for rural areas (5), recent approaches also emphasize the importance of this approach in more densely populated areas in developing countries (33, 34). Our own research in Kunming, a large, rapidly growing Chinese city, indicates that also in fast industrializing countries with severe water pollution problems, local experts may favor source separation (35). In coastal areas without wastewater infrastructure, where N removal is needed to protect the ocean from eutrophication and hypoxic “dead zones”, urine source separation is the technology of choice (36). Separating up to 80% of waste-
water N at the source by collecting urine can compete with most denitrifying treatment plants normally achieving about 50-60% nitrogen removal.
On-Site Technologies: Economic Potential in a Changing World An often heard criticism of decentralized solutions is that there are no economies of scale. Decentralized technologies are normally only considered where it is too expensive to build sewers. Technical development increasingly challenges this assumption, particularly because of the progress in membrane technology (37). This progress is not only technical, but also based on an “economy of numbers”: membranes are increasingly produced in large numbers, resulting in decreasing prices. Obviously, this is valid for any decentralized treatment technology that becomes broadly accepted (and consumer desired). Already today some source separating technologies are economically competitive (25). For urine, a break-even compared to conventional technology is achieved at additional investments in NoMix household technology of $260-440 USD/ person (38). In our opinion, this is possible provided that toilets and treatment technologies are produced in large numbers. Much more critical is the requirement that these technologies should function with minimal maintenance. Here we see the need for additional research. Micropollutants present a large uncertainty of urban water management: do we need new treatment steps at all wastewater treatment plants? If yes, what are the consequences for those areas worldwide that depend on simple wastewater treatment? Again, measures at the source may be more efficient (39-41). About 60-70% of human pharmaceuticals and hormones end up in urine (42). However, the more problematic ones tend to end up in feces, resulting in an approximate half-half distribution of the ecotoxicological potential in urine and feces (43). The elimination of these organic micropollutants in different urine treatment processes is discussed elsewhere (44). Additionally, the high concentrations in undiluted urine are very favorable for adsorption processes (45). In conclusion, removing half of the pharmaceutical burden with urine separation and improving the sorption capacity by higher sludge production in central non-nitrifying treatment plants could be a costefficient response to the increasing problem of micropollutants. Similarly for other contaminants, interventions at the source as opposed to central measures are gaining ground, e.g., reducing the micropollutant load in surface waters by controlling the use of pesticides (46) and biocides in building materials (47).
People Like Source Separation, But Convenience and Food Security are More Important A frequent objection is that people will not accept source separation because of an anticipated feeling of revulsion. However, many research results indicate the opposite: most people like source separation and are not disgusted by the idea. Currently available NoMix toilets are well accepted by most users if others are responsible for maintenance (i.e., in public buildings), and many people can imagine living in a home with a NoMix toilet (48). Similarly, a comparison of NoMix pilot projects in Austria, Denmark, Germany, Luxemburg, The Netherlands, Sweden, and Switzerland concludes that urine source separation is usually a well-accepted idea, although users often have to accept some nuisance, e.g. blockages, smell, or more time-consuming cleaning (unpublished review in progress). To be widely accepted, bathroom comfort must be as today, which is not the case. Furthermore, if health or other risks are excluded, the public mainly sees recycling of nutrients from urine to agriculture positively ((49), unpublished data). Also, farmers view urine VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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fertilizers pragmaticallysas long as they are safe and convenient, many accept them (50).
Shifting Technical Paradigms May Be Worthwhile, But Not Easy The potential advantages of source separation are mirrored by a remarkable interest from practitioners. Unsolicited interest by different parties to run pilot projects, nota bene at own expenses, resulted in the launch of several NoMix pilot projects associated with Novaquatis (51). In the German speaking countries, the many initiatives from universities in practice are today efficiently coordinated through the German Association for Water, Wastewater and Waste, resulting in an important report on the state of the art (11). In The Netherlands, there is an impressive boom of source separating activities, with around 20 pilot projects being implemented in collaboration with STOWA, the Dutch Foundation for Applied Water Research that coordinates research on behalf of local water administrations (52). Despite these and similar developments in other countries, source separating technologies are still considered immature and risky by most wastewater professionals. This turns into a self-fulfilling prophecy. The present centralized system was developed over decades by innumerable researchers and practitioners; source separating technologies are a priori considered “low-tech” and hardly any resources were allocated to their development. So long as no one invests in source separating technologies, such stay low-tech and are at best produced locally in small numbers, there being no market. We are convinced that this vicious circle can and should be broken. If we want to, it is possible to develop on-site source separating technologies that are just as reliable and easy to maintain as any modern household espresso machinesand just as affordable. All authors are senior scientists at Eawag, the Swiss Federal Institute of Aquatic Science and Technology, Du ¨ bendorf, Switzerland, and were part of the Novaquatis project management team. Tove A. Larsen is a chemical engineer and was head of the entire Novaquatis project together with Judit Lienert, biologist, who also coordinated the social science projects in Novaquatis. Today, Tove Larsen is group leader of “Future Concepts” in the Urban Water Management Department and Judit Lienert is group leader of “Decision Analysis” in the System Analysis, Integrated Assessment and Modelling Department. Alfredo C. Alder is an analytical chemist in the Environmental Chemistry Department and coordinated a project analyzing the fate of various pharmaceuticals in urine. Today he is a group leader on the fate of emerging contaminants in the urban water cycle. Rik I. L. Eggen is a molecular biologist and former head of the Environmental Toxicology Department. In Novaquatis, he coordinated the projects on the potential ecotoxicological effects of micropollutants in urine; today he is Deputy Director of Eawag. Max Maurer is a process engineer and coordinated the projects exploring methods to process the urine solution. Today he is head of the Urban Water Management Department and is also group leader of “Water Infrastructures”.
Acknowledgments Novaquatis was financed by Eawag, with substantial support from the Swiss National Science Foundation (SNF), the Swiss Agency for Development and Cooperation (SDC), a number of Swiss authorities, different institutions within the ETH domain, and the European Union. Several smaller financial contributions are also acknowledged. Most of all, we would like to thank the many researchers, students, practitioners, and people using the NoMix pilot installations, and friends of Novaquatis who contributed to the success of the overall project. We also thank four anonymous reviewers for their constructive comments on the manuscript.
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