Environ. Sci. Technol. 2007, 41, 5085-5089
Questioning the Excessive Use of Advanced Treatment to Remove Organic Micropollutants from Wastewater O L I V E R A . H . J O N E S , †,‡ P A T G . G R E E N , § NIKOLAOS VOULVOULIS,† AND J O H N N . L E S T E R * ,| Centre for Environmental Policy, Imperial College London, London SW7 2AZ, United Kingdom, Anglian Water Services, Thorpe Wood House, Thorpe Wood, Peterborough PE3 6WJ, United Kingdom, and Centre for Water Science, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, United Kingdom
Pollution from endocrine disrupting compounds and related micropollutants is widely regarded as a major environmental issue on both a regional and a global scale, largely due to concerns over risks to human and ecological health. Between 2005 and 2010, the United Kingdom is conducting a demonstration program, costing ∼40 million (∼$80 million at the time of writing), to evaluate technologies to remove these compounds from wastewater. However, while such advanced treatment techniques will undoubtedly reduce the discharges of micropollutants, they will also inevitably result in large financial costs, as well as environmentally undesirable increases in energy consumption and CO2 emissions. Here we calculate the price of treating urban sewage with two of the major options specifically proposed in the U.K. demonstration program: (i) granular activated carbon and ozone and (ii) membrane filtration and reverse osmosis. Economic analysis indicates that treating wastewater with these advanced technologies may be economically and environmentally undesirable due to the increased energy consumption and associated economic costs and CO2 emissions. Since the costs of advanced treatment of sewage would most likely have to be passed on to customers (both domestic and industrial), we propose that national demonstration programs should not only compare and contrast the most advanced treatment methods but also consider alternative techniques, such as increased sludge ages and hydraulic retention times in conjunction with nutrient removal stages and the varying redox conditions associated with them, which potentially may be almost as effective but with much lower environmental and financial costs.
Introduction Research on the impact of chemical pollution during the last three decades has focused almost exclusively on conventional * Corresponding author phone: +44 (0)1234 754905; fax: +44 (0)20 8995 4695; e-mail:
[email protected]. † Imperial College London. ‡ Present address: Department of Biochemistry, University of Cambridge, Tennis Court Rd., Cambridge CB2 1QW, U.K. § Anglian Water Services. | Cranfield University. 10.1021/es0628248 CCC: $37.00 Published on Web 06/15/2007
2007 American Chemical Society
priority pollutants, (e.g., heavy metals and poly aromatic hydrocarbons). Today, these compounds are less relevant for most first world countries since emissions have been substantially reduced through the adoption of appropriate legal measures and the elimination of many of the dominant pollution sources. The focus has consequently switched to compounds present in lower concentrations and that have only comparatively recently been thought of as pollutants (1). Endocrine disrupting compounds and pharmaceuticals are a broad and diverse group of biologically active compounds that are used in large quantities in both human and veterinary medicine around the world. The presence of these substances in the environment, and their potential to induce adverse biological effects, has been known for many years (2-4). Indeed, the estrogenic activity of certain synthetic chemicals has been known since the 1930s (5), and the potential for synergistic interactions has also been investigated (6, 7). Estrogens in the environment have been implicated in adverse health effects in both animals and humans for some years (8, 9), and there is increasing evidence that other pharmaceutical compounds may also cause harm to overall ecosystem health (10). A particularly pertinent example is the pharmaceutical drug diclofenac, which was recently shown to be the prime cause for the drastic falls in vulture populations in the Indian subcontinent (11). Nevertheless, these compounds are extremely important both in maintaining human health and to the economy. Therefore, it is unlikely that pollution by these substances can be controlled by legal measures or reduction at the source as other classes of pollutants have been. Studies in both the U.S. and Europe have shown that endocrine disrupting compounds and pharmaceuticals are present in sewage effluents at the nanogram to microgram per liter range (12, 13). While numerous compounds have been reported to preferentially absorb onto suspended solids, many are sufficiently soluble to be amenable to biotransformation and bioconcentration (14, 15); hence, they also have the potential to bioaccumulate (16). Complex issues for environmental health arise as a consequence of this behavior (17). Secondary biological treatment of wastewater significantly reduces the concentrations of many of these substances (18-20). However, as presently configured and operated, these processes do not afford total protection of the aquatic environment (21). Prompted in part by such observations, the U.K. has initiated a demonstration program (proposed and directed by the Environment Agency (EA) of England and Wales) to evaluate technologies, primarily aimed at eliminating endocrine disrupting compounds (and related organic micropollutants such as pharmaceuticals) from wastewater (22). Here, two major options specifically proposed in the EA program (namely, (i) granular activated carbon and ozone and (ii) membrane filtration and reverse osmosis) are assessed. These techniques are commonly used in the treatment of potable water but not for wastewater where their use remains unproven (23). Economic analysis has been undertaken in this study to determine if the application of these advanced technologies to the treatment of wastewater is economically or environmentally desirable when financial costs, energy consumption, and associated CO2 emissions are taken into consideration.
Materials and Methods First, the equipment that would be necessary for a standard Sewage Treatment Plant (STP) serving towns with small, VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Principal Dimensions and Operating Parameters of the Three Hypothetical Works process population size grit tank (m) storm tank (m) primary tank (m) aeration tanks (m) (plug flow, multichannel, and diffused air) clarifier (m) return act sludge pumps (L s-1) blowers (m-3 h-1)
small works
medium works
large works
5000 2×2 12 × 3 × 3 (one tank) 10 diameter (one tank) 12 diameter × 5 deep
50000 7×7 9 × 37 × 3 (two tanks) 21 diameter (two tanks) 32 × 32 × 5 deep
200000 13.5 × 13.5 18 × 73 × 3 (two tanks) 30 diameter (two tanks) 64 × 64 × 5 deep
13 diameter (one tank) 30 507
27 diameter (two tanks) 283 5076
40 diameter (two tanks) 1130 20304
medium, and large populations (with hypothetical population equivalents (PE) of 5000, 50 000, and 200 000, respectively) was calculated. This was done with the aid of two computer programs developed and owned (and widely used) by Anglian Water PLC. The first was used to calculate the dry weather flow and the maximum flow to treatment, assuming a 25% infiltration rate and negligible industrial flow for simplicity. The second program was used to generate data on the size of a conventional nitrifying activated sludge plant needed to treat the flows and biological load predicted from the first program. The STPs were designed to treat for a typical minimum U.K. effluent consent (i.e., not to exceed the following values in 95% of samples tested: suspended solids ) 35 mg L-1, 5 day biochemical oxygen demand (BOD5) ) 25 mg L-1, and ammoniacal nitrogen ) 5 mg L-1). The principal dimensions of the three works are shown in Table 1. The financial costs for the conventional elements of the works were then calculated using information available from Anglian Waters’ cost databases and the TR61 program, version 7 (24). This program was developed, and is maintained, by the U.K. Water Research Center (WRC), in conjunction with various U.K. water companies. It is used for capital cost estimation, comparative performance assessments, asset valuation, and investment planning within the U.K. water and wastewater construction sector. It uses a mathematical model that is based on the actual costs incurred in the past and is updated regularly. The program takes the individual dimensions of each process and provides an estimate of the civil, mechanical, and electrical engineering costs. An estimate was also made of the cost of sludge treatment by mesophilic anaerobic digestion, storage, and mechanical dewatering, and this is included in the capital and operational expenses for each option. The costs of adding on the extra treatment processes to remove endocrine disrupting compounds and pharmaceuticals from the effluent stream, as proposed in the U.K. study, were also calculated. These proposals are based on techniques previously shown to remove these substances from aqueous matrices (25). The additional advanced treatment processes were costed using manufacturers’ quotes and historical information collected by Anglian Water PLC, utilizing cost curves from current or completed projects where possible. The two selected treatment options, which represent a major part of the U.K. demonstration program, are described next. Sand filters are included in both options as a polishing step, but their contribution to the total capital and operational costs of each system is minimal. Option 1 (GAC + O3). This method contains the following parameters: (i) sand filters and feed pumping; (ii) ozone (O3) contact tanks and generators; and (iii) granular activated carbon (GAC) contactors and wash water tanks. Ozonation involves passing ozone through the water for treatment. A dose of between 1 and 2 mg L-1 is the usual 5086
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practice (with a contact time of 15 min) for drinking water treatment. However, much larger concentrations must be used for waters of poorer quality. We assumed the necessary ozone dose for wastewater to be 15 mg L-1 based on data from the literature (25) and discussion with Anglian Water. The cost was assumed to be 3 pence m-3, based on data from Anglian Water PLC. These figures are also in general agreement with those reported by Ternes (25). Activated carbon works by adsorbing potentially harmful chemicals from water filtered through it. It will hold these chemicals until the carbon reaches its saturation point; when this occurs, it must be reactivated or replaced. Filtration with GAC has proven to be a very effective way to remove a variety of endocrine disrupting compounds and pharmaceuticals from drinking water (25). The costs for this option were calculated with the GAC unit providing a minimum contact time of 15 min, and the capital costs includes backwash tanks, generators, and vented ozone destruction. Option 2 (MF + RO). This method contains the following parameters: (i) sand filters and feed pumping; (ii) membrane filtration (MF); and (iii) reverse osmosis (RO). Membranes are semipermeable barriers that are used to isolate and separate constituents from a liquid stream. They can therefore play an important role in water purification. The separation process can be accomplished through a number of physical and chemical properties of the membrane, as well as the nature of the material being separated. Membrane materials are diverse and can consist of synthetic polymers, natural fabrics, porous metals, or porous ceramics. The obstacle for their practical application is the relatively high cost of the process, including the cost of the membranes themselves, the energy cost of the operation, and the cost of chemicals required for membrane cleaning. Reverse osmosis occurs when the water is moved across a membrane against the concentration gradient. This method has been used extensively to convert brackish or saltwater to a potable supply and to recover dissolved salts from industrial processes. The efficiency and life of an RO system depend on effective pretreatment of the feedwater. This includes any process that can minimize fouling, scaling, and membrane degradation to optimize product flow, salt rejection, product recovery, and operating costs. Operating costs for option 2 are based on 12 pence m-3 for the MF and 10 pence m-3 for the RO (these figures were provided by Anglian Water PLC but may vary between water companies). Waste from the MF process is assumed to be recycled to the head of the works, and this has been included in the costs. However, there will also be significant waste volumes to dispose of from the RO element for which no costs were available. Reverse osmosis will also remove divalent cations resulting in a significant pH reduction. Thus, while of high purity, the resulting water will be very aggressive and would require further treatment to raise the pH to that
3.17 1.55 1.17 3.89 2.41 1.65 0.21 0.28 1.16 0.31 1.08 4.23 0.02 0.14 0.54 0.12 0.94 3.61 0.19 0.14 0.62 0.19 0.14 0.62 2.73 13.64 40.70 3.33 20.51 54.9 0 (remote treatment) 4 12 0 (remote treatment) 4 12 0.70 2.70 8.00 1.30 9.57 22.20
operating cost per year of advanced treatment (£ million) operating cost per year for standard STP and sludge treatment (£ million) total capital cost (£ million) capital cost of digestion and dewatering (£ million) capital cost of advanced treatment steps (£ million) works size (PE)
5000 50000 200000 5000 50000 200000
treatments used
option 1 AS and sand filter GAC and ozone option 2 AS and sand filter MF and RO membranes
capital cost for standard STP (£ million)
TABLE 2. Total Costs of the Three Sizes of Works Calculated by Application of the TR61 Procedure
The costs for each works are summarized in Table 2. The results indicate that the cost of utilizing drinking water technologies to treat urban wastewater from areas with low PEs will likely be prohibitively expensive. This is even more pronounced for works with medium and large PEs since the additional capital expense required is almost as much as the total capital cost of a standard plant. For the medium and large sized plants, the capital cost of the sand filter and membranes in option 2 exceeded the cost of the basic AS plant by £2.63 and £1.5 million, respectively. The potential operating costs of the extra treatment processes are also significantly higher than standard treatments, with treatment via MF and RO being more expensive than GAC and O3 (see Table 2). There is, however, an economy of scale in the cost per cubic meter of sewage treated via the advanced treatments, which was found to decrease as the size of the plant increased. An important point associated with utilizing the advanced treatment technologies examined here is the inevitable environmentally undesirable increase in energy consumption. At present, this demand would be met mainly from non-renewable sources. An energy intensive GAC or ozone plant, running 24 h a day, 365 days a year, would therefore indirectly contribute a large amount of CO2 to the atmosphere, with associated ramifications for global warming and climate change. In addition, advanced treatments would increase sludge production, which would then have to be safely disposed of, ideally in an environmentally sustainable manner. At present, it is not clear how this could be achieved. Taken together, these issues can significantly increase the economic (and environmental) cost of the plant. No attempt has been made in this work to calculate the environmental benefits of removing endocrine disrupting compounds or pharmaceuticals from the wastewater stream. This clearly depends largely on a variety of site specific factors, and there is inherent difficulty in assigning economic value to environmental factors, especially since environmental problems related to this issue may take a long time to become fully apparent. A potential alternative option for removing these compounds is the reconfiguration of existing biological wastewater treatment processes. For instance, it has previously been shown that STPs utilizing both nitrification and denitrification treatment steps and/or high sludge ages (∼15 days) exhibit better removal rates for endocrine disrupting compounds and pharmaceuticals than those using standard operating conditions (25, 26). The reason for this is likely to be the role of co-metabolism and the presence of monooxygenase enzymes. Co-metabolism is probable because endocrine disrupting substances and pharmaceuticals are present at very low concentrations (ng to µg-l). Organisms with a high substrate affinity and low growth rates (sometimes referred to as K strategists) can very efficiently utilize low levels of resources and may therefore play an important role in the removal of xenobiotic compounds present at low concentrations (27). Monooxygenase enzymes from nitrifying organisms and methane utilizing bacteria (methanotrophs) have previously been hypothesized to enhance the removal of endocrine disrupting chemicals (28). Many species of bacteria commonly found in STPs are capable of expressing monooxygenase enzymes (previously identified as a major mechanism in the degradation of xenobiotics such as pesticides), and it is likely that these enzymes will also help degrade pharmaceuticals. Increasing the sludge age would help with this since there would be a greater amount of cell lyses that would
total operating cost per year (£ million)
Results and Discussion
2.03 6.94 20.70 2.03 6.94 20.70
total cost per m-3 wastewater treated (£)
of receiving waters before final discharge, which will likely increase the MF + RO treatment costs considerably.
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liberate these enzymes. Once freed from cellular control, their specificity is greatly reduced, and hence, they would potentially be available to interact with any compound with which they came into contact (29). Nitrification and de-nitrification of the wastewater would also expose compounds to different bacterial enzymes. This is because the facultative anaerobic organisms involved utilize different biochemical pathways (requiring different enzymes) in the former process, which is aerobic, than the latter, which is anoxic. Many modern treatment facilities already have these systems in place, often in conjunction with biological phosphorus removal (which includes anaerobic phases). This further increases the diversity of the bacterial population and the associated range of enzymes. The increased removal of endocrine disrupting compounds and pharmaceuticals is likely to be a positive side effect of this biochemical diversity. Increasing sludge age also results in a reduction of the sludge loading rate and an increase in the sludge retention time. This enables populations of slower growing bacteria to develop and also serves to increase the potential for the acclimatization of the population to the compounds encountered (28). For example, a study by Johnson et al. (30) showed that increased HRT and SRT increased the amount of estrone (E1) removal (and probably also other biodegradable steroids) within the STP studied. There is also the potential to use treatment options such as sequencing batch reactors and/or membrane bioreactors to remove these compounds since both operate at high sludge ages. A study by Clara et al. (31) compared the behavior of selected micropollutants in a membrane bioreactor and an activated sludge plant operating at a very high sludge age. Each gave high removal rates for several (although not all) different endocrine disrupting compounds and pharmaceuticals as well as related compounds. While cheaper than advanced treatment options, nitrification-de-nitrification and increased sludge age do not affect some compounds, for example, clofibric acid and gemfibrozil (32). These compounds are also known to be unaffected by GAC and/or ozone (33). Although they may be removed by RO treatment (34, 35), this is likely to be expensive (as demonstrated in Table 2). The question then becomes one of how much should be spent removing ever decreasing amounts of pollutants from wastewater while simultaneously elevating the amount of pollutants (such as CO2 released) through increasingly energy intensive treatment processes. It should also be noted that some STPs with a high nitrifying capacity do not remove all drugs, particularly if high sludge ages are not used (37, 38). There is therefore an interesting contrast in approaches. Expensive, advanced treatment is likely to remove the majority of drugs from the wastewater, but at a large environmental and financial cost, while existing technology is cheaper but removes slightly fewer compounds. It may be time to address a paradigm of wastewater treatment that has previously been unchallenged; namely, that increasing effluent quality can only be envi-
FIGURE 1. Diagrammatic representation of the current paradox in wastewater treatment and water quality planning (after Zakkour et al. (36)). 5088
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ronmentally beneficial. In fact, when subjected to life cycle analysis, large-scale investments into increasingly energy intensive treatments are seen to be environmentally unsustainable in many cases. This is because the benefits of improved effluent quality are often outweighed by the negative effects on the wider environment when process construction, operation (energy consumption and CO2 emissions), and increased sludge production from the advanced treatment technologies are taken into account (see figure 1). There is no doubt that the implementation of legislation such as the European Union (EU) Water Framework Directive will result in significant extra costs to both domestic and industrial customers in the U.K. While reducing the discharges of pollutants, such as endocrine disrupting compounds, advanced treatment methods will also require extra energy consumption, which in turn will lead to increased CO2 emissions. It is therefore recommended that national demonstration programs (such as the one ongoing in the U.K.) not only focus on the most extensive/expensive treatment techniques but also include less costly, yet still effective and environmentally sustainable, methods.
Acknowledgments The authors thank the three anonymous reviewers whose time, insight, and persistence greatly improved the quality of the final manuscript. We also thank Anglian Water Services for the use of the TR61 program and related economic data (although the views expressed here are those of the authors and not Anglian Water Services). O.A.H.J. is also grateful to the U.K. Engineering and Physical Sciences Research Council (EPSRC) for the award of a Ph.D. scholarship.
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Received for review November 29, 2006. Revised manuscript received April 18, 2007. Accepted May 14, 2007. ES0628248
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