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Strategies towards sustainable wastewater management
Urban Water 3 (2001) 63±72
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Strategies towards sustainable wastewater management Erik K arrman * Water Environment Transport, Chalmers University of Technology, SE-412 96, Goteborg, Sweden Received 13 June 2000; received in revised form 15 January 2001; accepted 30 March 2001
Abstract Sustainable development requires sustainable wastewater management (SWM). For this reason, strategies towards SWM have been formulated and discussed in the research literature during the past decade. System analysis has sometimes been used for simulation of dierent strategies for wastewater management, in order to compare sustainability aspects such as environmental impacts and usage of resources. This paper suggests strategies for SWM based on results from a Swedish environmental system analysis called Organic waste as a plant nutrient resource ± system study, and from other studies with similar approaches. As a summary of the study, four strategies for SWM are suggested: 1. 2. 3. 4.
Handle nutrient-rich ¯ows separately from other waste ¯ows. Recycle nutrients and use energy eciently. Avoid contamination of wastewater ¯ows. Put unavoidable pollution on land®ll. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction During the 100-year long history of urban drainage, the main strengths have been to remove wastewater from users in order to prevent unhygienic conditions and to remove stormwater to avoid damage from ¯ooding. All this should be done without harming the environment. Existing urban water systems in the Western world ful®l these fundamental requirements to a high degree. Over the last ten years, however, the existing systems have been increasingly criticised from the viewpoint of sustainability. The most well-known de®nition of sustainable development was formulated by the ``Brundtland report'' (World Commission on Environment Development, WCED, 1987): Development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs. The WCED de®nition is widely used and accepted. However, there is still a challenge to make the sustainability concept useful for concrete decisionmaking. Niemczynowicz (1993), who formulated a sixlevel action plan, made an attempt for the urban water
*
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sector. The list of the options below is ranked according to priority: 1. preventive actions during all human activities; 2. on-site treatment and reuse close to production; 3. o-site treatment and reuse; 4. on-site or o-site concentration and storage; 5. treatment at small-scale treatment plants using novel, low-tech technology; 6. high-technological treatment and end-of-pipe. Niemczynowicz (1993) further stresses that there are two possible scenarios. First, the high technological option implies continuation, development and complementation of present technology. The second is very dierent, containing low-cost, low-energy solutions based on application of biological systems and recycling of resources. Niemczynowicz (1993) emphasises that the second scenario will lead to a less vulnerable and more sustainable society, compared with scenario 1. Butler and Parkinson (1997), who focus on urban drainage, partly agree with Niemczynowicz (1993) but conclude that sustainable urban drainage is in fact an unattainable goal. In order to reduce the non-sustainability they suggest preventive actions such as separate handling of industrial waste to enable the reuse of sewage sludge and separate handling of stormwater to restore drainage patterns. A third strategy against non-sustainability by Butler and Parkinson (1997) is to reduce inappropriate
1462-0758/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 0 7 5 8 ( 0 1 ) 0 0 0 1 8 - 8
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E. Karrman / Urban Water 3 (2001) 63±72
``use'' of potable water as a carriage medium in sewers. These strategies may, according to the authors, be realised through the adoption of: domestic water conservation, small-scale recycling of greywater and rainwater, on-site storage of in®ltration of stormwater, utilisation of natural drainage patterns and local sanitation technology. Otterpohl, Grottker, and Lange (1997) agree that existing urban water management is not sustainable. The traditional sanitation concept has several severe disadvantages: it needs too much water, dilutes faeces and raises nutrient levels in the sea even after very advanced treatment. A more sustainable urban sanitation concept for urban areas according to the authors is: (1) separation of faeces and urine with vacuum toilets and treatment with biowaste in biogas plants, (2) decentralised aerobic treatment of greywater in constructed wetlands and (3) in®ltration of stormwater to avoid a centralised sewage system completely. K arrman and J onsson (2000) studied dierent sanitary systems for handling of household wastewater and organic solid waste with a systems analysis approach including normalisation. The normalisation was used to sort out the most crucial environmental aspects to be considered in wastewater management. The normalisation process was here de®ned as the percentage contributions from the sanitary system to the total national (Swedish) impact of severe environmental eects. Impacts caused by emissions and usage of resources were normalised. Further was the potential use of residuals normalised analogously ± where, for example, nutrients in residuals were considered as potentially replacing purchase of mineral fertilisers. The normalised impacts were sorted into priority groups, where Priority Group 1 contained the largest contributions, exceeding 10% of the total impacts, and Priority Group 2 contained contributions, exceeding 0.1% of the total national impacts. The impacts in Groups 1 and 2 are presented below: Priority Group 1 (>10%): · discharges of nitrogen to receiving waters; · discharges of cadmium, mercury and lead to receiving waters; · potential replacement of mineral fertilisers through nutrient recycling; · heavy metal ¯ows to arable soil (connected to nutrient recycling). Priority Group 2 (0.1%±10%): · discharge of phosphorus to receiving waters; · discharge of copper to receiving waters; · air emissions causing acidi®cation; · energy use; · air emissions forming photo-oxidants; · ¯ows of cadmium to land®ll; · air emissions causing global warming. A conclusion of this study is that the most urgent sustainability aspects of the systems studied concern
nutrient management and heavy metal management. Two studies from the Netherlands (Graaf, MeesterBroertjes, Bruggeman, & Vles, 1997; Roeleveld, Klapwijk, Eggels, Rulkens, & van Starkenburg, 1997) partly agree with this statement, using a similar normalisation procedure with national data of the Netherlands. Impacts exceeding 10% in the study by Roeleveld et al. (1997) were primary oxygen consumption and nutri®cation due to discharges of nitrogen and phosphorus. The discharges of heavy metals to water did not exceed 10% of the total anthropogenic impact. Graaf et al. (1997) studied scenarios for the total urban water cycle and gave the highest priority to emission of heavy metals to receiving waters, discharges of micro-pollutants to receiving waters, sludge production, groundwater management, emission of oxygen-binding materials via over¯ows (peak discharge), hygiene and reliability. In the present paper, the most crucial sustainability aspects according to Karrman and J onsson (2000), i.e. the nutrient and heavy metal management, were further analysed. These aspects were analysed using simulation results from the project Organic waste as a plant nutrient resource ± system study and these results provided a basis for the formulation of four strategies for SWM. 2. Methods In the project Organic waste as a plant nutrient resource ± system study, four dierent systems for handling household wastewater and biodegradable household waste were simulated to compare their overall performance. The four systems compared were: (A) conventional wastewater treatment in a wastewater treatment plant (WWTP), (B) irrigation of energy forests with biologically treated wastewater, (C) liquid composting of toilet wastewater mixed with organic waste and (D) conventional wastewater treatment supplemented with urine separation. The three alternative systems B±D were constructed with the aim of increasing nutrient recycling and sustainability. For the dierent wastewater systems the fractions collected, treatment, treatment products and their utilisation are described in Table 1. The management of solid biodegradable household waste was included in the study and in systems A, B and D this fraction was transported by garbage truck to a central windrow-composting plant. The treated compost was land®lled. In system C, the organic household waste was transported manually to a local vacuum station, where it was ground and then co-transported with toilet wastewater in a low pressure system to the liquid compost reactor. After co-processing, the liquid compost was transported to, and utilised on, arable land. The four systems were simulated for a hypothetically formulated settlement with 20 000 inhabitants (10 000 in ¯ats and 10 000 in detached houses).
E. Karrman / Urban Water 3 (2001) 63±72
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Table 1 Management of wastewater in the four systems (A) Conventional system
(B) Irrigation of energy forests
(C) Liquid composting
(D) Urine separation
Fractions collected
(1) Wastewater
(1) Wastewater
Collection system
(1) Sewer
(1) Sewer
(1) Toilet wastewater (2) Bath, dish and laundry water (1) Vacuum system (toilet wastewater) (2) Sewer (bath, dish, laundry water) (1) Liquid compost (Toilet wastewater)
(1) Urine (2) Faeces and bath, dish and laundry water (1) Urine pipe
(2) Mech., biol. & chem. treatment (bath, dish and laundry water) (1) Water to recipient
(2) Mech., biol. & chem. treatment (faecesbath, dish and laundry water) (1) Water to recipient
(2) Liquid compost to arable land (3) Sludge and waste from WWTP to land®ll
(2) Urine to arable land
Treatment
Residuals
(1) Mech., biol. (incl. N-removal) and chem. treatment
(1) Mech. and biol. treatment and winter storage
(1) Water to recipient
(1) Water to irrigation of energy forest (2) Sludge to arable land or land®ll (3) Pre-treatment sludge to land®ll
(2) Sludge to arable land (3) Pre-treatment sludge to land®ll
Processes included in this study were production and distribution of drinking water, collection and transport of wastewater and of solid organic waste to treatment, treatment of wastewater and solid organic waste, discharges of treated wastewater to receiving waters, transport of treatment products to, and their utilisation on, arable land and transport of other treatment products to, and degradation on, a land®ll. The study dealt only with the running of the systems, and aspects of their construction were not included. The four systems were simulated using ORWARE (Organic Waste Research model) ± a computer-based substance ¯ow simulation model. ORWARE simulates
(2) Sewer (faecesbath, dish and laundry water) (1) Storage tank (urine)
(3) Sludge and waste from WWTP to land®ll
the ¯ows of 43 substances, including plant nutrients such as phosphorus and nitrogen and heavy metals such as cadmium, through the system. Thus, from the in¯ow of waste and wastewater, described by their content of these 43 substances, ORWARE simulates the size and composition of the ¯ows of the system, i.e. emissions to air and water, ¯ow of recycled products and accumulation on land®ll. The use of energy and other resources by the system is also calculated. The ORWARE model is a collection of sub-models developed in several research projects. They are mainly based on data from full-scale or pilot-scale experiments. The sub-models of the conventional wastewater system are described in Dalemo
Table 2 Flows and composition of wastewater and organic solid waste from households. Data from Swedish EPA (1995a), unless otherwise stated
Flow Dry matters Total nitrogen Total phosphorus Potassium BOD7 Cadmium a
Unit
Urine
Faeces
Grey water
Organic waste
kg/p,d g/p,d g/p,d g/p,d g/p,d g/p,d mg/p,d
1.5a; b 60 11 1.0 2.5 20e 0.0024f
0.1b 35 1.5 0.5 1.0 20e 0.010
150 80 1.0 0.3c 0.5 28 <0.13g
0.22d 66 1.3d 0.25d 0.61d ± 0.0085h
Hellstr om and K arrman (1996). Flush water excluded. c Sundberg, personal communication. d Sonesson and J onsson (1996). e Urine + faeces. f J onsson, Burstr om, and Svensson (1998). g Average value of Cd in sewage sludge in 8 Swedish WWTP, reduced with the data of urine and faeces. h Eklind et al.(1997). b
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E. Karrman / Urban Water 3 (2001) 63±72
(1999), management of solid organic waste in Sonesson (1998), land®lling in Bj orklund (1998) and nitrogen turnover in arable land in Dalemo, Sonesson, J onsson, and Bj orklund (1998). Finally, the descriptions of the sub-models of the irrigation, liquid composting and urine separation systems are found in K arrman, J onsson, Gruvberger, Dalemo, and Sonesson (1999). Table 2 shows input data of ¯ows and composition of wastewater and organic solid waste. Model assumptions, with a large in¯uence on the results, are described in more detail in Section 3. 3. Results The simulation results are sorted into four Sections 3.1±3.4, where each represents a suggested strategy for SWM. The four suggested strategies are: 1. Handle nutrient-rich ¯ows separately from other waste ¯ows. 2. Recycle the nutrients and use energy eciently. 3. Avoid contamination of wastewater ¯ows. 4. Put unavoidable pollution on land®ll. Further, in Section 3.5, strategies from the literature are discussed and compared. 3.1. Handle nutrient-rich ¯ows separately from other waste ¯ows Discharges of nitrogen to receiving waters cause eutrophication. The total nitrogen ¯ows to water from the four systems simulated, normalised in relation to the total nitrogen discharges in Sweden, are presented in Fig. 1. The conventional system contains 70% removal of nitrogen mainly through denitri®cation. In spite of the high degree of nitrogen removal, the conventional system still contributes 18% of the total anthropogenic ¯ows of nitrogen to water. Fig. 1 also shows that alternatives B±C are bene®cial from eutrophication as-
Fig. 1. Percentage contributions to nitrogen discharge to water from the management of household wastewater and organic waste in relation to the total impacts in Sweden.
pect. The reason for this is that in these systems the major proportion of nitrogen in the wastewater is used for fertilising crops, where nutrient uptake by the crops together with microbial, ®ltration and storage processes in the soil decreases the transport of nitrogen to waters. In the irrigation system the secondary-treated wastewater is stored in ponds and used for irrigation of energy forests during the cultivation period. In the liquid composting system, the nitrogen-rich toilet fraction is source-separated and used as a fertiliser. The remaining greywater, which is treated in a conventional wastewater treatment plant, contains only 7% of the nitrogen in household wastewater and household organic waste (Table 2). In the urine separation system, the urine is source-separated and used as a fertiliser after storage. The largest part of nitrogen, 52%, is therefore transported to arable soil. In this simulation 78% of the urine is collected in the urine pipe system, according to measurements in a Swedish eco-village (J onsson et al., 1998). The remaining amount of urine 22% is assumed to be ``wrong-sorted'' and is led through the wastewater treatment plant. No nitrogen removal is assumed in the WWTP in the urine separation system, but even though, this system does not discharge more nitrogen to water than the conventional system (Fig. 1). Discharge of phosphorus also contributes to eutrophication. The four systems studied remove at least 95% of phosphorus and should therefore be considered well designed for this aspect. It is however also important to recycle phosphorus. The reason for this is that phosphorus is a limited resource, and recycled phosphorus should therefore replace mineral fertilisers in the long term. However, this is not a very urgent problem. The natural phosphorus resources of the world from rock are expected to last at least between 150 and 200 years (Swedish EPA, 1995b). In spite of the large asset of phosphorus, there are still arguments for recycling even today. Disposal of organic phosphorus-rich waste products causes nutrient leaching to water and the most ecient way of reducing this impact is to avoid land®lling. However, a serious problem with the recycling of residuals from organic waste and wastewater is the content of heavy metals. The cadmium uptake in plants is highly dependent on the cadmium level in topsoil (Hedlund, Eriksson, Petersson-Grawe, & Oborn, 1997) and therefore a balance between input and output of cadmium to arable land should be accomplished. Fig. 2 shows the eect of the cadmium balance of arable land when recycling various residuals from the systems simulated. Here, a fertilisation level of 100 kg N/ha,yr and 22 kg P/ha,yr is assumed, which is a normal dose for cultivating cereal crops in mid-Sweden. A large part of the cadmium in¯ow comes from deposition, but fortunately, the deposition is decreasing over time, thanks to cleaner combustion. Small amounts of cadmium are also added to arable soil through lime,
E. Karrman / Urban Water 3 (2001) 63±72
Fig. 2. In¯ow of cadmium to arable land. A dose of 100 kg N/ha,yr and 22 kg P/ha,yr is assumed for recycling of residuals. The line marked ``Yield'' represents the out¯ow of Cd through the harvest.
but the single most important factor for the farmer to care for is the choice of fertiliser. The line marked ``Yield'' in Fig. 2 shows the amounts of cadmium leaving the land with the harvest of crops in an example for mid-Sweden conditions by Hedlund et al. (1997). In order to avoid an increasing load of cadmium in plants, the in¯ow of cadmium must not exceed this level. Fig. 2 shows that source-separated urine in fact is the only residual that ful®ls this criterion, but liquid compost also comes close. Sewage sludge, on the other hand, contributes a considerable amount of cadmium to arable soil and improvements of the quality of sewage sludge seem to be needed. This could be achieved with preventive actions, such as source control, or removal of cadmium through treatment of sludge. Irrigation water has also a higher load of cadmium than liquid compost and urine. Irrigation of energy forests is, however, still interesting since this cultivation is not aimed at food production. Fig. 3 shows the potential of recycling nitrogen and phosphorus through the residuals studied in relation to the total content of the substances in household wastewater and organic solid waste (Table 2).
Fig. 3. Recycling of plant-available nitrogen and phosphorus in relation to the total amount of these substances in wastewater.
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Fig. 3 shows that fertilising with liquid compost means that a major proportion of nitrogen and phosphorus is recycled to arable land. Separate handling of urine also means that a major part of the nutrients can be recycled, although not to the same degree as liquid compost. Using treated wastewater for irrigation means that a major proportion of nitrogen (57%) is recycled but only a small fraction of phosphorus (8%). However, since the same requirements regarding heavy metals in¯ow do not have to be applied to energy forest cultivation, sewage sludge and compost can also be recycled in this alternative. The total phosphorus recycling in the system with irrigation of energy forest would then reach almost 100%. The results presented in this section show that separate handling of blackwater and urine seems to be an ecient system for protection of receiving waters from eutrophication and also for a high degree of simultaneous clean recycling of nutrients. However, a system with handling of mixed wastewater, where treated wastewater is used for irrigation of energy forests, can also be a very ecient system for protection of receiving waters from eutrophication and for nutrient recycling. A disadvantage of this system is that the nutrients in wastewater are not recycled to food production. 3.2. Recycle the nutrients and use energy eciently Nutrient recycling is important in order to avoid eutrophication and probably also for decreasing the production of mineral fertilisers. Phosphorus recycling is especially important since phosphorus is a limited resource, while nitrogen should be considered as a question of energy, since it is technically possible, by the use of energy, both to ®x nitrogen from air and to remove nitrogen from wastewater. In Figs. 4±6, the question is
Fig. 4. Primary energy consumption for a conventional system where sewage sludge is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
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E. Karrman / Urban Water 3 (2001) 63±72
Fig. 5. Primary energy consumption for a liquid composting system where the liquid compost is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
Fig. 6. Primary energy consumption for a urine separation system where the urine is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
raised as to whether it is bene®cial from an energy perspective to recycle nutrients (as sewage sludge, liquid compost and urine) or to dispose of them on land®ll.
Table 3 presents data used for the results presented in Figs 4±6. Data from the simulations used in the comparisons are presented in Table 3. It is assumed that the mineral fertiliser production requires 45.3 MJ/kg N and 40.8 MJ/kg P of primary energy, calculated from Davis and Haglund (1999). It is also assumed that phosphate rock is mined in Northern Africa and transported to Sweden by ship over a distance of 5000 km (Tillman, Lundstr om, & Svingby, 1996). The use of fossil fuels for the shipping requires 0.47 MJ/tonnes, km (Tillman et al., 1996). Triple superphosphate is then assumed to be produced in Sweden. The N-fertiliser (ammonium nitrate) is assumed to be produced in Sweden. As a ®nal transport to the arable land, it is assumed that both N- and P-fertilisers are transported 1000 km by truck. This transport requires 1.2 MJ/tonnes, km of fossil fuels (Sonesson, 1998). Finally, 162 MJ/ha, yr is used for spreading of mineral fertiliser on arable land (Sonesson, 1998). A dose of 100 kg N/ha, yr and 22 kg P/ha, year is assumed for sludge and urine recycling as well as usage of mineral fertilisers. Fig. 4 presents a comparison between two alternatives of management in the conventional system. The sewage sludge is recycled to arable land in the alternative ``sewage sludge'', and land®lled in the alternative ``mineral fertiliser''. The same amounts of nitrogen and phosphorus that are recycled in ``sewage sludge'' are produced and transported to arable land in ``Mineral fertiliser''. The comparison is made for primary energy consumption, where an eciency of 52% for electricity production is assumed representing the Swedish average electricity production (STEM, 1999). Further, an eciency of 90% for oil and diesel production is assumed. Fig. 4 shows that the recycling alternative is more energy ecient than the non-recycling alternative for distances to arable soil up to 1080 km, but also that the
Table 3 Electricity and oil use in the conventional, liquid composting and urine separation systems.
Electricity (MJ/p,yr) Drinking water production and distribution Sewer system Vacuum system Liquid composting reactor Wastewater treatment plant (WWTP) Oil (MJ/p,yr) Collection of organic waste Transport of waste from WWTP to land®ll Windrow composting of organic waste Transport to arable land (10 km) Spreading
Conventional
Liquid composting
Urine separation
174 109 0 0 185
150 99 216 335 98
160 104 0 0 126
39 0.4 1.2 1.2 5.7
0 0.3 0 31 33
39 0.4 1.2 7.4 11
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primary energy usage for the two alternatives does not dier very much. The recycling alternative is around 5% more energy ecient than the non-recycling alternative for local usage of sludge (less than 10 km to arable land). Fig. 5 presents a comparison of liquid composting systems with recycling of the liquid compost (sludge and pre-treatment waste is land®lled) versus conventional system (sludge, pre-treatment waste and compost land®lled) with the same amount of nitrogen and phosphorus as in the liquid composting system produced as mineral fertiliser production (sludge, pre-treatment waste and compost land®lled). The comparison is made for primary energy consumption, calculated in the same way as in Fig. 4. The liquid composting system uses considerably more primary energy than the conventional system. This is due to the large use of electricity for the vacuum system and for the reactor. The usage of primary energy in the liquid composting system increases by 18% comparing 1 km transports of residuals with 100 km transports. Fig. 6 shows ®nally that the liquid composting system uses more electricity than the other alternatives. It is however interesting to note that Bengtsson, Lundin, and Molander (1997) in a case study found a small-scale liquid composting system to be more energy ecient than a small-scale wastewater treatment plant. The reason for the contradictory ®nding in this study is probably that data of large-scale wastewater treatment plants are used. Large wastewater treatment plants are more energy ef®cient than small, as shown by among others Balmer and Mattson (1993). Fig. 6 presents a comparison of a urine separation system with recycling of urine (compost, sludge and pretreatment waste is land®lled) and a conventional system where sludge, compost and pre-treatment waste is land®lled. The comparison is made for primary energy consumption, calculated in the same way as in Fig. 4. The ``urine recycling'' alternative is 30% less energy demanding than the one with mineral fertiliser production for a case of 1 km transport of urine. From Fig. 6 it is, however, also obvious that an energy ecient urine recycling system must not be very local. With the assumptions used, the primary energy consumption for ``urine recycling'' exceeds ``mineral fertiliser'' for transport distances beyond 400 km. Previous systems analyses (Gujer, 1996; Stenberg, Andersson, & K arrman, 1996; Tillman et al., 1996; Bengtsson et al., 1997) also show that urine separation systems are potentially more energy ecient than traditional wastewater systems. Another option for nitrogen recycling is the system with irrigation of energy forests, where in fact no fossil fuels are used for applying nitrogen on the arable land. This system is favourable also for environmental protection since it does not discharge wastewater to receiving waters. A third advantage with the irrigation
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Table 4 Land use for storage of residuals and utilisation of nitrogen in agriculture Irrigation Land use for storagea m2 =person Land use for nutrient utilisationb m2 =person a b
34 300
Liquid compost 1.6 500
Urine 0.4 300
Assumed height/depth of facilities 2 m. Assumed dose NP (kg/ha,yr) 100:22.
system is the 1900 MJ/p,yr calculated extra heat production due to higher harvest of energy forests, comparing cultivating energy forests with and without irrigation of secondary treated wastewater. On the other hand, the energy forest system uses electricity for the irrigation, and the system is less ¯exible than the other systems since land for storage ponds and cultivating energy forests must be available near the wastewater treatment plant. Another important aspect to consider when analysing the pros and cons of on-site reuse is the land use requirement for storage and recycling. Table 4 shows that storage of liquid compost or urine is possible almost everywhere ± even in densely built urban areas, but ecient on-site reuse is only possible on agricultural land. Nowhere else are 300±500 m2 of land per person available for cultivation. With the results in mind of the use of primary energy, dependent on local transport (Fig. 4), it could be concluded that liquid composting system is most suitable for a village with nearby agricultural areas, while a urine separation system is favourable in small and large urban areas. Irrigation of energy forests is favourable on the border of urban areas. A summary of this section would be that o-site reuse is always needed, since there is not enough land within the urban area. On-site storage is favourable for urine, but treated wastewater for irrigation and liquid compost should preferably be stored o-site. End-of-pipe solutions such as wastewater treatment plants seem to be needed even further in urban areas. Wastewater has to be led o from the urban areas even if a blackwater or urine separation system is implemented. Independent of the system, there will also be some pollution to take care of at the end of pipe. A very promising improvement of this end-of-pipe solution is to store treated wastewater and use it for irrigation of energy forests. The pros of this are that there is no direct discharge to receiving waters and energy is recovered through the energy forests. However, large amounts of land are needed for storage and arable land (which rather preferably be used for food cultivation) has to be utilised for cultivation of energy forests.
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3.3. Avoid contamination of nutrient-rich and nutrientpoor ¯ows As shown in Section 3.1, an advantage of the systems with blackwater or urine separation is the clean ¯ow of nutrients (very low loads of heavy metals). If a separation system is installed, it is of great importance not to contaminate the nutrient-rich ¯ows with greywater or other potential sources of pollution because the pollution will end up in the food production. Therefore it is extremely important not to misdirect wastewater from bathing, dishwashing or laundry (greywater) or stormwater into a blackwater or urine collection system. However, it is also important not to discharge heavy metals into the sewer systems managing nutrient-poor ¯ows. This will not solve the problems of the heavy metals, but instead the heavy metals will be diused in the receiving waters or will end up in the sewage sludge. 3.4. Put unavoidable pollution on land®ll Independent of the measures taken, there will still be some unwanted substances such as heavy metals in all wastewater ¯ows. It is of great importance that these ¯ows are minimised to arable land as well as to the receiving waters. Unavoidable ¯ows of heavy metals should instead be put on land®ll. Fig. 7 shows that cadmium generally ends up in the receiving water or in the sewage sludge (and in the irrigation water in the energy forest system). Only a small fraction of cadmium is disposed if the pre-treatment sludge from the WWTP is put on land®ll. If the sewage sludge is disposed, then loss of phosphorus will appear in all four systems. These losses are however relatively small in the liquid composting and the urine separation systems (compare Fig. 3) and land®lling the sludge in these systems would therefore be a good strategy in order to decrease the ¯ows of cadmium to arable land, without seriously aecting nutrient recycling.
Fig. 7. The fate of cadmium in the four systems simulated.
4. Discussion The four strategies suggested are not really sorted in order of priority, but there are reasons to emphasise strategy 1. This is since the most strategic measure in creating SWM is to separate nutrient-rich ¯ows from nutrient-poor ¯ows in order to achieve the perquisite energy ecient and clean recycling of nutrients to arable land. However, in order to achieve this, systems for storage, transport and spreading of residuals must be energy ecient, resource ecient and emit minimal pollution. Furthermore, the addition of avoidable pollution to the wastewater ¯ows must be minimised and the system must be constructed so as to protect arable land and receiving waters from the pollution that is unavoidable. A conclusion regarding prioritisation is that strategy 1 should be seen as the ®rst step towards SWM, while strategies 2±4 are the subsequent steps that ensure the success of strategy 1. There are some dierences between the four strategies suggested and the six-level action plan by Niemczynowicz (1993). Niemczynowicz places the highest priority on preventive actions in order to avoid pollution. This is undoubtedly an important strategy on a general level and it is also important in wastewater management. The simulations in the present study show that an ecient preventive action for protection of receiving waters and clean recycling of nutrients is separation of nutrient-rich and nutrient-poor ¯ows. Further, Niemczynowicz prioritises local to o-site treatment and reuse. The simulations con®rm that short distance transportation is more sustainable than long distance, but one should have in mind that the reuse of nutrients requires large areas (Table 4) and therefore local reuse is not always possible. For the urine separation system (Fig. 6), transportation of residuals up to around 400 km can be relatively energy ecient. In the two lowest levels of Niemczynowicz's action plan, treatment plant solutions are to be found. Niemczynowicz argues that these kinds of end-of-pipe solutions are costly and energy consuming. The most negative aspects shown by the simulations regarding wastewater treatment plants are ®rstly that the sewage sludge gets too contaminated with heavy metals and therefore cannot be used as a fertiliser. If however the strategy 1 formulated in this paper is implemented, then some kind of end-of-pipe solution is needed for treatment of the nutrient-poor ¯ow. Thus this strategy means not abandoning the wastewater treatment plant ± it means an ecient combination of source-separation and end-of-pipe techniques. It is however not always necessary to lead greywater to a treatment plant. If there is land available, then the concept formulated by Otterpohl et al. (1997) could probably be favourable. This concept consists of: (1) separation of faeces and urine with vacuum toilets and
E. Karrman / Urban Water 3 (2001) 63±72
treatment with biowaste in biogas plants, (2) decentralised aerobic treatment of greywater in constructed wetlands and (3) in®ltration of stormwater to avoid a centralised system completely. An important sustainability aspect is the saving of clean water. Butler and Parkinson (1997) emphasise the importance of reducing the inappropriate ``use'' of potable water as a carriage medium in sewers. The liquid compost and the urine separation systems are example where the amount of ¯ush water is reduced. The vacuum toilets in the liquid composting systems use 0.8 l per ¯ush compared to 4 l for a modern conventional water closet. The urine separation toilet has two bowls, a front bowl for urine which uses 0.3 l/d and a rear bowl for faeces which uses 4 l per ¯ush. The total saving of water is 15% for the liquid composting system and 10% for the urine separation system, compared to the simulated conventional system. Graaf et al. (1997) highlight hygiene as one of the most important aspects. The hygiene aspects of the simulation systems were assessed in the main report from the project Organic waste as a plant nutrient resource ± system study (K arrman et al., 1999). The assessment ®rst of all focused on how pathogens in faecal material were handled in the dierent systems. It was found that the conventional system has some negative aspects both through discharging pathogens into receiving waters and through ¯ows of pathogens to arable land if the sludge is used. The irrigation system improves these conditions since the treated water and the sludge are used on a controlled area for cultivating energy forests. The liquid composting system is also favourable for the hygiene aspects since the faecal material is composted and sanitised in the composting reactor. Finally, the urine separation system is favourable in separating urine from faeces. The urine can safely be used on arable land after storage for 6 months (J onsson et al., 2000). The faeces on the other hand are led to a wastewater treatment plant where the same sanitary situation occurs as for the conventional system. Graaf et al. (1997) further emphasize reliability as a very important sustainability aspect. For this aspect, one has to remember that the systems with irrigation of energy forest, liquid compost and urine separation are in a developing stage. There are not yet many full-scale applications of the systems and the few examples that are in operation have only been running for a few years. Separation of either WC or separate handling of urine seems like a potentially sustainable wastewater system. If a WC or a urine separation system is to be planned, leading greywater to the stormwater sewer could be an interesting option. Another way to go towards SWM could possibly be to keep the traditional wastewater system with a mix of WC and greywater, and to sort out phosphorus from wastewater or sludge using separation techniques. The euent from the wastewater
71
treatment plant could bene®cially be used for irrigation of energy forests. 5. Conclusions On the basis of an environmental systems analysis comparing four dierent wastewater systems and prioritisation of impacts using normalisation, this paper suggests a four-point strategy towards sustainable wastewater management. The four points are formulated and exempli®ed: 1. Handle nutrient-rich ¯ows separately from other waste ¯ows. It is advisable, for environmental and energy eciency reasons, to recycle nutrients from wastes. Nutrient-rich ¯ows are urine, faeces and solid organic household waste and mixing these ¯ows with nutrientpoor ¯ows such as greywater and stormwater should be avoided. 2. Recycle nutrients and use energy eciently. The choice of technical system for nutrient recycling depends mainly on the density of the settlement and the distance to agriculture. Systems handling blackwater and solid organic household waste mixed have the highest nutrient recycling. The mixed ¯ow can be digested or composted, for further use as fertiliser. Biogas from the digester or heat from the compost process can be utilised e.g. for heating of buildings. This type of system is favourable in villages near agricultural land since the transport of residuals must be kept low for energy reasons. Greywater can preferably be in®ltrated or treated in sand®lters. Treated greywater could also be collected in ponds and used for irrigation. A promising solution for densely populated urban areas is urine separation. The urine is stored in tanks for a short time within the area and further transported by truck to agriculture for long-term storage and utilisation on arable land. Since the urine ¯ow is low and highly concentrated with nutrients, larger distances to arable land can be tolerated. Faeces, greywater and possibly stormwater are led together to a wastewater treatment plant, for biological and chemical treatment. Biogas, as well as heat from wastewater, is recovered. If there is land available nearby the treatment plant, there is a good potential for cultivation of energy forests, which is a very energy ecient alternative. 3. Avoid contamination of wastewater ¯ows. It is of great importance not to contaminate the nutrient-rich ¯ows, since the pollution will end up in the food production. Therefore it is extremely important not to misdirect wastewater from bathing, dishwashing or laundry (greywater) or stormwater into a blackwater or urine collection system. However, it is also of importance not to discharge heavy metals into the sewer systems managing nutrient-poor ¯ows. This will not solve
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the problems of the heavy metals, but instead the heavy metals will be diused in the receiving waters or end up in the sewage sludge. 4. Put unavoidable pollution on land®ll. Independent of the measures taken, there will still be some unwanted substances such as heavy metals in the waste ¯ows. It is of great importance that the ¯ows of heavy metals are minimised to arable land as well as to the receiving waters. The largest proportion of the heavy metals will be found in the nutrient-poor ¯ows, and therefore the heavy metals should be trapped in the treatment plant. The sewage sludge should, possibly after phosphorus extraction with separation techniques, be incinerated and the ashes disposed of safely.
Acknowledgements This study was sponsored by the Swedish Council for Building Research (BFR), which is gratefully acknowledged. References Balmer, P., & Mattson, B. (1993). Operational costs of wastewater treatment plants (Kostnader f or drift av avloppsreningsverk). VAForsk report 1993-15 (in Swedish). Bengtsson, M., Lundin, M., & Molander, S. (1997). Life cycle assessment of wastewater systems. Report 1997:9, Department of Technical Environmental Planning, Chalmers University of Technology, G oteborg, Sweden. Bj orklund, A. (1998). Environmental systems analysis of waste management with emphasis on substance ¯ows and environmental impact. Licentiate thesis, Royal Institute of Technology, Department of Chemical Engineering and Technology, Stockholm, Sweden. Butler, D., & Parkinson, J. (1997). Towards sustainable urban drainage. Water Science and Technology, 35(9), 53±63. Dalemo, M. (1999). Environmental systems analysis of organic waste management ± The ORWARE model and the sewage plant and anaerobic digestion sub-models. Dissertation, Swedish University of Agricultural Sciences, Agraria 146, Uppsala, Sweden. Dalemo, M., Sonesson, U., J onsson, H., & Bj orklund, A. (1998). Eects of including emissions from soil in environmental systems analysis of waste management strategies. Resources, Conservation and Recycling, 24, 363±381. Davis, J., & Haglund, C. (1999). Life cycle inventory of fertiliser production. The Swedish Institute for Food and Biotechnology. SIK-Report 654-1999, G oteborg, Sweden. Eklind, Y., Beck-Friis, B., Bengtsson, S., Ejlertsson, J., Mathisen, B., Nordkvist, E., Sonesson, U., Kirchmann, H., Torstensson, L., & Svensson, B. (1997). Chemical characterisation of source separated biological Household wastes. Swedish Journal of Agricultural Research, 27, 167±178. Graaf, J. H. J. M. V. D., Meester-Broertjes, H. A., Bruggeman, W. A., & Vles, E. J. (1997). Sustainable technological development for urban-water cycles. Water Science and Technology, 35(10), 213± 220. Gujer, W. (1996). Comparison of the performance of alternative urban sanitation concepts. Environmental Research Forum, 5±6, 233±240.
Hedlund, B., Eriksson, J., Petersson-Grawe, K., & Oborn, I. (Eds.) (1997). Cadmium, state of the art and trends (Kadmium, tillst and och trender). Report 4759, Swedish Environmental Protection Agency, Stockholm, Sweden (in Swedish). Hellstr om, D., & Karrman, E. (1996). Nitrogen and phosphorus in fresh and stored urine. Environmental Research Forum, 5±6, 221± 226. J onsson, H., Burstr om, A., & Svensson, J. (1998). Measurements on two urine separating sewage systems ± urine solution, toilet usage and time spent at home in an eco-village and an apartment district (Matning p atv aurinsorterande avloppssystem ± urinl osning, toalettanvandning och hemvaro i en ekoby och i ett hyreshusomr ade). Report 228, Department of Agricultural Engineering, SLU, Uppsala, Sweden. (in Swedish). J onsson, H., Vinner as, B., H oglund, C., Stenstr om, T.A., Dalhammar, G., & Kirchman, H. (2000). Recycling source separated human urine (Kallsorterad humanurin i kretslopp). VA-Forsk report 2000-1, Stockholm, Sweden (in Swedish). Karrman, E., J onsson, H., Gruvberger, C., Dalemo, M., & Sonesson, U. (1999). Environmental systems analysis of household wastewater and solid organic waste (Milj osystemanalys av hush allens avlopp och organiska avfall). VA-Forsk report 1999-15. (in Swedish). Karrman, E., & J onsson, H. (2000). Normalising impacts in an environmental systems analysis of wastewater management. Conference pre-prints of the 1st World Water Congress of the International Water Association (IWA), 3±7 July 2000, Paris, France, book 5 water resources and waste management pp. 276±283. Niemczynowicz, J. (1993). New aspects of sewerage and water technology. Ambio, 22(7), 449±455. Otterpohl, R., Grottker, M., & Lange, J. (1997). Sustainable water and waste management in urban areas. Water Science and Technology, 35(9), 121±133. Roeleveld, P. J., Klapwijk, A., Eggels, P. G., Rulkens, W. H., & van Starkenburg, W. (1997). Sustainability of municipal wastewater treatment. Water Science and Technology, 35(10), 221±228. Sonesson, U. (1998). Systems analysis of waste management ± the ORWARE model, transport and compost sub-models. Dissertation thesis. Swedish University of Agricultural Sciences, Agraria 130, Uppsala, Sweden. Sonesson, U., & J onsson, H. (1996). Urban biodegradable waste ± amount, and composition. Report 201, Department of Agricultural Engineering, SLU, Uppsala, Sweden. STEM. (1999). Energy in Sweden. The Swedish National Energy Administration, ET 83:1999, Eskilstuna, Sweden. Stenberg, M., Andersson, A., & Karrman, E. (1996). Environmental impact assessment applied to alternative wastewater systems in Bergsjon and Hamburgsund, report from the ECO-GUIDE project, Department of Sanitary Engineering, Report 1996:1, Chalmers University of Technology, G oteborg, Sweden (in Swedish). Swedish EPA. (1995a). What does household wastewater contain? Swedish Environmental Protection Agency, Report 4425, Stockholm, Sweden (in Swedish). Swedish EPA. (1995b). Environmentally compatible water and wastewater systems ± a proposal for criteria (Milj oanpassade vatten ± och avloppssystem ± f orslag till bed omningsgrunder). Swedish Environmental Protection Agency, Report 4429, ISBN 91-620-4429-X (in Swedish). Tillman, A.M., Lundstr om, H., & Svingby, M. (1996). Life cycle assessment of alternative waste water systems in Bergsjon and Hamburgsund. (Livscykelanalys av alternativa avloppssystem i Bergsj on och Hamburgsund). Report 1996:1, Department of Technical Environmental Planning, Chalmers University of Technology, G oteborg Sweden (in Swedish). WCED (1987). Our common future. World Commission Environment and Development. Oxford: Oxford University Press.
www.elsevier.com/locate/urbwat
Strategies towards sustainable wastewater management Erik K arrman * Water Environment Transport, Chalmers University of Technology, SE-412 96, Goteborg, Sweden Received 13 June 2000; received in revised form 15 January 2001; accepted 30 March 2001
Abstract Sustainable development requires sustainable wastewater management (SWM). For this reason, strategies towards SWM have been formulated and discussed in the research literature during the past decade. System analysis has sometimes been used for simulation of dierent strategies for wastewater management, in order to compare sustainability aspects such as environmental impacts and usage of resources. This paper suggests strategies for SWM based on results from a Swedish environmental system analysis called Organic waste as a plant nutrient resource ± system study, and from other studies with similar approaches. As a summary of the study, four strategies for SWM are suggested: 1. 2. 3. 4.
Handle nutrient-rich ¯ows separately from other waste ¯ows. Recycle nutrients and use energy eciently. Avoid contamination of wastewater ¯ows. Put unavoidable pollution on land®ll. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction During the 100-year long history of urban drainage, the main strengths have been to remove wastewater from users in order to prevent unhygienic conditions and to remove stormwater to avoid damage from ¯ooding. All this should be done without harming the environment. Existing urban water systems in the Western world ful®l these fundamental requirements to a high degree. Over the last ten years, however, the existing systems have been increasingly criticised from the viewpoint of sustainability. The most well-known de®nition of sustainable development was formulated by the ``Brundtland report'' (World Commission on Environment Development, WCED, 1987): Development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs. The WCED de®nition is widely used and accepted. However, there is still a challenge to make the sustainability concept useful for concrete decisionmaking. Niemczynowicz (1993), who formulated a sixlevel action plan, made an attempt for the urban water
*
Tel.: +46-31-7722-167; fax: +46-31-7722-128. E-mail address: [email protected] (E. Karrman).
sector. The list of the options below is ranked according to priority: 1. preventive actions during all human activities; 2. on-site treatment and reuse close to production; 3. o-site treatment and reuse; 4. on-site or o-site concentration and storage; 5. treatment at small-scale treatment plants using novel, low-tech technology; 6. high-technological treatment and end-of-pipe. Niemczynowicz (1993) further stresses that there are two possible scenarios. First, the high technological option implies continuation, development and complementation of present technology. The second is very dierent, containing low-cost, low-energy solutions based on application of biological systems and recycling of resources. Niemczynowicz (1993) emphasises that the second scenario will lead to a less vulnerable and more sustainable society, compared with scenario 1. Butler and Parkinson (1997), who focus on urban drainage, partly agree with Niemczynowicz (1993) but conclude that sustainable urban drainage is in fact an unattainable goal. In order to reduce the non-sustainability they suggest preventive actions such as separate handling of industrial waste to enable the reuse of sewage sludge and separate handling of stormwater to restore drainage patterns. A third strategy against non-sustainability by Butler and Parkinson (1997) is to reduce inappropriate
1462-0758/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 0 7 5 8 ( 0 1 ) 0 0 0 1 8 - 8
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``use'' of potable water as a carriage medium in sewers. These strategies may, according to the authors, be realised through the adoption of: domestic water conservation, small-scale recycling of greywater and rainwater, on-site storage of in®ltration of stormwater, utilisation of natural drainage patterns and local sanitation technology. Otterpohl, Grottker, and Lange (1997) agree that existing urban water management is not sustainable. The traditional sanitation concept has several severe disadvantages: it needs too much water, dilutes faeces and raises nutrient levels in the sea even after very advanced treatment. A more sustainable urban sanitation concept for urban areas according to the authors is: (1) separation of faeces and urine with vacuum toilets and treatment with biowaste in biogas plants, (2) decentralised aerobic treatment of greywater in constructed wetlands and (3) in®ltration of stormwater to avoid a centralised sewage system completely. K arrman and J onsson (2000) studied dierent sanitary systems for handling of household wastewater and organic solid waste with a systems analysis approach including normalisation. The normalisation was used to sort out the most crucial environmental aspects to be considered in wastewater management. The normalisation process was here de®ned as the percentage contributions from the sanitary system to the total national (Swedish) impact of severe environmental eects. Impacts caused by emissions and usage of resources were normalised. Further was the potential use of residuals normalised analogously ± where, for example, nutrients in residuals were considered as potentially replacing purchase of mineral fertilisers. The normalised impacts were sorted into priority groups, where Priority Group 1 contained the largest contributions, exceeding 10% of the total impacts, and Priority Group 2 contained contributions, exceeding 0.1% of the total national impacts. The impacts in Groups 1 and 2 are presented below: Priority Group 1 (>10%): · discharges of nitrogen to receiving waters; · discharges of cadmium, mercury and lead to receiving waters; · potential replacement of mineral fertilisers through nutrient recycling; · heavy metal ¯ows to arable soil (connected to nutrient recycling). Priority Group 2 (0.1%±10%): · discharge of phosphorus to receiving waters; · discharge of copper to receiving waters; · air emissions causing acidi®cation; · energy use; · air emissions forming photo-oxidants; · ¯ows of cadmium to land®ll; · air emissions causing global warming. A conclusion of this study is that the most urgent sustainability aspects of the systems studied concern
nutrient management and heavy metal management. Two studies from the Netherlands (Graaf, MeesterBroertjes, Bruggeman, & Vles, 1997; Roeleveld, Klapwijk, Eggels, Rulkens, & van Starkenburg, 1997) partly agree with this statement, using a similar normalisation procedure with national data of the Netherlands. Impacts exceeding 10% in the study by Roeleveld et al. (1997) were primary oxygen consumption and nutri®cation due to discharges of nitrogen and phosphorus. The discharges of heavy metals to water did not exceed 10% of the total anthropogenic impact. Graaf et al. (1997) studied scenarios for the total urban water cycle and gave the highest priority to emission of heavy metals to receiving waters, discharges of micro-pollutants to receiving waters, sludge production, groundwater management, emission of oxygen-binding materials via over¯ows (peak discharge), hygiene and reliability. In the present paper, the most crucial sustainability aspects according to Karrman and J onsson (2000), i.e. the nutrient and heavy metal management, were further analysed. These aspects were analysed using simulation results from the project Organic waste as a plant nutrient resource ± system study and these results provided a basis for the formulation of four strategies for SWM. 2. Methods In the project Organic waste as a plant nutrient resource ± system study, four dierent systems for handling household wastewater and biodegradable household waste were simulated to compare their overall performance. The four systems compared were: (A) conventional wastewater treatment in a wastewater treatment plant (WWTP), (B) irrigation of energy forests with biologically treated wastewater, (C) liquid composting of toilet wastewater mixed with organic waste and (D) conventional wastewater treatment supplemented with urine separation. The three alternative systems B±D were constructed with the aim of increasing nutrient recycling and sustainability. For the dierent wastewater systems the fractions collected, treatment, treatment products and their utilisation are described in Table 1. The management of solid biodegradable household waste was included in the study and in systems A, B and D this fraction was transported by garbage truck to a central windrow-composting plant. The treated compost was land®lled. In system C, the organic household waste was transported manually to a local vacuum station, where it was ground and then co-transported with toilet wastewater in a low pressure system to the liquid compost reactor. After co-processing, the liquid compost was transported to, and utilised on, arable land. The four systems were simulated for a hypothetically formulated settlement with 20 000 inhabitants (10 000 in ¯ats and 10 000 in detached houses).
E. Karrman / Urban Water 3 (2001) 63±72
65
Table 1 Management of wastewater in the four systems (A) Conventional system
(B) Irrigation of energy forests
(C) Liquid composting
(D) Urine separation
Fractions collected
(1) Wastewater
(1) Wastewater
Collection system
(1) Sewer
(1) Sewer
(1) Toilet wastewater (2) Bath, dish and laundry water (1) Vacuum system (toilet wastewater) (2) Sewer (bath, dish, laundry water) (1) Liquid compost (Toilet wastewater)
(1) Urine (2) Faeces and bath, dish and laundry water (1) Urine pipe
(2) Mech., biol. & chem. treatment (bath, dish and laundry water) (1) Water to recipient
(2) Mech., biol. & chem. treatment (faecesbath, dish and laundry water) (1) Water to recipient
(2) Liquid compost to arable land (3) Sludge and waste from WWTP to land®ll
(2) Urine to arable land
Treatment
Residuals
(1) Mech., biol. (incl. N-removal) and chem. treatment
(1) Mech. and biol. treatment and winter storage
(1) Water to recipient
(1) Water to irrigation of energy forest (2) Sludge to arable land or land®ll (3) Pre-treatment sludge to land®ll
(2) Sludge to arable land (3) Pre-treatment sludge to land®ll
Processes included in this study were production and distribution of drinking water, collection and transport of wastewater and of solid organic waste to treatment, treatment of wastewater and solid organic waste, discharges of treated wastewater to receiving waters, transport of treatment products to, and their utilisation on, arable land and transport of other treatment products to, and degradation on, a land®ll. The study dealt only with the running of the systems, and aspects of their construction were not included. The four systems were simulated using ORWARE (Organic Waste Research model) ± a computer-based substance ¯ow simulation model. ORWARE simulates
(2) Sewer (faecesbath, dish and laundry water) (1) Storage tank (urine)
(3) Sludge and waste from WWTP to land®ll
the ¯ows of 43 substances, including plant nutrients such as phosphorus and nitrogen and heavy metals such as cadmium, through the system. Thus, from the in¯ow of waste and wastewater, described by their content of these 43 substances, ORWARE simulates the size and composition of the ¯ows of the system, i.e. emissions to air and water, ¯ow of recycled products and accumulation on land®ll. The use of energy and other resources by the system is also calculated. The ORWARE model is a collection of sub-models developed in several research projects. They are mainly based on data from full-scale or pilot-scale experiments. The sub-models of the conventional wastewater system are described in Dalemo
Table 2 Flows and composition of wastewater and organic solid waste from households. Data from Swedish EPA (1995a), unless otherwise stated
Flow Dry matters Total nitrogen Total phosphorus Potassium BOD7 Cadmium a
Unit
Urine
Faeces
Grey water
Organic waste
kg/p,d g/p,d g/p,d g/p,d g/p,d g/p,d mg/p,d
1.5a; b 60 11 1.0 2.5 20e 0.0024f
0.1b 35 1.5 0.5 1.0 20e 0.010
150 80 1.0 0.3c 0.5 28 <0.13g
0.22d 66 1.3d 0.25d 0.61d ± 0.0085h
Hellstr om and K arrman (1996). Flush water excluded. c Sundberg, personal communication. d Sonesson and J onsson (1996). e Urine + faeces. f J onsson, Burstr om, and Svensson (1998). g Average value of Cd in sewage sludge in 8 Swedish WWTP, reduced with the data of urine and faeces. h Eklind et al.(1997). b
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(1999), management of solid organic waste in Sonesson (1998), land®lling in Bj orklund (1998) and nitrogen turnover in arable land in Dalemo, Sonesson, J onsson, and Bj orklund (1998). Finally, the descriptions of the sub-models of the irrigation, liquid composting and urine separation systems are found in K arrman, J onsson, Gruvberger, Dalemo, and Sonesson (1999). Table 2 shows input data of ¯ows and composition of wastewater and organic solid waste. Model assumptions, with a large in¯uence on the results, are described in more detail in Section 3. 3. Results The simulation results are sorted into four Sections 3.1±3.4, where each represents a suggested strategy for SWM. The four suggested strategies are: 1. Handle nutrient-rich ¯ows separately from other waste ¯ows. 2. Recycle the nutrients and use energy eciently. 3. Avoid contamination of wastewater ¯ows. 4. Put unavoidable pollution on land®ll. Further, in Section 3.5, strategies from the literature are discussed and compared. 3.1. Handle nutrient-rich ¯ows separately from other waste ¯ows Discharges of nitrogen to receiving waters cause eutrophication. The total nitrogen ¯ows to water from the four systems simulated, normalised in relation to the total nitrogen discharges in Sweden, are presented in Fig. 1. The conventional system contains 70% removal of nitrogen mainly through denitri®cation. In spite of the high degree of nitrogen removal, the conventional system still contributes 18% of the total anthropogenic ¯ows of nitrogen to water. Fig. 1 also shows that alternatives B±C are bene®cial from eutrophication as-
Fig. 1. Percentage contributions to nitrogen discharge to water from the management of household wastewater and organic waste in relation to the total impacts in Sweden.
pect. The reason for this is that in these systems the major proportion of nitrogen in the wastewater is used for fertilising crops, where nutrient uptake by the crops together with microbial, ®ltration and storage processes in the soil decreases the transport of nitrogen to waters. In the irrigation system the secondary-treated wastewater is stored in ponds and used for irrigation of energy forests during the cultivation period. In the liquid composting system, the nitrogen-rich toilet fraction is source-separated and used as a fertiliser. The remaining greywater, which is treated in a conventional wastewater treatment plant, contains only 7% of the nitrogen in household wastewater and household organic waste (Table 2). In the urine separation system, the urine is source-separated and used as a fertiliser after storage. The largest part of nitrogen, 52%, is therefore transported to arable soil. In this simulation 78% of the urine is collected in the urine pipe system, according to measurements in a Swedish eco-village (J onsson et al., 1998). The remaining amount of urine 22% is assumed to be ``wrong-sorted'' and is led through the wastewater treatment plant. No nitrogen removal is assumed in the WWTP in the urine separation system, but even though, this system does not discharge more nitrogen to water than the conventional system (Fig. 1). Discharge of phosphorus also contributes to eutrophication. The four systems studied remove at least 95% of phosphorus and should therefore be considered well designed for this aspect. It is however also important to recycle phosphorus. The reason for this is that phosphorus is a limited resource, and recycled phosphorus should therefore replace mineral fertilisers in the long term. However, this is not a very urgent problem. The natural phosphorus resources of the world from rock are expected to last at least between 150 and 200 years (Swedish EPA, 1995b). In spite of the large asset of phosphorus, there are still arguments for recycling even today. Disposal of organic phosphorus-rich waste products causes nutrient leaching to water and the most ecient way of reducing this impact is to avoid land®lling. However, a serious problem with the recycling of residuals from organic waste and wastewater is the content of heavy metals. The cadmium uptake in plants is highly dependent on the cadmium level in topsoil (Hedlund, Eriksson, Petersson-Grawe, & Oborn, 1997) and therefore a balance between input and output of cadmium to arable land should be accomplished. Fig. 2 shows the eect of the cadmium balance of arable land when recycling various residuals from the systems simulated. Here, a fertilisation level of 100 kg N/ha,yr and 22 kg P/ha,yr is assumed, which is a normal dose for cultivating cereal crops in mid-Sweden. A large part of the cadmium in¯ow comes from deposition, but fortunately, the deposition is decreasing over time, thanks to cleaner combustion. Small amounts of cadmium are also added to arable soil through lime,
E. Karrman / Urban Water 3 (2001) 63±72
Fig. 2. In¯ow of cadmium to arable land. A dose of 100 kg N/ha,yr and 22 kg P/ha,yr is assumed for recycling of residuals. The line marked ``Yield'' represents the out¯ow of Cd through the harvest.
but the single most important factor for the farmer to care for is the choice of fertiliser. The line marked ``Yield'' in Fig. 2 shows the amounts of cadmium leaving the land with the harvest of crops in an example for mid-Sweden conditions by Hedlund et al. (1997). In order to avoid an increasing load of cadmium in plants, the in¯ow of cadmium must not exceed this level. Fig. 2 shows that source-separated urine in fact is the only residual that ful®ls this criterion, but liquid compost also comes close. Sewage sludge, on the other hand, contributes a considerable amount of cadmium to arable soil and improvements of the quality of sewage sludge seem to be needed. This could be achieved with preventive actions, such as source control, or removal of cadmium through treatment of sludge. Irrigation water has also a higher load of cadmium than liquid compost and urine. Irrigation of energy forests is, however, still interesting since this cultivation is not aimed at food production. Fig. 3 shows the potential of recycling nitrogen and phosphorus through the residuals studied in relation to the total content of the substances in household wastewater and organic solid waste (Table 2).
Fig. 3. Recycling of plant-available nitrogen and phosphorus in relation to the total amount of these substances in wastewater.
67
Fig. 3 shows that fertilising with liquid compost means that a major proportion of nitrogen and phosphorus is recycled to arable land. Separate handling of urine also means that a major part of the nutrients can be recycled, although not to the same degree as liquid compost. Using treated wastewater for irrigation means that a major proportion of nitrogen (57%) is recycled but only a small fraction of phosphorus (8%). However, since the same requirements regarding heavy metals in¯ow do not have to be applied to energy forest cultivation, sewage sludge and compost can also be recycled in this alternative. The total phosphorus recycling in the system with irrigation of energy forest would then reach almost 100%. The results presented in this section show that separate handling of blackwater and urine seems to be an ecient system for protection of receiving waters from eutrophication and also for a high degree of simultaneous clean recycling of nutrients. However, a system with handling of mixed wastewater, where treated wastewater is used for irrigation of energy forests, can also be a very ecient system for protection of receiving waters from eutrophication and for nutrient recycling. A disadvantage of this system is that the nutrients in wastewater are not recycled to food production. 3.2. Recycle the nutrients and use energy eciently Nutrient recycling is important in order to avoid eutrophication and probably also for decreasing the production of mineral fertilisers. Phosphorus recycling is especially important since phosphorus is a limited resource, while nitrogen should be considered as a question of energy, since it is technically possible, by the use of energy, both to ®x nitrogen from air and to remove nitrogen from wastewater. In Figs. 4±6, the question is
Fig. 4. Primary energy consumption for a conventional system where sewage sludge is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
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Fig. 5. Primary energy consumption for a liquid composting system where the liquid compost is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
Fig. 6. Primary energy consumption for a urine separation system where the urine is recycled to arable land versus a conventional system where the same amount of nutrients in the form of mineral fertilisers is recycled.
raised as to whether it is bene®cial from an energy perspective to recycle nutrients (as sewage sludge, liquid compost and urine) or to dispose of them on land®ll.
Table 3 presents data used for the results presented in Figs 4±6. Data from the simulations used in the comparisons are presented in Table 3. It is assumed that the mineral fertiliser production requires 45.3 MJ/kg N and 40.8 MJ/kg P of primary energy, calculated from Davis and Haglund (1999). It is also assumed that phosphate rock is mined in Northern Africa and transported to Sweden by ship over a distance of 5000 km (Tillman, Lundstr om, & Svingby, 1996). The use of fossil fuels for the shipping requires 0.47 MJ/tonnes, km (Tillman et al., 1996). Triple superphosphate is then assumed to be produced in Sweden. The N-fertiliser (ammonium nitrate) is assumed to be produced in Sweden. As a ®nal transport to the arable land, it is assumed that both N- and P-fertilisers are transported 1000 km by truck. This transport requires 1.2 MJ/tonnes, km of fossil fuels (Sonesson, 1998). Finally, 162 MJ/ha, yr is used for spreading of mineral fertiliser on arable land (Sonesson, 1998). A dose of 100 kg N/ha, yr and 22 kg P/ha, year is assumed for sludge and urine recycling as well as usage of mineral fertilisers. Fig. 4 presents a comparison between two alternatives of management in the conventional system. The sewage sludge is recycled to arable land in the alternative ``sewage sludge'', and land®lled in the alternative ``mineral fertiliser''. The same amounts of nitrogen and phosphorus that are recycled in ``sewage sludge'' are produced and transported to arable land in ``Mineral fertiliser''. The comparison is made for primary energy consumption, where an eciency of 52% for electricity production is assumed representing the Swedish average electricity production (STEM, 1999). Further, an eciency of 90% for oil and diesel production is assumed. Fig. 4 shows that the recycling alternative is more energy ecient than the non-recycling alternative for distances to arable soil up to 1080 km, but also that the
Table 3 Electricity and oil use in the conventional, liquid composting and urine separation systems.
Electricity (MJ/p,yr) Drinking water production and distribution Sewer system Vacuum system Liquid composting reactor Wastewater treatment plant (WWTP) Oil (MJ/p,yr) Collection of organic waste Transport of waste from WWTP to land®ll Windrow composting of organic waste Transport to arable land (10 km) Spreading
Conventional
Liquid composting
Urine separation
174 109 0 0 185
150 99 216 335 98
160 104 0 0 126
39 0.4 1.2 1.2 5.7
0 0.3 0 31 33
39 0.4 1.2 7.4 11
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primary energy usage for the two alternatives does not dier very much. The recycling alternative is around 5% more energy ecient than the non-recycling alternative for local usage of sludge (less than 10 km to arable land). Fig. 5 presents a comparison of liquid composting systems with recycling of the liquid compost (sludge and pre-treatment waste is land®lled) versus conventional system (sludge, pre-treatment waste and compost land®lled) with the same amount of nitrogen and phosphorus as in the liquid composting system produced as mineral fertiliser production (sludge, pre-treatment waste and compost land®lled). The comparison is made for primary energy consumption, calculated in the same way as in Fig. 4. The liquid composting system uses considerably more primary energy than the conventional system. This is due to the large use of electricity for the vacuum system and for the reactor. The usage of primary energy in the liquid composting system increases by 18% comparing 1 km transports of residuals with 100 km transports. Fig. 6 shows ®nally that the liquid composting system uses more electricity than the other alternatives. It is however interesting to note that Bengtsson, Lundin, and Molander (1997) in a case study found a small-scale liquid composting system to be more energy ecient than a small-scale wastewater treatment plant. The reason for the contradictory ®nding in this study is probably that data of large-scale wastewater treatment plants are used. Large wastewater treatment plants are more energy ef®cient than small, as shown by among others Balmer and Mattson (1993). Fig. 6 presents a comparison of a urine separation system with recycling of urine (compost, sludge and pretreatment waste is land®lled) and a conventional system where sludge, compost and pre-treatment waste is land®lled. The comparison is made for primary energy consumption, calculated in the same way as in Fig. 4. The ``urine recycling'' alternative is 30% less energy demanding than the one with mineral fertiliser production for a case of 1 km transport of urine. From Fig. 6 it is, however, also obvious that an energy ecient urine recycling system must not be very local. With the assumptions used, the primary energy consumption for ``urine recycling'' exceeds ``mineral fertiliser'' for transport distances beyond 400 km. Previous systems analyses (Gujer, 1996; Stenberg, Andersson, & K arrman, 1996; Tillman et al., 1996; Bengtsson et al., 1997) also show that urine separation systems are potentially more energy ecient than traditional wastewater systems. Another option for nitrogen recycling is the system with irrigation of energy forests, where in fact no fossil fuels are used for applying nitrogen on the arable land. This system is favourable also for environmental protection since it does not discharge wastewater to receiving waters. A third advantage with the irrigation
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Table 4 Land use for storage of residuals and utilisation of nitrogen in agriculture Irrigation Land use for storagea m2 =person Land use for nutrient utilisationb m2 =person a b
34 300
Liquid compost 1.6 500
Urine 0.4 300
Assumed height/depth of facilities 2 m. Assumed dose NP (kg/ha,yr) 100:22.
system is the 1900 MJ/p,yr calculated extra heat production due to higher harvest of energy forests, comparing cultivating energy forests with and without irrigation of secondary treated wastewater. On the other hand, the energy forest system uses electricity for the irrigation, and the system is less ¯exible than the other systems since land for storage ponds and cultivating energy forests must be available near the wastewater treatment plant. Another important aspect to consider when analysing the pros and cons of on-site reuse is the land use requirement for storage and recycling. Table 4 shows that storage of liquid compost or urine is possible almost everywhere ± even in densely built urban areas, but ecient on-site reuse is only possible on agricultural land. Nowhere else are 300±500 m2 of land per person available for cultivation. With the results in mind of the use of primary energy, dependent on local transport (Fig. 4), it could be concluded that liquid composting system is most suitable for a village with nearby agricultural areas, while a urine separation system is favourable in small and large urban areas. Irrigation of energy forests is favourable on the border of urban areas. A summary of this section would be that o-site reuse is always needed, since there is not enough land within the urban area. On-site storage is favourable for urine, but treated wastewater for irrigation and liquid compost should preferably be stored o-site. End-of-pipe solutions such as wastewater treatment plants seem to be needed even further in urban areas. Wastewater has to be led o from the urban areas even if a blackwater or urine separation system is implemented. Independent of the system, there will also be some pollution to take care of at the end of pipe. A very promising improvement of this end-of-pipe solution is to store treated wastewater and use it for irrigation of energy forests. The pros of this are that there is no direct discharge to receiving waters and energy is recovered through the energy forests. However, large amounts of land are needed for storage and arable land (which rather preferably be used for food cultivation) has to be utilised for cultivation of energy forests.
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3.3. Avoid contamination of nutrient-rich and nutrientpoor ¯ows As shown in Section 3.1, an advantage of the systems with blackwater or urine separation is the clean ¯ow of nutrients (very low loads of heavy metals). If a separation system is installed, it is of great importance not to contaminate the nutrient-rich ¯ows with greywater or other potential sources of pollution because the pollution will end up in the food production. Therefore it is extremely important not to misdirect wastewater from bathing, dishwashing or laundry (greywater) or stormwater into a blackwater or urine collection system. However, it is also important not to discharge heavy metals into the sewer systems managing nutrient-poor ¯ows. This will not solve the problems of the heavy metals, but instead the heavy metals will be diused in the receiving waters or will end up in the sewage sludge. 3.4. Put unavoidable pollution on land®ll Independent of the measures taken, there will still be some unwanted substances such as heavy metals in all wastewater ¯ows. It is of great importance that these ¯ows are minimised to arable land as well as to the receiving waters. Unavoidable ¯ows of heavy metals should instead be put on land®ll. Fig. 7 shows that cadmium generally ends up in the receiving water or in the sewage sludge (and in the irrigation water in the energy forest system). Only a small fraction of cadmium is disposed if the pre-treatment sludge from the WWTP is put on land®ll. If the sewage sludge is disposed, then loss of phosphorus will appear in all four systems. These losses are however relatively small in the liquid composting and the urine separation systems (compare Fig. 3) and land®lling the sludge in these systems would therefore be a good strategy in order to decrease the ¯ows of cadmium to arable land, without seriously aecting nutrient recycling.
Fig. 7. The fate of cadmium in the four systems simulated.
4. Discussion The four strategies suggested are not really sorted in order of priority, but there are reasons to emphasise strategy 1. This is since the most strategic measure in creating SWM is to separate nutrient-rich ¯ows from nutrient-poor ¯ows in order to achieve the perquisite energy ecient and clean recycling of nutrients to arable land. However, in order to achieve this, systems for storage, transport and spreading of residuals must be energy ecient, resource ecient and emit minimal pollution. Furthermore, the addition of avoidable pollution to the wastewater ¯ows must be minimised and the system must be constructed so as to protect arable land and receiving waters from the pollution that is unavoidable. A conclusion regarding prioritisation is that strategy 1 should be seen as the ®rst step towards SWM, while strategies 2±4 are the subsequent steps that ensure the success of strategy 1. There are some dierences between the four strategies suggested and the six-level action plan by Niemczynowicz (1993). Niemczynowicz places the highest priority on preventive actions in order to avoid pollution. This is undoubtedly an important strategy on a general level and it is also important in wastewater management. The simulations in the present study show that an ecient preventive action for protection of receiving waters and clean recycling of nutrients is separation of nutrient-rich and nutrient-poor ¯ows. Further, Niemczynowicz prioritises local to o-site treatment and reuse. The simulations con®rm that short distance transportation is more sustainable than long distance, but one should have in mind that the reuse of nutrients requires large areas (Table 4) and therefore local reuse is not always possible. For the urine separation system (Fig. 6), transportation of residuals up to around 400 km can be relatively energy ecient. In the two lowest levels of Niemczynowicz's action plan, treatment plant solutions are to be found. Niemczynowicz argues that these kinds of end-of-pipe solutions are costly and energy consuming. The most negative aspects shown by the simulations regarding wastewater treatment plants are ®rstly that the sewage sludge gets too contaminated with heavy metals and therefore cannot be used as a fertiliser. If however the strategy 1 formulated in this paper is implemented, then some kind of end-of-pipe solution is needed for treatment of the nutrient-poor ¯ow. Thus this strategy means not abandoning the wastewater treatment plant ± it means an ecient combination of source-separation and end-of-pipe techniques. It is however not always necessary to lead greywater to a treatment plant. If there is land available, then the concept formulated by Otterpohl et al. (1997) could probably be favourable. This concept consists of: (1) separation of faeces and urine with vacuum toilets and
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treatment with biowaste in biogas plants, (2) decentralised aerobic treatment of greywater in constructed wetlands and (3) in®ltration of stormwater to avoid a centralised system completely. An important sustainability aspect is the saving of clean water. Butler and Parkinson (1997) emphasise the importance of reducing the inappropriate ``use'' of potable water as a carriage medium in sewers. The liquid compost and the urine separation systems are example where the amount of ¯ush water is reduced. The vacuum toilets in the liquid composting systems use 0.8 l per ¯ush compared to 4 l for a modern conventional water closet. The urine separation toilet has two bowls, a front bowl for urine which uses 0.3 l/d and a rear bowl for faeces which uses 4 l per ¯ush. The total saving of water is 15% for the liquid composting system and 10% for the urine separation system, compared to the simulated conventional system. Graaf et al. (1997) highlight hygiene as one of the most important aspects. The hygiene aspects of the simulation systems were assessed in the main report from the project Organic waste as a plant nutrient resource ± system study (K arrman et al., 1999). The assessment ®rst of all focused on how pathogens in faecal material were handled in the dierent systems. It was found that the conventional system has some negative aspects both through discharging pathogens into receiving waters and through ¯ows of pathogens to arable land if the sludge is used. The irrigation system improves these conditions since the treated water and the sludge are used on a controlled area for cultivating energy forests. The liquid composting system is also favourable for the hygiene aspects since the faecal material is composted and sanitised in the composting reactor. Finally, the urine separation system is favourable in separating urine from faeces. The urine can safely be used on arable land after storage for 6 months (J onsson et al., 2000). The faeces on the other hand are led to a wastewater treatment plant where the same sanitary situation occurs as for the conventional system. Graaf et al. (1997) further emphasize reliability as a very important sustainability aspect. For this aspect, one has to remember that the systems with irrigation of energy forest, liquid compost and urine separation are in a developing stage. There are not yet many full-scale applications of the systems and the few examples that are in operation have only been running for a few years. Separation of either WC or separate handling of urine seems like a potentially sustainable wastewater system. If a WC or a urine separation system is to be planned, leading greywater to the stormwater sewer could be an interesting option. Another way to go towards SWM could possibly be to keep the traditional wastewater system with a mix of WC and greywater, and to sort out phosphorus from wastewater or sludge using separation techniques. The euent from the wastewater
71
treatment plant could bene®cially be used for irrigation of energy forests. 5. Conclusions On the basis of an environmental systems analysis comparing four dierent wastewater systems and prioritisation of impacts using normalisation, this paper suggests a four-point strategy towards sustainable wastewater management. The four points are formulated and exempli®ed: 1. Handle nutrient-rich ¯ows separately from other waste ¯ows. It is advisable, for environmental and energy eciency reasons, to recycle nutrients from wastes. Nutrient-rich ¯ows are urine, faeces and solid organic household waste and mixing these ¯ows with nutrientpoor ¯ows such as greywater and stormwater should be avoided. 2. Recycle nutrients and use energy eciently. The choice of technical system for nutrient recycling depends mainly on the density of the settlement and the distance to agriculture. Systems handling blackwater and solid organic household waste mixed have the highest nutrient recycling. The mixed ¯ow can be digested or composted, for further use as fertiliser. Biogas from the digester or heat from the compost process can be utilised e.g. for heating of buildings. This type of system is favourable in villages near agricultural land since the transport of residuals must be kept low for energy reasons. Greywater can preferably be in®ltrated or treated in sand®lters. Treated greywater could also be collected in ponds and used for irrigation. A promising solution for densely populated urban areas is urine separation. The urine is stored in tanks for a short time within the area and further transported by truck to agriculture for long-term storage and utilisation on arable land. Since the urine ¯ow is low and highly concentrated with nutrients, larger distances to arable land can be tolerated. Faeces, greywater and possibly stormwater are led together to a wastewater treatment plant, for biological and chemical treatment. Biogas, as well as heat from wastewater, is recovered. If there is land available nearby the treatment plant, there is a good potential for cultivation of energy forests, which is a very energy ecient alternative. 3. Avoid contamination of wastewater ¯ows. It is of great importance not to contaminate the nutrient-rich ¯ows, since the pollution will end up in the food production. Therefore it is extremely important not to misdirect wastewater from bathing, dishwashing or laundry (greywater) or stormwater into a blackwater or urine collection system. However, it is also of importance not to discharge heavy metals into the sewer systems managing nutrient-poor ¯ows. This will not solve
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the problems of the heavy metals, but instead the heavy metals will be diused in the receiving waters or end up in the sewage sludge. 4. Put unavoidable pollution on land®ll. Independent of the measures taken, there will still be some unwanted substances such as heavy metals in the waste ¯ows. It is of great importance that the ¯ows of heavy metals are minimised to arable land as well as to the receiving waters. The largest proportion of the heavy metals will be found in the nutrient-poor ¯ows, and therefore the heavy metals should be trapped in the treatment plant. The sewage sludge should, possibly after phosphorus extraction with separation techniques, be incinerated and the ashes disposed of safely.
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