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ORWARE – A simulation model for organic waste handling systems. Part 2: Case study and simulation results
Resources, Conservation and Recycling 21 (1997) 39–54
ORWARE – A simulation model for organic waste handling systems. Part 2: Case study and simulation results U. Sonesson a,*, M. Dalemo b, K. Mingarini c, H. Jo¨nsson a a
Department of Agricultural Engineering, Swedish Uni6ersity of Agricultural Sciences, P.O. Box 7033, S-75007 Uppsala, Sweden b Swedish Institute of Agricultural Engineering, P.O. Box 7033, S-75007 Uppsala, Sweden c Department of En6ironmental Engineering and Work Science, Royal Institute of Technology, S-10044 Stockholm, Sweden Received 15 April 1997; received in revised form 28 May 1997; accepted 3 June 1997
Abstract Results from simulations with the ORWARE model (ORganic WAste REsearch) are presented. The model was tested on a medium-sized Swedish city. The scenarios were planned to illustrate the consequences of different waste handling systems. The modelled processes for organic solid waste were; incineration, landfilling, anaerobic digestion and composting, for the wastewater they were sewage plant and source separation of urine. Each transport and treatment facility was modelled with respect to incoming waste. Each process model generates an energy balance, liquid and/or gaseous emissions and residual products as outputs. The simulation results show that source separation of solid waste, followed by biological treatment, is beneficial with respect to the recycling of phosphorus and environmental effects. The negative results for these systems are their energy balances. Source-separating human urine seems to be the only way to get a high degree of nitrogen recycling. The results also stress the importance of including liquid waste to get an overall picture of the transports needed to get the residues to arable land. Landfilling organic waste generates the largest negative environmental effect. A model description is presented in a companion paper. © 1997 Elsevier Science B.V.
* Corresponding author. 0921-3449/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 3 4 4 9 ( 9 7 ) 0 0 0 2 1 - 9
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Keywords: Systems analysis; Modelling; Organic waste; Environmental impact; Plant nutrient recycling; Integrated waste management
1. Introduction Organic waste is a kind of waste that is not likely to be reduced by technical and political measures, as can be done with other types of waste. The organic waste handling systems have developed rather arbitrarily as a result of both actual and historical needs and technical innovations. As a result, very little has been done so far regarding the co-ordination of different waste management aspects. In this study, organic waste has been defined as waste originating from living organisms that are essentially free from synthetic components and for which the present recovery and recirculation systems can be significantly improved. The present study had several objectives. The objectives were: 1. To make a system comparison between an increased and improved source-separation of solid waste, followed by biological treatment, composting or anaerobic digestion, on the one hand, and today’s system, incineration or landfilling, on the other. 2. To find out the distribution of plant nutrient content between the different wastes, especially solid vs. liquid waste, and its consequences. 3. To evaluate what the effects would be if urine-separating toilets were introduced as a means of recycling more of the plant nutrients contained in wastewater. A system approach is necessary to analyse the complicated way organic wastes are collected, treated and disposed. A large number of materials have been considered in the investigated scenarios. Solid, liquid, and gaseous material flows have been included in the analysis. An increased recycling of waste containing high levels of contaminants will increase the environmental load to soil. We assume that a prerequisite for the increased recycling of plant nutrients is that the pollution level of the urban organic waste has significantly decreased. Therefore, we have used a lower level than today’s contamination both in solid and liquid waste in this study. This lower level of pollutants is estimated to be reached within 10 years. This estimation for each pollutant was done considering current trends and possibilities for substituting toxic substances, product development and better source separation of waste. In this paper, the application of the simulation tool ORWARE (ORganic WAste REsearch) in five scenarios for waste handling in a medium-sized hypothetical Swedish city is presented. The ORWARE model itself is presented in a companion paper.
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2. Description of the system
2.1. The ORWARE model The inputs to the system are waste flows (from kitchens, industries, etc.) and energy (fuel and electricity). Within the system there are different waste treatment processes and corresponding transports. The outputs are emissions (to air, water, and soil), energy (biogas, electricity and heat) and residues containing plant nutrients. By considering both liquid and solid organic waste in the ORWARE model, we can evaluate different possibilities for recycling plant nutrients from urban to rural areas. The following handling and treatment processes are included in the model: collection and transportation by trucks, transport in sewers, a sewage treatment process, incineration, landfilling, composting, anaerobic digestion, and application of residual products on farm land. A list of variables, a vector, describing waste, product flows, and emissions was defined (Table 1).
2.2. System boundaries The term ‘System boundaries’ not only includes the geographical borders but also what processes etc. are taken into account in the analysis. The choice of system boundaries is very important and has to be made in relation to the aim of the analysis. The system boundaries influence the results of the simulations and the interpretation of the results. The choice of system boundaries also has a large impact on the amount of work needed for data collection. The objective of the project was to study the handling of the organic waste in a densely populated area. The area chosen for the study was the city of Uppsala, 70 km north of Stockholm, with 140 000 inhabitants. However, the scenarios are not specific for Uppsala. We have chosen to omit some very ‘Uppsala-specific’ waste sources. Table 1 The variable vector, describing waste, products and emissions Total carbon Stable carbohydrates Semi-stable carbohydrates Rapidly degradable carbohydrates Fat
Dry matter (DM) CO2 of biological origin CO2 of fossil origin
PCB PAH
N2O NO− 3
Cu Cr
Phenols
Total Sulphur
Ni
CO
Total oxygen
SOX
Zn
CH4
Total hydrogen
Cl
Hg
P K
Cd Particles
Protein BOD
Organic volatiles H2O Halogenated volatiles Total nitrogen
COD
Halogenated organic compounds Dioxins
Volatile solids (VS)
NH3/NH+ 4
Ca
NOX
Pb
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Table 2 Amount and dry matter content of the waste from the different sources Waste source
Solid Households, Single family house Households, Apartments Gardens, Single family house Gardens, Apartments Parks Restaurants and caterers Trade Large-scale bakeries Liquid Grease water Wastewater (total of which is urine)
Amount (ton/year, wet weight)
Dry matter content (% of wet weight)
2400 7240 290 280 1900 2100 610 86
35 35 70 70 60 30 14 55
3000 22×106 (5×104)
2 10×10×−3 (3)
All scenarios have one common function, the handling of a certain amount of organic waste. They have neither the same energy conversions nor the same amount of plant nutrients brought back to arable land. Only direct emissions from the system are included. The emissions from constructing the necessary infrastructure are not included in the model. For truck transports, only the exhaust emissions are included in the study. The emissions from producing heat and electricity needed in the scenarios with net heat and electricity consumption are not included in the model. The model covers the transports necessary to collect the waste, to take it to the treatment plant to process it, and then to transport and spread the residual product on farm land or take it to a landfill, whichever is applicable. We have not included any effects that will occur after the sludge or compost has been applied to arable land.
2.3. Amount of waste The amount of waste from different sources considered in this case study are presented in Table 2. All waste fractions in the model are presented in detail by Sonesson and Jo¨nsson [1], except for the wastewater, which is presented in detail by Nybrant et al. [2].
2.4. Description of scenarios With the ORWARE model we have studied five different future scenarios for the handling of organic waste. In all scenarios all sewage sludge was land applied. All biogas produced was used in an internal combustion engine to produce electricity and heat. In all scenarios the produced heat was delivered to a district heating
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system. All scenarios except No. 4 contained a windrow composting facility for park waste. In scenario 1, the organic solid waste was incinerated. The wastewater was treated conventionally in the sewage plant. The grease water was landfilled, the low DM content precludes incineration. In scenario 2, the organic solid waste was landfilled. The wastewater was treated conventionally in the sewage plant. In scenario 3, municipal source-separated solid organic waste and grease water was fermented in an anaerobic digester for production of biogas. The wastewater was treated conventionally. The biogas residual sludge and the compost from park waste was spread on arable land. Scenario 4 describes composting of source-separated organic waste in a reactor compost with exhaust gas purification. This implies an addition of drier, carbonrich material, such as straw. The wastewater was treated conventionally. The mature compost was applied on arable land. Scenario 5 was a variation of scenario 1, where the only difference was in the wastewater treatment; the toilets were urine-separation toilets, using only 0.2 litre water per urine flushing. The stored urine was used as nutrients on arable land without processing and the faeces fraction was treated in the sewage plant.
3. Classification and characterisation of output vectors The emissions from a truck or incineration plant arise within some seconds after the waste has entered the process and the emissions from compost are extended for several months. However, the ORWARE model deals with averages over a year, which implies that all emissions, except those arising from the landfill, can be considered to be instantaneous and thus may be compared. The emissions from the landfill are dealt with as follows: The average yearly emissions that will occur for approximately 100 years, are compared directly with all other emissions as described above; the material that is left in the landfill after this surveyable time is considered as emissions that will occur after that time, during the remaining time, either to air or to water. This way of looking at the landfill can be considered as a worst case for the material left in the landfill after the surveyable time. For evaluating our emission output vectors we have used LCA methodology, effect categories, for adding the scenarios’ total contribution to environmental loads. It is generally suggested to use Global Warming Potentials (GWPs), expressed as CO2-equivalents, given for a time horizon of 100 years [3]. For evaluating the effect on human health, we have used the CML provisional method [3,4]. We have calculated the addition to acidification impact, by the aggregation of acid emissions to air, using a ‘protons released’ approach [3]. For the eutrophication we have calculated the maximum case, i.e., that both nitrogen and phosphorus contribute to the eutrophication effect, using an approach suggested by Finnveden et al. [5]. In our evaluation of the photo-oxidant formation, we have weighted four compounds as ethene-equivalents. NOX may also contribute to the photo-oxidant
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formation, but it cannot be compared with the other substances mentioned above. Hence, we also present the photo-oxidant formation from NOX on its own. Ozone depletion gases are, for example, CFCs, VOC and nitrous oxide. The CFC flow from our organic waste handling system has been assumed to be negligible, and we have not found any satisfactory weighting factors for ozone depletion from the other substances [5]. Therefore, we did not consider depletion of stratospheric ozone. The ecotoxicological effects were not considered.
4. Simulation results Since one goal of the study was to compare different handling systems for organic waste, the results (Figs. 1–10) are presented with relative values, where one is the average for all scenarios. The values are only comparable within one diagram. The actual figure is mentioned in the captions. The energy diagram, Fig. 1, shows the net energy balance, heat produced above the x-axis, and the consumed diesel oil and electricity below the x-axis. These energy forms have different qualities. The incineration scenarios produce the largest net gain of heat and the composting scenario produces the smallest energy conversion. In the anaerobic digestion scenario the process needed extra heat to hygienise some incoming waste fractions. The somewhat higher oil consumption in the urine separation scenario was due to the long transport distances for transporting the urine, low in DM but rich in plant nutrients, to arable land. The emissions of substances with global warming potential, Fig. 2, are lowest for biological treatment of organic waste. These figures also include emissions from the use of diesel oil for transports. The large amount of greenhouse gases emitted in the landfill scenario is due to the landfill process, where organic waste degrades and emits methane. The landfill in the model has a system for gas collection. However,
Fig. 1. The energy conversions in the five scenarios (relative values, 1 equals 21×107 MJ).
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Fig. 2. The global warming effect in the different scenarios (relative values, 1 equals 3.4 ×103 kg CO2 equivalents),
we estimated that approximately 40% of the methane produced during the surveyable time escaped, and thus became an air emission with a global warming potential. The health effects from handling organic waste originate mainly from air emissions, the contributions from soil and water pollution are negligible in comparison. Hoffmann et al. [6], also reached the same conclusion. The health effects from air emissions, Fig. 3, are strongly influenced by emissions from the sewage plant, but these are almost the same in all scenarios. Air emissions from the anaerobic digester consist mainly of NOX from the production of electricity and heat in an
Fig. 3. The health effect from air pollution (relative values, 1 equals 4.7×104 kg contaminated bodyweight).
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Fig. 4. The acidification effect in the minimum case, excl. nitrogen compounds (relative values, 1 equals 68 kMol H + equivalents).
internal combustion engine. The urine separation scenario also gives a higher score compared with the incineration scenario; the difference lies in the transport of the nutrients. This diagram indicates that composting is a method of treating organic waste with a lower risk of causing health effects. It must be observed that we have not considered the risk of health hazards from fungus formation and corresponding aerosols and that the working environment may be hazardous. This aspect is outside the scope of this study. Acidification effects from emission not only depend on the emitted substances but also on the receiving soil. Nitrogen substances such as NOX and NH3 only cause acidification effects in some areas. Therefore, we have calculated a minimum and a maximum case, with varying acidification contributions from nitrogen compounds. The acidification effect in the minimum case, Fig. 4, is most severe when incinerating organic waste, due to the release of hydrogen chloride and sulphur oxides. Urine separation increases the contribution, due to the increase in transport. In the maximum case, the differences between scenarios are small. For the eutrophication effect, Fig. 5, we consider a maximum case where both nitrogen and phosphorus are potential eutrophication agents. The greatest contribution to the eutrophication effect comes from the sewage plant. A scenario with a sewage plant combined with landfilling of the solid organic waste is the most severe concerning eutrophication. The effect was the lowest in the urine separation scenario, because the sewage plant received much less nitrogen in this case. The landfill contributed more to the eutrophication effect than the biological methods for solid organic waste, as did the incineration. Both NOX and volatile organic compounds result in the formation of photochemical oxidants, although they are not comparable. The photochemical oxidant effect from organic volatiles, Fig. 6, is highest in the landfill scenario, due to the methane releases. The anaerobic digestion had some emissions of methane. The sewage plant gave a contribution that was similar for all scenarios.
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Fig. 5. The eutrophication effect in the different scenarios, maximum case (relative values, 1 equals 4.5 × 104 kg O2 equivalents).
The level of photochemical oxidants from NOX, Fig. 7, is highest in the anaerobic digestion scenario and lowest in the composting scenario. The urine separation increases the emissions of NOX due to increased transportation. The emission of NOX is closely related to the conversion of energy, since the formation of NOX originates primarily from combustion. Recirculation of nitrogen to arable land, Fig. 8, is a ratio between the nitrogen spread on arable land and the nitrogen in the waste entering the system. The greatest difference is between urine separation and other systems. The recirculation of nitrogen is highest for urine separation, because urine is rich in nitrogen, and the sewage plant is not capable of capturing the major part of the nitrogen. Approxi-
Fig. 6. The release of organic photochemical oxidants from organic compounds (relative values, 1 equals 3.6 ×103 kg ethene equivalents).
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Fig. 7. The release of the photochemical oxidant NOX (relative values, 1 equals 5.6×104 kg NOX).
mately 50% of the incoming nitrogen is released as water emission, even when the sewage plant is equipped with nitrogen reduction. The recirculation of phosphorus to arable land, Fig. 9, is highest in the composting and anaerobic digestion scenarios. The return of phosphorus is high in all scenarios, since phosphorus is effectively captured in the sewage sludge and returned as fertiliser to arable land. Figs. 1 – 9 are summarised in Fig. 10. A scenario with incineration of solid waste and a regular treatment of the sewage in a sewage plant results in medium-sized environmental effects, except for acidification (minimum case) where the contribu-
Fig. 8. The recirculation of nitrogen (part of the nitrogen found in the waste that is returned to arable land).
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Fig. 9. The recirculation of phosphorous (part of the phosphorous found in the waste that is returned to arable land).
tion is relatively high, compared with the other scenarios. The recirculation of nitrogen is low in the scenario but the yield of heat is high. Landfilling of the solid waste increases the global warming effect, the emissions of organic compounds forming photochemical oxidants and the eutrophication effect. Choosing anaerobic digestion as treatment for the solid waste instead of incineration, gives a lower
Fig. 10. Comparison of the total environmental effects for each scenario and category (1 equals the average value for each effect category).
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contribution to the effect of global warming and eutrophication and a somewhat higher contribution of photochemical oxidants and the effect on human health. The composting scenario results in lower environmental effects for all categories in comparison with the incineration scenario. Both anaerobic digestion and composting resulted in a somewhat higher return of nitrogen and phosphorus. The anaerobic scenario had a lower yield of heat but produces more electricity. The composting scenario consumes heat and electricity. Source-separation of the urine and handling this fraction separately from the rest of the sewage, together with incineration of the solid waste resulted in a higher contribution to the global warming effect, the photochemical oxidant formation, and the health effect, but it also gives a lower contribution to the eutrophication effect. The greatest advantage of introducing source-separation of urine is the increased recirculation of nitrogen.
5. Discussion The ORWARE model describes the flow of material and energy. The outputs are emissions to air and water as well as residues to arable land and energy conversions. We have not considered, for example, the economical, sociological and working environmental aspects. The project group has become increasingly convinced of the versatility of the approach used as a tool to investigate and discuss various aspects of organic waste management. At the same time, a number of general problems with this type of analysis have become apparent during the work. The most important of these problems are: lack of reliable input data difficulties in defining and applying appropriate system boundaries selection of appropriate scenarios evaluation problems During the inventory phase of the project, information had been sought from a large number of sources such as data from published research reports, informal reports, and as information from people with practical experience. A general result from this inventory phase is that it has been difficult to find quantitative information of the type needed. Perhaps the most difficult problem is the overall evaluation of the different scenarios. The environmental impact has been evaluated in the effect diagrams according to methods developed for Life Cycle Assessments, Figs. 1–10. The intrinsic difficulties with such evaluations have been subjected to lively discussion. An example is how to deal with the problem of scenarios producing different amounts of utilities. We have considered it not appropriate to weight the different environmental effects to one general environmental load unit. When assessing the potential environmental load from waste handling systems, local aspects must often be taken into consideration. For example, the local environmental effects, such as eutrophication and the formation of photochemical oxidants, are more severe in some regions than others.
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Other results from the simulations are the amount of recirculated nutrients and energy conversions. These results have to be considered together with the environmental effects. Concerning energy, it is also important to evaluate the quality of the energy. In this study, we have chosen electricity, heat and oil as basic parameters by including the conversion of the biogas from the landfill, anaerobic digestion and sewage plant, to electricity and heat in stationary engines. One possible way to solve the problems with the recirculation of nutrients and energy exchange is to expand the studied system from just processing a certain amount of organic waste to also produce a specific amount of nutrients, heat and electricity. Production of the fertiliser-replacing nutrients in the waste that is not recycled in some scenarios can then be included in the model. The emissions from manufacturing the fertiliser can be added to the other emissions. The energy difference between the scenarios can be handled in a similar way. It is very important, though, to consider which fuel to chose for producing the extra heat and electricity needed to make the scenarios produce the same amount of heat and electricity. For example, choosing between oil or wood for production of heat has a large impact on the global warming effect. With these considerations the evaluation of the scenarios is reduced to the comparison of the environmental effect categories only [2]. Studies which have presented models that have a scope and performance similar to ORWARE can be seen in the literature [7–9]. The strengths of the ORWARE model, compared with these other models is the possibility to analyse systems with a high degree of plant nutrient recycling, its extensive vector for flow characterisation which facilitates thorough analysis of environmental effects, and the inclusion of liquid waste.
5.1. The incineration scenario The slag from the incineration contains plant nutrients, e.g., potassium and calcium, in concentrated form, but the nitrogen in the waste is lost to the atmosphere. Provided that the source-separated and unpolluted solid organic waste could be incinerated separately or together with other unpolluted big-fuels, it should be possible to recycle many plant nutrients by using the slag, not the fly ash due to high levels of toxic compounds, as a fertiliser. Thus, where the amount of available unpolluted waste is large and the haulage to farmland is over a considerable distance, incineration might have advantages. It is difficult to incinerate very wet waste, like grease water (2% DM). These wet wastes are often, as in scenarios 1 and 5, deposited in a landfill. In the landfill, these wastes result in emissions to air and water. Thus, even where incineration is the base of the waste handling system, it is very important to find alternatives to landfilling for very wet organic waste fractions.
5.2. The landfill scenario Biodegradable waste produces large quantities of methane when anaerobically degraded as in the landfill. In the model, we assume that 40% leaks into the
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atmosphere. This methane generates an important global warming effect, Fig. 2, and a large photochemical oxidant effect, Fig. 6. Wet waste increases the production of leachate water. The leachate water contains plant nutrients and results in a eutrophication effect, Fig. 5. In future studies, it would be interesting to have a scenario where the leachate is treated with a purification process within the landfill model. The results clearly illustrate that in a landfill, the resources contained in organic waste, plant nutrients and part of the energy, will be bound for a long time, and lost from productive use and instead produce emissions to the environment. With landfilling the source-separated organic waste in small cells, the gas can be collected under a more concentrated time period thus increasing the collected yield of landfill gas. Furthermore, organic waste disposed of in a landfill increases the risk of disturbances like fires and ruptures in the top seal layer due to gas produced and waste settling. Disturbances like these, which were not modelled, combined with the large storage of both toxic substances and plant nutrients contained in the landfill, mean that the landfill is associated with a certain risk of large and uncontrolled emissions.
5.3. The anaerobic digestion scenario Biogas and mineralised, plant-available, nutrients are produced when a substrate is anaerobically digested. In all scenarios, the biogas produced is used in an internal combustion engine to produce heat and electricity. The resulting NOX emissions are a main part of the contribution to the health effect, Fig. 3, the acidification effect, Fig. 4, and the photochemical effect, Fig. 7. If used to produce only heat, or if the engine was equipped with a catalytic exhaust gas cleaning system, the emissions would be lower, which could be beneficial for the anaerobic digestion scenario. Biogas may also be used as vehicle fuel. This would increase the NOX emissions in comparison with producing heat and electricity, but it would decrease them in relation to using diesel as fuel. In all scenarios the produced amount of biogas suffices to replace the diesel used.
5.4. The composting scenario The composting scenario combines a high recirculation of plant nutrients, Figs. 8 and 9, with little need for transportation. The amount of plant nitrogen recycled to arable land is almost the same as in the anaerobic digestion scenario. Nitrogen is often largely lost as ammonia from the composting process. However, in the composting model, ammonia is recovered in a biofilter, which is recycled to arable land after use. The dry matter content of the finished compost is high, thus fairly little transportation is needed for the mature compost.
5.5. The urine source separation scenario The sewage contains the major part of the plant nutrients entering the system. The electricity consumption of the sewage pipe transport and of sewage plant
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treatment is small per ton of sewage. However, due to the very large amount of sewage, these processes are the major consumers of electricity in the organic waste-handling system; these processes together use more than 90% of the total electricity used by the system. Human urine is the fraction of the urban organic waste, containing by far the largest amount of plant nutrients [10]. Thus, urine separation toilets in combination with storage tanks and spreading on farm land significantly increase the recirculation of plant nutrients, Figs. 8 and 9. The large amount of nutrients to be recirculated increases the transport work. This increase in transport work naturally leads to an increase in the amount of diesel fuel used and therefore in environmental effects induced by air emission, for example the acidification effect, Fig. 4. Urine separation decreases the plant nutrient load on, and emission from, the sewage plant. This decreases the eutrophication effect, Fig. 5.
6. Conclusions The main goal was to test the ORWARE model, to discover its applicability, strengths and weaknesses. We believe that the model has been demonstrated to be versatile and that it produces useful information about a complex system; information that can hardly be gained in any other way than using systems analysis, using modelling and simulation experiments. The lack of accurate data makes it difficult to use the full potential of ORWARE. Economics is not yet included, which is a drawback of ORWARE compared with the example of MIMES/WASTE [9]. Concerning the sub-goals, we could conclude that source separation and biological treatment are beneficial concerning the recycling of phosphorus, and to some extent nitrogen, compared with today’s system. Composting has the poorest energy conversions. Anaerobic digestion has a higher energy ratio than composting but a lower ratio than incineration. Part of the energy from anaerobic digestion is converted to electricity, while all the energy from the incineration plant was heat. The environmental effects, in contrast, indicated that composting was the best alternative, without considering working health hazards. The second sub-goal was to evaluate the proportion between solid waste and wastewater, concerning plant nutrients. It was clear that the major part of plant nutrients was found in the wastewater. This implies that it is of great importance that wastewater is included in a study that aims at evaluating systems with a high degree of plant nutrient recycling. The reason for this is that the distances for these transports to arable land depend on the total amount of plant nutrients to be recycled. Since the wastewater contains the major part, it will be dimensioning for transport distances for all waste to be recycled, including solid organic waste. The third sub-goal was studied in scenario 5, where the only change from the incineration scenario was the implementation of urine-separating toilets. The results showed that this measure was the only one studied that gave a high recycling ratio for nitrogen. This is explained by the fact that a major part of all nitrogen leaving the urban society is found in human urine. Concerning environmental effects, it was
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only the eutrophication effect that was positively influenced by urine separation. The sewage plant can only remove approximately 50% of the incoming nitrogen. The energy consumption increased somewhat, and shifted to the increased use of oil and the decreased use of electricity. This is explained by the increased transportation needs.
Acknowledgements We wish to thank our supervisors and colleagues, Prof. Thomas Nybrant and Prof. Bjorn Frostell, as well as all other persons who have been involved in developing the ORWARE model. We also want to express our gratitude to the Swedish Waste Research Council (AFR) for financially supporting this research project.
References [1] Sonesson U, Jo¨nsson H. Urban Biodegradable Waste Amount and Composition: Case Study Uppsala, Report 201, Department of Agricultural Engineering, SLU, Uppsala, Sweden, 1996. [2] Nybrant T, Jo¨nsson H, Frostell B, Sundqvist JO, Thyselius L, Dalemo M, Mingarini K, Sonesson U. System Analysis of Organic Waste: The ORWARE model, a Case Study, AFR-report 109, Swedish Environmental Protection Agency, Stockholm, Sweden, 1996. [3] LCA Nordic. Technical Reports No. 10 and Special Reports No. 1 – 2. Stockholm, Sweden: Tema Nord 1995;503. [4] Heijungs R, Guine´e JB, Huppes G, Lankreier RM, Udo de Haes HA, Wegener Sleeswijk A, Ansems AMM, Eggels PG, van Duin R, de Goede HP. Environmental Life Cycle Assessments of products, Guide and Backgrounds, CML Leiden University, Leiden, The Netherlands, 1992. [5] Finnveden G, Andersson-Sko¨ld Y, Samuelsson MO, Zetterberg L, Lindfors LG. Classification (Impact analysis) in Connection with Life Cycle Assessments – A Preliminary Study. In: Product Life Cycle Assessment – Principles and Methodology. Stockholm, Sweden: Nord (Nordic Council of Ministers), 1992;9:172–231. [6] Hoffmann L, Weidema BP, Christiansen K. Methodological aspects of life cycle screening of biological treatment of source separated household waste. In: Proceedings of the First International Symposium – Biological Waste Management ‘‘A Wasted Chance’’. University of Essen, Technical University of Hamburg-Harburg, Germany, April 1995;Suppl 10:1 – 8. [7] Gupta B, Shepherd P, editors. Data Summary of Municipal Solid Waste Management Alternatives, National Renewable Laboratory, Golden, Colorado, USA, 1992. [8] White PR, Franke M, Hindle P. Integrated Solid Waste Management, A Life Cycle Inventory. UK: Chapman and Hall, 1995. [9] Sundberg J. Generic Modelling of Integrated Material Flows and Energy Systems. Ph.D Thesis, Chalmers University of Technology, Gothenburg, Sweden, 1993. [10] Jo¨nsson H, Olsson A, Kirchmann H, Pettersson S. Kallseparerad humanurin (Source separated human urine, in Swedish), SLU Info rapporter, allma¨nt 187, SLU, Uppsala, Sweden, 1995.
ORWARE – A simulation model for organic waste handling systems. Part 2: Case study and simulation results U. Sonesson a,*, M. Dalemo b, K. Mingarini c, H. Jo¨nsson a a
Department of Agricultural Engineering, Swedish Uni6ersity of Agricultural Sciences, P.O. Box 7033, S-75007 Uppsala, Sweden b Swedish Institute of Agricultural Engineering, P.O. Box 7033, S-75007 Uppsala, Sweden c Department of En6ironmental Engineering and Work Science, Royal Institute of Technology, S-10044 Stockholm, Sweden Received 15 April 1997; received in revised form 28 May 1997; accepted 3 June 1997
Abstract Results from simulations with the ORWARE model (ORganic WAste REsearch) are presented. The model was tested on a medium-sized Swedish city. The scenarios were planned to illustrate the consequences of different waste handling systems. The modelled processes for organic solid waste were; incineration, landfilling, anaerobic digestion and composting, for the wastewater they were sewage plant and source separation of urine. Each transport and treatment facility was modelled with respect to incoming waste. Each process model generates an energy balance, liquid and/or gaseous emissions and residual products as outputs. The simulation results show that source separation of solid waste, followed by biological treatment, is beneficial with respect to the recycling of phosphorus and environmental effects. The negative results for these systems are their energy balances. Source-separating human urine seems to be the only way to get a high degree of nitrogen recycling. The results also stress the importance of including liquid waste to get an overall picture of the transports needed to get the residues to arable land. Landfilling organic waste generates the largest negative environmental effect. A model description is presented in a companion paper. © 1997 Elsevier Science B.V.
* Corresponding author. 0921-3449/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 3 4 4 9 ( 9 7 ) 0 0 0 2 1 - 9
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Keywords: Systems analysis; Modelling; Organic waste; Environmental impact; Plant nutrient recycling; Integrated waste management
1. Introduction Organic waste is a kind of waste that is not likely to be reduced by technical and political measures, as can be done with other types of waste. The organic waste handling systems have developed rather arbitrarily as a result of both actual and historical needs and technical innovations. As a result, very little has been done so far regarding the co-ordination of different waste management aspects. In this study, organic waste has been defined as waste originating from living organisms that are essentially free from synthetic components and for which the present recovery and recirculation systems can be significantly improved. The present study had several objectives. The objectives were: 1. To make a system comparison between an increased and improved source-separation of solid waste, followed by biological treatment, composting or anaerobic digestion, on the one hand, and today’s system, incineration or landfilling, on the other. 2. To find out the distribution of plant nutrient content between the different wastes, especially solid vs. liquid waste, and its consequences. 3. To evaluate what the effects would be if urine-separating toilets were introduced as a means of recycling more of the plant nutrients contained in wastewater. A system approach is necessary to analyse the complicated way organic wastes are collected, treated and disposed. A large number of materials have been considered in the investigated scenarios. Solid, liquid, and gaseous material flows have been included in the analysis. An increased recycling of waste containing high levels of contaminants will increase the environmental load to soil. We assume that a prerequisite for the increased recycling of plant nutrients is that the pollution level of the urban organic waste has significantly decreased. Therefore, we have used a lower level than today’s contamination both in solid and liquid waste in this study. This lower level of pollutants is estimated to be reached within 10 years. This estimation for each pollutant was done considering current trends and possibilities for substituting toxic substances, product development and better source separation of waste. In this paper, the application of the simulation tool ORWARE (ORganic WAste REsearch) in five scenarios for waste handling in a medium-sized hypothetical Swedish city is presented. The ORWARE model itself is presented in a companion paper.
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2. Description of the system
2.1. The ORWARE model The inputs to the system are waste flows (from kitchens, industries, etc.) and energy (fuel and electricity). Within the system there are different waste treatment processes and corresponding transports. The outputs are emissions (to air, water, and soil), energy (biogas, electricity and heat) and residues containing plant nutrients. By considering both liquid and solid organic waste in the ORWARE model, we can evaluate different possibilities for recycling plant nutrients from urban to rural areas. The following handling and treatment processes are included in the model: collection and transportation by trucks, transport in sewers, a sewage treatment process, incineration, landfilling, composting, anaerobic digestion, and application of residual products on farm land. A list of variables, a vector, describing waste, product flows, and emissions was defined (Table 1).
2.2. System boundaries The term ‘System boundaries’ not only includes the geographical borders but also what processes etc. are taken into account in the analysis. The choice of system boundaries is very important and has to be made in relation to the aim of the analysis. The system boundaries influence the results of the simulations and the interpretation of the results. The choice of system boundaries also has a large impact on the amount of work needed for data collection. The objective of the project was to study the handling of the organic waste in a densely populated area. The area chosen for the study was the city of Uppsala, 70 km north of Stockholm, with 140 000 inhabitants. However, the scenarios are not specific for Uppsala. We have chosen to omit some very ‘Uppsala-specific’ waste sources. Table 1 The variable vector, describing waste, products and emissions Total carbon Stable carbohydrates Semi-stable carbohydrates Rapidly degradable carbohydrates Fat
Dry matter (DM) CO2 of biological origin CO2 of fossil origin
PCB PAH
N2O NO− 3
Cu Cr
Phenols
Total Sulphur
Ni
CO
Total oxygen
SOX
Zn
CH4
Total hydrogen
Cl
Hg
P K
Cd Particles
Protein BOD
Organic volatiles H2O Halogenated volatiles Total nitrogen
COD
Halogenated organic compounds Dioxins
Volatile solids (VS)
NH3/NH+ 4
Ca
NOX
Pb
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Table 2 Amount and dry matter content of the waste from the different sources Waste source
Solid Households, Single family house Households, Apartments Gardens, Single family house Gardens, Apartments Parks Restaurants and caterers Trade Large-scale bakeries Liquid Grease water Wastewater (total of which is urine)
Amount (ton/year, wet weight)
Dry matter content (% of wet weight)
2400 7240 290 280 1900 2100 610 86
35 35 70 70 60 30 14 55
3000 22×106 (5×104)
2 10×10×−3 (3)
All scenarios have one common function, the handling of a certain amount of organic waste. They have neither the same energy conversions nor the same amount of plant nutrients brought back to arable land. Only direct emissions from the system are included. The emissions from constructing the necessary infrastructure are not included in the model. For truck transports, only the exhaust emissions are included in the study. The emissions from producing heat and electricity needed in the scenarios with net heat and electricity consumption are not included in the model. The model covers the transports necessary to collect the waste, to take it to the treatment plant to process it, and then to transport and spread the residual product on farm land or take it to a landfill, whichever is applicable. We have not included any effects that will occur after the sludge or compost has been applied to arable land.
2.3. Amount of waste The amount of waste from different sources considered in this case study are presented in Table 2. All waste fractions in the model are presented in detail by Sonesson and Jo¨nsson [1], except for the wastewater, which is presented in detail by Nybrant et al. [2].
2.4. Description of scenarios With the ORWARE model we have studied five different future scenarios for the handling of organic waste. In all scenarios all sewage sludge was land applied. All biogas produced was used in an internal combustion engine to produce electricity and heat. In all scenarios the produced heat was delivered to a district heating
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system. All scenarios except No. 4 contained a windrow composting facility for park waste. In scenario 1, the organic solid waste was incinerated. The wastewater was treated conventionally in the sewage plant. The grease water was landfilled, the low DM content precludes incineration. In scenario 2, the organic solid waste was landfilled. The wastewater was treated conventionally in the sewage plant. In scenario 3, municipal source-separated solid organic waste and grease water was fermented in an anaerobic digester for production of biogas. The wastewater was treated conventionally. The biogas residual sludge and the compost from park waste was spread on arable land. Scenario 4 describes composting of source-separated organic waste in a reactor compost with exhaust gas purification. This implies an addition of drier, carbonrich material, such as straw. The wastewater was treated conventionally. The mature compost was applied on arable land. Scenario 5 was a variation of scenario 1, where the only difference was in the wastewater treatment; the toilets were urine-separation toilets, using only 0.2 litre water per urine flushing. The stored urine was used as nutrients on arable land without processing and the faeces fraction was treated in the sewage plant.
3. Classification and characterisation of output vectors The emissions from a truck or incineration plant arise within some seconds after the waste has entered the process and the emissions from compost are extended for several months. However, the ORWARE model deals with averages over a year, which implies that all emissions, except those arising from the landfill, can be considered to be instantaneous and thus may be compared. The emissions from the landfill are dealt with as follows: The average yearly emissions that will occur for approximately 100 years, are compared directly with all other emissions as described above; the material that is left in the landfill after this surveyable time is considered as emissions that will occur after that time, during the remaining time, either to air or to water. This way of looking at the landfill can be considered as a worst case for the material left in the landfill after the surveyable time. For evaluating our emission output vectors we have used LCA methodology, effect categories, for adding the scenarios’ total contribution to environmental loads. It is generally suggested to use Global Warming Potentials (GWPs), expressed as CO2-equivalents, given for a time horizon of 100 years [3]. For evaluating the effect on human health, we have used the CML provisional method [3,4]. We have calculated the addition to acidification impact, by the aggregation of acid emissions to air, using a ‘protons released’ approach [3]. For the eutrophication we have calculated the maximum case, i.e., that both nitrogen and phosphorus contribute to the eutrophication effect, using an approach suggested by Finnveden et al. [5]. In our evaluation of the photo-oxidant formation, we have weighted four compounds as ethene-equivalents. NOX may also contribute to the photo-oxidant
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formation, but it cannot be compared with the other substances mentioned above. Hence, we also present the photo-oxidant formation from NOX on its own. Ozone depletion gases are, for example, CFCs, VOC and nitrous oxide. The CFC flow from our organic waste handling system has been assumed to be negligible, and we have not found any satisfactory weighting factors for ozone depletion from the other substances [5]. Therefore, we did not consider depletion of stratospheric ozone. The ecotoxicological effects were not considered.
4. Simulation results Since one goal of the study was to compare different handling systems for organic waste, the results (Figs. 1–10) are presented with relative values, where one is the average for all scenarios. The values are only comparable within one diagram. The actual figure is mentioned in the captions. The energy diagram, Fig. 1, shows the net energy balance, heat produced above the x-axis, and the consumed diesel oil and electricity below the x-axis. These energy forms have different qualities. The incineration scenarios produce the largest net gain of heat and the composting scenario produces the smallest energy conversion. In the anaerobic digestion scenario the process needed extra heat to hygienise some incoming waste fractions. The somewhat higher oil consumption in the urine separation scenario was due to the long transport distances for transporting the urine, low in DM but rich in plant nutrients, to arable land. The emissions of substances with global warming potential, Fig. 2, are lowest for biological treatment of organic waste. These figures also include emissions from the use of diesel oil for transports. The large amount of greenhouse gases emitted in the landfill scenario is due to the landfill process, where organic waste degrades and emits methane. The landfill in the model has a system for gas collection. However,
Fig. 1. The energy conversions in the five scenarios (relative values, 1 equals 21×107 MJ).
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Fig. 2. The global warming effect in the different scenarios (relative values, 1 equals 3.4 ×103 kg CO2 equivalents),
we estimated that approximately 40% of the methane produced during the surveyable time escaped, and thus became an air emission with a global warming potential. The health effects from handling organic waste originate mainly from air emissions, the contributions from soil and water pollution are negligible in comparison. Hoffmann et al. [6], also reached the same conclusion. The health effects from air emissions, Fig. 3, are strongly influenced by emissions from the sewage plant, but these are almost the same in all scenarios. Air emissions from the anaerobic digester consist mainly of NOX from the production of electricity and heat in an
Fig. 3. The health effect from air pollution (relative values, 1 equals 4.7×104 kg contaminated bodyweight).
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Fig. 4. The acidification effect in the minimum case, excl. nitrogen compounds (relative values, 1 equals 68 kMol H + equivalents).
internal combustion engine. The urine separation scenario also gives a higher score compared with the incineration scenario; the difference lies in the transport of the nutrients. This diagram indicates that composting is a method of treating organic waste with a lower risk of causing health effects. It must be observed that we have not considered the risk of health hazards from fungus formation and corresponding aerosols and that the working environment may be hazardous. This aspect is outside the scope of this study. Acidification effects from emission not only depend on the emitted substances but also on the receiving soil. Nitrogen substances such as NOX and NH3 only cause acidification effects in some areas. Therefore, we have calculated a minimum and a maximum case, with varying acidification contributions from nitrogen compounds. The acidification effect in the minimum case, Fig. 4, is most severe when incinerating organic waste, due to the release of hydrogen chloride and sulphur oxides. Urine separation increases the contribution, due to the increase in transport. In the maximum case, the differences between scenarios are small. For the eutrophication effect, Fig. 5, we consider a maximum case where both nitrogen and phosphorus are potential eutrophication agents. The greatest contribution to the eutrophication effect comes from the sewage plant. A scenario with a sewage plant combined with landfilling of the solid organic waste is the most severe concerning eutrophication. The effect was the lowest in the urine separation scenario, because the sewage plant received much less nitrogen in this case. The landfill contributed more to the eutrophication effect than the biological methods for solid organic waste, as did the incineration. Both NOX and volatile organic compounds result in the formation of photochemical oxidants, although they are not comparable. The photochemical oxidant effect from organic volatiles, Fig. 6, is highest in the landfill scenario, due to the methane releases. The anaerobic digestion had some emissions of methane. The sewage plant gave a contribution that was similar for all scenarios.
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Fig. 5. The eutrophication effect in the different scenarios, maximum case (relative values, 1 equals 4.5 × 104 kg O2 equivalents).
The level of photochemical oxidants from NOX, Fig. 7, is highest in the anaerobic digestion scenario and lowest in the composting scenario. The urine separation increases the emissions of NOX due to increased transportation. The emission of NOX is closely related to the conversion of energy, since the formation of NOX originates primarily from combustion. Recirculation of nitrogen to arable land, Fig. 8, is a ratio between the nitrogen spread on arable land and the nitrogen in the waste entering the system. The greatest difference is between urine separation and other systems. The recirculation of nitrogen is highest for urine separation, because urine is rich in nitrogen, and the sewage plant is not capable of capturing the major part of the nitrogen. Approxi-
Fig. 6. The release of organic photochemical oxidants from organic compounds (relative values, 1 equals 3.6 ×103 kg ethene equivalents).
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Fig. 7. The release of the photochemical oxidant NOX (relative values, 1 equals 5.6×104 kg NOX).
mately 50% of the incoming nitrogen is released as water emission, even when the sewage plant is equipped with nitrogen reduction. The recirculation of phosphorus to arable land, Fig. 9, is highest in the composting and anaerobic digestion scenarios. The return of phosphorus is high in all scenarios, since phosphorus is effectively captured in the sewage sludge and returned as fertiliser to arable land. Figs. 1 – 9 are summarised in Fig. 10. A scenario with incineration of solid waste and a regular treatment of the sewage in a sewage plant results in medium-sized environmental effects, except for acidification (minimum case) where the contribu-
Fig. 8. The recirculation of nitrogen (part of the nitrogen found in the waste that is returned to arable land).
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Fig. 9. The recirculation of phosphorous (part of the phosphorous found in the waste that is returned to arable land).
tion is relatively high, compared with the other scenarios. The recirculation of nitrogen is low in the scenario but the yield of heat is high. Landfilling of the solid waste increases the global warming effect, the emissions of organic compounds forming photochemical oxidants and the eutrophication effect. Choosing anaerobic digestion as treatment for the solid waste instead of incineration, gives a lower
Fig. 10. Comparison of the total environmental effects for each scenario and category (1 equals the average value for each effect category).
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contribution to the effect of global warming and eutrophication and a somewhat higher contribution of photochemical oxidants and the effect on human health. The composting scenario results in lower environmental effects for all categories in comparison with the incineration scenario. Both anaerobic digestion and composting resulted in a somewhat higher return of nitrogen and phosphorus. The anaerobic scenario had a lower yield of heat but produces more electricity. The composting scenario consumes heat and electricity. Source-separation of the urine and handling this fraction separately from the rest of the sewage, together with incineration of the solid waste resulted in a higher contribution to the global warming effect, the photochemical oxidant formation, and the health effect, but it also gives a lower contribution to the eutrophication effect. The greatest advantage of introducing source-separation of urine is the increased recirculation of nitrogen.
5. Discussion The ORWARE model describes the flow of material and energy. The outputs are emissions to air and water as well as residues to arable land and energy conversions. We have not considered, for example, the economical, sociological and working environmental aspects. The project group has become increasingly convinced of the versatility of the approach used as a tool to investigate and discuss various aspects of organic waste management. At the same time, a number of general problems with this type of analysis have become apparent during the work. The most important of these problems are: lack of reliable input data difficulties in defining and applying appropriate system boundaries selection of appropriate scenarios evaluation problems During the inventory phase of the project, information had been sought from a large number of sources such as data from published research reports, informal reports, and as information from people with practical experience. A general result from this inventory phase is that it has been difficult to find quantitative information of the type needed. Perhaps the most difficult problem is the overall evaluation of the different scenarios. The environmental impact has been evaluated in the effect diagrams according to methods developed for Life Cycle Assessments, Figs. 1–10. The intrinsic difficulties with such evaluations have been subjected to lively discussion. An example is how to deal with the problem of scenarios producing different amounts of utilities. We have considered it not appropriate to weight the different environmental effects to one general environmental load unit. When assessing the potential environmental load from waste handling systems, local aspects must often be taken into consideration. For example, the local environmental effects, such as eutrophication and the formation of photochemical oxidants, are more severe in some regions than others.
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Other results from the simulations are the amount of recirculated nutrients and energy conversions. These results have to be considered together with the environmental effects. Concerning energy, it is also important to evaluate the quality of the energy. In this study, we have chosen electricity, heat and oil as basic parameters by including the conversion of the biogas from the landfill, anaerobic digestion and sewage plant, to electricity and heat in stationary engines. One possible way to solve the problems with the recirculation of nutrients and energy exchange is to expand the studied system from just processing a certain amount of organic waste to also produce a specific amount of nutrients, heat and electricity. Production of the fertiliser-replacing nutrients in the waste that is not recycled in some scenarios can then be included in the model. The emissions from manufacturing the fertiliser can be added to the other emissions. The energy difference between the scenarios can be handled in a similar way. It is very important, though, to consider which fuel to chose for producing the extra heat and electricity needed to make the scenarios produce the same amount of heat and electricity. For example, choosing between oil or wood for production of heat has a large impact on the global warming effect. With these considerations the evaluation of the scenarios is reduced to the comparison of the environmental effect categories only [2]. Studies which have presented models that have a scope and performance similar to ORWARE can be seen in the literature [7–9]. The strengths of the ORWARE model, compared with these other models is the possibility to analyse systems with a high degree of plant nutrient recycling, its extensive vector for flow characterisation which facilitates thorough analysis of environmental effects, and the inclusion of liquid waste.
5.1. The incineration scenario The slag from the incineration contains plant nutrients, e.g., potassium and calcium, in concentrated form, but the nitrogen in the waste is lost to the atmosphere. Provided that the source-separated and unpolluted solid organic waste could be incinerated separately or together with other unpolluted big-fuels, it should be possible to recycle many plant nutrients by using the slag, not the fly ash due to high levels of toxic compounds, as a fertiliser. Thus, where the amount of available unpolluted waste is large and the haulage to farmland is over a considerable distance, incineration might have advantages. It is difficult to incinerate very wet waste, like grease water (2% DM). These wet wastes are often, as in scenarios 1 and 5, deposited in a landfill. In the landfill, these wastes result in emissions to air and water. Thus, even where incineration is the base of the waste handling system, it is very important to find alternatives to landfilling for very wet organic waste fractions.
5.2. The landfill scenario Biodegradable waste produces large quantities of methane when anaerobically degraded as in the landfill. In the model, we assume that 40% leaks into the
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atmosphere. This methane generates an important global warming effect, Fig. 2, and a large photochemical oxidant effect, Fig. 6. Wet waste increases the production of leachate water. The leachate water contains plant nutrients and results in a eutrophication effect, Fig. 5. In future studies, it would be interesting to have a scenario where the leachate is treated with a purification process within the landfill model. The results clearly illustrate that in a landfill, the resources contained in organic waste, plant nutrients and part of the energy, will be bound for a long time, and lost from productive use and instead produce emissions to the environment. With landfilling the source-separated organic waste in small cells, the gas can be collected under a more concentrated time period thus increasing the collected yield of landfill gas. Furthermore, organic waste disposed of in a landfill increases the risk of disturbances like fires and ruptures in the top seal layer due to gas produced and waste settling. Disturbances like these, which were not modelled, combined with the large storage of both toxic substances and plant nutrients contained in the landfill, mean that the landfill is associated with a certain risk of large and uncontrolled emissions.
5.3. The anaerobic digestion scenario Biogas and mineralised, plant-available, nutrients are produced when a substrate is anaerobically digested. In all scenarios, the biogas produced is used in an internal combustion engine to produce heat and electricity. The resulting NOX emissions are a main part of the contribution to the health effect, Fig. 3, the acidification effect, Fig. 4, and the photochemical effect, Fig. 7. If used to produce only heat, or if the engine was equipped with a catalytic exhaust gas cleaning system, the emissions would be lower, which could be beneficial for the anaerobic digestion scenario. Biogas may also be used as vehicle fuel. This would increase the NOX emissions in comparison with producing heat and electricity, but it would decrease them in relation to using diesel as fuel. In all scenarios the produced amount of biogas suffices to replace the diesel used.
5.4. The composting scenario The composting scenario combines a high recirculation of plant nutrients, Figs. 8 and 9, with little need for transportation. The amount of plant nitrogen recycled to arable land is almost the same as in the anaerobic digestion scenario. Nitrogen is often largely lost as ammonia from the composting process. However, in the composting model, ammonia is recovered in a biofilter, which is recycled to arable land after use. The dry matter content of the finished compost is high, thus fairly little transportation is needed for the mature compost.
5.5. The urine source separation scenario The sewage contains the major part of the plant nutrients entering the system. The electricity consumption of the sewage pipe transport and of sewage plant
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treatment is small per ton of sewage. However, due to the very large amount of sewage, these processes are the major consumers of electricity in the organic waste-handling system; these processes together use more than 90% of the total electricity used by the system. Human urine is the fraction of the urban organic waste, containing by far the largest amount of plant nutrients [10]. Thus, urine separation toilets in combination with storage tanks and spreading on farm land significantly increase the recirculation of plant nutrients, Figs. 8 and 9. The large amount of nutrients to be recirculated increases the transport work. This increase in transport work naturally leads to an increase in the amount of diesel fuel used and therefore in environmental effects induced by air emission, for example the acidification effect, Fig. 4. Urine separation decreases the plant nutrient load on, and emission from, the sewage plant. This decreases the eutrophication effect, Fig. 5.
6. Conclusions The main goal was to test the ORWARE model, to discover its applicability, strengths and weaknesses. We believe that the model has been demonstrated to be versatile and that it produces useful information about a complex system; information that can hardly be gained in any other way than using systems analysis, using modelling and simulation experiments. The lack of accurate data makes it difficult to use the full potential of ORWARE. Economics is not yet included, which is a drawback of ORWARE compared with the example of MIMES/WASTE [9]. Concerning the sub-goals, we could conclude that source separation and biological treatment are beneficial concerning the recycling of phosphorus, and to some extent nitrogen, compared with today’s system. Composting has the poorest energy conversions. Anaerobic digestion has a higher energy ratio than composting but a lower ratio than incineration. Part of the energy from anaerobic digestion is converted to electricity, while all the energy from the incineration plant was heat. The environmental effects, in contrast, indicated that composting was the best alternative, without considering working health hazards. The second sub-goal was to evaluate the proportion between solid waste and wastewater, concerning plant nutrients. It was clear that the major part of plant nutrients was found in the wastewater. This implies that it is of great importance that wastewater is included in a study that aims at evaluating systems with a high degree of plant nutrient recycling. The reason for this is that the distances for these transports to arable land depend on the total amount of plant nutrients to be recycled. Since the wastewater contains the major part, it will be dimensioning for transport distances for all waste to be recycled, including solid organic waste. The third sub-goal was studied in scenario 5, where the only change from the incineration scenario was the implementation of urine-separating toilets. The results showed that this measure was the only one studied that gave a high recycling ratio for nitrogen. This is explained by the fact that a major part of all nitrogen leaving the urban society is found in human urine. Concerning environmental effects, it was
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only the eutrophication effect that was positively influenced by urine separation. The sewage plant can only remove approximately 50% of the incoming nitrogen. The energy consumption increased somewhat, and shifted to the increased use of oil and the decreased use of electricity. This is explained by the increased transportation needs.
Acknowledgements We wish to thank our supervisors and colleagues, Prof. Thomas Nybrant and Prof. Bjorn Frostell, as well as all other persons who have been involved in developing the ORWARE model. We also want to express our gratitude to the Swedish Waste Research Council (AFR) for financially supporting this research project.
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