6 Aug 2000
Sponsors
Institute of Advanced
Studies, UN Univ., Japan
AEON Foundation, Japan |
Material Flow Analysis of Integrated Bio-Systems (March-October 2000) http://www.ias.unu.edu/proceedings/icibs/ic-mfa |
Organized by:
Integrated
Bio-Systems Network
UNU/IAS
Alumni Association, UN Univ.,Tokyo
with the assistance of : MFA
Conference Planning Group
UNESCO
Microbial Resources Centre, Stockholm |
A Systems Approach Evaluation
of Sludge Management Strategies
Case Study: Sludge Management
in Valparaíso and Aconcagua, Chile
Ramírez, J.I. #, Frostell, B. *, Galindo, R. ¤
* Dept. of Environmental Engineering and Work Science, Royal Institute of Technology, S-100 44 Stockholm, Sweden.
¤ Department of Civil Engineering,
Universidad Técnica Federico Santa María, Valparaíso,
Chile
| Abstract
In the 5th region, located in central Chile, with approximately 1.200.000 inhabitants, infrastructure projects are being implemented or in a planning phase in order to increase the capacity to treat and dispose sewage. The proposed installations include new or significant retrofitting of sewage treatment plants as well as sea outfalls, including mechanical and chemical treatment previous to disposal to the sea. It is expected that these installations will result in a significant sludge production. The ORWARE (ORganic WAste REsearch) model, a specific approach for municipal waste and waste-water systems analysis, is used to calculate material and energy flows (MFA: Material Flow Analysis) for various alternatives to organic waste handling and municipal wastewater treatment within a municipality. For evaluation, the results are translated to environmental effects using LCA-methodology. In order to analyse the sludge management alternatives mentioned above in a structured way, we have proposed to apply the ORWARE model, as well as relevant parts of the ISO 14000- guidelines. The research project was divided in two stages: In the first stage the base line has been established and the relevant data and documentation gathered. This stage also included definition of environmental impact categories and goals, as well as the sewage and sludge management strategies to be compared. The management alternatives chosen were for sewage chemical or biological treatment while for sludge the management alternatives were based on digestion, composting or lime stabilisation. Alternatives for final destination of the treated sludge were landfill disposal or utilisation as fertiliser in agriculture. The environmental impact categories chosen were pathogen emissions, toxicological effects of selected heavy metals and organic compounds, energy use and generation, and emission of green house gases. Costs of management alternatives are also analysed. The second stage included the simulation and analysis of the simulation results. The following conclusions were derived from this study:
Key words - Systems Analysis, Modelling,
Waste Management, Environmental Impact, Plant Nutrient Recycling, Integrated
Waste Management
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| Introduction
Waste and wastewater handling has been subject to different demands during the times. In Europe about a hundred years ago, when municipal waste was mainly organic, the reason for collecting waste was partly of hygienic nature, but it was also an important source of plant nutrients in the food production. Both food waste and night soil was sought after by gardeners and farmers. Later, the use of organic waste as soil amendment decreased. This was due to several factors. Inorganic material increased in the household waste, which made it less desirable to use in farming, cheap mineral fertilisers out-competed natural fertilisers, and the construction of water closet systems resulted in a great loss of nutrients to water recipients. Waste handling, including organic waste handling, became a question of getting rid of undesired materials. During the last fifty years, waste and wastewater handling systems have developed considerably in Europe. This was to combat the increasing negative effects of waste disposal and wastewater discharges that became more and more evident. Solid waste collection and treatment (landfill disposal or incineration, sometimes biological treatment) became standard, as well as wastewater collection and for many large cities treatment in central plants. A characteristic of this process, however, is that the waste and wastewater systems were developed without any real systems approach. The solid waste handling system was developed without consideration of potential integration possibilities with the wastewater collection and treatment system (Harremoës, 1997, Ref. [10]). New objectives for organic waste handling has, however, emerged. The UN Agenda 21 document (1992) proposes to re-use organic waste, in order to enhance society’s ecological sustainability. Such a goal for organic waste handling necessarily calls for great changes both at the system level and at the specific technology level. A more system-oriented approach to organic waste handling will therefore be necessary, since the waste handling system of today is so complex. In this process, it will among other things be important to evaluate the overall effects of introducing new solutions. This is to ensure that negative effects with new solutions don’t override the positive ones (Otterpohl et al, 1998). In planning and implementing a sustainable society, it may be expected that the relative importance of organic waste handling systems will increase. This is since product and process developments will have to increase the degree of recycling of products and materials. This in turn will result in a decrease in the amounts of many wastes for disposal. Organic wastes (food wastes, toilet wastes, animal wastes, organic industrial wastes and park and garden wastes) will be much more difficult to decrease, since they are essentially coupled to the size of the human population and its food habits. The objective of the present program is to evaluate the sustainability of possible future systems for handling organic waste, including wastewater, especially systems with a high degree of plant nutrient recycling from urban areas to agriculture. To include wastewater is essential, since a major part of all plant nutrients that leave the urban society is found in this waste fraction. We have chosen to address this by using environmental systems analysis, which we define as: …" models and methods for the integrated quantification and presentation of material and energy flows through various parts of society and nature, and assessment of the sustainability of different development alternatives". A specific approach for municipal waste and wastewater systems, developed in Sweden is the model, ORWARE (ORganic WAste REsearch model; Nybrant et al, 1996), which calculates material flows and energy turn over for various alternatives to organic waste handling and municipal wastewater treatment within a municipality. For evaluation, the results are translated to environmental effects using LCA-methodology (LCA= Life Cycle Assessment). Several models of waste handling system can be found in the literature: Gottinger (1985), Jenkins (1982) and Kaila (1982) are all examples of economic models that are used to optimise localisation of landfills, incineration plants and transfer stations. Baetz & Neebe (1994), Everett & Modak (1996) and Anex et al. (1996) have mainly the same approach, but have included material recycling as an opportunity. Chang & Wang (1996) address the problem with the uncertainties the decision-maker faces, by introducing non-exact, or fuzzy, variables in the objective function to be minimised by the model. None of these models consider environmental effects or energy matters. Environmental effects and energy aspects are addressed by Gupta & Shepherd (1992), who present a model that is based on an emission database from different actual plants in USA. Sundberg (1993) has developed an optimisation model, MIMES/WASTE, for solid waste handling systems. Material flows in this model are characterised by a limited vector, mainly concerning organic pollutants and plant nutrients. It considers several waste fractions that are not included in ORWARE, whereas wastewater is not included. MIMES/WASTE considers the landfill as a sink, i.e., emissions from the landfill are not included in the model. White et al. (1995) presents a Life Cycle Inventory (LCI) concerning municipal solid waste. Approximately the same substances and processes are included as in ORWARE. Transports of residues to farmland are not included. Neither is wastewater, which also leaves out the sewage plant. The study is an inventory of the emissions and energy turnover in the system today, but it may also be used for simulating and evaluating different scenarios. The overall program goal is to further improve the approach used in the ORWARE model and other computer models mentioned above to a general tool for municipal waste and wastewater management systems and to implement the model through specific studies in the participating countries. In Latinamerica there is a growing demand of waste and wastewater handling systems in order to satisfy several demands, mainly from the point of view of health of the population. In order to evaluate correctly the different technical alternatives it is necessary to promote a knowledge transfer both at the system level and at the specific technology level. In the 5th region, located in central Chile, with approximately 1.200.000 inhabitants, infrastructure project are being implemented or in a planning phase in order to increase the capacity to treat and dispose sewage. The proposed installations include new or significant retrofitting of sewage treatment plants as well as sea outfalls, including mechanical and chemical treatment previous to disposal to the sea. It is expected that these installations will result in a significant sludge production. In order to analyse the sludge management alternatives in a structured way it is proposed to apply the ISO 14.000- guidelines, as well as the ORWARE model. Approach General A simulation model, ORWARE (ORganic WAste REsearch), for the handling system for organic waste in urban areas has been constructed. The model provides a comprehensive view of the environmental effects, plant nutrient utilisation and energy turnover for this large and complex system. The ORWARE model consists of several sub-models; sewage plant, incineration, landfill, compost, anaerobic digestion, truck transport, transport by sewers, residue transport and spreading of residues on arable land. The model is intended for simulating different scenarios, and the results are; emissions to air and water, energy turnover and the amount of residues returned to arable land. All results are presented both as the gross figure for the entire system and figures from each process. Throughout the model all physical flows are described by the same variable vector, consisting of 43 substances. Additionally the ORWARE Model calculates the energy use and production at different nodes within the system. This extensive vector facilitates a thorough analysis of the results, but involves some difficulties in acquiring relevant data. As shown in Figure 1 below, the material
and energy flow data generated by the ORWARE model is aggregated in a form
of "Effect Categories", which indicate the potential for environmental
damage and resource use, such as human toxicity, nitrification, acidification
global warming, etc. This approach is similar to an LCA (Life Cycle Analysis)
methodology.
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Description of the ORWARE SIMULINK Model
The model was constructed using the software MATLAB/SIMULINK [15]. An example of a scenario in ORWARE is presented in Figure 2.
Figure 2: The ORWARE model realised in
MATLAB / Simulink
A vector describes the physical flows between
sub-modules ("ORWARE vector"). A further description of this vector is
presented in Appendix 1.
Estimation of the pathogen emission
Based on the sewage and sludge flows (water and DS), the pathogen emissions could be estimated using the following concentrations of faecal coliforms (Reference [18]):
Table 1: Faecal Coliforms (FC) content
| Material Flow | Faecal Coliforms (FC) content |
| Wastewater to recipient – chemical treatment | 5 x 108 FC/l |
| Wastewater to recipient – biologic treatment | 2,50 x 106 FC/l |
| Compost | 1 x 106 FC/kg dry |
| Biosolid to soil | 2 x 106 FC/kg dry |
Background
In the 5th region, located in central Chile, with approximately 1.200.000 inhabitants, infrastructure projects are being implemented or in a planning phase in order to increase the capacity to treat and dispose sewage. The proposed installations include new or significant retrofitting of sewage treatment plants as well as sea outfalls, including mechanical and chemical treatment previous to disposal to the sea. It is expected that these installations will result in a significant sludge production.
In order to analyse the urban water systems in the 5th Region, two main systems have been chosen, the Valparaíso and the Aconcagua Systems, as shown in the Figure 3 below:
Figure 3: 5th Region Chile
The Valparaíso System includes the two adjacent cities of Valparaíso and Viña del Mar, located by the coast, and which have a total population of approximately 800.000 inhabitants. The sewage net coverage is over 90%. The existing sewage treatment comprises mechanical treatment and sea outfall. By the year 2003 it is expected that the sewage treatment will even include an additional step such as chemical precipitation.
The Aconcagua System includes a number of middle-sized cities located along the Aconcagua River, such as San Felipe, Los Andes, Quillota, and La Calera. The aggregated population of these cities is approximately 400.000 inhabitants. The sewage net coverage in the urban areas is around 90%. The existing sewage treatment consists of stabilisation ponds or none, while sewage treatment plants of active sludge type are expected to be built between the years 2000 to 2007.
These planned investments, aimed at reducing the hygienic risks for the population as well as the ecotoxicological load on the environment, require an integral, systems approach in order to choose the most suitable combination of sewage treatment, sludge management and sludge disposal or use. An appropriate analysis methodology should therefore study the aggregated hygienic risks, environmental impacts, as well as overall energy use and costs associated to each combination.
Approach used in the Case Study
The research project was divided in two stages; in the first stage the base line has been established and the relevant data and documentation gathered. This stage also included definition of environmental impact categories and goals, as well as the sewage and sludge management strategies to be compared. The second stage included the simulation and analysis of the simulation results
The management alternatives chosen were for sewage chemical or biological treatment while for sludge the management alternatives were based on digestion, composting or lime stabilisation. Alternatives for final destination of the treated sludge were landfill disposal or utilisation as fertiliser in agriculture. These combinations are shown Table 2 in below:
Table 2: Wastewater and sludge management strategies considered
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The environmental impact categories chosen are shown in Table 3 below:
Table 3: Environmental effects considered
Additionally, the costs of each alternative, relative to the base line was also considered.
The energy production and use was estimated at the following nodes:
Table 4: Nodes of energy use / production
considered in the simulation
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Additionally, based on the results of the ORWARE model, the expected concentration of some substances in the effluent from the sewage treatment plants as well as the sludge were estimated. The estimated concentrations in the sewage effluent were compared to the present Chilean standards regarding discharge to sea, rivers and lakes; while the expected concentrations in the sludge were compared to relevant standards in the US (40 CFR 503) and Europe (86/278/ EEC Appendix 1B
Simulation Results
The different strategies presented in Table 2 (Wastewater and sludge management strategies considered) were simulated in the ORWARE /Simulink model.
The resulting materials and energy flows were weighted by factors in order to estimate the total contribution to environmental loads associated to each strategy.
In the case of Global Warming Potentials (GWPs), it is expressed as CO2-equivalents, given for a time horizon of 100 years [17]. For evaluating the toxicity, the CML provisional method [17], [11] has been used. In order to estimate the eutrophication effect, both nitrogen and phosphorus contributions were considered, using the approach suggested by [7]. The hygienic aspect (i.e. pathogen reduction) is considered by adding the estimated quantity of faecal coliforms present in the treated sewage effluent and the treated sludge.
These aggregated results are shown in Figures
4 – 10 below:
A summary of the impact categories considered for the Valparaíso System is shown in Figure 9 below. The environmental load of each strategy is shown relative to the strategy with least environmental load or cost. However, it would be misleading to compare different effect categories based on this diagram. Note also that the effect category Faecal Coliforms is expressed in a logarithmic scale.
Figure 9: Summary Impact Categories
Considered – Valparaíso System
Figure 10: Impact Category: Eutrophication
– Aconcagua System
Moreover, from the material flows calculated by ORWARE, it was possible to make estimates of the expected heavy metal content in the sludge for the different strategies that consider spreading to soil. These estimates are compared in the tables below with the concentration allowed in the US and European legislation.
Table 5: Concentration sludge to soil (mg/kg) for each strategy
| Trace |
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| metal |
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| As |
75
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-
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-
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-
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-
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-
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| Cd |
85
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20
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19
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44
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23
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53
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| Cu |
4.300
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1.000
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1.210
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2.810
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1.620
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3.770
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| Hg |
57
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16
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1
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1
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1
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2
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| Mo |
75
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-
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-
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-
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-
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-
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| Ni |
420
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300
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63
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130
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7
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-
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| Pb |
840
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750
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255
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564
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325
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723
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| Se |
100
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-
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-
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-
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-
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-
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| Zn |
7.500
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2.500
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815
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1.710
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1.140
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2.460
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| Cr |
-
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1000
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551
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1.270
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670
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1.540
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| (1) 86/278/EEC Appendix 1B | ||||||
Table 6: Ratio Concentration Sludge to Soil / Limit CFR 503
| Trace |
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| metal |
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| As |
-
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-
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-
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-
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| Cd |
22%
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52%
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27%
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62%
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| Cu |
28%
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65%
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38%
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88%
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| Hg |
1%
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2%
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1%
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3%
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| Mo |
-
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-
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-
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-
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| Ni |
15%
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31%
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2%
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-
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| Pb |
30%
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67%
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39%
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86%
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| Se |
-
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-
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-
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-
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| Zn |
11%
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23%
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15%
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33%
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| Cr |
-
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-
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-
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-
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Table 7: Ratio Concentration Sludge to Soil / Limit 86/278/EEC App.1B
| Trace |
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| metal |
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| As |
-
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-
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-
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-
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| Cd |
95%
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222%
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114%
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264%
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| Cu |
121%
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281%
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162%
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377%
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| Hg |
4%
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8%
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4%
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10%
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| Mo |
-
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-
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-
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-
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| Ni |
21%
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43%
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2%
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-
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| Pb |
34%
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75%
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43%
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96%
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| Se |
-
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-
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-
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-
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| Zn |
33%
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68%
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45%
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98%
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| Cr |
55%
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127%
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67%
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154%
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As it can be seen from the tables above,
the concentration of copper and cadmium may present some problem for application
to soil.
Discussion and Conclusions
In the following discussion, some observations are made about the results of this particular application of the ORWARE model. The discussion includes even some comments about aspects not completely covered by the model, and general suggestions on how to reduce the environmental impact of the waste management system in the studied region. These general suggestions do not necessarily come from the results of the model, but from observations of the conditions of the existing waste management system.
After the discussion, the conclusions inferred from the results of the simulation are presented.
When analyzing the result corresponding to net energy use (Figure 5), it can be observed that the energy use even in the Base Line is rather high. This is due that the model even considers sewage pumping, which applies also for the Base Line. Moreover, in Figure 6 it can be observed that the increase of energy use due to chemical treatment is marginal. This result is in the same line with Ødegaard (Ref. [30]). However, it would seem interesting to further refine the model with regards to energy use when applying chemical treatment.
As it can be seen from the tables 6 and 7 above, the concentrations of heavy metal in the treated sludge are lower when applying composting. This is probably due to the fact that composting requires mixing with garden waste, which has a lower content of heavy metals.
Some aspects not fully considered in the ORWARE model, are for example the subject of public acceptance and environmental impact variation for different uses and place of disposal.
Acceptance of sludge application by farmers is not to be taken for granted, even if the existing sludge standards are met. The farmer may not accept the risks implied when setting the standards, as it has been the case in Sweden. In developed countries, applying sludge component separation may eventually solve this. In developing countries, however, this would seem to be a far too expensive technique. A more feasible solution is applying the sludge to forests where there is less risk of damage to human health and still sufficient improvement of productivity in order to justify the cost of sludge application.
In any case, control at source should be stressed to avoid heavy metals and toxic organic compounds in the sludge. This will require an active introduction of cleaner production techniques and limitation of use of hazardous chemicals in the society, in order to gradually eliminate the sources of contaminants that eventually turn up in the sludge.
With regards to the effects on human health and the ecotoxicology, the ORWARE methodology estimates a factor for these categories based on the quantity of toxic substances generated. These effects can vary, however, depending on the place of disposal. This issue is at present not fully considered in the ORWARE model.
As general observations of the waste management system in Valparaíso and Aconcagua, it can be commented that the landfills should include appropriate liner, recollection and treatment of leachate, as well as recollection and use of biogas.
As conclusions of the simulation the following can be drawn:
We wish to thank the Swedish International
Development Co-operation agency, Department of Research Co-operation (Sida
/ SAREC) for financially supporting the present research project.
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