Environmental and economic analysis of management systems for biodegradable waste

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Abstract

The management system for solid and liquid organic waste affects the environment and surrounding technical systems in several ways. In order to decrease the environmental impact and resource use, biological waste treatment and alternative solutions for sewage treatment are often advocated. These alternatives include increased agricultural use of waste residuals. To analyse whether such proposed systems indicate improvements for the environment and its sustainability, systems analysis is a useful method. The changes in environmental impact and resource use is not only a result of changes in waste treatment methods, but also largely a result of changes in surrounding systems (energy and agriculture) caused by changes in waste management practices. In order to perform a systems analysis, a substance-flow simulation model, the organic waste research model (ORWARE), has been used. The results are evaluated by using methodology from life cycle assessment (LCA). An economic analysis was also performed on three of the studied scenarios. The management system for solid organic waste and sewage in the municipality of Uppsala, Sweden, was studied. Three scenarios for different treatments of solid waste were analysed: incineration with heat recovery, composting, and anaerobic digestion. These three scenarios included conventional sewage treatment. A fourth scenario reviewed was anaerobic digestion of solid waste, using urine-separating toilets and separate handling of the urine fraction. The results are only valid for the case study and under the assumptions made. In this case study anaerobic digestion result in the lowest environmental impact of all the solid waste management systems, but is costly. Economically, incineration with heat recovery is the cheapest way to treat solid waste. Composting gives environmental advantages compared to incineration methods, without significantly increased costs. Urine separation, which may be implemented together with any solid waste treatment, has great advantages, particularly in its low impact on the environment. However, there is a large increase in acidification.

Introduction

Organic waste management systems have traditionally aimed at getting rid of the waste in the most economically efficient way. This tradition has lead to incinerating the solid waste or dumping it into landfills, and treating sewage from urban areas in large sewage plants. In Sweden, today, approximately 50% of the solid biodegradable waste is incinerated, 45% is put into landfill, and 5% is biologically treated (mainly composted, but anaerobic digestion is gaining interest). Almost 100% of the sewage water from urban areas are treated in three-stage municipal sewage plants. Over the last two decades environmental constraints have been implemented to decrease the negative environmental impacts caused by these systems. In Sweden there has been increased demands for reducing emissions from incinerators and landfills, as well as for increasing cleaning efficiency for the sewage plants. Furthermore, a tax on the organic waste put into landfills is to be introduced. However, refining the emission controls from incinerators and sewage plants are typically end-of-pipe solutions that does not solve the problems, but merely moves them in space (to landfills) and time (leaching from landfills in the future). This has lead to interest in inflow management solutions instead, such as source separation of solid organic waste to maintain the low pollutant level of kitchen waste. Another example is the increasing interest for alternative methods of treating sewage water, such as separating the toilet waste from other wastewater flows or source separating the urine by introducing specially constructed toilets.

If the pollutant contents (e.g. heavy metals in waste streams) are low enough, organic waste can be used as fertilisers in agriculture, thus diminishing the problem of misplaced plant nutrients. Moreover, the way in which fertiliser is manufactured for agricultural use is closely connected to the ecological sustainability of society. Today’s system, using large amounts of fossil fuels to produce nitrogen fertilisers and exploring virgin phosphate and potassium sources, is not sustainable. However, increased agricultural use of waste residuals would probably result in more transports, other waste treatments, and changed plant nutrient supply in agriculture.

The core question is if a change in the waste management system towards more agricultural utilisation of waste residuals is truly a gain both for the environment and its sustainability. Today’s waste management system produces heat for district heating systems. The proposed systems, on the other hand, with a high degree of plant nutrient utilisation, will supply plant nutrients and methane gas. Because the waste management system is multifunctional, it is necessary to use a systematic approach when analysing it. The most effective way to accomplish such a systematic approach is to make a model of the system and perform simulations of different scenarios, comparing alternative waste management systems.

There are several optimisation models for solid waste described in literature. Most models are focused only on different economic aspects [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Other studies include some environmental aspects, but the effects of using waste in the agricultural sector are not included [14], [15], [16]. In [17], [18], [19] the environmental impact is a major issue, along with economics, but neither include the full effects of using the residual waste products as fertiliser. Finally, there are some waste management models developed using life cycle assessment (LCA) methods [19], [20], [21], [22]. These LCA models focus only on environmental and resource use issues. All the models and studies mentioned have one thing in common; not one includes sewage water in the system.

The environmental impact and sustainability of sewage systems have been discussed in several articles, e.g. [23], [24], [25]. They all advocate separating household wastewater from the remaining water flow in order to make the system more sustainable. Günther [24] also advocated using urine separation. Tillman et al. [26] performed an LCA on three alternative sewage systems in two different areas in Sweden. In that study no solid waste was included. Bengtsson et al. [27] used LCAs for three planned housing areas in Sweden, with varying technical solutions for the sewage.

We have found only one study that examines both solid waste and sewage in the same analysis [28]. Here it states that urine separation and co-treatment of solid- and liquid-organic waste is beneficial for the environment and sustainability of the system, but no quantitative results are presented, however.

In Dalemo et al. [29] the organic waste research model (ORWARE), which includes both solid waste and sewage, is presented. The ORWARE model has been used in several case studies [30], [31], [32], [33], [34], [35]. The first study was performed to test the model and methodology, the others were mainly practical applications, but all had some element of model development. In Sonesson et al. [32] some model- and methodological improvements were featured.

The Swedish Environmental Protection Agency (SwEPA) have, since the early 90s, been funding several research projects aiming at the development of methods and models for analysing changes in waste management strategies. The reasons were the ones mentioned in the introduction, a growing concern for environmental issues throughout society. The ORWARE model was developed within this programme. After the initial case study [32], the interest for using the model grew among municipalities in Sweden. As a result of that, the municipality of Uppsala and the SwEPA together funded a systems analysis project including simulation experiments with the ORWARE model. The first objective of that study was to analyse the environmental impact and the economic outcome of four different systems for managing biodegradable waste (including sewage) for the municipality of Uppsala, Sweden. The results should give information on how to plan the waste management system on a long time perspective, i.e. strategic planning. This was the main question for the authorities in Uppsala. A second objective was to compare the results of this study with the results of the previous study performed on the same region [32] to see if the changed systems boundaries, and other changes proposed in that paper, affected the conclusions. This was the main question for the research group and the SwEPA, as a methodological development of the model. The project was reported by Björklund et al. [35].

This paper has three objectives; first to describe what type of results and conclusions can be made when using systems analysis and modelling methods within this area; second to make some general conclusions on the subject of environmental impact from management systems for biodegradable waste; third for methodological discussions regarding system boundaries and assumptions that have to be made. The indata and scenarios from the main study was used, but some very specific local circumstances were omitted. The rationale for that was that more general conclusions were desired.

Section snippets

The ORWARE model

The management of organic waste in the municipality of Uppsala, Sweden, was studied with the ORWARE simulation model. This is a static material flow model where the inputs to the system are waste flows (from kitchens, industries, etc.) and energy (fuel and electricity). Within the system are waste treatment processes and transport. The outputs are emissions to air, water, and soil, energy turnover, and amount of plant nutrients used by crops. Also, the economic turnover is calculated. The

Studied area

The area studied was the municipality of Uppsala. Uppsala is situated 70 km north of Stockholm and has 165 000 inhabitants. Nineteen thousand households live in one family dwellings in the city, 55 000 households are flats and 9000 one family dwellings are found in rural areas. The surroundings consists of approximately 25%, agricultural land, the remainder is forest. The amount of waste generated and thus treated is presented in Table 2.

The collecting of household waste is performed with

Results

The energy turnover for the four scenarios is presented in Fig. 2. COM produces least usable energy from waste. The heat deficiency must be replaced by supplementary production. COM produces less biogas (bus fuel) than ANA, thus diesel for busses must be added. COM also must be supplemented with production of mineral fertilisers because URI supplies crops with more nitrogen and phosphorus. COM is the scenario that has the least electricity consumption, the differences between scenarios are

Discussion

Even though the objective of this paper is fairly straightforward, interpreting the results is difficult. It is possible to analyse the results from several angles, none of which, taken alone, are correct. We have chosen to analyse the results from two different angles. First we will discuss each scenario, comparing it with the other scenarios and analysing their pros and cons. Second, we will analyse them on the system and methodology level, where choices of supplementary production and

Results

The conclusions from this study cannot be regarded as general in any way. The specific circumstances in the region studied will determine many of the conclusions drawn. These region-specific circumstances are both scientific (such as the soil’s sensitivity towards acidification) and societal (such as politically decided goals for environmental aspects). Below the types of conclusions that can be made is presented. It is also discussed how general the conclusions are, their dependence on

Acknowledgements

We wish to thank our supervisors and colleagues, Professor Thomas Nybrant, assistant Professor Håkan Jönsson, and Professor Björn Frostell. We also want to express our gratitude to the Swedish Waste Research Council (AFR), within the Swedish Environmental Protection Agency, for financially supporting this research project.

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