Chapter 3 Environmental Life Cycle Assessment
3.1. Goal and scope
This includes definition of system boundaries, details, accuracy and data quality, functional units and impact models to be used for the analysis. The goal of this study is to assess the environmental impacts associated with production of water and to highlight the processes contributing the most to these impacts.
3.1.1. Theory and methodology
The goal definition and scoping stage of LCA defines the purpose of the study, the expected product of the study, the boundary conditions and the assumptions. (SETAC, 1992) Typically LCA studies are performed in response to specific questions. The nature of the questions determines the goals and scope of the study.
Once the general goals and purpose of the LCA study are understood, the boundaries of the study must be determined. It is common practice to define the life cycle of the product, process or activity being studied as a system (Boustead, 1992). In this study the system is defined as a water and sewage network within the eThekwini Municipality. The boundaries for the LCA encompass the acquisition of raw materials, manufacture of intermediate materials, manufacture of the product being studied, use of the final product and final disposition. Recycling or reuse of the product is part of the methodology. When conducting an LCA, the design/development is usually excluded, since it is often assumed not to contribute significantly (Rebitzer et al., 2004).
After all steps that fall within the system boundaries are identified, the LCA practitioner may
choose to simplify the LCA by excluding some steps from the study. Hypotheses have been outlined and where steps have been excluded these are explained.
Heinz and Baisnee (1992) and Weidema (1993) suggested that two distinct categories of LCA goals exist. These are;
• to describe a product system and its environmental exchanges or
• to describe how the environmental exchanges of the system can be expected to change as a result of actions taken in the system.
This study shares both goals. In this chapter the first goal is addressed that is to describe the environmental profile of each unit. A stand alone LCA was performed on each unit in the studied system. All four steps of the ISO methodology were carried out including an improvement analysis.
The second goal is dealt with in Chapter 6 where different scenarios are considered. Chapter 6 links the individual LCAs performed in this chapter by using a different functional unit. This allows one to understand the environmental burden of the system as a whole and to then understand the effect changes will have to the system's environmental profile.
Part of the formal goal definition process is a requirement that the reasons for the study and the intended audience be identified. Currently in South Africa there is a drive to expand the provision of water and sanitation services. However in order to this in a sustainable manner one needs a clear understanding of the associated environmental burdens. There are many options that can be considered for these expansions. This LCA study in this chapter aims to provide detailed information on the environmental burdens of the processes currently in use in the eThekwini Municipality. This will aid planners and water professionals (the intended audience) when considering the different expansion options. Chapter 6 considers expansion scenarios and examines options that are not currently in use in the eThekwini Municipality.
Time and spatial boundaries must also be chosen for the study. The goal of the LCA is to model, as closely as possible the life cycle of the product, process or activity being studied. Spatial boundaries need to be evaluated in this context. Time boundaries are also important because industrial practices, legislative requirements and consumer habits vary over time.
3.1.1.1. Consequential vs Attributional LCA
It is important to decide, during the goal definition phase of the LCA, whether to conduct a consequential or attributional LCA. During a workshop held in 2003, specifically on life cycle inventory for electricity generation the term attributional life cycle assessment was defined as an attempt to answer how are things (i.e. pollutants, resources and exchanges among processes) flowing within the chosen temporal window while consequential life cycle assessment attempts to answer how will flows beyond the immediate systems change in response to decisions? For example, an attributional LCA would examine the consequences of using green power compared to conventional sources. A consequential LCA would consider the consequences of this choice in that only a certain amount of green power may be available to customers, causing some customers to buy conventional energy once the supply of greener sources was gone (Curran, Mann & Norris, 2005).
The is much debate amongst LCA practitioners on which type of LCA is better. Attributional LCAs are generally retrospective assessments of the accountancy type and are typically applied for hot-spot identification, product declarations and for generic consumer information.
Consequential LCAs study the environmental consequences of possible (future) changes to the system being studied.
One of the stated aims of this thesis is to determine the net effect of using recycled water.
Therefore it is necessary for the consequential method to be used. This means that processes or data used should represent the consequential technologies (i.e. as a consequence of recycling).
For example the consequential production technology should be included i.e. the electricity production technology that will respond to a small increase or decrease in demand. In South Africa this is coal based electricity generation and this was therefore used when compiling the life cycle inventory.
3.1.1.2. Functional unit
The functional unit is defined as a quantified description of the performance of the product system for use as a reference unit. Special attention has to be given to choosing the functional unit because it provides a reference to which the input and the output data in the inventory phase will be related. The functional unit of this study is defined as 1 kL of water at the qualities stipulated by the Umgeni Water guidelines for potable water and the eThekwini Municipality for
wastewater treatment. Each sub-system produces a different product (i.e. different quality of water) therefore it is not possible to use a constant /consistent functional unit throughout the study of each unit. It was decided that the functional unit would be one kilolitre of the water produced from each sub-system at the stipulated quality. For example when studying the wastewater treatment plant the functional unit is one kilolitre of treated wastewater produced and for the recycling plant the functional unit is one kilolitre of recycled water produced. Rihon (2002), in a similar study which investigated the environmental impacts of an 'anthropic water cycle' from the pumping station to the wastewater treatment plant, also chose to use a similar functional unit. Friedrich (2001) used a similar functional unit when considering water treatment processes. When considering the complete system it was necessary to use a different functional unit as one cannot simply consider the volume weighted sum of the different impact categories.
Having defined the functional unit, it is necessary to quantify the amount of product which is necessary to fulfil the function. The result of this quantification is the reference flow.
3.1.2. Assumptions
A study on the influence of system boundaries (Lundin, 2000) highlighted the fact that very different choices can be made for system boundaries in models of water and wastewater systems.
These choices inevitably affect the results. Most LCAs include only the operation of the studied technical systems and overlook the environmental load of the construction phase. Consequently questions related to the scale and longevity of the systems are also overlooked.
For the purpose of this study, the construction, operation and decommissioning phase of each sub-system was studied. Equipment was assumed to last the entire predicted design lifetime. For example Inanda Dam is designed to operate for a period of 70 years. However this is highly dependent on the silting rate of the dam but for calculation purposes 70 years was the figure used.
Each sub-system produces a different product therefore it is not possible to use a constant /consistent functional unit throughout the study. It was decided that the functional unit would be one kilolitre of the water produced from each sub-system. For example when studying the wastewater treatment plant the functional unit is one kilolitre of treated wastewater and for the recycling plant the functional unit is one kilolitre of recycled water. Rihon (2002), in a similar study which investigated the environmental impacts of an 'anthropic water cycle' from the pumping station to the wastewater treatment plant, also chose to use a similar functional unit. In
considering the complete system, the volume weighted sum of the different impact categories was calculated.
Neither production nor demand is always fully elastic, which means that the demand for one unit of product in the life cycle investigated affects not only the production of this product but also the consumption of the product in other systems (Ekvall, 2002). It was assumed that for the purposes of this model the system would be operating under what would be considered normal conditions.