This paper describes recent experience with integrated resource planning (IRP) and the application of least cost planning (LCP) for the evaluation of demand management strategies in urban water. Two Australian case studies, Sydney and Northern New South Wales (NSW) are used in illustration. LCP can determine the most cost effective means of providing water services or alternatively the cheapest forms of water conservation. LCP contrasts to a traditional approach of evaluation which looks only at means of increasing supply. Detailed investigation of water usage, known as end-use analysis, is required for LCP. End-use analysis allows both rigorous demand forecasting, and the development and evaluation of conservation strategies. Strategies include education campaigns, increasing water use efficiency and promoting wastewater reuse or rainwater tanks. The optimal mix of conservation strategies and conventional capacity expansion is identified based on levelised unit cost. IRP uses LCP in the iterative process, evaluating and assessing options, investing in selected options, measuring the results, and then re-evaluating options. Key to this process is the design of cost effective demand management programs. IRP however includes a range of parameters beyond least economic cost in the planning process and program designs, including uncertainty, benefit partitioning and implementation considerations.
The non-potable reuse of treated sewage in urban areas provides significant conservation of potable supplies beyond that available through water use efficiency. Effluent reuse is also an inevitable requirement in novel decentralised wastewater systems. At present, urban water reuse, where pursued, usually involves large-scale schemes based on new or existing centralised sewage treatment plants. This is despite the diseconomy of scale inherent in pipe networks that balances economies of scale in sewage treatment and negates any cost advantage for wastewater systems with more than around 1,000 connections. In light of this, the theoretical relationship between effluent reuse system scale and pathogen risks was examined at various effluent qualities. Waterborne disease was seen to be a significant factor when reusing effluent in urban areas and smaller systems were found to pose a lower risk of waterborne infection, all other things being equal. Pathogen risks were then included within an economic analysis of system scale. It was concluded that with the inclusion of pathogen risks as a costed externality, taking a decentralised approach to urban water reuse would be economically advantageous in most cases. This conclusion holds despite an exact evaluation of increased waterborne disease due to effluent reuse remaining problematic.
Decentralized wastewater treatment has the potential to provide sanitation that meets criteria for sustainable urban water management in a manner that is less resource intensive and more cost effective than centralized approaches. It can facilitate water reuse and nutrient recovery and can potentially reduce the ecological risks of wastewater system failure and the community health risk in a wastewater reuse scheme. This paper examines the potential role of membrane technology in sustainable decentralized sanitation. It is argued that the combination of membrane technology within decentralized systems can satisfy many of the criteria for sustainable urban water management. In particular, the role of membranes as a dependable barrier in the wastewater treatment process can increase system reliability as well as lowering the latent risks due to wastewater reuse. The modular nature of membranes will allow plant size to range from single dwellings, through clusters to suburb size. It is concluded that realization of the potential for membrane-based technologies in decentralized wastewater treatment will require some progress both technically and institutionally. The areas where advances are necessary are outlined.
This paper explores the use of levelised cost in planning for infrastructure networks. Levelised cost provides a useful measure comparing supply or conservation options on varying scales on an equivalent basis. Comparison is made to annualised cost, a metric often used as a means of comparing different supply side options. Urban water supply is used as the primary example, however levelised cost is equally applicable to other infrastructure networks, such as electricity or gas. The levelised cost is calculated as the ratio of the present value of projected capital and operating cost of an option to the present value of the projected annual demand supplied or saved by the option. The paper demonstrates that levelised cost is the constant unit cost of supply, provided by an option at present value. It is also the average incremental cost of the option at the point of implementation. When translated to a unit cost, annualised cost does not account for unutilised capacity in large scale schemes, systematically under representing actual costs. By using levelised cost this inherent bias is removed. Use of levelised cost would facilitate the inclusion of smaller scale and more incremental supply options into infrastructure networks providing both economic and environmental benefits.
Decentralised systems have the potential to provide a viable option for long term sustainable management of household wastewater. Yet, at present, such systems hold an uncertain status and are frequently omitted from consideration. Their potential can only be realised with improved approaches to their management, and improved methods to decision-making in planning of wastewater systems. The aim of this paper is to demonstrate the value of a novel framework to guide the planning of decentralised systems so that asset management and risk management are explicitly considered. The framework was developed through a detailed synthesis of literature and practice in the area of asset management of centralised water and wastewater systems, and risk management in the context of decentralised systems. Key aspects of the framework are attention to socio-economic risks as well as engineering, public health and ecological risks, the central place of communication with multiple stakeholders and establishing a shared asset information system. A case study is used to demonstrate how the framework can guide a different approach and lead to different, more sustainable outcomes, by explicitly considering the needs and perspectives of homeowners, water authorities, relevant government agencies and society as a whole.
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