Introduction: An advanced urban water and wastewater network – from the source of raw water to the sink for the treated effluent wastewater – is, to say the least, a complex one. The interdependencies and interrelationships among the constituent network components make an integrated network analysis as necessary for an as-thorough-as-possible understanding of the system, as a separate analysis of each of the different network components. If sustainable development is to be pursued by urban water and wastewater utilities, a foreknowledge of the evolution of the network to its configuration at the time of the analysis, is a sine qua non. In simple terms, what is observed now, is the result of all that has been done in the past. More specifically, this evolution over time, has called for, and has been associated with, material inflows and outflows, energy consumption and related emissions, environmental impacts along the way, periodic capital investments to extend, expand and upgrade the systems, annual expenses on operation and maintenance, and changes in policies, rules and regulations at the administrative level.
Materials and chemicals, energy and money, in addition to time and manual labour, are the ‘factors of production’ employed to fulfil the twin goals of water supply and wastewater treatment. The anthropogenic network components managed and operated by the utilities, are the water treatment plants (WTPs) and wastewater treatment plants (WWTPs), water pipelines, sewage, storm-water and combined-flow pipelines, and water and sewage pumping stations. (It goes without saying that the consumers ‘mid-stream’ linking the water supply subsystem to the wastewater handling sub-system, constitute the raison d’être of the network). Utilities should aim at providing acceptable levels of service to the consumers, while optimising the expenditure of money, the consumption of energy, chemicals and materials, and reducing environmental impacts. This is the triple bottom line approach (social-economic-environmental) which needs to be incorporated into asset management of the 21st century.
Background of Oslo: The city ofOslo – the focus of this research – is inhabited by about 600,000 people (as in year-2010); and is serviced by three WTPs of different capacities - Oset, Skullerud and Langlia - drawing raw water from the lakes Maridalsvannet, Elvåga, and Langlivannet, respectively. The treated water from the three WTPs reaches the consumers in the domestic, industrial and commercial sectors of the city through approximately 35,000 water pipes with a total length of over1,500 kilometres. The sewage discharged by the consumers and the storm-water (rainwater and snowmelt) are transported to two WWTPs – BEVAS (Bekkelaget Vann AS) and VEAS (Vestfjorden Avløpselskap) – through more than 54,000 pipes with a total length of around2,200 kilometres. Water and sewage pumping stations pressurise the respective flows. The treated effluent wends its way into theOslofjord, which is contiguous with theAtlantic Ocean.
IE tools and methods: The longest time-span considered for the time series analysis is 16 years – for the water and wastewater pipelines. For WTPs and WWTPs, the time window is much shorter - from year-2000 onwards. Material flow analysis (MFA) is performed to study the inflows of pipeline materials into the water and wastewater pipeline networks inOslo. The phenomenon of pipeline stock saturation is discussed vis-à-vis two other Norwegian cities –Trondheim and Tromsø; and an embodied energy analysis (EEA) is performed. Environmental life-cycle assessment (LCA) is carried out with the results of the MFA serving as the platform, to translate the past annual inflows into their associated environmental impacts, and to forecast the impacts that would occur in the future. Life-cycle costing (LCC) is performed in order to emphasize the importance of future investment decisions in, and rehabilitation approaches to the wastewater pipeline network. The flows of, expenses on, and the impacts associated with, chemicals and energy consumption at the WTPs and WWTPs, are analysed as time series. Energy, environmental and economic analyses are performed for the water and sewage pumping stations. Based on the sub-system studies, the system is visualised as a whole, and comparisons among the subsystems are done. The elaborateness of the studies, when it comes to historical (time-series in other words) analyses, is limited only by the non-availability of detailed data, and the aversion to make too many assumptions.
Measuring sustainable development: Indicators are useful as metrics in order to measure a water-wastewater utility’s progress towards sustainability. Sustainability or sustainable development, when considered holistically with regard to the urban water and wastewater system, may be looked upon as fourpronged. Social, economic, environmental and functional indicators can be aggregated by using suitable weighting factors to arrive at criteria indices and a grand sustainability index. Time series analyses like the ones referred to in the earlier paragraph will yield indicators as a time series, and enable a systematic measurement of ‘sustainable development’. Targets and benchmarks can be set in order to stimulate progress. There are benefits and pitfalls associated with such an aggregation.
Key findings: Useful insights are obtained from the analyses referred to, in the earlier paragraphs. As the water and wastewater pipeline networks evolve towards saturation, the annual environmental impacts decrease over time, and are increasingly dominated by the operation, maintenance and rehabilitation phases. Concrete is the dominant pipe-fabrication material in the wastewater pipeline network, while ferrous metals dominate the water pipeline network. LCC enables one to prove the superiority of a physical lifetime approach over the in-vogue economic lifetime approach, when it comes to economising and managing/utilising the pipeline assets more efficiently. The comparison among Trondheim,Oslo and Tromsø yields an interesting correlation between the population density and the mass of pipeline materials per capita of the population, which needs to be confirmed by obtaining more datasets – from cities within Norway firstly and foreign cities thereafter.
The economic and environmental analyses of WTPs and WWTPs in the city give interesting results, when the energy consumption, costs and associated environmental impacts, expressed in terms of per-unit-service-delivered – unit volume of water supplied in the case of water treatment and unit volume of wastewater treated in the case of wastewater treatment – are compared with the corresponding values for chemicals. Eutrophication emerges as the dominant environmental impact when wastewater treatment and effluent discharge are considered, pointing to the possibility of channelling funds towards nutrient removal in the WWTPs, or looking upstream to initiate source control measures to impede the release of nitrogen and phosphorus into the wastewater. The capture and utilisation of biogas has played a significant role in avoiding the production of natural gas and electricity, and the associated environmental impacts.
Gleanings: Thinking of the urban water and wastewater system as a single entity composed of interrelated components may possibly be easier on paper, but translating the knowledge of the interconnectedness to the adoption of new approaches to the management of the assets, is beset with numerous challenges. In a complex system in which there are ‘wheels within wheels’, changes or modifications made in one part, may have immediate or delayed effects on the others. Just as the component parts of the system are interconnected, so are the social, economic, environmental and functional aspects of sustainability. The priorities are never the same over time. There are innumerable external factors beyond the control of the utilities – prices of energy and chemicals for instance – which need to be taken into consideration. Sustainable development of urban water and wastewater systems is verily a tight-rope walk. Sustainability studies are never completed. This one is no exception. There are numerous aspects which have not been integrated into the research, owing to time constraints, paucity of data, and the subsequent need for narrowing down the scope. This study would however form the bedrock for consolidations, extensions and forays into more detailed examinations of the system.