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As water and energy are becoming limited resources, water footprints in the energy sector and energy footprints in the water sector are increasingly concerning in development and planning processes. In the context of cities, energy is of primary importance for urban water system management. From source abstraction, conveyance, treatment, distribution, waste water collection and treatment to recycle and disposal, every element of urban water system relies on energy. Typically, fossil fuels are the primary sources of energy, which produce considerable amounts of carbon dioxide and other greenhouse gases (GHGs) in the atmosphere. The relevance of an energy-carbon footprint lies not only at the operational stage of water management but also at the construction of infrastructure in the form of embodied energy. This gives rise to the concept of a “nexus,” where water, energy and carbon can be managed under the same domain. Cities are a significant place to study this nexus because of high population density, complex agglomeration of infrastructure, economy, industry, technology and their overall dynamics. The high energy demand for water utilities is one of the issues in sustainable management of water and sanitation services in developing and developed countries. There is limited research in Asia, and few efforts have been made in development and planning to address the water-energy-carbon nexus.

Drivers that Influence Water-Energy-Carbon Nexus in Cities

Authors: Dr. Shobhakar Dhakal, Asian Institute of Technology; Dr. Sangam Shrestha, Asian Institute of Technology; Mr. Ashish Shrestha, Asian Institute of Technology; Prof. Arun Kansal, TERI University; and Prof. Shinji Kaneko, Hiroshima University.

The geophysical, climatic, technological, social and economic environments in cities affect their water-energy development. As a result, the energy-carbon footprint per person with respect to per unit of water used will differ within cities and between countries. It is vital for water sector planners to understand the drivers that influence the water-energy nexus in order to formulate policies for optimising energy-carbon footprint. Urban settlements support more than half the global population and 2.8 billion more will be added by 2050. Energy use in the water sector will continue growing to meet the increasing water demand. Climate change will influence water availability, water quality, salt water intrusion, water and energy demand, and may impact built infrastructure, which have further implications on energy. Technological change, innovation and other factors affect the adoption of new water treatment technologies. This might have positive or negative implications on energy. However, newer technologies, which are less energy intensive, could be emphasised. For example, most households in New Delhi have been purchasing reverse osmosis water purification units in recent years, which will eventually increase the energy footprint of water. Further, water consumption patterns, physical loss of water in distribution networks, ageing infrastructure and inevitable maintenance will have more energy implications. 45–88 million m3 of water is lost every day due to leakages in water supplies worldwide, half of which are in developing countries. This amount is sufficient to serve 200–400 million people (Olsson, 2012).

Various factors determine the design and construction of water infrastructures. Among them are water sources, quality, future water demand, water/waste water standards, environmental regulations, natural hazards, feasible technologies and budgets, etc. Any new water infrastructure design and construction should be assessed and regulated by responsible institutions. The energy footprint of water infrastructure depends not only on its design but also on the embodied energy of construction materials in those systems. Therefore, different scenarios should be studied and planned—for example, selection of lined canal versus natural conduits for drinking water transport, or open drainage versus closed pipes for waste water systems. There is a need to assess or model every element of an urban water system to foresee its possible energy-carbon implications.

In Bangkok and Tokyo, surface water is a major water source. In Delhi, however, excessive extraction of ground water is in practice, which will eventually result in higher energy footprints of water abstraction. Moreover, in most of the East, West and South parts of Delhi, ground water depth has increased in recent years, causing more pumping energy requirements. The energy intensity in drinking water treatment is slightly higher in Tokyo than in Bangkok or Delhi due to its higher water quality standards, although this is not an accurate comparison as the value chain of energy footprints will be higher in cities like Delhi if energy intensive end-use water purification is involved.

Opportunities to Reduce Energy and Carbon Footprints in Urban Water Systems

Apart from efficient design and optimum operation in urban water system management, other options to reduce net energy-carbon footprints must be explored. The majority of energy in many cities still comes from fossil fuels. Some water and waste water treatment plants in Tokyo use solar energy to meet part of their energy demands. Similarly, the chemical energy of waste water treatment by-products are utilised as a means of resource recovery. For example, in Tokyo’s Tobu Sludge Plant, carbide products manufactured from sludge are being used as fuel in coal-fired power plants. A small percentage of treated waste water is also utilised in Delhi for gardening. The application of decentralised green energy systems in water/waste water systems and resource recovery from treated waste water shall be scaled up.

In most cities, policies and practices related to water/waste water, energy and carbon management exist independently outside the framework of a water-energy-carbon nexus. Few city-level governments are taking actions to reduce the net energy-carbon footprint related to urban water sector, such as the design of energy-carbon efficient infrastructure, promoting the use of alternative energies and recovering energy from treatment by-products. Integrating water, energy and carbon policies with direct participation from different government departments will enhance their overall goals and help achieve the common objectives to meet GHG reduction targets and ensure energy and water security.