Under the IPCC definition (e.g., IPCC, 2007a), mitigation is action to reduce emissions of greenhouse gases in order to reduce overall climate change. Notably the IPCC Fourth Assessment Working Groups II and III conclude that "mitigation primarily involves the energy, transportation, forestry, and agricultural sectors, whereas actors involved in adaptation primarily represent a large variety of sectoral interests, including agriculture, tourism, and recreation, human health, water supply, coastal management, urban planning and nature conservation" (IPCC, 2007a). Most of the references in the Working Group III 2007 Assessment Report (IPCC, 2007c) to urban water and wastewater refer to improvements in wastewater management as an approach to mitigating emissions (primarily of methane). Nonetheless, in the urban-water sector, there are several important mitigation options that can be incorporated into planning and operations because, in urban areas, water and energy are inextricably linked.
Water conservation/demand reductions
Reductions in water use can have multiple benefits, including cost reductions, increased overall supply reliability, and mitigation of greenhouse gas emissions (e.g., Cohen et al., 2004; Klein et al., 2005a). Less water used can mean less water needing to be captured at, and drawn from, various reservoirs and aquifers, less water to be transported and lifted over obstacles, less water to be treated, less water to be heated, less wastewater to be treated, and less wastewater to be transported and disposed of (Figure 5.1). Each of these steps in the water system generally requires energy (e.g., Cohen et al., 2004; Klein et al., 2005a). For example, in California, the State Water Project, which transports water from the wetter northern parts of the State to urban southern parts, is the largest single electrical energy user in the State (Cohen et al., 2004). In many cities, the energy used for water supply, treatments, and disposal has come from burning fossil fuels that emit greenhouse gases into the atmosphere. In many – perhaps, most – urban systems and situations, water conservation and demand reductions can provide greenhouse gas emissions mitigation benefits.
Urban water-use demands for residential supplies are typically largest in cities where housing is most dispersed, because outdoor uses of water are frequently the largest demands (e.g., Mayer et al., 1999). Thus, in many dense urban areas, achieving substantial demand reductions can be difficult. Nonetheless, one particularly important mechanism for controlling water waste in the cities of many developed and developing nations is reduction of large-scale leakage from the water-supply infrastructures. Lallana (2003b) compiled urban water-supply leakage estimates from 15 European nations, and found leakage rates ranging from about 4 percent of the total water supplies to 50 percent (Figure 5.4).
In informal urban settlements, planning and maintenance of water delivery systems are presumably less rigorous than in the formal areas, and leakage losses may be even larger. Leakage losses also represent opportunities for contamination of water supplies, so that efforts to reduce leakage will provide multiple benefits (Lallana, 2003b). Reduction of leakage is likely to depend on pressure in the water mains, soils, topography, and age of the water systems. Nonetheless, progress is possible and, indeed, being made in many urban water systems.
Water reclamation and recycling
Reclamation and recycling offer opportunities for reducing the energy used to provide water supplies (see e.g., Furumai, 2008). Recycled water generally still requires treatments that demand energy, but otherwise many of the initial extraction and transport energy demands can be reduced or eliminated because the reused water is already in the municipality (Figure 5.1).
Attention to energy efficiency of water supply expansion
More generally, most actions to expand or improve water supplies have ramifications in terms of overall energy use, which in turn need to be carefully assessed in terms of greenhouse gas emissions. Development of some urban water sources – such as groundwater pumpage or, more recently, desalination of brines or seawater – can require amounts of energy or conditions of energy development that may be problematic in terms of mitigating greenhouse gas emissions, especially if they are allowed to degrade (e.g., overdraft of aquifers with attendant increases in pumpage lift). Energy requirements for treatment of some water sources can also be decreased or increased depending on whether the water quality of the source is managed or mismanaged.
Figure 5.4: Urban water supply leakage losses from 15 European nations. Note that leakage rates will vary by city within countries.
Hydropower and reservoirs
Hydropower and surface water reservoir-based water supplies can have implications for mitigation of greenhouse gas emissions, although those benefits remain difficult to specify. Most reservoirs emit varying amounts of greenhouse gases through processes involved in the natural carbon cycle (Battin et al., 2008), although some reservoirs also absorb these gases. In particular, greenhouse gas emissions can be significant from shallow tropical reservoirs; e.g., Fearnside (1995) calculated emissions from two reservoirs in Brazil amounting to tens of millions of tons of CO2 and tens of thousands of tons of CH4 in a single year, three to six years after their initial inundation. It is believed that deeper, cooler reservoirs emit less gas (IPCC, 2008). However, wetlands and floodplains that were inundated in the establishment of reservoirs may also have been methane emitters, and those wetland emissions may have been substantially reduced by their inundation (Mata and Buhooram, 2007; IPCC, 2008). Thus, opportunities for greenhouse gas emissions mitigation may also be available in the planning, operations, and use of hydropower in general, hydropower as energy supplies for urban water and wastewater systems, and reservoir-based water supplies, but the extent of these opportunities generally needs more study.
Urban water heating
A significant amount of the energy associated with urban water supplies is dedicated to water heating for residential, commercial, and some industrial purposes. For example, in California, over half of urban water uses are residential, and over half of those uses involve water heating (Cohen et al., 2004, Figures 3 and 4, and p. 26). Actions and opportunities that favor the expansion of solar water heating have been identified as a useful part of urban mitigation programs (e.g., Razanajatavo, 1995; IPCC, 1996b; Nadel et al., 1998) in both developed and developing nations. Localized small-scale wind power could also help minimize the centralized energy needed to pump, distribute, and heat water for urban and domestic uses. Even if solar heating is not deployed, it is often possible to reduce greenhouse gas emissions by heating with fuels (or electricity) that are less carbon intensive, or by using tankless water heating.
Watersheds and river basins
Many traditional urban water systems tap into water supplies derived from broad hinterland watersheds and river basins (Kissinger and Haim, 2008; Broekhuis et al., 2004). Many cities, recognizing the vulnerabilities of these hinterlands to contamination and disruption, are moving to institute better land use and watershed management practices in these resource areas. Land use and watershed management impact overall land- and water-surface emissions or sequestrations of greenhouse gases, and thus need to be assessed in terms of their mitigation impacts or benefits, along with other costs and benefits.
Wastewater
Cities are large and concentrated producers of wastewater (Satterthwaite, 2008). Methane emitted during wastewater transport, treatment, and disposal, including from wastewater sludge, amounts to 3 to 19 percent of global anthropogenic methane emissions (IPCC, 1996a). Globally, the major sources of the greenhouse gas nitrous oxide (N2O) are human sewage and wastewater treatment (IPCC, 2007b). Methane emissions from wastewater are expected to increase by about 50 percent in the next several decades, and N2O emissions by 25 percent. Thus, one of the most direct ways to mitigate greenhouse gas emissions is through improvements in collection and management of urban wastewaters, using technologies most appropriate to the economies and settings involved (IPCC, 2007b). Technologies already exist for reducing, and perhaps reversing, these emissions growth rates.
In cities in developed nations, wastewater treatment facilities are sometimes major greenhouse gas emitters but those emissions have been identified as important avenues for overall greenhouse gas emissions reduction.A large proportion of the greenhouse gas emission from urban waste-water, however, is expected to take place in developing countries (e.g., Al-Ghazawi and Abdulla, 2008) and from informal urban settlements. In many of those developing countries and informal urban settlements, rapid population growth and urbanization without concurrent development of sufficient wastewater collection, treatment, and disposal infrastructures results in very large and unmitigated greenhouse gas emissions. Non-existent sewer systems, open sewers, ponding, and unchecked releases of untreated wastewaters are a fact of life in the informal sectors of cities in both developed and developing countries (IPCC, 2007b; Foster, 2008). Improved sanitation facilities, infrastructures, treatment and disposal systems in these settings would not only mitigate emissions but would offer substantial public health benefits as well (e.g., Al-Ghazawi and Abdulla, 2008; IPCC, 2007b).
