M.
Lafforgue
*a and
V.
Lenouvel
b
aSuez Environnement Consulting, Le Bruyère 2000 - Bâtiment 1, Zone du Millénaire, 650 Rue Henri Becquerel, CS79542, 34961 Montpellier Cedex 2, France. E-mail: michel.lafforgue@safege.fr; Tel: +33 4 67 81 63 44
bSuez Environnement Consulting, Parc de l'Ile, 15-27 rue du Port, 92022 Nanterre cedex, France
First published on 27th August 2015
More and more cities are dealing with the concurrent trends of water scarcity and growth in water demand. It has thus become imperative to find alternative resources. Both Singapore and Windhoek – two very different cities in terms of the context in which they have to provide water and the options available to them – have addressed this problem by developing efficient operational solutions for supplying water to their populations. It is therefore instructive to compare their approaches, similarities and differences as a means of identifying the possibilities that might be put to use elsewhere in the world. Water reuse, rainwater harvesting, dual reticulation systems, limitation of water losses, and water saving policies are some of the solutions identified as having the potential to significantly improve the water balance in cities. In practice, one quarter of the water used in Singapore and Windhoek is derived from recycled wastewater. In Singapore, most of this reclaimed water is used for industrial needs, whereas in Windhoek, it is used for drinking purposes. However, other potential problems need to be addressed such as energy consumption and the population's ability to pay for their water supply. Indeed, in Singapore, the energy consumption for water production is on the rise, mainly on account of the treatment processes used, and it is imperative to halt this trend. In Windhoek, on the other hand, a critical issue – potentially affecting the sustainability of the water supply system – is the population's ability to pay. There is every indication therefore that water reuse may be part of the solution for supplying water in water-stressed contexts. A comparative study of the urban water cycles in Windhoek and Singapore sheds light on the importance of addressing the issue of water reuse within a framework that is broader than a strictly technical approach. Closing the water loop is always undertaken along with water demand reduction policies combining education, information and financial incentives, and with urban planning and land use control in order to protect water resources and expand storage capacities.
Water impactMore and more cities are dealing with the concurrent trends of water scarcity and growth in water demand. Both Singapore and Windhoek, two very different cities, have addressed this problem by developing efficient operational solutions for supplying water to their populations. These solutions are applicable in a diverse range of contexts: rich and poor cities, temperate and tropical climates, as well as uniformity and diversity of population and housing. In the context of global warming, water reuse has emerged alongside water demand management as one of the key solutions for meeting water needs going forward, and Singapore and Windhoek have some valuable lessons to offer when it comes to implementing such solutions. |
Against this background, a critical issue is to identify how to reduce water demand and how to limit the abstraction of water from conventional natural sources such as lakes, rivers and aquifers. One of the options often considered is desalination. However, two factors may potentially preclude the use of this technology.
The first factor is the distance between the points of consumption and the sea.2 For cities located a long way from the sea, the cost of transferring water may lead to other options being adopted. This also holds true for other supply sources. In California, for example, energy requirements for water transfer systems can be as great as 4 kW h m−3.3
The second factor that may potentially rule out the use of desalination is the cost and availability of energy. Where energy is expensive or less available, desalination may not be a viable option, for, despite having decreased significantly, the amount of energy required to produce one cubic metre of drinking water from seawater is still in the 2.5–8 kW h m−3 range – around 4 kW h m−3 on average, as compared to 0.5 kW h m−3 for water produced from lakes, rivers or underground sources.3–5
Indeed, the energy requirements of desalination make it one of the most expensive sources of freshwater, which is why many research programmes are looking for ways to minimize the energy costs. One way of achieving energy savings is the cogeneration of energy and water,2 while research centres in Singapore and elsewhere are looking at producing membranes with lower energy requirements.6 Further opportunities for energy savings are to be found within the urban water cycle itself.5 With global energy prices set to increase over the long term, the issue of cost will be even more critical going forward.
As a consequence, other options have to be considered such as a small-loop approach to the water cycle and the demand-side management of water.7–11 The cities of Singapore and Windhoek have many lessons to offer in this respect. Both have limited water resources of their own and both have developed their own unique solutions to address water scarcity. The fact that these solutions are being implemented in completely different contexts opens up a wide range of possibilities for adapting water management solutions to the specifics of other cities around the world.
This paper presents the specific characteristics of water management in the two cities and then goes on to look at the lessons that can be learned in each instance. This approach has also been adopted in the Syracuse project,11 with the aim of identifying the potential for small loops and synergies in the water, energy and waste cycles.
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Fig. 1 Population and water demand in Singapore.18 |
Against this background, Singapore has adopted a water policy with two main objectives: reducing the water demand and increasing the availability of water.
Reductions in water demand have been achieved by means such as incentive-based billing, communication campaigns and water-saving devices.17 These actions have been successful in stemming the rise of per capita domestic water consumption (Fig. 2), which peaked at 175 l per capita per day in 1994 before dropping steadily to 152 l per capita per day in 2012. The current objective is to stabilize the consumption at around 140 l per capita per day by 2030.17,19
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Fig. 2 Per capita domestic water consumption and distribution network efficiency in Singapore.18 |
When it comes to reducing water losses, the distribution network efficiency has been stabilised at around 95% – an impressive result compared to levels found in other cities of the world. Singapore is accordingly on a par with Paris20 and Phnom-Penh21 on this parameter, whereas average water losses worldwide (as estimated by the World Bank in 2006) are in the 35% range and 15% in developed countries.22 By way of comparison, Non-Revenue Water (NRW) levels were estimated at 42% in Manila in 2011,23 25% in Bangkok in 2012,24 and 39% in Jakarta in 2014.25
Singapore's four main supply sources of water, known as the Four National Taps, are detailed below (Fig. 3).
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Fig. 3 The urban water cycle in Singapore (red: wastewater, yellow: raw water, green: treated water for non-potable uses, blue: drinking water). |
The first source of water is imported water from Johor. A supply source used by Singapore for many years now, the raw water imported from Malaysia meets roughly 40% of Singapore's current needs (250 Mm3 per year). The government's objective is to minimize the amount of water supplied from this source by 2061.
The second source of water is rainwater harvesting. Singapore receives roughly 2400 mm of rainfall per year. A harvesting policy covering a large part of the watershed has led to some major urban planning adjustments (involving rebuilding parts of the city and relocating residents and industries to improve the collection of water), as well as measures such as storing water in purpose-built reservoirs (17 in 2013), protecting water bodies to prevent the deterioration of supply sources and building new water treatment plants. Strict regulations and controls have also been introduced to support these measures.15,17,26,27 With the recent addition of the Marina, Punggol and Serangoon reservoirs, the amount of water supplied by this source has increased to 150 Mm3 per year for water collected from two-thirds of the total watershed area, the long-term target being to harvest rainwater from 90% of the catchment area.
The third source of water is reclaimed water. Going by the name of NEWater, this consists of water produced by a combination of microfiltration, reverse osmosis and UV treatment.28 In 2012, the total NEWater production was 110 Mm3 per year, catering for roughly 18% of the total water demand and 32% of industrial and commercial needs.12 2% of this reclaimed water is sent to rainwater storage tanks for treatment to drinking water standards. Municipal effluent is also reused (at a rate of 25 Mm3 per year), more specifically to supply the petrochemical industry in Jurong Island.
The fourth source of water is seawater desalination. Two desalination plants were built in 2005 and 2013 and used microfiltration and reverse osmosis with production capacities of 50 Mm3 and 115 Mm3 of water per year, respectively. As this process is highly energy-consuming, the plants are used essentially as backup facilities to handle spikes in demand.
The following are examples of prices charged for a cubic metre of drinking water produced from each of the four supply sources:29,30 S$ 0.78–US$ 0.49 (indexed to the price of oil and the inflation rate) for water from the Tuas desalination Plant in 2003; S$ 0.45–US$ 0.35 for water from the Tuas II desalination Plant in 2013; S$ 0.3 for water from the Ulu Pandan NEWater Plant in 2007; and S$ 0.3 for water supplied by the Changi NEWater Plant in 2009. Moreover, the price of raw water from Johor as established under the 1961–1962 agreement between Singapore and Malaysia was US$ 0.0026 per m3 (at the 2012 exchange rate).
Against this background, estimates suggest that increasing the share of local resources in the water balance will drive up energy consumption in the water cycle from 0.66 kW h m−3 in 2012 to 1.31 kW h m−3 by 2030.6 For this reason, Singapore is looking for ways to reduce the energy footprint of water at each stage of the water cycle.
Replacing part of the activated sludge process (aeration and final clarification) and the RO pre-treatment process (microfiltration or ultrafiltration) with a membrane bioreactor (MBR) in the NEWater production process will reduce the energy requirement of wastewater reclamation.6,31 The ongoing rehabilitation of the wastewater system and current research on ways of improving the sludge digestion process are also factors that will help the wastewater sector to reduce its energy footprint.32
Yet another solution adopted is to increase the overall rainwater catchment area to up to 90% of the total land area through the use of Variable Salinity Plants. The average energy consumption of these plants, which treat brackish water from undammed estuaries, is 1.9 kW h m−3, and eight suitable sites have already been identified by Singapore's national water agency.33
In the longer term, Singapore is counting on biomimetic technologies to achieve energy consumption levels of 0.75 kW h m−3 in desalination.6,34
Lastly, the city-state is moving in the direction of greater integration of the water and energy sectors. The new Tuas desalination plant houses a 411 MW power plant whose capacity exceeds that required for water production. Although the facility does not actually reduce the energy footprint of desalination, it may, by generating surplus energy, help reduce the energy costs associated with desalination.35 Additional energy sources for desalination such as geothermal energy are also being considered.36
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Fig. 4 The urban water cycle in Windhoek (red: wastewater, yellow: raw water, green: treated water for non-potable uses, blue: drinking water). |
The city's surface water sources consist of the Omatako, Swakoppoort and Von Bach dams, located 60 km to 200 km away from the city, which provide roughly 20 Mm3 of water a year, of which 17 Mm3 is available for Windhoek.37,38 This source of supply is totally dependent on meteorological conditions (explaining the year-to-year variations as seen in Fig. 5). Moreover, the price of the m3 of water supplied by NamWater has increased in recent years, up from N$ 8.29 in 2011–2012 to N$ 9.03 in 2012–2013, N$ 12.94 in 2013–2014 and N$ 14.23 in 2014–2015.39–42
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Fig. 5 Trends in water production in Windhoek as a function of supply source.18 |
The aquifer, which may contribute anything from 0 Mm3 to 5 Mm3 a year to the city's water supply, is located beneath the city (Fig. 5). This source has been improved by the Managed Aquifer Recharge programme recently introduced in the southern part of the city – an initiative in which human activity is controlled to prevent the contamination of groundwater.
In the sphere of water reuse, domestic effluent is treated in activated sludge and biofilter systems before being sent to:
• The old Goreangab water reclamation plant (“old GWRP”) built in 1968 with a capacity of 3300 m3 per day and expanded to a capacity of 7500 m3 per day in 1997. The plant was originally used for drinking water purposes, but now produces semi-purified water for the irrigation of the city's parks and gardens (at 1.3 Mm3 per year, the plant's current output accounts for only 8% of total water needs43).
• The new Goreangab water reclamation plant (NGWRP), a 21000 m3 per day facility built in 2002,44 incorporates the treatment process shown in Fig. 6. The reclaimed water, which is blended with treated surface water in a ratio of one-third to two-thirds,45 accounted for 26% of the total amount of water produced in 2003.38
In rainy years, the surplus potable water is injected into boreholes south of the city after an additional Granular Activated Carbon (GAC) filtration and disinfection treatment to prevent bacterial growth and clogging.
Alternative supply sources considered by the end of the 1990s included pumping from the Tsumeb aquifer (located 490 km from the city), pumping from the Okavango river (750 km from the city), Managed Aquifer Recharge (MAR) in the southern part of Windhoek (offering storage possibilities of up to 11 Mm3 per year), and additional treatment to increase the proportion of reclaimed water in the treated water mix. Table 1 presents water treatment costs and energy consumption per m3. MAR clearly emerges as the optimal solution, providing, for a relatively low cost, an aquifer-based storage capacity of between 33 Mm3 and 60 Mm3.43,46,47 Ideally, these processes should be improved to address groundwater quality issues (salinity, nitrate and Dissolved Organic Carbon levels, among others),38,45 a possible solution being the use of membrane technologies.
Options | Distance (km) | Energy consumption (kW h m−3) | Treatment cost (euro per m3) |
---|---|---|---|
NB: Energy consumption is given for water treatment and conveyance up to the entrance of the water distribution system. | |||
Current supply sources | 0 to 200 | 0.8 to 1.5 | 0.35 to 0.95 |
Okavango river | 750 | 4.94 | 3.33 |
Tsumeb aquifer | 490 | ND | 2.22 |
Reclaimed water with additional water treatment process | 0 | 1.8 | 1.26 |
Managed Aquifer Recharge at Windhoek | 0 | 1.78 | 1.2 |
Singapore | Windhoek | |
---|---|---|
Objective | To be self-sufficient | To meet the needs of the population |
Water sector organisation | Entire urban water cycle managed by the PUB, presence of a top-class private and academic water sector | Entire urban water cycle managed by the CoW, high level of expertise within the municipality |
Economic constraints | An increasingly wealthy population, thus strong capacity to invest | Limited capacity to invest in conjunction with growing CAPEX and OPEX requirements |
Cost of feasible solutions | On the rise because the water produced today is more expensive to produce (in both CAPEX and OPEX terms) | On the rise because the water produced today is more expensive to produce (in both CAPEX and OPEX terms) |
Impacts on energy consumption | Increasing because of desalination and water reuse needs | Increasing because of the need to increase water treatment and pumping |
How the problem has been/is being solved | Improving energy efficiency in the technologies used | Developing alternative local resources |
Possible improvements | Promotion of synergies between water and energy cycles | Reduction of water losses in the informal suburbs |
Greater energy efficiency at wastewater treatment plants | ||
Limitations | Energy dependency | Changes in water quality in reuse loops that necessitate the use of membrane technologies to increase reuse capacities |
Capacity to improve technologies at the same time as water needs increase | Need to consider long-distance transfers if the population continues to grow | |
Capacity to finance water services in the event of continuing growth of the poorest segment of the population |
In both cities, the water authority is responsible for water supply, wastewater collection and treatment, and rainwater and resource management. Although an integrated organizational structure of this kind is not essential in closing the water loop, its simplicity facilitates a joined-up approach to the urban water conundrum. Singapore has also been successful in forging a dynamic private water sector in which domestic and foreign companies work with local universities to drive innovation and the development of best-available-technologies.
As a city-state, Singapore provides a setting conducive to uniform policies when it comes to water management, urban planning, regulation and pricing, while rising GDP facilitates water management through higher capital investment and water tariffs. In Windhoek however, although the CoW (City Of Windhoek) has a substantial degree of autonomy, the decision-making process is in part constrained by central government policies and social acceptance in a post-apartheid context. A key issue is the supply of water to people in low-income areas. In these areas, the population has tended to grow, in part due to migration from rural areas. So with low-income people accounting for a growing share of water demand, there is a growing risk of financial instability in the system – a risk that is all the more critical in the context of the rising price of water supplied by NamWater, as mentioned above.
The solutions adopted are a consequence of these contextual factors. Since its independence, Singapore has embarked on a huge programme of resizing its urban infrastructure in order to collect, store and protect rainwater, and to collect and treat wastewater. The creation of estuary dams is just one example of such infrastructure projects. As a rich city with a wealthy population, Singapore has the resources needed to finance these huge capital work programmes.
Windhoek faces a more complicated situation. Its sole options are to bring in water from distant locations (500 km to 800 km away) and foot the associated transfer bill or to increase the reclaimed portion of the treated water (necessitating increasingly complex and costly treatment processes). To make water more affordable, the CoW has continually striven to optimize the urban water cycle. Since roughly 50% of the water stored in dams is lost through evaporation, it has made sense to store part of this water in an aquifer. This type of storage was found to be feasible beneath the city of Windhoek, which is why the CoW took the decision to recharge the aquifer with the surplus water obtained in rainy years and to store it for use during the dry years. The result is a low-investment solution to the water demand equation for the coming 5 to 10 years.
The second part of the solution has involved reducing water losses and water demand. When it comes to water losses, the CoW has initiated a programme of leak detection, resulting in a distribution system efficiency of currently around 89–90%, an impressive level given the city's low population density. The CoW's aim is to improve the efficiency by a further 4%, with the focus on the poorest parts of the city where losses are likely to be higher. The third part of the solution is the use of membrane technologies for water reuse purposes. This will indeed be necessary if the plan is to increase the share of reclaimed water in the water mix – an area in which Singapore can be considered a precursor.
While Singapore can be considered a precursor in the field of membrane treatment processes, Windhoek merits attention as a precursor in the reuse of wastewater for drinking purposes. Whereas in most cities the possibility of using wastewater for drinking water production runs into problems of acceptability among the population,48 the CoW has been reusing wastewater for some 40 years now with the approval of its citizens. Singapore, for example, reuses a huge part of its wastewater but only 2% of this is used as drinking water at present, the rest being sent to industrial facilities. Over time, Singapore is planning to increase the proportion of reused water in its distribution system as a key route to meeting its objective of water self-sufficiency.
Despite the significant differences between the two cities in terms of the context in which water is supplied and the approaches adopted for water management, there are many areas in which similarities can be identified.
It is through a combination of these measures that the cities of Singapore and Windhoek have brought their per capita water usage down to 152 l per day and 155 l per day, respectively – in both cases achieving roughly a 20% to 30% reduction in the total water consumption (Fig. 7 covers all water uses including municipal and commercial uses (roughly 20%) and water losses (10%), whereas Fig. 2 includes only domestic consumption).
A key issue is the need to reduce energy consumption in the water cycle. Singapore's goal of water self-sufficiency has indeed pushed up its energy requirements as demonstrated clearly by Lenouvel et al.6 Reducing energy consumption in the water cycle has thus become imperative for Singapore, and the city has embarked on a huge programme of energy savings (achieved by means such as improved membrane technologies and energy recovery in wastewater treatment plants). Reducing energy usage is also a key objective for CoW, since it directly impacts the cost of water as seen in Table 1. Most cities that have adopted water reuse technologies have to address the need to optimise energy usage in the water cycle, and both Singapore and Windhoek have made substantial progress in this direction.
Windhoek's approach to water management is particularly insightful as it demonstrates that even in a stressed environment and with a large poor population, there are indeed feasible solutions to meet the population's water needs, flexibility and pragmatism being the bywords for effective water management. Singapore is a leading light when it comes to technological research, while providing a good example of how urban planning and water management can be interlinked.
These two complementary cases provide some valuable insights and demonstrate the need to adapt solutions to local contexts and constraints. Although very different in terms of their economic, social, urban and water contexts, Singapore and Windhoek show that success in closing the water loop is not only a technical matter, but that water reuse has to be integrated in a more comprehensive policy, involving a savvy mix of urban planning and land use control, water demand reduction and development of investment capacities.
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