At risk: the world’s water supply

By David Lui, Ian Rowbottom and Larry Vandeventer

Several recent assessments forecast a looming global water supply crisis, in both the developed and undeveloped worlds. According to a 2002 United Nations report, nearly two-thirds of the Earth’s population is at risk of water shortage. Currently, one person in three is affected by water scarcity due to overuse, pollution, or insufficient sanitation and infrastructure.

The root cause of this problem is essentially a concatenation of simultaneous events: rapid global population growth, expanding urbanization and industrial development, and global climatic change, the latter of which has suddenly affected geographic abundance of supply.

In the U.S., the states of California, Texas and Florida have been particularly affected by urbanization and industrialization.

In Europe, where the generally lower quality water requires more extensive treatment, intensive remediation efforts may not be enough to maintain adequate supply.

Along Spain’s Mediterranean coast, in Australia, and in China, significant droughts have seriously impacted water resources. Estimates suggest that by 2025, 4 to 5 billion people will be living in mega-cities – which will not necessarily be located in areas of renewable or sustainable water supply.

In addition, the predicted geographic distribution of water supply will not effectively match existing and future potential demand. As a result, water has achieved parity with energy as a precious resource requiring careful stewardship.

The UN report summed up the situation tersely: “The simple fact is that there is a limited amount of water on the planet, and we cannot afford to be negligent in its use. We can’t keep treating it as if it will never run out.”

Water industry response

In places where there is a high naturally-occurring availability of water, the principal barrier to additional supply is water quality.

To replace conventional settling which is in many cases too slow, the water industry is introducing advanced high-rate treatment processes capable of producing a larger volume of water using a smaller infrastructure footprint, with enhanced removal of impurities and disinfection of unwanted bacteria and viruses.

The development of a membrane treatment using high pressure reverse osmosis (RO) in the mid-1970s made it feasible to remove salts from brackish ground water, such as in aquifers in Florida, referred to as brackish water reverse osmosis (BWRO). These same RO membranes are used for seawater reverse osmosis (SWRO), when operated at higher pressures.

A further advancement in membrane technology in the last ten years includes low pressure micro-filtration and ultra-filtration membranes developed as filters that remove natural turbidity, pathogens and some viruses. These low-pressure membranes can be used to replace the function of clarification and filtration and provide an absolute barrier to pathogens.

Current trends

As water scarcity increases, the industry is responding with a multifaceted approach that relies on different techniques for different situations.

Conservation
The first emerging trend will be increased emphasis on further conservation in potable, agricultural and industrial use, to directly reduce demand on potable supply and increase available water supply to support growth.

Reuse
The second emerging trend is reuse of treated wastewater for landscape, agricultural and industrial uses, thereby reducing demand on potable water supplies. Wastewater reuse in its simplest form includes using treated wastewater for landscape and agricultural irrigation. Wastewater can also be reused for a large variety of industrial applications, following additional treatment tailored to the specific industrial water quality objectives.

Highly treated wastewater can also be re-injected into groundwater aquifers to rejuvenate diminished groundwater supply (aquifer storage and recovery), or added to large existing surface water reservoirs, and thereby ‘banked’, for future potable supply.

In both cases, the wastewater is ‘aged’ in the groundwater or surface water reservoir prior to additional treatment to meet potable water supply standards. This is commonly referred to as planned indirect potable reuse.

Direct potable reuse is also possible, whereby advance treatment processes are used to produce potable water from treated wastewater, without ‘aging’ as an intermediate step. Reuse is certainly not without its challenges. Of particular concern for indirect and direct potable reuse are newly discovered endocrine-disrupting chemicals detected in raw and treated wastewater. The potential human health affects of these chemicals are not yet fully understood but removal requires advanced treatment such as reverse osmosis and advanced oxidation processes.

The implementation of indirect and direct potable reuse will definitely be significantly affected by public health perceptions, with indirect potable reuse standing a better chance of public acceptance. At present, Singapore’s New Water Project is the only functioning direct potable reuse project in the world, and the ‘potable’ water produced from it is blended with a large quantity of genuinely potable water prior to public consumption.

Desalination
The third emerging trend to increase water supply is desalination of brackish groundwater and seawater.

Brackish water treatment (BWRO) is widely practiced in the world today, and is favored over seawater desalination (SWRO) primarily due to lower cost. SWRO is more expensive than BWRO because of the higher operating pressures and associated energy cost, and the need for greater pre-treatment in the form of clarification and filtration or low pressure membrane treatment, to prevent fouling of the RO membranes.

However, with continuing technological advancements, there is reason to believe this cost differential could be neutralized within the next 10 years. For example, a major trend in implementation of SWRO is co-location of the facilities with power plants that use seawater for cooling. This can eliminate the need for a new seawater intake and outfall for brine disposal, thereby reducing permitting and capital costs. Nevertheless, at present, subsidies to the cost of water of around $300/acre-ft. are required to facilitate a SWRO project in the U.S.

The relationship between desalination and reuse is particularly interesting and deserves careful scrutiny. Reuse and wastewater reuse and desalination, particularly SWRO, will continue to emerge and evolve globally in parallel, with implementation of reuse accelerating more quickly than seawater desalination. General wastewater reuse will continue to be used to offset potable water demands. As opportunities for general wastewater reuse are satisfied, and as water scarcity and groundwater capacity decreases, aquifer storage and recovery and indirect potable reuse will increase. At the same time, as BWRO capacity becomes limited, SWRO will become more of a necessity.

How are these trends playing out?

In the U.S., the first large-scale seawater desalination plant has been installed in Tampa Bay, Florida. However, it suffered from pre-treatment and membrane fouling problems, and was recently redesigned. At present, there are 18 active SWRO projects in California in various stages of implementation. In Texas, the Texas Water Development Board has funded studies to evaluate potential desalination projects in Brownsville, Corpus Christi, and Freeport. In Florida, the possibility of co-locating desalination plants with existing power plants is being evaluated.

In Australia, demand management, operational changes and alternative sources of supply (often requiring additional treatment and therefore extra cost) are increasingly being assessed and used to ‘drought-proof’ urban and rural communities across Australia. The required solutions need to be better integrated and combined to provide a more robust and flexible supply system.

Planning controls are also being reinforced to enforce more efficient water use in residential areas. For example, in parts of Australia (including Sydney) any new residential development must now demonstrate a 40% water saving over conventional usage rates.

Alternative supply sources now coming into consideration include rainwater tanks, desalination (groundwater and seawater), storm water harvesting and recycling of sewage effluent and grey water. Aquifer storage and recovery is also being used in suitable locations for the temporary storage of treated storm water and recycled effluent.

End use is also changing. For example, treated sewage effluent has historically been considered for non-potable uses such as irrigation, industrial cooling water, garden watering and toilet flushing. However, it is now also being considered for indirect and/or direct potable reuse – although considerable community concern has prevented this from occurring. However, as the relative scarcity of water resources increases and as the health concerns and current negative public perceptions are resolved, this may become feasible in the future.

In Hong Kong, the scarcity of water has been a chronic problem since the city was founded. Rapid urbanization and economic development has driven up demand and a series of droughts have exacerbated the supply situation.

At present, 70% of raw water supply is imported from Guangdong, China while local impounding reservoirs provide the remaining 30%. In order to save fresh water, 80% of the population uses seawater for toilet flushing, alleviating 23% of the demand for fresh water. In the last 30 years, China’s own urbanization and economic expansion has constrained the export of water to Hong Kong.

“Lack of access to water – for drinking, hygiene and food security – inflicts enormous hardship on more than a billion members of the human family,” said ex-UN Secretary-General Kofi Annan. “It is likely to become a growing source of tension and fierce competition between nations – but it can also be a catalyst for cooperation.” At AECOM, our goal is to use our skills to ensure that the world’s precious resource of water is used in the most efficient way to the benefit of all.

AECOM is well aware that the challenge of providing sustainable global water supply in the next 50 years is very serious, and we believe through the application of sustainable technologies we can make a major difference, and help to improve global distribution of clean, safe water.

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