Maximizing the synergies between energy and water

He also examines seawater and other alternative sources and explains how they can be treated and re-used by advanced filtration and separation technologies to reduce pressure on drinking water sources.


Water management involves intercepting the hydrological cycle at the
municipal level. (Image courtesy of varuna/shutterstock) 

 

We read about water scarcity and climate change issues in the news regularly. What used to be a topic restricted to technical specialists has now become a common focus for conversation in political and economic arenas.

Water scarcity is a significant and well documented problem that continues to cause concern as the human population grows and industrialization increases. But all the media attention has led to many advances, both in terms of technology and in manufacturing processes, to improve the situation and provide reliable sources of drinking water.

Seawater provides an obvious resource to tackle the problem of water scarcity, especially and obviously in coastal areas, but alternative sources of water can also be utilized where a readily available conventional supply is unavailable. We can define alternative sources of water as those which have not been significantly exploited, due to the need for advanced technology to remove elements such as organic and inorganic pollutants and toxins.

Examples of alternative sources of water include highly turbid river water, eutrophic lake water or secondary treated effluent. Alternative sources have been successfully used for indirect drinking applications, ultrapure water processes such as semiconductors or high purity water applications in manufacturing industry or power generation systems and we describe examples of these applications below.

Compounding the problems associated with water scarcity is the expanding global energy consumption issue. Water usage and energy consumption are intimately linked - the production of water requires a significant amount of energy and the production of energy requires a significant amount of water. As a result, process solutions to produce drinking water must also take into account the need for energy conservation.

Advances in water reuse technologies have allowed designers and engineers to make better use of water that has already served one need but with effective treatment could go on to serve one or more additional purposes. However, advances in desalination technology can make it more economical to turn seawater directly into drinking water or, with further treatment, into water for industrial purposes.

Industrialization and population growth lead to water scarcity problems. (Image courtesy of leungchopan/shutterstock)

 

Hydrological cycle

But the opportunities for increasing environmental sustainability can be overlooked by concentrating on the separate applications of water and wastewater treatment without considering the whole hydrological cycle, which needs to be managed in an integrated way so we avoid focusing on these disciplines as unrelated subjects. The benefits of utilizing alternative water sources have become better understood over the last decade.

The responsibility for the management and control of water consumption throughout the hydrological cycle typically rests with local governments, agencies and their regulators. Their priorities are to reduce waste, control demand, minimize leakage and evaporation losses and maximize the efficiency of the water management process.

On a planetary scale, the hydrological cycle involves evaporation from sea and land, the formation of clouds and then the water falling back to Earth as precipitation, being collected in rivers and entering the sea again before being intercepted for use as drinking water and discharged from municipalities as treated effluent. We can think of the concept of shrinking this cycle and just consider the local perspective, with treated wastewater being recycled into drinking water in adjacent treatment plants, or planned indirect potable use of wastewater, where treated wastewater is used to replenish groundwater. The groundwater is then used as a drinking water source.

Often, when seawater is not readily available, we find power generation facilities located on the outskirts of municipalities along with wastewater treatment facilities and drinking water treatment systems fed from alternative sources. We could consider using these alternative sources of water for cooling and process water in the power station rather than using treated water but this requires us to be able to manage the hydrological cycle for the city as a whole. If we integrate power generation and water treatment processes as a single management activity, we can focus on various benefits including reduced aqueous pollution into receiving waters and retention of high quality water for drinking supply.

Hybridization

Hybridization is an approach to increase the efficiency of an overall system by taking advantage of the synergy between different unit process solutions.
In a municipal water management plan, the efficiency of power generation systems could be increased by reducing the inefficient period which occurs during periods of low demand such as during the night or in the summer months. This unused power generation capacity could be used to increase water treatment rates if there is sufficient water demand or storage capacity.

Also, any surplus energy generated from intermittent renewable sources, such as wind or solar photovoltaic systems, during periods of low demand and high production could be directed towards water treatment processes. Hybridization could be used to exploit this unused capacity as part of a comprehensive water management plan.

Using the example of treating wastewater for planned indirect drinking use, demand periods would tend to vary on diurnal cycles. During the low demand period, water could be “banked” into groundwater or reservoir storage at a high rate to create fresh water aquifers in brackish or saline zones, or supplement surface water supplies. Also, where appropriate, this could help control seawater ingress due to over abstraction of groundwater or, in the future, help combat rising seawater levels expected due to climate change.

We can argue that we could then effectively convert unsold off peak electricity or excess renewable production into valuable fresh groundwater in the low demand period, for future use in the high demand period, hence effectively storing energy and using the groundwater or reservoir “bank” like a battery.

Technology

Desalination of seawater can be achieved by using thermal or membrane processes, or a hybrid combination of the two types. In the late 19th century, the first major technical advance in desalination technology was the development of the multiple effect distillation (MED) process. Here, pre-heated feed water flowing over tubes in the first effect is heated by prime steam, resulting in evaporation of a fraction of the water content of the feed.

The water vapor generated by brine evaporation in each effect flows to the next effect, where heat is supplied for additional evaporation at a lower temperature. There the vapor condenses, giving up its latent heat to evaporate an additional fraction of water from the brine. The process of evaporation-plus-condensation is repeated from effect to effect, hence the term ‘multiple effect.’ Each effect operates at successively lower pressure and temperature.

In the mid-1960s multistage flash (MSF) distillation became popular. In MSF, seawater (after mixing with the recycle stream) is pressurized and heated to the maximum top brine temperature (TBT). When the heated brine flows into a stage maintained at slightly below the saturation vapor pressure of the water, a fraction of its water content flashes into steam. The flashed vapor passes through a mist eliminator and condenses on the exterior surface of heat transfer tubing.

The condensed liquid drips into trays as a product water. The unflashed brine enters a second stage where it flashes again to vapor at a lower temperature, producing a further quantity of product water. The flashing-cooling process is repeated from stage to stage until both the cooled brine and the cooled distillate are finally discharged from the plant as blow-down brine and product water.

Reverse osmosis

At the same time as MSF was being developed the first reverse osmosis (RO) membrane was produced. In RO, treated water termed ‘permeate’ passes from the feed to the product side of the membrane when a pressure exceeding the osmotic pressure of the feed water is applied. This ‘reverses’ the natural osmotic flow and concentrates salt ions into a waste concentrate stream.

In the early years of desalination, positive displacement and centrifugal pumps provided 100% of the energy to power a seawater RO plant, but innovations in the field of energy recovery have improved efficiency. Waste energy from RO systems can be recovered and can account for 25-30% of the energy required to overcome the osmotic pressure of seawater. This lowers the total energy requirement of desalination plants dramatically.

Nearly all membrane-based desalination plants today utilize some form of energy recovery. Future innovations in pumping and energy recovery, combined with innovations in membrane technology, hold the key to lowering the operating cost of desalination even further.

An equally important aspect of lowering the costs of desalination is the reduction made in the capital cost of membrane-based RO systems. This cost bears a direct relationship to the overall cost of water, as most plants are financed and their initial costs are amortized into the overall cost of water produced.

It is likely over the coming years that we will see a continuing reduction in energy costs per unit treated water due to more efficient energy recovery systems, the introduction of new types of membranes and higher fluxes through the RO membranes. It is also likely that new types of membranes will be far more fouling resistant.

Water entering RO systems can be pre-treated to reduce or eliminate the potential for fouling using other technologies such as ultra-violet (UV) sterilization and the membrane technologies ultrafiltration (UF), microfiltration (MF) and nanofiltration (NF). UF and MF technologies can be incorporated into membrane bioreactor (MBR) systems. Here, a feed supply high in organic fouling potential can be treated in an aerobic bioreactor to reduce total suspended solids and chemical oxygen demand, with the solids removed in a separate ‘side-stream’ or an integrated membrane system.

The International Desalination Association (IDA) advises that the effective use of MBR, UF, MF, NF and RO technologies is likely to dramatically increase the number and size of water reuse projects over the next decade and we could envisage the direct coupling of advanced water reuse technologies with seawater desalination as a means of reducing energy and cost still further.

Notable examples of advanced water reuse techniques also incorporating the concept of water banking described above are currently installed at various sites around the world. Two examples in Singapore and California, United States, are summarized below. We also include a smaller UK example where secondary treated effluent is processed for feed directly as required into a power station as demineralized water.

A number of large scale hybrid seawater desalination plants are under construction utilizing the thermal and membrane technologies described above. Many of the recent plants in the Middle East are being built as integrated hybrid solutions where the best properties of thermal processes are combined with the best features of membrane technology.

This provides optimum solutions for power production and the production of fresh water from the desalination systems as demand varies throughout the day and year. These hybrid projects combine power generation capability with MED or MSF, and NF and RO systems are added to raise efficiency and water recovery rates.

Singapore

The Singapore Water Reclamation Study was initiated in 1998 by the Singapore Public Utilities Board (PUB) to determine the suitability of using high-purity water as an alternative source of ground and surface water to supplement Singapore's water supply. The treated NEWater (the braded high quality water) meets the USA Environmental Protection Agency and World Health Organization drinking water standards. It is produced from municipal secondary treated effluent using MF, RO and UV.

Following extensive pilot testing, full scale NEWater factories at Bedok and Kranji water reclamation plants were commissioned in 2002. The largest plant at Kranji was designed to initially treat 40 MLD with a capacity to expand to 72 MLD in the future.

Since February 2003, treated water has been supplied to industries for non-drinking use in power generation and other applications. In 2004, the third NEWater factory at Seletar was commissioned, which began supplying treated water to the microelectronics industry. Then, high purity water produced directly from the secondary treated effluent was being used in semiconductor wafer fabrication plants at the Ang Mo Kio industrial area in Singapore.

The recent addition of the Sembcorp NEWater Plant marks a major step in Singapore’s water sustainability activities with its total capacity increasing significantly to 228 MLD, a similar capacity to the GWRS in California (see below). The NEWater Project meets more than 30% of Singapore’s total water demand with the majority of the reclaimed water supplementing surface water reservoirs.

The contract for the latest NEWater facility was awarded to Sembcorp in January 2008 by the PUB under a public-private partnership initiative. The project was engineered in two phases over a two-year period. The initial phase of the plant, producing 69 MLD, began operations in 2009.

This was the second and largest NEWater plant to be designed, built, owned and operated by the private sector, while PUB owns another three plants. This new plant uses the same established and proven water reuse technology pioneered by PUB. NEWater from this plant meets the same quality standards as those applicable to the other NEWater plants.

California

The Groundwater Replenishment System (GWRS) in California also processes municipal secondary treated effluent that would have previously been discharged into the Pacific Ocean. The ‘waste’ is treated using a three-step process consisting of MF, RO and UV with hydrogen peroxide. The process produces high quality water that exceeds all Californian and US federal drinking water standards. Treated water is used to replenish groundwater reserves and increase the supply of lower saline sources for subsequent drinking water production, irrigation and agriculture.

Operational since January 2008, this advanced water purification project can produce up to 265 MLD of high quality water, enough, when treated further, to meet the needs of nearly 600,000 residents in north and central Orange County, California.

The design and construction of the GWRS was a project jointly funded by the Orange County Water District and the Orange County Sanitation District (OCSD). These two public agencies have worked together for more than 30 years, previously on the Water Factory 21 project that led to the development of the GWRS.

Water used in the GWRS is first treated at OCSD, which collects more than 757 MLD of wastewater and removes a high degree of impurities through several processes. A stringent source control program limits metals and chemicals flowing into OCSD's plants in the Fountain Valley and Huntington Beach areas. The wastewater undergoes treatment through bar screens, grit chambers, trickling filters, activated sludge systems, clarifiers and disinfection processes before it is sent to the GWRS.

Following treatment by MF and RO in the GWRS, the treated water is exposed to high intensity UV light with hydrogen peroxide to disinfect and destroy any trace organic compounds that may have passed through the RO membranes. This provides an effective disinfection and advanced oxidation process that eliminates these compounds.

UK

A smaller scale project in the UK has been in operation for over a decade and incorporates MF and RO technology with secondary treated municipal effluent again used as an alternative source, processed into high purity water for feeding directly into the Centrica-owned Peterborough gas turbine power station. The station is located adjacent to Flag Fen Sewage Treatment Works. ACWA Services built and installed the plant, commissioning the system in 2000.

The RO plant produces water with a conductivity of less than 60 μS/cm enabling demineralized water production at the power station to increase by 20% with a reduction of over 90% in the costs of ion exchange regeneration. A total of 1.25 MLD of drinking water is saved (previously purchased from Anglian Water) which initially reduced the power station’s total water usage by 11%.

Whilst the Flag Fen project is not on the same throughput scale as the NEWater Project or the GWRS, it does show demand for drinking water can be reduced at the municipal or smaller scale by industries utilizing alternative sources of water directly, using proven advanced filtration and separation technology. The geographical location of the power station in Peterborough and Flag Fen sewage treatment works was a main driver for the project. But the opportunity for synergies and hybridization were limited due to the type of power station in existence and the water quality and volume requirements.

Saudi Arabia

The largest operational hybrid desalination and power production plant in the world is currently the Ras Al-Khair hybrid plant in Saudi Arabia that incorporates RO membranes and MSF thermal technology to produce 308 MLD and 728 MLD respectively. The plant is dual purpose, with an export production capacity of 1,025 MLD desalinated water and an electricity production capacity of 2,650 MW.

The construction of the plant commenced in February 2011 and was completed earlier this year. The power plant comprises five 600 MW combined cycle gas turbine units and two 220 MW single cycle gas turbine units. Maaden's new alumina refinery, located nearby, will utilize up to 1,350 MW of the electricity and 25 MLD of the water produced.

The largest operational hybrid thermal desalination and power production plant is currently the Jubail Water and Power plant (JWAP) incorporating MED technology, also located in Saudi Arabia. This is a Marafiq plant built by SIDEM with 800 MLD production capacity from 27 MED units. The cost was US$ 1 bn. This is also a dual purpose plant generating 2,744 MW electricity in addition to the desalinated water.

United Arab Emirates

The largest hybrid MED-RO plant currently is the Fujairah II project, a SIDEM/Veolia project built as a green field development in the United Arab Emirates and producing 2000 MW of power and 591 MLD of desalinated water. The hybrid system includes five high-efficiency gas turbines operated in combined cycle mode.

The Fujairah I project, owned by Emirates Sembcorp Water and Power Company and commissioned in 2004, comprises a hybrid MSF-RO system again combined with power production, with an electricity generation capacity of 893 MW and a seawater desalination production capacity of 455 MLD.

We have suggested that comprehensive water management, which examines the water cycle as a whole, is required to make the most effective overall use of seawater and alternative sources of water whist optimizing power generation capacity. In addition, there is the need to reduce waste, demand, leakage and evaporation losses so we can maximize water management efficiency.

The GWR system and NEWater plants provide excellent examples of how alternative sources can be used on a municipal scale to expand drinking water supplies and reduce the pressure on traditional raw water sources. Filtration and separation technology for the treatment of secondary effluent is now well proven and consistently high quality, high purity water can be produced.

Based on the large scale experiences in Saudi Arabia and the United Arab Emirates, we can envisage the possibility of future projects where alternative water and seawater systems can become integrated with power generation systems to provide cooling, process and freshwater for banking and storage when required, using various local water sources.

This would make the most effective use of hybridization in the selection of process technologies, reducing the overall energy requirement and environmental impact of a municipality by making full use of generation capacity during periods of low power demand and high renewable energy production.

The challenge is for the development of integrated solutions at the municipal scale where both water efficiency and energy efficiency can be maximized whilst maintaining security of drinking water supply.

The world will continue to face pressure on its water supply. Population growth, climate change and industrial development with increased energy requirements will inevitably force the trend to continue. But the high visibility of the water scarcity issue today has driven and will continue to drive innovation and investment in the water and energy sectors, with the development of emerging technologies and improved manufacturing processes.