Governments and their regulators have the responsibility for management and control of water consumption throughout the hydrological cycle. The priority for municipalities is to maximise water management efficiency by reducing wastage, demand, leakage and evaporation losses.
According to Bruce Durham, technical secretary of the EUREAU Water Recycling and Reuse Working Group (www.eureau.org) and alternative resource project manager at Veolia Water (www.veoliawater.com), “Throughout Europe, water stress is increasing, due to changing population density and climate, diffuse pollution, unreliable precipitation, short-term population increases due to tourism, and increased demand for irrigation to improve agricultural productivity.
“At the same time, the EU Water Framework Directive could lead, in some regions, to a reduction of 15-20% of abstraction to protect surface and groundwater quality and quantity.”
At a recent meeting of EU water directors, held in Mondorf-les-Bains, Luxembourg, it was concluded that, under the terms of the EU Urban Wastewater Treatment Directive, wastewater reuse was now firmly on the agenda for discussion at future meetings. Hence, we can expect the debate on integrated water management to intensify over coming months.
Scales of the cycle
Reclaimed water is used for indirect potable applications, ultrapure processes or high-purity water applications in manufacturing industry or power generation. Where consumption is in excess of water available, seawater desalination normally makes up the deficit, at high cost.
The benefits of water reuse are not well understood by municipal decision-makers, and the important opportunities for increasing environmental sustainability can be overlooked. The hydrological cycle needs to be managed so that we avoid the common mistake of focusing on the issues of potable water quality and wastewater treatment for environmental protection as if they were unrelated subjects.
This hydrological cycle can be viewed as having three scales:
In most parts of the world, regular precipitation is required in order to satisfy potable water requirements. In many areas, historical records for droughts and floods have been broken almost every year over the course of the last decade. Increased urbanisation and risks to the supply of imported water reduce the availability of potable water. As a consequence, we cannot always rely on regular precipitation or imported sources of water.
Many countries with similar climates and economies have embraced reuse for industrial applications, saline ingress control, groundwater recharge and irrigation. According to Durham, this has resulted in an increase in the water volume used for reuse projects of 25 to 60% per year over the last five years, compared to desalination growth of approximately 17% per year in the most active regions. Reuse projects have helped local economies develop more robust and flexible water management strategies. Experience and technological developments are building export business and providing solutions needed to respond to climate change.
Water reuse is of fundamental importance to environment and economy. Benefits are numerous and include reduced aqueous pollution into receiving waters and retention of high-quality water for potable supply. Reuse links the large water cycle and the smaller local water cycle together and can be used as a catalyst to help understanding of the hydrological cycle as a whole.
Lower water production costs on reuse and desalination projects are the key to increasing water availability. Energy costs are usually the largest contribution to cost (25-40%), due to the physical weight of water and the need to remove dissolved pollutants with the aid of membrane or thermal systems.
Hybridisation is an approach to increasing the efficiency of a process by taking advantage of the synergy between the strengths and the weaknesses of different process solutions. These process solutions can include competing technologies such as thermal and membrane desalination techniques.
One approach being studied is how to increase the efficiency of power generation systems by reducing the inefficient, off-peak, low-power demand period. This unused power generation capacity could be used to increase valuable freshwater production if there is sufficient demand or storage (Figure 1).
Says Bruce Durham, “UNEP have demonstrated that more water is consumed by evaporation from reservoirs globally than the volume of water used by man. Therefore groundwater storage, or water banking, is an attractive proposition”.
New freshwater storage aquifers are being extensively used around the world for freshwater banking. Water banking can increase the security of supply during droughts or disasters. Durham argues that converting off-peak electricity into valuable fresh groundwater for future use is equivalent to energy storage.
This type of hybrid solution could include large wastewater re-purification systems to produce fresh water and thus control over-abstraction of groundwater or seawater ingress, or to create new freshwater aquifers in brine or seawater zones. Water banking is already being undertaken in a number of municipal applications worldwide, and looking at a few of these applications is instructive.
In the United States, for instance, control of over-abstraction of groundwater and seawater ingress has been practised in several areas for more than 25 years. At Orange County Water District (OCWD, www.ocwd.com), southern California, high-quality water reclaimed from treated wastewater has since 1976 been injected into ground water as an alternative source.
Currently, OCWD is constructing the Groundwater Replenishment (GWR) System. Their advanced membrane purification facility will
provide 265Ml/d of treated water, all of it to be treated using micro-filtration, reverse osmosis and UV/hydrogen peroxide. Half the water produced will be injected to expand the seawater intrusion barrier; the remainder will be sent through a 20km pipeline to percolation ponds, where it will naturally enter and recharge the groundwater basin.
OCWD say that the GWR system will save money by improving the quality of the water in the groundwater basin, and it will provide a new drought-proof water source whilst using about half the energy compared to the alternative of importing potable water.
Recycled water was chosen by OCWD because, in addition to economic advantages, environmental benefits included:
Elsewhere in the USA, the Scottsdale Water Campus is a water resources management facility located in Arizona, providing water reuse technology which is used to reclaim wastewater for large-scale turf irrigation, as well as aquifer recharge. The 45Ml/d advanced water treatment facility treats tertiary effluent, prior to subsurface discharge, for indirect potable reuse.
The Water Campus has been in operation since 1998. It possesses a significant knowledge base with respect to water quality, performance, and supply issues associated with water reuse. Koch Membrane Systems’ thin film composite 20cm-diameter high-rejection reverse-osmosis membranes have been used at Scottsdale since 1999.
This site has also participated in successful field tests of 44cm-diameter elements. The owners are looking at large elements as a way of increasing the production capacity within the existing building. In autumn 2004, the 17.25-inch pilot unit was retrofitted to accept 18-inch Mega-Magnum elements. The larger membranes operate at a similar productivity and efficiency to the traditional 20cm-diameter membranes with a lower capital cost per unit volume of water produced.
With a mean annual precipitation of only 590mm, there is insufficient rainfall in the German capital, Berlin, to replenish the groundwater supply used for the potable source. To ensure a sustainable water supply for Berlin’s citizens, Veolia Water, Berliner Wasser Betriebe and their partners have been engaged in the NASRI (natural and artificial systems for recharge and infiltration) project, which uses complex resource evaluation and analysis methodology (CREAM) in order to evaluate groundwater management strategies and the benefits and limitations of the well-established artificial groundwater recharge system.
Quite apart from the lack of rainfall, another problem facing thirsty Berliners is the presence of a salt water aquifer which lies beneath the fresh water aquifer. This can be seen in Figure 2 (left). The diagram shows the three different types of aquifer recharge used in Berlin: bank filtration along rivers and lakes, natural recharge, and artificial recharge. Two of the three existing bank filtration lakes are blended with Berlin’s treated wastewater for aquifer recharge.
Bank filtration and artificial groundwater recharge have been used as a treatment process in Berlin for many years. Here, soil-aquifer treatment works in a similar way to slow sand filters, except that the media are the sand and gravel in the sub-soil through which the surface water slowly moves over a period of about 50 days. The only visible footprint is the infiltration ponds and abstraction wells. The process is so effective that it is not necessary to chlorinate the abstracted water prior to distribution for potable use.
Comprehensive water management – that is to say a system which examines the water cycle as a whole – is required to make use of alternative water sources. In addition, there is the need to reduce waste, demand, leakage and evaporation losses so we can maximise water management efficiency.
Increasing the performance efficiency of process systems provides obvious economic benefits. Membrane processes such as RO are subject to membrane fouling, which increases power demand as well as other operating costs and reduces treated water output. By reducing and managing efficiently the rate of fouling, costs can be reduced. Advances in membrane technology, such as the Mega-Magnum, can also help to reduce capital costs.
The GWR, Scottsdale and Berlin municipal projects described above are excellent examples of how alternative water sources can
be harnessed to the expansion of potable water supplies in large-scale water cycles and reduce the requirement for traditional raw water sources. Future projects can be envisaged where similar systems become integrated with power generation systems to provide cooling, process and freshwater for banking and storage when required, making use of hybridisation in the selection of process technologies. We can aim to reduce the overall energy requirement and the environmental impact by making full use of generation capacity during periods of low power demand.
The challenge is to develop integrated municipal solutions at the large-scale water cycle level where water and energy efficiency can be maximised.
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