Aquifer storage and recharge technology can be used to make treated effluent fit for re-use. By Bassem Halabi.
Supporting five per cent of the world’s population, with only one per cent of the world’s freshwater supply sums up the water scenario in the Middle East & North Africa (MENA) region. The gradual decline in the availability of naturally renewable water resources in the MENA region can be attributed to two factors – rapidly growing population and mismanagement of water. However, modern technology offers a number of sustainable solutions, including innovative waste-to-tap programmes that could help replenish fresh water supplies.
Aquifer Storage and Recovery System (ASRS) is a rapidly growing sustainable option for water management. It is a process whereby treated wastewater is injected into an aquifer to undergo further natural purification prior to its recovery for potable re-use. In this scheme, wastewater undergoes extensive biological treatment followed by tertiary treatment, culminating in reverse osmosis (RO) and disinfection for complete purification. At this stage, the water becomes suitable for potable use. However, public opposition to using the water due to its origins has been a significant barrier to its acceptance. A possible way out is artificial injection of the water into a soil aquifer system.
The injection process, in addition to its aesthetic appeal, enhances the treatment of the injected water through a natural process within the soil layers. The location and extent of aquifers is dependent upon the geological conditions of the underlying rock. Therefore, careful selection of the aquifer and related retention period achieves additional polishing features that includes natural re-mineralisation (as soil layers may include certain minerals that could improve the quality of the water), removal of trace organics (certain natural soil aquifers can remove up to 99% of trace organic materials such as benzene, toluene and chloroform) and ion exchange (some natural soils posses ion exchange characteristics that assist in the removal of some heavy metals and ammonia).
Additional benefits include natural filtration, reduction of nitrogen and phosphorus, and de-nitrification. But these features are applicable only in cases where water is injected without RO treatment.
Water injection for aquifer recharge can be done at different depths depending on the receiving aquifer’s depth. Injection can be done into confined, semi-confined or unconfined aquifers, though normally it is done into semi-confined aquifers that are being depleted by over pumping.
Confined aquifers can still be recharged either by direct injection into the aquifer or by injection into remote locations allowing sufficient transfusion time for the necessary treatment.
ASRS technology was identified as one of the sources of future water supply in El Paso, Texas. The scheme proved to be so successful that the city is considering the construction of a surface water treatment plant for treating excess Rio Grande water and injecting it into the aquifer for recharge. The water could then be drawn during the dry season. Obviously, in addition to its strategic storage application, the scheme reduces evaporation losses and is superior to above-ground reservoirs. In the case of reclaimed wastewater, ASRS definitely has the benefit of breaking the psychological barrier for using reclaimed wastewater for potable purposes.
Most aquifer recharge systems are easy to operate and the technology is generally well understood. Recharge can significantly increase the sustainable yield of an aquifer particularly in arid regions. In some cases, the aquifer water can be improved by recharging with high quality injected water.
Commercial feasibility
From the commercial side of the reclamation and injection process, Metito Overseas has carried out several studies to determine the viability of the process. In one case, the cost of seawater desalination was compared to the cost of reclamation of wastewater for potable use using commercially available biological treatment processes followed by micro-filtration and RO by taking into consideration a typical 10,000 m3/ day plant. It was estimated that the cost of water production using reclaimed wastewater was $0.42/m3, which is much lower than the cost of water production using seawater desalination. This figure was confirmed by the cost of $0.46/m3 for the Sulaibiya Wastewater Treatment and Reclamation Plant, Kuwait, the largest facility of its kind in the world. While the water produced in Sulaibiya is not used for potable purposes, its use for irrigation and industrial purposes eases the pressure on costly desalinated water which was being used earlier for both applications.
Water is central to sustainable development. While some countries are blessed with abundant natural water resources, many have no option but to search for alternative sources of water. Namibia, one of the most arid countries in sub-Saharan Africa, is an example of how careful management utilising high-tech ASRS technology can sustain precious water resources. The capital city Windhoek relies on three dams built on temporary rivers that run only in the rainy season for its water supplies. Full utilisation of the water was difficult, since evaporation rates were high and the utilised water was 15.7m3, while evaporation accounted for a loss of 35m3. The water supply was being augmented from a few boreholes that had low recovery rates. To overcome this problem, in 1992, the authorities initiated a successful water resources management programme. Four years later, overall demand was maintained at the 1989 level, despite a 35% increase in population.
Less dependence on desalination
For the Gulf region, with easy access to a plentiful supply of seawater, desalination is a natural alternative and as such, the region is the world’s largest market for desalination. However, dependence on desalination has to be carefully managed since desalination and its byproducts have an harmful impact on the sea’s delicate eco-system and marine life. Furthermore, there is a limit in terms of financial capacity to the number of plants that can be constructed. For countries with no access to seawater, there is no option but to improve management of existing water resources and to augment them by increasing the use of treated wastewater.
Sustainable development and modern technology is key to addressing the world’s water shortage and careful water management can ensure that shortages are kept to the minimum. Recycling water is one way of minimising the use of freshwater supply and its benefits to the environment are plentiful. Recycled water has many uses, from direct potable use to indirect, such as industrial use – in cooling towers, refineries and petrochemical plants – to agriculture and for municipality tasks such as road washing. The degree to which water is treated is often determined by its end use – whether it is for potable or non-potable use – which determines the technology used.
Across the Gulf region, Treated Sewage Effluent (TSE) is being extensively used for landscape irrigation. But steps have also been taken to promote the use of TSE for applications like district cooling which needs high-quality potable-grade water. The Palm Jumeirah sewage treatment plant, for example, will process TSE through a membrane-based polishing unit to make it suitable for district cooling. Moreover, highly advanced water re-use technology has now made it possible to treat wastewater to a degree of purity that is suitable for direct human consumption. In Singapore, for example, a pioneering project has been put in place where the government is promoting high-grade reclaimed water called NEWater which is treated wastewater further purified using advanced membrane technologies. Ultra-clean and safe to drink, NEWater has passed more than 30,000 scientific tests and surpassed the World Health Organisation (WHO) requirements. However, while advanced membrane treatment reduces the strain on natural water resources, a potential downside is that the entire process is energy intensive. Furthermore, the psychological barrier of drinking treated wastewater is hard to overcome.
Aquifer recharge using high quality reclaimed water can be a viable option in the search for new potable water resources, especially in the arid regions. While new technologies are frequently coming to the fore, the best technology continues to be human capacity for using water efficiently. Simple measures such as tackling dripping faucets, harvesting rainwater, avoiding contamination of municipal sewage with hazardous chemicals and educating consumers about being ‘water smart’ can go a long way in protecting the earth’s most precious natural resource.
(The author is Group Business Development Director, Metito)
