In the region today, there are three main ways to deal with treated sewage effluent (TSE): channel it into the sea or a local watercourse; reuse it for irrigation of green spaces and golf courses; or reuse it in industrial applications. The final destination of the wastewater and its composition defines the type and level of treatment required to ensure that the final effluent is fit for its purpose and is safe for discharge.
Historically, throughout the region and the world, treatment has involved the use of preliminary screening to remove large objects and grit removal to take out sand and stones, before moving into the primary treatment phase.
The sewage then sits in sedimentation tanks where the solids settle to the bottom and are taken away, and the oils and fats rise to the top and are skimmed off. The remaining liquor is then sent for secondary treatment, which is usually a biological process. In this secondary phase, more organic material is removed, for example by encouraging naturally present bacteria to feed on the pollutants in the liquor.
After a sufficient period to allow the reduction of organic loading, the suspended matter is either allowed to settle out of the liquor or is filtered out, leaving a cleaner liquid.
For TSE that is to be deposited in watercourses or the sea, the effluent is usually then disinfected to kill viruses. The method of choice has traditionally been chlorine addition, but alternative approaches such as ultra-violet (UV) disinfection are gaining popularity.
“Chlorine itself is considered to have health risks and generates by-products that are detrimental to health, so recently we have moved on to UV light for disinfection,” says Jim Lozier, head of global technology for desalination and membranes at US consultancy CH2M Hill, which is a member of the International Private Water Association (IPWA). “We produce light of a certain wavelength and that wavelength inactivates viruses and other pathogens.”
Another potential use for TSE is artificial aquifer recharge, or aquifer storage and recovery
The advantage of using UV light is that it can tackle viruses that are resistant to chlorine, such as cryptosporidium, and it does not leave any residual chemicals in the water stream. On the downside, the energy costs are higher. However, chlorine must be purchased and transported, so on a lifecycle basis the costs can be comparable.
Some water treatment processes use ozone as a disinfectant. “It is a stronger oxidant than chlorine and you can generate it onsite, so you don’t have storage and transport issues,” says Lozier.
For TSE that is to be reused, the treatment options depend on its ultimate purpose. Non-potable uses such as landscape irrigation, which is favoured in the Middle East, mean that the additional treatment focuses on controlling health risks and ensuring that the water is bacteriologically safe. From a treatment perspective, this usually means taking the secondary effluent from the treatment works and filtering it further, a step known as tertiary treatment. The filters used for this can be granular, cloth or membrane, which is also called micro- or ultra-filtration. Disinfection is then carried out to clean the water further.
|Year||Membranes||Energy consumption (kilowatt hours/cm)|
|*=Dow’s reverse osmosis membranes have become 44 per cent more efficient since 1990; cm=Cubic metres; BW,HR,HRLE=Membrane names. Source: Dow Water & Process Solutions|
A significant advance in treatment technology that has attracted interest in the Middle East is the incorporation of membrane filters into the secondary treatment process, a technology known as membrane bioreactor (MBR).
“This is probably the fastest growing use of micro- and ultra-filtration in the industry,” says Lozier. “It produces higher-quality water, and reduces the footprint and energy consumption required.” He says its lifecycle is now efficient enough for the market to adopt it on a large scale.
Oman’s Haya Water has been a regional pioneer in the use of MBR, with a 55,000 cubic-metre-a-day (cm/d) unit installed at Al-Ansab, another under construction in Darsait and a further unit planned for Amerat.
Saudi Arabia has also invested in the technology, with the Rabigh refinery MBR plant commissioned in December 2010. Dubai selected the technology to treat wastewater at Palm Jumeirah and, most recently, Bahrain Petroleum Company (Bapco) announced in June 2012 that it would use MBR at its oil refinery in Sitra.
For industrial reuse of wastewater, effluent treated to a tertiary standard may be sufficient, but there is a range of other options available to further improve quality.
Processes such as reverse osmosis to remove salts, nano-filtration for further organic removal, and advanced oxidation to target specific substances are increasingly finding their way into the regional treatment portfolio. This is driven in part by a fourth potential use for TSE: artificial aquifer recharge, or aquifer storage and recovery (ASR).
ASR is gaining ground worldwide and involves treating water to an ultra-high standard before using it to refill aquifers. These aquifers are then used as storage facilities, allowing the new water to mix with the existing groundwater, and is treated again before being used.
In chemical terms, the quality of the TSE is better than the naturally occurring groundwater, but the idea that treated sewage could be a potable water source is unacceptable to many regional consumers.
Despite its tendency to delay major infrastructure projects, Kuwait is leading the way on regional applications of ASR and has been using the process to generate desalinated water since the 1980s. After embarking on a public-private partnership (PPP) back in 2002 to create the world’s largest wastewater treatment plant with reverse osmosis membranes at Sulaibiya, the country now has a $430m, 425,000 cm/d facility, which pumps out ultra-high quality effluent that can be used to recharge aquifers and boost groundwater resources.
Kuwait lacks surface water, so groundwater is its only natural resource. However, this groundwater is brackish due to the ingress of salt water and can only be used for non-potable purposes.
ASR is being used successfully around the world, says Lozier. “It is popular in California because we have had a lot of seawater intrusion over the years, so we treat the wastewater to a very high level, then put it in the ground to create a seawater intrusion barrier. This also replenishes the aquifer,” he says.
This water eventually becomes mixed with the groundwater that is being extracted for potable use. “You can also place it into reservoirs and mix it with other water, and that is what we call an environmental buffer,” he says. “This is then treated by a treatment plant as part of a mixed supply of natural water and high purity wastewater.”
Along with Kuwait, other states are considering the ASR approach. Qatar, for example, has already identified potential sites for a 30 billion-gallon storage reserve in the north of the country.
However, there is a long way to go before the region can begin using treated wastewater for potable sources, no matter how indirectly. “All the water that we have is used water,” says Lozier. “It has been around forever. You spend all this money to desalinate water and then use it once and throw it away. Why not reuse it? But the question is, what is the best way?”
That is the question that all Middle East countries need to answer.