The water-starved countries of the Middle East are continuously looking for new and innovative ways to supplement their water resources. The reuse of treated sewage effluent generated by municipal sewage treatment plants is already widespread across the region.
The Gulf states recycle up to 70 per cent of their treated effluent, predominantly for irrigation, and most are targeting increases in the volume of total wastewater muse.
But there is also increasing interest in water conservation and reuse on a local level, driven to a large extent by state-backed campaigns to build more environmentally friendly buildings, especially in the emirates of Dubai and Abu Dhabi.
This has thrown the concept of recycling grey water, or untreated household wastewater, into the spotlight. But it is still a subject of much debate as the benefits of the technology are not clear cut.
Broadly speaking, the term grey water refers to water that has not been contaminated by toilet waste. It accounts for about 70 per cent of residential wastewater, with black water, or sewage water, comprising the remainder.
Grey water itself can be subdivided into two types: dark grey water, which is produced by washing machines and kitchen utilities; and light grey water, which comes from showers, bathtubs and wash basins.
The latter makes up nearly 40 per cent of all domestic wastewater and contains the least amount of pathogens and contaminants.
If separated out at the point of generation, light grey water is a valuable resource that can be used instead of drinking water for activities such as best flushing toilet and irrigation.
The average household generates 586 litres a day of wastewater, according to the World Health Organisation. With toilet flushing accounting for 32 per cent of domestic water consumption, significant volumes of desalinated water could be saved through the installation of grey-water recycling technology, particularly if fitted in the basements of high-rise buildings. Using reclaimed water for toilet flushing and irrigation translates not only into additional credits for real estate developers seeking green buildings ratings but also provides savings on bills for consumers.
Grey water can be reclaimed using one of two methods, with the choice of technology determining how the water can subsequently be reused. The simplest method is to use a coarse primary treatment, often a sand filter, which screens off oil, grease and solids.
In this instance, to minimise the risk to public health, the reclaimed water can only be used for subsurface irrigation. It cannot be used for toilet flushing, so the financial advantages are lost.
Although grey water is the least contaminated component of the entire waste stream, it still has the potential to transmit disease, so care has to be taken to avoid washing heavily soiled or bloodstained clothing and nappies in basins to limit the chances of this happening. Primary treated grey water may also contain chemicals from soaps and shampoos that are harmful to certain plant species or alter the alkalinity of the soil.
But potentially more troublesome is the impact that removing 40 per cent of the waste stream could have on the operations of municipal sewerage networks if grey water recycling for irrigation becomes commonplace.
"Existing gravity sewers are designed on a certain velocity based on factors like the density of the liquid itself," says Saad Alani, director for water and environment at Abu Dhabi-based Hyder Consulting Middle East. "if you remove half the grey water, you are left with a thick waste, which is not going to travel as designed in the network. As a result, you will have odour problems and blockages to deal with."
The more sophisticated method of grey-water recycling involves a secondary treatment and disinfection of the water. This cleans the water to a much higher standard and means it can be used internally for toilet flushing and externally for unrestricted irrigation, car cleaning, and even for use in water features.
In addition to this flexibility in usage, the impact on the sewerage networks is much less when secondary treatment systems are employed as most of the water re-enters the waste stream after being used for toilet flushing. However, this is a more costly option than the primary treatment process.
A further consideration when assessing the benefits of grey-water recycling systems is that they require double the amount of pipe work needed by regular wastewater infrastructure, which not only adds to the capital cost of a building, but also requires more intricate designs and additional space.
For property developers who want to be seen to reduce the environmental impact of their developments but also to have water for landscaping purposes, it might be simpler to install a decentralised sewage treatment plant, which does not have an impact on the internal configurations of the building.
There is currently little economic incentive for UAE developers that intend to install an onsite sewage treatment plant to combine it with a grey-water recycling system, even though this would be the best solution environmentally.
Although reusing grey water for toilet flushing would reduce by one third the amount of desalinated water consumed by a building, given the UAE's low water tariffs--$0.008 a gallon compared with $0.012 a gallon in Germany--it would take years for the savings made on water bills to compensate for the additional capital investment needed to install two separate treatment systems, along with the extra power required to run them and the additional land requirements, which in each case can be almost 20 per cent higher.
"The issue of grey water is important," says Alani. "However, everything is important if it comes with the right price. It has to be properly costed."