In the UAE most drinking and building water is produced by desalination, which takes energy and money. Yet a large share of the water used in a building only needs to be good enough to flush a toilet or water a garden. Treating and reusing water on site can reduce how much new water a building needs. This article explains the options and the main design questions.
Greywater and wastewater are not the same
Greywater is the lightly used water from showers, baths, hand basins and laundry. It contains soap, skin oils, hair and some organic matter, but not toilet waste. Blackwater, or full wastewater, includes toilets and usually kitchens. It carries much more organic load and pathogens.
Because greywater is cleaner, it can be treated with simpler systems. Greywater recovery typically uses screening, filtration, a biological or physical treatment stage and disinfection. Full wastewater treatment needs biological processes such as MBBR and more polishing before reuse.
Where the water can go
The most common reuse duties in buildings are toilet and urinal flushing, landscape irrigation, and in some cases cooling tower make-up or washing. The permitted uses and the quality each requires depend on the applicable local requirements, which should be confirmed for each project. Treated water is always kept in its own pipework and tanks, clearly separated and marked from the drinking water supply.
Which buildings suit reuse
Reuse works best where a lot of greywater is produced and there is a matching non-potable demand. Hotels and resorts are good candidates: guest showers and laundries produce greywater steadily, and flushing and landscaping need water every day. Residential towers, staff accommodation and some commercial buildings in cities such as Dubai and Doha follow the same pattern.
A quick water balance
Before choosing equipment, estimate how much greywater the building produces each day and how much non-potable water it uses. If greywater supply is much larger than demand, the system can be smaller and the surplus discharged. If demand is larger, the treated greywater covers part of it and the rest comes from the normal supply.
Designing it in
Reuse is far easier to include at design stage than to retrofit. Separate greywater drains, a dual supply network for flushing and irrigation, space for treatment and storage, and access for maintenance all need to be planned. In existing buildings it is still possible, but drain separation is often the hardest part.
Disinfection and storage
Treated water should not sit warm and stagnant. Storage is sized to balance daily cycles without holding water for long, and the final water is disinfected, for example with UV, often with a residual disinfectant for the distribution network where required. Tanks and pipework need regular checks.
Operation and maintenance
A reuse system is a small treatment plant and needs routine attention: cleaning screens, checking filters, monitoring the biological stage, replacing UV lamps and testing the treated water. Automation reduces the workload, but someone on site should understand the system. Planning for this from the start keeps the system running.
Common pitfalls
- Mixing kitchen water into greywater, which adds fats and organic load the system was not designed for
- Undersizing storage, so treated water runs out at peak times, or oversizing it, so water stands too long
- Leaving no access for maintenance in a crowded plant room
- Stopping water quality testing once the system is running
Most of these are avoided by agreeing the water balance and the reuse duties before the drainage design is fixed. A short review with the drainage designer at that stage saves costly changes later.
The benefit
The main gain is lower demand for desalinated water and the cost that comes with it. Reuse also reduces the volume sent to the sewer and can make landscaping possible where water would otherwise be limited.
AFG manufactures greywater recovery and wastewater treatment systems for buildings across the region. To check whether reuse fits your building, send the occupancy, daily water use and planned reuse through the quote form.