As the district cooling industry sets up new plants or expands capacity of existing ones, reclaimed water can cater to the industry cooling water needs as a costeffective substitute says Bassam Halabi

The Arabian Gulf is known to have one of the warmest climates on earth with hot summer conditions prevailing for over six months a year. With deserts comprising more than 77% of its land, or 2,330,000 m2, it is also one of the most arid zones in the world.

Modern living in the region necessitated the provision of indoor climate control and thereby, extensive use of air conditioning. Up to the end of the last decade, individual or central heat pump type air conditioning systems were provided in dwellings, but these consumed a lot of power, almost 50% of the peak power load, and used environmentally harmful gases such as Freon. The search for more energy efficient and environmentally friendly solutions resulted in the development of District Cooling Plants (DCPs). In district cooling, the cooling energy is produced in centralised manner, which enables greater unit sizes. As a result, energy can be utilised in a more cost effective way. Compared to buildingspecific cooling, district cooling is a more environmentally friendly alternative and is an easy and reliable source of cooling energy.

Water-based cooling systems follow either once-through or closed-cycle designs. Once-through cooling systems use large volumes of water, typically in the range of thousands of cubic metres per day from a water source. Water is pumped through the condenser in a single pass and returned to the same source. Although the water withdrawal rates are very high, most of the water is subsequently returned to the water body. Closed-cycle cooling systems, on the other hand, receive their cooling water from a cooling tower and basin, to which it is subsequently returned. In such a system, it is mandatory to periodically discharge part of the recirculating water to remove the build-up of undesirable solids. As a result of evapouration and planned cooling tower blowdown, regular addition of ‘makeup’ cooling water is needed. Makeup volumes are much lower than daily once-through volumes and may go up to hundreds of thousands of cubic metres per day. Although makeup water withdrawal volume is lower, virtually all of the water is consumed without being returned to the water body. As such, there is a continuous demand for fresh makeup water, which puts stress on fresh water resources. Reclaimed water is a viable alternative source of water available throughout the UAE. Unfortunately, only one per cent of the wastewater generated in the country is currently being reused, which makes Treated Sewage Effluent (TSE) an unexploited water resource.

Reuse of water, including reclaimed water, has been the central focus of several governmental bodies. Reclaimed water is treated at municipal wastewater treatment plants to secondary treatment standards in order to comply with environmentally safe disposal limits. In some cases, reclaimed water is treated to tertiary treatment levels. The starting treatment levels and additional treatment required for reclaimed water used in district cooling ranges from secondary to tertiary. The additional treatment steps may include chemical addition, clarification, disinfection, pH adjustment, and membrane filtration.

An effective water re-use scheme

After water is polished through a membrane system its quality becomes as good as desalinated water but far less expensive than the latter. Technically speaking, after this stage of treatment, water is suitable for human consumption. But religious and psychological barriers prevent its use for such applications. These barriers or hurdles must be overcome when considering the use of recycled water, and people need to be educated on the facts and not the perceptions. Even with such barriers, secondary treated water can be effectively used in a multiple of applications including irrigation, industrial, and cooling purposes.

An innovative polishing scheme

Advanced water treatment technologies such as MBR (Membrane Bio-Reactor) and ZLD (Zero Liquid Discharge) have made the use of recycled water increasingly feasible and viable. The use of TSE has a positive impact on the number of cycles in a cooling tower. In discharging the blowdown though, careful consideration must be given to the point of discharge in terms of irrigation or sewer lines, as the quality required by each dictates the number of blowdown cycles, as shown below:

The advantages of implementing TSE polishing may be summarised thus:

  • Environmental benefits of re-use
  • Eliminating the disposal of reject and blowdown by employing ZLD Technology
  • Polished water quality meets and exceeds potable water standards, allowing for equivalent or higher cycles of concentration for district cooling
  • Economic feasibility ensures cost savings compared to the use of desalinated water and disposal of reject

Typical cost savings resulting from the use of TSE in district cooling applications are shown far right:

There are some key challenges in opting for an innovative polished water scheme.

These are:

  • Availability of sufficient TSE for polishing with the increasing pressure on requirement for irrigation purposes
  • Area availability for the sewage treatment and polishing plants. Area required is approximately 20 – 30 m2/m3 of treated water
  • Fluctuations in winter and summer for makeup water requirements
  • Disposal of blow-down and reject from the polishing plant (though less than reject from desalination process)

• Employment of the proper ZLD process Reclaimed water represents a valuable water resource with many potential applications. As the district cooling industry sites new plants or expands capacity of existing ones, it must identify sufficient supplies of water for cooling. Reclaimed water can help meet that need. However, it needs to be treated for process reasons as its chemical composition may create problems of scaling, corrosion, bio-fouling, or stress cracking of some metal surfaces.

To conclude, although reuse of reclaimed water is not a new practice, it is likely to become increasingly popular throughout the GCC in the near future. Benefits of the use of TSE in the DC process may be summarised as follows:

  • Environmental:
    Using innovative polished (recycled) water is in line with Dubai’s vision. Applying the optimum ZLD process is a recommendation for future improvement to minimise impact on the environment
  • Technical:
    High quality polished water allows for higher cycles of concentration for blowdown, and enables the reduction of potable water consumption
  • Economic:
    The use of polished water results in significant cost savings when compared to potable desalinated water


The author is Group business Development Director, Metito.