Innovative Reuse of "Waste" Water for Positive Impact

Away from the Atrium, a central cooling plant lies on the roof of the second floor near Lift 3.

The cooling tower project was borne as a complement to the existing seawater cooling plant with the objective of using both synergistically to save energy.  The only catch: cooling towers use a lot of water.

The cooling tower plays a critical role in HKUST's central campus air conditioning system. It works through a process of evaporative cooling, in which ambient air is in contact with falling water, thereby exchanging heat. While part of the water is being evaporated, the remaining cooled water will travel down to the pores lining outside of the tower, where filtration takes place to separate impurities from the fresh water.  While effective, this process consumes an enormous amount of water – roughly 200,000 cubic meters of water per year. To give a sense of scale, an Olympic sized swimming pool holds around 2,500 cubic meters.

Because of this high use, our Campus Management Office (CMO) is always on the lookout for ways to reduce this water consumption.  For example, the code of practice by the Electrical and Mechanical Services Department (EMSD) requires the condensing water to be continuously bled off to maintain the concentration of total dissolved solids and suspended solids at an acceptable level. This results in around 8,724 m3 of water that would ordinarily go down the drain.  However, instead of wasting it, CMO was able to reroute this water into the university library to supply water for toilet flushing.

Even after reusing the bleed off water for toilets, the system still consumes a huge amount of water.  This led to another insight from the CMO team; recognizing that when humid air blows over the cold pipes in the air conditioning units (called air handling units, or AHUs) the result is condensation. The AHUs are large enough that the drops of condensation actually result in a steady flow of water that can be captured and pumped back into the cooling tower. CMO developed an innovative system to capture the water and now roughly 6 cubic liters per day – or 2,000 m3 per year – is captured and reused in the system.  

This still leaves a large appetite for water, but every little bit helps.  With innovations like these, the goal is to reduce enough water on campus to offset completely the entire cooling towers by 2028.