Posted on 8/8/2012 8:20 AM by Jeffrey Kling
Water efficiency seems secondary to the always demanding energy efficiency, but through consideration and proper design, significant savings await. The collection of various site water sources, purposed for various needs, will result in an economic return.
Analogous to other systems, efficiency advances require capital investments to collect, treat and distribute. The associated operating and maintenance routines increase in importance. The rise in utilities should motivate all building owners to, at the very least, consider the concepts.
Implementation is more demanding than a traditional design, therefore owners should decide what water to harvest and reuse in the early design stages. Owners need to be aware of the major components and the interdisciplinary impact. Any water system beyond the norm might require storage, filters, pumps, treatment, additional piping, and space.
The common inputs are rainwater, process water, grey water, cold condensate, fire suppression, humidification, and raw source. The best uses require the most volume with minimal treatment or movement such as irrigation, evaporative cooling, and flushing fixtures. Traditional evaporative cooling system designs require a variety of these system components already and may use materials suited to handle the harvested water.
Evaporative cooling towers will require storage, pumps and treatment. Every building requires a storm water system, but the path of conveyance can be manipulated for the interests of repurposing. Outdoor air requirements result in constant cold condensate flows and with planning may be returned to the system for evaporation.
Generally consider 50,000 cu. ft. per minute of outdoor air, which by code will equate to 2,500 people across 357,500 sq. ft. A multi-story building with 100,000 sq. ft. of roof area might be roughly expected. With an average precipitation of 4 inches per month, a total of 250,000 gallons might be collected across the month. The same outdoor air stream requires 350 tons per hour to condition.
If the building is occupied for twenty days in the month, assuming 3.6 gallons per ton per hour of water evaporation will occur, in the extreme will requires 1,260 gallons per hour or 200,000 gallons across the month. The same airstream starting at a 70 degree dew point might produce around 150 gallons per hour or 24,000 gallons across the occupied month in cold condensate. The vast number of specifics, inefficiencies, and varying conditions were not included, but the measures show the water saving potential.
Non-potable water use in any facility equates to a large majority of the utility bill, and the example proves proper planning with minimal additions to typical systems results in significant savings. It is possible to join some portions of multiple systems together for a holistic approach.
Consider the same number of occupants flushing 1.6 gallons around three times a day results in 12,000 gallons flushed per day or 240,000 gallons per occupied month. It could be possible to collect and store water for flushing and diverting the excess to other systems as necessary. Owners cognizant of the possibilities and willing to deviate from the norm should be requiring design teams to present the obtainable for any project.
Jeffrey Kling is Mechanical Engineer, Gibbens Drake Scott Inc.