Oct 09, 2014

Water Reuse Goes to School

K-12 school uses rainwater for toilet flushing & irrigation

heapy engineering_rainwater harvesting_milton union
heapy engineering_rainwater harvesting_tank

This article will review the benefits and challenges of implementing water reuse strategies, focused on building heating, ventilation and air conditioning (HVAC) and plumbing systems at the new 217,000-sq-ft Milton Union K-12 School. Public policy changes (codes, etc.) needed to be addressed during a project to more readily integrate these strategies into the building design. A summary of the water savings associated with incorporating these strategies is described herein. Taking advantage of the availability of “wasted” and “site water” is based on considering critical design elements coupled with technological advances that have occurred in the water treatment industry.

Rainwater & Regulations

Only 3% of the world’s water resources are freshwater, yet millions of gallons of potable water are flushed every day. In fact, toilet flushing makes up approximately one-fourth of the water consumed in the average home. And, according to American Water Works Assn. studies, flushing in commercial buildings accounts for almost 50% of indoor water consumption. Additionally, almost 4% of the energy consumed in the U.S. is for water pumping and distribution — just so people can have water available at the faucet or to flush a toilet. That is a considerable amount of energy use and water consumption just to flush a toilet, let alone the cost expended to treat, store and distribute that water.  

There are multiple alternative sources of water that could be reused for flushing. These include subsurface water, rainwater, greywater and blackwater. Each of these presents different challenges, including treatment, storage and pumping. With each of these challenges, the cost for reuse increases significantly from using subsurface water or rainwater to using greywater or blackwater. The cost for potable versus non-potable water reuse also increases due to the treatment required.

Typically, the most economical and sustainable water solutions involve using collected and treated subsurface and rainwater for flushing (water closets and urinals), cooling coil condensate for cooling tower makeup water and either source for irrigation.

To start any project, one needs to research which codes or local regulations need to be addressed. Current plumbing codes do not typically permit using untreated outside-sourced water in a building. Additionally, water rights are a major concern in many areas of the country where collecting and using the water landing on or passing through (via streams, creeks, etc.) a property may not be allowed by local law. 

Public water systems typically charge for sewer use based on metered potable water use. Water reuse presents a challenge to these local utilities because there is no rainwater metering, thus they do not know how much wastewater is coming from a home or office using rainwater for toilet flushing. This brings up the issue of whether outside-sourced water needs to be metered.

The American Rainwater Catchment Systems Assn. (ARCSA) and the American Society of Plumbing Engineers (ASPE) are currently co-developing a new American National Standard to provide design and installation requirements for rainwater catchment systems. ARCSA/ASPE 63: Rainwater Catchment Systems, an existing standard first published in July 2009, has gone through extensive revision using the ASPE standards development process accredited by the American National Standards Institute. 

This standard has received support from the International Assn. of Plumbing and Mechanical Officials and NSF Intl. Once these standards are in place, they can provide consistency in regulating, reviewing and approving the alternative means to make use of rainwater for building plumbing systems.

Implementing Reuse

Heapy Eng. designed the mechanical-electrical systems and provided LEED consulting services for Milton Union School Districts in West Milton, Ohio, for its new LEED Gold K-12 school. The new building consolidates the district’s elementary, middle and high schools into one building. The project includes classrooms, a cafeteria, a gymnasium, a library, a media center and an auditorium.

Some of the green strategies used in this building initiative include LED lighting, large window areas to utilize natural lighting with a system that automatically dims inside lighting when it is bright outside, solar thermal and photovoltaic panels, and a 75,000-gal reservoir that collects rainwater for use in restrooms and watering the athletic fields. 

High-efficiency chillers make ice inside storage tanks during the night (when the energy required to run them is less costly) and the thermal energy is used during the day to cool the water that circulates through the building’s air conditioning system. Energy management monitoring will be used in all four of the building’s wings, allowing for location-specific adjustments.

The specification on the school’s rainwater catchment system can be summarized as follows:

  • Collection from a partial roof area and interior courtyard drains is directed into the 75,000-gal storage tank.
  • The collected water supplies water closet and urinal flushing through a cistern, treatment and re-pressure system, and provides for some irrigation.
  • The initial cost was approximately $170,000.
  • The estimated savings are approximately $6,000 per year.  (Initial savings of $12,000 were cut in half after the city began to require sub-metering for sewage charges.)
  • Baseline case annual consumption would have been 3,487,780 gal per year. This includes total water consumption by toilets, urinals, bathroom and classroom lavatories, kitchen hand-washing sinks and showers. 
  • Total design case—offset from baseline due to low-flow fixture use only, not rainwater harvesting—is annual water consumption of an estimated 1,977,251 gal per year.
  • Toilets and urinals are estimated to use 1,445,836 gal per year.
  • With rainwater harvesting providing all of the water for toilet and urinal flushing, total potable water savings are 84.8%.

Plumbing designers need to realize the effect on total building water consumption when employing a low-flow fixture approach along with water reuse. Education on these alternatives and adjustments to current codes also will assist in moving the industry forward on the path of lowering overall water use in the built environment. 

About the author

Michael J. Berning, P.E., CEM, LEED AP BD+C, is director of sustainable design for Heapy Eng. Berning can be reached at [email protected] or 937.224.0861.

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