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Precision gray water management is easier thanks to new technology such as this density insertion meter.

{ QUESTION } Our company wants to be more involved in marketing our product as a sustainable material. We are interested in trying to help the building owners on whose projects our concrete is used to earn LEED points. We are considering trying to use more process water in our mixes.

What is the proper method of incorporating more process water, especially “gray” water, into our mixes? How much water should we treat as we begin our effort? Also, is there an effective way to measure the gray water's density?

{ ANSWER } Every year, NRMCA estimates an average of 5% of all concrete batched for delivery in the U.S. is returned to the plant for either disposal or treatment. A portion of this is slurry-like residue from the drum cleanout after the pour. Production managers also estimate that drivers use about 150 gallons of water to wash out a ready-mix truck every day. And when you add in the stormwater which must be treated as process water if allowed to commingle with process water, the amount of water to treat daily is substantial.

Working to incorporate recycled water into your concrete mix can be an effective way to document your commitment to sustainable construction while dealing with this production constraint. Sometimes, using recycled water could be a possible source for LEED points. But there are other reasons to use process water in concrete mixes.

Before beginning on a gray water program, consult ASTM C-94, Specification for Ready-Mixed Concrete. The standard gives guidance on the procedures to document using gray water in concrete delivered in conformance to the specification.

Also, participate in the “Managing Returned Concrete and Wash Water at a Ready Mixed Concrete Plant” Webinar offered periodically by NRMCA. This Webinar discusses the options in reusing water from concrete production operations. It also covers handling returned concrete, particularly the recent research conducted at NRMCA on using crushed concrete aggregate for new concrete. Led by Karthik Obla, NRMCA's vice president of technical services, the Webinar focuses on technical and operational issues. To learn more, e-mail Obla at kobla@nrmca.org.

The decision to incorporate gray water requires a total commitment, says Jeff Metz, president of Enviro-Port Inc., a Gratiot, Wis., manufacturer of reclaiming systems. Metz has worked with hundreds of clients.

A realistic goal helps to properly size the gray water agitation (holding) tanks needed for incorporation. For example, a plant averaging 500 yards of production per day would have the possibility of batching a maximum of 12,500 gallons per day at 25 gallons per yard input.

The tanks would then be sized accordingly, allowing for future growth and surge capabilities. “Implementing precision gray water technology has allowed producers to stay within the boundaries of the daily production water required,” says Metz. “This approach is the most energy-efficient.”

A thorough testing program requires testing documentation before, during, and after the transition period. It is the most important step to getting started. This should include increased frequency of strength, material, and process water quality testing, especially for density.