Water Quality Research by David
Dave Rexing, Southern Nevada Water Authority’s water quality research and development manager,
assists staff in conducting a variety of research and laboratory experiments.
In 1994, Las Vegas experienced an outbreak of
Cryptosporidiosis (crypto). At that time, ozone was the only
known disinfectant capable of handling crypto. Because of
this, SNWA decided to add ozone as its primary disinfectant in
the treatment process. However, also at the time, there was an
Environmental Protection Agency (EPA) maximum contaminant
level of 10μg/L on bromate, which is a by-product of ozonation.
It requires very little naturally occurring bromide in the source
water to produce 10μg/L of bromate upon ozonation. At least for
the moment, it appeared that the bromate maximum contaminant
level (MCL) might prevent the implementation of ozone. SNWA’s
research and development (R&D) group started researching and
piloting the chemistry behind the formation of bromate by ozone
and ultimately formulated the Chlorine-Ammonia Process for
Bromate Mitigation. This was patented in 2003 by SNWA and
offered complimentary to the industry.
18 SOURCE fall 2018
More recently, SNWA has engaged in additional bromate
mitigation research with The Water Research Foundation Project
4588, “Effect of Ozone Dissolution on Bromate Formation,
Disinfection Credit, and Operating Cost.” It would appear that
the side stream injection of ozone versus fine bubble diffusion
increases CT values, as well as decreases bromate formation.
On-Site Hypochlorite Generation
SNWA uses chlorine as its secondary disinfectant in the
distribution system. At the River Mountains Treatment Facility,
salt is used to generate on-site sodium hypochlorite. While the
R&D group investigated how to optimize the on-site generation
of hypochlorite, the team discovered that bromate could be pro-duced
in the process. Again, this complicated compliance with
the bromate MCL. It was decided that no more than 5μg/L of
bromate (one half the MCL) should come from the on-site gen-eration
of the hypochlorite.
WHEN MY FRIENDS IN THE WATER INDUSTRY FIND OUT WHAT I DO, they generally have three
questions. Isn’t water quality research expensive and complicated? Isn’t it difficult to recruit the caliber
of personnel necessary to carry out water quality research? And finally, isn’t it difficult to justify to your
ratepayers the cost of water quality research? Even though the answer to all of the above questions is “yes,”
my experience has convinced me that water quality research is absolutely necessary, and that it has very real
long-term payoffs. By citing some real-world examples of water quality research carried out by the Southern
Nevada Water Authority (SNWA), I hope to demonstrate those payoffs.
ALL PHOTOS COURTESY OF SOUTHERN NEVADA WATER AUTHORITY