Algal bloom affects Lewisville water


For the past few days, when residents turned their spigots on, they may have noticed an odor accompanying their flow of water. As the summer temperatures climb into the triple digits, algal bloom affects the smell and flavor of lake water, but not the safety.

Lewisville residents get a portion of the municipal water supply from Lewisville Lake. As of late last week, the lake has turned, churning up algae from the bottom of it.

“It’s not harmful, it’s just not palatable,” Utilities Manager Karen Emadiazer said Monday, July 25. “We expect that this will be gone by tomorrow.”

IMG_4595At Lewisville’s C.R. Feaster Water Plant, raw water is pumped from the lake, then chlorine, ammonia and carbon are added to it, she said. Water is pulled from two locations on the south side of the lake immediately off of the the dam that runs east and west, Director of Public Services Keith Marvin said.

“We use the carbon for the taste and odor issues we have,” Emadiazer said. “It’s a very, very small amount [of algal secretion] but the human palate is really sensitive to the two particular things that are part of that.”

Nearly a year ago, the plant purchased an instrument to detect the nanograms of Geosmin and 2-methylisoborneol, or MIB, the organic compounds which algae produces that affects the smell and taste of lake water. The piece of equipment has been out of service though, thus the facility has to send samples to a Florida lab to test for the blooms, she said. Florida is the nearest location for this type of testing.

“It’s like driving without an indicator because we can’t detect necessarily when that comes in,” Emadiazer said.

The most recent test sample was sent Tuesday, July 19. Lake water wasn’t affected until Thursday or Friday.

Water at different temperatures have a different density. For example, ice floats to the top of a glass, she said.

“You have that inversion because the cooler [water] wants to come up. They call that the lake turning,” Emadiazer said. “It brings up everything that’s mucky and yucky.”

Emadiazer said the facility knows to look for and monitor the turning. It typically happens in July and August as well as in the winter around January and February.

“I don’t think we’ve had any problems for about two years, this one just got by us,” she said.

The flavor and odor caused by algae used to last all summer. Approximately two years ago the plant made improvements to its carbon system, allowing it to feed higher dosage rates than it was previously able to.

“Now we can feed almost 20 miligrams per liter where before we only fed 4 miligrams per liter,” Emadiazer said.

Chemical coagulants ferric sulfate, lime and polymer are gently mixed with water to promote flocculation, the process that occurs when particles adhere together to form larger particles called floc, according to materials from the Lewisville Citizen’s University.

The floc then settles to the bottom of the water, leaving clear water at the top. The dirty water is disposed of and the clean water goes through the filter system.

A kayaker passes the Dallas Water Utilities intake on the Elm Fork of the Trinity River just above Sandy Lake.
A kayaker passes the Dallas Water Utilities intake on the Elm Fork of the Trinity River just above Sandy Lake.

Marvin said the city also purchases treated water from the City of Dallas. This allows the city to have a backup supply of water and meet demand during summer months for the entire city.

Dallas Water Utilities contractually provides Lewisville with nine million gallons of treated water every day, according to materials from Lewisville Citizen’s University.

The Texas Commission on Environmental Quality gave the City of Lewisville a Superior Drinking Water System rating. In 2015, the city received recognition from the commission for its tenth year in the Texas Optimization Program, which recognizes participants that meet extremely stringent water quality criteria, according to materials from the citizen’s university.

Update: The article previously referred to MIB and Geosmin as algae. The two substances are organic compounds produced by algae. The article has been changed to reflect this information.