Disposal of sewage from urban areas contributes to phosphorus load to lake

OKEECHOBEE — Florida farmers are sometimes blamed for nutrient loading that originated in the coastal urban areas, according to Dr. Paul Gray of Florida Audubon.

Dr. Gray gave a presentation on the issues affecting Lake Okeechobee’s water quality at the July 31 meeting of the Okeechobee Rotary Club.

For many years, the coastal communities were hauling treated sewages and dumping it north of the lake, Dr. Gray explained.

“Some of the places were getting 100 tons of phosphorus in one year,” he added.

A 2009 Florida Audubon report found 109 tons of phosphorus in biosolids had been applied to a 1,700 acre property in one year. To put that in perspective, Lake Okeechobee has a drainage basin of approximately 2.8 million acres, according to the South Florida Water Management District. The Florida Department of Environmental Regulation target goal for phosphorus entering Lake Okeechobee from all runoff into the lake is 105 metric tons per year.

Because the sewage dump sites were on land that was zoned “agriculture,” this nutrient load in the runoff has been attributed to agriculture.

“This urban nutrient stream gets blamed on the agricultural community,” said Dr. Gray.

In 2007, the Florida Legislature banned the landspreading of Class B biosolids in the environmentally-sensitive Lake Okeechobee watershed, but a loophole in the law allows unrestricted spreading of Class AA biosolids to continue.

Florida Senate Bill 392, passed in 2007, states: “After December 31, 2007, the department may not authorize the disposal of domestic wastewater residuals within the Lake Okeechobee watershed unless the applicant can affirmatively demonstrate that the phosphorus in the residuals will not add to phosphorus loadings in Lake Okeechobee or its tributaries. This demonstration shall be based on achieving a net balance between phosphorus imports relative to exports on the permitted application site. Exports shall include only phosphorus removed from the Lake Okeechobee watershed through products generated on the permitted application site. This prohibition does not apply to Class AA residuals that are marketed and distributed as fertilizer products in accordance with department rule.”

In the 2009 report, Audubon Florida called on the State of Florida, wastewater utilities, and landowners “to end the practice of land disposal of dried sludge from sewage treatment plants (also referred to as wastewater residuals and bio-solids) in the Lake Okeechobee watershed. Nutrients in the lake and its tributaries exceed state and federal standards and pollution from phosphorous has reached a crisis point. As a result, Florida’s largest lake faces ecological collapse. Sludge contributes nearly one-fourth of the phosphorous in the Lake Okeechobee watershed and is the most preventable source of pollution.

“The costs of cleaning up the phosphorus from sewage sludge alone have not been calculated but will be in the billions of dollars. The existence of so much additional phosphorous in the watershed will complicate meeting state and federal standards and continue to contribute pollution for many years to come. The disposal of sludge and most of its use as a fertilizer in the Lake Okeechobee watershed should come to an immediate end,” Audubon wrote in 2009.

In the February 2013 publication “Expediting Lake Okeechobee Pollution Control,” Audubon again called on the Florida Legislature to “remove the loophole that allows the application of Class AA biosolids in the watershed.”

At the Rotary meeting, Dr. Gray explained that in the 1970s, the lake water had a phosphorus level of about 40 parts per billion (ppb). “The goal is to get it back to 40 ppb.” In recent years the level has more than doubled or even tripled that target. (Note, phosphorus levels vary in different parts of the lake, with higher levels in the open water and lower levels in the marshes.)

High levels of phosphorus in water entering the lake make it impossible to lower the phosphorus level in the lake, Dr, Gray said.

In 2017, water entering the lake averaged 146 ppb phosphorus, according to the South Florida Water Management District.

In addition, high water levels damage or destroy the aquatic vegetation around the edges of the lake; that vegetation is the lake’s natural filter system which helps clean the water.

Nutrient load in the runoff from the Lake Okeechobee watershed continues to be many times higher than the 105 metric ton limit set by the Florida Department of Environmental Protection in 2000. In 1986, 421 metric tons of phosphorus entered the lake via runoff flowing into the lake. In 2017, the annual total was 484 metric tons. The most recent 5-year average was 531 tons. Some 95 percent of the nutrient load comes from the watershed north of Lake Okeechobee.

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