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Salinity May Emerge as New Challenge
For Dischargers, Wastewater Officials Told
BOSTON--Salinity is emerging as a pollutant that may challenge wastewater
treatment officials in arid states and in areas that use deicing agents during
the winter, several municipal wastewater officials said July 16.
Salinity is defined as total dissolved solids or minerals that may include
sodium, bicarbonate, sulfate, potassium, chloride, and various metals such as
iron, manganese, and selenium, Henry Day, the pollution control superintendent
for Phoenix, said. He spoke at the summer conference of the Association of
Metropolitan Sewerage Agencies focusing on emerging pollutants.
"Utilities need to think about hidden or potential pollutants and begin to think
about control strategies to minimize or eliminate them," Day said.
Agricultural lands used to grow crops are the most affected by high salt
concentrations in irrigation water, although in urban areas, high salt content
can cause erosion of water and wastewater infrastructure as well as industrial
equipment, Day said. In the United States, 14 million acres of land has been
lost or limited because water with high salinity was used for irrigation, Day
said.
Phoenix Becoming 'Salt Trap.'
The problem is getting worse in the Colorado River Basin, he said, in part
because dams prevent the flow of salt to the sea. Other sources of salt include
groundwater, fertilizers, and consumer discharges. Although salts are removed
from the Phoenix area via the Gila River and other streams, about 1 million tons
of salts are added to the valley from the Colorado River and the Salt River
annually. This has caused a large buildup in agricultural areas where water used
for irrigation or other purposes evaporates leaving behind the salts.
"Phoenix is becoming a salt trap," Day said.
Because of the high demand for water, he said, more desalinization plants are
being built. Treatment technologies typically involve microfiltration and
reverse osmosis that essentially separates the salts from the rest of the
effluent. The disposal becomes another problem, Day and other officials said.
For example, the resulting brine may contain other contaminants of concern.
In Phoenix, about 40 percent of the resulting brine is disposed of into the
sewer system. In other areas, "brine lines" are constructed to carry salty
mixtures to desalinization plants or simply to be disposed of in the ocean.
Other disposal methods include evaporation ponds, which require a lot of land
area, and deep well injection, Day said.
Salinity in the form of chloride has become a problem for sanitation officials
in Los Angeles County, which had a cleanup plan, or total maximum daily load,
imposed for the Santa Clara River.
Effluent From Treatment Plant
Margaret Nellor, assistant head of technical services for the Los Angeles County
Sanitation District, said the TMDL was adopted July 10 for the Santa Clara
River, which is fed primarily by effluent from a county-owned sewage treatment
plant and is used mostly as the water supply to irrigate nearby farms, including
avocados, which are "salt-sensitive."
The wastewater treatment plant in question, she said, "is the headwater for the
Santa Clara River."
However, the TMDL set a chloride standard of 100 milligrams per liter, a
concentration that Nellor said can never be met. According to the U.S.
Geological Survey, water with dissolved solids concentrations of less than 500
mg/l, about a quarter of a teaspoon of salts per gallon of water, generally is
suitable for most uses, including drinking.
"Salinity is a sleeping issue," Nellor said. "And it may become an issue for
areas where there is deicing."
The concentration set in the TMDL is not scientifically sound, she said. In
fact, there has been a "bumper crop of avocados" the past several years, she
said.
One of the major sources of chloride to the treatment plant is residential water
softeners, which until 1997 were banned. Once residents were allowed to begin
using them again, chloride levels went up. High salinity is also a problem
during drought periods, Nellor said.
The sanitation district has banned the use of water softeners by businesses and
is trying to impose a ban on residents, but a lawsuit is expected, she said.
While residents pay about 10 cents per pound to add water softeners, the
treatment plant pays about $5 per pound to remove the resulting salts.