One-hundred-year storms: Mitigate adverse effects of high flows

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A one-hundred-year storm has a one in one hundred chance of occurring in a one-year period. Hurricane Sandy hit the East Coast last year and packed a punch that knocked out critical infrastructure facilities — including wastewater treatment plants. 

The storm surge raised the sea level to 12 feet above normal, and millions of gallons of water poured into wastewater treatment facilities. Flooding events are some of the most challenging issues treatment plant operators must manage. 

Soon after the hurricane hit the East Coast, the south was hit with rainstorms. The 11 mgd (design) wastewater treatment facility operated by the Loxahatchee River Environmental Control District (LRECD) in Jupiter, Fla., experienced torrential downpours. During October and December 2012, as much as 11 inches of rain fell in a four-hour period.

At the LRECD, it is all hands on deck when a storm event occurs. 

Follow the steps

The district followed specific process controls to avoid untreated sewage discharge into the public sector. The LRECD experiences the best treatment when it maintains a low sludge retention time. When the heavy rains occurred and facility capacity was exceeded, operators made process control changes to contend with the onslaught of influent flow. Waste was slowed, blowers for the aeration system were shut down, and return activated sludge (RAS) pumps were shut off to allow biosolids to settle so microorganisms were not washed out. 

Operators changed the cycle for the bar screens that remove debris and rags from incoming flow from intermittent to continuous to prevent clogging. All of the effluent was diverted to deep well injection storage until the flow subsided and treatment stabilized so quality effluent could be sent out for irrigation purposes. Operators continually monitored lift stations, and emergency generators were kept running to keep power to the treatment plant and collection systems. 

As part of a coastal community, the LRECD plant also frequently experiences saltwater intrusion. Saltwater intrusion from intracoastal flooding and storm surges can cause damage to pumps and other key components. Saltwater destroys pumps, motors and electronics. Replacement parts and pumps are sufficiently kept in stock. 

Back to normal

When flows to the plant returned to design parameters the blowers were reactivated along with return activated sludge pumps to re-establish aerobic organisms for process control. Waste pumps that send sludge to the dewatering facility were re-energized to maintain proper sludge retention time (SRT). After a 36-hour period quality effluent was restored to pre-flooding conditions. 

Using the LRECD as a model, operators should follow process controls to ease the effects of high flows during heavy rains and flooding when facility capacity is exceeded. First, adequate staffing is essential during storms and heavy flows. Keep equipment running and monitor lift stations to ensure the public is safe from pollution and sewage spills. All loose items around the facility should be secured to keep possible airborne projectiles from flying around during high winds. 

The saying in Florida about tropical storms is “hide from wind and run from water.” Treatment plant operators neither run nor hide during emergency weather conditions. Personnel are on the front lines keeping machinery operating and the public safe. 

Thomas Cavanaugh, Compliance/Safety Officer
Gary McClure, Chief Operator
Loxahatchee River Environmental Control District
Jupiter, Fla.


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