A little more than four years ago, Rehoboth Beach began pumping highly treated effluent from its wastewater treatment plant into the ocean via an outfall pipe that terminates one mile due east of Deauville Beach.
As part of the Delaware Department of Natural Resources and Environmental Control permit allowing the outfall, the city is required to have the outfall inspected every two years. A Baltimore-based company recently completed the biennial inspection and, minus two missing diffuser bolts, things seem to be in good working order.
Ballard Marine Construction conducted the inspection June 28 to July 1. According to the report, two of the 10 diffusers at the end of the outfall are missing one bolt each, but otherwise the outfall pipe “was intact and showed no signs of deterioration or misalignment.”
Rehoboth Public Works Director Kevin Williams said the city is pleased with the report and the two diffusers are secure because each have enough remaining bolts. The city plans to replace any missing or damaged bolts when the next required inspection is completed in two years, he said.
Williams said a complete diffuser assembly costs roughly $30,000 and is designed to last 50 years, but would be expected to last longer. The city had to replace one two years ago after it was damaged, he said, guessing the damage was caused by a passing ship anchor.
In addition to looking at the diffusers, which help dilute the concentration of the effluent, the report says all articulating concrete block mats are intact and in good condition. There is no sign of erosion, scouring or displacement of armoring materials, said the report.
Williams said each mat has a submerged weight of 4,000 pounds and is used to prevent erosion of the seabed beneath the piping. It’s similar to the bed prepared for railroad tracks to sit on, he said.
The report also says there are no signs of leakage or erosion along the bottom of the sea floor above the pipe.
During the peak season, Williams said there are typically 2 million to 2.5 million gallons of treated effluent coming out of the outfall per day. Prior to the outfall, the effluent was piped into the Lewes-Rehoboth Canal, which abuts the city’s wastewater treatment facility to the west.
The original discharge permit from DNREC expired at the end of June. Williams said the city submitted its permit application to DNREC in December 2021 and is still awaiting approval. However, he said, the city continues to be in compliance with the discharge limits established.
The city’s permit allows for a daily average of 425 pounds each of biological oxygen demand and solids to be discharged from the outfall. During a commissioner discussion in January, testing showed less than 50 pounds of each pollutant is being pumped daily since the outfall began working in 2018.