Marlowe’s Creek outfall insufficient, study finds

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According to a study by architectural, engineering and surveying company Alley, Williams, Carmen and King, Inc., the Marlowe’s Creek outfall is insufficient for future growth in Roxboro.

The Duncan and Southside pump stations pump wastewater north over a ridge that splits the city into a northern and southern half and into the outfall which then carries the wastewater to the wastewater treatment plant.

Josh Johnson, an engineer with Alley, Williams, Carmen and King, presented the report that concluded the outfall can handle daily dry weather flows, but wet weather causes problems that lead to sanitary sewer overflows.

A further complication is the proposed 400-home Satterfield Farms subdivision on the southern boundary of the city which would require around 144,000 gallons of sewer service per day, projected to be pumped by the Duncan Pump Station, according to Johnson’s report.

Johnson said that Southside is pumping near capacity, but Duncan has significant capacity available.

According to the report, the Southside pump station is pumping at or near its 1,140 gallons per minute capacity while Duncan is carrying approximately 130,000 of 460,000 possible gallons per day - around 28 percent of its capacity.

Another complication for the pump stations is their combined force main which limits the total combined gallons per minute from 2,500 to 2,036.

“The 2,036 is an important number for the modeling of the existing system because it means that it’s all you can get from the pump stations to go into the outfall,” Johnson said. “2,500 gallons per minute would be well more than the capacity of the outfall. The question is how that plays in the scenario.”

According to the report, the outfall line works as intended during dry weather with daily gravity flow and both pump stations, but it begins to fail during stormwater events and the force main maxed out.

“So what’s there now?” Johnson said. “Pump stations are restricted by the combined force main. The existing outfall is insufficient for the existing conditions. It does not satisfy the flow that it needs in a minimal wet weather situation. As the wet weather gets worse, the situation gets worse. The results of that are that you have overflows and you have very, very limited future capacity for the outfall.”

Public Services Director Andy Oakley said the outfall’s stormwater problems will be addressed in a study planned for January.

Johnson offered five improvement options with the goals of effectively carrying wet weather flows, allowing for future growth, being economically and regulatory feasible and opening up future growth area to gravity sewer, if possible.

“The simple way, solution wise, is to separate one pump station from the outfall,” he said. “Specifically, if we could separate Southside from the outfall. Southside is pumping further, it’s a longer force main. That’s a nice, easy way to take significant flow off the outfall line.”

The first option would be to split the force main.

With separate force mains, 668,160 additional gallons per day would be able to flow to the outfall. However, this solution must be combined with improvements to the outfall or it will experience the same overflows.

A split force main would cost $800,000.

Option two would be a larger or second, parallel outfall.

Johnson said replacing the outfall would be infeasible due to environmental concerns because the line is in close proximity to wetlands, streams and structures. It would also be infeasible to build a second outfall due to similar environmental concerns.

The second option would also require a split force main and would total around $5.1 million.

The third option would be a parallel force main all the way to the wastewater treatment plant.

Johnson said the 34,000 lineal feet of the line would raise the project cost significantly. The option would also require a split force main, but would bypass the Marlowe’s Creek Outfall. This option would also provide limited future growth improvements and no additional service area.

The fourth option would be a second wastewater treatment plant.

Johnson said there were many prohibitive factors including an interbasin transfer of wastewater between the Neuse and Roanoke River basins, nutrient limitations and a new outfall line.

A new wastewater treatment plant would cost approximately $107 million.

The fifth and favored option would be a gravity sewer along the western edge of the city – the Western Sewer Interceptor.

The $10 million project would add gravity sewer to the city’s western boundary.

“Southside is on the plans to be upgraded anyway, but you would tie the Southside pump station force main into the new outfall line, so it would solve the Marlowe’s Creek problem and at the same time you’d be laying a new sewer outfall line to provide another area for growth.”

The WSI was previously designed in 1998 but was never constructed due to cost concerns at the time.

“Obviously, that is not a cheap alternative, but there is a difference in economically feasible and cheap,” Johnson said. “Sometimes economically feasible is what you need to do and sometime cheap is not what you need to do.”

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