Assateague Island is a barrier island that stretches from the Eastern shore of Maryland down Atlantic coast to Virginia. The island is a National Park, administered by the National Park Service, under the National Seashore division. Assateague Island is a vital resting and feeding area for birds and also is home to more than 300 wild ponies. The park receives about 1 million visitors per year. The sensitive tidal marsh and surrounding waterways require nitrogen and phosphorus to be removed from wastewater produced on the island and the existing treatment plant was failing to meet effluent requirements.
The Existing Plant:
The existing Wastewater Treatment Plant was constructed in 1994 with an upgrade of facilities in 2006. The wastewater treatment plant serves the Headquarters/Visitor Center, collects highly concentrated waste from pit toilets on the island and also the University of Maryland Ecology Learning Center. Effluent from the plant was discharged directly into Sinepuxent Bay, an area protected for shellfish harvesting. The intent of the project was to not only improve effluent quality, but also to allow discharge into the adjacent man-made wetlands area proffering additional improvements to the effluent quality prior to reaching the Bay.
A Membrane bioreactor (MBR) was selected as the new treatment method because no other technology could demonstrate consistent achievement of the high quality discharge required given the variable flows and loads that the system would encounter. Dynatec was selected to provide an out-of-basin MBR system to be applied with minimal disturbance to the area and maximum use of existing facilities. Because the existing basins could be retrofitted and reused with the out of basin MBR, the Dynatec system was the lowest cost solution for the project.
The existing treatment system consisted of the following components:
- A lift station to transfer waste into the system
- Two sequencing batch reactors
- One sludge holding basin
- One UV sterilization basin and
- One final re-aeration basin
Dynatec converted the system by adding a screen to remove solids as the waste entered the system, converted one of the existing SBR basins to an equalization tank, and added coarse bubble aeration for mixing, converted the other SBR basin to an aerated reactor with fine bubble aeration, converted the sludge holding tank to an anoxic basin, the re-aeration basin to a post-anoxic reactor, and the UV basin to re-aeration.
In addition, new PD blowers were added with VFD control to allow good control of the dissolved oxygen content of the aerated reactor. An out-of-basin membrane system was added to clarify the mixed liquor and a permeate holding tank with aeration was added to ensure that the DO discharge requirements would be met and to meet a future requirement for reuse of the treated water for flushing. Finally, chemical feed systems were added for pH control, auxiliary carbon feed and for phosphorus precipitation.
After the installation was complete, and the system was seeded with sludge, the plant was able to meet discharge requirements relatively quickly. One of the challenges that the treatment system faces is the wide variety of load and volume that is experienced. For example, in winter, there is low flow, since only members of staff are present, with normal waste strength. In summer, visitor attendance is at a peak, so flows are high. When the pit toilets are discharged to the system, very high loadings are experienced. In addition, flows can vary considerably between weekdays and weekend days.
The system meets the new and more rigorous discharge requirements. The staff plays close attention to the system process parameters in order to optimize system operations
The upgraded system is designed to treat up to 12,000 gals per day, and was built with further capacity in mind for the future, should new facilities be added that will require