The Alliance for Water Efficiency (AWE) and ...
Nearly every wastewater utility confronts its own unique operational considerations. However, few rival those on Martha’s Vineyard, the premier vacation island off the coast of Cape Cod, Mass. The historic offshore retreat has been the setting for countless beach bonfires, a movie about a great white shark and the playground for the rich and politically powerful. Millions more have traipsed the island, which holds a legacy tracing back centuries.
The island’s population is 15,000 but that number explodes up to 100,000 when mainland vacationers arrive.
Joe Alosso, facilities manager for both Edgartown and Oak Bluffs wastewater utilities, oversees the treatment plants. The utilities have a modest number of connections but they serve the island’s restaurants, lodgings, shops and homes and are subject to unusual seasonal peaks.
“We have adopted a proactive stance in managing our facilities that consists of an SBR package plant and an EIMCO carrousel plant with a dual collection system comprised of 10-in. and 12-in. gravity lines and small force main sewers,” Alosso said. “We tried to use a small diameter system that would get into areas with close quarters or shallow conditions. We put grinder pumps on many of these connections where they serve two or three homes or businesses.”
An estimated 85% to 90% of the island’s development remains dependent on septic systems. In fact, Oak Bluffs and Edgartown are two of only three towns that operate sewage treatment plants on the island. The Edgartown plant treats all septic waste pumped from the island, while the Oak Bluffs SBR plant has had the surplus capacity to resolve problems for two major institutions.
Ocean Park is where effluent subsurface discharge deposits into leaching beds.
Because laws prohibit ocean discharge of even the highest grade of treated effluent, the Oak Bluffs plant operates under a Department of Environmental Protection groundwater discharge permit. The subsurface discharge occurs beneath Ocean Park, the centerpiece of the town located less than 10 yards from the shore. The effluent percolates through 28 leaching beds, sized at 50 by 100 ft, before reaching the underlying groundwater. The receiving system has brought nitrogen levels down to consistently less than the typical 40 mg/L, Alosso said.
“We meet drinking water standards with the treated effluent,” Alosso noted. “The coliform is always less than 2 mg/L, whereas they don’t even close the beach here unless it reaches 200 mg/L. We put out less than 5 mg/L of total nitrogen, although 10 mg/L would be considered safe.”
Until recently, the Oak Bluffs plant primarily served the commercial businesses and in-town residences. Facilities such as the local hospital and high school operated standalone package plants to treat wastewater. That changed earlier this year when the local hospital connected to the Oak Bluffs utility in order to treat the .02- to .03-million gal per day (mgd) wastewater stream, instead of continuing to operate a standalone package plant. The high school is the next candidate for a connection, which will accept and treat the school’s sewage and return the high-quality effluent to the school’s existing onsite subsurface receiving system under the track facility.
The hospital and high school have presented special considerations to Scholfield, Barbini & Hoehn, Inc., the island-based engineers for two new projects that would include a new lift station. Most utility operators would concur that their biggest operational headaches are hospitals, schools and restaurants whose wastewater streams contribute to clogged lift stations and operational problems even at their down-line wastewater treatment plants.
The hospital, now building a new replacement facility, agreed to build the necessary lift station and related infrastructure before relinquishing ownership and maintenance to Oak Bluffs. Early in the planning, it became evident that the new lift station needed the ability to overcome chronic clogging that plagued the hospital’s previous lift station. Hospital wastewater often carries improperly disposed hand towels, bandages and other medical waste flushed into the system.
The pump is found clean and unclogged after the first time it is removed from the pit.
“[At] the old lift station they operated, they would experience clogging episodes at least twice a week,” said Richard J. Barbini, P.E, who led the design of the new lift station. “Until recent years, we would have lived with the problem by installing a grinder pump, sometimes with screening, at the new lift station. We initially considered this solution until it became apparent we couldn’t get the needed head capacity.”
An award-winning pump technology developed by ITT Flygt was a better alternative. The company that developed the submersible pump decades ago had brought the new “N-Pump” technology to the U.S. several years before Barbini's firm took on this project.
The energy-efficient pump units feature a volute with the self-cleaning impeller. A stationary relief groove in the volute clears the semi-open impeller of all debris so that it can be carried away through the pump. The patented system virtually eliminates built-up debris that induces clogging. Through installations around the globe, the N-Pump has proven inherently more reliable than a traditional chopper pump that can grind rags and other debris into pieces small enough to pass through lift stations but were resistant to rocks, wood or other hard foreign materials. Also, the shredded material could cause problems at the down-line treatment plant unless the plant had screens at the headworks to capture it.
Hospital discharge is transferred to the WWTP by two pumps.
Barbini sized the new hospital lift station to serve present and future flows and those from the potential medical office development nearby. He sized the ITT Flygt Model NP 3171 pump unit using design points of 80 to 115 gal per minute (gpm). The long force main served by the new station required 240 TDH at 100 gpm. Despite the challenging low-flow, high-head conditions, the installation has passed the test at the hospital. The N-Pump is a likely candidate for a similar lift station at the high school in the future.
“I haven’t heard of a clog yet,” Barbini said.
Alosso, equally impressed, has ordered a 5-hp N-Pump to replace a larger horsepower chopper pump at the plant.