To serve the needs of its Cape Cod resort town, the Provincetown (Mass.) Wastewater Treatment Facility relies on a unique vacuum sewer system to get the water to the facility, as well as talented operators to manage drastic flow variations common to a seasonal weekend getaway. The facility is operated by Woodard & Curran (Portland, Maine) as a subcontractor to the firm that designed the plant, AECOM (Los Angeles). Operating Provincetown’s wastewater treatment system not only requires all the training and know-how that any facility its size demands, it requires an understanding of the economy of the town and the vagaries of the tourist trade.
Until the facility began operation in July 2003, residents and businesses in Provincetown were responsible for managing their own wastewater, usually with private septic systems. The leach fields of most septic systems in town had to be elevated to ensure 5 ft (1.5 m) of separation between the bottom of the soil-absorption system and the maximum groundwater elevation, as required by state regulations.
However, with the installation of the vacuum sewer system, residents have a choice. Vacuum sewers are well suited to regions with high water tables and, with some modifications, highly variable flows. The typical vacuum system is designed to operate within the normal variations of a residential area, but Provincetown’s flows are far from normal. The area is a resort community with a relatively small year-round population. This means that in the summer months, influent is dominated by flows from seasonal restaurants, hotels, and guest houses. In winter, the flow is primarily residential. Flows range from as
Provincetown, Mass., relies on a vacuum sewer system to collect its wastewater for treatment. The photo shows construction of the town’s central vacuum station, including the vacuum tank, which makes the system work. (Credit: Woodard & Curran [Portland, Maine])
low as 60,000 gal/d (227 m³/d) in winter to higher than 400,000 gal/d (1500 m³/d) on peak summer days.
After a brief period of trial and error, the facility staff learned to make seasonal adjustments to the system and rely heavily on biannual calibration of valves and controllers to ensure proper and compliant system operation. Significant improvements to remote vacuum monitoring, as well as communication and supervisory control and data acquisition (SCADA) systems, also have been essential.
Sequencing Batch Reactors
Once the wastewater reaches the facility, it passes through an in-channel microstrainer, where a flowmeter measures and records the volume. The influent then enters a pair of sequencing batch reactors (SBRs) for nitrification–denitrification. Afterward, the solids are allowed to settle, and the supernatant is decanted and passed through an equalization tank. Next, two disc filters remove additional suspended solids before ultraviolet disinfection. The final effluent is discharged through five rapid infiltration basins
Here again, the variable nature of Provincetown’s flow requires extra effort from the operators. The complications caused by the drastic fluctuations mean that the experience of the operators is the most essential element of the treatment system. They must understand when and how to change the SBR aeration cycles, modify wasting strategies, and determine appropriate settings for the vacuum sewer
Biosolids management also is a constantly changing equation. Staff must gear up for busy weekends and anticipate much higher levels of influent and associated levels of biosolids. Maintaining an appropriate inventory of solids in the SBR tank requires careful planning based on experience and is a factor of facility design, past performance, and projected flow.
With these considerations in place, the facility uses a proprietary program in its SCADA system to control the 6-hour process of treating a batch of influent. The entire process is automated. When the facility is unattended, any operation problems automatically trigger notification of the on-call operator.
In 2008, the original process instrumentation for the SBR monitoring and control system was nearing the end of its useful life, so the temperature, dissolved-oxygen, and pH equipment was replaced, and oxidation-reduction potential monitors were added to improve facility performance. New SCADA screens were added to accommodate the new data being collected.
Additionally, Provincetown’s service area continues to grow. In 2008, one new service area consisting mostly of residences was added, and another is planned for 2009. These additions expand the reach of the facility’s “checkerboard district.” In each service area, connecting to the vacuum sewer system is voluntary. Property owners with a functional septic system can opt out of the system and instead join a townwide septic system inspection program.
However, connection to the vacuum sewer is an extremely popular choice. When the system was proposed in 2001, about 340 property owners respond-ed that they wanted to be connected to the new system. By the time construction was finished, more than 500 customers were on-line. Now, more than 800 customers are connected.
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