Septic waste from houses in East Falmouth loads more than 20 tons of nitrogen into Great, Green, and Bournes ponds each year. This huge amount of nitrogen accounts for more than half of the total nitrogen that enters the ponds, and is the main reason that water quality in the ponds has declined over the past twenty-five years.
A central sewage treatment plant offers a cost-effective way to reduce nitrogen inputs from waste-water, and to begin to restore health to the ponds.
Nitrogen is a nutrient, and at low levels (< 0.3 ppm) sustains the growth of aquatic plants that form the base of healthy pond ecosystems. However, at the high levels observed in the ponds today (> 0.7 ppm), nitrogen stimulates the growth of algae that shade out native vegetation, which in turn destroys fish habitat and occasionally uses up so much oxygen that fish kills occur. Shellfish, unable to survive without oxygen, disappear.
The Ashumet Plume Citizens Committee has been working to identify solutions to the nitrogen-overloading problems, and they quickly realized that any meaningful solution would have to reduce very sharply the nitrogen inputs from septic waste. They explored several waste-water treatment options and judged these options according to three criteria: nitrogen removal efficiency, reliability, and cost-per-household.
On-site Treatment
The option that scored lowest for all three criteria is on-site denitrifying systems. These self-contained units are installed in individual homes, much like the current Title V septic systems. However, unlike Title V systems that are not designed to remove nitrogen, these units convert nitrogen from organic material into gas that is released into the atmosphere.
The drawbacks to on-site systems are that they only remove 65% of the nitrogen in the waste, and they are expensive: $18,000 per house. Furthermore, the systems require regular maintenance; otherwise, the nitrogen removal efficiencies decrease rapidly.
On-site systems also require year-round use in order to operate efficiently. The process that converts organic nitrogen into gas is driven by bacteria that need a constant supply of waste (which to them is food). For the many East Falmouth residents who are here only during the summer, on-site denitrifying systems would be extremely inefficient.
Neighborhood Treatment
Neighborhood sewage treatment plants offer several advantages over those on-site systems. These small plants process up to 10,000 gallons of waste per day, which is enough to sewer approximately 25 homes. They remove 80% of the nitrogen in waste-water at a per-house cost that is only slightly higher than the on-site systems.
Neighborhood plants, unlike on-site denitrifying systems, can operate efficiently year- round even if some of the users are only in Falmouth for the summer. As long as a reasonable number of the users stay year-round, there is enough flow of waste to the plant to sustain the bacterial populations the system needs to function properly.
Central Treatment
A central treatment plant offers the same advantages as the neighborhood plant and several more. Because central plants use commercial quality materials and are run by professionals, they are highly efficient and reliable. More importantly, they remove 90% or more of the nitrogen from waste water.
Treatment plants range in size, with the smallest processing 0.2 millions gallons a day, approximately 400 homes. As the size of the plant and the number of homes connected increases, the cost per house decreases. A small plant (400 homes) for some of the homes within the lower watersheds of the ponds, south of Route 28, would cost $18,500 per home, only slightly more than on-site de-nitrification. A larger plant (2700 homes) would cost $16,500 per home, a significant per-home savings over the other options.
For areas that are densely-developed, then, central wastewater treatment appears best from the standpoint of all three criteria: greatest efficiency in nitrogen removal, best reliability and lowest cost-per-household. That conclusion would apply to the lower watersheds of the ponds where there now are more than 4000 houses.
That conclusion does not apply, however, to the 2300 homes located in the upper watersheds of the ponds. The upper watersheds are much less densely-developed, and fewer houses can be reached per segment of connecting pipe. Therefore the cost-per-house to connect to a central plant would be considerably higher in those areas. Most of the added homes to reach buildout conditions will be located in the upper watershed [in total, the home count is expected to increase from 6300 today to more than 7700 at buildout].
The optimum approach for the upper watersheds appears to be a combination of neighborhood and on-site denitrifying systems, that despite their drawbacks, operate autonomously and thereby avoid the expense to construct lengthy collection lines. An alternative or supplementary approach for the upper watersheds would be to construct wetlands at the base of each river that feeds into the ponds. The wetlands would act like sponges by absorbing some nitrogen flowing into the ponds from the upper watersheds.
Nitrogen Load Impact
To restore reasonably-acceptable water quality to the three ponds, the nitrogen loading at buildout must be cut in half. That will require sharp reductions in nitrogen loading from septic systems and fertilizer use that represent the addressable sources of nitrogen. For example, if nitrogen loading from fertilizer can be cut in half, then the nitrogen load from septic systems would have to be cut by 80%. Cutting fertilizer use will not be easy, but it will save money for property owners. That’s not true of wastewater treatment, of course.
For example, a small plant to service 400 homes in the lower watersheds would cost about $7.5 million. That plant essentially could be financed from the US Air Force commitment and it would remove at least twice as much nitrogen as will come from the Ashumet nitrogen plume. But the total nitrogen load will grow by 15%, at least, when the watersheds are developed to the maximum capacity allowed by current zoning restrictions. Already poor water quality would get considerably worse.
Increasing the size of the plant to remove 15% of the nitrogen load would keep water quality from deteriorating further. The plant would connect 1100 homes, and cost about $20 million. After the first $7.5 million from the US Air Force, the Town or property owners would have to pick up the difference.
Applying nitrogen-removal treatment to most of the homes in the watersheds would put the total cost in the range of $75 to $100 million or more. The expense would cover a large central treatment plant for the lower watersheds, and a combination of neighborhood and on-site de-nitrification for the upper watersheds.
November Town Meeting resolved to pursue solutions to nitrogen pollution in the three ponds to the extent required to achieve meaningful improvements in water quality, recognizing that such solutions will require expenditures substantially in excess of US Air Force funding. The Committee is working to define such alternatives, including the potential to create a nitrogen management district that would be essentially self-funding.
Meanwhile, it is useful to put the cost of wastewater treatment in context. There is no free lunch; even Title V systems are expensive and they do not last forever. The next article in the series will look at such costs over the next 20 to 30 years, and also examine the risks of deteriorating water quality in our coastal ponds to property values, the town’s tax base, and Falmouth quality of life for residents and our ability to attract visitors.
Web Site
Thanks to the efforts of George Trudeau, Falmouth’s webmaster and manager of information technology, and Brad Stumcke who is the Committee’s media contact, the Committee now has its own website that can be reached via the Meetings link at www.town.falmouth.ma.us or directly at www.geocities.com/ashumet2001. The site contains the Committee Report, Falmouth Enterprise articles and complete reports of the consultants [Horsley & Witten, UMASS and Applied Coastal Research].