In the proposed West Acton Sewer District, two different types of sewer would be used. Most houses would be served by gravity sewer. In gravity sewer, wastewater flows through large, sloping pipes which are buried 10 or so feet underground. Pumping stations are used to lift wastewater uphill where needed.
However, about 15% of the proposed West Acton Sewer District would be served by low-pressure sewer. Properties on Mass Ave between Villageworks and the High School, as well as some properties on Arlington Street, would be served by this type of sewer (map).
In this technology, a small-diameter hose is buried at shallow depth along the street. In order to transport sewage through small, non-sloping hoses, each house is equipped with a holding tank and grinder pump. The 70-gallon holding tank is installed at ground level with a visible tank cover. The grinder pump inside the tank chops up solids and pumps sewage under pressure to the main hose at the street. If a problem occurs and the sewage level in the tank becomes elevated, an alarm goes off in the house:
Hooking up to low-pressure sewer is frequently more expensive than hooking up to gravity sewer. This is because the homeowner must pay for 1) the grinder pump unit and 70-gallon holding tank, 2) installation of the tank and pump, 3) electrical work to install the control panel in the house, and 4) hookup of the pump to the main hose at the street.
After installation, a number of other drawbacks present themselves:
1. Grinder pumps are susceptible to failure if anything besides sewage or toilet paper enters the system. This includes disposable wipes, feminine hygiene products, floss, sand, plastic toys, etc. Even with normal use, grinder pumps only last 7-10 years. A grinder pump failure represents a household emergency, as little water can be used until a new pump is installed. Otherwise, raw sewage can overflow into one’s yard (the top of the tank is at ground level). The cost of grinder pump replacement would be $2,000 - $4,000 (including emergency installation), every 7-10 years.
2. Unless the homeowner maintains a backup generator, a power outage also represents a household emergency. Depending on the sewage level when the power went out, the holding tank could have just 38-46 gallons of remaining storage capacity. Homeowners are advised to flush toilets sparingly to reduce the risk of raw sewage overflowing.
3. Low-pressure sewers have been known to cause odor problems. One way to mitigate the odor is to add chemicals to the sewage at periodic treatment stations. Some towns even assess an additional fee for low-pressure systems, as these systems "create hydrogen sulfide gasses that result in nuisance odors and deterioration of sewer infrastructure."
4. Going away for the winter? Homeowners may need to pour 10 gallons of anti-freeze down the drain before leaving, in order to protect grinder pumps from freezing during winter months.
Some homes, with Board of Health approval, may be allowed to use an existing septic tank with the grinder pump, depending on the design and structural integrity of the tank. This would give the homeowner a longer grace period during pump failure and power outages. However, permission from the Board of Health is not guaranteed. A hoisting mechanism may need to be installed to get the grinder pump out of the tank for repair or replacement.
More information about grinder pumps can be found here.
More information about low-pressure technology can be found in the EPA Factsheet.