What is a nitrogen-reducing septic system?
It is a septic system with an added treatment step that removes nitrogen before the water reaches the soil. Rhode Island's septic rules define nitrogen-reducing technology as equipment that cuts total nitrogen by at least 50% and discharges no more than 19 mg/l. You may also hear it called an advanced or I/A system.
A conventional system settles solids in a tank and sends the liquid to a leach field, where soil does most of the treatment. That works for bacteria and solids, but much of the nitrogen passes through to groundwater. An advanced system adds a treatment unit between the tank and the field to remove more of it.
Where are nitrogen-reducing systems required in South County?
They are required inside two Critical Resource Areas. The Salt Pond region covers the pond watersheds of Westerly, Charlestown, South Kingstown and Narragansett. The Narrow River area covers that river's watershed in Narragansett, North Kingstown and South Kingstown. Inside both, the rule reaches new systems, alterations and repairs.
CRMC puts it directly: advanced nitrogen-reducing technology is required for any new installation, repair, modification or replacement of a septic system in those areas. The rules allow limited exceptions for repairs that only replace the septic tank. A cesspool in either area has to be replaced with an advanced system, according to RIDEM.
To check a specific lot, use RIDEM's Critical Resource Area boundary tool. Our guide to the salt pond nitrogen rules explains how the boundaries were drawn and which ponds they protect.
Why the salt ponds need less nitrogen
Nitrogen from septic systems travels through groundwater into the salt ponds, where it feeds algae and lowers water quality. Researchers found that 70 to 95% of the nitrogen entering the ponds from land and air arrives through groundwater, which is why the state rules focus on what leaves each septic system.
A white paper prepared for CRMC by URI researchers estimated that septic systems supplied about 85% of the nitrogen reaching groundwater at Point Judith Pond and about 80% at Potter Pond. The same paper put septic nitrogen at almost 75% of the input to Green Hill Pond and 60% to Ninigret Pond. The Salt Pond plan links dense development to polluted wells and eutrophic ponds.
What does a nitrogen-reducing system cost?
RIDEM's published range for an advanced nitrogen-reducing system is $23,000 to $32,000, compared with about $10,000 to $15,000 for a conventional one. The figures on RIDEM's septic page are undated, so treat them as a starting point; your soils, groundwater and lot size set the real number.
For comparison, Westerly's 2015 facilities plan estimated about $30,000 per lot for advanced onsite systems in Misquamicut, against about $34,000 per property to extend sewers there. The Community Septic System Loan Program can help: RIIB's current plan sets the cost at 1% a year plus a $300 fee, up to 10 years, and most towns cap loans at $25,000. Confirm current terms with your town or RIHousing.
This is general information, not legal or tax advice.
Looking after an advanced system
An advanced system needs more attention than a conventional one, because the treatment unit has to keep working to meet the nitrogen limit. EPA's septic care guidance suggests inspecting alternative systems about once a year, compared with at least every three years for a typical conventional system.
Some towns build that into their rules. South Kingstown notes that innovative and alternative systems need more frequent maintenance under its inspection program, and Charlestown's ordinance requires every system to be inspected at least once every three years. Keep a service contract and records on file. See septic inspections and the septic maintenance schedule guide.
Planning the project with the right people
An advanced system is designed by a RIDEM-licensed designer and built by a RIDEM-licensed installer, as the state rules require for any septic work. The designer chooses a nitrogen-reducing technology that fits your lot and files the plan with RIDEM. URI's professional registries let you look up both.
If you are replacing a cesspool inside a salt pond watershed, start with cesspool replacement; if an existing system has failed, see septic system replacement. The sale rules are covered in our guide to Rhode Island's cesspool law, and the full list of services shows related work.
To talk through your lot, call 401-535-7951 for a free estimate or send your details through the contact page.
Nitrogen-reducing septic questions
How do I know if my property is in a Critical Resource Area?
RIDEM publishes an online Critical Resource Area boundary tool that shows whether a lot sits inside the Salt Pond or Narrow River area. Your designer will also confirm it during the site work. In South County the areas touch parts of Westerly, Charlestown, South Kingstown, Narragansett and North Kingstown.
Do I need a nitrogen-reducing system for a small repair?
Inside the Salt Pond and Narrow River areas, the rules require nitrogen-reducing technology for repairs as well as new systems and alterations. There are limited exceptions for repairs that only replace the septic tank. Your designer can tell you whether your job fits one of them.
Can I use a CSSLP loan for an advanced system?
The Community Septic System Loan Program covers repair or replacement of failing systems and cesspool replacements in participating towns, which include all five South County towns touched by the salt pond and Narrow River areas. Loan caps vary by town, so confirm the current limit with your town or RIHousing.
How much nitrogen does an advanced system remove?
To qualify under Rhode Island's rules, the technology must reduce total nitrogen by at least 50% and discharge effluent at 19 mg/l or less. Actual performance depends on the unit, how it is installed, and how well it is maintained, which is why regular inspection matters.
Is Block Island's Great Salt Pond covered by these rules?
Block Island's Great Salt Pond is not among the ponds RIDEM names for the Salt Pond Critical Resource Area, which covers mainland watersheds. Block Island runs its own wastewater management program to protect the island's sole source aquifer, with inspections at least every three years under the town code. Zoning there also sets treatment standards by location and soils.