The short version: inside the Chesapeake Bay Critical Area, every new or replacement septic system in Anne Arundel County must be a nitrogen-reducing "BAT" unit. Outside it, BAT is usually optional, except when an addition needs more capacity than your current system has. The state's Bay Restoration Fund can cover most or all of the cost, and the county's Health Department reviews any project that adds living space or bedrooms.
1. What a BAT system is
BAT stands for Best Available Technology for removing nitrogen. It is a pretreatment unit installed in place of, or ahead of, a conventional septic tank, which cuts the nitrogen your system sends toward the Bay. The Maryland Department of the Environment keeps the list of approved units.
2. When BAT is required
Under state rules, BAT is required for new construction and for replacement systems in the Critical Area (land within 1,000 feet of tidal water), and for very large systems. Maryland dropped the statewide requirement for new homes outside the Critical Area in 2016, and Anne Arundel County confirms that new and replacement systems outside the Critical Area are not required to use BAT. The county exception: if your existing system can't handle the extra living area of an addition, a BAT unit is required inside or outside the Critical Area.
3. Bay Restoration Fund grants
Septic users pay a $60 a year Bay Restoration Fee, and that fund pays for upgrades. In Anne Arundel County, grants go in priority order: failing systems in the Critical Area first, then failing systems elsewhere, then systems with no permit record, then working conventional systems (Critical Area first each time). Funding is first-come, first-served and there may be a waitlist.
| Household | Grant toward the BAT upgrade |
|---|---|
| Income up to $300,000 | 100% of the maximum allowable cost |
| Income over $300,000 | 50% |
| New construction on a vacant lot | Reduced by $1,200 |
Low- and moderate-income owners may also get help replacing the drainfield. You still pay permit fees (the county lists a $450 percolation test application fee and a $625 installation permit fee), any work beyond the unit, and anything above the grant. Apply through the Health Department's Sanitary Engineering Program (410-222-7324 for the grant program) and use a contractor from its pre-qualified list.
4. The ongoing cost: maintenance
A BAT unit must be operated and maintained for its whole life by a certified service provider, with at least one inspection a year. New units come with a two-year maintenance contract and warranty; after that, you need to keep a service contract in place. Budget for it.
5. Adding on or adding bedrooms
On a septic lot, the building permit goes to the Health Department, and you list existing and proposed bedrooms and square footage. The system must be sized for the larger home, and the lot must have room for two future replacement systems. If a conventional system can't be expanded but your current one works, the county allows a one-time limited addition: up to 50% more living space, capped at 1,000 square feet, with an advanced pretreatment unit and no added bedrooms. Check this before you design a bedroom suite.
6. Septic to sewer
If public sewer runs along your property and your system is failing, a Bay Restoration Fund grant of up to $25,000 can help pay to connect. The county's long-term Our wAAter septic-to-sewer program aims to convert thousands of systems in dense waterfront neighborhoods over 30 years, with neighborhood petition projects paid back through an annual assessment. Check your address with the county before you replace a system that may soon be on sewer.
