Storm Drain vs. Sanitary Sewer: What Goes Where

Two systems, two destinations

EPA explains that most U.S. communities have separate systems. One set of pipes carries wastewater from homes and businesses to a treatment plant, and a separate set collects stormwater from drains at the ends of driveways, around parking lots and along streets and carries it to a local waterway (EPA). Atlanta's watershed department describes its storm system as pipes, catch basins, yard inlets and open ditches that carry runoff to the nearest stream, lake or river, picking up pollutants on the way (Atlanta DWM).

What belongs in each

Wastewater from your plumbing fixtures and appliances belongs in the sanitary sewer, even when it looks clean. Atlanta lists wastewater piped from a washing machine or floor drain to a creek or ditch among the illicit discharges residents should report (Atlanta DWM).

Storm drains are for rain. Atlanta's watershed department says that discharging any polluted water, sewage, oil or other material into a storm drain, catch basin, roadside ditch, stream or manhole is strictly prohibited (Atlanta DWM). EPA's advice is to keep litter, pet waste, leaves and debris out of gutters and storm drains, because those outlets drain directly to lakes, streams, rivers and wetlands (EPA).

Some water falls in between, and local rules decide where it goes. Atlanta, for example, suggests draining chlorinated pool water to the sanitary sewer, or holding it until chlorine and temperature levels are acceptable and then spreading it over a permeable surface before it can reach the storm system (Atlanta DWM). Other utilities set their own rules, so ask yours before you drain a pool or reroute a pipe.

Why downspouts and sump pumps stay off the sewer

Sewer pipes are sized for wastewater, not rain. Gwinnett County's Department of Water Resources says stormwater connections put an unnecessary load on treatment plants and could cause sanitary sewer backups. It runs smoke tests to find faulty pipes and illegal connections such as storm drains or roof drains, leaves door hangers at least 24 hours before testing, and follows up wherever smoke appears from something that shouldn't be connected (Gwinnett DWR). If smoke rises from your downspout during a test, it is probably tied into the sewer.

Local sewer codes spell out the ban. Atlanta's code, for example, prohibits sending ground water, roof drainage, basement drainage, subsurface drainage or yard drainage to the sanitary sewer through a direct or indirect connection, with a narrow exception tied to industrial discharge permits (Atlanta Code § 154-299). That covers a downspout piped into the sewer line and a sump pump that empties into a floor drain or laundry sink. Other Georgia utilities have their own versions, and several provider pages in the metro Atlanta directory cite them. Georgia's plumbing code has a parallel rule, covered with a checklist for your property in why drains back up when it rains.

Don't confuse a sump pump, which moves groundwater, with a sewage ejector, which lifts wastewater from a basement bathroom up to the sewer line; see sewage ejector pumps.

Combined sewers in downtown Atlanta

In a combined system, wastewater and stormwater share the same pipes. EPA explains that in dry weather everything goes to the treatment plant, but heavy rain can overwhelm the system, and permitted outfalls then release untreated or partially treated flow into nearby waters. About 700 U.S. communities deal with these combined sewer overflows (EPA).

Atlanta is one of them. The city says its storm sewers connect to the sanitary system only in its remaining combined areas, where most wet-weather flow goes to combined sewer overflow treatment facilities and, in the heaviest rain, a small portion may go directly to streams (ATL311).

When Georgia EPD reissued the city's combined sewer permits in 2015, it said Atlanta had separated storm and sewage pipes in most areas, leaving about 10 percent of the city on combined sewers that serve the highly developed downtown area, where removing them isn't practical. The permits limit overflows to four a year, averaged over three years, and EPD named Albany and Columbus as Georgia's other cities with combined systems at the time (Georgia EPD). Combined area or not, Atlanta's ban on dumping into storm drains and catch basins applies citywide (Atlanta DWM).

Who to call: storm drain, ditch or sewer

The answer depends on where you live. Inside a city, the city may run its own storm drains even where a county runs the sewer.

DeKalb's reporting page sends blocked storm drains, catch basins and routine drainage problems to Roads & Drainage, and treats sewage system emergencies, water main breaks and unexplained water flowing in a storm drain during dry weather as watershed emergencies (DeKalb County). Gwinnett's water department maintains the public storm drains and pipes along with the sewer system (Gwinnett DWR).

Not every pipe or ditch is public. Gwinnett says many pipes, ponds, drainage channels and creeks are not part of the public storm system and are the property owner's responsibility (Gwinnett DWR). In general, Atlanta is responsible for stormwater infrastructure in its right-of-way along city streets, on city property, or formally deeded to and accepted by the city; other parts are the responsibility of property owners or homeowners' associations (Atlanta DWM). If you aren't sure who owns a ditch or pipe, ask your city or county before you pay for work on it.

Never put anything but rain in a storm drain

EPA's advice for homeowners: dispose of used oil, antifreeze, paint and other household chemicals properly, never in storm drains, and clean up spilled oil, brake fluid, grease and antifreeze instead of hosing them into the street (EPA). Blowing leaves and grass clippings into the gutter counts, too.

Report what you see. Atlanta asks residents to report grass clippings or leaves dumped into or near storm drains, auto fluids or chemicals poured into a catch basin, and foam, bubbles or a milky look in a ditch or stream (Atlanta DWM). DeKalb routes illicit discharges into storm drains, chemical spills, paint or oil in waterways and sewage overflows to its environmental line (DeKalb County). For grease, wipes and medicines, which belong in neither system, see what not to flush or pour.

This is general information, not legal advice; your city, county or sewer provider can tell you which rules apply at your address.

Bottom line

Wastewater goes to the sewer and a treatment plant; rain goes to the storm drain and the nearest creek. Keep downspouts and sump pumps off the sewer, keep everything but rain out of storm drains, and when something clogs, figure out which system it belongs to before you call.