Combined and Separate Sewers, and Which One Is Under Your Street

One county, two kinds of sewer

WEP states it plainly: in Onondaga County the City of Syracuse has a combined sewer system, in which sewage and stormwater share a single pipe, while other municipalities have separate stormwater and sewer systems. During heavy rainfall or snowmelt the volume can exceed the capacity of the combined system, and combined sewer outfalls discharge the surplus into local tributaries. Those outfalls are worth understanding rather than being alarmed by, because they tell you what the failure mode is.

The surplus has to go somewhere, and a discharge to a creek is the designed alternative to it coming back up through the lowest opening in the nearest building. That is the city. It is also why the same rainstorm is a completely different event on a Manlius street, where the rain has no legitimate route into the sanitary sewer at all.

Inside the city, rain is part of the load your pipe carries

Under the NOAA 1991 to 2020 normals, Syracuse receives 38.47 inches of precipitation a year at Hancock International Airport, spread across 173.6 days with at least a hundredth of an inch, 22.7 days with half an inch or more and 5.3 days with an inch or more. The last two figures are the ones a combined system feels.

A hundred and seventy days of drizzle is a rounding error to a sewer; a handful of days a year with an inch or more is a capacity event. Add the 123.8 inches of snow the same station averages, and a ground surface that crosses the freezing point in both directions on roughly 84 days a year, and the melt becomes a second season of the same problem, arriving over hours instead of minutes.

If your basement backs up only in those conditions, you are looking at a different diagnosis from a house that backs up on a dry day.

How much the overflow picture has changed, in the county's own numbers

Table 3-1 of the county's 2018 Ambient Monitoring Program report records that Onondaga County had 72 operational combined sewer overflows in 1998, before the Amended Consent Judgment, and that as of December 31, 2018 those 72 had become 25 abated, 27 closed and 20 still operational, with 10 on Harbor Brook, 9 on Onondaga Creek and 1 on Ley Creek.

The Amended Consent Judgment set a schedule of combined sewage capture rising to 95 percent by volume by the end of 2018; the county reported 97.7 percent capture and the state Department of Environmental Conservation determined in November 2019 that it had achieved compliance with that milestone. Both halves are true at the same time.

The system is far better than it was, and twenty outfalls still operate, which is why a heavy rain in the city is still a heavy rain for the pipe under your street.

Outside the city, rain has no route in, and the codes work to keep it that way

In a separate system the storm drains and the sanitary sewers are two different networks, so rainfall does not surcharge the pipe carrying your waste. The villages write that separation into their own law. Village of Fayetteville Code section 133-17 prohibits connecting roof downspouts, exterior foundation drains or areaway drains to a building sewer. Village of East Syracuse Code section 277-5E prohibits any source of surface runoff or groundwater from being connected to a building sewer.

Those clauses are not decoration. Where they get broken, usually by a downspout somebody tied into a convenient pipe decades ago, a separate system starts behaving like a combined one at one address, and the house that floods is the house that made the connection. On a suburban property that only backs up in rain, that is the first thing worth looking for.

So why do separate sewer towns still flood?

Because a sanitary sewer that rain cannot surcharge is only half of the drainage picture. The Village of East Syracuse annex to the 2025 county hazard mitigation plan records flooding on several streets and residential properties from poor drainage, and notes that some Village streets have no storm sewers at all, with Hartwell Avenue and East Second Street among the places named.

Skaneateles records storm sewers on the eastern side of the Village surcharging in a storm that dropped two and a half to five inches in under two hours, with properties on the west side flooding from surcharge. Cicero records a storm of up to six inches in a short time in which many homes around Brewerton flooded when sump pumps were overwhelmed. Three towns, three mechanisms, and not one of them a blocked lateral.

Inflow and infiltration: the pipe fills with water that was never meant to be in it

A sanitary sewer is not supposed to carry groundwater, and an old one does anyway, through cracks, open joints and leaking manholes. The county is actively chasing it.

WEP states that its right-of-way maintenance and condition assessment along the Bishop Brook Trunk Sewer supports compliance with an Order on Consent between Onondaga County and the state Department of Environmental Conservation requiring investigation and reduction of inflow and infiltration sources, with a closed-circuit television crew inspecting manholes and sewer lines for condition and for signs of it.

A parallel abatement project is in progress across the Meadowbrook-Limestone plant's service area, lining sewers and replacing and rehabilitating manholes to reduce excessive wet weather flows. If you live in Manlius, Fayetteville, DeWitt or Jamesville, that work is aimed squarely at the wet weather behavior you notice in your own basement.

What is under a city street is older than most people picture

WEP records that the Midland and Colvin trunk sewer, which links the Southside of Syracuse to the county system and the Metropolitan plant, was originally constructed in 1896 out of brick, is egg-shaped, measures 87 inches tall by 58 inches wide, lies 28 feet underground and carries over 100 million gallons a day. A routine inspection in winter 2024 found the structure compromised, and it was repaired with a new lining that July.

That is the scale of the thing your lateral eventually joins, and it is the context for the smaller work you may see going on: WEP is reconfiguring the flow regulator structure at Evans Street to improve sewer hydraulics and increase capacity, excavating to nearly 30 feet, with the stated aim of reducing the frequency of overflows at CSO 066, on site from January 2026 and expected to be substantially complete by September 2026.

When an overflow happens, the public has to be told within four hours

New York's Sewage Pollution Right to Know Act, Environmental Conservation Law section 17-0826-a, requires a publicly owned treatment works or the operator of a publicly owned sewer system to report a discharge of untreated or partially treated sewage, including a combined sewer overflow, to the Department of Environmental Conservation and the local health department no later than two hours after discovery, and to notify the general public through electronic media no later than four hours after discovery.

That gives you a cross-check nobody mentions. If your basement took sewage during a storm and a public notice went out covering your area at the same time, the event and the system are connected. Write the date and the time down before you speak to anybody, because the record ages faster than the memory of it.

What to tell whoever you call

The single most useful sentence you can give a contractor is when it happens. A house that backs up only in heavy rain or during a thaw is telling a different story from a house that backs up on a dry Tuesday when the washing machine empties. Inside the city, rain in the pipe is normal and the questions are capacity and your own protection.

Outside it, rain in the sanitary sewer is a fault, and the questions are which connection or which crack is letting it in. Say which municipality you are in, say what the weather was doing, and say which fixtures were affected and in what order. Those three answers cost nothing to give and they put whoever comes out on the right line of inquiry before they arrive.

Checked against Onondaga County Department of Water Environment Protection and NOAA National Centers for Environmental Information (checked September 8, 2026).