combined sewer overflow technology fact sheet: inflow ... · combined sewer overflow technology...

United StatesEnvironmental ProtectionAgency

Office of WaterWashington, D.C.

832-F-99-035September 1999

Combined Sewer OverflowTechnology Fact SheetInflow Reduction

DESCRIPTION

This fact sheet describes inflow reduction, apractice designed to minimize the volume of stormwater runoff that enters a combined sewer system(CSS). Inflow reduction can result in a lowercombined sewer overflow discharge volume andmay reduce the number of combined seweroverflows.

CSSs are wastewater collection systems designed tocarry both sanitary sewage and storm water runoffin a single pipe to a wastewater treatment plant.During wet weather periods, the hydraulic capacityof the CSS may become overloaded, causingoverflows to receiving waters at discharge pointswithin the CSS. These overflows are calledcombined sewer overflows, or CSOs. Inflowreduction refers to a set of control technologies thatare used to reduce the amount of storm waterentering the CSS from surface sources. Inflowreduction can be a cost-effective way to reduce thevolume of flow entering the CSS and the volumeand/or number of CSOs. It is particularlyapplicable in CSO communities where open land isavailable to accommodate redirected flow forinfiltration or detention, or where storm water canbe diverted to surface waters.

By helping to reduce the overall flow volumes intoa CSS, inflow reduction technologies help tooptimize the system’s storage capabilities.Maximizing storage in the collection system is oneof the nine minimum controls that every CSOcommunity is expected to implement in accordancewith the Environmental Protection Agency's 1994CSO Control Policy.

Technologies used to reduce inflow include:

• Roof drain redirection.

• Basement sump pump redirection.

• Flow restriction and flow slipping.

• Storm water infiltration sumps.

• Stream diversion.

All of these technologies have relatively low costs,and most require little maintenance in comparisonwith other CSO controls.

APPLICABILITY

Roof Drain Redirection

Roof drains often convey rainfall directly fromresidential and commercial roofs into a CSS. Flowinto the CSS can be reduced by redirecting roofdrains onto lawns or into dry wells or drainfieldswhere flows can infiltrate into the soil. Roof drainredirection (redirection) works best in residentialareas where homes have open yards. Telephone,mail, or door-to-door surveys are necessary todetermine the extent of roof drain connections tothe CSS. Because the net volume reduction perhousehold is small, implementation needs to bebroad and consistent throughout the service area.The cumulative reduction effects of such a programcan be substantial. Redirection can be eithervoluntary or mandatory. Cash or other incentivescan be instrumental in achieving widespreadredirection.

Roof drain redirection is a relatively simple taskthat can be done easily by individual homeowners.Therefore, it is essential to prepare guidance forhomeowners who elect to perform the redirectionthemselves. This guidance material should alsoinclude a list of contractors able to provideredirection for a reasonable cost. Homeownersshould be reminded to periodically check to makesure that redirected water is infiltrating and is notcontributing to other surface water problems. Thecommunity should develop a schedule and checkthat homeowners who have agreed to disconnecthave remained disconnected. Roof drainredirection should be combined with basementsump pump redirection where possible.

Basement Sump Pump Redirection

Many buildings have sump pumps to pumpfloodwater from basements. Often this water isdischarged directly into the CSS. Redirecting thisflow away from the CSS and onto lawns or drywells or drainfields reduces the volume of stormwater entering the CSS.

Unlike roof drains, which are common to mostbuildings, basement sump pumps usually exist onlyin discrete areas. Therefore, surveys (preferably on-site) are necessary to determine where sump pumpsare located, whether they discharge directly to aCSS, and whether it is feasible to redirect them intoa yard, a dry well, or an infiltration field. It isimportant to note that the feasibility of dischargingdownspout and basement sump discharges into ayard, a dry well, or an infiltration field depends onthe soil type, the slope, and the drainage conditionsaround the home.

As with roof drain redirection, guidelines forbasement sump pump self-redirection should bedistributed, and the area should be checkedregularly to ensure that redirected water isinfiltrating and is not contributing to other surfacewater problems.

Flow Restriction and Flow Slipping

Flow restriction and flow slipping methods utilizeroadways and overland flow routes to temporarilystore storm water on the surface, or to convey storm

water away from the CSS. Flow restriction isaccomplished by installing static flow or "braking"devices in catch basins to limit the rate at whichsurface runoff can enter the CSS. Excess stormflow is retained on the surface and enters the systemat a controlled rate, eliminating or reducing thechance that the system will be hydraulicallyoverloaded and overflow. The volume of on-streetstorage is governed by the capacity of the staticflow device, or orifice, used for restriction, as wellas surface drainage patterns.

As opposed to flow restriction, where flow ratesinto the CSS are reduced but all storm floweventually flows into the storm sewer system, flowslipping refers to the intentional blocking of stormwater from entering the CSS at catch basins for thepurpose of routing, or "slipping", it elsewhere.Flow slipping is accomplished by partially orcompletely blocking the entry of surface runoff atcatch basin inlets and letting the runoff followoverland flow routes.

Flow restriction and flow slipping can effectivelyreduce inflow during peak runoff periods and candecrease CSO volume. Use of these methods mustbe carefully planned to ensure that sufficient surfacestorage or overland throughput capacity exists inthe drainage area. These methods are almostalways used in conjunction with other practices,such as roof drain and basement sump pumpredirection.

Flow restriction works best in relatively flat areaswhere temporary ponding and detention of water onstreets is acceptable. Extensive public educationand testing are required to build support andaddress concerns that residents and elected officialsmay initially have regarding on-street storage. Flowslipping is an option where opportunities foron-street storage are not available. The "slipped"flow can be diverted along natural drainage routesto separate receiving waters, separate storm sewersystems, or even to more optimal locations withinthe CSS.

The use of flow restriction and flow slippingrequires a detailed evaluation of the collectionsystem and catch basins. The community mustassess the potential for unsafe travel conditions,

Source: Modified from city of Portland, 1997.

FIGURE 1 TYPICAL INFILTRATION SUMPASSEMBLY

flood damage, and damage to roadways. Pilotstudies and monitoring are recommended to identifyimpacts and confirm performance.

Storm Water Infiltration Sumps

Storm water infiltration sumps are below-groundstructures used to collect storm runoff and pass itinto the soil. The infiltration sumps collect runoffin standard storm water inlet structures at theground surface, and rout it to a two-chamberedsystem consisting of a manhole structure and anattached sump chamber. The manhole chamberserves as a sedimentation basin. As this chamberfills, flow reaches an overflow point and begins tofill the second chamber, the perforated sump. Theperforations allow the water to percolate outwardinto the soil. The sump chambers are typically 8 to12 meters (25 to 35 feet) deep and are surroundedwith granular backfill to promote infiltration. Atypical infiltration sump assembly is shown inFigure 1.

Infiltration sumps can reduce inflow in areas wherethe underlying soils are moderately to highlypermeable, and the water table is well below theground surface. They are generally more applicablein residential areas that are less than 50 percentimpervious. It is important to get the surface runoffto streets where sumps have been installed, and

implementation in conjunction with roof drain andbasement sump pump redirection is recommended.Due to the potential for chemical contamination ofground water, infiltration sumps are notrecommended for commercial or industrial areas.

Before starting construction of a storm waterinfiltration sump, the community should develop asump management plan that addresses the policy,design, construction, maintenance, and publiceducation aspects of the infiltration sump program,as well as addressing spill response, wellmonitoring, storm water runoff quality, and stormwater sampling protocols. Consideration should begiven to local traffic disruptions during sumpinstallation. Sumps will need to be cleaned everytwo to three years to remove material that collectsin the sedimentation basin.

Stream Diversion

As cities grew during the nineteenth and earlytwentieth centuries, many small streams wererouted into pipes to facilitate development. Incommunities where streams have been routed intoCSSs, the surface runoff once conveyed in thesestreams reduces capacity in the CSS and contributesto overflows. Rerouting natural streams andsurface runoff away from the CSS and back to theiroriginal watercourse or to other receiving waterscan have a significant impact on CSS capacity.

Urban stream diversion is one of the moreexpensive inflow reduction options since it typicallyrequires design and construction of new storm drainlines. Stream diversion resembles CSO separationin that new alternative flow routes are required forsurface runoff. It is typically employed in situationswhere less expensive and less disruptive options forinflow reduction are not feasible, or do not providesufficient inflow reduction. The potential amountof inflow to be diverted from the CSS needs to bewell documented in order to assess itscost-effectiveness.

The first step in implementing a stream diversionprogram is to identify and evaluate alternativeroutes from the stream to nearby receiving waters.It may be possible to "daylight" streams by allowingthem to flow in an open channel in some semblance

of their natural condition. Daylighting can providegreenspace and can serve as an amenity to thecommunity. However, daylighting opportunitiesare limited in most CSS communities because thestreams were originally piped in order to supportproperty development. Diversion through separatepipe systems is also an option, and may be morepractical where open space is limited and thepotential for flooding is high.

ADVANTAGES AND DISADVANTAGES

Inflow reduction can be a very important part of aCSO management program. By reducing theamount of extraneous inflow to a system, CSSs mayavoid CSO problems. Specific advantages anddisadvantages of each of the inflow reductionmethodologies discussed in this fact sheet arepresented below.


Roof drain redirection is a relatively simple and lowcost (per unit) option for reducing storm waterinflow to a CSS. However, in order for a roof drainredirection program to be successful, the publicmust be educated as to the benefits and methods forimplementing such a program. This can be time-consuming and will most likely require some sort ofrebate program or other incentive for compliance.In addition, because the effect per individual roofdrain redirection is small, this program must beimplemented in a wide service area to be effective.


Advantages and disadvantages of a basement sumppump redirection program are similar to those ofroof drain redirection programs. A basement sumppump redirection program can be a relativelyinexpensive and easy way to reduce flow to CSSs.However, unlike roof drains, basement sump pumpsmay occur less frequently and their locations maybe harder to determine. And as with roof drainredirection, a basement sump pump redirectionprogram can be time-consuming and will mostlikely require some sort of rebate program or otherincentive for compliance. And similarly to roofdrain redirection, because the effect per individualsump pump redirection is small, this program must

be implemented in a wide service area to beeffective.


Flow restriction and flow slipping methods can beeffective ways to manage flow in specific parts ofthe CSS. They are relatively inexpensive and easyto implement. However, implementation willrequire a public awareness campaign to inform thepublic of the purpose of the flow diversions. Inaddition, these inflow reduction methods requirelarge surface areas to temporarily store flow beforeit enters the CSS.


While more expensive than roof drain or basementsump pump redirection, storm water infiltrationsumps can be very effective in areas where there ishighly permeable soil and little chance ofgroundwater contamination. However, infiltrationsumps are not recommended for use in areas withhigh groundwater tables or in soils with lowpermeability.

Stream Diversion

Stream diversion can be an expensive method forreducing storm water inflow into the CSS.Changing a stream channel may also requirenumerous permits and may not meet with publicapproval. However, stream diversion will oftencause a significant decrease in the amount of stormwater flowing into a CSS.

PERFORMANCE

The performance of inflow reduction programs isusually measured by the reduction in CSS volume.As sources of inflow are identified and eliminated,the overall flow volume in the CSS decreases, andthe likelihood of CSOs decreases as well. Severalspecific cases studies for inflow reduction arepresented below.


In St. Paul, Minnesota, an estimated 20 percent ofCSO volume came from roof drains. As a result of

a $40 rebate for voluntary redirection and otherinnovative outreach efforts, approximately 18,000homes redirected their roof drains over a three yearperiod. Presently, 99 percent of all residentialproperties are disconnected. In addition, the city isevaluating creative funding options to assistcommercial and industrial property owners todisconnect roof drains.


South Portland, Maine, conducted a visual surveyof the 6,000 residential buildings in the city andfound 380 homes with roof drains and 300 homeswith sump pumps discharging directly into the CSS.The city mailed letters offering to reimburse eachproperty owner for redirecting their roof drains andsump pumps. After the work was completed andinspected by the city, homeowners were reimbursed$75 for each redirected roof gutter and $400 foreach redirected sump pump. The program hasredirected more than 379 roof drains and 304basement sump pumps. The resulting reduction isapproximately 58 million gallons of water per year.Because of the variables involved (rainfall patterns,differing drainage areas, etc.), the city has notdetermined a direct correlation between theseprograms and CSO events; however, overall flow tothe city’s wastewater treatment plant has beenreduced by 2 percent through these efforts.


Gently graded berms can be added to roads andused in conjunction with flow restriction tomaximize on-street temporary storage. In Skokie,Illinois, a fully developed suburb of Chicago servedby a CSS that covers 8.6 square miles, an integratedprogram was developed that emphasized berms andflow restrictors to both increase on-street storageand to reduce the peak rate of flow entering theCSS. In order to accomplish this, 2,900 flowrestricting devices were installed at catch basins,and 871 berms were constructed on streets. Inaddition, 100 percent of the roof drains previouslyconnected to the CSS were disconnected.

South Portland, Maine, conducted a detailedevaluation of 750 catch basins within the CSS anddetermined that 30 catch basins could be eliminated

without any adverse safety or flood damageconsequences. Solid covers were placed on thesecatch basins to direct flow to separate storm sewersor natural drainage. Flow slipping in this mannerhas reduced flow in the CSS by approximately 12million gallons per year.

In Hartford, Connecticut, a pilot flow slippingproject was implemented as part of the city's CSOabatement plan. The method employed catch basinsthat completely crossed the streets, incorporatingsumps that could be located anywhere in theroadway. This strategy allowed the city to workaround the myriad of existing subsurface utilitylines. To ensure safe pedestrian crossings, the catchbasins were located upgradient of pedestriancrosswalks.


Infiltration sumps can be retrofitted withincombined sewer areas, usually beneath the streetsystem. In Portland, Oregon, much of thecombined sewer area has highly permeable soilswith a high hydraulic capacity. Portland iscurrently wrapping up a program that installedapproximately 4,000 infiltration sumps between1994 and 1998. Combined with other CSO controlprograms, including a successful roof draindisconnection program, sewer separation, andstream diversion, it is predicted that the total CSOvolume will be reduced by 3 billion gallons per year(approximately half of the total CSO volume).Portland is initiating another program to maximizestorage capacity in the CSS to further reduce theoverflow volume.

Stream Diversion

Portland, Oregon's Ramsey Lake Wetland Projectinvolved completely separating an urban streamfrom the CSS, creating a wetland to treat theseparated storm water flow prior to discharge intoa receiving stream. The project also providescommunity involvement opportunities such asschool field trips for the planting of wetland plants,and there are future plans to use the facility as anature interpretive center. Portland has sixadditional stream diversion projects planned.

COSTS

Cost comparisons for the wide range of inflowreduction techniques are difficult to make withoutconsideration of site specific factors. The followingsection discusses the range of costs representativeof inflow reduction programs. Findings aresummarized in Table 1:


Some programs have offered rebates or incentivesof from $40 to $75 to homeowners and businessesparticipating in voluntary redirection programs tooffset typical costs for redirection.


Typical costs for basement sump pump regulationsare $300 to $500 per home. Full rebates can beused to encourage homeowners to participate.


Flow restricting orifice devices for catch basins arepriced from $500 to $1,200 each depending on thesize and number ordered. In contrast, simple steelplates to cover catch basins can be fabricated andinstalled for approximately $100 each. Detailedengineering studies to evaluate the collection

system and catch basins require substantial in-houseexpertise or consultant assistance.


Costs are closely related to the type of soil, thedensity of the sumps, and the desired amount ofinflow reduction. Depending upon site specificconditions, total costs are expected to range from $2to $8 per 1,000 gallons per year.

Stream Diversion

Costs for the Ramsey Lake Wetland Projectincluded $500,000 for purchasing the 29.75 acrewetland area; $2.6 million for wetland planning andconstruction costs; $10 million for sewer separationand $3 million for new storm sewer trunklines; and$1.3 million for sump disconnection. This wetlanddrains 640 acres.

REFERENCES

1. City of Hartford, Connecticut, 1999.Denise Horan, Hartford MetropolitanDis t r i c t Commiss ion , pe r sona lcommunication with Parsons EngineeringScience, Inc.

2. Hides, S., 1997. “Quantity is the Key to

TABLE 1 COSTS OF INFLOW REDUCTION ACTIVITIES

Technology Cost Comments

Roof Drain Redirection Costs range from $45-$75 forindividual homeowners to disconnect

Rebates for individual homeowners todisconnect

Basement Sump Pump Redirection Disconnection costs approximately$300-$500 per home

Incentive programs usually used toreimburse homeowners

Flow Restriction and Flowslipping Flow restrictors can cost between$500 and $1,200; covers for catchbasins to encourage flow slipping cancost as little as $100

Indirect additional costs may beincurred because of additional waterpooling on surface streets

Storm Water Infiltration Sumps $2-$8 per 1,000 gallons inflowremoved

Costs are closely related to the typeof soil, the density of the sumps, andthe desired amount of inflow reduction

Stream Diversion $17.4 million for 29.75 acre streamdiversion into a created wetland inPortland, OR

Costs included land acquisition, stormsewer separation and new trunk lines,and wetland design and construction.

Source: Compiled by Parsons Engineering Science, Inc., 1999.

For more information contact:

Municipal Technology BranchU.S. EPAMail Code 4204401 M St., S.W.Washington, D.C. 20460

Improving Quality: a Common Sense Approach forReducing Wet Weather Impacts.” Presented at the26th Annual WEAO Technical Symposium andOPCEA Exhibitions, London, Ontario, Canada.

3. McKelvie, S., 1996. Flowslipping: AnEffective Management Technique for UrbanRunoff. Parsons Brinckerhoff Quade &Douglas, Inc. Proceedings from Urban WetWeather Pollution Conference, QuebecCity.

4. City of Portland, Oregon, 1994. CSOFacilities Plan. Prepared by CH2M HILL.

5. City of Portland, Oregon, 1999. Lester Lee,City of Portland Bureau of EnvironmentalServices, personal communication withParsons Engineering Science, Inc.

6. City of South Portland, Maine, 1999. JayReynolds, City of South PortlandEngineering Department, personalcommunication with Parsons EngineeringScience, Inc.

ADDITIONAL INFORMATION

City of Hartford, ConnecticutDenise HoranMetropolitan District Commission, Engineering andPlanning Department555 Main Street, P.O. Box 800Hartford, CT 06142-0800

City of Portland, OregonLester LeeCity of Portland Bureau of Environmental Services1211 Southwest 5th Avenue, Suite 800Portland, OR 97204

City of St. Paul, MinnesotaMike KassanSewer Utility, Department of Public Works1000 City Hall AnnexSt. Paul, MN 55102

City of Skokie, IllinoisEddie NikaiVillage of Skokie Department of Public Works

9050 Gross PointSkokie, IL 60077

City of South Portland, MaineJay ReynoldsCity of South Portland Engineering Department25 Cottage RoadSouth Portland, ME 04106

The mention of trade names or commercialproducts does not constitute endorsement orrecommendation for the use by the U.S.Environmental Protection Agency.

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