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Abtract: Sanitary Sewers and Stormwater Management Systems

Introduction to Sanitary Drainage Systems In the United States, the Environmental Protection Agency (EPA) regulates wastewater and stormwater released into the sanitary or storm sewer systems or into bodies of waters (e.g., lakes, rivers, bays, ocean) through the National Pollutant Discharge Elimination System wastewater permitting program. Environment Canada regulates water discharges through the Wastewater Systems Effluent Regulations. State, provincial, and local regulatory agencies can issue even more stringent guidelines. Facilities managers must be familiar with all of the relevant governing regulations.

Federal regulations govern both direct dischargers that release sewage into bodies of water and indirect dischargers that release sewage to local wastewater treatment plants.  Most campuses are indirect dischargers and are subject to pretreatment regulations. In an atmosphere of  increasing  regulation, understanding the federal, state, provincial, and local rules is essential.

Components of Sanitary Sewage Design Flow Average Flow. Historical flow information extrapolated across the number of users of facilities is the best predictor of per-person flow. Typical per-unit sewage generation is used in the absence of actual flow measurements.

Peaking Factors. Wastewater collection and treatment system design require consideration of both average and peak sewage flow. Historical information is the best determinant of future needs, but in the absence of historical data, the Harmon peaking factor is a suitable metric.

Infiltration Rates.  In addition to sewage, the  infiltrations and inflows of water from cracked pipes, maintenance holes, damaged joints, drains, roof leaders, and cross- connections must be taken into account in estimating system flow.

Hydraulic Design. Sewer capacity is normally derived using the Manning formula, which calculates flow as a function of pipe area, hydraulic radius, pipe perimeter, pipe slope, and Manning’s roughness factor (which depends on pipe material).

Sewage Treatment Facilities

Most universities directly discharge sewage  into municipal sewage systems, which regulate the type and amount of biological oxygen demand, total suspended solids, specific metals, pH, temperature, and specific inorganic compounds that can be discharged into the system. Alternatively, a specific wastewater treatment facility must be built.

Sanitary Sewage Pumping Station

A sewage pumping system is designed to continue operating if a power outage or equipment failure occurs. Design flow is the capacity of the pumping station with the largest unit out of service. Sewage pumping stations must be well maintained, easily accessible, and protected from the elements. Sewage system design must emphasize operator safety and compliance with local regulations and with the explosion prevention protocol detailed in code requirements.

Pump  Selection.  A  sewage  pumping  station  must  have a minimum of two pumps to ensure peak instantaneous design flow. A system head  curve  model,  which evaluates forcemain C factors and wet-well  sewage levels, is used to facilitate exact constant or variable  speed pump design requirements. Constant pumps are sufficient for sewage release to  a  large  municipal system. A variable-speed  pump,  which  creates  a steadier flow, is best if sewage is released to a small municipal system or treatment plant.

Wet-Well Design. Sewage flows through the wet  well from the collection system  to  the  pumps.  Design focuses on maintaining system flow by minimizing solid deposits, ensuring adequate pump cycle times and providing access for O&M. The minimum pump cycle is usually 10 minutes to protect the pump motor. Wet-well size is a function of pump flow rate and cycle time. Sizing must account for detention storage during power failures.

Standby Power. Sewage systems will continue to receive sewage during power outages. A standby power generator is ideal, but increasing the size of the wet-well and its holding capacity can suffice.

Control  Systems.   Sewage  pumping  stations  are normally controlled by an electronic level control system, which measures water level in the wet-well with type pressure sensors or  ultrasonic  level  devices  and activates the pumps  as  needed.  A  backup  control system (floats installed in the wet-well) also is used. A flowmeter enables the operations  staff  to  evaluate actual flow and system reserve capacity.

Forcemain Design. A forcemain transfers sewage from the pumping station to the outlet sewer. The proper pressure rating of the forcemain is a function of the maximum pump head and the length of the pipe; a longer pipe will result in a lower required pressure rating.

Operations. System maintenance for the sewage pumping station includes regular system checks, cleaning of the wet-well, annual equipment maintenance, and rewinding of the electrical motors (every 10 years). Control system calibration and testing of backup systems are also recommended.

Oil Water Separators

Building codes and municipal sewer use bylaws can require the removal of fats, oil, and grease before   transfer of sewage is allowed to the municipal plant. An  oil water separator, which a third-party disposal company regularly cleans and maintains, is typically installed.

Combined Sewer Overflows

Combined sewers that collect both sanitary sewage and stormwater runoff are less expensive but risk overburdening the system in the event of  a  severe storm. In the 1950s, municipalities began to build separate sewage and stormwater runoff systems, but many still operate legacy combined sewage systems. Interceptors (which treat incoming flow) and regulators (which actively discharge excess flow into water bodies) are used to minimize the risk of system overload, sewer backups, and flooding. To mitigate the environmental impact of the discharge, many municipalities built secondary sewer systems,  constructed  overflow facilities, disinfected sewer water with sodium hypochloride, and installed physical barriers so that solid waste is not released into the waterways.

Introduction to Storm Drainage Systems

Storm drainage systems capture runoff from rainfall and direct it to an appropriate receiving system, another storm drainage system or a receiving body  of  open water. Components include (1) a minor  system  to capture runoff from rainfall events typically expected once every 5 years, (2) a major system to capture runoff from rainfall events experienced on average once every 100 years, and (3) components of stormwater management, which considers water quality and environment issues and standard stormwater drainage issues.

Design Rainfall

Drainage infrastructure design requires a hydrologic analysis of (1) actual rainfall peak flows, depth, and estimation of flow to determine requirements (natural design); or (2) estimated design flows from statistical analysis of local rainfall data (synthetic design). The natural design method (which is more accurate long term) uses the single largest rainfall during the duration model of either the minor or major system. The  synthetic method creates a hyetograph based on total rainfall volume during the model time.

Minor and Major Systems

Criteria. Flow depth and velocity  are  considered  in system design, and government regulations specify criteria. Planning requires understanding the statistical relationship between risk and probability. A rainfall event that has a 1 percent chance of happening once in 100 years has a 22 percent chance of happening in the next   25 years. Climate change also  should  be  factored  into the model.

Flow Estimates. Land usage, surface details (runoff coefficient C), overland flow, and travel time characteristics impact flow. Surface characteristics are largely a function of soil permeability.

Rational Method. The rational method for estimating flow typically uses the formula Q=2.78CiA, where Q is peak flow, A is drainage area, I is rainfall intensity, and C is the runoff coefficient.

Hydrograph Methods. When runoff volume is a  concern, a hydrograph method that includes catch basin capture rates, depression storage, infiltration, spatial and temporal variations, and concentration time must be used. Storm hyetographs, continuous rainfall records, detailed physical characteristics of the system infrastructure (e.g., soil permeability; slope and area of drainage area; control systems; depression storage and infiltrations rates; groundwater, pond, or tide levels) are considered.

Depth and Velocity Estimates. Flow depth and velocity must be estimated to model design flows. The hydraulic grade line, the elevation to which water will rise when the system is operating under design specifications, should be at or below the crown of the storm sewer.

Simplistic. Manning’s formula is a  function  of  flow area, size of the drainage area (hydraulic radius) and slope and is used for drainage systems that are above ground (e.g., storm sewers, roads). The Darcy-Weisbach formula is a function of head loss (resulting from friction), flow velocity, pipe length and diameter, and hydraulic radius and is used for surcharged conditions.

Dynamic. Dynamic hydrotechnical models,  which combine hydrologic and hydraulic modeling techniques, are the most effective way to identify surcharge impacts and depth and velocity concerns.

Design. System design includes control devices in catch basins, minimum storm sewer size, velocity and slope, flow velocity, overland flow depth and velocity, storage facilities, and control structures.

Stormwater Management

The development of a university  campus  typically changes usage of the surrounding land from a rural to an urban environment. Stormwater management, which seeks to restore the surrounding area to its previous hydrograph pattern, can help to mitigate any negative effects on the local water quality.

Water Quantity Criteria

Flooding and Erosion. Reducing the variance between unusual extreme event peak flows and frequently occurring minor flows involves reducing post- development peak flows to predevelopment peak flows.

Baseflow, Water Balance, and Groundwater. Implementing adequate stormwater management techniques (which focus on surface runoff, infiltration, and evapotranspiration) can help ensure groundwater availability for water supply and fish habits. Stormwater runoff guidelines are issued by federal, state, provincial, and municipal authorities.

Methods.  Best management practices increasingly use a distributed approach to control source, conveyance, and outlet (end-of-pipe) flows. Low impact development guidelines are emerging.

Source Control. The primary processes used in source control are flushing storm sewers, increasing soil and pavement permeability, and creating onsite water storage systems.

Conveyance Control. Stormwater management techniques used in conveyance control are infiltration basins, exfiltration trenches, permeable paving, perforated pipe, curb-and-gutter systems, and oil and gas separators.

End-of-Pipe Control. Oil and grit traps and water detention facilities are the main tools used to address water quality and quantity issues.

Sanitary and Storm Sewer Materials and Construction

Types of Materials. (1) Pipe design is a function of  dead, structural, and live loads. (2) Sewer pipe bedding (which brings the trench up to grade) and backfill design (which addresses surround and support) must account for soil type, soil density, and differential frost heave. (3) Sewer pipe materials (typically PVC or PE) must be durable, cost-effective, and produced according to government regulations.

Performance Testing. Suppliers are often required to test product design compliance with regulations.

Methods of Construction

Sewer installations are typically open cut (for smaller projects) or trenchless (for larger projects).

Open Cut. Open cut installations (which are cost- effective but disruptive) involve excavating trenches, laying and jointing pipe, refilling trenches, compacting filling material, and restoring above-ground soil.

Trenchless Technologies. Trenchless methods (which reduce the need for surface excavation but can be costly) include horizontal drilling, micro tunneling, pipe ramming, and pipe and bore.

Horizontal Directional Drilling. This process involves drilling a pilot hole, monitoring position electronically, attaching a back reamer at the appropriate depth, and enlarging the hole on drill removal.

Micro Tunneling. This technique pushes a new pipe or conduit behind a boring machine (pipe jacking), while using a bentonite slurry for adequate lubrication.

Pipe Ramming. This pneumatic process involves using percussive blows to drive the pipe into the ground and flushing the resulting soil packed into the pipe with an auger, compressed air, or water jet.

Jack and Bore. Also known as auger bore, this process consists of using rotating cutting heads and augers to clear soil from the pipe as it is being jacked into place.

Maintenance and Repairs

Sewer system maintenance and upkeep must be included in institutional budgets; maintaining the system is far less expensive and disruptive than system replacement.

Preliminary Sewer System Analysis.  Outside specialists in hydraulic and condition assessment monitor infiltration and inflow, review maintenance records, and visually inspect the system in response to system issues (e.g., overflows, odors, corrosion, excessive power costs, anticipated flow increases).

Sewer System Evaluation Survey

The SSES monitors the amount of rainfall-induced infiltration and inflow that is caused by natural system deterioration and evidenced by extraneous flow to identify potential cost savings.

SSES  for  Assessing  Infiltration.  Tasks   involved  in SSES infiltration assessment include (1) failure impact assessment using a set of failure impact ratings to identify inspection and rehabilitation priorities; (2) inspection using CCTV technology and a robotic camera; (3) sewer condition assessment using data from the CCTV inspection to prioritize maintenance and repair of structural and serviceability deficiencies.

Maintenance Holes Condition Assessment. Structural and serviceability condition assessments for maintenance holes assign structural (infrastructure) and serviceability (system access) defect ratings.

SSES for Detecting Inflow. This process includes (1) a smoke test, which uses smoke blowers; (2) a dye test, which introduces dye at the suspected source of inflow; and (3) CCTV visual inspections.

Methods of Repairing Sanitary and Storm Sewers

(1) Excavation and replacement involves an external system inspection and excavation (if system is not visibly damaged) to address system repair or replacement options. (2) Chemical grouting (joint sealing) can be used to repair leaky pipe joints, cracks, and holes but will not cure structural damage. (3) Slip lining is the insertion of a new pipe inside of an existing pipe. (4) The fold-and-formed process, which does not require excavation, uses either U-Liner (high-density polyethylene resin) or NuPipe (PVC) technology and is more limited than slip lining. (5) Inversion lining (cured- in-place process) involves installing a flexible resin impregnated liner, which cures in place. (6) Specialty concrete can reinforce weakened concrete pipes and maintenance holes by providing an acid-resistant  coating.  (7) Pipe bursting is a technique that replaces  old pipes with larger higher-capacity pipes by using an expander head to break and expand old pipe and then reruns new pipe in the old pipe’s original footprint.

Maintenance    Hole    Rehabilitation   Techniques. Methods to rehabilitate and maintain sewer holes typically replace the frame and cover and repair the sidewall by applying waterproof and corrosion-resistant compounds and grouting, installing fiberglass liners, and inserting polyethylene liners.

Service Lateral Rehabilitation Techniques. Service laterals, which connect buildings to main sewer lines, should be rehabilitated with chemical grouting or inversion lining in the event of infiltration.