Sanity sewers are hydraulic conveyance structures that carry wastewater to a treatment plant or other authorized point of discharge. A typical method of conveyance used in sewer systems is to transport wastewater by gravity along a downward-sloping pipe gradient. These sewers, known as conventional gravity sewers, are designed so that the slope and size of the pipe is adequate to maintain flow towards the discharge point without surcharging manholes or pressurizing the pipe.

Conventional wastewater collection systems are the most popular method to collect and convey wastewater. Pipes are installed on a slope, allowing wastewater to flow by gravity from a house site to the treatment facility. Pipes are sized and designed with straight alignment and uniform gradients to maintain self-cleansing velocities. Manholes are installed between straight runs of pipe to ensure that stoppages can be readily accessed. Pipes are generally eight inches or larger and are typically installed at a minimum depth of three feet and a maximum depth of 25 feet. Manholes are located no more than 400 feet apart or at changes of direction or slope.

Can StormNET be used to design sanitary sewer systems?

Yes. StormNET offers a complete solution for both sanitary and stormwater sewer design in a single software package. With StormNET sanitary lift stations, pumps and other hydraulic structures are easily incorporated into any design. Since StormNET is fully hydrodynamic, complicated situations can be modeled with confidence, and the user-friendly graphical interface allows projects to be completed quickly and efficiently.

Where is it appropriate to use a gravity sewer system?

Conventional gravity sewers are typically used in urban areas with consistently sloping ground because excessively hilly or flat areas result in deep excavations and drive up construction costs. Conventional gravity sewers remain the most common technology used to collect and transport domestic wastewater. In hilly areas that exhibit a large amount of topographic relief, a pressure sewer system may be more appropriate.

What are some of the important design criteria for gravity sewers?

The design of conventional gravity sewers is based on the following design criteria:

• Long-term serviceability : the design of long-lived sewer infrastructure should consider serviceability factors, such as ease of installation, design period, useful life of the conduit, resistance to infiltration and corrosion and maintenance requirements. The design period should be based on the ultimate tributary population and usually ranges from 25 to 50 years.
  
  
• Design flow : sanitary sewers are designed to carry peak residential, commercial, institutional, and industrial flows, as well as infiltration and inflow. Gravity sewers are designed to flow full at the design peak flow.
  
  
• Minimum pipe diameter: a minimum pipe size is dictated in gravity sewer design to reduce the possibility of clogging. The minimum pipe diameter recommended by the Ten State Standards is 8 inches (200 mm). Though the Ten State Standards are adopted by ten specific states (Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, New York, Ohio, Pennsylvania and Wisconsin) and the Province of Ontario, they often provide the basis for other state standards.
  
  
• Velocity: the velocity of wastewater is an important parameter in a sewer design. A minimum velocity must be maintained to reduce solids deposition in the sewer, and most states specify a minimum velocity that must be maintained under low flow conditions. The typical design velocity for low flow conditions is 1 ft/s (0.3 m/s). During peak dry weather conditions the sewer lines must attain a velocity greater than 2 ft/s (0.6 m/s) to ensure that the lines will be self cleaning (i.e., they will be flushed out once or twice a day by a higher velocity). Velocities higher than 10 ft/s (3.0 m/s) should be avoided because they may cause erosion and damage to sewers and manholes.
  
  
• Slope: sewer pipes must be adequately sloped to reduce solids deposition and production of hydrogen sulfide and methane.
  
  
• Depth of bury: depth of bury affects many aspects of sewer design. Slope requirements may drive the pipe deep into the ground, increasing the amount of excavation required to install the pipe. Sewer depth averages 3 to 6.5 ft (1 to 2 m) below ground surface. The proper depth of bury depends on the water table, the lowest point to be served (such as a ground floor or basement), the topography of the ground in the service area and the depth of the frost line below grade.
  
  
• Appurtenances: appurtenances include manholes, building connections, junction chambers or boxes and terminal cleanouts, among others. Regulations for using appurtenances in sewer systems are well documented in municipal design standards and/or public facility manuals.

What are some typical design flows for sanitary sewer systems?

The following table lists average design flows for gravity systems based on the development type:

What are the minimum slopes required for various pipe sizes?

The following table lists suggested minimum slopes for various pipe lengths:

What are some guidelines for manhole size and spacing in a gravity sewer?

Manholes for small sewers (24 inches [610 mm] in diameter or less) are typically 4 ft (1.2 m) in diameter. Larger sewers require larger manhole bases, but the 4 ft (1.2 m) barrel may still be used. Manhole spacing depends on regulations established by the local municipality. Manholes are typically required when there is a change of sewer direction. However, certain minimum standards are typically required to ensure access to the sewer for maintenance. Typical manhole spacing ranges between 300 to 600 ft (90 to 180 m) depending on the size of the sewer and available sewer cleaning equipment. Municipalities often have their own specific guidelines that should be consulted.

What are the advantages of a gravity sewer system?

Conventional gravity sewer systems have been used for many years and procedures for their design are well established. When properly designed and constructed, conventional gravity systems perform reliably.

Properly designed and constructed conventional gravity sewers provide the following advantages:

• Can handle grit and solids in sanitary sewage.
  
• Can maintain a minimum velocity (at design flow), reducing the production of hydrogen sulfide and methane. This in turn reduces odors, blockages, pipe corrosion and the potential for explosion.
  

What are the disadvantages of a gravity sewer system?

Some of the disadvantages of gravity sewer systems are:

• The slope requirements to maintain gravity flow can require deep excavations in hilly or flat terrain, driving up construction costs.
  
• Sewage pumping or lift stations may be necessary as a result of the slope requirements for conventional gravity sewers, which result in a system terminus at the tail of the sewer, where sewage collects and must be pumped or lifted to a collection system. Pumping and lift stations substantially increase the cost of the collection system.
  
• Manholes associated with conventional gravity sewers are a source of inflow and infiltration, increasing the volume of wastewater to be carried, as well as the size of pipes and lift/pumping stations, and, ultimately, increasing costs.
  

What are some of the potential problems that can arise if gravity sewer systems are not properly maintained?

Effective operation of a conventional gravity sewer begins with proper design and construction. Serious problems may develop without an effective preventative maintenance program. Potential problems include:

• Explosions or severe corrosion due to discharge of uncontrolled industrial wastes.
  
• Gravity sewers may produce unpleasant odors.
  
• Corrosion of sewer lines and manholes due to generation of hydrogen sulfide gas.
  
• Collapse of the sewer due to overburden or corrosion.
  
• Poor construction, workmanship or earth shifts may cause pipes to break  or joints to open up. Excessive infiltration/exfiltration may occur.
  
• Protruding taps in the sewers can substantially reduce line capacity and contribute to frequent blockages.
  
• Excessive settling of solids in the manhole and sewer line may lead to obstruction, blockage or generation of undesired gases.
  
• The diameter of the sewer line may be reduced by accumulation of slime, grease and viscous materials on the pipe walls, leading to blockage of the line.
  
• Faulty, loose or improperly fit manhole covers can be a source of noise as well as inflow.
  
• Ground shifting may cause cracks in manhole walls or pipe joints at the manhole which become a source of infiltration or exfiltration.
  
• Debris (i.e., rags, sand, gravel, sticks, etc.) may collect in the manhole and block the lines.
  
• Tree roots may enter manholes through the cracks, joints or a faulty cover and cause serious blockages.
  

What are some potential causes of recurring sanitary sewer overflows?

Some problems that can cause recurring sanitary sewer overflows include:

• Infiltration and inflow: too much rainfall or snowmelt infiltrating through the ground into leaky sanitary sewers not designed to hold rainfall or to drain property, and excess water inflowing through roof drains connected to sewers, broken pipes, badly connected sewer service lines.
  
  
• Undersized systems: sewers and pumps are too small to carry sewage from newly-developed subdivisions or commercial areas.
  
  
• Pipe failures: blocked, broken or cracked pipes; tree roots grow into the sewer; sections of pipe settle or shift so that pipe joints no longer match; and sediment and other material builds up causing pipes to break or collapse.
  
  
• Equipment failures: pump failures, power failures.
  
  
• Sewer service connections: discharges occur at sewer service connections to houses and other buildings; some cities estimate that as much as 60% of overflows comes from the service lines.
  
  
• Deteriorating sewer system: improper installation, improper maintenance; widespread problems can be expensive to fix develop over time, some municipalities have found severe problems necessitating billion-dollar correction programs, often communities have to curtail new development until problems are corrected or system capacity is increased.

How can sanitary sewer overflows be prevented?

Many avoidable sanitary sewer overflows are caused by inadequate or negligent operation or maintenance, inadequate system capacity, and improper system design and construction. These sanitary sewer overflows can be reduced or eliminated by:

• Sewer system cleaning and maintenance.
• Reducing infiltration and inflow through system rehabilitation and repairing broken or leaking service lines.
• Enlarging or upgrading sewer, pump station, or sewage treatment plant capacity and/or reliability.
• Construction wet weather storage and treatment facilities to treat excess flows.

Communities also should address sanitary sewer overflows during sewer system master planning and facilities planning, or while extending the sewer system into previously unsewered areas.

A few sanitary sewer overflows may be unavoidable. Unavoidable overflows include those occurring from unpreventable vandalism, some types of blockages, extreme rainstorms and acts of nature such as earthquakes or floods.

What is a combined sewer system?

Combined sewer systems are designed to carry sanitary wastewater and storm water in the same pipe to a sewage treatment plant during dry weather. In periods of rainfall or snow melt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, lakes or estuaries.

Approximately 772 cities in the United States have combined sewer systems. These systems are found mostly in older urban areas that experienced rapid population and industrial growth. Combined sewer systems were built to convey sewage, industrial wastewater, and stormwater directly to nearby receiving bodies, which in turn, created sanitary living conditions. Combined sewers have fallen out of favor being replaced by separate sanitary and stormwater systems.

How can combined sewer overflows be prevented?

Storage is often the best measure for attenuating peak combined sewer flows. Specific retention methods include underground storage (e.g., tunnels), in-receiving water storage and retention basins.

Retention basins capture and store some of the excess combined sewer flow that would otherwise be bypassed to receiving waters. Stored flows are subsequently returned to the sewer system during dry weather periods, when in-line flows are reduced and capacity is available at the treatment facility.

Where can I find more information about sewer overflows?

The following documents provide additional information about sewer overflows:

Retention Basins (EPA Combined Sewer Overflow Fact Sheet)
Sanitary Sewer Overflow Solutions
Managing Sanitary and Combined Sewer Overflows (EPA Fact Sheet)

Can StormNET help prevent sewer overflows?

Yes. The advanced modeling capabilities of StormNET can pinpoint potential sewer overflow situations. By identifying problem areas during the design process, StormNET can help avoid costly retrofitting operations and prevent flooding during high flow periods.

What is a lift station?

Wastewater lift stations are facilities designed to move wastewater from lower to higher elevation through pipes. Key elements of lift stations include a wastewater receiving well, often equipped with a screen or grinding to remove coarse materials; pumps and piping with associated valves; motors; a power supply system; an equipment control and alarm system and an odor control system and ventilation system.

Can StormNET model pumping and sanitary lift stations?

Yes. StormNET can model an array of hydraulic structures, including sanitary sewer lift stations.

What are the design criteria commonly used for lift stations?

Cost effective lift stations are designed to:

• Match pump capacity, type, and configuration with wastewater quantity and quality.
• Provide reliable and uninterruptible operation.
• Allow for easy operation and maintenance of the installed equipment.
• Accommodate future capacity expansion.
• Avoid septic conditions and excessive release of odors in the collection system and at the lift station.
• Minimize environmental and landscape impacts on the surrounding residential and commercial developments.
• Avoid flooding of the lift station and the surrounding areas.

What are the advantages of sanitary lift stations?

Lift stations are used to reduce the capital cost of sewer system construction. When gravity sewers are installed in trenches deeper than 10 ft (3 m), the cost of sewer line installation increases significantly because of the more complex and costly excavation equipment and trench shoring techniques required. The size of the gravity sewer lines is dependent on the minimum pipe slope and flow. Pumping wastewater can convey the same flow using smaller pipeline size at shallower depth, and thereby, reducing pipeline costs.

What are the disadvantages of sanitary lift stations?

Compared to sewer lines where gravity drives wastewater flow, lift stations require a source of electric power. If the power supply is interrupted, flow conveyance is discontinued and can result in flooding upstream of the lift station, It can also interrupt the normal operation of the downstream wastewater conveyance and treatment facilities. This limitation is typically addressed by providing an emergency power supply.

Key disadvantages of lift stations include:

• High cost to construct and maintain and the potential for odors and noise.
• Lift stations also require a significant amount of power, are sometimes expensive to upgrade, and may create public concerns and negative public reaction.

The low cost of gravity wastewater conveyance and the higher costs of building, operating, and maintaining lift stations means that wastewater pumping should be avoided, if possible and technically feasible. Wastewater pumping can be eliminated or reduced by selecting alternative sewer routes or extending a gravity sewer using direction drilling or other state-of-the-art deep excavation methods. If such alternatives are viable, a cost benefit analysis can determine if a lift station is the most viable choice.

Where can I find more information about sanitary lift stations?

The following document provides more information about the design of sanitary sewer lift stations:

Sewer Lift Stations (EPA Collection Systems Fact Sheet)

What is a pressure sewer?

Pressure sewer systems move wastewater via pressure flow. They are used in sparsely populated or suburban areas in which conventional collection systems would be expensive. These systems generally use smaller diameter pipes with a slight slope or follow the surface contour of the land, reducing excavation and construction costs.

How are pressure sewers different than gravity sewers?

Pressure sewers differ from conventional gravity collection systems because they break down large solids in the pumping station before they are transported through the collection system. Their watertight design and the absence of manholes eliminates extraneous flows into the system. Thus, alternative sewer systems may be preferred in areas that have high groundwater that could seep into the sewer, increasing the amount of wastewater to be treated. They also protect groundwater sources by keeping wastewater in the sewer.

The disadvantages of pressure sewage systems include increased energy demands, higher maintenance requirements and greater on-lot costs. In areas with varying terrain and population density, it may prove beneficial to install a combination of sewer types.

Where can I find out more information about pressure sewers?

The following document contains additional information about the design of pressure sewer systems:

Pressure Sewers (EPA Wastewater Technology Fact Sheet)

What materials are commonly used in the construction of sanitary sewers?

There are several different pipe materials available for wastewater collection systems, each with a unique characteristic used in different conditions. The four common pipe materials are ductile iron, concrete, plastic and vitrified clay. A description of each material along with the advantages and disadvantages of each can be found in the following document:

Pipe Construction and Materials (EPA Wastewater Fact Sheet)

What are some sources of additional information about gravity sewers?

The following documents provide additional information about conventional gravity sanitary sewer systems:

Conventional Gravity Sewers (EPA Collection Systems Fact Sheet)
Small Diameter Gravity Sewers (EPA Decentralized System Fact Sheet)