Sanitary Sewer Overflows (SSOs) occur when the capacity of a separate sanitary sewer is exceeded, normally during storm events. There are several factors that may contribute to SSOs from a sewerage system, including pipe capacity, operations and maintenance effectiveness, sewer design, age of system, pipe materials, geology and building codes.
Enforcement actions by USEPA and the States for SSOs range from requiring billion dollar programs to no involvement. Due in part to this new emphasis on SSO regulation, communities will be giving increased attention to managing the effects of rainfall-induced infiltration and inflow on their sewerage systems.
The Water Environment Federation (WEF) recommends the following for prevention and control of sanitary sewer overflows:
These principles are described in greater depth below:
Wet weather SSOs are best managed through a watershed approach
The current approach to water quality and ecosystem protection relies on a patchwork of legislative and regulatory requirements that focus narrowly on individual pollution sources. A watershed approach is needed to more effectively address wet weather water quality problems.
WEF supports USEPA's renewed emphasis on the watershed approach for addressing surface water, ground water and other ecosystem issues. The watershed approach is an effective method for assessing the relative importance of individual pollution sources within drainage areas. WEF urges the application of watershed principles to the evaluation and control of wet weather water pollution sources, including SSOs. SSOs must be considered relative to other water quality problems within specific geographical areas. Such an approach ensures that communities, regulatory agencies, and other stakeholders have a clear understanding of the individual and collective impacts of all sources of pollution. Use of the watershed approach also provides a mechanism for setting priorities to control various pollutant sources and for selecting controls that achieve maximum benefit at an affordable cost.
Risk assessment can be used to determine SSO control priorities
Assessment of the risks to human health and to environmental quality associated with SSOs should be foremost in the development of any national, regional, or state policy for control or abatement. Priority must be given to overflows that directly impact human health, such as basement backups and other SSOs that have likely human exposure. In order to accomplish meaningful assessment of risks from SSOs to human health or the environment, the exposure pathways for contaminant migration from sanitary sewer overflows must be thoroughly understood.
Knowledge of all factors influencing the sewerage system is essential
A thorough understanding of the performance of sanitary sewerage systems during wet weather is essential to the mitigation of rainfall-induced SSOs. Since complex hydraulic conditions often exist in sanitary sewerage systems during large rainstorms, isolated SSO control efforts will often simply shift the problem to an adjacent area. WEF suggests the following study components for development of an SSO mitigation program:
- wet and dry weather flow measurements
- sewer system model
- receiving water model
- sewer system evaluation surveys
- O& M assessment
- assessment of water quality, ecosystem, and public health impacts
- evaluation of collection system rehabilitation and other options
- financial capability analyses
- assessment of storm drainage impact (noncombined systems)
Due to regional hydrogeological and meteorological conditions, each community's problems must be evaluated on a site specific basis.
O&M related SSOs should be minimized
Effective operations and maintenance (O&M) programs begin with the planning and design of sanitary sewerage systems. Correct sizing of the system, the selection of materials and equipment, and thorough construction and inspection methods can help avoid permanent system deficiencies that affect the operations and maintainability of the system, as will involving O&M staff in the planning stages of each project.
System records and a system inventory are essential elements of an O&M program. Rapid advances in computer technology have made the development and maintenance of databases easier and more cost effective than ever before.
Physical inspections of sanitary sewerage systems remain the foundation of an effective O&M program. An effective collection system O&M program depends on the willingness of utility managers and elected officials to recognize that costs of reactive maintenance often exceed the costs of effective and proactive O&M programs.
WEF recommends the development of performance indicators for collection systems through a benchmarking process which can take into account variations in size and geographic region. This is preferable to national, one-size-fits-all O&M Standards.
Private property Inflow/Infiltration (I/I) issues should be addressed
Private service laterals, downspouts, and area drains belonging to private residences and businesses are a significant source of I/I in many cities. The overall success of an I/I correction program to reduce SSOs is limited if these private sources are not addressed. A means to overcome the institutional and legal barriers to reduce extraneous inflows from private property sources needs to be established. Education of the public is important to the success of any service lateral maintenance or repair program.
Use appropriate treatment to meet water quality standards
Proper planning, design, and operations and maintenance should alleviate the vast majority of SSOs in a community. There may exist, however, some SSOs which are impractical to address other than through treatment. WEF recommends that alternate treatment processes be considered before requiring secondary treatment of SSOs. In general, biological or secondary treatment is not appropriate for wet weather flows, due to the dilute character and variable nature of the flows. Treatment of SSOs during wet weather can be implemented to meet water quality standards without the high capital costs associated with secondary treatment.