CWR - Sewers 101

A sewer system is made up of a collection system and a treatment system. A sewer collection system is comprised of many "basins", which may also called sub-basins or mini-basins. In simple terms, a basin provides the wastewater infrastructure for a number of customers in a defined area and feeds into a larger sewer main. A basin generally covers an area of 50-100 acres or 100-300 customers.

A city or sewer district may have anywhere from a few basins to hundreds of basins. By way of example, King County, WA has 775 basins, (which they term “mini-basins”).

The performance of a sewer system is dependent on the condition of its collection system, which includes its pipes, manholes and service laterals.

What is Inflow and Infiltration (I/I)?

Infiltration is the seepage of groundwater into sewer pipes through cracks and joints. Inflow is typically flow from a single point, such as a discharge from sump pumps and foundation drains, or stormwater entering openings in the sewer access covers. I/I is considered an unnecessary addition to the volume of water being "collected" and conveyed by the collection system and then treated at the treatment plant.

What Causes I/I?

What is Excess I/I and Why Should I Care?

Excess I/I represents the amount of water that is getting into the collection system in excess of the actual waste (sewage) water that is generated per connection plus a minimum or target I/I factor of approximately 25% of the per connection volumes.

Once water - clean or waste - is in the collection system, there is a cost to the taxpayer. All water in the collection system is treated. Therefore, the more clean water that is being "treated," the greater the unnecessary spending. In wet weather events, where I/I utilizes collection system capacity, there may be discharge into basements, streets and/or public waterways.

See "Hidden Costs" below and example of unnecessary I/I costs in Taxpayers section.

Why are There "Excess Costs" Within All Collection Systems?

Why Does System Performance in Rain Events Indicate an Inefficient Collection System?

Rain events stress a sewer system at both the collection and treatment ends if the collection system is inefficient. The inefficiency of the collection systems leads to rapid and significant "excess I/I" which may utilize all available collection capacity. In this event, no more water (clean or sewage) can enter the system and discharges occur. These discharges of contaminated water occur in basements, streets and/or public water ways. These discharges are unacceptable to citizens, environmental groups and the EPA and are increasingly expensive to respond to -- and the real problem may not even be addressed after significant dollars are spent.

What is a CSO...and How Big Is the Problem? (Source: 2004 EPA Report to Congress)

A combined sewer system is a wastewater collection system, owned by a state or municipality,
that is specifically designed to collect and convey sanitary wastewater (domestic sewage from
homes as well as industrial and commercial wastewater) and storm water through a single pipe.
During precipitation events (e.g. rainfall or snowmelt), the systems are designed to overflow
when collection system capacity is exceeded, resulting in a combined sewer overflow (CSO) that
discharges directly to surface waters.

Today, there are 746 communities with combined sewer systems with a total of 9,348 CSO
outfalls that are identified and regulated by 828 NPDES permits. Combined sewer systems are
found in 32 states (including the District of Columbia) and nine EPA Regions. CSO
communities are regionally concentrated in older communities in the Northeast and Great Lakes
regions. EPA estimates that about 850 billion gallons of untreated wastewater and storm water
are released as CSO each year in the United States.

What is an SSO...and How Big Is the Problem? (Source: 2004 EPA Report to Congress)

A sanitary sewer system is a wastewater collection system, owned by a state or municipality, that
is specifically designed to collect and convey only sanitary wastewater (domestic sewage from
homes as well as industrial and commercial wastewater). In such systems, storm water is
conveyed through an additional set of pipes. These systems can overflow when collection
system capacity is exceeded due to wet weather (as the result of infiltration and inflow), when
normal dry weather flow is blocked for any of several reasons, or when mechanical failures
prevent the system from proper operation.

In the Report to Congress, EPA estimates that between 23,000 and 75,000 SSOs occur each year
in the United States, resulting in releases of between 3 billion and 10 billion gallons of untreated
wastewater. These events take place throughout the United States.

What's Required to "Fix" the Problem?

The performance of a sewer system is dependent on the condition of its collection system, which includes its pipes, manholes and service laterals.

The view of many engineers is that the performance of a sewer system can be reduced to the “sum of the parts” performance of each basin. For example, if the Average Dry Weather Flow from a basin is 300,000 gallons per day, but the RDII, (rain dependent inflow and infiltration), is 2,400,000 gallons per day, then the basin has a high rate of Inflow and Infiltration (I/I) into its collection system. This occurs when the pipes, manholes and service laterals have cracks and other defects which allow groundwater and stormwater to enter the sewer system. Moreover, all excess I/I must be conveyed, stored and treated at a high cost to ratepayers.

The goal is to get the sewer system to perform as originally designed, by eliminating the entry of unwanted water through collection system rehabilitation.

The first issue is to get the majority, (i.e. greater than 70%), of the I/I eliminated through collection system renewal, (using mostly Trenchless Technologies), out of all problem basins. The second issue is how cost-effectively the I/I can be removed. (Cost-effectiveness alone will not solve the problem).

If both issues can be addressed, (>70%), and cost-effective (i.e., price guarantee per gallon removed), then the successful removal strategy can be rapidly replicated throughout the entire sewer system within a very short time frame, (i.e. less than 3 years). This is the Ideal Method for addressing problem issues and will ensure that the municipal utility enjoys the full benefits of rehabilitation.