CEMP-E
EI 11C201
1 March 1997
CHAPTER 10
EVALUATION OF EXISTING SEWER SYSTEMS
10-1. PROBLEM. Obviously before a sanitary sewer system is expanded, the adequacy of the
existing sewer and treatment system must be evaluated. This is generally the responsibility of
the installation where the system is located. Large amounts of extraneous water now enter
portions of many of these systems through infiltration of groundwater and through inflow from
direct sources. This excess water has often lead to unnecessary expenses from pumping and
treatment, reduced treatment efficiencies, treatment process upsets due to hydraulic shocks, and
overflowing manholes and wet wells when the system's capacity is exceeded. This has lead to
health hazards, permit violations, and fines to installations.
a. Infiltration. Water leaking from the ground into sewer systems is called infiltration. Its
major sources are cracks in pipes, joints, manholes, and building laterals. The more a system
deteriorates, the more infiltration occurs. Main trunk lines are often located in low lying areas
such as swamps to allow gravity flow from the collection system. Most of these trunk lines now
are major sources of infiltration.
b. Inflow. Direct connections such as roof drains, basement drains, yard drains, and storm
sewer cross connections cause the inflow of relatively clean water into the system. Since it
occurs during and immediately after a rainfall event, it can be a large quantity of water in a short
period of time. Systems along waterways can experience inflow problems if the waterway floods
into unsealed manholes.
10-2. SOURCES OF SEWER SYSTEM DETERIORATION. Deterioration of systems can be
caused by stresses from surrounding soils and groundwater, surface loads, differential settling,
temperature variations causing expansions and contractions, frost heave, the corrosive nature of
the wastewater, and roots growing through joints. Such factors can lead to stress fractures in
pipes and joints, crushed pipes, displaced joints, and cracked or collapsed manholes. Chemical
degradation can result from microbial action releasing sulfide gas into the air where it oxidizes
biologically to sulfuric acid. The acid is destructive to concrete and in old brick manholes has
destroyed the mortar around the bricks. The presence of sulfates also leads to microbiological
induced corrosion.
10-3. INFILTRATION/INFLOW ANALYSIS. An infiltration/inflow (I/I) analysis is used to
demonstrate the non-existence or possible existence of excessive I/I in the system. Rainfall and
flows within the study area or subareas must be monitored during dry, low groundwater
conditions, during and after major rainfall events, and during high groundwater conditions. This
is often accompanied with monitoring groundwater gauges in the immediate vicinity of the
system.The increase in flows during and immediately after rains is considered to be inflow and
the increases after surface water has runoff and during high groundwater conditions are
infiltration. The analysis includes a comparison to water records and interviews with key
operating personnel. An estimate of the cost of eliminating the I/I conditions is compared to the
costs of transporting and treating the I/I. Other factors to be considered include reducing the
probability of sewer cave-ins and adverse effects of overflows. If the analysis indicates that it
may be worthwhile to eliminate the I/I sources the system is evaluated in more detail in small
areas. Means for closer evaluations include smoke testing, physical inspections and remote
inspections.
10-1
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