Page 579 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 535

14.15 Operation and Maintenance of Drainage Systems 535
14.14.6 Performance Force main performance is closely tied to the performance of the lift
station to which it is connected. Pump-force main performance curves are used to define and
compare the operating characteristics of a given pump or set of pumps along with the associated
force main. They are also used to identify the best combination of performance characteristics
under which the lift station-force main system will operate under typical conditions (flows and
pressures). Properly designed pump-force main systems usually allow the lift station pumps to
operate at 35% to 55% efficiency most of the time. Overall pump efficiency depends on the
type of pumps, their control system, and the fluctuation of the influent wastewater flow.

14.15 OPERATION AND MAINTENANCE OF DRAINAGE SYSTEMS

14.15.1 Gravity Sewer System
The principal problem in the operation and maintenance of sewers is the prevention and relief
of stoppages. Education and pollution prevention can enhance operation and maintenance pro-
grams by informing the public of proper grease disposal methods. Effective operation and haz-
ards control of a conventional gravity sewer begins with prevention through design (PtD) and
construction. Tree roots and debris accumulation are the main causes of problems. Important,
too, in areas of cohesionless soil is the entrance of sand and gravel through leaky joints and
pipe breaks. Cement, mortar, and lime-mortar joints will not keep out roots as effectively as bi-
tumastic hot-poured or factory-installed rubber joints. The plastic and other newer jointing
materials work well. Understandably, debris is more likely to accumulate in the upper reaches
of sewers where flows are low and unsteady. Sharp changes in grade and junctions at grade are
danger points. Grease from eating places, oil from service stations, and mud from construction
sites, often discharged intermittently and in high concentration, are leading offenders. Well-
scheduled sewer flushing is an obvious answer when system design cannot be altered.
Interruptions in sewer service may be avoided by strict enforcement of sewer ordi-
nances and timely maintenance of sewer systems. Regular inspection and maintenance
minimize the possibility of damage to private property by sewer stoppages as well as the
legal responsibility of the sewer authority for any damages. An operation and maintenance
program is necessary and should be developed to ensure the most trouble-free operation of
a sanitary sewer system. An effective maintenance program includes enforcement of sewer
ordinances, timely sewer cleaning and inspection, and preventive maintenance and repairs.
Inspection programs often use closed-circuit television (CCTV) cameras and lamping to
assess sewer conditions. Sewer cleaning clears blockages and serves as a preventive main-
tenance tool. Common sewer cleaning methods include rodding, flushing, jetting, and bail-
ing. Serious problems may develop without an effective preventive maintenance program.
Occasionally, factors beyond the control of the maintenance crew can cause problems.
Potential problems include the following:
1. Explosions or severe corrosion due to discharge of industrial wastes without
pretreatment.
2. Odors.
3. Corrosion of sewer lines and manholes due to generation of hydrogen sulfide gas.
4. Collapse of the sewer due to overburden or corrosion.
5. Poor construction, workmanship, or earth shifts may cause pipes to break or joints
to open up. Excessive infiltration/exfiltration may occur.
6. Protruding taps in the sewers caused by improper workmanship (known as
plumber taps or hammer taps). These taps substantially reduce line capacity and
contribute to frequent blockages.