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BEIRUT ARAB UNIVERSITYFACULTY OF ENGINEERING
CIVIL & ENVIRONMENTAL ENGINEERING DEPT.
SANITARY COURSE(CVLE453)
DESIGN OF WASTEWATER SYSTEMS
FALL 2015‐2016
Combined & Separate Systems
Most Sewers Rely on Gravity Flow
• Most sewers are designed to flow by gravity (water flows down‐hill)– Includes sewer pipe from home to septic tank or to a municipal collector pipe
– Gravity sewers must follow the topography of the land
– Where gravity flow is not possible, pumps are used
• Individual unit pump• Large municipal lift (pump) station
Basic Sewer Design
• Collector pipes (pipe in street) is minimum 200mm diameter (to allow cleaning)
• Service pipes (home or building to collector is 150mm diameter
• Gravity sewer pipes have no bends, manholes used to make transitions in direction and pipe size
• Pipe sections between manholes are at a constant grade or slope (S)
Introduction
The design of sewer system generally includes the following steps:
‐ Preliminary investigation,‐ Detailed Survey‐ Actual Design,‐ Preparation of final drawings.
Name Description
Lateral They collect the WW from buildings and convey it to a main sewer
Main Conveys WW to trunk line
Trunk Large diameter sewers to convey WW to STP
Forcemain Pressurized WW pipe
Underground Infrastructure Allocation1 ‐ electricity cables; 2 ‐ telephone cables; 3 ‐ gas pipes; 4 ‐ water supply pipes; 5 ‐ sanitary sewer
pipes; 6 ‐ street inlets; 7 ‐ drainage pipe
Small Diameter Sewer LayoutPipe will flow full, under pressure in these areas
Bell Spigot
Manholes• Are the most familiar appurtenance of a WW collection system.
• They have been built of brick and cast in place.
• Current practice, precast concrete.
• The manhole cover is Heavy duty cast iron. It should not be perforated because of the potential for inflow from storm water. This also minimizes escape of odors. When there is a potential for the manhole to be submerged, the cover is provided with a gasket and is bolted down. If the manhole is not in a roadway, the top level should be above flood level.
Typical Manhole
Fig 5.1, p 134
Drop Manhole is used when the inflow and outflow sewers differ in elevation by more than 60cm. This protects the workers from taking a sewer shower while they work. It also reduces volatilization of odor compounds. The entire outside of the drop connection is encased in concrete to minimize differential settlement between the drop pipe and the manhole.
Drop Manhole
Lift Stations & Pumping Stations
• Flat Terrain; long pipe to reach the STP may result in deep sewers.
• Hilly Terrain; When hills present an obstacle • Obstacles; bedrock• Groundwater.
Pump Stations
• Wet Well: The pumps are placed inside the WW collection sump. The pumps should be of submersible type.
• Dry Well: The pumps are placed separately from the wastewater collection sump.
Large Pump Station Small Pump Station