Drop manholes should be used when the difference in elevation between the influent and the effluent of the manhole cannot handle the drop into the barrel without creating excessive turbulence which reduces the production of H2S
Back flow preventers are used to stop the accidental backflow or reverse flow of wastewater into buildings. They are used when the lowest overflow point in the buildings plumbing is above the rim elevation line of the manhole in the lateral or street main immediately upstream of the buildings sewer connection. There are two types, the mechanical check valves and the atmospheric overflow device with floatable ball seal.
Junction structures are used to join large diameter sewers without a manhole. They usually contain a structure that “breaks” the turbulence thus reducing the possibility of H2S production.
Inverted siphons are sewer lines installed lower than the normal gradient to pass under obstructions such as streams and roadways. Wastewater is “pushed” thru the siphon by the pressure because the upstream sewer, at a junction box, is higher than the downstream sewer. Siphons need more maintenance due to slowing of flow and solids build-up.
Air Jumpers are used to equalize air pressure on inverted siphons.
Interruptions in sewer service may be avoided by strict enforcement of sewer ordinances and timely maintenance of sewer systems. Regular inspection and maintenance minimizes 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. Effective operation of a conventional gravity sewer begins with proper design and construction. Serious problems may develop without an effective preventative maintenance program. Occasionally, factors beyond the control of the maintenance crew can cause problems.
Potential problems include:
1. Explosions or severe corrosion due to discharge of uncontrolled industrial wastes.
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.
7. Excessive settling of solids in the manhole and sewer line may lead to obstruction, blockage, or generation of undesired gases.
8. The diameter of the sewer line may be reduced by accumulation of slime, grease, and viscous materials on the pipe walls, leading to blockage of the line.
9. Faulty, loose, or improperly fit manhole covers can be a source of noise as well as inflow.
10. Ground shifting may cause cracks in manhole walls or pipe joints at the manhole, which become a source of infiltration or exfiltration.
11. Debris (i.e., rags, sand, gravel, sticks, etc.) may collect in the manhole and block the lines.
12. Tree roots may enter manholes through the cracks, joints, or a faulty cover, and cause serious blockages.