When it comes time to assess the results you have obtained, utilizing maintenance employees from the public works department and major industries and a thorough records review can help to determine known trouble spots. Some information of use could be:
- Where blockages usually occur
- Where there is flooding or sewer backups in conjunction with storm events
- Whether there are areas of the city that have buildings with downspouts going into the ground, but with no nearby storm sewers
- Which areas have had major repair work
- The quality of the drawings you are basing your work on
- Should your first step be to have the field drawings corrected?
- Are there sewer areas not mapped?
- Are all the valves located on the plans?
The records review should include sewer service requests, work orders, repair projects that have been completed or proposed, correspondence with regulators or the wastewater plant about excess flows, internal facility plans and built plans and drawings of the storm and sanitary sewers. It should also include any engineering reports or applicable sample or flow information concerning the collection system.
Another tool used for assessment is visual inspection of manholes and sewer lines. A lot of Inflow and Infiltration enters the sewer through deteriorated manholes. Manhole defects are readily apparent upon visual inspection. The manhole can be physically entered if the steps are in good condition and confined space entry precautions are observed.
The following are common sources of infiltration and inflow through manholes (all are made worse if ponding occurs over the manhole):
- holes in manhole covers
- poor fit between manhole cover and rim
- cracks and holes in the pavement around the manhole rim
- cracks or misalignment between bricks in the manhole
- loss or absence of mortar between the bricks
- cracks in the invert
- gaps or misalignment of connecting pipes
The lines can be visually inspected through the manhole, by either lamping the lines and looking up them while in the manhole or by using a remote halogen light and mirror while standing above the manhole. Inspections of all manholes and sewer lines should be documented.
An important word of caution: Remember that even while working over an open manhole, you must observe confined space precautions. Sewer gases can render you unconscious before you detect them with your unaided senses and many people have been killed by falling unconscious into manholes. In addition, methane gas is common in sewers and very explosive, so sparks and open flames must always be kept away from sewers or manholes.
Informative interviews and a thorough records review are two very useful assessment tools that are inexpensive and can be accomplished by in-house personnel. The use of these tools along with flow monitoring, smoke testing, dye testing and cleaning and TVing the lines will give you the information needed to begin the assessment of your collection system.