By James A. Sneed, P.E.
Our first article discussed the four major steps involved in establishing a successful, sustainable inflow & infiltration (I/I) reduction program. The following two articles covered the first two steps, flow monitoring & prioritizing.
Now that you have the data to know where to focus your efforts, the fun begins! The heart of the I/I cycle is the Sanitary Sewer Evaluation Survey (SSES).
The SSES consists of four primary areas, each of which would merit its own article. Here, we will simply identify the task, purpose and some industry standards to use as guidelines. The four primary tasks in a good survey of your collection system are: physicals inspections, smoke testing, flow isolations, and Closed-Circuit Television Inspections (CCTV).
Physical inspection of the system is an essential and foundational part of the SSES. It is used to inventory the system, verify or develop accurate maps, access the condition of manholes, and provide nodes for data capturing and connectivity. Physical inspection is accomplished by opening each manhole, capturing the necessary physical data, and assigning a rehab priority. My suggestion for the first round is to keep it simple. Develop a simple numbering system, a basic inventory sheet, and keep the initial condition assessment to good, fair, or poor.
Smoke testing of main lines and laterals occurs by pushing smoke into the sanitary sewer collection system where rain induced I/I ranked high. The smoke will follow the path of least resistance, escape the system where there are deficiencies, and thus identify areas where extraneous flow can enter. There are various types of smoke, typically liquid or candle varieties are used in conjunction with blowers designed to push 1,700 to 3,000 cubic feet per minute (CFM) of air into the system. Three keys necessary for success are:
1) Notify the public and fire department
2) Choose a dry day where the ground is not saturated
3) Use sandbags so that you can direct the smoke, controlling the direction it moves for each set up
Flow isolations are conducted using weirs, 48 hours or more after a rain event. Use the hydrograph to verify that you are working with groundwater. Calculate the gallons per day per inch per mile (gpd/inch*mile) by measuring flow during late-night low-use periods when the groundwater table is elevated. Flow isolations can be extremely important in identifying the 10% of your system that should be inspected using CCTV.
CCTV is the most expensive SSES tasks and can also tell you the most information about your underground asset. In addition to condition assessment, CCTV is used to pinpoint the type and location of customer service connections. It is important to focus CCTV work in the most critical areas in order to get the most “bang for your buck.”
The table below shows industry standards for performing the first round of an SSES in an I/I cycle. Remember the I/I cycle is a never ending and necessary process in the responsible operation of your collection system.
|Physical Inspection||Smoke Testing||Flow Isolations||Television Inspection|
The above table shows general guidelines intended to assist in planning and resource allocation in line with USEPA recommendations.