Storms highlight the need to reduce infiltration and inflow into the collection system so as not to overwhelm the piping system causing plant damage or sewage overflows into streets, so much of the focus has been on dealing with removal of infiltration and inflow through televising the sewer system and sealing or lining sections where leaks are noted. However, many miles of videotape show virtually nothing, so significant money is spent to find “nothing.” Part of this is because “infiltration” and “inflow” are not the same, and storm events do not highlight infiltration nearly as much inflow.
The manholes and clean-outs are required for access and removal of material that may build up in the piping system and for changes in direction of the pipe. Manholes are traditionally pre-cast concrete or brick, with brick being the method of choice until the 1960s. Brick manholes suffer from the same problems as vitrified clay sewer lines – the grout is not waterproof so the grout can leak significant amounts of groundwater. The manhole cover may not seal perfectly, becoming another source of infiltration. Pre-cast concrete manholes resolve part this problem, but concrete is not impervious either. While elastomeric or bituminous seals are placed between successive manhole rings, the concrete is still exposed. Many utilities will require the exterior of the manholes to have a coal-tar or epoxy covering the exterior which helps to keep water out.
Inflow results form a direct connection between the sewer system and the surface. The removal or accidental breaking of a cleanout, unsealed manhole covers, laterals on private property, connected gutters or storm ponds, damaged chimneys from paving roads, or cracking of the pipe may be a significant source of inflow to the system. All are potential sources of inflow which can be identified easily during storm events. The peaking that correlates with the rainfall is inflow, not infiltration since infiltration is part of the base flow that creeps upward with time. When operators see peaks, this is not indicative of infiltration which is groundwater. Think inflow. Inflow causes peaks in run time on lift station pumps, and create potential overflows at the plant. The good news is that simple, low tech methods can be used to detect inflow, which should be the precursor to any infiltration investigation.
The following outlines a basic program for inflow detection and correction for any utility system. The order is important, and pursuing all steps will resolve the majority of issues. The first step is inspection of all sanitary sewer manholes for damage, leakage or other problems, which while seeming obvious, usually surprises. The manhole inspection should include documentation of condition, GPS location, and some form of numbering if not currently available. Most manholes have limited condition issues, but where the bench or walls are in poor conditions, that should be repaired with an impregnating resin.
Next is repair/sealing of chimneys in all manholes to reduce inflow from the street during flooding events. The chimney includes the ring, cement extensions, lift rings, brick or cement used to raise the manhole ring. Manhole covers are often disturbed during paving or as a result of traffic. The crack between the ring and cover can leak a lot of water. The intent of the chimney seal is to prevent inflow from the area beneath the rim of the manhole, but above the cone.
The next step is to put dishes into the manholes. One might think that only manholes in low lying areas get water into them, but surprisingly every manhole dish that is properly installed has water in it. Hence assume that all manholes leak water between the rim and cover. Most collection system workers are familiar with dishes at the bottom of the manhole where they are of limited use. This is because the dish deforms when filled with water or is knocked in when the cover is flipped. The solution is a deeper dish with reinforcing ribs. No ribs, don’t use it. A gasket is required.
Once the manholes are sealed, smoke testing can identify obvious surface connections. The normal notifications, inspection and documentation will identify broken or missing cleanout caps, surface breaks on public and private property, connection of gutters to the sewer system, and stormwater connections. All should be documented via photograph, by associated address and public or private location. The public openings at cleanouts can be corrected immediately. However, if the cleanout is broken, it may indicate mower or vehicle damage, that can occur again. If missing, the resident may be using the cleanout to drain the yard. In either case the collection system needs to be protected. USSI (http://www.elastaseal.com/about_us.html), located in Venice, FL developed a solution, called the LDL plug to correct those commonly broken or commonly opened cleanouts to reduce inflow.
Notices should then be sent to property owners with documentation of the inflow connections to their property. This is sometimes the most difficult part of the program due to political will, but it is necessary. This finishes the inflow correction portion of the project, but one more step will help focus efforts for the second “i”.
The final step is a low flow investigation, which is intended to focus on the infiltration piece of the problem. Such an event will take several days and must be planned to determine priority manhole to start with and sequencing.
Based on a projected plan and route:
- Open the manholes
- Inspecting them for flow
- Determining if flow is significant. If investigation of basin will end and new basin will be started. If flow exists, open consecutive manholes upstream to determine where flow is derived from. Generally a 2 inch wide bead of water is a limit of “significant” infiltration.
Documentation of all problems and corrections in a report to utility that identifies problem, location and recommended repair. Identification of sewer system leaks, including those on private property (via location of smoke on private property).
The example in Dania Beach, FL was that the last step indicated that only 15% of the sewer system needed to be televised. This saved the City almost $1.2 million. Their total costs is under $1.4 million for all parts of the project, spread over several years and contracts. Overall the hope is that the inflow and infiltration programs together will save $400,000/yr, a five year payback. But the key is to insure you get the inflow as well as the infiltration… Otherwise storms will continue to overwhelm plants, creating public health concerns and ruining your reuse program.