Infiltration and Exfiltration

Infiltration

             "Infiltration" may be defined as water which enters the sanitary sewer system through defective joints, cracked or broken pipes, the walls of manholes, manhole tops, and yard, areaway and foundation footing drains. Usually the accumulation of ground and surface water accompanying a rainy period can be a factor; infiltration may also take place during dry weather if the sanitary sewer is near a creek bed or spring. In recent years, infiltration has become more important to engineers, health officials, water treatment and sewage treatment plant officials. 

             Sewage treatment plants are usually designed for dry-weather flow with a nominal allowance for infiltration during the wet season. This allowance is usually from 20 to 25 percent in well designed systems. Reports indicate that some treatment plants receive 100, 200, and even 300 percent of capacity during periods of heavy rain. A report from a County Engineer of the State of New York makes the following observations concerning infiltration. 

The quantity of ground, storm and surface waters discharging into county trunk sewers from sanitary lateral connections is highly excessive, resulting in overloading the trunk sewers, pumping stations and treatment plants, and increasing treatment and maintenance costs. This situation has resulted in the flooding of buildings and homes caused by surcharged sewers and in pollution of adjacent streams, potable waters and bathing areas. Unless this condition is alleviated, the ultimate capacity of the trunk sewers will be reached many years before the date contemplated by the sewer design, and the time will be brought nearer when the costly job of constructing additional facilities must be undertaken, the county is undertaking the preparation of construction standards, aimed to prevent leakage in sewers, for presentation to the contributing municipalities for their consideration.

Infiltration has been with us almost since the first pipes were laid, but with the increasing provision of treatment plants, the problem becomes more serious and costly. There is practically no way to cure it if it occurs. Careful specifications, the use of the best materials and rigid inspection during construction are the only preventatives. In many cases, the major part of infiltration enters through the house connections, emphasizing the need for our preventative factors of specifications, good materials and strict inspection in their construction. In one study it was estimated that house connection infiltration represented about 80% of the total. It should be remembered that in a residential area, the footage of house sewers may be twice as great as the footage of laterals.("Infiltration into Sewers Can Cost Lots of Money", Public Works, August, 1958.)

Exfiltration

             During dry seasons a sewer that leaks may allow sewage to flow out into the soil and find its way into underground streams, thereby contaminating ground water. This is called "exfiltration". A watertight sewer line is essential to eliminate this condition. Cast iron soil pipe systems are watertight and durable, and assure adequate protection against the exfiltration. 

             In May 1980, an infiltration test was conducted by the Cast Iron Soil Pipe Institute and witnessed by an independent testing laboratory. The test was conducted to determine the effect of infiltration of water through NO-HUB® soil pipe and hubless couplings. The hubless couplings used in this test were of a design upon which the Institute previously held patent rights. The testing procedure involved connection 4, 6, 8 and 10" size reducers with hubless couplings and measuring the amount of water seepage into the interior cavity of the soil pipe system when water pressure was placed on the exterior of system. A pressure of 50 PSI was exerted on the system for 30 minutes and no leakage was found. The pressure was then reduced to 20 PSI, and allowed to remain for 24 hours. Still no leakage was found.