Infiltration-and-Inflow Abatement Made (Relatively) Easy

The hidden and not-so-hidden costs of infiltration-and-inflow (I/I) can occur for many reasons, but most of the causes stem from aging infrastructure that need maintenance or replacement.

Infiltration-and-Inflow Abatement Made (Relatively) Easy

Ed Grieser and Andy Buchan — Oct 10, 2007

The hidden and not-so-hidden costs of infiltration-and-inflow (I/I) can occur for many reasons, but most of the causes stem from aging infrastructure that need maintenance or replacement.

The most common means of infiltration is through cracked or deteriorated manhole chamber walls due to long-term exposure to hydrogen sulfide gas and chamber fracture that is caused by ground movement and constant trafficking. Other causes include surface water runoff through manhole covers and down the sides of the frames because of cracked road surfaces, broken grade rings between the chamber cone and the manhole frame and cracked pipes and leaking joints.

Essentially, I/I becomes a headache for municipalities looking to reduce or maintain costs and ensure their processing facilities remain within the design capacity. When rain or groundwater enters the sewer system, the necessary processing of the additional water in sewage treatment facilities often costs more than $3 per 1,000 gal.

Exacerbating the cost issues are the strict environmental standards municipalities must meet. These include capacity management operations and maintenance (CMOM) regulations that require elimination of sanitary sewer overflows (SSOs) and improved overall system efficiency, and new U.S. Environmental Protection Agency (EPA) requirements for protection of floodplains, creeks, ponds and other sensitive environmental areas.

Towns trying to resolve chamber leakage often apply coatings to the inside of the chamber and the grade rings, protect the outside of the chamber with rubber shields and repair cracks on the outside of the chambers through external repair. These tactics need to be carefully considered to ensure they are appropriate for achieving the desired outcome. While these solutions may be effective, they often require specialist installation or external chamber application. This makes maintenance on existing systems difficult without expensive excavation around the chamber.

Options

What repairs can the municipality perform that are quick, efficient and still reduce infiltration? There are many options available, but for the purposes of this article we will consider solutions that are 'in the line of sight.' Line of sight means being able to evaluate the chamber from aboveground. While the bottom of a manhole chamber is not literally in the line of sight, a manhole chamber is the easiest part of the sanitary sewer system to assess its condition. The manhole chamber is also the easiest and fastest part of the system to repair.

It's estimated that as much as 50 percent of the total I/I enters the system via leaking and deteriorating manhole chambers. While there are many options for manhole renewal or repair, with each having advantages in specific situations, chemical grouting has several distinct advantages that are worthy of discussion.

If complete replacement of the infrastructure is impractical, chemical grouts can be used. There are many different types of polyurethane grout available. Closed cell grouts are like a series of bubbles that are attached to each other. Therefore, closed cell grouts do not allow water to pass through the material. Open cell grouts are similar to a sponge.

Without being overly technical, the two main types of polyurethane are hydrophilic and hydrophobic. Hydrophilic grouts require water to be present to react and to maintain the expanded state. Dual component hydrophobic grouts do not require additional water to react and have the additional advantage of not requiring water to be present to remain expanded. Pre-polymeric hydrophobic (single component) grouts use water to begin the reaction but expel it in the final phase of reaction. Hydrophilic grouts typically shrink over time.

One such product, SealGuard II, is a two-component polyurethane grout. SealGuard II has been used for more than 25 years in the mining industry, as well as in water sealing applications for the past eight years, in both above- and below-grade structures.

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