March 27, 2009

Rehabilitating Sewage Structures

by Kevin Morris

Have you ever heard the term “Out of sight out of mind”? Sewer collection systems qualify as “just that” to the general public. After spending the last several years in this segment of the water and wastewater industry, I am amazed at the current state of most of our buried infrastructure in the United States.

It has been estimated that 25-30% of water infiltration enters through manholes. Treating this excess water adds significant cost to the treatment of wastewater, for any municipality. Couple the additional treatment cost with the cost of maintenance and repairs to these structures from microbial induced corrosion and your current budget has been exceeded.

Microbial Induced Corrosion acts to¬†breakdown the sulfates in the waste stream, producing hydrogen sulfide gas (H2S) and carbon dioxide (CO2). These acidic gases¬†reduce the pH of the concrete and Sulfur Oxidizing¬†Bacteria¬†(SOB’s) attach to the surface as sulfates are produced and these¬†SOB’s (Thiobacillus Thioxidans) consume H2S and¬†discharge sulfuric acid (H2SO4).

Severly Corroded Manhole

For years the industry has addressed this problem with the lowest cost option and repaired these structures with similar cementitious products to those used for the initial construction. While these repair products (microsilica modified repair mortars and calcium aluminates repair mortars) slow the corrosion rates from an average of 18% loss by weight (Portland Cement) over an 8-year period to as little as 3% loss by weight (Calcium Aluminates)over the same period, they do not stop the process.

Correcting the problems that exist in the structures should incorporate a system of products for repair and protection.

  • Infiltration – Typically corrected with hydrophilic or hydrophobic polyurethane grouts installed through trenchless technology.
  • Structural Rehabilitation – The rehabilitation of the structure is typically addressed with the use of cementitous repair products that have increased physical properties over that of the original substrate. The right selection of these materials can be left as a stand alone liner in a mild sewer environment.
  • Corrosion Resistant Linings – Epoxies,¬†urethanes, and¬†polyureas¬†offers the greatest improvement to the life cycle of these assets. The selection of the right products should incorporate some specific physical performance requirements and in most cases are not those that the industry has¬†promoted for years.

Sewer structure rehabilitation can be a long-term solution if the specified products are designed to provide the maximum life cycle protection. Considering¬†solutions of this nature can be cheaper in the long run than what the low bid offers. The best point of view that I have heard in a long time may have been that of Mike Hicks, Program Manager – Sewer Repairs & Replacement for the City of Fort Wayne, Indiana in a recent Trenchless Technology article. Hicks stated “I don’t want to leave my granchildren with problems that effect the quality of their lives.”

Comments (5)

  1. August 26th, 2009 at 9:58 am

    Will SherFlex resist hydrostatic (head) pressure? Does any urethane or polyurea resist head pressure? what is the adhesion of polyurethanes or polyureas in moist environments during application?

  2. August 27th, 2009 at 1:04 pm

    Head pressure is not the failure mechanism that causes blistering in coatings when applied to concrete. In order to blow a properly mixed, applied, and cured coating off of concrete via head pressure, the pressure would exceed the tensile strength of the substrate making the substrate the failure mechanism.

    Blisters in coatings are due to cyclic water migration through the substrate. Each time the water comes to the surface through the capillaries of the concrete in brings salts or chlorides with it. Over a period of time the salts affect the bond line of the coating, creating blisters. If mositure emission through the concrete is occuring rapidly enough, it can affect the cure of the product and the resulting blisters will happen at a more rapid pace.

    Consider this information with regards to concrete. Mositure will always exist in the substrate (Moisture Vapor Transmission) as static moisture moving around but not out of the concrete. The concrete is always seeking to reach a state of equilibrium with it’s environment. An example would be, new concrete placed in a building that has not been become a conditioned space would yield X% mositure in the slab and would not emit this mositure if the environmental conditions were similar. However, when you close up the building and turn on the air conditioning, the relative humidity in the building will decrease and cause the mositure in the concrete to become (Mositure Vapor Emmision) dynamic and move out of the substrate.

    If the sewer structure is buried, then mositure in the environment will be relatively constant and the temperature will not see a drastical flucuation in temperature and the issue of MVE (Moisture Vapor Emission) is drastically minimized. If the structure is above grade then mositure will always travel out of the structure through the path of least resistance or the exterior of the substrate. Most problems are seen when the structure is an open top design that is buried. The sun will warm the non-submerged areas of the tank drawing the water to the heat source increasing the chances for an MVE related failure.

    Will SherFlex resist hydrostatic pressure? Yes, but this is not the failure mechanism. Will any urethane or polyurea resist head pressure? Yes. However, all products including epoxies will fail due to MVE issues unless they are specifically designed to mitigate these issues. Some products are designed, so that if they disbond from the substrate they will free stand in the structure. With products of this nature you will not see a visable failure mechanism but you also will not know what is going on between the lining and the structure. You could have water inlfow & infiltration that could continue to take place without being noticed and this is one of the primary reasons (along with corrosion control) that you would line a sewer structure.

    What is the adhesion of polyurethane or polyures in moist environments during application? Polyurethanes and polyureas do not like moist substrates and therefore require the use of a primer (generally an epoxy, but not always) to adhere properly. The adhesion of the primer will be greater than the tensile strength of the concrete or masonry substrate to which it was applied. The substrate should never be greater than SSD (Surface Saturated Dry) for the application of the primer. Active or flowing leaks are typically stopped through methods of trenchless technology application with a hydrphobic or hydrophilic polyurethane grout. Airborne mositure, in the amount of a mist, that can be felt on ones skin while in a structure is not a good canidate for the application of polyurethanes or polyureas unless you can bypass the structure during application and cure.

  3. October 26th, 2009 at 5:59 am

    Dear Sir,

    For a big project for sewage channel lining we suggest poly urea, do you know poly urea used in sewage channel till now for a big channel or No? and if this lining can resist against negative pressure of water or No?
    In advance thank for your cooperation.

  4. November 1st, 2009 at 10:17 am

    Polyurea will work very well as a lining in a sewage channel. It has excellent chemical resitance as well as abrasion resistance. Yes, Polyurea will withstand negative side pressure,however, all products including epoxies will fail due to MVE issues unless they are specifically designed to mitigate these issues. Some products are designed, so that if they disbond from the substrate they will free stand in the structure. With products of this nature you will not see a visable failure mechanism but you also will not know what is going on between the lining and the structure. You could have water inlfow & infiltration that could continue to take place without being noticed and this is one of the primary reasons (along with corrosion control) that you would line a sewer structure.

  5. October 28th, 2009 at 9:35 am

    For a big project for sewage channel lining we suggest poly urea, do you know poly urea used in sewage channel till now for a big channel or No? and if this lining can resist against negative pressure of water or No?
    In advance thank for your cooperation.

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