introduCtion Acids When in contact with portland cement concrete, acid will attack the exposed surface and be neutralized by the alkalinity of the concrete. A given quantity of an acid will destroy a given mass of concrete, that mass being inversely proportional to the total alkalinity of the concrete. Without acid replenishment, the reaction stops. Interior Acid Attack ConCrete pipe Two types of interior acid attack are encountered. One is the biochemical phenomenon which can occur within a sanitary sewer. The second type relates to acidic effluents. The mechanics of biochemical acid attack in sanitary sewers are those stated for acid attacks in general. There are, however, certain unique aspects that are experienced with this phenomenon: • The acid involved is always sulfuric, H2SO4. • Attack is confined to the interior, unsubmerged perimeter of the pipe. • Although the total volume of acid available is small, the concentration is high, up to 5 percent, and a high percentage will react with the exposed concrete surfaces. The development of this type of aggressive acidic environment depends upon a complex, highly unusual and specific set of sewage conditions. Present engineering technology provides the means for predicting the development and prevention or control of these specific conditions. The application of technology to prevention is an integral part of the design of sanitary sewer systems. Where designs call for control rather than prevention of this phenomenon, predictable levels of acidic exposure will be present. In these instances, countermeasures for increasing the resistance of the concrete to acidic environments are available. Highly acidic materials are usually prohibited from entering a public sewer system because acidic effluents are harmful to the sewage treatment process and may be harmful to the sewer. When acid is encountered, the attack is confined to the wetted interior surface of the pipe. The measure of aggressiveness of the acidic effluent is the pH, which is-defined as the logarithm of the reciprocal of the hydrogen ion concentration, H+, of a solution. A pH of 7 is neutral, neither basic nor acidic. Below 7 the material is acidic and above 7 it is basic. Since the numbers are logarithms, a pH of 5 is ten times as acidic as a pH of 6. With flowing effluent, there is constant replenishment of the acid, and the rate of attack is inversely proportional to the pH and the total alkalinity of the pipe wall. Continuous flow of effluent with a pH below 5.5 is considered aggressive and below 5.0 is highly aggressive. Intermittent flow, and cyclical increases in pH to higher values, will mitigate the rate of attack. All of these factors need to be considered in evaluating the severity of acidic conditions for any specific installation and the need for countermeasures. stormwater treatment teChnologies box seCtions manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 47