Design Considerations for Lining Concrete Sludge Mixing and Storage Tanks in Wastewater Treatment Plants: Issues that do not meet the Eye

Experience with condition assessment, testing, and failure diagnosis of protective linings in sludge storage and mixing tanks at facilities in Massachusetts, Connecticut, New York, New Jersey, Florida, and throughout the eastern seaboard, has led to the identification of certain considerations that, while are not always readily apparent, can substantially affect performance of the linings. In projects where these considerations were ignored, premature failure of the linings has occurred, often within a few years of installation. These considerations go beyond an understanding of the biogenic hydrogen sulfide corrosion mechanism occurring in the headspace, which is the primary corrosion driver in such structures. Similarly, they go beyond knowledge of the specialty protective linings specifically formulated to resist this mechanism. Selection of a lining with an appropriate level of resistance for the severity of the corrosive conditions alone is not an assurance of acceptable performance. Among the considerations identified in this discussion will be an understanding of the proper substrate condition, including the depth and degree of contamination. Testing, inspection, and analysis techniques necessary to quantitatively establish this condition are presented, including pH and sulfate content. Understanding of the substrate condition will also enable the selection of appropriate resurfacing materials from among the various cementitious and polymer alternatives. Consideration of the tank geometry, as well as the type and extent of concrete cracking, is required to determine the degree of rigidity or flexibility the lining must provide to facilitate selection of the appropriate lining type. Understanding of the thermal and moisture transmission characteristics of the structure, particularly those resulting from below ground construction and ground water exposure, will enable appropriate design selections to prevent premature failure from their effects. Assessment of the movement characteristics and corresponding treatment of cracks is necessary to ensure lining continuity and lining film quality. Finally, consideration of the proper application, curing, and quality assurance testing is necessary to assure the integrity and longevity of the lining in service. Experience has demonstrated that consideration of these factors will enable successful lining installations able to provide long-term protection to concrete sludge storage tanks in wastewater treatment plants.