Hydraulic Analysis of Gas Pockets in Wastewater Rising Mains

Oscar Pozos-Estrada, Oscar Fuentes, Alejandro Sanchez, Faustino De Luna, Eduardo Rodal

Thursday 2 july 2015

9:45 - 10:00h at South America (level 0)

Themes: (T) Hydro-environment, (ST) Rehabilitation of water systems

Parallel session: 10C. Engineering - Industrial

Hydrogen sulfide corrosion can occur in wastewater rising mains in places where the sewage does not continuously contact the top of the pipe. Corrosive gas pockets can accumulate at high points of rising mains when the hydraulic grade line is below the pipe, or a gas pocket cannot be dragged downstream owing to the buoyancy of the pocket. Hydrogen sulfide gases are generated within the anaerobic slime layer formed on the submerged pipe walls. Hydrogen sulfide gas released from the slime layer rise into the airway portion of the sewer pipe and is oxidized by bacteria on the un-submerged portion of the pipe in the presence of moisture forming sulfuric acid. It is this sulfuric acid that corrodes ferrous metals and concrete. This paper presents a methodology to locate the points where the pockets are likely to accumulate in rising mains and subsequently compute the length of the pockets. The method can be used by the engineers to suggest solutions during the design stage or for existing rising mains to replace or rehabilitate the line with inert pipe materials on areas prone to corrosion attack. In addition, experimental investigations have been developed with the aim of demonstrating that the flow under air pockets with a pressure greater than the atmospheric is similar to the flow in open channels. From this investigation it was concluded that the gradually varied flow theory is applicable to compute the flow profiles underneath gas pockets. A case study is analyzed by using the proposed methodology.