Gas pockets in a wastewater rising main: a case study.
This paper presents a case study of an existing wastewater rising main (WWRM) in which an extreme transient event produced by simultaneous power failure of the pumps caused the rupture of a 1.2 m (48 in) prestressed concrete cylinder pipe (PCCP), causing an important leakage of sewage. The event and the methodology followed in order to validate the diagnostics of the failure are described. The detail study included in situ observation of the system, experimental investigation in a setup, hydraulic analysis, as well as details of the structural strength of the WWRM. After the extensive investigation and several simulations of fluid transients for different scenarios and flow conditions, it was found that stationary small gas pockets accumulated at high points of the WWRM were identified as the principal contributory factor of the failure. This case study serves as clear warning of the consequences of operating a WWRM with gas pockets at its high points.
Removal of Air from Water Lines by Hydraulic Means
Air is introduced into water transmission lines through vortices at intakes, pumps, vents, and other causes. This results in loss of capacity, surges and blow backs, difficulties in filtering operations, corrosion of lines, and reduction of pump efficiency to name a few. The best solution to air problems is to prevent the introduction of air at the intake. Other means of air removal are air relief valves and by the inertia of the flowing water. No conventional similitude exist to study the time of clearance of isolated air pockets and results of Froudian scale model tests are very conservative. For pipe sizes below 1 ft. (0.30m) the clearance of isolated pockets may require higher than practical velocities.
Growth and physiology of Thiobacillus novellus under nutrient-limited mixotrophic conditions
Thiobacillus novellus was cultivated in a chemostate under the individual limitations of thiosulfate, glucose, and thiosulfate plus glucose. At dilution rate (D) of 0.05 h-1 or lower, the steady-state biomass concentration in mixotrophic medium was additive of the heterotrophic and autotrophic biomass at corresponding D values. The ambient concentrations of thiosulfate, glucose, or both in the various cultures were low and were very similar in mixotrophic, heterotrophic, and autotrophic environments at a given D value. At D = 0.05 h-1, mixotrophic cells possessed higher activities of sulfite oxidase and thiosulfate oxidation compared to autotrophic cells, as well as higher activities of glucose enzymes and glucose oxidation than heterotrophic cells. Thus, in contrast to nutrient-excess conditions, in nutrient-limited mixotrophic environments at these D values, T. novellus did not exhibit characteristics of uncoupled substrate oxidation, inhibition of substrate utilization, and repression of enzymes of energy metabolism. It is concluded that T. novellus responds to mixotrophic growth conditions differently in environments of different nutritional status, and the ecological and physiological significance of this finding is discussed.