Effect of nitrite and nitrate on biogenic sulfide production in sewer biofilms determined by the use of microelectrodes

The effects of O2 and NO3− concentrations on in situ sulfate reduction and sulfide reoxidation in microaerophilic wastewater biofilms grown on rotating disk reactors were investigated by the use of microelectrodes for O2, S2−, NO3−, NO2−, and pH. Microelectrode measurements showed the vertical microzonation of O2 respiration, NO3− respiration, H2S oxidation and SO42− reduction in the biofilms. The microelectrode measurements indicate that sulfate reducing activity was largely restricted to a narrow anaerobic zone located about 500 μm below the biofilm surface. An addition of nitrate forced the sulfate reduction zone deeper in the biofilm and reduced the specific sulfate reduction rate as well. The sulfate reduction zone was consequently separated from the O2 and NO3− respiration zones. Anaerobic H2S oxidation with NO3− was also induced by addition of nitrate to the medium. Measurements of the reduced inorganic sulfur compounds (FeS, FeS2 and S0), total-Mn and total-Fe in the biofilm indicated that the produced H2S became immediately oxidized with O2, NO3− and other oxidants, mainly ferric/ferrous hydrates. On the basis of the present results, it was estimated that of all sulfide produced, 13% of the sulfide was precipitated by metal ions as FeS and S0 just above the sulfate reduction zone, 65% was anaerobically oxidized to SO42− with NO3− as an electron acceptor and 22% was aerobically oxidized within the biofilm incubated in 70 μmol l−1 of DO and 280 μmol l−1 of NO3−.

Temperature Measurements Using In Thermocouple In The Mocvd Rotating Disk Reactors

Rotating Disk Reactors used for Metalorganic Chemical Vapor Deposition have evolved into a leading manufacturing technology for several materials, including nitrides, compound semiconductors, metals, and oxides. One of the major issues to be resolved in bringing this technology into routine high yield manufacturing has been precise and repeatable wafer temperature measurement and control. The conventional approach to the rotating wafer temperature measurements by a stationary thermocouple located near the rotating wafer carrier suffers from low accuracy and repeatability. We have implemented a rotating thermocouple with a junction located close to the wafer for the temperature measurements in the MOCVD Rotating Disk Reactor. This approach allowed us to obtain reliable and accurate wafer temperature measurements with minimum dependence upon variable process parameters and to protect the thermocouple from degradation in the aggressive reactor environment. The temperature difference between wafer and thermocouple for the rotating and stationary thermocouple designs as a function of process parameters will be discussed.