A new method of autotrophic denitrification with spent sulfidic caustic as substrate and alkalinity source

A new method based on sulfide utilizing autotrophic denitrification was adopted to remove nitrate from wastewater and to reuse spent sulfidic caustic containing high sulfide and alkalinity levels. The experiments were performed using a bench-scale upflow anoxic hybrid growth reactor (UAHGR) and an upflow anoxic suspended growth reactor (UASGR) to characterize the stoichiometric relationship between sulfur and nitrate in the process as well as the performance of the reactors. The level of nitrate removal from the UAHGR and IJASGR were maintained at over 90% at a nitrate loading rate ranging from 0.15 similar to 0.40 kgNO(3)(-)/m(3).d and no significant nitrite accumulation was observed in either reactor. Although the influent pH values were higher than the optimum range of autotrophic denitrification at 8.7 similar to 10.1, the effluent pH was stable at 7.2 similar to 7.9 due to the production of hydrogen ions during operation. The stoichiometric ratio of sulfate production to nitrate removal was 1.5 similar to 2.1 MgSO42-/mgNO(3)(-) in both reactors. A comparison of the reactor performance revealed that the chemical parameters of the UAHGR operation corresponded to a plug flow like type reactor while the chemical parameters of the UASGR operation corresponded to a completely stirred tank reactor like type reactor. UAHGR did not require sludge recycling due to the packed media while UASGR required 300 similar to 700% sludge recycling. Therefore, spent sulfidic caustic could be used in the sulfur utilizing autotrophic denitrification processes as substrate and alkalinity sources. (c) KSBB.