Nitrate Promotes Biological Oxidation of Sulfide in Wastewaters: Experiment at Plant-Scale Juan Garcı´a de Lomas, 1 Alfonso Corzo, 1 Juan M. Gonzalez, 2 Jose A. Andrades, 3 Emilio Iglesias, 4 Marı´aJose ´ Montero 5 1 Dpto. Biologı´a, Facultad de Ciencias del Mary Ambientales, Pol. Rı´o San Pedro s/n, 11510-Pto. Real, Ca ´diz, Spain; telephone: 34-956-016177; fax: 34-956-016019; e-mail: juan.garciadelomas@uca.es 2 IRNAS, CSIC. Avda. de Reina Mercedes 10, Sevilla, Spain 3 Aguas de Jerez, EMSA, C/Ca ´diz 1, 11402-Jerez de la Frontera, Ca ´diz, Spain 4 Yara Iberian. Po de la Castellana 130-50, Madrid, Spain 5 Dpto. de Ciencias de los Materiales e Ingenierı´a Metalu ´rgicay Quı´mica Inorga ´nica, Facultad de Ciencias del Mar y Ambientales, Pol. Rı´o San Pedro s/n, 11510-Pto. Real, Ca ´diz, Spain Received 11 August 2005; accepted 29 September 2005 Published online 27 October 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/bit.20768 Abstract: Biogenic production of sulfide in wastewater treatment plants involves odors, toxicity and corrosion problems. The production of sulfide is a consequence of bacterial activity, mainly sulfate-reducing bacteria (SRB). To prevent this production, the efficiency of nitrate addition to wastewater was tested at plant-scale by dosing concentrated calcium nitrate (Nutriox TM ) in the works inlet. Nutriox TM dosing resulted in a sharp decrease of sulfide, both in the air and in the bulk water, reaching maximum decreases of 98.7% and 94.7%, respectively. Quantitative molecular microbiology techniques indi- cated that the involved mechanism is the development of the nitrate-reducing, sulfide-oxidizing bacterium Thio- microspira denitrificans instead of the direct inhibition of the SRB community. Denitrification rate in primary sedi- mentation tanks was enhanced by nitrate, being this almost completely consumed. No significant increase of inorganic nitrogen was found in the discharged effluent, thus reducing potential environmental hazards to receiv- ing waters. This study demonstrates the effectiveness of nitrate addition in controlling sulfide generation at plant- scale, provides the mechanism and supports the environ- mental adequacy of this strategy. ß 2005 Wiley Periodicals, Inc. Keywords: wastewater; sulfide; nitrate; sulfide-oxidizing bacteria INTRODUCTION In septic sewage, the use of sulfate as final electron acceptor by sulfate-reducing bacteria (SRB) results in H 2 S production. This may involve toxicity, foul odors, corrosion problems (Boon, 1992; Hamilton and Lee, 1995; Pomeroy and Boon, 1990) and reduced treatment efficiency (Einarsen et al., 2000). The expenses and derived nuisances of sulfide motivate a growing interest to minimize its production. A number of methods for controlling sulfide production in different environments has been proposed (Fig. 1). Collect- ing off-gases and physicochemical removal of H 2 S (e.g., with addition of iron salts) may be considered as late stage treatments for sulfide reduction (Cadena and Peters, 1988; Montgomery et al., 1990). A second approximation involves preventing septic conditions either by inhibition of biological activity (IBA) or by specific prevention of anaerobic activity (PAA). Considering IBA, the addition of biocides is the most common method (Reinsel et al., 1996), but other inhibitors (e.g., nitrite, molybdate) have been also succesfully tested (Nemati et al., 2001c). PAA may involve elimination of sulfate from the water source (Bakke et al., 1992) or en- couraging microbial competition via addition of thermo- dynamically favorable compounds such as oxygen (Hobson and Yang, 2000), nitrate (Bentzen et al., 1995; Hobson and Yang, 2000), or nitrite (Londry and Suflita, 1999; Telang et al., 1997). The addition of nitrate and nitrite may also favor biological oxidation of sulfide generated by nitrate-reducing, sulfide-oxidizing bacteria (Jenneman et al., 1997; McInerney et al., 1996; Nemati et al., 2001a,b). This last mechanism consumes sulfides as it is produced, eliminating the need for additional current physico-chemical sulfide removal strate- gies (Fig. 1), prevents this gas from diffuse to the overlying water in contrast with off-gases collection and physico- chemical removal of H 2 S. The ‘‘inhibitory’’ effect of nitrate on sulfide production has been known for many years, however the involved mechan- ism remains controversial. Both transient and long-term inhibition of sulfide production have been reported long-time ago (Allen, 1949; Beardslay et al., 1956; Carpenter, 1932; ß 2005 Wiley Periodicals, Inc. Correspondence to: Juan Garcia de Lomas Contract grant sponsors: Ministry of Education and Science (MEC), Spain; MEC Contract grant numbers: MAT2000-0261, REN2002-01281