Journal of Hazardous Materials 148 (2007) 485–490 Biological nitrate removal from wastewater of a metal-finishing industry Carmen Gabald´ on, Marta Izquierdo, Vicente Mart´ ınez-Soria, Paula Marzal, Josep-Manuel Penya-roja ∗ , F. Javier Alvarez-Hornos Department of Chemical Engineering, University of Valencia, Dr. Moliner, 50, 46100 Burjassot, Spain Received 24 July 2006; received in revised form 26 February 2007; accepted 28 February 2007 Available online 3 March 2007 Abstract An upflow packed bed reactor at laboratory scale has been operated for a continuous period of 5 months to investigate the technical feasibility of biological nitrate removal applied to the effluent of the coagulation–sedimentation wastewater of a metal-finishing industry. The reactor was fed with industrial wastewater in a five-fold dilution to reproduce the global spill in the factory (20/80, industrial wastewater/domestic wastewater) with a concentration of nitrate between 141 and 210 g NO 3 -N/m 3 . Methanol was added as a carbon source for denitrification. Inlet flow rate was progressively increased from 9 to 40 L/day (nitrogen input load from 45 to 250 g NO 3 -N/(m 3 h)). The highest observed denitrification rate was 135 g NO 3 -N/(m 3 h) at a nitrate load of 250 g NO 3 -N/(m 3 h), and removal efficiencies higher than 90% were obtained for loads up to 100 g NO 3 - N/(m 3 h). A mass relation between COD consumed and NO 3 -N removed around 3.31 was observed. Better results were achieved in a previous stage using tap water with nitrate added as a sole pollutant as a synthetic feed (critical load of 130 g NO 3 -N/(m 3 h) and denitrification rate of 200 g NO 3 -N/(m 3 h) at a nitrate load of 250 g NO 3 -N/(m 3 h)). This fact could indicate that the chemical composition of the industrial source hinders to some extent the performance of the biological process. Whatever case, results demonstrated the viability of the denitrification process for the global industrial wastewater. A simple model based on Monod kinetics for substrate consumption, and constant biomass concentration was applied to model the industrial wastewater treatment, and a reasonably good fitting was obtained. © 2007 Elsevier B.V. All rights reserved. Keywords: Biological nitrate removal; Denitrification; Metal-finishing process; Industrial wastewater; Fixed-bed biological reactor 1. Introduction Industrial wastewater from metal-finishing processes con- tains heavy metals and cyanide as main toxic com- pounds. Conventional treatments used to remove these hazardous materials include physical–chemical methods such as cyanide oxidation, reduction of hexavalent chromium, and coagulation–flocculation followed by sedimentation for heavy metals removal. In some metal surface-finishing industries, the acid pickling step generates high levels of nitrate and/or sul- phate which increase the conductivity values of the wastewater streams. As nitrate and sulphate are very high soluble substances, they cannot be removed by conventional physical–chemical treatments. Nitrogen discharge to ponds and lakes could con- tribute to enhance the eutrophication process, so many regions ∗ Corresponding author. Tel.: +34 963543131; fax: +34 963544898. E-mail address: josep.penarrocha@uv.es (J.-M. Penya-roja). including EU countries, have developed restricted regulations for nitrogen concentration in wastewater effluents. Among several non-biological available methods, selective inorganic ion exchangers can be used to remove nitrate ions from metal-finishing wastewaters, but this technique is expensive and generates a saline waste from resin regeneration which becomes difficult to dispose of. By other side, biological methods are low cost and non-generating hazardous residues. Nitrogen removal via biological nitrification–denitrification processes are usually applied for municipal wastewater treatment with a high number of world-wide applications [1]. In the last decade, studies on biological nitrogen removal using synthetic wastewater to simulate industrial discharges have been performed in order to evaluate the potential appli- cation of the nitrification–denitrification process to industrial wastewaters [2], but few studies focused on real industrial wastewater have been published in the metal-processing indus- try. Schuch et al. [3] and Buchhesiter et al. [4] operated a pilot plant for nitrification and denitrification of different industrial 0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.02.071