ELSEVIER Bioresource Technology 75 (2000) 1--6 Simple methods for the determination of the denitrifying activity of sludges M. Sánchez, A. Mosquera-Corral, R. Méndez *, J.M. Lema 1 Abstract In this work, the conditions for measuríng the specific denitrifying activity (SDA) of sludgeswere establishedas simple batch tests based on the measurementof Nz production. The effects of carbon to nitrogen ratios (C/N =1.3 and 2.0), sludge con- centrations (0.35, 0.73, 1.5 and 3.0 g volatile suspended solids (VSS) 1-1) and shaking (0-180 rpm) on SDA were studied by means of the application of two factoríal designs. A very simple method (Method A), based on the displacement of an alkaline solution by the Nz generated, was used. In the 0.35-0.73 g VSS 1-1 range, the SDA was strongly affected by variable VSS, but not in the range 1.5-3.0 g VSS 1-1. The optimum conditions to obtain the SDAs, evaluated by means of a statistical tool, were a C/N = 1.3, 1.5 g VSS 1-1 and shaking at 180 rpm. Method A was statistically compared to a second method (Method B), based on the measurement of partial pressureand composition of the gas produced in the sameconditions. @ 2000 Elsevier Science Ltd. All ríghts reserved. Keywords: Denitrification; Batch assays; Specific denitrifying activity (SDA); N2 production 1. Introduction Denitrification is a biological processoften applied to remove nitrates or nitrites from wastewaters by their reduction to molecular nitrogen. Large varieties of het- erotrophic bacteria are able to denitrify wastewaters in anoxic conditions (Pseudomonas, Paraccocus,Alcalig- enes, Thiobacillus, Bacillus). To enable this process to occur, the presence of an organic carbon source which functions as an electron donor is required, while nitrate acts as the last electron acceptor in the respiratory chain. The effects of different carbon substrates(glucose, ace- tate, methanol, wastewater), and the C/N ratio, on the denitrification process have been studied (Tam et al., 1992;Akunna et al., 1993). It has been observedthat the use of acetic acid as the carbon source ensures high denitrification rates (Carley and Mavinic, 1991; Mateju et al., 1992). Different intermediates (NO;:-, NzO, NO) mar accurnulate during denitrification, depending on the kind and concentration of substrate, operational conditions, the presence of toxic substances and the competition between microorganisms (Knowles, 1982; Polprasert and Park, 1986).Therefore, it is necessary to control operational conditions to ensure a proper deni- trification. Knowledge of the specificdenitrifying activity (SDA) of sludge makes it possible to calculate the maximum nitrogen load that may be treated by a system. There are two main groups of methods to evaluate the denitrifi- cation cate of a system. One of them is based on mea- surement of substrate (NO} or NO2) depletion cate or alkalinity production by denitrification; the other is based on the detection of the final product, Nz gas or NzO using acetylene as an inhibitor (Kaspar et al., 1981; Bradley et al., 1992;Rozzi et al., 1998). Several values of SDAs are found in the literature ranging from 0.02 to 1.5 g NO} -N g VSS-1'day-l (Akunna et al., 1994; Beaubien et al., 1995;Sánchez, 1996). In the present work, the effects of C/N ratio, VSS concentration and shaking on the determination of the SDA were evaluated to establish the optimum opera- tional conditions. A very simple method (A) based on the displacement of an alkaline solution by the Nz generated, was compared to another method (B), based on the measurement of pressure and composition of the gas produced in the sameconditions. .Corresponding author. Tel.: +34-981-563100 ext. 14227;fax: +34- 981-595012. E-mail address: eqrmndzp@uscmail.usc.es (R. Méndez). 0960-8524/00/$ -see front matter @ 2000 EIsevier Science Ltd. AII rights reserved.PII: S O 9 6 O -8 5 24 ( O O ) O O O 3 3 -X Department of Chemical Engineering, /nstitute of Technology, University of Santiago de Compostela. Avda. de las CienciassIn, E-/5706 Santiago de Compostela, Spain Received 25 February 1999;received in revised fonn 15 February 2000; accepted 22 February 2000