RESEARCH NOTE NITROUS OXIDE PRODUCTION UNDER TOXIC CONDITIONS IN A DENITRIFYING ANOXIC FILTER J. M. GARRIDO, J. MORENO, R. ME Â NDEZ-PAMPI Â N and J. M. LEMA* * M Department of Chemical Engineering, University of Santiago de Compostela, Avda. das Ciencias s/n., E-15706 Santiago de Compostela, Spain (First received November 1995; accepted in revised form October 1997) AbstractÐProduction of nitrous oxide in a multifed anoxic ®lter was found during denitri®cation of wastewaters containing nitrite and nitrate. In the same reactor three dierent actions take place: Deni- tri®cation of nitrite and nitrate, degradation of formaldehyde (which is used as carbon source for deni- tri®cation) and urea hydrolysis. Nitrous oxide was detected during overloading periods being its concentration increasing in the gas phase when formaldehyde accumulates. 1.0% of nitrous oxide in the gas phase was measured when formaldehyde concentration was increased up to 1500 mg l À1 in the feed- ing and the loading rate from 1.5 to 6 g l À1 d À1 . Formaldehyde was found to be toxic or inhibitory for denitri®cation, also observing a 90% urea hydrolysis inhibition at formaldehyde concentration over 300 mg l À1 . The presence of nitrous oxide in the gas phase might be used for detecting overloading or anomalous working of denitrifying reactors. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐnitrous oxide, formaldehyde, denitri®cation INTRODUCTION Wastewaters from factories of urea-formaldehyde adhesives for ®breboard manufacturing have a high content in nitrogen, due to the presence of urea, and COD, basically from formaldehyde. The use of a denitri®cation-nitri®cation process is required for nitrogen and organic load removal. In the biologi- cal treatment of these wastewaters, formaldehyde can exert a toxic or inhibitory eect, therefore caus- ing a failure in the biological process. In this paper the production of nitrous oxide in a denitrifying anoxic ®lter in periods of overloading or under toxic conditions is studied. MATERIALS AND METHODS Experimental An up¯ow multifed anoxic ®lter (Veiga et al., 1994) with a working volume of 1.5 liters was employed (Fig. 1). Temperature was maintained at 378C by keeping the reac- tor in a thermostated chamber. Gas production was regis- tered by using an electric step counter and a gas ¯owmeter (Veiga et al., 1990). A synthetic medium with a similar composition to that obtained by mixing the industrial wastewater with a solution of nitrite and nitrate coming from a nitrifying reactor was employed. A medium con- taining 500 mg l À1 of formaldehyde, 230 mg N l À1 of urea and 230 mg NO x -N l À1 , and a mineral solution (Bennemann, 1990) was fed by four entrances along the reactor. In order to study the stability of the reactor, for- maldehyde concentration was increased from 500 to 1500 mg l À1 , simulating the eect of spills of formaldehyde in the factory (days 235 and 256), which represented a change of the loading rate from 1.5 to 6 g l À1 d À1 . From day 257 formaldehyde was not added in the feeding. Analysis The euent was analyzed 2±3 times a week for pH, total organic carbon (TOC), inorganic carbon (IC), NH 4 - N and NO x -N. Formaldehyde was determined using a spectrophotometric method based on the Hantzsch reac- tion (Nash, 1953). Nitrous oxide was determined by gas chromatography using a Hewlett Packard 5890 Series II, equipped with a thermal conductivity detector (TCD). Temperature of the injector, oven and detector were 110, 35 and 1108C, respectively. 10 ml min À1 helium was used as carrier gas. TOC and IC were determined using a TOC- 5000 Shimadzu equipment. Ammonia was analyzed by a selective electrode (APHA, 1985). Nitrite and nitrate were analyzed by capillary electrophoresis using a Waters Quanta 4000 system. RESULTS AND DISCUSSION In Fig. 2 the formaldehyde concentration in the euent and the nitrous oxide concentration in the gas phase are shown. Until day 210, the anoxic sys- tem worked properly, with the formaldehyde used as carbon source by the denitrifying microorgan- isms. At the same time urea was almost completely hydrolysed to CO 2 and ammonia. During this period, formaldehyde concentrations lower than 1 mg l À1 and NO x -N between 3±30 mg l À1 were Wat. Res. Vol. 32, No. 8, pp. 2550±2552, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00 PII: S0043-1354(97)00433-8 *Author to whom all correspondence should be addressed. 2550