IDENTIFICATION AND QUANTIFICATION OF NITROGEN NUTRIENT DEFICIENCY IN THE ACTIVATED SLUDGE PROCESS USING RESPIROMETRY ZUOJUN NING 1 , GILLES G. PATRY 1 * M and HENRI SPANJERS 2M 1 University of Ottawa, Department of Civil Engineering, 161 Louis Pasteur, PO Box 450, Station A, Ottawa, Ontario, K1N 6N5, Canada and 2 Department of Environmental Technology, Wageningen Agricultural University, PO Box 8129, NL-6700 EV, Wageningen, The Netherlands (First received 1 August 1998; accepted in revised form 1 August 1999) AbstractÐExperimental protocols to identify and quantify nitrogen nutrient de®ciency in the activated sludge process were developed and tested using respirometry. Respirometric experiments showed that when a nitrogen nutrient de®cient sludge is exposed to ammonia nitrogen, the oxygen uptake rate (OUR) of the sludge increases while an initial nitrogen nutrient uptake takes place at the same time. Our investigation suggests that this initial nitrogen uptake is metabolically mediated. The protocols developed in this study can be used: (a) to assess whether a sludge sample is nitrogen nutrient de®cient or not; and (b) to estimate the amount of nitrogen required to remedy a nutrient de®cient condition in the sludge. Finally, a respirometry-based strategy to control nutrient addition to the activated sludge process is proposed. 7 2000 Published by Elsevier Science Ltd Key wordsÐrespirometry, activated sludge, nitrogen, nutrient control, pulp and paper wastewater INTRODUCTION Nutrient control is of critical importance in the bio- logical treatment of nutrient de®cient wastewaters, such as those generally encountered in the pulp and paper industry (SaunamaÈki, 1997; Ramamurthy et al., 1996). In addition to carbon and energy sources, microorganisms require an adequate supply of nitrogen and other potentially growth-limiting nutrients. Depending on the industrial processes involved, wastewater from the pulp and paper industry can be nutrient de®cient. In fact, most plants tend to exhibit large variations in nutrient concentrations, and generally require the addition of nitrogen and/or phosphorus in order to promote a healthy activated sludge environment (SaunamaÈki, 1997, 1994). Consequently, it may be necessary to monitor very closely the nutrient balance of the mixed liquor in the aeration tanks. Among the nutrients required by microorganisms, nitrogen is particularly important since it is needed in the greatest quantity. The addition of an excessive, uncontrolled, amount of nitrogen is obviously not desirable. Such practice is not only expensive because of the waste of excessive nutrient and the cost resulting from the extra oxygen required for nitri®cation, it is also environmentally detrimental to receiving waters. On the other hand, an insu- cient amount of nitrogen will result in incomplete oxidation of the waste and deteriorating sludge settling properties (Grady and Lim, 1980). Eorts have been made in recent years to control nitrogen nutrient addition in the activated sludge process treating wastewater from the pulp and paper industry. Under steady-state in¯uent con- ditions (i.e., little or no variation in ¯owrate and or- ganic load), nitrogen in the form of urea can be added to the waste stream. This approach requires that a certain amount of ammonia nitrogen be pre- sent in the treated euent (1±2 mg/l) in order to prevent the onset of nutrient de®cient conditions (MoÈbius, 1991; Grau, 1991; SaunamaÈki, 1994). However, the actual release of nitrogen in the eu- ent can be as high as 10 mg N/l due to the in¯uent load ¯uctuations (MoÈbius, 1991). This approach is largely a compensation for the lack of a well- de®ned proven nutrient control strategy. To cope with ¯uctuations in in¯uent organic loads which are generally the result of upstream process failure, equipment washouts, wastewater production vari- ations, and temporary process shutdown, nitrogen nutrient addition based on in¯uent ¯ow or in¯uent organic load conditions has been suggested and applied (Jansson et al., 1994). Conductivity was used in measuring the in¯uent organic load vari- Wat. Res. Vol. 34, No. 13, pp. 3345±3354, 2000 7 2000 Published by Elsevier Science Ltd Printed in Great Britain 0043-1354/00/$ - see front matter 3345 www.elsevier.com/locate/watres PII: S0043-1354(00)00020-8 *Author to whom all correspondence should be addressed. E-mail: patry@uottawa.ca