RAPID COMMUNICATION AEROBIC GRANULAR SLUDGEÐA CASE REPORT PENG DANGCONG 12 , NICOLAS BERNET 1 *, JEAN-PHILIPPE DELGENES 1 and RENE MOLETTA 1 1 Laboratoire de Biotechnologie de l'Environnement, INRA, Avenue des Etangs, 11100 Narbonne, France and 2 Department of Environmental Engineering, Xi'an University of Architecture and Technology, Xi'an, P.R. China (First received August 1998; accepted in revised form October 1998) AbstractÐAerobic granular sludge was observed in a Sequencing Batch Reactor (SBR) in which a syn- thetic urban wastewater containing sodium acetate as an organic substrate was fed, and dissolved oxy- gen (DO) was controlled at low concentration (0.7±1.0 mg/l) Microscope examination showed that the morphology of the granules was nearly spherical (0.3±0.5 mm in diameter) with a very clear outline. The granular sludge had a good settleability (Sludge Volume Index (SVI) between 80±100 ml/g) and high COD removal and nitri®cation activities (2.16 g(TOC)/g(SS) Á d and 0.24 g(NH 3 -N)/g(SS) Á d). With granular sludge, high quality euent was obtained for treatment of synthetic wastewater under low DO. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐgranular sludge, aerobic SBR, low dissolved oxygen, morphology, activity INTRODUCTION The performance of an activated sludge process highly depends on the quality of the sludge formed in the reactor. The sludge should be easily separated from the liquid and maintained in the reactor. Generally, a dense, ¯oc-like sludge is observed in normally-operated activated sludge processes. The shape, porosity and density of the ¯oc are deter- mined by both composition of the wastewater to be treated and operating parameters, such as loading rate, DO concentration and sludge retention time. Granular sludge may be developed in some anaero- bic (Lettinga et al., 1980) and anoxic (Green et al., 1994) processes like up-¯ow sludge blanket (USB). This kind of sludge has a good settleability and can be easily maintained in the reactor. Therefore, high loading rates can be gained. In anaerobic condition, only ¯oc-like sludges are reported up to now. In this paper, we report the formation of granu- lar sludge in an aerobic sequencing batch reactor (SBR) fed with a synthetic urban wastewater. Some preliminary characteristics of the granules are pre- sented. A lot of sewage treatment plants are subject to short-time or long-time overloads which lead to either a short hydraulic retention time (HRT) (when ¯ow rate exceeds design values) or low DO in the aeration tanks (in high loading conditions). In both circumstances, process treatment eciency will be reduced. In the second situation, ®lamentous bacteria like Sphaerotilus natans/Type 1701 and Haliscomenobacter hydrossis (Wanner, 1994) may grow, which will lead to a decrease of the sludge settleability, even to the process failure. One poss- ible solution may be to change the in¯ow pattern from continuous to discontinuous. The primary objective of the study was to investigate the eect of low DO concentration on the performance of the process. MATERIALS AND METHODS Reactor description A 5 l SBR was inoculated with a low DO bulking acti- vated sludge (SVI: 250±300 ml/g) containing ®laments (Fig. 1a) and fed with a synthetic wastewater whose com- position was the following: sodium acetate, 800 mg/l; am- monium chloride, 250 mg/l; K 2 HPO 4 /100 mg/l; CaCl 2 Á 2H 2 O/70 mg/l; MgSO 4 Á 7H 2 O/30 mg/l, microelement sol- ution/50 ml/l. The microelement solution contained: CaCl 2 Á 2H 2 O/7.34 g/l; MgCl 2 Á 6H 2 O/25.07 g/l; FeCl 3 Á 6H 2 O/ 4.8 g/l; MnCl 2 Á 4H 2 O/1.03 g/l; ZnCl 2 Á 2H 2 O/0.01 g/l; CuCl 2 Á 2H 2 O/0.112 g/l; NaMoO 4 Á 2H 2 O/0.0025 g/l. Operating parameters were as follows: hydraulic reten- tion time (HRT): 8 h; sludge retention time (SRT): 20 days; DO in the bulk liquid: 3.5±4 mg/l at the beginning of the experiment (about 20 days). The ®lling, reaction, settling and withdrawing periods were respectively 0.5, 0.75, 2.5 and 0.25 h long, which means 6 cycles a day. The reactor was thermostatically regulated at 258C and stirred at 400 rpm to ensure good mixing and oxygen transfer. Wat. Res. Vol. 33, No. 3, pp. 890±893, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98/$ - see front matter PII: S0043-1354(98)00443-6 *Corresponding author. Tel.: (33) 4 68 42 51 51; fax: (33) 4 68 42 51 60; e-mail: bernet@ensam.inra.fr. 890