The eect of kaolin particles on the behavior of nitrifying activated sludge units J.L. Campos, J.M. Garrido, R. M endez * , J.M. Lema Department of Chemical Engineering, Institute of Technology, University of Santiago de Compostela, Avda. das Ciencias s/n, E-15706 Santiago de Compostela, Spain Received 6 April 2001; received in revised form 20 July 2001; accepted 25 July 2001 Abstract The eects of the addition of powered particles of kaolin to nitrifying activated sludge systems were studied. Kaolin was added to a nitrifying activated sludge reactor, during the operational phase, to observe the eects of this clay on reactor performance. The results were compared to those obtained from a similar unit operated without kaolin. The settling properties of the sludges from both units were similar sludge volume index SVI) of 14.5 ml/g VSS; zone settling velocity ZSV) of 7.5 m/h), but the speci®c nitrifying activities of ammonia and nitrite oxidizing processes were enhanced up to 75% and 50%, respectively, when kaolin was added. The mechanism of action of kaolin was not clear. Additional ammonia, nitrite and nitrate adsorption tests showed that these compounds were not adsorbed by kaolin. This demonstrated that no bene®cial eect was caused by adsorption of either substrates or products. Short-term activity tests also showed that the stimulating eects of kaolin on speci®c activity were not immediate. The eects of kaolin when nitrifying units were operated under unfavorable conditions were also evaluated: In a second set of experi- ments, a nitrifying unit was operated with low levels of dissolved oxygen DO), with and without kaolin. The presence of kaolin exerted practically no eect on ammonia oxidation but nitrite oxidation slightly diminished. In a third set of experiments, a ni- trifying unit was subjected to pH shocks 9, 10 and 11) over 3 h with pH then restored to 7.8. A pH shock of 11 caused a decrease of 60% in nitrifying activity for 12 days. When kaolin was added to this unit the eciency of the system was completely restored in 4 days. Therefore, kaolin might be useful to restore damaged units. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Activated sludge; Ammonia adsorption; Kaolin; Nitri®cation; pH shock; Speci®c nitrifying activity 1. Introduction Removal of ammonia by biological nitri®cation, us- ing activated sludge systems, is a process that is widely used in the treatment of domestic and industrial waste- waters. For a long time, upgrading this process, in order to treat high loads either to eliminate or to avoid in- hibitory eects, has been the goal of many works. Sev- eral authors found that suspended particles had a stimulating eect on nitri®cation, both in natural eco- systems and in wastewater treatment plants. Studies with lake and river water emphasized the important role of suspended inorganic particles in the activity of nitri- fying bacteria. Seppanen 1970) found the lag phase of ammonia oxidation in river and lake waters was longer when the water was clear. Laurent 1971) also found that the activity of nitrifying bacteria was greater in muddy zones than in clear water. The activated sludge process with powdered activated carbon PAC) consists of the addition of powdered carbon to conventional activated sludge processes in order to biologically degrade organic compounds, en- hance the settling properties of sludge and increase the nitrifying capacity Bettens, 1979). This addition im- proves nitri®cation, especially in unfavorable conditions such as low temperatures and the presence of inhibitory compounds Ng et al., 1987) but system costs rise be- cause of the high cost of PAC. Another alternative to this process is continuous or batch addition of powdered clay, which reduces costs compared to PAC. The addition of powdered clay may improve biomass retention, so increasing sludge con- centration, sludge sedimentation properties sludge volumetric index SVI) and zone settling velocity ZSV)) and the stability of the system during overloads. Therefore, this method may be applied to wastewater Bioresource Technology 81 2002) 225±231 * Corresponding author. Tel.: +34-981-563100x14227; fax: +34-981- 595012. E-mail address: eqrmndzp@usc.es R. M endez). 0960-8524/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0960-852401)00136-5