Available online at www.sciencedirect.com Journal of Hazardous Materials 157 (2008) 367–373 Hydrodynamic characteristics of airlift nitrifying reactor using carrier-induced granular sludge Ren-Cun Jin a,b , Ping Zheng a,∗ , Qaisar Mahmood a , Lei Zhang a a Department of Environmental Engineering, Zhejiang University, Hangzhou 310029, PR China b Department of Environmental Science, Hangzhou Normal University, Hangzhou 310036, PR China Received 21 June 2007; received in revised form 28 December 2007; accepted 3 January 2008 Available online 8 January 2008 Abstract Since nitrification is the rate-limiting step in the biological nitrogen removal from wastewater, many studies have been conducted on the immobilization of nitrifying bacteria. A laboratory-scale investigation was carried out to scrutinize the effectiveness of activated carbon carrier addition for granulation of nitrifying sludge in a continuous-flow airlift bioreactor and to study the hydrodynamics of the reactor with carrier-induced granules. The results showed that the granular sludge began to appear and matured 60 and 108 days, respectively, after addition of carriers, while no granule was observed in the absence of carriers in the control test. The mature granules had a diameter of 0.5–5 mm (1.6 mm in average), settling velocity 22.3–55.8 m h -1 and specific gravity of 1.086. The relationship between the two important hydrodynamic coefficients, i.e. gas holdup and liquid circulation velocity, and the superficial gas velocity were established by a simple model and were confirmed experimentally. The model also could predict the critical superficial gas velocity for liquid circulation and that for granules circulation, with respective values of 1.017 and 2.662 cm min -1 , accurately. © 2008 Elsevier B.V. All rights reserved. Keywords: Airlift reactor; Granular sludge; Nitrification; Hydrodynamic characteristic; Activated carbon 1. Introduction Nitrogenous compounds like ammonium are prevalent in many wastewaters and need to be removed to prevent oxygen depletion and eutrophication of surface waters. Biological nitro- gen removal from wastewater using nitrification–denitrification is a well-known and cost-effective treatment process [1,2]. Because of their extremely low growth rate, it is generally accepted that retaining a large number of nitrifying bacteria within the reactor is difficult to achieve, thereby making the nitrification a rate-limiting step in the entire nitrogen removal process [3,4]. Much work has been conducted on the development of phys- ical or ecological methods of immobilizing nitrifying bacteria including cell-entrapping and cell-attaching techniques [5–7]. However, the immobilized cells created by gel entrapping tech- niques are easy to be suffered from mass transfer resistance [5]. ∗ Corresponding author. Fax: +86 571 86971709. E-mail address: pzheng@zju.edu.cn (P. Zheng). Previous researches also demonstrated that it takes a long time to construct a nitrifying biofilm on the surface of carrier mate- rials, particularly when the wastewater contains few organic compounds [8]. Moreover, the matrices and carriers used for cell immobilization inevitably occupy significant space in the reactor, limiting cell density. To avoid these problems, granu- lar sludge was generated to enhance cell retention and biomass concentration simultaneously. Sludge granulation has been intensively studied in anaerobic systems such as upflow anaerobic sludge blanket (UASB) [9,10] and in aerobic systems [11–13], but these reports have mostly focused on organic pollutant removal and the aerobic granular reactor used in these tests were mainly sequencing batch reactors (SBR) [11,12,14–17]. Until now, fewer attempts have been made to culture aerobic granular sludge in a continuous-flow system. Three-phase airlift reactors (ALR) are being applied fre- quently in chemical, biotechnological and environmental processes as simple and effective gas–liquid–solid phase con- tactors. ALR offers advantages over traditional three-phase contactors, namely, a lower gas requirement for complete sus- pension of the solid, elimination of dead volumes, rapid mixing 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.01.003