Chemical Engineering Science 61 (2006) 3951 – 3961 www.elsevier.com/locate/ces A fed-batch design approach of struvite system in controlled supersaturation Md. Imtiaj Ali ∗, 1 , Philip Andrew Schneider School of Engineering, James Cook University, Townsville, Queensland, QLD-4811, Australia Received 25 November 2004; received in revised form 23 August 2005; accepted 20 January 2006 Available online 20 March 2006 Abstract This paper focuses on struvite (MgNH 4 PO 4 · 6H 2 O) crystallization in controlled supersaturation. Struvite can be used as a slow-release fertilizer. Crystallization experiments were conducted using supersaturated solutions. The secondary focus of this paper is the design of a struvite recovery system in fed-batch-controlled supersaturation mode. The design and commissioning of fed-batch struvite crystallization included the determination of operating supersaturation of struvite crystallization, suitable seed materials and the composition of feed solution. Determination of operating supersaturation of struvite crystallization was conducted by two steps including thermodynamic simulation using gPROMS 2 (process simulation software) along with a set of batch experiments. Investigation of suitable seed materials was also conducted using set of batch experiments. Two types of seed materials including quartz sand and struvite seeds were used in the investigation of seed materials. Composition of feed solution included the investigation of struvite solution chemistry using PHREEQC 3 thermodynamic modeling package. Based on the previously investigated design approach, struvite crystallization in fed-batch system was conducted using a 44-L of reactor with 15-L of initial reactant volume.  2006 Elsevier Ltd. All rights reserved. Keywords: Struvite; Control; Supersaturation; Reactor; Design; Thermodynamics 1. Introduction Magnesium ammonium phosphate hexa-hydrate (Mg NH 4 PO 4 · 6H 2 O), commonly known as struvite, is a threat for nutrient-rich wastewater systems when the nutrient composi- tion exceeds stable saturation limit (Muramatsu et al., 2000). Crystalline deposits of struvite characteristically form in the wastewater treatment infrastructure in high turbulence zone. The very sensitive zones of forming struvite are valves, bends in pipe, separating screens, pumps, etc. The mass of crystalline deposits formed can be extensive and can lead to operational failure by clogging water distribution pipes. Struvite satisfies ∗ Corresponding author. Tel.:+61 07 4781 43463; fax: +61 07 4775 1184. E-mail address: Md.Ali@jcu.edu.au (Md.I. Ali). 1 After December 2006, corresponding email is phil.schneider@jcu.edu.au (Dr. Philip A. Schneider), Tel.: +61 07 4781 5427. 2 gPROMS process simulation software, Process System Enterprise, London, UK. 3 US Geological Survey, Hydrologic Analysis Software Support Program, 437 National Centre, Reston VA 20192, E-mail: h2osoft@usgs.gov. 0009-2509/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2006.01.028 a need of slow-release fertilizer and has many uses in horticul- ture such as in nurseries, golf course, etc. as boutique fertilizers (Nelson et al., 2000). The basic technique of growth type crystallizer was derived from mixed suspension mixed product removal (MSMPR). The key endeavor of this paper is to design a crystallization pro- cess capable of reducing spontaneous precipitation and allow- ing crystals to grow. Pilot scale-controlled crystallization, in the presence of suspended seeds, has been investigated in this paper. Thermodynamically, the metastable zone is defined as the critical zone of supersaturation of solution where crystalliza- tion is not governed by nucleation and thus avoids rapid and/or spontaneous precipitation. Hirasawa (1996) documented an ex- perimental approach to determine metastable zone width for hydroxyapatite crystallization, relating to solution supersatura- tion, pH value, and minimum solubility limit of crystallization. Crystallization in the metastable zone is heterogeneous. In in- dustrial crystallization, the metastable zone technique is widely practiced due to smoother continuous operation in controlled supersaturation (Thaller et al., 1981; McPherson, 1988; Srini- vasakannan et al., 2002).