## 1 Introduction  
In recent decades the wastewater treatment industry has identified the discharge of phosphorus (P) from industries, agriculture and municipal effluents into waterways as a risk to natural environments due to the serious effects of eutrophication and health of aquatic organisms. Eutrophication is the abundance of aquatic plants, growth of algae, and depletion of dissolved oxygen [1; 2]. In cases of severe eutrophication, dissolved oxygen gets depleted causing fish death when algae decay [3; 4]. The algal biomass produced by eutrophication can also hinder water treatment by blocking filters or penetrate through them causing bad odour and taste to the treated water [5]. The blue-green algae produced can release compounds that can be toxic to fish and other aquatic organisms [6]. These conditions are also of potential risk to human health, if shellfish contaminated with algal toxins is consumed by humans [7]. As the global supplies of clean water diminish and people's demand for water rises, advanced wastewater treatment is becoming an international focus for the rational use of scarce water resources and as a means of safeguarding aquatic environments from the damage caused by wastewater disposal. In recent times the treatment of domestic urban wastewater using the membrane bio-reactor (MBR) process has become popular as effluent standards are more stringent and current water conservation measures strongly emphasise water reuse applications. The MBR system consists of a suspended-growth biological reactor combined with a membrane unit process [8]. MBR is a promising process in wastewater treatment for its ability in completely removing solids, superior removal of nutrients and organic matter, high loading rate capabilities, low/zero sludge production and small footprint. This makes the MBR particularly suitable for water reuse. Although the MBR process can remove most organic matter and nutrients from water, it fails to remove P when exposed to peak and variable loads depending on the operating conditions [9]. Phosphate is an important resource used in agriculture and many other industries. However, P resources are limited and there have been some alarming reports that deposits of high-grade P ones are likely to be depleted within the next few decades [10; 11]. Phosphate has no substitute in food production and in a world of 9 billion people by 2050, securing sufficient P will be critical for future food security. Yet the World's main source of P--phosphate rock--is non-renewable and becoming increasingly scarce and expensive. Peak P was estimated to occur by 2035, after which the demand would outstrip the supply [11]. As P use efficiency is low, only 15%-20% of applied P is used by crops and animals, and the remaining P is shunted into various waste streams [12]. The P in these waste streams, if economically recovered, can contribute to a sustainable management of P resources [12]. Therefore, the recovery of P from P-containing wastewater is essential for developing an alternative source that can help overcome the global challenge of P scarcity. To illustrate this point, a P recovery scenario in Australia can be considered. The daily amount of wastewater generated in Sydney, Australia is over 1200 ML. Assuming a typical raw sewage P concentration of 11 mg/L, Sydney's wastewater system generates 13.2 tonnes of P on a daily basis or 4820 tonnes of P annually. Phosphorus is present in wastewater as orthophosphates, condensed phosphates and various organic phosphate fractions. In the MBR system only a small amount of P is used for cell metabolism and growth (1%-2% of the total suspended solids (TSS) mass in the mixed liquor) [13]. Since significant amounts of P cannot be removed in MBR treatment, another post-treatment strategy is required along with MBR for P removal. Phosphorus can also be removed through enhanced biological phosphorus removal (EBPR), however, the successful operation ofEBPR depends on many process operational factors, especially variations in wastewater quality [14].