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The present review provides an understanding of principles of struvite crystallisation and examines the techniques and processes experimented to date by researchers at laboratory, pilot and full scale to maximise phosphorus removal and reuse as struvite from wastewater effluents. Struvite is mainly known as a scale deposit causing concerns to wastewater companies. Indeed struvite naturally occurs under specific condition of pH and mixing energy in specific areas of wastewater treatment plants (e.g. pipes, heat exchangers) when concentrations of magnesium, phosphate and ammonium approach an equimolar ratio 1:1:1. However, thanks to struvite composition and its fertilising properties, the control of its precipitation could contribute to the reduction of phosphorus levels in effluents while simultaneously generate a valuable end by-product. A number of processes such as stirred tank reactors, air agitated and fluidised bed reactors have been investigated as possible configurations for struvite recovery. Fluidised bed reactors emerged as one of the promising solutions for removing and recovering phosphorus as struvite. Phosphorus removal can easily reach 70% or more, although the technique still needs improvement with regards to controlling struvite production quality and quantity to become broadly established as a standard treatment for wastewater companies.

Advanced oxidation comprises a range of similar but different chemical processes aimed at tackling pollution in water, air and soil. Over the past few decades, multidisciplinary research has been carried out to study a broad spectrum of topics such as understanding of process fundamentals, elucidation of kinetics and mechanisms, development of new materials, modelling, process integration and scale-up. This article identifies and discusses certain directions that seem to advance R&D on advanced oxidation for water/wastewater treatment.

The main focus of the paper is to review current understanding of floc structure and strength. This has been done by reviewing current theoretical understanding of floc growth and breakage and an analysis of different techniques used for measuring floc strength. An overview has also been made of the general trends seen in floc strength analysis. The rate of floc formation is a balance between breakage and aggregation with flocs eventually reaching a steady-state size for a given shear rate. The steady-state floc size for a particular shear rate can, therefore, be a good indicator of floc strength. This has resulted in the development of a range of techniques to measure floc size at different applied shear levels using a combination of one or more of the following tools: light scattering and transmission; microscopy; photography; video and image analysis software. Floc strength may be simply quantified using the initial floc size for a given shear rate and the floc strength factor. More complex techniques have used theoretical modelling to determine whether flocs break by large-scale fragmentation or smaller-scale surface erosion effects, although this interpretation is open to debate. Impeller-based mixing, ultrasound and vibrating columns have all been used to provide a uniform, accurate and controllable dissipation of energy onto a floc suspension to determine floc strength. Other more recent techniques have used sensitive micromanipulators to measure the force required to break or compress individual flocs, although these techniques have been limited to the measurement of only a few hundred flocs. General trends emerge showing that smaller flocs tend to have greater strength than larger flocs, whilst the use of polymer seems to give increased strength to only some types of floc. Finally, a comparison of the strength of different types of floc (activated sludge flocs, organic matter flocs, sweep flocs and charge neutralised flocs) has been made highlighting differences in relative floc strength.

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