Struvite (MgNH4PO4·6H2O) solution chemistry was studied in order to understand a struvite scaling problem which exists in the pipe network of a series of 4 effluent lagoons at a piggery. The conditional solubility constant (PS=SMG·SNH3−N·SPO4−P, equilibrium S values (M)) was determined over a range of pH values (6.8-8.5) and solution concentrations of Mg, NH3−N and PO4−P close to field conditions, and at a temperature of 30°C. A comparison of field and laboratory data indicates struvite precipitates from the second lagoon onwards. The solubility data obtained also indicate potential for precipitation as a method of nutrient removal from wastewaters.

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A number of waste solutions from processes operating in Western Australia (anaerobic digester supernatants, facultative lagoon treated piggery and abattoir waste effluents) were characterised chemically and by automated titration to determine acid-base characteristics. The results indicate conditions for optimising the removal of N and P by precipitation (predominantly struvite) as well as the way forward in determining the full scope of N and P waste streams for which recycling by precipitation (either magnesium or calcium based salts) may be feasible. INTRODUCTIONThe solution chemistry of struvite (MgNH 4 PO 4 ·6H 2 O) precipitation from waste solutions is not well described, despite an increasing number of pilot and full scale precipitation plants being developed [1][2][3][4]. This solution chemistry is fundamental to the development of optimal performance of pilot plant and full scale struvite P recovery operations. Ideally, a molar ratio in solution of Mg:N:P of 1 [12][13][14] are some of the more common human and animal wastewater streams which may be associated with P recovery by struvite precipitation. This list is by no means comprehensive and there are numerous wastes not covered including pharmaceutical, chemical and food waste streams, biosolids (e.g. municipal organic waste) and landfill leachates [4,15]. It is apparent from the published component analyses of the above listed waste streams that present with the components of choice (COC) for struvite precipitation (NH 3 -N, PO 4 -P and occasionally Mg) are usually a number of components which react competitively with one or more of these COC (viz. Ca, CO 3 -C and in some cases volatile fatty acids -VFAs). These competing components prevent the ideal 1:1:1 Mg:N:P condition being directly met without the loss of COC by counter reactions. Given this situation, the only realistic option is to adjust pH so that its value is conducive to struvite formation over other products and then introducing a good reactive form of excess magnesium, or some variation of this approach (e.g. alkaline sources of Mg), to allow precipitation of MgNH 4 PO 4 ·6H 2 O to a P or N limited solution state (usually P limited). Similarly, if calcium phosphates (apatites) are the desired P recovery product, a solution pH conducive to these products needs to be achieved, followed by introduction of a reactive form of excess calcium or a variation of this approach.