The excessive use of phosphorus (P) contributes to the depletion of resources and can lead to eutrophication of surface waters. We developed a new phosphorus recycling process at lab scale on treated pig manure. Several parameters determine the quality of the final effluent as well as the quantity and the composition of the solid. In this study, the influence of the HRT (hydraulic retention time) and the ratio N:P were tested in an experimental pilot lab scale designed using synthetic pig effluent. P precipitation occurred in the reactor by adding magnesia (MgO), followed by a separation of struvite crystals on a sieve. The phosphorus abatement rate was 91-100 %. A longer residence time promote the growth of crystals and their agglomeration. Likewise, a high ratio N:P influenced positively the crystal size and the proportion of struvite in the solid. To maximise struvite purity, the ratio N:P should be more than 1:1.However this ratio should not be too high to avoid a discharge of N in the surface water and to comply with the Nitrates Directive. An HRT of 12 hours is a good deal to treat a large volume of slurry while keeping a suitable crystal size for the filtration step.