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In some intensive animal production areas, the accumulation of nutrient surpluses (N, P,) from livestock effluents has led to severe pollution problems (water, air, soil).

The control of this potential pollutant load requires the development of processing methods to remove the excess nutrients. In France, biological treatment based on aeration (nitrification/denitrification) is the most widespread technology on farms. Nowadays this treatment has to be adapted to include phosphorus removal as well as nitrogen removal.

For this purpose, the characteristics of pig slurry from different farms (with or without phytase in pig diets) were studied through phosphorus, magnesium and calcium fractionation including ortho-P, organic dissolved P, precipitated P, biomass-P and residual-P. Moreover, the influence of mechanical separation (press-auger and centrifugation) was studied.

In raw slurry, 4-10 % of phosphorus was soluble, 60-85% was precipitated and 3-20% was phosphorus linked to the biomass. The total phosphorus concentration is slightly decreased when diets with phytase are used (12%). Without phytase, around 20% of total phosphorus was residual, i.e. on a very insoluble form, probably as calcium phytate. Up to 50% of this form remained after the separation step. No residual phosphorus was found with phytase in the diet.

Both separators studied (press auger and centrifugation) did not affect the concentration of soluble compounds in the separated slurry. When TSS concentration in raw slurry is high (>3,5%), the abatement of the TSS concentration is similar with centrifugation or press-auger. In contrast, centrifugation and press-auger decreased the total phosphorus concentration in the effluent by up to 50% and 15% respectively. Most of the phosphorus removed by centrifugation is precipitated phosphorus.

The difference between the amount and the quality of the phosphorus present in the effluents for each type separator should be considered in proposing a relevant dephosphorisation strategy for pig slurry.