Mechanical separation of liquid animal manure can be an effective technique for production of a liquid and a nutrient-rich solid fraction. The efficiency of separators depends on the physical and chemical composition of the animal manure. Therefore, the particle size composition was measured for different types of manure before treatment with a decanting centrifuge and a screw press. Storage of pig manure reduces the total dry matter content, and the content of small particles (< 0.0016 mm) is reduced more than the content of large particles. In consequence, the proportion of large particles will increase, while the portion of small particles will decrease. The separation efficiency of the screw press was found to be low, as this separator only retains particles > 1 mm. The decanter centrifuge retained all the particles > 0.02 mm and was therefore much more efficient than the screw press. Separation efficiency was also found to be highly dependent on the type of manure used.