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Mechanical Separation of Slurry

Mechanical separation takes coarse solids and fibres out of the slurry. The liquid fraction will have 1 - 6 % dry matter and so have the following possible advantages:-

• Crusts or sediment are less likely to form during storage and the volume of the liquid that needs to be stored is reduced by about 20%.
• Liquids are easily pumped and can be spread by a travelling irrigator, 70 - 80% of the plant nutrients from the slurry will be in the liquid fraction.
• You can apply it to growing crops with less risk of smothering them or of solids building up.
• If you want to use aerobic treatment the power needed to do this is reduced.
• Separated solids are either spread on the land as solid manure's or composted and sold as a soil conditioner if you can find a market for it.
• Separation adds an extra treatment, more equipment and costs. You will still need storage facilities for the liquid. There are a variety of separator types. These can use perforated screens, perforated belts or the principle of the centrifuge.
• Slurry that needs to be separated is collected to one point in a tank. You might need to dilute and mix cattle slurries or slurries containing bedding before separating them. A pump is used to move liquids from the tank to the separator. The separator is often up on a platform so solids can fall into a heap or trailer and liquids can flow down to a store.

Choice of mechanical separator

For pig slurry of 2 to 4% dry matter a wedge wire screen or vibrating screen will work satisfactorily. The dry matter content of the separated solids will be 8 to 12%, these solids will self-drain if they are held in a suitable store.

Separators which press, squeeze or screw the slurry against a fabric belt or perforated stainless steel screen will produce a solid with dry matter ranging from 18 to 30%, these solids can be composted.