Types of Manure Separation
March 8, 2000 ISSUE #M4
There are four different types of machines commonly seen separating dairy manure: sidehill screens, drum screens, drag flight conveyors, and screw presses.
Sidehill screens are the least expensive and require the least maintenance. These are simple sloped screens (alternatively called static and gravity screens) with a weir box at the top. The manure is pumped to flow over the dam of the weir box and down the screen. The filtered liquid goes through the screen, while the solids (tailings) fall off at the bottom.
The disadvantage of a sidehill screen is that even under the best of operating conditions the solids come off as a wet sludge. This sludge is almost too wet to compost, and it is difficult to handle for spreading on the field. Additionally, sidehills tend to blind over, requiring a farm hand to wash down the screen. A sidehill will keep a lot of solids out of the pond, but it is definitely not a strong manure separator.
Drum screens use a drum made of perforated metal to separate the solids from the liquid. Alfa Laval previously offered one, and Houle currently has a unit in their line. These will produce drier cake than a sidehill. However they are rarely popular. Maintenance is frequent because of all the moving parts and adjustment; screens can be damaged by large solid tramp materials; and they are exceptionally dirty.
Drag flight conveyor separators come in two varieties. The Blossom and Agpro machines are single pass: the lower end is submerged in the manure pit, and the solids are dragged up perforated or wedgewire screens to where they fall into a small screw squeezer. The Albers unit is a double pass design. The manure starts by being dragged down a sidehill. It makes a U-turn in a sump and is dragged back up to where it falls into a roller squeezer.
Drag flight separators have the disadvantage of many moving parts: chain, sprockets, drag flights, bushings, besides an optional squeezer at the discharge. Leakage and large size are negatives. The manure cake is dryer than achieved with a sidehill screen, but not up to that produced by a screw press.
The screw press, which produces the driest cake, is the fourth category of manure separator. These are designed to operate intermittently, unattended, with a minimum of maintenance. They will be further described in later Pressing News issues.