If compost could breathe, the double screws compost turner would be its finest personal trainer—it forces compacted material to inhale deeply, stretch, and then exhale fluffy clouds. Without it, aerobic fermentation is like running a marathon with a hand over the mouth.

Step into a modern fermentation facility, and you’ll likely see a steel giant pacing back and forth on rails, its two massive screw augers tirelessly churning the material and releasing plumes of steam. This is the double screws compost turner—the undisputed “heart” of the aerobic fermentation process.

Its invention stemmed from a simple question: How do you rapidly and uniformly compost mountains of livestock manure, straw, and sludge without producing foul odors? The answer: let them breathe freely. Aerobic microorganisms need oxygen to decompose organic matter, generate heat, and kill pathogens and weed seeds. The core mission of the double screws turner is to “force-ventilate and thoroughly mix” the piled material.

What does “double screws” mean? As the name suggests, it features two parallel screw shafts rotating in opposite directions, with continuous spiral blades welded onto them. As the machine travels along the fermentation trench rails, the two shafts rotate simultaneously, lifting the deepest material from the trench bottom forcefully upward and throwing it to both sides. This sequence instantly fluffs the material, greatly increasing its porosity. Oxygen is “pumped” deep into the pile with each turn, while hot steam and excess carbon dioxide are released—one turn equals one deep “breath exchange”.

Never underestimate this action. During the 15-to-25-day fermentation cycle, the turner must make multiple appearances. It rushes in to cool the pile when temperatures exceed 60–70°C; it promotes evaporation when moisture is too high; it acts as a giant fertilizer mixer when raw materials and additives need to be evenly blended. Thanks to the double screws turner, fermentation time can be shortened by more than 30%, and odors are virtually eliminated—anaerobic zones are destroyed, drastically reducing hydrogen sulfide and ammonia emissions.

Modern advanced models take the “breathing coach” role to the extreme. Some screw shafts are designed to be hollow, with numerous air holes on the surface. Integrated with internal fans or heaters, they can force oxygen or adjust temperature deep within the pile while turning  It”s like giving the material a massage while gently blowing warm or cool air into it, keeping the microorganisms in their most comfortable “gym” at all times.

Suitable for: manure, straw, distiller”s grains, sludge Turning depth: 1.5m – 3m Efficiency: >10x faster than manual turning Fermentation cycle: as short as 12 days

Within a complete organic fertilizer production line, the double screws turner usually forms a “trio” with fermentation trenches and a transfer car. By switching rails via the transfer car, a single turner can serve multiple trenches—”one machine, multiple trenches”—greatly reducing equipment investment. The mature compost is then conveyed to the next stage—for instance, the disc granulator we previously explored—to be shaped into uniform pellets. In short, without the deep “training” of the turner, the subsequent granulation would lack the “physical foundation” of consistent, mature material.

So, when you admire the disc granulator making powder dance a waltz, don’t forget the giant machine pacing day and night in the fermentation trench, silently pumping oxygen. It may not be elegant, but it is precisely this powerful “breathing” that gives waste organics a new life, transforming them into the dark, fertile nutrients in our fields.