Why can’t the screening process for bio-organic fertilizer be designed the same way as it is for conventional organic fertilizer?

While screening is a standard procedure in conventional organic fertilizer production, it faces additional constraints when applied to bio-organic fertilizer: because the material contains live microbial cultures, the screening process must be precise without damaging the microbes or causing contamination.

1. Handling wet, sticky materials: Preventing screen blinding

After granulation, bio-organic fertilizer typically retains a moisture content of 15%–25%. Due to the high stickiness of the organic matter, fine-mesh screens on standard vibrating sieves tend to clog (blind) rapidly. It is preferable to use a rotary screener machine or a vibration screener machine equipped with a "bouncing ball" mesh-cleaning system. The mesh size should be selected based on the target granule size—typically 8–20 mesh; using a mesh that is too fine will only exacerbate clogging.

2. Multi-stage screening and a closed-loop return system

Since the yield of qualified product is generally between 85% and 95%, the process must separate the output into three streams: finished product, oversized particles, and fine powder. The oversized particles and fines are routed back into the granulation or crushing stage, creating a closed-loop system. Dedicated return inlets should be used to prevent cross-contamination with fresh, incoming raw materials.

3. Sealing, dust control, and preventing microbial contamination

Dust generated during screening contains live microbes, which can pollute the workshop air and lead to cross-contamination between production batches. The fertilizer screener machine should be fully enclosed and fitted with a pulse-jet dust collector, with flexible connections at the feed and discharge points. When switching between batches, the screen drum and chutes must be easily accessible for rapid cleaning, ensuring no "dead zones" remain.

4. Gentle handling to minimize mechanical damage

Impact (dropping) and compression can damage the surface of the granules, negatively affecting the environment required for the microbes to survive. To reduce granule breakage, the inclination of chutes should be minimized, and belt conveyors should be used instead of screw conveyors (augers) for material transport. In summary: opt for a rotary screener machine or an anti-clogging vibration screener machine with an 8–20 mesh size; implement a three-stage screening process with a closed-loop return system; ensure the equipment is fully sealed and equipped with dust collection; prioritize gentle transport to minimize drops; and design for easy cleaning to prevent cross-contamination.