Screw press or decanter centrifuge (screw press separator)

A screw press, also called a screw extruder press, is a mechanical solid-liquid separator used to dewater digestate from anaerobic biogas plants. It works by feeding liquid digestate (4–10% dry matter) into a perforated cylindrical screen, where a slow-rotating screw progressively compresses the material against the screen. Free water passes through the perforations and exits as a liquid fraction, while solids accumulate, are further compressed against an adjustable end cone, and discharge as a stackable solid cake.

Screw presses typically produce a solid fraction at 20–30% dry matter and a liquid fraction at 4–8% dry matter. Throughput depends on screen size and feedstock — a 7.5 kW unit handles 3–8 m³/hr of digestate, sufficient for a 5–10 TPD CBG plant. Capex runs ₹15–35 lakh for the unit plus ancillaries (feed tank, polymer dosing, conveyors). Energy consumption is modest at 0.5–1.5 kWh per m³ processed.

The alternative technology is the decanter centrifuge, which uses high-speed rotation (3,000–4,500 rpm) to fling solids to the outer wall of a horizontal bowl, where a scroll conveyor removes the cake at one end while clarified centrate exits at the other. Decanters achieve higher dry-matter content in the solid fraction (25–35% DM) and lower solids in the centrate (1–3% DM) than screw presses, but they cost 3–5× more (₹60 lakh to ₹2 crore), draw 3–5× more power (3–6 kWh per m³), and require a polymer flocculant for stable operation. Decanters are the choice when downstream processes — such as evaporation, pelletising, or composting — need very dry input, or when liquid fraction is to be discharged to land or further-treated for surface release. Screw presses dominate the Indian SATAT-scheme small and mid-scale segment because of their lower capex, simpler operation, and ability to handle fibrous agro-residue digestate that would otherwise foul a centrifuge. The trade-off is straightforward: capex versus dry-matter content and centrate clarity.