Dewatered (Dewatered)

Dewatering is the mechanical separation of digestate into a solid cake and a liquid filtrate (or centrate), reducing the volume of digestate that must be transported, stored or further processed. Raw digestate from a wet anaerobic digester typically contains 3-7% total solids — it is mostly water by mass. Dewatering raises the solid cake to 20-30% TS and produces a separate liquid stream that can be recycled to the digester for moisture management or land-applied directly as a nutrient-rich liquid fertiliser.

Three mechanical technologies dominate Indian practice. Screw presses compact slurry through a tapered screw against a perforated screen; they are simple, low-energy, suit fibrous digestate, and achieve 22-28% TS in the cake. Capex is ₹15-40 lakh for plant-scale units; throughput is 1-5 m³/hour. Decanter centrifuges spin slurry at high speed in a horizontal bowl, separating denser solids from liquid; they handle finer particles, achieve 25-32% TS, but consume more energy (5-12 kWh/m³ versus 1-3 for screw press) and cost ₹40-150 lakh. Belt-filter presses squeeze slurry between two porous belts under tension; they suit specific slurries with prior flocculation, achieve 18-25% TS, and are common in sewage-sludge applications but less so in CBG.

The economic case for dewatering is straightforward. Raw digestate at 5% TS contains 95% water that costs ₹3-7 per tonne-km to transport. A 5 TPD CBG plant produces roughly 80-120 m³/day of raw digestate; dewatering this to 25% TS reduces solid-cake volume by 75-80%, cutting transport cost by a similar fraction and effectively quintupling the economically viable delivery radius for fertiliser sale.

The trade-offs centre on liquid-stream management and polymer use. The liquid filtrate, containing 1-2% TS and most of the soluble nitrogen and potassium, must go somewhere — recycled to the digester as moisture make-up (reducing freshwater consumption by 40-60%), applied to land as liquid fertiliser (requiring tanker logistics and FCO compliance), or treated in an effluent plant before discharge. Polymer flocculants used to improve dewaterability cost ₹150-400 per kg at 2-5 kg/tonne dry solids, adding ₹500-1,500 per tonne to operating cost — a real expense that must be balanced against the transport savings.