settle (settlement)

Settling refers to the gravity-driven separation of suspended solids from liquid digestate, in which dense particles migrate downward through the liquid column and accumulate at the base of a tank, lagoon, or pipe section. The behaviour is governed by Stokes' law: settling velocity rises with particle diameter and density difference, and falls with liquid viscosity. In compressed biogas (CBG) plants, controlled settling in lagoons or clarifiers is a low-cost alternative to mechanical separators such as decanter centrifuges or screw presses, producing a thickened sludge at the bottom and a clarified supernatant on top.

The same physics becomes a hazard inside pipework and digesters. When slurry flow velocity drops below the minimum self-cleansing threshold — typically 1.0-1.5 m/s for digestate carrying 5-8% total solids — heavier fractions such as grit, sand, struvite crystals, and fibrous material drop out of suspension and accumulate at low points, bends, and along the bottoms of horizontal runs. Over weeks to months, these deposits harden into cement-like layers that throttle flow, accelerate pipe corrosion under deposit, and impose pressure penalties on pumps.

Operators manage settling through several levers, each with trade-offs:

• Higher flow velocity keeps solids suspended but increases pump power consumption and pipe wear.
• Continuous agitation inside digesters prevents bottom sludge and surface scum but adds 5-15 kWh per tonne of feedstock to the electricity load.
• Periodic flushing or pigging clears accumulated deposits but requires plant shutdown and slurry handling capacity.
• Grit removal upstream traps sand and stones before they enter the digester, protecting both pumps and the active digester volume.

Unmanaged settling inside a digester is particularly damaging because accumulated grit reduces the effective working volume, shortens the actual hydraulic retention time below design, and lowers biogas yield. CBG plants typically lose 10-20% of digester volume to inert deposits within 3-5 years if grit removal and mixing are inadequate, forcing costly desludging campaigns.