Anaerobic Digester Product

Overview

An anaerobic digester is a biological reactor that runs on the same microbial consortium found in a cow's rumen, scaled to a tank the size of a small swimming pool stack. Organic feedstock — manure slurry, energy crops, food waste — is held warm, sealed, and stirred for several weeks while four guilds of microorganisms successively hydrolyze, acidify, and finally methanize it. The outputs are biogas at roughly 55% methane, burned in a CHP engine or sent to an upgrading plant, and digestate, a stabilized slurry that returns to the fields as fertilizer with most of its nitrogen intact and its odor and pathogens largely destroyed.

The farm-scale standard is the mesophilic CSTR: a Digester Tank of 1000–4000 m³ run at 38–42 °C with a hydraulic retention time of 25–40 days. Throughput is set by biology, not machinery — push the feed rate too high and the acid-forming bacteria outrun the slower methanogens, the pH slides, and the reactor "sours," a failure that takes weeks to recover from.

The vessel

The Tank Shell is cast concrete or glass-fused bolted steel, wrapped in Tank Insulation because the biology must be kept ~25 °C above ambient year-round at minimum heat cost. The zone above the slurry line sees warm, wet gas with hydrogen sulfide, which attacks concrete as biogenic sulfuric acid; the Gas-Zone Liner protects exactly this band. Access Hatches allow grit removal during the rare shutdowns, and operators check foam and mixing pattern daily through the Sight Glass — a crust forming on the surface is the first visible sign that mixing has degraded.

Keeping it stirred and warm

Mixing serves the biology: it keeps fresh feed in contact with the microbial population, holds temperature uniform, prevents grit settling and scum crust, and lets gas bubbles escape. The Submersible Mixers are propeller units on Mixer Guide Masts so they can be winched up for service without emptying the tank; the slow Paddle Agitator, driven through a Servo Motor and Helical Gear Pair, folds the surface layer specifically against crust. Mixers run intermittently — typically minutes per quarter hour — because methanogens prefer gentle flow and because mixing is a major parasitic load.

Heat comes from the CHP engine's jacket water through the Wall Heating Coil, balanced by the Heating Manifold and tempered by the Heating Mixing Valve so the wall never exceeds ~60 °C and cooks the biofilm against it. Three Slurry Temperature Sensors at different heights confirm the Coolant Pump loop is holding the set point within ±0.5 °C; methanogens tolerate slow drift but punish rapid swings.

Feeding and discharge

Feeding is little and often. The Feed Hopper holds a day of silage, the Feed Auger meters it into the liquid stream, and the Macerator chops fiber below ~12 mm so the Feed Pump — a progressive-cavity unit that handles 12% solids — can push it through the trace-heated Feed Pipe, which enters below the surface so no gas escapes during charging. Each fresh charge displaces digestate out through the Overflow Siphon, a U-trap that keeps the gas seal while liquid leaves. The Screw Press Separator then splits the digestate into stackable fiber and a liquid fraction that the Digestate Pump sends to covered storage.

Gas storage and safety

The Gas Holder Roof doubles as the roof: the Inner Gas Membrane rises and falls with stored gas, while the Outer Weather Membrane, kept tensioned by the continuous Support Air Blower at 2–5 mbar, carries wind and snow. Both membranes clamp gas-tight to the rim at the Membrane Clamp Rail. Storage of a few hours' production decouples the digester from the consumer; the Gas Level Sensor reports the inventory and the hydraulic Pressure Relief Valve protects the membranes at +5/−1 mbar — biogas systems run at pressures a hand can produce, so even small overpressure matters.

On the way out, gas passes the Gas Safety Train: the Condensate Trap drains water from the saturated gas, the Desulfurization Unit cuts H2S from thousands of ppm to under 200 before it can corrode the engine, the Flame Arrester blocks any flashback, and spring-close Gas Shutoff Valves isolate the line on alarm. When the holder is full and the engine offline, the Emergency Flare burns the excess rather than venting methane.

Process supervision

The Monitoring System PLC watches the slow variables that matter: gas composition from the Biogas Analyzer (a falling CH4 fraction signals stress days before anything else), pH from the pH Probe (souring shows below 6.8), level and foam from the Fill Level Sensor, and pressures via Pressure Sensors on gas and slurry. Its main actuation is restraint — when the biology shows stress, the correct response is almost always to feed less and wait.