Solid Feeding in Anaerobic Digestion: What is it? When to use it, and how to run it
Solid feeding means delivering higher dry-matter materials into a digester through a mechanical dosing system rather than pumping a slurry. Think silage, crop residues, shredded green waste, poultry litter, or fibrous manures. The goal is simple, get more energy-dense material into the biology in a controlled, safe way that protects mixing, gas yield and plant uptime.
Solid feeding sits on a spectrum from “wet AD with some solids” through to dry anaerobic digestion where total solids are 20 to 40 percent, and systems may be plug-flow or batch tunnels. Understanding where your plant sits on this spectrum is the first strategic choice.
Why solid feeding matters now
Feedstock reality. Many UK plants handle fibrous or packaged wastes, agricultural silages, and manures that are hard to slurry reliably.
Commercial pressure. Gate-fee markets move. Being able to accept a wider range of solid wastes protects revenue. WRAP’s gate fee reports show how AD competes with other treatment routes, so flexible intake helps your margin.
Policy and markets. Biogas and biomethane remain central to circular waste treatment and decarbonisation. Robust, versatile feeding broadens your eligibility for offtake and waste contracts.
Plants that can reliably feed solids widen the funnel of acceptable wastes, improve resilience to market swings and protect specific methane yield.
What is Solid Feeding?
At its core, solid feeding is the metered addition of high-TS substrates via purpose-built equipment into a digester or pre-mix, usually with recirculated liquor. The equipment does four jobs.
Buffer and store a day’s worth of solids under cover.
Meter material at a steady rate matched to biology and mixing.
Condition the substrate. Chop, de-air, pre-mix with recirculate so it sinks and disperses.
Deliver directly into the digester headspace, a pre-mix tank, or a pressurised injection point.
Dry AD plants push this further by running the whole process at 20 to 40 percent TS with plug-flow or batch tunnels. Wet AD usually aims for 2 to 12 percent TS in the digester, so solids are conditioned into a pumpable mix.
Common solid-feeding technologies
Solid-feeding kit is mature and widely deployed. The right choice depends on fibre length, density, contamination risk and desired feed rate.
Walking floor or moving-floor bunkers. High-volume storage that indexes material to an extraction screw. Good for silage and shredded organics. agrikomp.com+1
Auger or screw-dosing units. Compact systems that compress and de-air material as they dose. Often paired with a recirculate ring main. boerger.com
Conveyor and belt feeders. Useful where material is free-flowing and pre-shredded. ZORG
Pre-mixers with recirculate. Many systems wet and chop material with digestate liquor before injection which reduces floaters, helps mixing and reduces air entrainment. boerger.com
Manufacturers publish duty ranges to help specification, for example corn silage dosing at 7 to 9 tonnes per hour with 30 to 35 percent dry solids on a modern auger unit. Treat these as indicative, then prove on your feedstock with a trial.
| Factor | Solid Feeding | Liquid Feeding |
|---|---|---|
| Typical substrates | Silages, fibrous manures, shredded food waste, crops | Slurries, dissolved wastes, screened liquids |
| TS in feed line | 12 to 40 percent into pre-mix, lower if liquored | 2 to 12 percent |
| Equipment | Bunkers, floors, augers, choppers, injection | Tanks, macerators, pumps, valves |
| Pros | Higher VS per m³, flexible intake, lower dilution, potential for higher specific gas per reactor volume | Simple hydraulics, lower risk of blockages, easier CIP |
| Cons | Bridging and ratholing risk, scum layers if poorly conditioned, higher wear on screws and seals | Dilution lowers organic loading rate, transport of water, may limit feedstock choices |
Design choices that make solid feeding work
1. Pre-treatment and preparation
Shredding and size reduction. Target a chop length that avoids ropey tangles and wraps around shafts.
Depackaging where relevant. Film and flexible plastics cause floaters that resist mixing.
De-airing. Pressing the mat to expel trapped air helps the feed sink. Many auger systems do this by design.
2. Pair the feeder with the biology
Hydraulic retention time and OLR. Higher VS per cubic metre lets you raise organic loading rate, but only step up once volatile fatty acids are stable and alkalinity is healthy.
Recirculate integration. A dedicated line for hot digestate liquor aids wetting, reduces oxygen ingress and helps push through sticky plugs.
3. Think TS strategy, not just hardware
Wet AD with solids. Keep reactor TS modest, use liquor in the pre-mix to avoid scum.
Dry AD. Embrace higher TS with plug-flow or batch systems, plan for percolate recirculation and leachate handling.
The feeder is one part of a system. Success depends on pre-treatment, liquor management and a measured ramp-up of loading rate.
Compliance, quality and digestate. Do not leave money on the table
If you are handling food waste or animal by-products, your feeding and pasteurisation steps must align with your quality and compliance plan. In the UK, PAS 110 underpins the Biofertiliser Certification Scheme and the Government’s digestate resource framework points directly to PAS 110 as the reference standard. Your sampling, pathogen control and HACCP should reflect the materials you feed and the way you feed them
Troubleshooting. What to do when solids fight back
Floating layer or scum
This usually points to trapped air, plastic films, long fibre, or insufficient mixing. First, increase liquor injection during dosing so the feed is wetted and sinks, run the de-air compression if your auger provides it, and temporarily raise mixer duty to disperse the mat. Longer term, improve pre-shredding to shorten fibres, stop films at intake, tune the auger compression for better de-airing, and consider adding a surface-breaking liquor jet.
Solution - Remove air, shorten fibre, and help the feed disperse quickly.
Bridging in the bunker
Over-dry material, long fibre, or poor levelling are the usual culprits. Safely rake the surface and adjust pusher speed to keep material moving as an immediate fix. For a durable solution, change chop length to prevent mats, blend with wetter material to improve flow, and re-calibrate the pushers so the floor indexes smoothly.
Solution - Control moisture and fibre length, then match the pusher duty to the material.
Rising VFAs after introducing a new waste
This is often over-rapid ramp-up or weak characterisation. Pause solid dosing and switch to liquid feed for 24 to 48 hours while biology catches up. Then strengthen your feedstock matrix, tighten acceptance testing, and reintroduce the material with smaller doses spread across more cycles.
Solution - Stabilise first, then come back with “little and often”.
High motor current on the dosing screw
Expect a foreign object, compaction, or dull knives. Stop, lock out, and clear the obstruction, then inspect or replace knives before restarting. To prevent repeats, tighten intake controls, add a stone trap or magnet ahead of the auger, and keep a maintenance schedule for blades and bearings.
Solution - Protect the machine from debris, keep cutters sharp, and monitor load trends.
Grit accumulation
Sand, eggshells, and soil settle in low points and rob mixing energy. Increase purge frequency and inspect known grit pockets as a quick response. For the long haul, fit a grit trap on the intake, adjust pipework to avoid dead legs, and work with suppliers to limit contamination at source.
Solution - Capture grit early and design out settling zones.
Most solid-feeding issues trace back to three factors. Fibre length, trapped air, and inconsistent dosing cadence. If you control those, most problems shrink to routine adjustments.
Frequently asked questions
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No. Many wet CSTRs successfully feed solids by conditioning in a pre-mix with hot liquor, keeping reactor TS modest while capturing the energy density of solids.
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For wet AD, operate the reactor near 8 to 12 percent TS and manage solids in the pre-mix. For dry AD, design the process around 20 to 40 percent TS with plug-flow or batch tunnels.
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Only if fibre wraps or floating layers build up. Control chop length, de-air the feed and sync dosing with mixer duty to protect assets.
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Possibly. If your intake changes to include new ABP categories or packaged food wastes, review your HACCP, sampling and pasteurisation validation to remain aligned with PAS 110 and regulatory guidance.
Need support with Solid Feeding? Contact our team today.
If you are assessing solid feeding or planning a conversion, book a short optimisation review with our team. We will benchmark your intake, recommend a feeder and dosing strategy, and map the compliance steps so digestate quality and output stay protected.
