A vertical in-vessel fermentation tank composts manure in an enclosed, aerated and stirred vessel, reaching thermophilic temperatures that break down organic matter and reduce pathogens in days rather than weeks. Good results depend on adjusting feed moisture to about 55–65% and the C/N ratio to about 25–30:1, and on treating the exhaust air in a deodorization system.
What is a manure composting fermentation tank?
Key takeaway: A vertical in-vessel fermentation tank is an enclosed, insulated reactor that mixes and aerates manure so aerobic microorganisms heat it into the thermophilic range and stabilize it quickly. With feed adjusted to roughly 55–65% moisture and a C/N ratio of about 25–30:1, the active phase typically takes about a week, and enclosed exhaust air can be treated to control odor.
Livestock and poultry farms produce large volumes of manure that must be managed to protect water, air and neighbors. Composting turns it into a stable, lower-odor soil amendment that can be used or sold as organic fertilizer. This guide explains how vertical in-vessel tanks work, which process parameters matter, and how they compare with windrow and trough composting.
How does a vertical in-vessel fermentation tank work?
The tank is a cylindrical, insulated steel vessel with a central rotating shaft carrying stirring arms. Air is blown through the shaft and arms into the material, and an exhaust collects the warm, moist air from the top.
A typical operating sequence:
- Pre-mixing: fresh manure is blended with bulking material (and sometimes recycled compost) to adjust moisture and C/N ratio.
- Feeding: the mixture is lifted into the top of the tank by a bucket elevator or conveyor, usually once a day.
- Aerobic fermentation: forced air and slow stirring keep the material oxygenated; microbial activity raises the temperature.
- Plug flow: material gradually moves downward through warmer zones as new feed is added on top.
- Discharge: fermented material is discharged from the bottom outlet, usually daily, after its residence time in the vessel.
- Exhaust treatment: air from the top of the tank is drawn through a deodorization tower before release.
- Curing and processing: the discharged material is aged, then screened, and may be granulated or bagged.
Many tanks include auxiliary heating to help start-up in cold weather, and a control panel that monitors temperature and runs the aeration and stirring cycles.
What are the key process parameters?
Composting is a biological process, so control comes from giving microorganisms the right conditions.
| Parameter | Typical target | Why it matters |
|---|---|---|
| Moisture | About 55–65% | Too wet blocks air flow; too dry slows microbial activity |
| C/N ratio | About 25–30:1 | Too low releases ammonia; too high slows decomposition |
| Oxygen | Aerobic conditions throughout | Prevents anaerobic odors such as hydrogen sulfide |
| Temperature | Thermophilic, roughly 55–70 °C | Drives fast decomposition and pathogen reduction |
| pH | About 6.5–8.5 | Very high pH increases ammonia loss |
| Particle size / structure | Porous, crumbly mix | Allows air to move through the material |
These are commonly accepted composting ranges; the best values for your manure should be confirmed during commissioning.
Moisture adjustment
Fresh manure is often 70–85% moisture, depending on the animal and housing system. Mixing in dry bulking materials — straw, sawdust, rice husk, mushroom residue — or recycled compost brings it into the working range. Where manure is very wet, a screw press separator or other sludge dewatering equipment can remove water before composting and reduce the amount of bulking material needed.
C/N ratio
Poultry and pig manure are nitrogen-rich; cattle manure is somewhat more balanced. Carbon-rich amendments raise the ratio. If the ratio is too low, nitrogen escapes as ammonia, which both causes odor and reduces the fertilizer value of the product.
The thermophilic phase
Once conditions are right, temperatures typically rise into the thermophilic range within a day or two of feeding. Holding the material at elevated temperature is what reduces pathogens and weed seeds. Many compost standards specify minimum temperature-time combinations (for example, a set number of days above 55 °C); check the requirements in your market for the product you intend to sell.
How long is the cycle time?
The active in-vessel phase typically takes about 7–10 days. Material is fed and discharged continuously in daily batches, so a tank's daily capacity is roughly its working volume divided by the residence time.
The discharged material has been substantially stabilized, but many producers cure it for a further period in a covered area before screening and sale to improve maturity and reduce phytotoxicity. Total time from fresh manure to finished fertilizer therefore depends on your product specification as well as the tank.
How is odor controlled?
Odor is the most common reason composting operations face complaints. An enclosed tank makes it practical to capture all the process air. Typical options:
- Spray (wet) deodorization tower: water or a weak acid solution scrubs ammonia and some odorous compounds.
- Biological deodorization (biofilter or bio-trickling tower): microorganisms on a packing medium break down odorous compounds.
- Combined systems: a scrubber followed by a biological stage for higher removal.
Good process control is equally important: adequate aeration avoids anaerobic zones, and a correct C/N ratio limits ammonia generation. Leachate and wash water from the site should be collected and treated, for example in a compact sewage treatment plant.
What is the output used for?
The discharged material is a stabilized organic product that can be:
- applied directly to fields as an organic soil amendment after curing;
- screened and sold as powdered organic fertilizer;
- granulated, sometimes blended with mineral nutrients, to make organic-inorganic compound fertilizer;
- used as a growing medium component.
Product quality depends on the manure, the bulking material and process control. Sellers must meet the organic fertilizer standards in their market, which typically cover organic matter, nutrient content, moisture, pathogens and heavy metals.
How does a fermentation tank compare with windrow and trough composting?
| Factor | Vertical fermentation tank | Trough (channel) composting | Windrow composting |
|---|---|---|---|
| Footprint | Smallest | Medium | Largest |
| Active phase | About 7–10 days | Typically a few weeks | Typically several weeks or longer |
| Odor control | Enclosed; exhaust can be treated | Partially enclosed in a building | Open; hard to control |
| Weather influence | Low (insulated vessel) | Moderate | High |
| Labor | Low; automated feeding and discharge | Moderate; turning machine operation | Higher; turner operation and pile management |
| Capital cost | Higher per tonne | Medium | Lowest |
| Best fit | Farms with limited space or odor-sensitive neighbors | Medium to large fertilizer plants | Large sites with space and few neighbors |
A vertical tank is usually the strongest option when land is scarce, the climate is cold or wet, or neighbors are close. Windrows remain the lowest-cost option where large open land is available and odor is less of a concern.
How do you size a fermentation tank?
Gather the following before requesting a proposal:
- Daily manure quantity (tonnes/day) by animal type.
- Moisture content of the manure as collected.
- Available bulking materials and their quantities.
- Climate at the site (minimum winter temperature).
- Intended product: field application, powder or granular fertilizer.
- Space, power supply and any local odor or emission rules.
Daily feed volume (after mixing with bulking material) multiplied by the residence time gives the required working volume, which determines tank size or the number of tanks. For example, if 10 m³ of mixed material is fed per day and the residence time is 8 days, the tank needs a working volume of about 80 m³, plus freeboard. Larger farms often install two or more tanks rather than one very large vessel, which gives operating flexibility and allows one tank to continue running during maintenance. Also plan the space and equipment around the tank: a mixer for pre-blending, conveyors or a bucket elevator, a covered curing area and, if you intend to sell product, screening and packing equipment.
Request a composting system proposal
Send us your animal type, daily manure quantity, moisture content and site conditions at inquiry@waste-water-equipment-factory.com, on WhatsApp at +86 176 2154 1688, or through our contact page. Our engineers will recommend tank size, deodorization system and auxiliary equipment from our range of composting equipment and other environmental equipment.
FAQ
How long does manure take to compost in a fermentation tank?
The active in-vessel phase typically takes about 7–10 days, depending on the material and the vessel design. The discharged material is often cured or aged further before it is sold as finished organic fertilizer.
What moisture content is needed for in-vessel composting?
Feed material is typically adjusted to about 55–65% moisture. Fresh manure is usually wetter, so it is mixed with dry bulking material such as straw, sawdust, rice husk or recycled compost.
Do fermentation tanks smell?
Because the process is enclosed, exhaust air can be collected and treated, typically in a spray or biological deodorization tower. This makes odor much easier to control than in open windrows.
Can poultry manure be composted in a fermentation tank?
Yes. Poultry manure is nitrogen-rich, so it usually needs more carbon-rich bulking material to bring the C/N ratio up to the target range and to limit ammonia release.
