Process parameter control and routine operation and maintenance of vertical sealed aerobic fermentation tanks

 

For a farm, purchasing a fermenter is not the hardest part—using it well, keeping it stable, and turning it into real profit is where the true skill lies.

The author recently obtained a set of factory operating requirement documents for a certain brand of fermenter. Combined with field experience from actual operation sites, the following key technical details have been organized. If you are currently evaluating equipment, or have already installed a fermenter but are experiencing unstable output quality, this article is worth reading.

I. Temperature Control: Higher Is Not Always Better

The core of aerobic fermentation is microbial activity, and the temperature range where microorganisms are most active is between 55°C and 65°C. In actual operation, attention should be paid to temperature readings at three locations—the upper, middle, and lower sections of the tank.

The normal temperature distribution is ”high at the top, low at the bottom.” A middle-section temperature maintained between 55°C and 60°C is generally considered ideal. This temperature window ensures the metabolic rate of aerobic bacteria while avoiding temperatures high enough to kill the microbial population. The system's overall safety temperature limit is 70°C—if this value is exceeded, there is a risk of igniting the biogas inside the tank. This is not an exaggeration; it is clearly stated in the operating manual.

What should be done if the temperature exceeds the limit?

The operating procedure is: first turn on the spray pump (the spray tower water pump of the deodorization system), then start the high-pressure blower at the bottom of the tank, blowing into the tank for approximately 15 minutes. The temperature generally returns to around 55°C. Note that the sequence must not be reversed—if the blower is started before the spray pump, exhaust gas without a water film will be discharged directly, and odor complaints from the surrounding area will be virtually unavoidable. After the temperature drops, turn off the blower first; the spray pump should only be turned off after the deodorization blower has also been stopped.

Some operators take shortcuts and simply open the observation hatch on the tank top to dissipate heat when temperatures rise. This method works for emergency situations, but doing this long-term has two consequences: first, the oxygen level in the fermentation environment becomes unstable, affecting microbial activity; second, odors escape into the atmosphere, failing environmental compliance.

II. Moisture Content: Two Critical Control Points for Feedstock and Output

The moisture content requirement for feedstock is ≤65%. In actual practice, pig manure typically has a moisture content of 70% to 80%, so before loading, bulking agents such as straw powder, rice husks, and wood shavings must be mixed in to adjust moisture and the carbon-to-nitrogen ratio. After mixing, squeeze a handful of the mixture—if it forms a ball but no water seeps between the fingers, it is ready to be loaded into the tank.

Output material is only considered qualified when moisture content is controlled at around 30%. The testing method is crude but effective—grab a handful of the output material, squeeze it into a ball, and then see if it can be easily crumbled. If it crumbles easily, the degree of maturation is sufficient; if it comes out as wet, sticky lumps that feel like modeling clay and cannot be crumbled by hand, fermentation is incomplete and discharging should be stopped immediately.

The opening of the discharge port also requires careful attention. Normally, an opening of 10 to 20 centimeters is sufficient. Opening it too wide can cause upper-layer material that has not been fully fermented to be carried out together with the finished product, compromising quality. When discharge is not smooth, the discharge door can be opened and closed slightly several times, but frequent repeated operation should be avoided—the hydraulic system is highly sensitive to such rapid reciprocating action, which can easily trigger pressure alarms or even damage the discharge hydraulic components.

III. Feedstock Input and Output Ratio

The industry empirical value is 3:1—that is, approximately 1 cubic meter of semi-finished organic fertilizer can be produced from 3 cubic meters of fresh manure after 7 to 9 days of in-tank fermentation.

This ratio fluctuates depending on the type of raw material. Chicken manure, due to its relatively lower moisture content (generally 50% to 60%), yields a slightly higher output ratio; dairy cow manure, with its high fiber content and greater moisture, tends to yield a lower ratio. If the output volume is found to be significantly lower than expected (for example, 4:1 or even lower), this usually indicates that moisture content was not properly controlled—water evaporated inside the tank rather than being converted into fertilizer.

The material layer height inside the tank must also be controlled. The maximum level should not exceed 1 to 1.2 meters from the tank top. Overfilling leads to inadequate mixing, and the load on the central main shaft and mixing blades increases significantly; underfilling wastes thermal energy—although the fermenter's insulation layer reduces heat loss, the tank's own heat storage capacity is fixed, so the less material there is, the higher the unit cost.

IV. Hydraulic System and Mixing: Three Daily Inspection Items

The mixing system is driven by a hydraulic power unit, using 46# wear-resistant hydraulic oil. Before daily operation, there is a non-negotiable inspection checklist:

First, check the hydraulic oil level. If the oil level is low, it indicates possible leakage or aging seals—simply adding oil is not sufficient; the root cause must be identified. Topping up when low is basic practice, but frequent oil loss always indicates an underlying problem.

Second, check the pressure gauge. Under normal operating conditions with a normal material level in the tank, the hydraulic system's operating pressure should remain stable. If pressure suddenly rises—the system's alarm setpoint is 15 MPa—it is most likely that a single-side cylinder is advancing (one pawl is pushing while the other has not returned to position). In this case, the machine must be stopped immediately for inspection. Do not push through it, as the result of forcing operation is often the scrapping of the main shaft or the ratchet mechanism.

Third, listen to the sound. Normal operation produces a uniform, rhythmic ”click-click” sound (the sound of the ratchet mechanism advancing). If irregular abnormal noises are heard—such as metallic impacts or dull ”clunk-clunk” sounds—it is highly likely that the mixing blades are scraping against the tank wall or that foreign objects are lodged in the mixing system. In such cases, stop the machine first, look through the observation port on the tank top, and only restart after confirming safety.

Additionally, lubrication must not be skimped on. The grease pump should be operated at least once daily to ensure that every lubrication point has grease overflowing. Particular attention should be paid to the lubrication point on the main shaft bearing at the tank bottom—that bearing uses a copper sleeve with graphite lubrication structure. If grease supply is not maintained, the main shaft's service life will be drastically shortened, and the cost of replacing one main shaft is enough to buy half a year's supply of grease.

V. Deodorization System: A Critical Link Easily Overlooked

The deodorization of fermenters generally follows a two-stage approach: ”water spray tower + biofilter.” The water in the spray tower should be changed every 1 to 3 days. During hot summer weather, daily changes are recommended. Microbial deodorizing agents can be added to the water, which perform considerably better than plain water.

The ventilation ducts also require daily inspection. During the fermentation process, dust and debris are generated inside the tank and will accumulate over time, eventually clogging the ducts. Once ventilation becomes inadequate, oxygen supply inside the tank is insufficient, and aerobic fermentation turns into anaerobic fermentation—not only will the output quality suffer, but malodorous gases such as hydrogen sulfide and ammonia will also be produced.

The exhaust from the chimney should also be observed. Normal exhaust should appear as white water vapor (steam) with virtually no noticeable irritating odor. If the exhaust has a yellowish tint or a distinct ammonia smell, this indicates incomplete fermentation or an imbalanced carbon-to-nitrogen ratio in the feedstock—the formula needs adjustment.

VI. Safety and Maintenance: Several Easily Overlooked Details

The steel cable of the loading hoist is one of the most failure-prone areas. The operating documentation specifically emphasizes daily inspection—particularly at the load-bearing bend position of the loading hopper, where the steel cable is prone to broken wires and strands. Once broken strands are detected, do not entertain the wishful thinking of ”pushing through a few more days”—stop the machine immediately and replace the cable. A hopper loaded with one ton of material falling from a height of over three meters is no laughing matter.

Another concern is foreign objects in the raw materials. Before loading, materials must pass through a screen or be visually inspected by designated personnel. Feed bags, plastic film, stones, and wire—if any of these enter the tank, at best they will wrap around the mixing blades and force a tank cleanup; at worst, they can damage the mixing shaft or even tear the tank's weld seams. The cost of one tank cleanup (downtime, labor, and restart) ranges from several thousand to tens of thousands of yuan.

The fermenter must be completely emptied and cleaned at least once per year. It is not enough to simply discharge the finished material—the tank door must be opened and personnel must enter to inspect whether the mixing blades have any tangled material, whether the vent ports are clogged, and the wear condition of the main shaft bearing. This one day of shutdown for maintenance pays for 364 days of stable operation.

VII. Installation Foundation: Things to Consider Before Equipment Arrival

Finally, a note on installation. A common problem on many farms is: the equipment is purchased and the site is selected first, only to discover later that the foundation does not meet specifications.

The foundation requirement is a concrete platform measuring 8.5 meters in length, 6.2 meters in width, and 0.8 meters in height, with a load-bearing capacity of at least 25 tons (the heaviest component weighs approximately 13 tons and is 6 meters in diameter). Three-phase power must be run to the equipment location, fitted with a 300A circuit breaker. For models FM-90 and above, the cable must be at least 4-core with 70 mm² cross-section.

Hydraulic oil (46# wear-resistant hydraulic oil, approximately 400 liters) and lubricating grease (0# grease, approximately 10 kg) should be prepared in advance. CO₂ gas-shielded welding equipment and at least two certified welders are required during installation. These may seem like minor details, but missing even one of them will stall the entire installation schedule.

Conclusion

A good piece of equipment, if not used properly, is only half as good. The fermenter—its principle is not complicated: heating, mixing, aeration, and time—but producing consistently high-quality organic fertilizer from every batch requires day-in, day-out standardized operation. Checking temperatures a few more times, applying grease a few more passes, inspecting the steel cable more frequently—these seemingly insignificant routine efforts ultimately reflect in the equipment's service life and the quality of the output.