High fertilizer prices have hog producers scrambling to maximize fertilizer value of their manure. Pit slurry systems are the best at capturing the fertilizer value of manure. In contrast anaerobic lagoon systems have been designed to minimize the nutrients applied in un-agitated lagoon effluent. Typically only 10% of the nitrogen, 5% of the phosphorus and 60% of the potassium excreted by the pig is in un-agitated lagoon effluent.
Our objective here is to list possible options for farmers with lagoon systems to increase nutrients available for land application. We are not endorsing any specific approach nor have we addressed the economics which may be prohibitively expensive for some of the listed approaches.
All farmers with a lagoon can increase nutrient value of the manure from un-agitated lagoons by agitating the lagoon. Lagoon agitation allows you to access all the phosphorus and potassium excreted by the hogs depending on how efficiently you re-suspend the solids in the lagoon. Agitation will also remobilize organic nitrogen in the bottom of the lagoon increasing nitrogen concentration in the effluent. Factors to consider include:
o Difficult to fully agitate a lagoon to access all nutrients.
o Incomplete agitation creates variability in the nutrient concentration during application reducing fertilizer value.
o Difficult to predict ahead of time manure nutrient concentration of agitated lagoon effluent.
o Results in a material with a low N:P ratio.
All farmers with a lagoon also have the option of periodically dredging sludge from the bottom of their lagoon. Factors to consider include:
o Expense of dredging?
o Nutrient concentration can vary during dredging reducing fertilizer value.
o Difficult to predict ahead of time nutrient concentration of dredged sludge.
o Dredged sludge is a phosphorus rich product with quite low N:P ratio.
Both of these scenarios have the advantage that they require no engineering changes to your buildings and allow you to continue managing your barn as you have done in the past.
Other options for increasing nutrient recovery from lagoon systems are more specific to the type of manure handling system used in the barn. Two types of manure removal systems are commonly built into hog facilities using a lagoon for manure storage: I. Pull-plug gravity drain gutter systems and II. Sloped gutter flush systems. We will address these separately:
I. Pull plug gravity drain gutters emptying into a lagoon.
These systems offer the opportunity to convert to a slurry-based manure handling system without changing manure handling in the barn. There are two possible options:
1. Eliminate the lagoon and convert to a slurry system by building an outside tank and drain the building pits into the new tank instead of the lagoon.
2. Alternatively build a transfer tank that has a minimum capacity capable of holding at least one cycle of emptying the barn gutters. This slurry manure could be agitated and spread when the tank is full. Alternatively the tank could be managed to let the solids settle while the liquids are decanted to the lagoon. The settled solids would be spread separately when the tank is full of settled solids. The anaerobic lagoon would be maintained for decanted liquid storage and as backup storage capacity for the entire system.
II. Sloped gutter flush systems.
The challenge in this situation is that you need to maintain a source of flush water unless you can re-engineer the building to use a different method to remove manure from the barn.
Options for systems that continue using recycled flush water:
1. Solid separation with flush systems is relatively ineffective for hog manure systems and unlikely to extract many nutrients or much volume.
2. Build two cement tanks. The first large enough to hold at least six to 12 months of manure production. The second should be large enough to supply at least a week of flush water volume. Pump lower solids liquid off the top of the main manure tank into the second tank after a period of time for solids settling (1 to 3 days with no manure addition). The second tank is mechanically aerated to reduce odors and polish (improve quality of) recycle flush water. This approach may increase challenges with struvite buildup in the recycle water pumps and piping systems.
Your one option for eliminating the use of flush water to clean the barn is to retrofit flush gutters to a mechanical scraper system. You will end up with two manure streams: a liquid stream will continually occur and the solids stream would occur when the gutter is scraped. Factors to consider include:
o This approach would require re-engineering the bottom end of the barns to collect both streams of manure and transfer them to a slurry manure structure that would replace your lagoon.
o Odor generation from dry-scraped gutters often degrades indoor air quality. Insufficient space exists to reasonably install a pit ventilation system to help mitigate the odors to improve building indoor air quality.
There is no way to reasonably convert a sloped gutter recycle water flush system into a gravity drain gutter system. Typically the gutter slope is too great and the gutter depth is insufficient to create a gravity drain basin system.
The value of the nitrogen, phosphate and potassium excreted by a pig has a fertilizer value of close to $14.00 per pig. In any manure handling system some of the nitrogen will be lost in the barn and during handling plus some organic nitrogen never becomes available to the plant. The highest potential nutrient recovery is with slurry systems where the manure is injected. Fertilizer value in these systems can reach $10.30 per pig.
The lowest nutrient recovery is with surface applied un-agitated lagoon effluent. Fertilizer value in these systems can drop below $2.00 per pig. The difference, over $8.00 per pig, is the potential return on investment from increasing manure value in a lagoon system. To get these returns assumes that you apply manure to crops that can utilize all the nitrogen, phosphorus and potassium in the manure.
Assumptions for above calculations: Grow-finish pigs are raised from 40 to 280 pounds with an average daily gain of 1.75 pounds, fed a diet that averaged 13.7% crude protein, 0.37% phosphorus and 0.55% potassium and fertilizer prices of $0.70 per pound nitrogen, $1.10 per pound phosphate and $0.55 per pound potash.
Submitted by:
John A. Lory
Joe Zulovich
University of Missouri
