Prior to the World Pork Expo I met with a group of Brazilian equipment dealers to review US swine ventilation for both curtain and tunnel wean-finish facilities. Part of the presentation included a discussion of deep pit manure storage versus anaerobic lagoon storage.
After returning to Brazil, one of the dealers contacted me for help in understanding the sizing of our deep pits. He apparently has several producers who are considering deep pits over fully slatted floors as an option to lagoons in order to better utilize the manure as a fertilizer resource.
Have you ever thought about why our grow-finish pits are 8 ft deep (other than this is the size of the standard cast concrete form)? Here is the math.
A typical grow-finish facility grows about 2.7 groups of pigs per year though the facility. Data from the 2015 MetaFarms grow-finish summary says each group of pigs averages 120 days on feed. This then becomes 324 days/year with pigs producing manure. The rest of the days are used up with 3-4 week market periods, cleaning, repairs, slow fills, etc.
If the facility has wean-finish bowl drinkers (AquaChief, Drik-o-Mat, Kane, etc) or wet/dry shelf feeders (think CrystalSprings and their copies) manure production averages about 1 gal/pig/day. I have had producer reports as low as 0.8 gal/pig/day. At 1 gal/pig/d this means each pig space generates approximately 324 gallons of manure per year.
A typical grow-finish facility is permitted and contracted for 1248 pigs (not 1250 because a double room barn then houses 2500 head which is 1000 animal units in most states which triggers a different level of record keeping and regulation). This makes total manure production 404,352 gal/yr.
A typical barn has room dimensions of 50 ft x 192 ft. With a fully slatted floor and full pit, the pit floor has 9600 square feet of surface area. Since 1 cubic foot is 7.48 gallons, this means each foot of pit depth translates into 71,800 gallons. This means each year 5.6 feet of manure is generated. In fact it is closer to 6 feet by the time you account for all of the slat support columns, etc.
When you consider that after a few years 6-18” of heavy sludge and manure remains in the pit after pumping, suddenly we’re up to over 7 ft of manure depth prior to fall pumping and land application. An 8 ft deep pit isn’t ‘too deep’ when you add it all up.