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Utilizing Acid-tolerant Nitrifying Bacteria to Generate the Acidity Needed to Operate Ammonia Scrubbers on Poultry and Swine Facilities


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Although acid scrubbers can capture ammonia (NH3) in exhaust air from animal facilities, they are not cost-effective because the nitrogen (N) captured has roughly the same value as the acid. The objective of this work was to develop an NH3 scrubber that uses acid-tolerant nitrifying bacteria to generate the acid needed for scrubbing NH3. Nitrification is an acid-forming process, with two moles of acid formed for each mole of ammonium (NH4) nitrified. A laboratory microcosm that held a clay substrate was inoculated with sewage from the aeration basin of a local wastewater plant. A feed solution containing ammonium chloride (NH4Cl) and sodium bicarbonate (NaHCO3) was then pumped through the system. Ammonium, nitrate (NO3) and pH were measured periodically in the influent and effluent. After a few weeks, NO3- analysis indicated >95% of the NH4 was being nitrified to NO3-, at which point the NH4Cl and NaHCO3 concentrations were doubled. This process was repeated until the feed solution contained 4 g NH4Cl/L. Then the NaHCO3 was reduced over time so that the molar ratio of base to potential acidity (from nitrification of NH4) was less than one, causing the effluent pH to decrease. Even though the influent pH was 8.2, the effluent pH was reduced to 4.2 over time, indicating acid-tolerant nitrifying bacteria were at work. Clay substrate from the microcosm was then used to inoculate slats of an NH4 scrubber with these bacteria at a commercial broiler farm in NW AR, by encasing the clay substrate in mesh that was attached to the slats. Within weeks, bacteria slime mats formed on the slats and nitrification of NH3 and acid generation began. Over 20 kg of N were captured in 35 days, about half of which was NO3-. This technology has the potential to make the use of NH4 scrubbers a cost-effective best management practice.

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Utilizing Acid-tolerant Nitrifying Bacteria to Generate the Acidity Needed to Operate Ammonia Scrubbers on Poultry and Swine Facilities

  1. 1. Utilizing Acid-tolerant Nitrifying Bacteria to Generate the Acidity Needed to Operate Ammonia Scrubbers on Poultry and Swine Facilities Philip Moore USDA Agricultural Research Service Poultry Production and Product Research Unit Fayetteville, Arkansas
  2. 2. • According to the EPA, the biggest source of atmospheric NH3 in the U.S. is animal manure. Roughly 27% of the total NH3 emissions are believed to originate from poultry manure (Battye et al., 1994). • We found over half of the N excreted by broilers is lost to the atmosphere as NH3 before the manure is removed from the barns (Moore et al., 2011). • The NH3 emission factor for 50 day old broilers found in that study was 46 g NH3/bird (about 1/10th of a pound per bird). • This not only causes air and water pollution, it results in the loss of about 100 million pounds of NH3/year in Arkansas alone, where over one billion broilers are produced. This represents a huge waste Ammonia emissions from poultry manure
  3. 3. • Nitrogen entering lakes and rivers via atmospheric wet and dry NH3 deposition contributes to eutrophication in the same manner as N entering from runoff. • When NH3 reacts with NOx or SOx compounds in the air, it can form fine particulate matter (PM-2.5), which can cause human health problems when inhaled. • When NH3 is deposited onto soil via wet or dry fallout, it is converted to NO3 - (nitrate) via nitrification, which is an acid-forming reaction. • In Holland, the national forests begin dying in the 1980s due to soil acidification. Researchers found that 45% of the acid deposition was due to NH3 and 85% of the NH3 originated from livestock Eutrophication, PM-10s, and soil acidification
  4. 4. • Currently, the only technology used in the U.S. to control NH3 emissions from poultry houses is litter acidification using chemicals like aluminum sulfate or sodium bisulfate. • Another technology used to control NH3 emissions from animal rearing facilities in Europe is the use of acid scrubbers. • With acid scrubbers, exhaust air from barns is passed through a reactor and an acid solution is sprayed through the air, transferring the NH3 from the gas to liquid phase. • Acid scrubbers also remove odors and dust from air. Dutch scrubbers have shown a 30% removal efficiency for odor and a 62- 93% removal efficiency for PM-10 particles from air (Melse and Ogink, 2005). Capturing Ammonia with Acid Scrubbers
  5. 5. • Dutch acid scrubbers are very effective, but cost about $250,000/house. • These scrubbers use sulfuric acid to scrub ammonia. • Almost all of the scrubbers on farms in Holland are on swine farms, since heavy dust loadings in air from poultry farms clogs the packing material in scrubbers. European Acid Scrubbers
  6. 6. • The goal of our research on acid scrubbers was to develop a scrubber that can handle very heavy dust loadings, which is typical from exhaust air from broiler houses, without clogging. • Another goal was to develop a scrubber that was both simple and safe enough that a grower could operate it without outside technical assistance. • In a perfect world, the scrubbers would be cost-effective to operate. Capturing Ammonia with Acid Scrubbers
  7. 7. Fertilizing Crops with N from Scrubbers • In some watersheds, growers cannot apply litter because of soil test P thresholds and must purchase commercial fertilizer to meet the N needs of their pasture. • If a scrubber could be developed that would trap an average of 5 lbs N per day, then the grower would recover 1825 lbs N/unit/year. • Assuming the grower has 2 units on each chicken house and a total of four houses, then he/she could recover 14,600 lbs N/year. This would be almost enough N to supply 150 lbs N/acre to 100 acres of cropland. • Air and water quality would also benefit.
  8. 8. The current version has two scrubbers; one for dust and the second for ammonia. The dust must be removed first or acid will be wasted neutralizing the dust. The shells are made out of fiberglass. Each has a 360 L reservoir. These scrubbers are made for minimum vent fans on broiler houses in order to get the most N per dollar spent (Melse et al., 2006). Dust scrubber Ammonia scrubber
  9. 9. The dust scrubber can hold up to 8 rows of 11 wooden slats at 45 degree angles.
  10. 10. The dust scrubber has a screen above the reservoir to catch feathers and very large particulate matter. feather trap
  11. 11. water curtains acid curtains cool cell water reservoir acid reservoir wooden slats ARS Air Scrubber feather trap dust scrubber ammonia scrubber
  12. 12. Initially we included plastic cool cell material on the exhaust end of the scrubber. It was enclosed in a stainless steel frame and was on hinges, allowing for easy access into the scrubber. This material is an effective medium for acid scrubbing and is used in Dutch and German scrubbers. We thought it would improve performance. However, it clogged after 3-4 weeks.
  13. 13. • Currently studies are underway to evaluate the scrubbers at commercial broiler farms in AR, DE and PA. • We have used several different types of acid in the scrubbers, including sodium bisulfate, alum and sulfuric acid. • This work is funded by a Conservation Innovation Grant (CIG) from NRCS with Hong Li at Univ. of Delaware. On-farm Evaluation of Ammonia Scrubber
  14. 14. 1. Evaluate the effectiveness of the scrubbers in reducing ammonia, dust, and odors from the exhaust air of poultry houses. 2. Determine how much nitrogen is captured with the scrubber under various conditions. 3. Determine the amount of acid, electricity, water and labor needed to operate the scrubber. 4. Determine the cost-effectiveness of this BMP. Objectives
  15. 15. Energy and water use by the scrubbers • The average electricity use for the scrubbers was 544 Kwh/scrubber-flock ($27.20/scrubber-flock assuming $0.05/Kwh). • The average amount of water used by a scrubber for one flock was 970 gallons (3,671 L).
  16. 16. Ammonia capturing efficiency. Each run was conducted with 22.7 kg (50 lbs) of sodium bisulfate (PLT). Sampling time Mass NH3-N in exhaust (kg ) Mass NH3-N captured (kg) Efficiency,% Run 1 12 hr 1.82 1.41 77 24 hr 3.60 1.59 44 Run 2 24 hr 3.98 2.20 55 48 hr 7.77 2.92 38 Run 3 12 hr 1.98 1.18 60 48 hr 7.94 4.54 57
  17. 17. Economics • Although the efficiency of the scrubber when we used PLT wasn’t too bad (average = 55% reduction in NH3), the cost-effectiveness was ugly. • The average amount of N captured with 50 lbs of PLT was 3.02 kg N or 6.64 lbs N. A 50 lb bag costs $15.50. • This is equivalent to $2.33/lb N. Of course that price doesn’t include the price of the scrubber or the cost of electricity, water and labor to operate it. • Since poultry production is not affected with scrubbers, the only economic benefit is the capture of lost nitrogen.
  18. 18. Free acid is needed to make this cost-effective • The economics of using any type of acid to capture ammonia from poultry houses looks questionable, particularly since the scrubbers themselves are so expensive. • Sulfuric acid poses a significant safety risk when used by typical producers and is not cheap in small quantities. When bought in quantities manageable by a grower (30 gal drum), sulfuric costs $10-15/gal, which is about the same cost as PLT when converted to moles acid/dollar. Most growers don’t want a semi load. • We looked for “free” acids to use for this purpose, but waste acids are often contaminated and transportation costs alone makes them unattractive. • Therefore we decided to try to develop a biological system in
  19. 19. Nitrification of ammonia produces acid • Nitrification is a process carried out by bacteria in soil and water, where ammonium (NH4 + ) is converted to nitrate (NO3 - ). • Two moles of acid are formed for every mole of ammonium oxidized: NH4 + + 1.5O2  NO2 - + 2H+ + H2O NO2 - + 0.5O2  NO3 - • We hypothesized that a system could be developed to use ammonia-oxidizing bacteria, such as Proteobacteria, Nitrosomonas and Nitrosococcus to generate the acid needed to capture ammonia in the scrubbers.
  20. 20. Can bacteria produce the acid via nitrification? • The idea was as follows: if every mole of ammonia scrubbed from the air resulted in two moles of acid, and the new acid then captured more ammonia that was nitrified, and the cycle continuously repeated itself, then we would have a free acid source. • The only problem is the bacteria responsible for nitrification will not grow under acidic or even slightly acidic conditions. In nature, nitrification is greatly inhibited below pH values of 6.5. • However, in order for scrubbers to work, the pH needs to be less than 4.5. • Hence, we needed to develop acid-tolerant nitrifying bacteria.
  21. 21. Developing acid-tolerant nitrifying bacteria • In order to do this we built a reactor in the lab which was inoculated with sewage taken from the aeration basin of the wastewater treatment plant in Rogers, AR. • Then a feed solution containing ammonium chloride and a base (sodium bicarbonate) was slowly pumped through the reactor. Oxygen was metered into the system at rate needed to achieve at least 10 ppm dissolved O2. • The reactor effluent was analyzed for ammonium, nitrate and pH. As the bacteria grew, the ammonium concentration was increased to very high levels. Then, over time, we reduced the sodium bicarbonate in the feed solution, which caused the pH to drop. • The organisms that survived were very acid tolerant.
  22. 22. Oxygen Tank feed solution Feed Pump Circulation Pump Filtralite flow flow flow flow flow meter Reactor Dissolved oxygen sampling port waste Mixing vessel NH Cl4 Laboratory reactor for developing acid-tolerant nitrifying bacteria
  23. 23. Laboratory reactor for developing acid-tolerant nitrifying bacteria
  24. 24. 0 50 100 150 200 250 4 5 6 7 8 Time (days) EffluentpH Influent pH The effluent pH decreased to values as low as 4.2 as the sodium bicarbonate concentration was reduced in the feed solution. Chemical analysis of the water indicated that 98% of ammonia was nitrified.
  25. 25. Clay medium was used to inoculate scrubbers with acid-tolerant nitrifiers The clay was wrapped in cheesecloth, which was stapled to the slats
  26. 26. Three slats containing the inoculum were then placed in the scrubber.
  27. 27. A short time later we observed this bacterial slime growing on the slats. We also measured high levels on nitrate in the scrubber for the first time.
  28. 28. The system worked. In just over a month we captured 20 kg (44 lbs) of N, even though only 3 slats out of 88 were used. About half of the nitrogen was ammonium and half was nitrate.
  29. 29. feather trap wooden slats water curtains water curtains Dust Scrubber Ammonia Scrubber poultry house exhaust fan air flow bacterial biofilms growing from nylon mesh Nitrification-enhanced ammonia scrubber (patent pending)
  30. 30. Conclusions • Acid scrubbers can remove NH3 from broiler house exhaust air, but they are currently not cost-effective. • Although sodium bisulfate is safer than strong acids, it is not practical because of sodium toxicity. We’re currently testing sulfuric acid and alum, but the economics are much better. • Poultry production is not improved when acid scrubbers are used to capture NH3in air exhausted from poultry houses, so the only economic benefit is the value of the N, which is cheap. • Bottom line – It’s much easier and more cost-effective to prevent NH3 emissions from broiler houses using litter amendments like alum, than to use scrubbers to capture the N.
  31. 31. Problems/Future Research • The acid-tolerant organisms we developed thrive at very warm temperatures (80 - 90o F). The scrubbers are located on the outside of the poultry houses. To keep the water heated to 90o F in the winter would take an enormous amount of energy. • Hence, our next step is to try to develop a strain of acid-tolerant nitrifying bacteria that are also tolerant of cold conditions. So we have set up the lab microcosm again and are using a big aquarium cooling system to slowly reduce the temperature. • We are hoping to develop a nitrification-enhanced ammonia scrubber that is practical and works year round. • However, bacteria are living things which require just the right combination of temperature, oxygen, and nutrients to thrive. Hence, this is challenging.