The Premium Standard Farms Nitrification and Denitrification Experience

By David Townsend, P.E., Brian Paulsen, W. James Wells, P.E.

 

Premium Standard Farms, Inc. (PSF) operates a Concentrated Animal Feeding Operation (CAFO) named the Whitetail Farm in northwest Missouri. The farm houses grow-finish pigs that range in weight from 55 to 260 pounds while on the farm. A wastewater treatment system known as the advanced nitrification and denitrification (AND) system has been designed, constructed and placed into operation for six individual sites of 8,800 pigs each for a total of 52,800 head. Nitrification and denitrification occurs at a single wastewater treatment plant centrally located on the farm. The process description for the system is as follows:

• Permeable covers on each of six existing anaerobic lagoons for odor control and emissions reduction.
• Transfer of the daily inflow (on average) from each existing lagoon to a central nitrification and denitrification system.
• Anoxic basin for nitrate and biochemical oxygen demand reduction
• Aeration basin designed for ammonia conversion to nitrate through nitrification (w/ recycle to anoxic basin)
• Biosolids storage basin for settling and further denitrification
• Irrigation storage basin for storage of treated effluent prior to land application

 

The system is designed to reduce property line odor concentrations to less than regulatory thresholds, air emissions of ammonia and hydrogen sulfide from wastewater treatment sources, and effluent nitrogen concentration of total nitrogen before land application by at least 50 percent. The reduction of land area required for effluent irrigation is expected to reduce the spill risk associated with irrigating large acreages on soils with high clay content and slopes of up to 20 percent.

 

Introduction

 

PSF is a leading pork producer with facilities located in the north central Missouri, Texas and North Carolina. Headquartered in Kansas City, Missouri, Premium Standard Farms is the nation’s second largest pork producer as well as a truly vertically integrated pork producer.

 

PSF has approximately 107,000 sows and 800,000 finishing spaces on company owned farms in Missouri. The grow finish sites are either 8,000 head or 8,800 head facilities (8 barns per site) each with an individual lagoon. With few exceptions, the lagoons are single stage anaerobic systems designed for treatment, sludge storage and irrigation storage.

 

The production barns are flush type with fully slatted floors and shallow gutters beneath the slats. Lagoon effluent is continuously recycled to the barns for flushing purposes 24 hours per day seven days per week. Multiple flushing reservoirs containing between 800 and 1600 gallons are located in each production building. Periodically, a flush valve releases the contents of a reservoir to flush individual lanes within the building. A small pit at the low end of each building collects the drainage from the flushing lanes and has a single outlet connected to a gravity sewer system. The gravity sewer system connects a series of production barns and has a single outlet pipe to the farm’s lagoon.

 

PSF has experienced a number of wastewater spills that mostly occurred during land application. PSF’s Missouri farms are located in a region of rolling hills with slopes approved for irrigation of up to 20% and soils with high clay content. The topographic relief of the area results in a number of small waterways located between the closely spaced ridge tops. Any small problem during irrigation, if not detected immediately, can lead to a spill into a waterway. Hose failure, equipment failure, aboveground piping, human error, etc generally cause spills that occur during land application. These types of spills have been mostly a few hundred to a few thousand gallons that are quickly contained by emergency response procedures. As a result of these spills and odor concerns, a number of lawsuits were filed against the company.

 

In September 1999, PSF entered into an agreement with the State of Missouri to install “Next Generation Technology” on the Missouri farms. The agreement committed PSF to spend up to $25 million over a period of five years to develop and implement the technology. The technology was undefined, but must be approved by a court-appointed expert panel that reviews the work proposed by the company. In November 2001, the PSF entered into an agreement with the U.S. EPA that requires substantially eliminating atmospheric emissions of ammonia and hydrogen sulfide and to reduce the effluent nitrogen concentration by at least 50 percent. The effluent nitrogen baseline was three years of data from the existing anaerobic lagoons.

 

Goals

 

These agreements have resulted in the company formulating the following goals:

• Reduce property line odor concentrations to less than regulatory thresholds
• Reduce air emissions of ammonia and hydrogen sulfide from wastewater treatment sources
• Reduce the effluent nitrogen concentration of total nitrogen before land application by at least 50%
• Reduce the spill risk associated with irrigating large acreages on soils with high clay content and slopes of up to 20%.

 

The Whitetail farm is the first of approximately twelve large CAFOs that PSF will retrofit with new technology during the implementation period covered in the settlement agreements. In many ways, this project, although on a rather large scale, is a prototype system that will better define the design and operational parameters for the subsequent systems.

 

Treatment System Selection

 

In order to design a treatment system that would be able to reduce the nitrogen in the flow from the existing anaerobic lagoons by 50 percent or more without increasing the release of ammonia nitrogen to the atmosphere, a nitrification/denitrification treatment system was contemplated. A determination was made that the treatment system would operate year around because of the problems of trying to start up and then shut down the treatment system every year. Because of the year around operation, there are a wide variety of conditions, including the wastewater characteristics and the temperature of the wastewater removed from the anaerobic lagoons. The anaerobic lagoon effluent temperature is highly variable and effects both nitrification and denitrification rates. The system was designed to allow for varying the flow rate each month to achieve the sludge age needed to accomplish nitrification. Denitrification from the system will be limited by the available biochemical oxygen demand (BOD₅) in the anaerobic lagoon effluent.

 

HDR Engineering and PSF designed the resulting nitrification and denitrification system specifically for this project. The system has been termed the advanced nitrification and denitrification (AND) system. It has been designed, constructed and placed into operation in April 2002 for six individual sites of 8,800 pigs each for a total of 52,800 head located at the PSF Whitetail Farm in North Central Missouri. As Shown in Figure 1, nitrification and denitrification occurs at a single wastewater treatment plant centrally located on the farm. The process description for the system is as follows:

• Permeable covers on each of six existing anaerobic lagoons
• Transfer of flow from each existing lagoon to the anoxic basin
• Anoxic basin for nitrate and biochemical oxygen demand reduction
• Aeration basin designed for nitrification (w/ recycle to anoxic basin)
• Biosolids storage basin
• Irrigation storage basin

 

Permeable Covered Lagoons

 

Anaerobic treatment is logical for swine waste given the high strength characteristics. Anaerobic systems have many advantages over fully aerobic systems. These advantages include minimal power requirements, low sludge production, potential energy production, and less operator attention. Anaerobic lagoons perform similarly to impermeable covered anaerobic digesters with respect to BOD₅, chemical oxygen demand (COD), total solids (TS) and volatile solids (VS) removal, and can perform better with respect to total Kjeldahl nitrogen (TKN), ammonia and phosphorus removal (Cheng 2000). Although anaerobic biology is a good fit with swine waste, anaerobic lagoons are the subjects of much political and regulatory criticism today. PSF’s approach is to modify the lagoons for odor control.

 

Covers can be effective in reducing odors from lagoons, but gas tight covers will cause difficulties with other parts of the PSF operation. A gas tight cover contains odorous gases in the lagoon, but increased odors will be released from the barns when recycled lagoon water is used for flushing purposes. In addition, increased odors will occur during irrigation of lagoon liquid. Four to five times the amount of land may be needed for land application due to increased nitrogen retention.

 

In 1999, PSF became aware of an innovative lagoon cover material. The BioCap™ cover is made of a permeable material that is a felt-like polypropylene that floats on the lagoon surface. Several producers in Minnesota had installed the cover over their manure basins or lagoons as an odor control method. Good results were being reported on the reduction of odor and hydrogen sulfide from these first installations. PSF made a visit to Minnesota and were impressed with the results. Unlike impermeable covers, the permeable covers were providing odor control without causing a buildup of odorous compounds or nitrogen in the treated wastewater. The permeable cover was believed to be reducing odor in two ways: by physically limiting the emission of odorous chemicals from the lagoon (including elimination of wind and wave action) and by creating an aerobic, biologically active zone on top of the cover where odorous chemicals emitted from the lagoon are oxidized by microorganisms. Algae were observed on the surface of the covers.

 

After a site visit to the farms in Minnesota, PSF decided to install a permeable cover as a test on one of the Missouri farms. A permeable cover was installed on lagoon 24 at the Homan Farm during the fall of 1999. At the Whitetail Farm, all nine existing anaerobic lagoons were covered in the spring of 2000.

 

PSF has been evaluating the odor control effectiveness of permeable covers using ambient scentometry measurements at the lagoon berm. Results from the ambient monitoring show a significant improvement in ambient odor and hydrogen sulfide measurements from the covered lagoon compared to the control lagoon. The permeable cover test site is Homan 24; the control site is Homan 20. Twenty-five pairs of scentometry observations were made between October 7, 1999, and October 6, 2000. The scentometry data were analyzed in terms of the frequency of observations that fall into certain classes. Two classes were defined as: observations of two dilutions to threshold (D/T) or below detection, and observations of seven D/T or greater. The scentometry data show that observations of 2 D/T or less were significantly more frequent at the covered lagoon than at the control lagoon (p=0.0002). Eighty percent of the observations (twenty out of 25) at the covered lagoon were minimal compared to 28 percent at the control lagoon (seven out of 25). On all 25 sampling days, the odor observed at the covered lagoon was either less than or equal to the odor observed at the open lagoon.

 

Research at the Whitetail farm has been conducted to evaluate the effectiveness of covered lagoons installed on a site-wide basis as an odor control option. Between June 2 and October 16, 2000, 27 sampling visits were made to the Whitetail site. Scentometry observations were made at the property. Odor was below detection on 78% of sampling events (21 out of 27).

 

A study in Colorado (Baumgartner 2000) evaluated the effectiveness of a permeable cover for reducing the emission of ammonia, hydrogen sulfide, and total volatile organic compounds (VOCs) from a swine lagoon. Emission comparisons were performed between treatment areas that were covered with the BioCap™ and an opening in the cover on the same lagoon. The emission tests showed that the permeable cover achieved an average reduction for hydrogen sulfide, ammonia, and total VOCs of 81%, 96%, and 90%, respectively during the December 1999 sample collection period. Reductions of hydrogen sulfide, ammonia, and total VOCs during the April 2000 period were 98%, 74% and, 88%, respectively and 97%, 61% and, 79%, respectively for the June 2000 period. These results indicate that the emissions reduction performance of the permeable covers appears to be independent of seasonal conditions.

 

Design Loadings for AND System

 

The design effluent discharging from the existing lagoon at each of the six sites is based upon an analysis of the existing data for the months of June and July and is anticipated to be as shown in the following table. The effluent quality assumes that the point of withdrawal is just above the sludge level near the bottom of the lagoon. The purpose of this location is to maximize the BOD₅ concentration of the wastewater in order to provide the food source for the denitrifying bacteria in the wastewater treatment system. An additional advantage is the warmer wastewater temperature during the winter months.

 

Large Lagoon Effluent Design Conditions (June and July)

 

The wastewater will be pumped from six of the existing covered lagoons. In the winter months the wastewater temperature in the lagoons is anticipated to be 6° C (based on actual measurements near the bottom of the lagoon this past winter). Although this is below the optimum temperature where nitrifying organisms function well, by increasing the sludge age some nitrification should continue. By operating all year around, instead only 6 months per year, the problem of starting up a nitrification treatment system each spring is avoided. The startup of such a system could easily take 4 to 6 weeks because the wastewater temperature is cold and the ammonia concentration is high.

 

Based upon a year-round operation and the goal of nitrifying and denitrifying the maximum amount of nitrogen discharging to the AND system, the rate of flow treated during each month will vary depending upon the temperature of the wastewater. During the summer months (wastewater temperature of 25° C or greater) the flow rate will be 0.144 mgd (6 sites @ 0.024 mgd/site) to the aeration basin which is greater than the average flow rate of 0.090 mgd (6 sites @ 0.015 mgd/site). During the colder winter months with wastewater temperatures of 6° C, the flow rate will be only 0.041 mgd. The aeration basin will be sized to provide a 21-day detention time at 0.144 mgd, which provides a detention time of 74 days at 0.041 mgd.

 

Anoxic Basin

 

In wastewater treatment systems where the intent is to nitrify and then denitrify the ammonia nitrogen, there must be a sufficient quantity of BOD₅ as a food supply for the bacteria stripping oxygen from the nitrates in the wastewater. The concentration of BOD₅ needed to denitrify 1.0 mg/l of nitrate-nitrogen is estimated to be in the range of 4 to 5 mg/l (McKinney 2000). The amount of BOD₅ available to denitrify nitrates varies with different waste streams with typical relationships being as follows:

 

For municipal wastewater, the ratio of BOD₅:NO₃-N of 10:1 shows that there is more than enough BOD₅ for denitrification. For CAFO lagoon effluent, the BOD₅ varies substantially depending upon the degree of anaerobic treatment that occurs. Anaerobic decomposition of BOD₅ is highly dependent upon the temperature of the wastewater with the higher wastewater temperatures producing respectively a greater degree of BOD₅ reduction. The ratio of BOD₅:NO₃-N from a lagoon at a CAFO will vary substantially throughout the year. In the summer, the ratio could be as low as 0.6:1 and in the winter the ratio could be as high as 2:1, which is still less than the desired ratio of 4.5:1 for complete denitrification. Another complicating factor is that only a minimum amount of wastewater can be treated in the winter when the ratio is higher because of the need for an increased sludge age in the aeration basin to accomplish nitrification. Depending upon the required degree of denitrification, methanol can be added to provide a source of BOD₅ without adding nitrogen.

 

In the AND system, the step where denitrification occurs is the anoxic basin, which precedes the aeration basin and receives the flow from the six existing anaerobic lagoons. The purpose of the anoxic basin, which precedes the aeration basin, is to utilize the available BOD₅ from the influent for denitrification. This step also recovers alkalinity to help sustain nitrification and reduce the BOD₅ in the influent to the aeration basin.

 

Flow from the aeration basin is recycled to the anoxic basin at a ratio of 1.6:1 to return nitrates for denitrification in the anoxic basin. The anoxic basin has a detention time of 1.53 days at a combined flow of 0.374 mgd (0.144 mgd inflow and 0.230 mgd recycle from aeration). The anoxic basin allows the bacteria to utilize the oxygen from the nitrates in the recycle flow to oxidize the incoming BOD₅. After the bacteria have stripped the oxygen from the nitrates (NO₃), the remaining nitrogen is released to the atmosphere as nitrogen gas. There will be two 20-HP floating directional mixers to maintain solids in suspension.

 

Assuming that 4.5 lb of BOD₅ are needed to denitrify 1.0 lb of nitrate nitrogen, there is not sufficient influent carbonaceous BOD₅ to the anoxic basin for complete denitrification Design calculations predict removal of 50 – 60 percent of the nitrate. Addition of methanol could further reduce the nitrates at the rate of 1.25 lbs of nitrate nitrogen per gallon of methanol. Since the settlement agreement with the USEPA requires at least a 50 percent effluent nitrogen removal, a methanol addition system has been provided as a backup system to provide supplemental carbon should it be needed. In the longer term, a method of directing raw waste from the barns to the anoxic basin to provide supplemental carbon may be developed.

 

The following table, which includes the anticipated effluent from the anoxic basin, is based upon oxygen being available from 2,208 lb/day of nitrate nitrogen (May, June and July), which has been nitrified in the aeration basin. The table also assumes that the flow from the settling basin does not contain a significant amount of BOD₅.

 

Anoxic Basin Effluent

 

Aeration Basin

 

The effluent from the anoxic basin discharges to the aeration basin that has a detention time of 21 days at the design flow of 0.144 mgd. The basin has an aeration system to supply the oxygen demand of 1,013 lbs of O2/hour under standard conditions. This is based upon an effluent BOD₅ from the anoxic basin of 187 lbs/day and a TKN of 2,402 lbs/day. The waste loads to the aeration basin vary monthly as does the temperature of the wastewater. This results in varying oxygen requirements for the bacteria. The following table presents that information, along with the blower horsepower operating at full load.

 

Oxygen and Blower Requirements for Aeration Basin

* Four blowers needed for mixing (160 hp)

 

At design conditions, 0.288 mgd of mixed liquor from the aeration basin will flow by gravity to the adjoining settling basin. Supernatant from this settling basin will enter a perforated-pipe effluent collection device, flow to an effluent collection manhole and then flow by gravity to the subsequent biosolids storage basin.

 

Biological sludge that settles in the quiescent settling basin will flow by gravity to a sludge draw-off manhole. If necessary to increase the sludge age in the aeration basin, return activated sludge (RAS) will be pumped from this manhole back to the aeration basin influent with a 100 gpm (0.144 mgd) pump. Waste activated sludge will flow by gravity from this manhole and mix with the settling basin effluent as it flows to the biosolids storage basin. At design conditions, influent to the biosolids storage basin is projected to be as shown in the following table:

 

Biosolids Storage Basin Influent at Design Conditions

 

Biosolids Storage Basin

 

The biosolids storage basin serves three functions. The first function is to separate the biosolids from the effluent. The second function is to provide storage for the biosolids to encourage the reduction in volume of the biosolids through anaerobic decomposition. The third function of the storage basin is creating a benthic demand from the settled sludge. However, the benthic demand has been estimated to be only 120 pounds BOD₅/day because of the long sludge age in the aeration basin. The 120 pounds of BOD₅ would have the capability of denitrifying only 45 pounds of NO₃-N/day or approximately 16,200 pounds of nitrogen/year. The effluent from the biosolids storage basin will discharge to the irrigation storage basin.

 

Irrigation Storage Basin

 

The wastewater will be pumped to the irrigation storage basin after the biosolids have settled in the biosolids storage basin. The volume of water to be stored will be the water treated when land application is not occurring. This will be during the months of November (2.14 m-gal.), December (1.78 m-gal.), January (1.27 m-gal), February (1.15 m-gal.), March (1.78 m-gal.) and April (2.14 m-gal.) for a total of 10.27 mgallons. In addition there is a reserve storage volume of 2.73 million gallons for a total of 13.0 m-gallons or 40 acre-feet to allow for unusually high precipitation during these 7 months (22 inches from November through April through May, which is 30% more than the average precipitation for that 7 month period of 16.8 inches.). There will be a minimum depth of two feet remaining in basin at all times. The stored water will be pumped into the irrigation piping system, which will discharge primarily to center pivot irrigation systems.

 

Air Emissions

 

The project will involve collection of high-frequency, semi-continuous emission measurements, using micrometeorological and wind tunnel flux measurement methods (Zahn 2000), for ammonia (NH₃), hydrogen sulfide (H₂S), non-methane total volatile organic compounds (NMVOC), and odor, over a 9 month period from a control lagoon at the PSF Locus Ridge #2 swine facility, and five treatment or storage cells, which are components of the AND system at the Whitetail site. The five sampling locations from the AND system are an existing lagoon (Whitetail #4) covered with permeable cover (BioCap™), an anoxic basin, an aeration basin, a biosolids storage basin, and an irrigation storage basin.

 

The goal of this project is to obtain estimates of emission rates of several compounds from “baseline” facilities, as well as facilities where new technologies have been installed. These estimates will be made over a long time period to provide information on seasonal variations in emissions.

 

Summary

 

A nitrification and denitrification system has been designed, constructed and placed into operation 52,800 head of finishing pigs at PSF’s Whitetail Farm in Missouri. Nitrification and denitrification occurs at a single wastewater treatment plant centrally located on the farm. The process description for the system is as follows:

• Permeable covers on each of six existing anaerobic lagoons for odor control and emissions reduction.
• Transfer of the daily inflow (on average) from each existing lagoon to a central nitrification and denitrification system.
• Anoxic basin for nitrate and biochemical oxygen demand reduction
• Aeration basin designed for ammonia conversion to nitrate through nitrification (w/ recycle to anoxic basin)
• Biosolids storage basin for settling and further denitrification
• Irrigation storage basin for storage of treated effluent prior to land application

 

The six existing anaerobic lagoons on the farm were retained for pretreatment of the raw manure. The existing lagoons are covered with permeable covers for cost-effective odor and emissions reduction. The cover is a non-woven geotextile with a felt-like texture.

 

The first basin in the central treatment plant is the completely mixed anoxic basin where nitrates in the recycle flow from the aeration basin are denitrified to nitrogen gas. There is not sufficient BOD₅ for complete denitrification. The design calculations predict removal of 50 – 60 percent of the nitrate. A methanol addition system has been provided as a backup system to provide supplemental carbon should it be needed to meet a minimum 50 percent reduction requirement.

 

An aeration basin with submerged diffusers was designed for nitrification. The influent rate is varied with changing temperature to provide hydraulic detention times in the aeration basin from 21 days at 25° C to 74 days at 6°. Design calculations predict that there will not be sufficient alkalinity in the basin to maintain nitrification. A magnesium hydroxide addition system has been included in the design to provide this alkalinity. After the true supplemental alkalinity requirements are determined from operational history, a lime feed system may be added if determined economically justifiable.

 

The biosolids generated in the aeration basin will settle in a biosolids storage basin. The basin will provide storage for the biosolids for a several year period. The effluent from the biosolids storage basin will discharge to an irrigation storage basin. Approximately 6 months of irrigation storage will be provided. The stored water will be pumped into the irrigation system (primarily center pivots).

 

Air emissions from one permeable covered lagoon and each of the subsequent treatment and storage cells will be monitored for ammonia, hydrogen sulfide, and non-methane VOCs.

 

References

 

Baumgartner Environics, Inc. 2000. Final Report: Emission tests of the BioCap biocover installed on a Colorado swine lagoon. September 8, 2000.
 
Cheng, J., J. Pace, K. D. Zering, J.C. Barker, K.F. Roos, and L. M. Saele. 2000. Evaluation of alternative swine waste treatment systems in comparison with a traditional lagoon system. pp.679-686. In Moore, J. A. (ed). Proceedings of the Eighth International Symposium on Animal, Agricultural, and Food Processing Wastes. ASAE:St. Joseph, Michigan.
 
McKinney, R.E. 2000, personal communication.
 
Zahn, J.A., A. E. Tung, B.A. Roberts, and J.L. Hatfield. 2001, Abatement of Ammonia and Hydrogen Sulfide Emissions from a Swine Lagoon Using a Polymer Biocover, J. Air & Waste Management. Association, 51:562-573.