Effects of centrifuges and screens on solids/nutrient separation and ammonia emissions from liquid dairy manure

Purpose

Some Idaho dairies use liquid manure handling systems that result in large amounts of manure applied via irrigation systems to adjacent cropland during the growing season. Solids and nutrients presented in liquid dairy manure pose challenges to manure handling. Separating solids and nutrients from liquid dairy manure is a critical step to improve nutrient use efficiency and reduce manure handling costs. Most Idaho dairies have primary screens that separate coarse particles from their liquid streams. A few dairies have incorporated secondary solid separation technologies (centrifuge and secondary screen) into their manure handling systems to achieve higher solids and nutrient removal rates. Idaho dairymen want to know more information about solid and nutrient separation efficiencies by centrifuges and screens to make informed decisions on upgrading their solid/nutrient separation technologies. The objectives of this study were to evaluate centrifuges and screens in terms of removing solids and nutrients from liquid dairy manure and affecting ammonia emissions from the treated liquid dairy manure.

What Did We Do?

A year-long evaluation of on-farm centrifuges and screens on removing solids and nutrients and affecting ammonia emissions from centrifuge- and screen-separated liquid dairy manure was conducted. Triplicate fresh liquid dairy manure samples were collected monthly from before and after screens and centrifuges on a commercial dairy meanwhile triplicate screen- and centrifuge separated solids were collected from the same dairy. Figure 1 shows the dairy’s liquid manure flow diagram and locations where the liquid and solid manure samples were collected. The collected solids were analyzed for nitrogen (N), phosphorus (P), and potassium (K) concentrations by a certified commercial laboratory. The collected liquid samples were analyzed for total and suspended solids based on Methods 2540B and D (APHA, 2012) in the Waste Management Laboratory at the UI Twin Falls Research and Extension Center. Ammonia emissions from the monthly collected liquid dairy manure were evaluated using Ogawa ammonia passive samplers outside the Waste Management Lab for a year. Ammonia emission rate was calculated based on the duration and NH4-N concentrations from the Ogawa ammonia passive sampler tests. Ogawa passive ammonia sampler and Quickchem 8500 analysis system are shown in Figures 2 and 3.

Figure 1. Liquid manure flow diagram (liquid manure samples were collected at points 1 (before screens), 3 (after screens), and 5 (after centrifuges), solid samples were collected at points 2 (screen separated solids) and 4 (centrifuge separated solids).
Figure 2. Ogawa ammonia passive sampler.
Figure 3. Quickchem 8500 analysis system (Lachat Instruments, Milwaukee, WI).

What Have We Learned?

Centrifuge can further remove finer particles than cannot be removed by primary screens. Figure 4 shows both the screen- and centrifuge separated solids.

Figure 4. Centrifuge separated (left) and screen (right) separated solids.

Total nitrogen, phosphorus, and potassium in screen- and centrifuge separated solids are shown in Figures 5, 6, and 7. It was noticed that centrifuge separated solids had significantly (P<0.05) higher N, P, and K than that in screen separated solids. Yearlong averages of 9.2 lb/ton of total nitrogen, 8.0 lb/ton of P2O5, and 7.2 lb/ton of K2O were in the centrifuge separated solids while yearlong averages of 5.4 lb/ton of total nitrogen, 2.0 lb/ton of P2O5, and 4.4 lb/ton of K2O were in the screen separated solids.

Figure 5. Total nitrogen in screen separated and centrifuge separated solids.
Figure 6. Phosphorus in screen separated and centrifuge separated solids.
Figure 7. Potassium in screen separated and centrifuge separated solids.

Liquid dairy manure total solids and suspended solids are shown in Figures 8 and 9. Both the total solids and suspended solids in the liquid stream were significantly (P<0.05) reduced after the screen and centrifuge treatment.

Figure 8. Total solids in raw (before screens), after screens, and after centrifuges.
Figure 9. Suspended solids in raw (before the screens), after the screens, and after the centrifuges.

It was found that there was no significant difference (p≥0.05) between treatments for the ammonia emission rate in Figure 10 Which indicates that further treatment is needed to reduce ammonia emissions.

Figure 10. Ammonia emission rate during the test period.

In Figure 11 a correlation was determined between ammonia emission rate and suspended solids. As suspended solids were reduced within liquid dairy manure the ammonia emission rate increased among the treatments.

Figure 11. Ammonia emission rate vs. suspended solids.

In Figure 12 a correlation was determined between ammonia emission rate and ambient temperature. As the ambient temperature increased, so did the ammonia emission rate among the treatments.

Figure 12. Ammonia emission rate vs. suspended solids.

The test results showed:

    1. Centrifuge can further remove finer particles that can’t be removed by primary screens.
    2. Centrifuge separated solids contained higher N, P, and K contents, especially P (at an average of 8 lb/ton of P2O5 in centrifuge separated solids vs. 2 lb/ton of P2O5 in screen separated solids).
    3. Ammonia emissions from raw liquid manure, screen- and centrifuge separated liquid manure did not show significant differences.
    4. The most influential factors for ammonia emissions from liquid dairy manure were ambient temperatures and suspended solids within the liquid dairy manure.

Future Plans

We will hold workshops and field days to communicate the results with producers and promote on-farm adoption of advanced separation equipment such as centrifuge.

Authors

Lide Chen, Waste Management Engineer, Department of Soil and Water Systems, University of Idaho

Corresponding author email address

lchen@uidaho.edu

Additional author

Kevin Kruger, Scientific Aide, Department of Soil and Water Systems, University of Idaho.

Additional Information

APHA. (2012). Standard Methods for the Examination of Water and Wastewater. Washington D.C. : American Public Heath Association., Pp. 2-64 and Pp. 2-66

Acknowledgements

USDA NIFA WSARE financially supported this study. Thanks also go to Scientists at USDA ARS Kimberly Station for their help with analyzing ammonia emission samples.

 

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.

Discharge Quality Water from Dairy Manure: A Summary of the McLanahan Nutrient Separation System

Why Study Dairy Manure Treatment?

Dairy manure has historically been land applied consistent with the agronomic requirements of growing crops.  Due to consolidation of the dairy industry over the last 40 years, animal density has increased dramatically creating logistical, storage and environmental challenges.  Also, environmental constraints and water scarcity is more recognized.  Manure maintains tremendous nutrient value; however, water comprises approximately 90% of the manure stream.  Development of new and innovative methods for extracting nutrients for beneficial reuse while preparing water for beneficial on-site reuse is of paramount importance to the future of the US dairy industry.

What Did We Do?

Figure 1. The 4 steps that make up the McLanahan Nutrient Separation System

Figure 1. Diagram of the four steps that make up the McLanahan Nutrient Separation System

Research was initiated in 2004 to evaluate the potential of coupling a traditional complete mix digester with an ultrafiltration system to create what is commonly referred to as an anaerobic membrane bioreactor (AnMBR).

The AnMBR acts to separate hydraulic retention time from solid retention time while producing a high quality effluent. There are opposing views in the literature with respect to the impact of pump/membrane shear on biological activity and our objective was to add clarity.  An outcome of the research was that biogas production is negatively impacted by high shear forces but at practical flow rates, the impact is negligible. An additional finding, and the focus of this paper, is the recognition that the AnMBR is a logical starting point for a comprehensive nutrient recovery and water reuse process.

photo of pretreatment digester
Figure 2. Pretreatment Digester
photo of ultrafiltration
Figure 3. Ultrafiltration

Based on this early research, a comprehensive Nutrient Management System has been developed that seeks to improve the social and environmental sustainability of the dairy industry, while reducing the cost and liability associated with manure management.  In general, nutrients are separated and concentrated, allowing for application where and when they are needed. The separated water can be land irrigated, re-used or even discharged. The system is comprised of four steps (as depicted in Figure 1):  pretreatment under anaerobic conditions (Figure 2), ultrafiltration (UF) (Figure 3), air stripping (Figure 4) and reverse osmosis (RO) (Figure 5).

photo of air stripping equipment
Figure 4. Air Stripping

The pretreatment system (anaerobic digester) and UF system are coupled together (AnMBR).  The manure fed to the AnMBR first undergoes sand separation (only for dairies bedding with sand) followed by solid separation to remove coarse solids to prevent plugging of the UF system.    The digester portion of the AnMBR produces a homogeneous feedstock while producing biogas useful for energy production, although its production is a secondary concern of the process.   There are two outputs from the UF process: permeate and concentrate.  The permeate stream, often referred to as “tea water”, is devoid of suspended solids and contains the dissolved constituents found in manure including ammonia and potassium.  The concentrate stream contains 95%+ of the phosphorus and 88%+ of organic nitrogen with a total solid content of 6-7%.  Due to the shearing action of the pump/membrane system coupled with anaerobic degradation, the resulting concentrate stream is readily pumped and the solid fraction tends to stay in suspension.

photo of reverse osmosis equipment
Figure 5. Reverse Osmosis

Permeate from the UF flows to an air stripping process for ammonia removal.  The equilibrium relationship between un-ionized and ionized NH3+/NH4 is controlled by pH and temperature.  As a general rule, the air stripper is used to remove as much ammonia as practical through the addition of waste heat (such as from an engine generator set or biogas boiler).  The stripped ammonia is combined with dilute sulfuric acid to produce liquid ammonium sulfate (approximately 6% nitrogen and 7% sulfur).

The air stripped water is fed to a RO process which produces clean water suitable for direct discharge and a concentrated liquid fertilizer containing the potassium.  The clean water represents approximately 55% of the starting volume of manure.  As an option, a plate and frame press can be used to dewater UF concentrate to produce a solid product containing phosphorus and organic nitrogen.  Inclusion of this technology offers the potential of increasing the percentage of clean water produced by the complete process to more than 60% of the starting volume of manure.

What have you learned?

  • UF membrane excludes 95%+ of phosphorus and 88%+ of organic nitrogen.
  • Stable flux rates at operating total solid concentrations of 6.0-7.0%.
  • Ammonium sulfate concentrations of 28-30% were readily achieved.
  • Overall, approximately 55% of water is recovered as discharge quality
  • Through the use of solid-liquid separation, the potential exists to increase volume of recovered water to 60%+.

Impact of Technology

The technology is flexible and can be applied to meet farm specific goals objectives.  Separated and concentrated nutrients can be land applied where and when they are needed and the production of clean water creates new and improved opportunities for water management.  Overall, the process vastly improves the farmer’s control of the manure management process.

Authors

James Wallace, P.E., PhD, Environmental Engineer, McLanahan Corporation JWallace@mclanahan.com

Steven Safferman, P.E., PhD, Associate Professor, Michigan State University

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.