In October 2007, ECS began conducting trials at California State University Fresno (CSUF) to explore using the new patent pending AC Composter™ (ACC) covered aerated static pile system to manage dairy manure solids and comply with airquality standards. A two-zone portable pilot system was delivered by ECS and installed at the CSUF research dairy. This research is being conducted under a Phase I USDA CSREES Small Business Innovative Research grant. The goal is to determine how well the ACC (Figure 1) can meet the operational and economic demands of manure management at a dairy and provide adequate capture and control to limit air emissions for compliance with California regulations.

Composting is a proven technique to process organic wastes into a more stable and valuable form. On-site composting of dairy manure allows a dairy to recycle manure solids into a safe bedding material and sell finished product. Recycling to create bedding reduces the expense of importing straw or other materials, and exporting compost creates a means to manage excess nutrients. However, increasingly stringent air emissions regulations have made open windrow composting less tenable.

One of the key goals of this research has been to quantify the ability of the ACC to capture and control air emissions. During the composting trials air emissions samples were collected by a CSUF team under the direction of Dr. Charles Krauter. Samples were taken from the exhaust duct, the surface of the biofilter, and the surface of the covered piles. Samples were analyzed using a real-time photoacoustic gas analyzer (LumaSense INNOVA 1314). Sampling from a tap in the exhaust, along with flow measurements at the same point, were used to determine the actual air emissions flux in the duct. Samples at the biofilter and pile surfaces were taken using Surface Isolation Flux Chambers. These chambers are placed over the surface to be measured and charged with clean air at a known rate to measure gas emissions flux emanating from the enclosed surface (Radian, 1986).

Initial data for air emissions from the system are summarized in Tables 1 and 2. The data shows that the capture rate for the ACC is 100%. It also shows that the biofilter control rates are significantly higher than BACT. The high control rate is largely due to the low loading rate of the biofilter (i.e. a longer retention time). Because the ACC makes low aeration rates feasible, it becomes cost-effective to have a biofilter with a low loading rate.

ECS has applied for a Phase II USDA CSREES SBIR grant to continue this research and collect more comprehensive emissions data from a larger prototype system operating at a large commercial dairy. Phase II research is scheduled to begin in September of 2008. For more information on the AC Composter™ system, please contact ECS or go to www.compostsystems.com.

ND represents a measured concentration below the detection limit of the INNOVA device for a given species. For methane, the detection limit is 0.3ppm.

References
Hao, X., C. Chang, F.J. Larney, and G.R. Travis. 2001. Greenhouse gas emissions during
cattle feedlot manure composting. J. Environ. Qual. 30: 376-386.


Radian Corporation. 1986. Measurement of Gaseous Emission Rates from Land Surfaces
Using an Emission Isolation Flux Chamber, Users Guide, EPA /600/8-86/008, February
1986.


San Joaquin Valley Air Pollution Control District (SJVAPCD). 2007. Rule 4565 -
Biosolids, Animal Manure, and Poultry Litter Operations

Inventive minds have come up with an amazing number of variations to channel air through a floor and into, or out of, a compost pile. Designs that have been based on sound engineering calculations and experience have generally worked. However, since all aeration floors are constrained by physics and Murphy, the floors that were less rigorously designed have not faired well.

The characteristics by which compost aeration floors should be evaluated are:
• Uniformity of air distribution
• Capital cost
• Durability
• Operational Cost
• Safety
• Functionality as a floor drain

The Physics

Some mal-distribution of air flow in aeration floors is guaranteed since air flowing through any confined space loses pressure due to frictional losses. It turns out that the mechanisms of this frictional loss are non-linear, somewhat counter-intuitive, and, on close inspection, complicated; some very smart engineers with crew-cuts and slide rules have spent careers characterizing the physics of channeled air-flow. At the simplest level we can say that as air flows down a pipe it looses pressure. If that pipe is perforated, we can be assured that the pressure forcing the air through those orifices will vary along that length, and therefore the flow through the orifices will vary as well. The extent of the flow variations amongst the orifices is the mal-distribution.
Broadly speaking, the only method the aeration floor designer has to minimize mal-distribution is to make the distribution frictional losses “small” in relationship to the combined frictional losses at the orifices and through the compost pile (compost pile frictional losses are notoriously overestimated). This means that a suspended aeration floor with tiny high speed orifices over a huge plenum volume of slow moving air would be great; except when it came time to pay for it. In the spectrum of aeration floors designed from basic principles we see at one end the “sparger” type design that relies on very high orifice pressure (few holes spread far apart), and on the other end designs with very low speed air flows through the distribution channels and low orifice pressure loss. There are also designs that try to balance these two effects and operate at medium pressures. These designs can be classified as High, Medium, and Low Pressure; examples of each are shown below.

Cost, Durability, and Operational Considerations

Below is a table that attempts to evaluate and compare different aeration floor designs that are assumed to be competently designed to minimize mal-distribution. This analysis is by necessity generalized, but some helpful definitions and ranges can be given. For example: High pressure systems generally operate between 10-30 static inches; Low pressure systems often run between 1-4 static inches; Medium generally operate between those ranges. Energy costs follow pressure ranges. The O&M costs for above grade perforated pipe is considered high because it generally has to be handled each time a pile is built. High pressure aeration floors generally work well only in positive aeration since the orifices tend to plug in suction (negative); this also means they are not suitable for reversing aeration.

The right choice of aeration floor design will depend on your cost structure and your facility needs.