Anaerobic Digestion at Dairy Farms

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Courtesy of BioCycle Magazine

The need to upgrade dairy waste management practices to overcome pollution problems is leading more farmers to seek solutions with anaerobic digestion technology. Two recent examples of this trend are underway at California sites.

The Cal Poly Dairy is located adjacent to the California Polytechnic State University campus in San Luis Obispo. The dairy milks 180 cows with a total population of over 350 animals, including heifers and calves. Most of the herd is housed in freestall barns. About 90 percent of the manure is deposited on concrete and flushed with fresh or recycled water to a single-cell lagoon. The remaining ten percent is deposited in the corrals and collected only seasonally. Solids are separated from the flushed wastewater prior to storage in the lagoon, which has a volume of 19,000 m3, translating to 50 to 90 days of storage, depending upon the water used by the dairy.

A methane recovery system has been constructed at the dairy. The initial design was based upon the present and anticipated herd size. The new lagoon, which has a liquid volume of 14,000 m3, was constructed next to the existing one. The first lagoon is being used for storage and the second for methane recovery. The new lagoon was covered with a flexible membrane incorporating buoyant material so that the cover floats on the surface, and a gas collection system was attached. The project will reduce odors, keep greenhouse gas out of the atmosphere, and cut down on water pollution through reduction of organic matter in the lagoon.

COVERED LAGOON DESIGN

Design of a constant volume methane-producing lagoon must consider all of the volume flowing into the lagoon to avoid hydraulic washout of bacteria. The inclined screen will capture 15 percent of the manure volatile solids from the liquids flowing to the lagoon. The estimated daily wastewater is 350,000 liters/day at 0.3 percent total solids and 0.25 percent volatile solids (VS).

The predicted output of the lagoon will average 320 m3/day of biogas. As the gas bubbles form in the lagoon, they are channeled along 23-cm diameter flotation blocks under the cover to the gas manifold at the bank of the digester. This manifold exits the cover via a 7.5-cm flange, which is connected to a 7.5-cm diameter PVC buried pipe. The pipe then connects to the rest of the gas handling system. Eventually, the gas system will be piped to a gas handling/utilization building containing the electric generation system. Methane production from the covered lagoon is adequate to produce 20 to 25 kW on a continuous basis from the present animal population.

COSTS AND SAVINGS

A biogas fired microturbine with 25 kW maximum electrical output will operate in parallel with the utility system at a constant level of output controlled by the biogas supply. The microturbine will be supplied at no cost by Reflective Technologies.

The annual electrical use by the dairy operation is approximately 234,000 kWh with a value of $21,000, averaging almost $0.09/kWh. Benefits from using biogas to produce electricity are based upon an average of up to 23 kW, which is the estimated electrical generation. The completed methane recovery system will produce 170,000 kWh of electricity and 77,000 kJ of hot water annually, worth approximately $16,000. Total project cost is expected to be $200,000 with a simple payback of 13 years, not including the environmental benefits of odor control and water pollution prevention.  
 Although there is no capital cost of electric generation for this project, the cost of the state-bid lagoon construction was very high $60,000, compared with typical farm construction of under $25,000. Since the estimated cost of a 25 kW engine-generator is over $30,000, the higher lagoon cost offsets the absence of an electric generation capital cost for the Cal Poly system.

Preliminary gas measurements taken this summer indicate approximately 200 m3 of biogas produced daily from a partial cover of less than 50 percent of the total lagoon surface area. The biogas is being continuously flared and is maintaining a self-supporting flame. Preliminary gas analysis has indicated a methane percentage of 75 percent, with the balance of 25 percent being carbon dioxide. The biogas quantity and quality will continue to be monitored, and the microturbine generator will be incorporated sometime in 2000. Cover installation will be completed this year, at which point electricity output will be at its maximum.

ORGANIC DAIRY

As “the first organic dairy west of the Mississippi,” implementing a sustainable manure management system is important for the six year old Straus Family Creamery in Marshall, California. The dairy barn is cleaned and washed three times daily with each milking. The freestall barn, where the cows rest and eat after milking, also is flushed. The waste water goes into several holding ponds for separation of solids and liquids. The solids are then composted.

Straus is experimenting with new manure management methods. Its first project was development of a system using bacterial inoculation to break down waste from the barn and reduce solid matter and toxins. The system helps increase absorption by cover crops and stop harmful contamination from runoff. Some of the treated water is recycled to flush the freestalls.

ADVANCED INTEGRATED POND SYSTEM

Last August, the dairy was selected as a site by the Marin County Resources Conservation District for the application of an innovative process, Advanced Integrated Pond System (AIPS) with a methane digester. Dairies in the Tomales Bay watershed are potential sources of nopoint pollution. The AIPS process reduces pollution loads before disposal on land or surface waters of the basin. The operation will demonstrate low-cost treatment technology and provide opportunities for replication at other locations. Funding has been made available by the Clean Water Act, and Swanson International Engineering, Inc. of Martinez, California is the technical consultant.

Five existing ponds will form a system of anaerobic and aerobic processes to reduce wastewater organics, solids, nutrients, coliform bacteria, etc. Due to limited funds, modifications will be confined to Ponds 2 and 3. Ponds 1, 4 and 5 will be operated as part of the system, but no work will be included under the project. The effluent disposal system will be used without an upgrade.

In Ponds 2 and 3, dikes, inlet structures and pipes/valves will be modified or installed as needed. The ponds will be deepened for anaerobic digestion chambers. In Pond 2, a cover will be installed to capture methane and a meter installed to measure gas production and for atmosphere venting. The inlet for creamery wastewater will be modified for loading into Pond 3. Outlet structures and a pump station will be modified as needed for pumping wastewater to Pond 4.

The system will be operated and monitored for a six-month period. A report will be prepared on the monitoring, evaluation, findings and conclusions of the technology demonstration. At the completion of the study, the facilities will revert to the owners.

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