Covered Lagoon Digesters
Design & Operation
Covered lagoon digesters consist of large earthen or lined ponds that are tarped with a flexible or rigid cover. The cover captures biogas (primarily methane and carbon dioxide) produced from the anaerobic digestion of manure and other various farm waste products. Manure and/or organic waste is added to the lagoon, where it undergoes anaerobic digestion over a period of time, typically several months. The cover helps in capturing the biogas while preventing odors and minimizing greenhouse gas emissions. The captured biogas is then processed for power or injected into pipelines for further downstream use while the digestate is further treated via solid-liquid separation.
The liquid digestate, or effluent, is pumped to a treatment facility where polymers (flocculants) are typically added to help speed up the solid-liquid separation process. After polymer addition, the solids are separated using specialty equipment such as centrifuges, belt presses, or screw presses. The dry sludge can then be disposed of by means of trucking to landfill or a compost facility, used as a solid fertilizer, or provided to the barns for bedding. The liquid fraction can be further treated for reuse, or used directly for irrigation. The correct polymer is essential in digestate treatment as it reduces the moisture content of the sludge, resulting in a denser, more manageable solid fraction. Efficient dewatering minimizes the volume of sludge, thereby lowering transportation and disposal costs. Additionally, the improved separation reduces the load on downstream processing equipment, further cutting operational expenses and enhancing overall cost-effectiveness in waste management.
Advantages
Low Cost and Simple Design: Covered lagoon digesters offer a cost-effective solution for waste management, requiring minimal investment compared to more complex systems. Their straightforward design makes them easy to construct and maintain, making them an accessible option for many farms and facilities. This simplicity also translates to reduced operational costs and ease of use, allowing for efficient biogas production with minimal technical expertise.
Disadvantages
Limited Temperature Control and Efficiency: Covered lagoon digesters have limited temperature control, which can affect their efficiency. They rely on ambient temperatures to maintain the anaerobic digestion process, making them more suitable for warm climates. In colder regions, the digestion process can slow down significantly, reducing biogas production and overall efficiency. This limitation necessitates careful consideration of the geographical location and climate before implementation.