Complete Mix Digesters

Complete mix digesters consist of a large tank equipped with mixers/impellers that continuously stir the contents to maintain uniform conditions and prevent settling of solids. Manure and other organic waste are continuously or semi-continuously fed into the tank. The mixing ensures even distribution of heat and microorganisms, promoting efficient digestion and production of gases that can then be used for power generation or pipeline injection.  

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

High efficiency and Better Process Control: Complete mix digesters offer high efficiency in biogas production due to continuous mixing, which ensures even distribution of heat and microorganisms. This process control results in more consistent and effective digestion, leading to higher yields of biogas. Additionally, the ability to maintain uniform conditions within the digester allows for better management of the digestion process and improved overall performance.

Disadvantages

Higher Cost and More Complex Operation: Compared to covered lagoons, complete mix digesters require a higher initial investment and have more complex operational needs. The equipment for continuous mixing and temperature control increases both the capital and maintenance costs. Additionally, the operation of these digesters demands more technical expertise, making them less accessible for smaller farms or those with limited resources.