This AD technology provides advanced digestion for sludge, as well as technology for the digestion of food waste.
The available digester volume is a factor in determining the additional material, but so is the organic loading rate. Digester loading rates are designed conservatively around sludge because of its dilute nature. When implementing codigestion, loading rates can be practically increased, because external organic materials can be more concentrated with solids, especially volatile solids. This would allow for the digester volume to be more efficiently used provided the digester equipment, such as heat exchangers and mixers, can handle the new conditions.
With increased solids, existing digester mixing systems may be inadequate to properly mix the tank contents, which could impact the biogas yield and the potential for settling. Digester heating systems must also be able to tolerate the increased solids content.
At the onset of a codigestion program, the types of waste streams that will be available to the site, the condition of the material and the need to condition and separate contaminants must be considered. This is key to ensure the long-term success of any codigestion program. Separation equipment should be selected to treat the material that it receives. For instance if a plant received fats, oil and grease exclusively, the separation equipment considerations are different than if it received food waste from a grocery store. In addition, some contaminants (such as plastics and metals) will need to be separated. The organic material will have inherent grit and grit created during the separation process. To ensure that this grit does not affect the digester mechanical equipment or deposit in the digester, it must be effectively removed. A clean, consistent slurry, which can be provided to a digestion system at reliable rates along with the sludge is necessary to ensure long-term stability.
While the purpose of codigestion is that external organic material is digested within the same tank as sludge, the impact on makeup of the resultant biosolids must be considered. Depending on local regulations or the requirements of potential off-takers, a preference for separate sludge biosolids and the external organic material digestate, may need to be considered. In this case, operational practice must allow for dedicated and isolated digester volume for the external material. Plant owners can still benefit from the additional biogas, and to some this principle may simplify implementation.
A natural fit worth implementing
While challenges certainly exist when implementing a codigestion program, the natural fit is obvious in terms of the optimal use of existing assets and the positive impact on long-term environmental concerns. Some AD technology provides biological hydrolysis for sludge and for the digestion of food waste. The economic and environmental drivers are here to stay and technology is able. Transforming wastewater treatment plants into energy centers is not the future, it is now.
Michael Theodoulou, PE, is a senior product manager with GE Water & Process Technologies, with more than 16 years of experience in developing, commercializing and integrating innovative technologies, including advanced anaerobic digestion solutions. He can be reached at [email protected].