Anaerobic co-digestion is referred to the process of simultaneously managing and processing several organic waste feedstocks in one digester. It is known as a useful approach to increase the performance and eventually the profitability of biogas plants. Co-digestion has been shown to increase biogas yield by as much as 170% only by the addition of 5% of a different suitable feedstock [Maragkaki et al., 2018]. Another study reported 100% increase of biogas and 120% increase in the methane content of the produced biogas by co-digestion [Rabii et al., 2019]; nevertheless, without process optimization, introducing co-digestion is no guarantee of performance improvements.
There are several technical and financial parameters that need to be considered in order to make the best decision of adding one or several feedstocks to a digester. In co-digestion, the plant runs on a higher organic loading rate and faces significant variation in feedstock composition which might lead to process instability. Technical and economic optimization have to be performed before adding one or several feedstocks into the mixture. Proper selection of feedstocks, optimizing the recipe and process criteria in the digester tremendously reduce the risk of failure and increases the process performance. The following are among the notable factors to be considered for co-digestion of biogas plants:
Feedstock prices (tipping fee, cost or free) including the transportation cost and their availability are factors that can have a tremendous impact on the financial success of a biogas plant, often being the determining factor. There might be multiple types of suitable feedstocks available in close proximity to a biogas plant; however, availability and seasonality of feedstock supply along with the price of feedstock are parameters that can impact the choice of feedstock selection and are vital to the financial viability and stability of biogas plants.
Digester working volume which would include feedstock storage limitation and biogas handling capacity are other parameters to consider when adding new feedstocks for anaerobic co-digestion. It is important to consider how adding new feedstock will influence the hydraulic retention time and, eventually, biogas production. The digester should be able to fulfill the increase in input capacity and there should be enough storage available to handle the excess feedstock and extra biogas produced from co-digestion.
The feedstock biodegradability, total solids, volatile solids and chemical oxygen demand also highly influence the process outputs including the produced biogas and digestate. Different feedstocks have different biodegradability, chemical oxygen demand and volatile solid content; this influences the hydraulic retention time, organic loading rate and eventually the expected outputs. The feedstock total solid impacts the process targeted total solid and the volume of water that needs to be added to the digester in order to run the process at an optimal total solid.
Anaerobic co-digestion impacts the nutrient balance in the process such as Carbon over Nitrogen ratio (C/N). The difference in the nutrient content of each individual feedstock influences the overall process nutrient balance. Adding feedstock with high or low nutrient content can be an approach to balance the mono or co-digestion process.
Operating the biogas plants with a single feedstock or a mixture of feedstock does not guarantee the maximum return for outputs and investment. Process simulation and the right estimation of the process outputs significantly reduce or may even eliminate the risk associated with creating the optimal recipes and plant operational conditions. Performing calculations by hand to find the optimal recipe and process criteria are onerous and complex, time-consuming, and impractical given the immense scale of the undertaking. Biogas operators have commonly relied on using educated guesses or “biogas by trial and error” as it is commonly referred to in the industry. Finding ideal recipes and process criteria becomes even more difficult once multiple feedstocks are potentially considered to be part of the mixture going into the digester. A more cost effective and time efficient way is to estimate production outputs and returns and utilize the digester at a much more optimized level. Such a process can be called a simulation and is a robust technological approach to overcome these problems to advance plant performance.
For more information about AD simulation and optimization, don’t forget to check back at our blog as the topic will be expanded in the near future.