(166f) Economic Feasibility of Evaporation-Based Liquid Digestate Treatment for Biogas Plants

Biogas plants (BGP) are important renewable energy sources and they contribute to sustainable development. In BGP, organic waste is used to produce biogas which is either utilized for on-site electricity generation or is upgraded to biomethane and then distributed to a natural gas network or used as a fuel (compressed biomethane - CBG).

The core technology is the anaerobic digestion unit. Its development has currently achieved maturity. However, the current frontier of biogas technologies research is focused on liquid digestate processing which supports circular economy of biogas industry.

Evaporation-based systems are very suitable for further liquid digestate treatment especially when storage and transportation are associated with significant costs for operators. It reduces liquid digestate volume by thickening and at the same time preserves its fertilizing potential. Further, it can produce clean water suitable for discharge into the environment, i.e. is able to prevent ammonia from entering the outgoing freshwater stream.

On the other hand, such treatment unit has higher investment cost and reasonable payback period is not always achievable. It depends on many parameters such size of the plant, liquid digestate production, transportation and storage cost, waste heat availability, thickening-related parameters, etc.

This contribution provides detailed sensitivity analysis to various parameters in order to show what is important to consider when making decision about an investment into an evaporation-based liquid digestate treatment system. Since BGPs are costly, supporting tools are frequently applied by governments. These include investment support, higher feed-in tariffs for electricity, CHP incentives, etc. The conditions provided by such tools are also considered.

Tools supporting the biogas plant operation and investments are crucial. Reasonability of investment into an evaporation-based system for liquid digestate significantly decrease without them. However it is not the necessary condition. Beside the investment cost, the parameters that play key role are transportation cost, dry matter content in liquid digestate and cost of chemicals. Less important parameters, but still significant, are application cost of the liquid digestate, energy consumption of an evaporation unit, dry matter content in thickened concentrate and how much waste heat is available. For a specific combination of values of these parameters, the evaporation-based system for liquid digestate treatment is feasible.