Process Simulation of Ammonia Recovery from Biogas Digestate by Air Stripping with Reduced Chemical Consumption.

Nitrogen can be recovered from the liquid fraction of biogas digestate by air stripping in order to make concentrated fertilizers which can be transported more efficiently from an agricultural area with manure nitrogen surplus to an area where it can replace inorganic fertilizers. The product is ammonia sulfate, as sulphuric acid absorption is used to produce the concentrate in a liquid form. Process simulations are carried out to investigate the operational and capital costs of a process treating 30,000 kg/h liquid digestate. Two designs with and without NaOH addition, respectively, are compared. Increased pH of the liquid digestate by NaOH addition favors NH3 over NH4+, and hence increases process efficiency reducing capital costs, but increases the operational costs three times due to the cost of NaOH. Further, the total treatment cost is highly dependent on potential sales price of ammonia sulfate. Without NaOH, the treatment cost is 3.2-7.6 US$/m(3) and with 210kg/h NaOH the treatment cost is 5.2-9.6 US$/m(3).