ABSTRACT
Conventional plant and meat protein production have low nitrogen usage efficiencies and high energy needs. Microbial protein (MP) is an alternative that offers higher nitrogen conversion efficiencies with low energy needs if nitrogen is recovered from a concentrated waste source such as source-separated urine. An electrochemical cell (EC) was optimized for ammonia recovery as NH3/H2 gas mixtures usable for MP production. Undiluted hydrolyzed urine was fed to the caustic-generating cathode compartment for ammonia stripping with redirection to the anode compartment for additional ammonium extraction. Using synthetic urine at 48 A m-2 the nitrogen removal efficiency reached 91.6 ± 2.1%. Tests with real urine at 20 A m-2, achieved 87.1 ± 6.0% and 68.4 ± 14.6% requiring 5.8 and 13.9 kWh kg N-1 recovered, via absorption in acid or MP medium, respectively. Energy savings through accompanying electrolytic H2 and O2 production were accounted for. Subsequently, MP was grown in fed-batch on MP medium with conventional NH4+ or urine-derived NH3 yielding 3.74 ± 1.79 and 4.44 ± 1.59 g CDW L-1, respectively. Dissolution of gaseous NH3 in MP medium maintained neutral pH in the MP reactor preventing caustic addition and thus salt accumulation. Urine-nitrogen could thus be valorized as MP via electrochemical ammonia recovery.
