Reclaiming Ammonia From Anaerobic Digestate As A Profitable Product

Most physical/chemical processes utilized to recover nutrients from anaerobic digestate are uneconomical since they produce nutrient products that have little value. On the other hand, numerous processes have been developed to economically destroy ammonia nitrogen by converting it to N2 gas. Those processes also produce NOx, a precursor to ground level ozone, and N2O a powerful greenhouse gas at 310 times the impact of CO2. This paper presents the initial investigations of a process that strips and recovers ammonia from anaerobic digestate as a valuable product while recovering most of the phosphate. The concentrated ammonia recovered by the process has value since it can be used in: (1) lignocellulosic pretreatment for biofuel production, (2) stripping CO2 from biogas through the “chilled ammonia process” to produce biomethane, a transportation quality fuel, and ammonium bicarbonate solids for use as a renewable fertilizer, and (3) diesel exhaust Fluid used for the destruction of NOx from combustion sources through selective catalytic reduction (SCR). Those products have sufficient value to justify the use of a nutrient recovery process as opposed to nutrient destruction processes. Nitrogen is recovered with low energy inputs and without the use of chemicals, by stripping ammonia from the digestate and condensing it as an ammonium hydroxide product. The process uses a fixed film of phototrophic organisms to increase the pH of the digestate while consuming bicarbonate, a prerequisite for the economic recovery of phosphate and ammonium hydroxide. The process can be designed to operate using several different configurations. The basic configuration produces a consistent 98% plus removal and recovery of ammonia at relatively low loading rate (0.2± Kg N/ m3/d). The second configuration produces a variable (75% to 85%) ammonia recovery at significantly higher loading rates. Over 90% of the phosphate is recovered under both operating configurations utilizing calcium chloride as a calcium source if sufficient calcium is not available in the substrate. This paper describes a four month pilot investigation that demonstrated the recovery of 98% of the influent ammonia as a concentrated (6%±) ammonium hydroxide solution, without the use of chemicals, and 90% of the phosphate with the use of calcium chloride if sufficient calcium is not present in the substrate. The pilot operation concentrated 98%+ of the influent ammonia to 3% of the influent flow. Over 90% of the influent phosphate was recovered as a rapidly settling concentrated amorphous tricalcium phosphate precipitate.