Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry

Abstract Biogas slurry, the by-product of anaerobic digestion, represents 90 %–95% of the input feed for digesters. Treatments of biogas slurry to reduce its volume and add value to it are difficult and costly. This study used the humidification-dehumidification process to concentrate biogas slurry and find a more economical and feasible method to provide appropriate conditions for sustainable utilization of biogas slurry. The effect of heating temperature, heating time, air flow rate and initial pH on the efficiency of water removal and nutrients (ammonia nitrogen, total phosphorus, chemical oxygen demand, soluble salt) recovery was investigated. The results showed that the humidification-dehumidification system could be effectively used to concentrate the biogas slurry. The recovery of ammonia nitrogen, total phosphorus, and soluble salt in the condensed phase reached 96% when the initial pH of biogas slurry was 6, the heating temperature was 70 °C, the air flow rate was 10 L/min, and the heating time was 30 min. Under these conditions, the mass concentrations of ammonia nitrogen, total phosphorus, chemical oxygen demand, and electrical conductivity in the dilute phase were 157.49 mg/L, 0.66 mg/L, 8.70 mg/L, and 0.55 mS/cm, respectively, which was far below the mass concentrations of them in raw biogas slurry. Water removal efficiency of 34.12% and ammonia nitrogen recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 °C, heating time was 48.54 min, air flow rate was 10 L/min, and initial pH was 4.80. Treatment of biogas slurry by the humidification-dehumidification system can result in a higher efficiency of water removal and nutrient recovery in a relatively short time. Moreover, the simple process, low operation cost and requirement for influent quality is conducive to engineering applications of the humidification-dehumidification process and the value-added utilization of biogas slurry.