Performance evaluation of a solar evaporation system for liquid digestate concentration

The safe and efficient treatment and resource recovery of the large amount of liquid digestate (LD) produced by biogas plants has become an important factor limiting the development of the biogas industry. In this study, a solar evaporation system was developed based on low cost, high efficiency, and high nutrient recovery of LD concentration. The effects of different parameters, including solar radiation, LD flowrate, and daily treatment capacity, on the concentration efficiency, nutrient recovery, and thermal efficiency were investigated. Economic and enviroeconomic analyses were conducted to comprehensively assess the feasibility and environmental benefits of the system for large-scale biogas plants. The results showed that the higher the solar radiation, the higher the air temperature and its saturation vapor pressure, which led to an increase in the concentration efficiency of LD. The LD concentration efficiency can reach up to 43.33% when the average daily solar radiation was 773.08 W/m². High solar radiation also contributed to high LD temperature, leading to an increase in the percentage of free ammonia (NH₃) in the LD, which was not conducive to the recovery of ammonia nitrogen (NH₃N). By increasing the LD flow rate, the heat and mass exchange between air and LD in the spray chamber was enhanced, resulting in higher concentration efficiency. The variation of LD flow rate and daily treatment capacity had little effect on nutrient recovery. The recovery of NH₃N and soluble salts in the concentrated phase could be as high as 78.39% and 84.19%, respectively, when the initial pH of LD was 5, the air flow rate was 0.03 m³/s, and the LD flow rate was 6 L/min. In addition, the economic analysis indicated that the concentration cost of the system in treating a daily LD production of 100 tons was 1.08 USD/ton. The solar evaporation system can reduce CO₂ emissions by 2.40 × 10⁵ tons and earn carbon credits of 3.48 × 10⁶ USD over its lifetime. This study provides a new direction and reference for LD concentration.