Microwave assisted pretreatment of urban green waste with more efficiency and less energy cost

Existing mainstream treatment methods for urban green plant wastes are unable to achieve near-zero carbon emissions nor provide full and efficient utilization of their biological value. Here, a green and efficient pretreatment method was developed that can greatly save treatment time from 24 h to 6 min by increasing the efficiency for nearly 40 times compared with conventional alkali pretreatment, while maintained a similar rates of > 65% lignin removal for woody plant wastes. Results indicated that the microwave-assisted pretreatment solution had higher utilization potential with a yield of 6592.4 mg/L soluble sugar content, which was 283.7% of that in the traditional alkali pretreatment. It also generated lower types and contents of by-products. For herbaceous wastes, the microwave-assisted pretreatment enabled the samples to achieve the similar degradation effect with lower reaction temperature and less by-products. This approach significantly reduces the energy consumption of the process by 75%. It also yields an appreciable content of reducing sugars as well as small molecule organic acids, which is favorable for subsequent bioavailability. In addition, both woody and herbaceous wastes can be treated effectively in microwave at 300 and 350 W for 6 min, respectively. This study provides a novel insight of microwave assisted pretreatment of urban green waste by accelerating the efficiency and reducing energy cost of conventionally hydrothermal pretreatment of lignocellulose bioresources. It opens up a new horizon for the use of green waste to produce sugars, small molecule acids, reducing the need for fossil fuels and food crops.