To resolve the problems such as the relatively low utilization value of hydrothermal products and excess processing liquid

this study proposed a multiple recycling process scheme with hydrothermal processing liquid as the HTC medium. By analyzing the transformation and distribution of nitrogen (N)

phosphorus (P)

and organic matters in hydrothermal solid and liquid phases

the feasibility and advantage of the hydrothermal recycling approach for improving the utilization value of sludge by-products and saving water resources were investigated. It was found that under the HTC condition of 250 ℃

N-containing components in hydrothermal liquid increased with recycling. After three cycles

the concentration of NH+4-N and P in liquid reached 17 000 mg/L and 200 mg/L respectively (being 4 and 5 times the primary hydrothermal processing liquid)

significantly improving the efficiency of hydrothermal resource recovery. The conversion of organic components to solid phase promoted carbon formation

with the utilization value of hydrochar products increased. In conclusion

regulating the intensity of hydrothermal reactions and increasing the recycling cycles of processing liquid tend to maximize the utilization value of hydrothermal products and improve the efficiency of water use. The results provide a fresh insight for popularizing the hydrothermal technique to achieve sludge reduction and resource utilization.