###### Abstract
Hydrothermal liquefaction (HTL) and supercritical water gasification (SCWG) are distinguished from HTC by the process temperature and thus by the main products obtainable. During HTL, biomass is processed at 300-350 \({}^{\circ}\)C and 15-20 MPa for the production of a liquid 'biocrude' [29], while during SCWG biomass is treated at temperature reaching 600-700 \({}^{\circ}\)C in supercritical water condition, with the main aim to obtain gas products (mainly hydrogen and carbon dioxide). [29; 30]. The present study focused on the upgrading of waste biomass to yield a solid bio-fuel, and thus HTL and SCWG, higher energy demanding technologies, were not taken in consideration. Since HTC occurs in aqueous medium, large amounts of carbon in feedstock distributes to the by-product aqueous phase known as process water (PW) [31]. PW consists mainly of organic acids (formic, acetic, lactic, and propionic), sugars (glucose, fructose, and xylose), furans (furfurral and 5-HMF), and phenols [32; 33; 34; 35; 36; 37]. In order to make the process more economically viable and environmentally friendly, a feasible way to treat or utilize the PW needs to be figured out. The recirculation of PW as reaction medium of the hydrothermal process is a promising approach, which could not only minimize the hydrothermal wastewater discharge but also increase the yield of biofuel and thus carbon recovery. On an industrial scale, reuse of PW could be also an efficient method of heat recovery; moreover, it could significantly reduce the water demand for the process. Recirculation of process water is a potential method for increasing the overall efficiency of the HTC system. However, it should not be the only alternative and methodologies such as biological degradation [38], oxidation treatment [39], and other possible utilization paths [40] should be considered. Recent investigations on the recirculation of PW show promising results for a cleaner production of hydrochar. [PERSON] et al. [41] determined the yield and fuel properties of hydrocarbons from HTC of three different biomass types (grape pomace, orange pomace and poultry litter) for three recirculation steps. HTC test were performed at 225 \({}^{\circ}\)C with a residence time of 10-30 min. By recirculation, mass and energy yields of the hydrochar increased. The elemental composition and heating value of hydrocarbons did not change but their combustion characteristics did: The ignition temperature and combustion reactivity decreased. The authors [41] suggested that PW might lead to a resistance to diffusion of the degraded soluble fragments from surface of biomass to the aqueous solution, leading to an increase in hydrocarbon yield. [PERSON] et al. [42] studied the effects of the recirculation of process water during HTC of poplar wood chips (temperature of 220 \({}^{\circ}\)C and residence time of 4 h). Organic acids in the liquid phase catalyzed dehydration reactions and provoked a rise of the carbon content and higher heating value (HHV) of hydrocarbons. In contrast to what reported by [PERSON] et al. [41] hydrochar yield increase was not observed. Recirculation of PW during HTC of chlorella and soybean straw was investigated by [PERSON] et al. [43] for four cycles. The yield of hydrochar was increased from 20.5% to 26.7% for chlorella and from 47.7% to 54.7% for soybean straw; the energy yield was increased by about 10%. Other studies [44; 45; 46; 47] confirmed that for the majority of the studied biomasses, the biofuel yield and properties could be improved by PW recirculation. There are few studies describing the effects of temperature during water recycling on HTC. [PERSON] et al. [48] examined effect of PW recycling at 180 and 220 \({}^{\circ}\)C. Results showed that hydrochar mass yield increases with progressing PW recirculation and higher temperatures strengthen this effect. [PERSON] et al.