*Article* **The Influence of Low-Temperature Food Waste Biochars on Anaerobic Digestion of Food Waste**

**Kacper Swiechowski \*, Bartosz Matyjewicz, Paweł Telega and Andrzej Białowiec ´**

Department of Applied Bioeconomy, Wrocław University of Environmental and Life Sciences, 37a Chełmo ´nskiego Str., 51-630 Wrocław, Poland; 110829@student.upwr.edu.pl (B.M.); pawel.telega@upwr.edu.pl (P.T.); andrzej.bialowiec@upwr.edu.pl (A.B.) **\*** Correspondence: kacper.swiechowski@upwr.edu.pl

**Abstract:** The proof-of-the-concept of application of low-temperature food waste biochars for the anaerobic digestion (AD) of food waste (the same substrate) was tested. The concept assumes that residual heat from biogas utilization may be reused for biochar production. Four low-temperature biochars produced under two pyrolytic temperatures 300 ◦C and 400 ◦C and under atmospheric and 15 bars pressure with 60 min retention time were used. Additionally, the biochar produced during hydrothermal carbonization (HTC) was tested. The work studied the effect of a low biochar dose (0.05 gBC <sup>×</sup> gTSsubstrate−1, or 0.65 gBC <sup>×</sup> <sup>L</sup>−1) on AD batch reactors' performance. The biochemical methane potential test took 21 days, and the process kinetics using the first-order model were determined. The results showed that biochars obtained under 400 ◦C with atmospheric pressure and under HTC conditions improve methane yield by 3.6%. It has been revealed that thermochemical pressure influences the electrical conductivity of biochars. The biomethane was produced with a rate (k) of 0.24 d<sup>−</sup>1, and the most effective biochars increased the biodegradability of food waste (FW) to 81% compared to variants without biochars (75%).

**Keywords:** methane fermentation; biogas; biomethane; biochar; pyrolysis; hydrothermal carbonization; biochemical methane potential; biogas production kinetics
