In this study, the fate of nitrogen during catalytic hydrothermal liquefaction (HTL) of sewage sludge (SS) is investigated using three different catalysts (CuNi/SiO$_2$, HCOOH, CuSO$_4$) with 5 wt% loading. The bio-crude yields obtained from HTL experiments catalyzed with CuNi/SiO$_2$ are similar to those obtained through non-catalyzed experiments. HCOOH slightly increases the bio-crude yields, while maximum yields of 24.5 wt% is obtained in the presence of CuSO$_4$, which also reduces the nitrogen content by 15 % and enhances the hydrocarbons compared to the non-catalyzed HTL. Mechanistic investigations regarding the interaction of amino acids and carbohydrates by Maillard reactions are carried out using model compounds, namely lysine and lactose. CuSO$_4$ effectively increases the yield with 50 % and reduces the nitrogen content by 24 % in the bio-crude during HTL of lysine alone. In the case of the model mixtures, bio-crude yields, nitrogen content, and Maillard reactions products behaved similarly both for catalytic and non-catalytic HTL, a slight reduction of amines was found in the presence of CuSO$_4$. Hydro-char and some organic compounds are assumed to act highly reductant during catalytic HTL. Coke deposition and adsorbed poisoning by Maillard reaction products are proposed as the main reasons for the deactivation of catalysts.