A system for on-line monitoring of a supercritical water oxidation (SCWO) process, the system including an SCWO reactor, a feedstock supply line which supplies a feedstock to the SCWO reactor, an oxidant supply line which supplies an oxidant to the SCWO reactor, at least one sensor which measures at least one parameter of the feedstock and the oxidant, and a controller which determines a Chemical Oxygen Demand (COD) and a Heating Value (HV) of the feedstock based on the at least one parameter, such that the controller adjusts the amount of the oxidant supplied to the SCWO reactor based upon the COD and the HV of the feedstock.