Abstract: A cooling system in an aftertreatment unit for a multi-cylinder internal combustion engine is disclosed. The aftertreatment unit includes a selective catalyst reduction (SCR) module, an exhaust conduit portion, and a number of reductant injectors. Each reductant injector includes an injector coolant outlet. The cooling system supplies coolant to the reductant injectors by a coolant pump. A phase separation tank, positioned at a higher gravitational potential downstream of the reductant injectors, includes a number of compartments. Each compartment includes at least one inlet portion and at least one outlet portion. Each inlet portion is in fluid communication to the injector coolant outlet and each outlet portion is in fluid communication with the coolant reservoir. A coolant anti-siphon line portion facilitates fluid communication between the reductant injectors and the phase separation tank.