A process for preparing a multimodal ethylene copolymer in suspension in a reactor cascade including a first polymerization reactor and subsequent polymerization reactor(s) including polymerizing ethylene and C3-C12-1-alkenes and forming a suspension of multimodal ethylene copolymer particles in a suspension medium made from or containing a diluent, transferring the suspension into a separator, wherein the suspension is separated into multimodal ethylene copolymer particles and recovered suspension medium, purifying part of the recovered suspension medium for producing purified components of the recovered suspension medium, and recycling part of the purified components of the recovered suspension medium to the first polymerization reactor of the reactor cascade, wherein the purified components of the recovered suspension medium recycled to the first polymerization reactor and made from or containing the diluent, undergo catalytic hydrogenation before being introduced into the first polymerization reactor.