Abstract: An effluent treatment system 1, 70, 80 for treating effluent which has passed through a primary treatment step such as a septic tank 50 comprises a container 5 having inlets 7 for effluent and outlets 8 for filtered effluent. The container 5 contains a biological treatment medium 41 comprising a homogeneous mixture of young Sphagnum peat and an organic peat fibre. The medium 41 is covered by a layer 40 of peat fibre for distributing the effluent and another layer 44 of peat fibre is provided below the medium 41 to induce flow of treated effluent to the outlets 34.