An apparatus for separating oil, water, gas and solid particles from a hydrocarbon-containing fluid produced from an oil and/or gas production facility, the apparatus comprising a separation tank, the separation tank comprising: an inlet for a multiphase hydrocarbon-containing fluid comprising oil, water, gas and solid particles to be separated into its constituent oil, water, gas and solid phases, the inlet being at an upstream side of the separation tank; a solids separator in fluid communication with the inlet; a solids outlet connected to the separation tank such that solids separated by the solids separator from the multiphase hydrocarbon-containing fluid can be removed from the separation tank through said solids outlet; a first reservoir for containing an oil/water liquid mixture, from the multiphase hydrocarbon-containing fluid, which has been separated from the solids separated by the solids separator; an upper part of the separation tank being located above the first reservoir for receiving gas separated from the oil/water liquid mixture; a second reservoir for containing oil separated from the oil/water liquid mixture, the second reservoir being located downstream of the first reservoir; a weir at an end of the first reservoir that divides the first reservoir from the second reservoir, the weir being provided to separate oil from the oil/water liquid mixture; a first liquid outlet of the first reservoir connected to a lower part of the separation tank; a second liquid outlet of the second reservoir connected to the lower part of the separation tank; and a gas outlet connected to the upper part of the separation tank; characterised by a solid particles collector located beneath the solids separator such that the separated solid particles from the solids separator are collected in said solid particles collector; and a fluidisation flushing system located in the vicinity of the solid particles collector for flushing solid particles, which have collected in the solid particles collector, from the solid particles collector though the solids outlet. an inlet for a multiphase hydrocarbon-containing fluid comprising oil, water, gas and solid particles to be separated into its constituent oil, water, gas and solid phases, the inlet being at an upstream side of the separation tank; a solids separator in fluid communication with the inlet; a solids outlet connected to the separation tank such that solids separated by the solids separator from the multiphase hydrocarbon-containing fluid can be removed from the separation tank through said solids outlet; a first reservoir for containing an oil/water liquid mixture, from the multiphase hydrocarbon-containing fluid, which has been separated from the solids separated by the solids separator; an upper part of the separation tank being located above the first reservoir for receiving gas separated from the oil/water liquid mixture; a second reservoir for containing oil separated from the oil/water liquid mixture, the second reservoir being located downstream of the first reservoir; a weir at an end of the first reservoir that divides the first reservoir from the second reservoir, the weir being provided to separate oil from the oil/water liquid mixture; a first liquid outlet of the first reservoir connected to a lower part of the separation tank; a second liquid outlet of the second reservoir connected to the lower part of the separation tank; and a gas outlet connected to the upper part of the separation tank; characterised by a solid particles collector located beneath the solids separator such that the separated solid particles from the solids separator are collected in said solid particles collector; and a fluidisation flushing system located in the vicinity of the solid particles collector for flushing solid particles, which have collected in the solid particles collector, from the solid particles collector though the solids outlet.