A centrifugal filter unit (300, 500, 700) for an apparatus, wherein the apparatus is for use in the treatment of a substrate with a treatment formulation, the centrifugal filter unit (300, 500, 700) comprising; a) a housing (302, 502, 702) having an outlet (304, 504, 704) configured to allow a filtrate to exit the centrifugal filter unit (300, 500, 700); b) a filter (308, 508, 708) rotatably mounted in the housing (302, 502, 702), wherein the filter (308, 508, 708) comprises a first end (310, 710) proximate a first end (312, 712) of the housing, a second end (314, 714) proximate a second end (316, 716) of the housing and a side wall (318, 718) connecting the first end (310, 710) of the filter (308, 508, 708) and the second end (314, 714) of the filter (308, 508, 708), wherein the first end (310, 710), second end (314, 714) and side wall (318, 718) of the filter (308, 508, 708) define a filter chamber (320, 720), and wherein the side wall (318, 718) comprises perforations configured to allow liquid to pass through but to prevent solid material from passing through; c) a lid (324, 524, 724) configured to substantially seal the first end (312, 712) of the housing (302, 502, 702), wherein the lid (324, 524, 724) is openable; d) an inlet (326, 526, 726) configured to allow a feed to enter the filter chamber (320, 720); e) a cap (328, 528, 728) at the first end (310, 710) of the filter (308, 508, 708), wherein the cap (328, 528, 728) comprises an aperture (330, 530, 730) that cooperates with the inlet (326, 526, 726) to allow the feed to enter the filter chamber (320, 720); f) a drive means (390, 432, 732) for rotating the filter (308, 508, 708); g) a removable portion (360, 560, 760) configured to be positioned in the filter chamber (320, 720) during operation of the centrifugal filter unit (300, 500, 700) and which when removed from the filter chamber (320, 720) following operation of the centrifugal filter unit (300, 500, 700) takes with it at least a portion of the solid material prevented from passing through the perforations; and h) an impeller (334, 534, 734) wherein the impeller is comprised in the filter chamber (320, 520, 720), wherein the removable portion comprises the impeller (334, 534, 734) and the cap (328, 528, 728). a) a housing (302, 502, 702) having an outlet (304, 504, 704) configured to allow a filtrate to exit the centrifugal filter unit (300, 500, 700); b) a filter (308, 508, 708) rotatably mounted in the housing (302, 502, 702), wherein the filter (308, 508, 708) comprises a first end (310, 710) proximate a first end (312, 712) of the housing, a second end (314, 714) proximate a second end (316, 716) of the housing and a side wall (318, 718) connecting the first end (310, 710) of the filter (308, 508, 708) and the second end (314, 714) of the filter (308, 508, 708), wherein the first end (310, 710), second end (314, 714) and side wall (318, 718) of the filter (308, 508, 708) define a filter chamber (320, 720), and wherein the side wall (318, 718) comprises perforations configured to allow liquid to pass through but to prevent solid material from passing through; c) a lid (324, 524, 724) configured to substantially seal the first end (312, 712) of the housing (302, 502, 702), wherein the lid (324, 524, 724) is openable; d) an inlet (326, 526, 726) configured to allow a feed to enter the filter chamber (320, 720); e) a cap (328, 528, 728) at the first end (310, 710) of the filter (308, 508, 708), wherein the cap (328, 528, 728) comprises an aperture (330, 530, 730) that cooperates with the inlet (326, 526, 726) to allow the feed to enter the filter chamber (320, 720); f) a drive means (390, 432, 732) for rotating the filter (308, 508, 708); g) a removable portion (360, 560, 760) configured to be positioned in the filter chamber (320, 720) during operation of the centrifugal filter unit (300, 500, 700) and which when removed from the filter chamber (320, 720) following operation of the centrifugal filter unit (300, 500, 700) takes with it at least a portion of the solid material prevented from passing through the perforations; and h) an impeller (334, 534, 734) wherein the impeller is comprised in the filter chamber (320, 520, 720),