A screw press for separating liquid from a slurry mixture, the screw press comprising: a generally tubular body with an axially spaced-apart inlet section and outlet section, and a filter section between said inlet section and outlet section; said filter section including an axially extending stack of stationary coplanar filtration plates defining a core passage; and a rotatable screw mounted in said tubular body and extending axially through said core passage for conveying the slurry mixture from the inlet section to the outlet section while compressing and dewatering the slurry mixture by forcing at least part of the liquid of the slurry mixture to be expelled out of the tubular body through liquid passages of said filter section surrounding the core passage, said liquid passages being formed by inter-plate gaps defined between each pair of adjacent filtration plates; characterized in that the stationary filtration plates are continuously maintained fixed relative to one another and clamped in direct intimate face-to-face contact by a clamping assembly operable for applying a constant axial clamping pressure substantially uniformly about said core passage, and in that the filtration plates have a predetermined surface roughness, the inter-plate gaps being function of said predetermined surface roughness and said axially clamping pressure such that some of the discs cannot be forced apart to create preferential passages and result in intermittent decrease of the slurry pressure inside the core passage, wherein the surface roughness of the filtration plates corresponds to an average of highest and lowest points on mutually engaging surfaces of the filtration plates, said surface roughness being between 1 to 30 microns, and wherein the clamping assembly further includes first and second clamping plates (42, 44) at opposite ends of the stack of filtration plates and a plurality of stay bolts threadably engaged to a nut at opposed distal ends of the stay bolts to axially clamp the stack of filtration plates between_the first and second clamping plates. a generally tubular body with an axially spaced-apart inlet section and outlet section, and a filter section between said inlet section and outlet section; said filter section including an axially extending stack of stationary coplanar filtration plates defining a core passage; and a rotatable screw mounted in said tubular body and extending axially through said core passage for conveying the slurry mixture from the inlet section to the outlet section while compressing and dewatering the slurry mixture by forcing at least part of the liquid of the slurry mixture to be expelled out of the tubular body through liquid passages of said filter section surrounding the core passage, said liquid passages being formed by inter-plate gaps defined between each pair of adjacent filtration plates; characterized in that the stationary filtration plates are continuously maintained fixed relative to one another and clamped in direct intimate face-to-face contact by a clamping assembly operable for applying a constant axial clamping pressure substantially uniformly about said core passage, and in that the filtration plates have a predetermined surface roughness, the inter-plate gaps being function of said predetermined surface roughness and said axially clamping pressure such that some of the discs cannot be forced apart to create preferential passages and result in intermittent decrease of the slurry pressure inside the core passage, wherein the surface roughness of the filtration plates corresponds to an average of highest and lowest points on mutually engaging surfaces of the filtration plates, said surface roughness being between 1 to 30 microns, and wherein the clamping assembly further includes first and second clamping plates (42, 44) at opposite ends of the stack of filtration plates and a plurality of stay bolts threadably engaged to a nut at opposed distal ends of the stay bolts to axially clamp the stack of filtration plates between_the first and second clamping plates.