De-inking paper sludge (DPS) and primary sludge (PS) containing 20% secondary sludge from a paper mill were characterized as to their suitability for the manufacture of medium density fiberboard. Compared with DPS, PS had a lower ash content, higher holocellulose content, more and longer fibers, lower pH, and higher buffering capacity. These characteristics make PS a better fiber resource for fiberboard than DPS. Fiberboards were manufactured at the Pilot Plant of Forintek (Québec City, QC, Canada) using virgin spruce-pine-fir fiber (SPF) and PS or DPS at different sludge/SPF weight ratios with 12% ureaformaldehyde resin. At an equal sludge/SPF weight ratio, PS-SPF panels had much higher mechanical properties than did DPS-SPF panels. At a PS/SPF weight ratio of 7:3, the mechanical properties of PS-SPF panels were higher than the requirements of ANSI A208.2-2002 MDF standard for Grade 120 in terms of internal bond strength, modulus of rupture, modulus of elasticity, and thickness swelling. With DPS/SPF weight ratios as low as 3:7, the tested mechanical properties of DPS-SPF panels could meet the requirements of ANSI A208.2-2002 MDF standard for Grade 120
