A method is provided for thickening sludge produced through a ballasted
flocculation process. The sludge and granular material from the ballasted
flocculation process are directed to a sludge separator where the sludge
is separated from the granular material. Thereafter the separated sludge
is sent to a degassing area and the sludge is degassed. A flocculating
agent is mixed or injected into the sludge. The sludge is directed to a
sludge thickener with a load at a sill calculated on the surface of the
thickener greater than 200 kgMIS/m.sup.2/day. This process produces a
thickened sludge having a concentration greater than 10 gm/l.